WO2002026485A1

WO2002026485A1 - Thin wall injection molding

Info

Publication number: WO2002026485A1
Application number: PCT/US2001/042400
Authority: WO
Inventors: Levi A. Kishbaugh; Juan A. Cardona; David E. Pierick
Original assignee: Trexel, Inc.
Priority date: 2000-09-29
Filing date: 2001-10-01
Publication date: 2002-04-04
Also published as: AU2002211833A1

Abstract

Systems and methods for forming various thin-walled injection molded polymeric material, including microcellular polymeric material, are described. Injection-molded containers having significant void volume (weight reduction) and surprising top load strength are produced. Deviation from average (nominal) wall thickness is reduced since injection pressure, and resulting mold core deflection, are reduced, resulting in more uniform wall thicknesses. High length-to-thickness ratios, containers having locations remote from the gate that are thicker than average wall thickness, reduced cycle times, low molding temperatures, and other features are presented.

Description

Thin Wall Injection Molding

Field of the Invention

The present invention relates generally to injection molding of polymeric articles, and more particularly to the injection molding of ultra-thin microcellular articles. Background of the Invention

Polymeric foam articles can be produced by injecting a physical blowing agent into a molten polymeric stream, dispersing the blowing agent in the polymer to form a mixture of blowing agent and polymer, injecting the mixture into a mold having a desired shape, and allowing the mixture to solidify in the mold. A pressure drop in the mixture can cause the cells in the polymer to grow. As an alternative to a physical blowing agent, a chemical blowing agent can be used which undergoes a chemical reaction in the polymer material causing formation of a gas. Chemical blowing agents generally are low molecular weight organic compounds that decompose at a critical temperature and release a gas such as nitrogen, carbon dioxide, or carbon monoxide. Under some conditions cells can be made to remain isolated in such materials, and a closed-cell foamed material results. Under other, typically more violent foaming conditions, the cells rupture or become interconnected and an open-cell material results. Polymeric foam molding is well known. U.S. Patent No. 3,436,446 (Angell) describes a method and apparatus for molding foamed plastic articles with a solid skin by controlling the pressure and temperature of the mold.

Microcellular material typically is defined by polymeric foam of very small cell size. Various microcellular material is described in U.S. Patent Nos. 5,158,986 and 4,473,665. These patents describe subjecting a single-phase solution of polymeric material and physical blowing agent to thermodynamic instability required to create sites of nucleation of very high density, followed by controlled cell growth to produce microcellular material.

Microcellular molding techniques are described in the patent literature. U.S. Patent No. 4,473,665 (Martini- Vvedensky) describes a molding system and method for producing microcellular parts using polymeric pellets pre-pressurized with a gaseous blowing agent which are melted in an extruder and extruded into a pressurized mold cavity. U.S. Patent No. 5,158,986 (Cha et al.) describes a system in which polymeric pellets are introduced into an extruder barrel and melted, a supercritical carbon dioxide blowing agent is mixed with the polymer in the barrel and to form a homogenous solution of blowing agent and polymeric material, thermodynamic instability is induced in the system, thereby creating sites of nucleation in the molten polymeric material, and the nucleated material is extruded into a mold cavity. Nucleation and cell growth occur separately according to the technique; thermally-induced nucleation takes place in the barrel of the extruder, and cell growth takes place in the mold.

International Patent Application No. PCT/US99/26192 of Pierick, et al. filed November 4, 1999 and entitled "Molded Polymeric Material Including Microcellular, Injection-Molded, and Low-Density Polymeric Material", and International Patent Application No. PCT US98/00773 of Pierick, et al., filed January 16, 1998, published July 23, 1998 (WO 98/31521) and entitled "Injection Molding of Microcellular Material" describe a variety of polymeric molding techniques, systems and molded articles including reciprocating screw systems, accumulator systems, blowing agent injection systems for formation of skin/foam/skin articles, structural foam molded parts, and thin walled parts. Microcellular polymeric articles or non-microcellular polymeric foam articles can be produced having thicknesses, or cross-sectional dimension, of no more than about 0.010 inch, via injection molding. Articles of polymer with melt flow rate of less than about 40 are produced with length-to-thickness ratio at least 250: 1.

Polymeric containers are made, typically, by one of two methods: injection molding or thermoforming. Thermoforming can, in many cases, result in a lower weight, lower cost container. For commercial viability in light of thermoforming, injection molded containers typically are reduced in wall thickness. This can have two adverse effects, one being a reduction in top load strength, and the second being an increase in injection pressure required during molding. Increased pressure can result in core deflection in the mold, resulting in containers with uneven wall thicknesses. As top load strength is dictated by the thinnest part of the wall, top load strength for a container having uneven wall thickness will be lower than expected based upon average, or optimal wall thickness.

While the above and other reports represent, in some cases, useful techniques associated with the manufacture of microcellular material and the manufacture of material via injection molding, a need exists in the art for polymeric injection molding systems and processes that result in improved articles.

Summary of the Invention The present invention provides articles, methods and systems related to molded polymeric material. In one aspect, the invention provides a series of articles. One article is a molded microcellular polymeric article having a shape corresponding to that of a molding chamber. At least one portion of the article has a cross-sectional dimension of about 0.0075 inch or less. In another embodiment an article of the invention comprises a molded polymeric foam structure including at least one portion having a cross-sectional dimension of about

0.0075 inch or less.

Another article of the invention comprises a molded microcellular polymeric article having a shape corresponding to that of a molding chamber, including at least one portion having a length-to-thickness ratio of at least about 300: 1.

Another article of the invention comprises an injection molded polymeric article having an average wall thickness of less than about 0.030 inch. The distalmost location of the article relative to the gate location includes at least one section having a thickness at least 20% greater than the average wall thickness of the article. Another article of the invention comprises an injection molded polymeric container including an opening bounded by a rim having a thickness at least 20% greater than the average wall thickness of the container. The container includes a gate location at a location remote from the rim.

In another embodiment the invention provides a molded polymeric article having a shape corresponding to that of a molding chamber. The article has an average wall thickness of less than 0.030 inch and a minimum wall thickness of at least 0.028 inch. Another aspect of the invention provides a series of methods. One method comprises injecting a precursor of microcellular polymeric material into a molding chamber including at least one portion having a thickness of about 0.0075 inch or less. Then a molded microcellular polymeric article is removed from the molding chamber.

The article includes at least one portion, corresponding to the at least one portion of the molding chamber, having a cross-sectional dimension of about 0.0075 inch or less. Another embodiment involves a method for forming a molded polymeric part from a polymer molding system including an extruder and a mold. The system is constructed and arranged to deliver blowing-agent-free molten polymeric material from the extruder into the mold, to solidify the polymeric material in the mold, and to eject from the mold a first molded polymeric article having a void volume of essentially zero, all at a minimum mold clamp force. Polymeric material mixed with a supercritical fluid additive is delivered from the extruder into the mold, and the polymeric material is allowed to solidify in the mold. A second molded polymeric article then is ejected from the mold, all at a second mold clamp force no more than 95% the first clamp force. Another method of the invention involves injecting molten polymeric material into a mold, solidifying the polymeric material in the mold and ejecting from the mold a molded polymeric article having an average wall thickness of no more than 0.125 inch. The method is carried out while maintaining clamp force on the mold of no more than about 1 ton/in². Another method of the invention involves injecting polymeric material into a mold, solidifying the polymeric material in the mold and ejecting from the mold a molded polymeric article having an average wall thickness of no more than 0.100 inch, and carrying out the method while maintaining clamp force on the mold of no more than about 1.5 ton/in². In another embodiment this same method is carried out where the average wall thickness is of no more than 0.060 inch, and the clamp force is no more than about 1.75 ton/in². In another embodiment the average wall thickness is no more than 0.030 inch, and the clamp force is no more 3 ton/in². In yet another embodiment the average wall thickness is no more than 0.015 inch and the clamp force is no more than 3.5 ton/in². Another embodiment involves injecting a mixture of molten polymeric material and a supercritical fluid additive into a mold having an interior shape which, in the absence if internal pressure, defines an article having an average wall thickness of no more than 0.030 inch. The method involves solidifying the polymeric material in the mold and ejecting from the mold a molded polymeric article having an average wall thickness of no more than 0.031 inch.

In another aspect the invention provides a series of systems. One system includes polymer processing apparatus constructed and arranged to form a fluid precursor of molded microcellular polymeric material, and a molding chamber in fluid communication with the processing apparatus. The molding chamber includes at least one portion having a dimension of about 0.0075 inch or less.

Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.

Brief Description of the Drawings

Fig. 1 is a schematic illustration of injection molding apparatus useful for the invention; Fig. 2 is a schematic illustration of alternate injection molding apparatus, including an auxiliary accumulator, useful for the invention;

Fig. 3 is a photocopy of a scanning electron micrograph (SEM) image of a product produced according to the invention;

Fig. 4 is a photocopy of an SEM image of a product of the invention; and Fig. 5 is a photocopy of an SEM image of a product of the invention.

Detailed Description of the Invention

Commonly-owned, co-pending international patent publication nos. WO 98/08667, published March 5, 1998 and WO 98/31521, published July 23, 1998, are incorporated herein by reference.

The present invention provides systems, methods, and articles in connection with intrusion and injection molding of polymeric material, including microcellular polymeric material, and other techniques. For example, although injection and intrusion molding are primarily described, the invention can be modified readily by those of ordinary skill in the art for use in other molding methods such as, without limitation, low-pressure molding, co-injection molding, laminar molding, injection compression, and the like.

The present invention involves, in one aspect, the surprising discovery that particularly thin-walled parts, parts having a large length-to-thickness ratio, and parts having readily-controlled thicknesses at various locations, including thicker sections distant from mold gate locations (particularly useful in connection with food containers) can be made.

The invention involves in another aspect the use of a supercritical additive in polymeric injection molding processes. This reduces the viscosity of material injected into the mold and results in thin parts, high length-to-thickness ratios, lower-temperature molding, lower cycle times, and reduced injection pressure. The reduced injection pressure results in little or no mold core deflection and more uniform parts (better correspondence between part dimension and interior mold dimension). Even in view of prior art techniques involving supercritical fluid blowing agents, the systems, methods, and articles of the invention are surprisingly advantageous. The invention also involves molding that is carried out at significantly reduced clamp tonnage (without flash - parts are free of plastic outside of the area defined by the mold cavity), saving cost and minimizing complexity. The various embodiments and aspects of the invention will be better understood from the following definitions. As used herein, "nucleation" defines a process by which a homogeneous, single-phase solution of polymeric material, in which is dissolved molecules of a species that is a gas under ambient conditions, undergoes formations of clusters of molecules of the species that define "nucleation sites", from which cells will grow. That is, "nucleation" means a change from a homogeneous, single-phase solution to a mixture in which sites of aggregation of at least several molecules of blowing agent are formed. Nucleation defines that transitory state when gas, in solution in a polymer melt, comes out of solution to form a suspension of bubbles within the polymer melt. Generally this transition state is forced to occur by changing the solubility of the polymer melt from a state of sufficient solubility to contain a certain quantity of gas in solution to a state of insufficient solubility to contain that same quantity of gas in solution. Nucleation can be effected by subjecting the homogeneous, single-phase solution to rapid thermodynamic instability, such as rapid temperature change, rapid pressure drop, or both. Rapid pressure drop can be created using a nucleating pathway, defined below. Rapid temperature change can be created using a heated portion of an extruder, a hot glycerine bath, or the like. "Microcellular nucleation", as used herein, means nucleation at a cell density high enough to create microcellular material upon controlled expansion. A "nucleating agent" is a dispersed agent, such as talc or other filler particles, added to a polymer and able to promote formation of nucleation sites from a single-phase, homogeneous solution. Thus "nucleation sites" do not define locations, within a polymer, at which nucleating agent particles reside. "Nucleated" refers to a state of a fluid polymeric material that had contained a single-phase, homogeneous solution including a dissolved species that is a gas under ambient conditions, following an event (typically thermodynamic instability) leading to the formation of nucleation sites. "Non-nucleated" refers to a state defined by a homogeneous, single-phase solution of polymeric material and dissolved species that is a gas under ambient conditions, absent nucleation sites. A "non-nucleated" material can include nucleating agent such as talc. A "polymeric material/blowing agent mixture" can be a single-phase, non-nucleated solution of at least the two, a nucleated solution of at least the two, or a mixture in which blowing agent cells have grown. "Nucleating pathway" is meant to define a pathway that forms part of microcellular polymeric foam extrusion apparatus and in which, under conditions in which the apparatus is designed to operate (typically at pressures of from about 1500 to about 30,000 psi upstream of the nucleator and at flow rates of greater than about 10 pounds polymeric material per hour), the pressure of a single-phase solution of polymeric material admixed with blowing agent in the system drops below the saturation pressure for the particular blowing agent concentration at a rate or rates facilitating rapid nucleation. A nucleating pathway defines, optionally with other nucleating pathways, a nucleation or nucleating region of a device of the invention. "Reinforcing agent", as used herein, refers to auxiliary, essentially solid material constructed and arranged to add dimensional stability, or strength or toughness, to material. Such agents are typified by fibrous material as described in U.S. Patent Nos. 4,643,940 and 4,426,470. "Reinforcing agent" does not, by definition, necessarily include filler or other additives that are not constructed and arranged to add dimensional stability. Those of ordinary skill in the art can test an additive to determine whether it is a reinforcing agent in connection with a particular material.

For purposes of the present invention, microcellular material is defined as foamed material having an average cell size of less than about 100 microns in diameter, or material of cell density of generally greater than at least about 10⁶ cells per cubic centimeter, or preferably both. Non-microcellular foams have cell sizes and cell densities outside of these ranges. The void fraction of microcellular material generally varies from 5% to 98%. Supermicrocellular material is defined for purposes of the invention by cell sizes smaller than 1 μm and cell densities greater than 10¹² cells per cubic centimeter.

In preferred embodiments, microcellular material of the invention is produced having average cell size of less than about 50 microns. In some embodiments particularly small cell size is desired, and in these embodiments material of the invention has average cell size of less than about 20 microns, more preferably less than about 10 microns, and more preferably still less than about 5 microns. The microcellular material preferably has a maximum cell size of about 100 microns. In embodiments where particularly small cell size is desired, the material can have maximum cell size of about 50 microns, more preferably about 25 microns, more preferably about 15 microns, more preferably about 8 microns, and more preferably still about 5 microns. A set of embodiments includes all combinations of these noted average cell sizes and maximum cell sizes. For example, one embodiment in this set of embodiments includes microcellular material having an average cell size of less than about 30 microns with a maximum cell size of about 50 microns, and as another example an average cell size of less than about 30 microns with a maximum cell size of about 35 microns, etc. That is, microcellular material designed for a variety of purposes can be produced having a particular combination of average cell size and a maximum cell size preferable for that purpose. Control of cell size is described in greater detail below. In one embodiment, essentially closed-cell microcellular material is produced in accordance with the techniques of the present invention. As used herein, "essentially closed-cell" is meant to define material that, at a thickness of about 100 microns, contains no connected cell pathway through the material.

Referring now to Fig. 1, a molding system 30 is illustrated schematically that can be used to carry out molding according to a variety of embodiments of the invention. Although Fig. 1 (as well as Fig. 2) is similar to figures shown in prior, commonly- owned, published patent applications, differences between this and prior applications will become apparent from the description herein. International Patent Publication WO 98/08667, referenced above, can be consulted for a detailed description of Figs. 1 and 2. System 30 of Fig. 1 includes a barrel 32 having a first, upstream end 34, and a second, downstream end 36 connected to a molding chamber 37. Mounted for rotation within barrel 32 is a screw 38 operably connected, at its upstream end, to a drive motor 40. Although not shown in detail, screw 38 includes feed, transition, gas injection, mixing, and metering sections.

Positioned along barrel 32, optionally, are temperature control units 42. Units 42 can be used to heat a stream of pelletized or fluid polymeric material within the barrel to facilitate melting, and/or to cool the stream to control viscosity and, in some cases, blowing agent solubility. Any number of temperature control units can be provided. Barrel 32 is constructed and arranged to receive a precursor of molded polymeric material, specifically, a precursor of molded polymeric microcellular material. As used herein, "precursor of molded polymeric material" is meant to include all materials that are fluid, or can form a fluid and that subsequently can harden to form a molded polymeric article. Typically, the precursor is defined by thermoplastic polymer pellets, but can include other species. For example, in one embodiment the precursor can be defined by species that will react to form microcellular polymeric material as described, under a variety of conditions, e.g. thermosetting polymers. Preferably, a thermoplastic polymer or combination of thermoplastic polymers is selected from among amorphous, semicrystalline, and crystalline material including polyolefins such as polyethylene and polypropylene, fluoropolymers, cross-linkable polyolefins, polyamides, polyvinyl chloride, and polyaromatics such as styrenic polymers including polystyrene. Thermoplastic elastomers can be used as well, especially metallocene-catalyzed polyethylene.

Typically, introduction of the precursor of polymeric material utilizes a standard hopper 44 for containing pelletized polymeric material to be fed into the extruder barrel through orifice 46, although a precursor can be a fluid prepolymeric material injected through an orifice and polymerized within the barrel via, for example, auxiliary polymerization agents. In connection with the present invention, it is important only that a fluid stream of polymeric material be established in the system.

Immediately downstream of downstream end 48 of screw 38 in Fig. 1 is a region 50 which can be a temperature adjustment and control region, auxiliary mixing region, auxiliary pumping region, or the like. For example, region 50 can include temperature control units to adjust the temperature of a fluid polymeric stream prior to nucleation, as described below. Region 50 can include instead, or in addition, additional, standard mixing units (not shown), or a flow-control unit such as a gear pump (not shown). In another embodiment, region 50 can be replaced by a second screw in tandem which can include a cooling region. In an embodiment in which screw 38 is a reciprocating screw in an injection molding system, region 50 can define an accumulation region in which a single-phase, non-nucleated solution of polymeric material and a blowing agent is accumulated prior to injection into mold 37. Microcellular material production according to the present invention preferably uses a physical blowing agent, that is, an agent that is a gas under ambient conditions (described more fully below). However, chemical blowing agents can be used and can be formulated with polymeric pellets introduced into hopper 44. Suitable chemical blowing agents include those typically relatively low molecular weight organic compounds that decompose at a critical temperature or another condition achievable in extrusion and release a gas or gases such as nitrogen, carbon dioxide, or carbon monoxide. Examples include azo compounds such as azo dicarbonamide.

As mentioned, in preferred embodiments a physical blowing agent is used. One advantage of embodiments in which a physical blowing agent, rather than a chemical blowing agent, is used is that recyclability of product is maximized. Thus, material of the present invention in this set of embodiments includes residual chemical blowing agent, or reaction by-product of chemical blowing agent, in an amount less than that inherently found in articles blown with 0.1% by weight chemical blowing agent or more, preferably in an amount less than that inherently found in articles blown with 0.05% by weight chemical blowing agent or more. In particularly preferred embodiments, the material is characterized by being essentially free of residual chemical blowing agent or free of reaction by-products of chemical blowing agent. That is, they include less residual chemical blowing agent or by-product that is inherently found in articles blown with any chemical blowing agent. In this embodiment, along barrel 32 of system 30 is at least one port 54 in fluid communication with a source 56 of a physical blowing agent. Any of a wide variety of physical blowing agents known to those of ordinary skill in the art such as helium, hydrocarbons, chlorofluorocarbons, nitrogen, carbon dioxide, and the like can be used in connection with the invention, or mixtures thereof, and, according to a preferred embodiment, source 56 provides an atmospheric gas, preferably nitrogen or carbon dioxide as a blowing agent.

In preferred embodiments a supercritical fluid additive is used in injection molding techniques, and is mixed with polymeric material in polymer processing apparatus such as that described with reference to Fig. 1 prior to injection of the resulting mixture into a mold. The supercritical fluid additive preferably serves also as a blowing agent for forming a molded polymeric foam article, preferably a molded microcellular polymeric article. Advantages associated with use of a supercritical fluid additive are described more fully below. As blowing agent, in one embodiment solely supercritical carbon dioxide, nitrogen, or a combination is used. Supercritical carbon dioxide or nitrogen additive can be introduced into the extruder and made to rapidly form a single- phase solution with the polymeric material either by injecting the additive as a supercritical fluid, or injecting it as a gas or liquid and allowing conditions within the extruder to render it supercritical, in many cases within seconds. Injection of the additive into the extruder in a supercritical state is preferred.

The mixture (preferably a single-phase solution) of supercritical additive and polymeric material formed in this manner has a very low viscosity which advantageously allows lower temperature molding, as well as rapid filling of molds having close tolerances to form very thin molded parts, parts with very high length-to-length thickness ratios, parts including thicker distal regions, molding carried out at low clamp force, etc., discussed in greater detail below. A supercritical fluid additive also provides an advantage in that it facilitates the rapid, intimate mixing of dissimilar polymeric materials, thereby providing a method for mixing and molding dissimilar polymeric materials without post-mold delamination. Dissimilar materials include, for example, polystyrene and polypropylene, or polystyrene and polyethylene.

A pressure and metering device 58 typically is provided between blowing agent (or additive) source 56 and that at least one port 54. Device 58 can be used to meter the mass of the blowing agent between 0.01 lbs/hour and 70 lbs/hour, or between 0.04 lbs/hour and 70 lbs/hour, and more preferably between 0.2 lbs/hour and 12 lbs/hour so as to control the amount of the blowing agent in the polymeric stream within the extruder to maintain blowing agent at a desired level. The amount of blowing agent or additive in the polymeric stream can be controlled to be at a variety of levels, including between about 0.1%) and 25% by weight of the mixture, or between about 1.0% and 25% by weight, or between about 6% and 20% by weight, or between about 8% and 15% by weight, or between about 10% and 12% by weight. The particular blowing agent used (carbon dioxide, nitrogen, etc.) and the amount of blowing agent used can be selected by those of ordinary skill in the art with benefit of the present disclosure, based upon the polymer, the density reduction, cell size and physical properties desired. In embodiments where nitrogen is used as blowing agent, blowing agent is present in an amount between 0.1% and 2.5%, preferably between 0.1%) and 1.0% in some cases, and where carbon dioxide is used as blowing agent the mass flow of the blowing agent can be between 0.1% and 12% in some cases, between 0.5% and 6.0% in preferred embodiments.

Although port 54 can be located at any of a variety of locations along the barrel, according to a preferred embodiment it is located just upstream from a mixing section 60 of the screw and at a location 62 of the screw where the screw includes unbroken flights. A preferred embodiment of the blowing agent port includes multiple ports, e.g. two ports on opposing top and bottom sides of the barrel. In this preferred embodiment, port 54 is located at a region upstream from mixing section 60 of screw 38 (including highly-broken flights) at a distance upstream of the mixing section of no more than about 4 full flights, preferably no more than about 2 full flights, or no more than 1 full flight. Positioned as such, injected blowing agent is very rapidly and evenly mixed into a fluid polymeric stream to quickly produce a single-phase solution of the foamed material precursor and the blowing agent. Port 54, in a preferred embodiment, is a multi-hole port including a plurality of orifices connecting the blowing agent source with the extruder barrel. A plurality of ports can be provided about the extruder barrel at various positions radially and can be in alignment longitudinally with each other. For example, a plurality of ports can be placed at the 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clock positions about the extruder barrel, each including multiple orifices. In this manner, where each orifice is considered a blowing agent orifice, the invention includes extrusion apparatus having at least about 10, preferably at least about 40, more preferably at least about 100, more preferably at least about 300, more preferably at least about 500, and more preferably still at least about 700 blowing agent orifices in fluid communication with the extruder barrel, fluidly connecting the barrel with a source of blowing agent. Also in preferred embodiments is an arrangement in which the blowing agent orifice or orifices are positioned along the extruder barrel at a location where, when a preferred screw is mounted in the barrel, the orifice or orifices are adjacent full, unbroken flights 65. In this manner, as the screw rotates, each flight, passes, or "wipes" each orifice periodically. This wiping increases rapid mixing of blowing agent and fluid foamed material precursor by, in one embodiment, essentially rapidly opening and closing each orifice by periodically blocking each orifice, when the flight is large enough relative to the orifice to completely block the orifice when in alignment therewith. The result is a distribution of relatively finely-divided, isolated regions of blowing agent in the fluid polymeric material immediately upon injection and prior to any mixing. In this arrangement, at a standard screw revolution speed of about 30 rpm, each orifice is passed by a flight at a rate of at least about 0.5 passes per second, more preferably at least about 1 pass per second, more preferably at least about 1.5 passes per second, and more preferably still at least about 2 passes per second. In preferred embodiments, orifices are positioned at a distance of from about 15 to about 30 barrel diameters from the beginning of the screw (at upstream end 34). Downstream of region 50 is a nucleator 66 constructed to include a pressure-drop nucleating pathway 67. As used herein, "nucleating pathway" in the context of rapid pressure drop is meant to define a pathway that forms part of microcellular polymer foam extrusion apparatus and in which, under conditions in which the apparatus is designed to operate (typically at pressures of from about 1500 to about 30,000 psi upstream of the nucleator and at flow rates of greater than about 5 lbs polymeric material per hour), the pressure of a single-phase solution of polymeric material admixed with blowing agent in the system drops below the saturation pressure for the particular blowing agent concentration at a rate or rates facilitating nucleation. Nucleator 66 can be located in a variety of locations downstream of region 50 and upstream of mold 37. In a preferred embodiment, nucleator 66 is located in direct fluid communication with mold 37, such that the nucleator defines a nozzle connecting the extruder to the molding chamber and the nucleated polymer releasing end 70 defines an orifice of molding chamber 37. According to one set of embodiments, the invention lies in placing a nucleator upstream of a mold. Although not illustrated, another embodiment of nucleator 66 includes a nucleating pathway 67 constructed and arranged to have a variable cross-sectional dimension, that is, a pathway that can be adjusted in cross- section. A variable cross-section nucleating pathway allows the pressure drop rate in a stream of fluid polymeric material passing therethrough to be varied in order to achieve a desired nucleation density. While pathway 67 defines a nucleating pathway, some nucleation also may take place in the mold itself as pressure on the polymeric material drops at a very high rate during filling of the mold.

The system of Fig. 1 illustrates one general embodiment of the present invention in which a single-phase, non-nucleated solution of polymeric material and blowing agent is nucleated, via rapid pressure drop, while being urged into molding chamber 37 via the rotation action of screw 38. This embodiment illustrates an intrusion molding technique and, in this embodiment, only one blowing agent injection port 54 need be utilized. In another embodiment, screw 38 of system 30 is a reciprocating screw and a system defines an injection molding system. In this embodiment screw 38 is mounted for reciprocation within barrel 32, and includes a plurality of blowing agent inlets or injection ports 54, 55, 57, 59, and 61 arranged axially along barrel 32 and each connecting barrel 32 fluidly to pressure and metering device 58 and a blowing agent source 56. Each of injection ports 54, 55, 57, 59, and 61 can include a mechanical shut- off valve 154, 155, 157, 159, and 161 respectively, which allow the flow of blowing agent into extruder barrel 38 to be controlled as a function of axial position of reciprocating screw 38 within the barrel.

The embodiment of the invention involving a reciprocating screw can be used to produce non-microcellular foams or microcellular foam. Where non-microcellular foam is to be produced, the charge that is accumulated in distal region 50 can be a multi-phase mixture including cells of blowing agent in polymeric material, at a relatively low pressure. Injection of such a mixture into mold 37 results in cell growth and production of conventional foam. Where microcellular material is to be produced, a single-phase, non-nucleated solution is accumulated in region 50 and is injected into mold 37 while nucleation takes place.

Although not shown, molding chamber 37 can include vents to allow air within the mold to escape during injection. The vents can be sized to provide sufficient back pressure during injection to control cell growth so that uniform foaming occurs. In another embodiment, a single-phase, non-nucleated solution of polymeric material and blowing agent is nucleated while being introduced into an open mold, then the mold is closed to shape a molded article.

According to another embodiment an injection molding system utilizing a separate accumulator is provided. Referring now to Fig. 2, an injection molding system 31 includes an extruder similar to that illustrated in Fig. 1. The extruder can include a reciprocating screw as in the system of Fig. 1. At least one accumulator 78 is provided for accumulating molten polymeric material prior to injection into molding chamber 37. The extruder includes an outlet 51 fluidly connected to an inlet 79 of the accumulator via a conduit 53 for delivering a mixture, such as a non-nucleated, single-phase solution of polymeric material and blowing agent to the accumulator.

Accumulator 78 includes, within a housing 81, a plunger 83 constructed and arranged to move axially (proximally and distally) within the accumulator housing. The plunger can retract proximally and allow the accumulator to be filled with polymeric material/blowing agent through inlet 79 and then can be urged distally to force the polymeric material/blowing agent mixture into mold 37. When in a retracted position, a charge defined by single-phase solution of molten polymeric material and blowing agent is allowed to accumulate in accumulator 78. When accumulator 78 is full, a system such as, for example, a hydraulically controlled retractable injection cylinder (not shown) forces the accumulated charge through nucleator 66 and the resulting nucleated mixture into molding chamber 37. This arrangement illustrates another embodiment in which a non-nucleated, single-phase solution of polymeric material and blowing agent is nucleated as a result of the process of filling the molding chamber. Alternatively, a pressure drop nucleator can be positioned downstream of region 50 and upstream of accumulator 78, so that nucleated polymeric material is accumulated, rather than non- nucleated material, which then is injected into mold 37.

In another arrangement, a reciprocating screw extruder such as that illustrated in Fig. 1 can be used with system 31 of Fig. 2 so as to successively inject charges of polymeric material and blowing agent (which can remain non-nucleated or can be nucleated while being urged from the extruder into the accumulator) while pressure on plunger 83 remains high enough so that nucleation is prevented within the accumulator (or, if nucleated material is provided in the accumulator cell growth is prevented). When a plurality of charges have been introduced into the accumulator, shut-off valve 64 can be opened and plunger 83 driven distally to transfer the charge within the accumulator into mold 37. This can be advantageous for production of very large parts. A ball check valve 85 can be provided near the inlet 79 of the accumulator to regulate the flow of material into the accumulator and to prevent backflow into the extruder, and to maintain a system pressure required to maintain the single-phase solution of non- nucleated blowing agent and molten polymeric material or, alternatively, to prevent cell growth of nucleated material introduced therein.

System 31 can also include a blowing agent-free conduit 88 connecting an outlet 90 of the extruder with an accumulator inlet 91. Inlet 91 of the accumulator is positioned at the face of plunger 83 of the accumulator. A mechanical shut-off valve 99 is positioned along conduit 88, preferably near outlet 90. Extruder outlet 90 is located in the extruder upstream of blowing agent inlet 54 (or multiple blowing agent inlets, as in the extrusion arrangement illustrated in Fig. 1) but far enough downstream in the extruder that it can deliver blowing-agent-poor fluid polymeric material which can be essentially free of blowing agent. Thus, the system includes a first outlet 90 of the extruder positioned to deliver fluid polymeric material essentially free of blowing agent, or at reduced blowing agent concentration, from the extruder to a first inlet 91 of the accumulator, and a second outlet 51 downstream of the mixing region of the extruder positioned to deliver a mixture of fluid polymeric material and blowing agent (a higher blowing agent concentration than is delivered from outlet 90, i.e. blowing-agent-rich material) to a second inlet 79 of the accumulator. The accumulator can include heating units 96 to control the temperature of polymeric material therein. The accumulator includes an outlet that is the inlet 69 of nucleator 66. A passage (or nozzle) defining nucleating pathway 67 connects accumulator 78 to the molding chamber 37.

A series of valves, including ball check valves 98 and 85 located at the first and second inlets to the accumulator, and mechanical valves 64 and 99, respectively, control flow of material from the extruder to the accumulator and from the accumulator to the mold as desired. Practicing the method according to one embodiment of the present invention involves injecting blowing agent-poor material into a mold to form a nearly solid skin, followed by injecting blowing agent-rich material into the mold to form a foamed core. Although not illustrated, with proper synchronization this method can also be used to form articles having a foamed exterior and a solid interior. The invention involves, in all embodiments, the ability to maintain pressure throughout the system adequate to prevent premature nucleation where nucleation is not desirable (upstream of the nucleator), or cell growth where nucleation has occurred but cell growth is not desired or is desirably controlled.

The invention provides for the production of molded microcellular polymeric articles or molded non-microcellular polymeric foam articles of a shape of a molding chamber, having a void volume of at least about 5%. Preferably, the void volume is at least about 10%), more preferably at least about 15%, more preferably at least about 20%, more preferably at least about 25%. The articles of the invention include the above- noted void volumes in those sections that are of cross-sectional dimensions noted herein. The invention also provides a system and method to produce foam molded parts with surfaces replicating solid parts. At least a portion of the surface of these parts is free of splay and swirl visible to the naked human eye. Such molded parts can be produced when the temperature of the melt and mold temperature and a blowing agent concentration is optimized to allow blowing agent to diffuse away from the surface of the part so that the surface includes a skin layer essentially free of cells. This skin layer is essentially solid polymer, thus the part appears as a solid polymeric part appears to the unaided human eye. Splay and a swirl, in foamed polymeric material, is caused by bubbles at the surface being dragged against a mold wall. Where bubbles at the surface are removed, due to temperature control, splay and a swirl is avoided. In these embodiments molded parts are produced having an outer skin of essentially solid polymeric material free of cells, having a thickness at least three times the average cell size of the foam material. Preferably, the outer skin thickness is at least about five times the average cell size of the material. Another reason that molded parts can be produced, according to the invention, that are free of visible splay and a swirl is that the diffusion rate of a supercritical fluid blowing agent is believed by the inventors to be more rapid than that of typical blowing agents, allowing diffusion at the surface of the article to occur, as described, to form a solid skin layer.

As mentioned, the invention provides for the production of molded foam polymeric material, preferably microcellular material having very thin sections. The invention also provides molded polymeric material having high length-to-thickness ratios. Moreover, molded polymeric articles having very thin sections and/or very high length-to-thickness ratios can be produced in which the parts that correspond very closely in dimension to the mold in which they were formed where the mold is in an unstressed configuration. In typical prior art arrangements, as wall thickness of molded parts is reduced, pressures in mold cavities must be increased (so that the mold can be filled) whereupon deflection of the mold core can result. This can result in parts that have uneven wall thicknesses with thin sections governing the strength of the part. For example, in the case of a molded container having a bottom, side walls, and a top opening, mold core deflection can result in uneven wall thicknesses with the thinnest section governing top load strength. In such a situation a container may actually have a lower top load strength than expected from part design, because one section of the container wall is thinner than the nominal (average) wall. The present invention, with use of supercritical fluid additive, allows for reduction of injection pressures and therefore cavity pressures, and core deflection is minimized resulting in a more uniform part. For example, a supercritical fluid additive mixed with polymeric material can be injected into a mold. The mold, in the absence of internal pressure, may define an article having an average wall thickness of a particular dimension (for example, no more than 0.030 inch). The polymeric material can be solidified in the mold and ejected from the mold as an article having an average wall thickness very close to that of the internal dimension of the mold absent internal pressure (e.g., no more than about 0.031 inch). The average wall thickness increases very little, or not at all, because lower internal pressures are allowed. This results in more uniform wall thickness (where uniformal thickness is desired), leading to stronger parts (as a function of average wall thickness). According to this embodiment, the average wall thickness preferably is no more than about 0.05% greater than the average wall thickness of the mold interior in the absence of internal pressure, more preferably no more than about 0.1 %, or preferably no more than about 0.2%, and more preferably still no more than about 0.3% greater. This arrangement also allows for molding of articles, having shapes corresponding to those of molding chambers, at average wall thickness very close to their minimum wall thicknesses. For example, an average wall thickness of a molded part of the invention may be no more than 0.05% greater than that of the minimum wall thickness of the part, more preferably no more than 1%, 2%, or 3%. For example, a molded polymeric article may have an average wall thickness of less than 0.30 inch and a minimum wall thickness of at least 0.028 inch. The arrangement of the invention allows for very thin molded parts. In particular, molded polymeric articles of the invention (preferably microcellular), having a shape corresponding to that of a molding chamber, include at least one portion having a cross-sectional dimension of about 0.0075 inch or less. "Having a shape corresponding to that of a molding chamber", as used herein, means a part that is formed within a mold, preferably an injection-molded part. The shape may be identical to, or similar to, that of the molding chamber. The shape may deviate slightly from that of the molding chamber due to very slight deflection caused by internal pressure, by mold-cracking techniques, etc. In preferred embodiments, the cross-sectional dimension is no more than about 0.005 inch, and the dimension can also be no more than about 0.004 inch, 0.003 inch, 0.002 inch, and other dimensions. In preferred embodiments, the maximum thickness of the molded article is no more than about 0.080 inch, or 0.040 inch.

Reduced viscosity of precursor material injected into molds also allows for methods involving injecting precursor into a molding chamber where the chamber includes at least one portion having a thickness of about 0.0075 inch or less. A molded polymeric article is removed from the molding chamber and the article includes at least one portion, corresponding to the portion of the molding chamber, that has a cross- sectional dimension of about 0.0075 inch or less. That is, the very thin section of the mold itself produces a part having a corresponding thin section; the mold is not distorted during injection. This method, and other methods of the invention can be carried out at low injection temperatures. The precursor can be injected into the molding chamber at a molding chamber temperature of less than about 100° C. That is, the interior walls of the molding chamber do not, at any point, exceed 100° C. The technique can be carried out at mold temperatures that are lower, as well, for example less than about 75° C, less than about 50° C, less than about 30° C, or less than about 10° C.

The invention provides injection molded polymeric materials having high length- to-thickness ratios. Length-to-thickness ratio, in this context, defines the ratio of the length of extension of a portion of a polymeric molded part extending away from the injection location in the mold (gate) and the thickness across that distance. That is, an injection-molded part will include a portion that is distalmost relative to the gate, and length is defined from the gate location (location on the part corresponding to the gate of the mold) to this distalmost location. An average thickness is defined along that length, i.e., an average thickness between the gate and the distalmost location from the gate. The length from the gate to the distalmost location, divided by the average wall thickness along that length, defines the length-to-thickness ratio. The invention provides molded polymeric materials having length-to-thickness ratios of at least about 300:1, or 450:1, or 600:1, or 750:1, or 900:1, or 1200:1, or 1500:1, or 1800:1, or even 2000:1. These length- to-thickness ratios can define at least one portion of the article, or can define the entire article. For example, the entire article may have a length-to-thickness ratio of at least about 300:1, or other ratios described above. A variety of polymeric materials can be molded in accordance with the invention, including polymer having a melt flow rate of less than about 40, or one having a melt flow rate of less than about 10. •

Parts can be formed that have essentially uniform wall thicknesses, or wall thicknesses that vary. In one embodiment, parts are molded in which wall thickness increases as a function of distance from the gate, either smoothly or abruptly at various locations. For example, in one embodiment, an injection molded polymeric article of the invention is thicker at a distalmost location relative to the gate location. A food container, for example, may desirably have thin walls but a relatively thicker rim at its opening, and may be gated at a location distal from the rim. For example, a container may have a gate location in the center of its bottom, thus the rim of the opening of the container is the distalmost location relative to the gate location. The thickness of the distalmost location can be at least 20% greater than the average wall thickness, or in other embodiments, 25% greater, 30% greater, 35% greater, 40% greater, or even 50% greater. The surprising versatility of techniques of the invention allows for complete filling of a mold having distal locations of wall thickness greater than more proximal locations (relative to the gate location). In many prior art arrangements, incomplete filling of such a mold would result. Techniques of the present invention can be used to form containers, gated at their bottom (or other location significantly removed from their container opening) that are thicker than average wall thickness and include a rim defining an essentially flat, outward-facing sealing surface having a width, and a cross-section that is linear through at least 90% of its width. That is, a cross-section of the rim has a sealing surface that is essentially flat, where the sealing surface defines the widest, essentially flat portion of the rim. The outward-facing edge (edge facing away from the gate), has a cross-section that is essentially linear (flat) through at least 90% of the width. This allows for easy sealing of removable polymeric sealing wrap, or the like, to a container rim. The container may be constructed and arranged to contain food (as would be understood by those of ordinary skill in the art), and may be packaged with instructions for storage of food in the container. It is a feature of the invention that durable polymeric storage containers are formed in accordance with the invention, particularly useful for storage of the container and its contents at a temperature of below 6° C, more preferably, below 0° C. Systems of the invention include nucleating pathways that have length and cross- sectional dimensions that create a pressure drop in a fluid, single-phase solution of polymeric material and blowing agent at a pressure drop rate sufficient to cause microcellular nucleation, when the solution is passed through the nucleating pathway at rates for which the system is constructed. Since the design of a molding system and the rate of introduction of polymeric material into a mold typically are planned in conjunction with each other, those of ordinary skill in the art will understand the meaning of reference to rates for which the system is constructed. Specifically, the nucleating pathway has length and cross-sectional dimensions that can create a pressure drop at a rate of at least about 0.3 GPa/sec in fluid polymeric material and blowing agent, as a single phase solution, for example when passing through the pathway at a rate of greater than 40 pounds fluid per hour. Other flow rates and pressure drop rates suitable for microcellular nucleation are apparent from reading the present application.

In one embodiment, a channel between polymer processing apparatus of the invention and a mold includes a cell growth region and a nucleating pathway.

Specifically, the channel can include a cell growth region between a nucleating pathway and the molding chamber which increases in cross-sectional dimension in the direction of the molding chamber. The channel can also include a divergent portion between its inlet and the molding chamber, specifically, between its inlet and the nucleating pathway. The divergent portion can increase in width in a downstream direction (toward the molding chamber) while decreasing in clearance (height). The result is an increase in width while maintaining a cross-sectional area that does not change significantly. Specifically, the divergent portion increases in width by at least about 100%, preferably at least by about 200%), and more preferably still by at least 300%, while maintaining a cross-sectional area that changes by no more than about 25%, preferably by no more than about 15%, and more preferably still by no more than about 10%). The divergent portion allows for introduction of precursor material through the inlet and delivery of the precursor to the nucleating pathway while widening the pathway flow to a dimension equal to the width of the molding chamber, while also maintaining a relatively constant pressure profile in the material. This arrangement allows the nucleating pathway to have a width-to-height ratio of at least about 1.5:1, more preferably at least about 2.0:1, more preferably at least about 5.0:1, more preferably at least about 10:1, and more preferably still at least about 20: 1. The system of the invention also allows very rapid cycle times of injection molding of polymeric material of void volume of at least about 5% (or higher values noted above). In particular, a cycle time (injecting precursor material, allowing the material to solidify in the molding chamber as a polymeric article, and removing the article from the mold and repeating) can be carried out at cycle time of less than about 1 minute, more preferably less than about 45 seconds, more preferably less than about 30 seconds, and more preferably still less than about 25 seconds.

The invention also allows for significantly reduced clamp force in injection molding processes. This aspect of the invention can be described by comparison of an arrangement set up to mold solid polymeric articles, with an arrangement of the invention for molding articles where a supercritical fluid additive is included. Specifically, a polymer molding system that includes an extruder and a mold constructed and arranged to deliver blowing-agent-free molten polymeric material from the extruder into the mold and to eject a molded polymeric article from the mold having a void volume of essentially zero, will be set up with a minimum mold clamp force. That is, the system will include a clamp force sufficient to keep the mold closed during injection. The process of the invention allows such an apparatus to operate at a mold clamp force no more than 95% of the clamp force at which the system is held during molding of solid (blowing-agent-free) material. Preferably, the second mold clamp force (that clamp force required using supercritical fluid additive), is no more than about 85%, or 75%, or 65%, 55%, 45%), or even no more than about 35% of the clamp force for the solid material. More specifically, a molded polymeric article can be made that has an average wall thickness of no more than about 0.125 inch, while maintaining a clamp force on the mold of no more than about 1 ton/in . The function and advantage of these and other embodiments of the present invention will be more fully understood from the examples below. The following examples are intended to illustrate the benefits of the present invention, but do not exemplify the full scope of the invention. The examples below demonstrate advantages of injection molding of a charge of polymeric material and supercritical fluid blowing agent, in that articles are formed that have a surface, corresponding to an interior surface of a molding chamber, that is free of splay and swirl visible to the naked human eye. Example 1 : Injection Molding 40 MFR Polypropylene Copolymer into a Container with a 285: 1 L/t Ratio

A two stage injection molding machine (Engel manufacture) with a 32:1 1/d, 40 mm plasticizing unit feeding melted polymer into a 40 mm diameter plunger was used to process a 40 MFR polypropylene copolymer. The machine was constructed to run solid parts at a minimum clamp tonnage of 130 tons. The machine was set up in this example to run microcellular material at a clamp tonnage of 80 tons. The plunger and plasticizing units were connected by a spring loaded ball check joiner assembly and the plunger injects into a mold through a typical pneumatically driven shut-off nozzle. The mold used for this trial produced a container with a uniform wall thickness of 0.022 inches and a maximum flow length of 6 inches, length to thickness ratio of 272: 1. While plasticizing the polymer, 1.0% by weight of nitrogen in a supercritical state was injected into the plasticizing unit at approximately 18 diameters from the feed section. This was accomplished using one radially positioned port containing 176 orifices of .02 inch diameter and an actuated control valve to meter a mass flow rate of blowing agent at a rate of0.5 1bs/hr.

The homogeneous single phase solution of polymer and supercritical nitrogen was maintained under a pressure of at least 2000 psi during the entire cycle to maintain the single phase solution. The single phase solution was injected into the container mold producing a container with a weight of 29 grams, 9.4% weight reduced from the solid container with a weight of 32 grams, and a cellular structure with cell size of less then 10 microns. Fig. 3 is a photocopy of an SEM image (500x) of a cross-section of the container. Example 2: Injection Molding a Talc Filled Polypropylene into a Part with a Wall Thickness of 0.0125 inches

A reciprocating screw injection molding machine (Arburg manufacturer) with a 27:1 1/d, 30 mm plasticizing unit that also served as the injection plunger, was used to injection mold a 10% talc filled polypropylene copolymer. The mold produced a circular speaker cone with uniform wall thickness of 0.0125 inches and a flow length 2.066 inches, a resulting L/t ratio of 165 : 1. During plastication of the polymer, nitrogen as a supercritical fluid was injected into the extruder barrel through a port located 18 diameters from the feed throat. The injection port contained 176 orifices of .02 inch diameter and was open based on the axial position of the screw and closed based on a set time period. The nitrogen flow rate was controlled by an actuated control valve to meter a mass flow rate of 0.25 lb/hr.

The resultant parts had a part weight of 5.3 grams, a weight reduction of 8.3%) as compared to the solid part with a weight of 5.8 grams. The cell size of for the foamed parts were less than 5 microns. Fig. 4 is a photocopy of an SEM image (lOOOx) of a cross-section of the resulting product.

Example 3: Injection Molding a 35 MFR Polypropylene Copolymer into a Part with a Wall Thickness of 0.020 Inches

A reciprocating screw injection molding machine (Husky manufacturer) with a 27: 1 1/d, 42 mm plasticizing unit that also served as the injection plunger, was used to injection mold a 35 melt flow rate polypropylene copolymer. The machine was constructed to run solid parts at a minimum clamp tonnage of 150 tons. The machine was set up in this example to run microcellular material at a clamp tonnage of 120 tons. The mold produced a drinking cup with uniform wall thickness of 0.020 inches and a flow length 3.9 inches, a resulting L/t ratio of 177 : 1. During plastication of the polymer, nitrogen as a supercritical fluid was injected into the extruder barrel through a port located 18 diameters from the feed throat at 0.35%. The injection port contained 176 orifices of .02 inch diameter and was open based on the axial position of the screw and closed based on a set time period. The nitrogen flow rate was controlled by an actuated control valve to meter a mass flow rate of 2.0 lb/hr.

The resultant parts had a part weight of 8.8 grams a weight reduction of 9% as compared to the solid part with a weight of 9.65 grams. The cell size for the foamed parts averaged less than 25 microns. Fig. 5 is a photocopy of an SEM image (lOOOx) of a cross-section of the resulting product. Example 4: Injection Molding 33% Reinforced PA 6/6

A two stage injection molding machine (Engel manufacture) with a 32:1 1/d, 40 mm plasticizing unit feeding melted polymer into a 40 mm diameter plunger was used to process a 33% filled PA 6/6. The plunger and plasticizing units were connected by a spring loaded ball check joiner assembly and the plunger injects into a mold through a typical pneumatically driven shut-off nozzle. The mold used for this trial was a 2 cavity cold runner mold that produced a flapper door for an automotive H/NAC system. The part has a nominal wall thickness of 0.080 inches and the projected surface area for the two cavities is 55.6 square inches. Parts were produced using the PA 6/6 without the use of supercritical fluid. Under these conditions, 150 tons of clamp force were required to produce a flash free part. Given the projected surface area for the mold cavities, approximately 55.6 square inches, this results in 2.7 tons per square inch. This part was then produced using a single phase solution which was created by introducing nitrogen as a supercritical fluid into the extruder barrel through a port located 18 diameters from the feed throat. The injection port contained 176 orifices of .02 inch diameter and was open based on the speed at which the screw was rotating. The nitrogen flow rate was controlled by an actuated control valve to meter a mass flow rate. Using this single phase solution, the clamp tonnage needed to produce a flash free part was 30 tons which results in 0.54 tons per square inch of clamp. Example 5: Injection Molding Polycarbonate

A two stage injection molding machine (Engel manufacture) with a 32:1 1/d, 40 mm plasticizing unit feeding melted polymer into a 40 mm diameter plunger was used to process a polycarbonate resin, Calibre 2061-15-FC850122 . The plunger and plasticizing units were connected by a spring loaded ball check joiner assembly and the plunger injects into a mold through a pneumatically driven shut-off nozzle. The mold used for this trial was a two cavity cold runner mold that produces a handle for a medical assembly. This part has a nominal wall thickness of 0.080 inches and the total projected surface area for the two cavities is 19.2 square inches.

Parts were produced using the polycarbonate without the use of supercritical fluid. Under these conditions, 120 tons of clamp force were required to produce a part free from flash. Given the projected surface area, approximately 19.2 square inches, this results in 6.25 tons per square inch. This part was then produced using a single phase solution which was created by introducing nitrogen as a supercritical fluid into the extruder barrel through a port located 18 diameters from the feed throat. The injection port contained 176 orifices of .02 inch diameter and was open based on the speed at which the screw was rotating. The nitrogen flow rate was controlled by an actuated control valve to meter a mass flow rate of 0.4 lb/hr. Using this single phase solution, the clamp tonnage needed to produce a flash free part was 15 tons which results in 0.78 tons per square inch of clamp.

Table 1, below, shows reduced clamp tonnage for injection molding of flash-free parts achievable using techniques of the invention involving a supercritical fluid additive. As can be seen, the supercritical fluid additive significantly reduces clamp tonnage required.

TABLE 1

Those skilled in the art would readily appreciate that all parameters listed herein are meant to be exemplary and that actual parameters will depend upon the specific application for which the methods and apparatus of the present invention are used. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described. In the claims the words "including", "carrying", "having", and the like mean, as "comprising", including but not limited to.

What is claimed is:

Claims

1. An article comprising: a molded microcellular polymeric article having a shape corresponding to that of a molding chamber, including at least one portion having a cross-sectional dimension of about 0.0075 inch or less.

2. An article as in claim 1, having a void volume of at least about 5%.

3. An article as in claim 1, having a void volume of at least about 10%.

4. An article as in claim 1 , having a void volume of at least about 15%.

5. An article as in claim 1 , having a void volume of at least about 20%.

6. An article as in claim 1 , having a void volume of at least about 25%.

7. An article as in claim 1, the article having a maximum thickness of no more than about 0.080 inch.

8. An article as in claim 7, the article having a maximum thickness of no more than about 0.040 inch.

9. An article as in claim 1, including at least one portion having a cross-sectional dimension of no more than about 0.005 inch.

10. An article as in claim 1, including at least one portion having a cross-sectional dimension of no more than about 0.004 inch.

11. An article as in claim 1 , including at least one portion having a cross-sectional dimension of no more than about 0.003 inch.

12. An article as in claim 1, including at least one portion having a cross-sectional dimension of no more than about 0.002 inch.

13. An article as in claim 1, at least one portion of the article having a length-to- thickness ratio of at least about 300:1.

14. An article as in claim 1 , at least one portion of the article having a length-to- thickness ratio of at least about 450: 1.

15. An article as in claim 1 , at least one portion of the article having a length-to- thickness ratio of at least about 600:1.

16. An article as in claim 1, at least one portion of the article having a length-to- thickness ratio of at least about 750:1.

17. An article as in claim 1, at least one portion of the article having a length-to- thickness ratio of at least about 900:1.

18. An article as in claim 1, at least one portion of the article having a length-to- thickness ratio of at least about 1200: 1.

19. An article as in claim 1, at least one portion of the article having a length-to- thickness ratio of at least about 1500:1.

20. An article as in claim 1 , at least one portion of the article having a length-to- thickness ratio of at least about 1800:1.

21. An article as in claim 1, at least one portion of the article having a length-to- thickness ratio of at least about 2000:1.

22. An article as in claim 1, the entire article having a length-to thickness ratio of at least about 300:1.

23. An article as in claim 1, the entire article having a length-to thickness ratio of at least about 750:1.

24. An article as in claim 1, the entire article having a length-to thickness ratio of at least about 1500:1.

25. An article as in claim 1, the entire article having a length-to thickness ratio of at least about 2000:1.

26. An article as in claim 1, comprising a polymer having a melt flow rate of less than about 40.

27. An article as in claim 1 , comprising a polymer having a melt flow rate of less than about 10.

28. An article as in claim 1, comprising cells of average size of less than about 100 microns in diameter, and a cell density greater than at least about 10 cells per cubic centimeter.

29. An article as in claim 1, comprising cells of average size of less than about 75 microns in diameter, and a cell density greater than at least about 10 cells per cubic centimeter.

30. An article as in claim 1, comprising cells of average size of less than about 50 microns in diameter, and a cell density greater than at least about 10° cells per cubic centimeter.

31. An article as in claim 1 , comprising cells of average size of less than about 30 microns in diameter, and a cell density greater than at least about 10⁶ cells per cubic centimeter.

32. An article as in claim 1 , comprising cells of average size of less than about 20 microns in diameter, and a cell density greater than at least about 10⁶ cells per cubic centimeter.

33. An article as in claim 1, comprising cells of average size of less than about 10 microns and having a cell density of greater than at least about 10 cells per cubic centimeter.

34. An article as in claim 1, formed by urging a flowable material into a molding chamber and allowing the microcellular article to form therein having a shape essentially identical to that of the molding chamber.

35. An article comprising: a molded polymeric foam structure including at least one portion having a cross- sectional dimension of about 0.0075 inch or less.

36. An article comprising: a molded microcellular polymeric article having a shape corresponding to that of a molding chamber, including at least one portion having a length-to-thickness ratio of at least about 300:1.

37. An article as in claim 36, wherein the entire molded article has a length-to- thickness ratio of at least about 300: 1.

38. An article as in claim 36, including at least one portion having a length-to- thickness ratio of at least about 450:1.

39. An article as in claim 36, including at least one portion having a length-to- thickness ratio of at least about 600:1.

40. An article as in claim 36, including at least one portion having a length-to- thickness ratio of at least about 750:1.

41. An article as in claim 36, including at least one portion having a length-to- thickness ratio of at least about 900:1.

42. An article as in claim 36, including at least one portion having a length-to- thickness ratio of at least about 1200:1.

43. An article as in claim 4, including at least one portion having a length-to- thickness ratio of at least about 1500:1.

44. An article as in claim 4, including at least one portion having a length-to- thickness ratio of at least about 1800:1.

45. An article as in claim 4, including at least one portion having a length-to- thickness ratio of at least about 2000:1.

46. An article as in claim 36, having a void volume of at least about 5%.

47. An article as in claim 36, the article having a length-to-thickness ratio of at least about 300:1 across a distance of at least about 1 inch measured in a direction extending away from a gate location on the article.

48. An article as in claim 36, comprising a polymer having a melt flow rate of less than about 40.

49. An article as in claim 36, comprising a polymer having a melt flow rate of less than about 10.

50. An article comprising: an injection molded polymeric article having an average wall thickness of less than about 0.030 inch, a gate location, and a distalmost location relative to the gate location, the distalmost location including at least one section having a thickness at least 20% greater than the average wall thickness.

51. An article as in claim 50, comprising an injection molded polymeric container including an opening bounded by a rim having a thickness at least 20% greater than the average wall thickness of the container.

52. An article as in claim 51, the rim defining an essentially flat, outward-facing sealing surface with a width, a cross section of the rim having an outward-facing edge that is linear through at least 90% of the width.

53. An article comprising an injection molded polymeric container including an opening bounded by a rim having a thickness at least 20% greater than the average wall thickness of the container, wherein the container includes a gate location at a location remote from the rim.

54. An article as in claim 53 wherein the container includes a bottom and walls, the rim being located at the top of the walls, and the gate location is in the bottom of the container.

55. An article as in claim 53, comprising a container constructed and arranged to contain food.

56. An article as in claim 53, further comprising instructions for storage of food in the container.

57. An article as in claim 53, further comprising instructions for storage of the container at a temperature of below 6°C.

58. An article as in claim 53, further comprising instructions for storage of the container at a temperature of below 0°C.

59. A method comprising injecting a precursor of microcellular polymeric material into a molding chamber including at least one portion having a thickness of about 0.0075 inch or less; and removing a molded microcellular polymer article from the molding chamber including at least one portion, corresponding to the at least one portion of the molding chamber, having a cross-sectional dimension of about 0.0075 inch or less.

60. A method as in claim 59, comprising injecting the precursor into the molding chamber at a molding chamber temperature of less than about 100°C.

61. A method as in claim 59, comprising injecting the precursor into the molding chamber at a molding chamber temperature of less than about 75 °C.

62. A method as in claim 59, comprising injecting the precursor into the molding chamber at a molding chamber temperature of less than about 50°C.

63. A method as in claim 59, comprising injecting the precursor into the molding chamber at a molding chamber temperature of less than about 30°C.

64. A method as in claim 59, comprising injecting the precursor into the molding chamber at a molding chamber temperature of less than about 10°C.

65. A method as in claim 59, comprising forming a molded microcellular polymeric article having a void volume of at least about 5%.

66. A method as in claim 65, comprising cyclically repeating the method at a cycle time of less than about 1 minute.

67. A method as in claim 65, comprising cyclically repeating the method at a cycle time of less than about 45 seconds.

68. A method as in claim 65, comprising cyclically repeating the method at a cycle time of less than about 30 seconds.

69. A method as in claim 65, comprising cyclically repeating the method at a cycle time of less than about 25 seconds.

70. A method as in claim 59, comprising injecting the precursor comprising a fluid, single-phase solution of polymeric material and blowing agent that is a gas under ambient conditions into the molding chamber while subjecting the solution to a rapid pressure drop at a pressure drop rate sufficient to cause microcellular nucleation, and essentially immediately thereafter allowing and controlling cell growth by subjecting the material to a second pressure drop that is less than the first pressure drop and is at a decreasing rate.

71. A method as in claim 59, comprising establishing a stream of molten polymeric material flowing in polymer processing apparatus; injecting a blowing agent that is gas under ambient conditions into the flowing polymeric stream to form the precursor; and injecting the precursor into the molding chamber.

72. A method as in claim 71 , wherein the blowing agent is an atmospheric gas.

73. A method as in claim 72, wherein the blowing agent comprises nitrogen.

74. A method as in claim 72, wherein the blowing agent comprises carbon dioxide.

75. A method as in claim 72, wherein the blowing agent comprises helium.

76. A method as in claim 72, wherein the blowing agent consists of carbon dioxide.

77. A method as in claim 71, comprising introducing the blowing agent from a blowing agent source into the flowing stream of polymeric material through a plurality of orifices in a blowing agent port fluidly connecting a barrel of the polymeric processing apparatus with the blowing agent source.

78. A method as in claim 77, wherein the blowing agent port includes at least 10 orifices.

79. A method as in claim 77, wherein the blowing agent port includes at least 100 orifices.

80. A method as in claim 77, wherein the blowing agent port includes at least 500 orifices.

81. A method as in claim 77, wherein the blowing agent port includes at least 700 orifices.

82. A method as in claim 78, further comprising periodically passing respective orifices with at least one flight of a screw constructed and arranged to rotate within the barrel.

83. A method as in claim 71, further comprising accumulating a charge of a single- phase solution of polymeric material and blowing agent in an accumulator after forming the solution in the polymer processing apparatus, and then injecting the charge from the accumulator into the molding chamber.

84. A system comprising: polymer processing apparatus constructed and arranged to form a fluid precursor of molded microcellular polymeric material; and a molding chamber in fluid communication with the processing apparatus, the molding chamber including at least one portion having a dimension of about 0.0075 inch or less.

85. A system as in claim 84, further comprising a channel between the polymer processing apparatus and the molding chamber, the channel including a divergent portion between the inlet and the molding chamber that increases in width by at least about 100%) while maintaining a cross-section area changing by no more than about 25%.

86. A system as in claim 85, the channel including a nucleating pathway between the divergent portion and the molding chamber having length and cross-sectional dimensions that, when a fluid, single-phase solution of polymeric material and blowing agent is passed through the pathway at rates for which the system is constructed, creates a pressure drop in the fluid polymer at a pressure drop rate sufficient to cause microcellular nucleation.

87. A system as in claim 85, the divergent portion decreasing in thickness while increasing in width between the inlet and the molding chamber.

88. A method for forming a molded polymeric part comprising: providing a polymer molding system including an extruder and a mold, the system constructed and arranged to deliver blowing-agent-free molten polymeric material from the extruder into the mold, to solidify the polymeric material in the mold, and to eject from the mold a first molded polymeric article having a void volume of essentially zero, all at a minimum mold clamp force; delivering polymeric material mixed with a supercritical fluid additive from the extruder into the mold, allowing the polymeric material to solidify in the mold, and ejecting a second molded polymeric article from the mold, all at a second mold clamp force no more than 95% the first clamp force.

89. A method as in claim 88, wherein the second mold clamp force is no more than 85%) the first clamp force.

90. A method as in claim 88, wherein the second mold clamp force is no more than 75%> the first clamp force.

91. A method as in claim 88, wherein the second mold clamp force is no more than 65%) the first clamp force.

92. A method as in claim 88, wherein the second mold clamp force is no more than 55%) the first clamp force.

93. A method as in claim 88, wherein the second molded polymeric article is flash free.

94. A method for forming a molded polymeric part comprising: injecting molten polymeric material into a mold; solidifying the polymeric material in the mold and ejecting from the mold a molded polymeric article having an average wall thickness of no more than 0.125 inch; carrying out the method while maintaining clamp force on the mold of no more than about 1 ton/in².

95. A method as in claim 94, wherein the injecting step comprises injecting a mixture of molten polymeric material and a supercritical fluid additive into the mold.

96. A method as in claim 95, the injecting step comprising injecting a single-phase solution of molten polymeric material and a supercritical fluid additive into the mold.

97. A method for forming a molded polymeric part comprising: injecting molten polymeric material into a mold; solidifying the polymeric material in the mold and ejecting from the mold a molded polymeric article having an average wall thickness of no more than 0.100 inch; carrying out the method while maintaining clamp force on the mold of no more than about 1.5 ton/in .

98. A method for forming a molded polymeric part comprising: injecting molten polymeric material into a mold; solidifying the polymeric material in the mold and ejecting from the mold a molded polymeric article having an average wall thickness of no more than 0.060 inch; carrying out the method while maintaining clamp force on the mold of no more than about 1.75 ton/in .

99. A method for forming a molded polymeric part comprising: injecting molten polymeric material into a mold; solidifying the polymeric material in the mold and ejecting from the mold a molded polymeric article having an average wall thickness of no more than 0.030 inch; carrying out the method while maintaining clamp force on the mold of no more than about 3 ton/in².

100. A method for forming a molded polymeric part comprising: injecting molten polymeric material into a mold; solidifying the polymeric material in the mold and ejecting from the mold a molded polymeric article having an average wall thickness of no more than 0.015 inch; carrying out the method while maintaining clamp force on the mold of no more than about 3.5 ton/in².

101. A method for forming a molded polymeric part comprising: injecting a mixture of molten polymeric material and a supercritical fluid additive into a mold having an interior shape which, in the absence of internal pressure, defines an article having an average wall thickness of no more than 0.030 inch; solidifying the polymeric material in the mold and ejecting from the mold a molded polymeric article having an average wall thickness of no more than 0.031 inch.

102. A molded polymeric article having a shape corresponding to that of a molding chamber, having an average wall thickness of less than 0.030 inch and a minimum wall thickness of at least 0.028 inch.