WO2001075558A1

WO2001075558A1 - A system operable to identify and access information about a user

Info

Publication number: WO2001075558A1
Application number: PCT/SE2000/000641
Authority: WO
Inventors: Thomas Broquist; Lars Eriksson
Original assignee: Targian Ab
Priority date: 2000-04-03
Filing date: 2000-04-03
Publication date: 2001-10-11
Also published as: WO2001075558A9; AU4961700A; EP1410130A1

Abstract

The present invention relates to a system operable to identify and access information about a user (109) of a distributed communication system. The system comprises at least one service device (108), and at least one access device (105a) operable to provide access to said distributed communication system. The system also comprises at least one control means (101; 103) connected to said at least one access device (105a) and to said at least one service device (108), a to said at least one access device (105a) connected identification device (113) operable to identify an address of a specific user (109). The system also comprises at least one storage device (102; 104) connected to said at least one control means (101; 103), and a to said at least one control means (101; 103) connected cache means (111) operable to store mappings of said addresses and identifications for said users (109).

Description

METHOD AND APPARATUS FOR MANUFACTURING PLASTIC FOAM PRODUCTS HAVING DIFFERENT

WALL STRUCTURES

The invention relates to a method for manufacturing plastic products. The invention relates in particular to a method for manufacturing plastic products by injection molding, with different wall parts of the product having different properties. 5 It is known to manufacture plastic products by injection molding. In such manufacture, under relatively high pressure, a slightly fluid plastic mass is introduced into a mold cavity, at elevated temperature, whereafter the mass is cooled in the mold and is subsequently removed from the mold. In such a method, the wall structure is substantially determined by the

10 injection pressure, the amount of mass introduced, the type of plastic and the heating and cooling path. The density of the walls of the product is then substantially the same in the whole product. In addition, it is preferred that such a product has substantially even, and preferably uniform, wall thicknesses, in order to prevent unwanted stresses. This means, however,

15 that such products often comprise portions which have an undesired thickness and/or density, owing to the method used and other parts necessitating such thickness and/or density. This is disadvantageous in that relatively great wall thicknesses with a massive structure require relatively much material and hence require a long cooling time, and moreover yield an

20 undesirably high weight. The small wall thicknesses, by contrast, have the disadvantage that they are often constructionally undesirable and moreover do not lead to the desired properties, such as thermally insulating action. The combination of relatively thick and relatively thin parts with a massive wall structure has the disadvantage of giving rise to stresses and

25 deformations in the product as a result of inter alia different cooling times.

The object of the invention is to provide a method for manufacturing products, in particular by injection molding, whereby the above-mentioned disadvantages of the known method are avoided while the advantages thereof are maintained. To that end, a method according to the invention is characterized by the features of claim 1.

In a method according to the present invention, a plastic mass is injected into an injection mold and allowed at least partially to cool in the mold, so as to obtain a form-stable product. According to the invention, in at least a portion of the product, there is provided for gas evolution in the form of a propellant, while, or in that, the volume of the respective mold cavity during cooling is at least partly enlarged. At that location, as a result of the gas evolution, the propellant gas will provide for foam formation in the respective part of the wall of the product, so that a wall portion is obtained which has a foamy wall structure. This results in a wall portion having a greater thickness and approximately the same weight as a wall portion formed from the same amount of plastic with a massive wall structure. Surprisingly, it has been found that this yields substantially no undesired thermal or mechanical stresses in the end product; that a product can be obtained in one injection molding cycle which has different portions with different wall structures, while the mechanical properties of the product are maintained or even improved over a comparable massive injection molded product, while a considerable saving in cycle time and weight can be achieved. The fact is, owing to the wall thickness of the parts of the foamed wall structure having increased considerably at the same shot weight, weight and cycle time decrease compared to a wall part of the same thickness when injection molded to be massive. Surprisingly, it has been found that with a method according to the present invention, in a particularly simple and economic manner, thermally insulating products such as containers for food products, beverages, medicines, biological products, chemical products and the like can be obtained. Also, with a method according to the present invention, plastic products of a different kind can be manufactured, such as construction elements, shell parts, consumer products, packages and the like.

It is preferred that in a method according to the present invention at least those parts of the product are formed with a relatively massive wall structure, by which the product is to be connected with other artifacts, such as, for instance, clamping edges of cooperating containers and covers, assembly elements for construction parts such as screw and clamp joints and the like. Moreover, it is preferred that those parts of products are injection molded with a relatively massive wall structure, that are substantially determinative of the outward appearance of the product, such as transitions between different wall parts, free longitudinal edges and the like. Advantageously, relatively large wall parts of products can be designed with a foamed wall structure, to raise the insulating value, optionally to enhance the stiffness and to adapt the appearance of the product. In a method according to the present invention, in the injected plastic mass, there may be included chemical components which upon pressure and/or temperature change in the mold lead to gas formation. However, it is also possible, for instance, to introduce the propellant into the plastic mass during injection. As propellant, for instance, particularly suitably, use can be made of water vapor, C0₂ or a CO2 containing gas such as air. Chemical components suitable for producing a propellant as a result of a chemical reaction are sufficiently known from practice and are used, for instance, for pressurizing beer in containers and as foaming agent in injection molding. In a method according to the present invention, material stress reducing means can be included in the mold in or adjacent at least a portion of the mold walls, such that upon enlarging the space for the mass, the propellant will expand substantially at the location of said stress reducing means and will induce foaming. In this way, the site and extent of formation of the foamy wall structure can be determined and controlled still better. In a method according to the present invention, said space for expansion can be obtained by slightly opening the mold during cooling of a product formed therein. However, it is also possible to use a mold with . movable parts, whereby, through partial retraction of the or each movable part, the space for the mass can be enlarged to some extent, such that the desired foaming can be obtained. To that end, also springing means can be utilized in the mold cavity which maintain their initial position during injection of the mass but under the influence of the gas pressure are pressed away, to enlarge the space referred to. With the aid of a method according to the present invention, simple wall parts can be formed with a foamed structure, which have a thickness 1.5 times the thickness of a wall with a massive wall structure injection molded from the same amount of plastic. Preferably, the parts with foamed wall structure are formed with a thickness of twice, more in particular 2.5 times or more, the thickness of a wall with a massive wall structure injection molded from the same amount of plastic. An increasing thickness will in particular have an increasing insulating action, while moreover the mechanical properties will be increasingly changed, in particular improved. In addition, a wall part foamed to a high degree will become slightly elastically compressible, so that a proper shock resistance and form memory are obtained.

It is preferred that a high percentage of the product has a foamed wall structure, in particular when a high volume with respect to the weight is desired, such as, for instance, in construction panels, covering panels and thermally insulating products such as containers and panels.

The invention further relates to a plastic product, characterized by the features of claim 16.

Such a plastic product, which can for instance be manufactured with a method according to the present invention, offers the above-mentioned advantages. For each part of the respective product, an optimum wall structure can be obtained, so that a constructionally advantageous, economically favorable and particularly useful product is obtained.

Preferably, in a product according to the present invention, fastening means, if any, are fitted in those portions of the product that have a substantially massive wall structure, in order to enable proper attachment. In a first advantageous embodiment, a product according to the invention is characterized in that at least a number of the portions of the product with a foamed structure is a shell part. This means that the respective parts have a wall thickness which is relatively small with respect to the further dimensions of the respective shell part, although the wall thickness mentioned is naturally greater than a wall thickness with a massive wall structure formed from the same amount of plastic. Such a product, for instance a body part of a vehicle, a covering panel for a facade or the like, provides the advantage that it can be made of particularly light design, has a short cycle time in manufacture and yet can offer the desired mechanical properties.

In a second advantageous embodiment, a product according to the invention is characterized in that it is substantially container-shaped, provided with a bottom and an upright longitudinal wall. Advantageously, a free longitudinal edge of the container is designed with a substantially massive wall structure, so that it is form-retaining and moreover provides a good possibility for forming fastening means, for instance a clamping edge or screw thread for a cover, while in particular the longitudinal wall and the bottom have a foamed wall structure, so that a good thermally insulating action is obtained. It is then preferred that the transition between the longitudinal wall and the bottom, as well as the free longitudinal edge have a substantially massive wall structure, so that dimensional stability and form retention of the container are improved still further, for instance for the purpose of stackability. Optionally, a cover can be provided with a closing edge having a substantially massive wall structure and a closing face, located within the edge, having a substantially foamed wall structure. The closing edge is then preferably designed such that it can cooperate through clamping or screws with corresponding fastening means on the longitudinal edge of the longitudinal wall of the container. It will be clear that optionally inserts can be incorporated in a product according to the present invention, in particular in those parts that have a substantially massive wall structure, for instance for attachment to other parts.

In a third advantageous embodiment, a product according to the present invention is characterized in that springing parts are provided having a substantially massive wall structure, on or to which springing parts clamping parts are provided having a substantially foamed wall structure. The clamping parts are then preferably slightly elastically compressible. Such a product can be suitably employed, for instance, for clamping artifacts, with a clamping force being substantially provided by a spring action of the or each springing part, while the or each clamping part abuts against said artifact. The slightly elastic compressibility of the clamping parts then affords protection against damage of the artifact, while moreover the springing means, if desired, can thereby be kept at a distance from the artifact.

The invention further relates to injection molds, suitable for use in a method or for the manufacture of a product according to the invention. In the further subclaims, further advantageous embodiments of methods, products and molds according to the invention are described. To clarify the invention, methods, products and molds according to the invention will be further elucidated with reference to the drawing. In the drawing:

Fig. 1 shows, in sectional side elevation, a portion of a container with cover; Fig. 2 schematically shows, on an enlarged scale, a corner portion of a container according to Fig. 1;

Fig. 3 shows, in sectional side elevation, a portion of a CD box, in particular the clamping means thereof, with a portion of a clamped CD; Fig. 4 shows, in sectional side elevation, an alternative embodiment of a CD box with clamping means and enclosed CD;

Fig. 5 shows, in sectional side elevation, a portion of a shell part with insert for attachment thereof;

Fig. 6 schematically shows, in sectional side elevation, a portion of a mold with mold cavity, in a first embodiment;

Fig. 7 schematically shows, in sectional side elevation, a portion of a mold with mold cavity, in a second embodiment; and

Fig. 8 schematically shows, in sectional side elevation, a portion of a mold with mold cavity, in a third embodiment. In this description, the same or corresponding parts have the same or corresponding reference numerals. The products shown are always at least substantially manufactured from plastic by injection molding or a like mold forming technique, whereby in a conventional manner use is made of plastic which is introduced into a mold cavity under elevated temperature and is cooled therein. In the drawing, product parts are shown having partly massive and partly foamed wall structures. In this description, 'massive wall structure' is to be understood to include at least a wall structure which, with the respective plastic, is normally achieved, at least can be achieved, by injection molding, while 'foamed wall structure' is to be understood to include at least a wall structure whose density is reduced with respect to the massive wall structure mentioned, in particular in that gas bubbles have been obtained therein. However, different wall structures may also be obtained through compaction of parts of the wall structure starting from the wall structure as initially obtained upon injection molding. In the drawing, the wall parts having a less massive structure, in particular having a foamed wall structure, are represented with cells therein, filled with a gas. These cells are drawn as being relatively large. It will be clear, however, that in practice they will typically be smaller, while moreover the density of the foamed parts can also be smaller without recognizable gas bubbles being necessarily included therein. The cells mentioned can be both open and closed.

In Fig. 1, in sectional side elevation, a (left-hand) part of a container 1 is shown, in particular an ice box, with cover 2. The container 1 has a bottom 4 and a continuous longitudinal wall 6 extending from the bottom 4, by which an inner space 8 is enclosed. The bottom 4 is connected with the longitudinal wall 6 by a corner part 10. The upper free longitudinal edge 12 of the longitudinal wall 6 is provided with an outwardly bent clamping edge 14. This edge 14 can cooperate with a closing edge 16 of the cover 2, in a manner know per se, so that the cover 2 can be sealingly secured onto the container 1. The cover 2 comprises a closing face 18 within the continuous circumferential closing edge 16.

In the embodiment shown in Fig. 1, the longitudinal edge 12, at least the clamping edge 14, the closing edge 16 and the corner part 10 are injection molded with a massive wall structure. The other parts (such as bottom 4, longitudinal wall 6 and closing face 18) have a less dense structure, in particular a foamed structure. In the drawing, the relatively massive parts are represented with a relatively dense hatching; the parts having a less massive, more particularly having a foamed wall structure, are indicated with a less dense hatching and any cells 20 drawn therein. In Fig. 2, by way of example, a magnified corner part 10 is shown, with an adjoining portion of the bottom 4 and of the longitudinal wall 6. In this embodiment, the bottom 4 and the longitudinal wall 6 have a wall structure with enclosed gas filled cells 20, while the corner part 10 is massive. The parts 4, 6, 18 having a less massive, preferably foamed, wall structure with enclosed gas bubbles, at least cells 20, have a relatively high insulating action, in particular thermally insulating action. These parts account for the greater part of container 1 and cover 2, for instance for more than 85% thereof, based on the external surface. The relatively massive parts 10, 14, 16 then provide, on the one hand, for dimensional stability of the product and, on the other, for a proper clamping possibility of the cooperating clamping edge 14 and closing edge 16. Moreover, the massive corner part 10 provides for good stackability.

A container 1 according to Fig. 1 can be manufactured by injection molding it in a mold, with a mold cavity initially defined by the closed mold, the mold cavity being shaped such that therein a container 1 can be molded with a completely massive wall structure. That is to say that, initially, the bottom 4 and the longitudinal wall 6 too are formed with a massive wall structure. Subsequently, during cooling of the container 1, the volume of the mold cavity is at least locally enlarged, such that at least adjacent the bottom 4 and the longitudinal wall 6 the distance between the relevant wall parts of the mold cavity is increased. The plastic mass from which the container 1 is manufactured is selected such that an internal pressure is built up therein, preferably in that a propellant under pressure is included therein, or is formed therein, in particular as a result of a change in the temperature and/or the pressure in the mold, at least in the mass, for instance through the use of a conventional foaming agent. Upon the above- mentioned enlargement of the space in, at least of, the mold cavity, the volume of the respective parts 4, 6 of the container will increase as a result of the internal pressure mentioned, so that the wall thickness D in situ can increase by, for instance, a factor of 1.5, 2, 3, or more, depending inter alia on the internal pressure, the additional space being presented, the plastic used, the closing pressures used, any propellant used, and the extent of cooling of the plastic before the above-mentioned extra space is presented. The greater wall thickness and any enclosed gases enhance the insulating action (K value) considerably. Fig. 3 schematically shows, in sectional side elevation, a portion of a storage device for plate-shaped information carriers, in particular a CD box as described in more detail in European patent application 0 886 863, which publication is understood to be incorporated herein by reference. This storage box comprises a central part 22 to which a cover part 26 is pivotally connected on opposite sides through an integral injection molded hinge 24 (living hinge). On the central part 22 a number of pairs of clamping fingers 28 are provided, two clamping fingers 28 being shown on opposite sides of a groove 30. Clamped between the clamping fingers 28 is a CD 32, such that it extends approximately at right angles to the plane of the central part 22 and can be held in this position. Each clamping finger 28 comprises a first springing part 34 connected with the central part 22 and having a substantially massive wall structure, and a clamping part 36 extending from the free end of the springing part 34 and having a less massive, in particular foamed, wall structure. The clamping part 36 has primarily been injection molded with a wall thickness D approximately equal to the wall thickness D of the springing part 34, viewed in the plane of the drawing, whereafter the space of the injection mold adjacent the clamping part 36 has been enlarged on the side facing the opposite clamping finger 28, such that expansion of the clamping part 36 has occurred, thereby reducing the density, in particular through the formation of gas bubbles, at least cells 20. In this embodiment, the clamping force for retaining the CD 32 is substantially supplied by elastic bending of the springing parts 34, with the slightly convex heads 38 of the clamping parts 36 abutting against the CD 32. As a result of the slightly reduced density of the clamping parts 36, these can, if desired, be made of slightly compressible design, so that roughnesses can be compensated and damage of the CD can be prevented still better.

As products according to the invention can be injection molded with relatively uniform, relatively small wall thicknesses, cycle times, which are substantially determined by cooling times, are relatively short, while through reducing the density of parts of the product, the wall thickness can be locally raised considerably. Injection molding wall parts of a comparable thickness and a massive wall structure would lead to much longer cycle times and moreover to undesired stress and deformations in the products. By the use of a method according to the invention, this is simply prevented, the more so since relatively low shot weights can be used for relatively voluminous products. It will be clear, incidentally, that such a method can also be used for enlarging the volume of walls of at least substantially a complete product.

In Fig. 4, a portion of a CD box of the jewelbox type is shown, with springing clamping fingers 40, shown in cross section, with a CD 32 fixed thereon. In this embodiment, the clamping fingers 40 are arranged as radials of a circle, as known per se from a jewelbox, mounted on a cover part 26. In Fig. 4A, such a clamping finger 40 is shown in cross section, in the condition in which it is initially molded in a mold cavity, with a massive wall structure. Subsequently, adjacent the corner 42 in the clamping finger 40 the space of the mold cavity is slightly enlarged, in the direction away from the arm 44, such that the material of the corner 42 can expand, thereby reducing the density, to the foamed corner 42 shown in Fig. 4. As a result, the corner 42 forms a clamping projection 46, under which the CD 32 can be locked, against the respective cover part 26. The CD can be released by depressing the arms 44, whereafter the CD 32 can be pulled up over the clamping projections 46 swung clear to some extent. It will be clear, incidentally, that also wall parts such as the cover parts 26 can have a less dense wall structure than the springing parts 34, for instance to lower the weight.

In Fig. 5, there is shown, in sectional side elevation, a portion of a shell part 50 such as a mudguard or facade plate, provided with a fastening means 52 in the form of an insert 54 with internal screw thread 56. Injection molded around the insert 54 is a plastic part 58 having a massive wall structure, while around the fastening means 52 extend wall faces 60 having a less dense, preferably foamed, wall structure, which wall faces 60 are substantially determinative of the outward form of appearance of the respective shell part 50. The 'shell part' should herein be understood to comprise at least faces and singly or doubly curved sheet parts of a wall thickness which is relatively small with respect to the other dimensions, although that wall thickness as shown in Fig. 5 can be greater than the wall thickness of the massive parts. It will be clear, incidentally, that it is also possible to initially injection mold the sheet parts with a smaller wall thickness and thereupon to cause the thickness thereof to be reduced, in particular by foaming, so that the wall thickness is increased, so that still lighter shell parts 50 can be obtained with desired mechanical properties, such as stiffness and strength, impact resistance and the like. It will further be clear that the stiffness of the shell parts 50 can increase by increasing the wall thickness, even when gas inclusions such as cells 20 occur therein, as a result of an increase of the distance to the neutral line.

In Figs. 1-5, a number of examples of products are shown, which products can be formed with a method according to the present invention. It will be clear that these are shown by way of example only, and that many products of a different kind can be formed in the same or a comparable manner. Thus, for instance, thermally insulating drinking cups, containers, plant pots, furniture parts, interior parts of vehicles, serving trays and many products of a different kind can be formed in a comparable manner. In the foregoing part of the description, it has been indicated that foaming can be obtained by enlarging the space of the mold cavity, at least locally, such that as a result of internal pressure in the mass from which the product is being injection molded a local increase of the wall thickness is obtained, without additional material having to be added. In principle, this can be obtained with an injection mold known per se. Figs. 6-8 show three alternative embodiments of portions of molds particularly suitable for that purpose, in partly sectional side elevation. These mold forms will be described by way of example.

Fig. 6 shows, in sectional side elevation, a portion of a first mold half 62 and a second mold half 64, which can close against each other on a closing face 66 and thereby enclose a mold cavity 68. In this embodiment, the mold walls 70 comprise small lowered portions or undercuts 72A, 72B. The undercut 72A in the first mold half 62 has been provided in the respective mold wall 70 by milling. In such a manner, for instance, an existing mold can be simply adapted for the practice of a method according to the present invention. The undercut 72A in itself may be small, for instance one-tenth or a few tenths of a millimeter, but can also be deeper. In the second mold half the lowered portion or undercut 72B is has been formed as an integral part. The lowered portions or undercuts 72A, 72B can serve as material stress reducing means, which can lead to the surprising effect that upon opening of the mold, when in the product to be formed at least between the lowered portions 72A, 72B a relatively high internal pressure has been built up, for instance by a propellant, the wall parts of the product that abutted against the lowered portions 72A, 72B will be urged outwards, thereby increasing the wall thickness of the respective part. The product will foam and increase in volume considerably, in a way comparable to popcorn. It will be clear, incidentally, that such a lowered portion 72 can also be provided on just one side or that several lowered portions can be provided, for instance for obtaining a ribbed structure. Without wishing to be bound to any theory, this seems to be the consequence of the greater mass between the lowered portions, and lower material stress. The propellant, at least the pressure increasing agents, will find the path of least resistance and thereby push the respective parts away to reduce pressure. Fig. 7 shows, in cross section, a portion of an alternative mold, again with first and second mold halves 62, 64 closing on a closing face 66. In a second mold half 64, there is provided a movable part 74, for instance in the form of a slide, which can be moved in a direction P into and out of the mold cavity 68. With such a mold, a product can be formed by filling the mold cavity 68 with plastic, with the slide 74 brought in the condition shown in Fig. 7, whereupon, after partial cooling of the product, the slide 74 can be pulled away, such that an end face 76 thereof is, for instance, approximately flush with the mold wall 70, so that local enlargement of the wall thickness can be obtained, while reducing the density thereof as a result of foaming. Incidentally, such a mold can also be employed in a converse manner, whereby the mold cavity 68 is filled with the slide 74 in the retracted position mentioned, whereafter during cooling of the product, the slide is moved to the position shown in Fig. 7, so that the density of the material between the end face 76 and the opposite mold wall 70 is increased and hence the wall structure thereof is compacted. Such a method is advantageous, for instance, when the starting material is plastic which in itself leads to a product having a relatively low density of the wall structure. In Fig. 8, a further alternative embodiment of a mold for use in a method according to the invention is shown, in which a springing wall part 78 is provided in the wall 70 of the second mold part 64, which springing wall part 78 is moved by springs 80 at least partly into the mold cavity 68 enclosed between the first mold part 62 and the second mold part 64. The springs 80 are then preferably designed to have a spring characteristic such that at the normal filling pressure during filling of the mold cavity 68 the springing wall part 78 will continue to extend in the condition shown in Fig. 8 but upon increase of the internal pressure, for instance through gas evolution in the plastic mass, will be pushed outwards in the direction P so as to enlarge the space available in the mold cavity 68. Then an increase of the respective part of the wall thickness will occur, thereby reducing the density of that part and forming cells therein.

In a method according to the invention, preferably use is made of standard foaming agent as known from the injection molding art. Conventionally, such agents are used to obtain additional filling pressure in an injection mold. Such foaming agents are commercially available in a wide variety, for use with different plastics. Dosage, that is, the amount of added foaming agent per amount of plastic can be simply chosen depending on the desired extent of foaming. This will be immediately clear to those skilled in the art.

By way of illustration, an application example will be described, which, however, should not be construed as being limitative in any way.

A container according to Fig. 1 was injection molded from Polypropylene (PP) , using a foaming agent of type MJ 2015, supplied by the firm M. J. Additive GmbH, Germany. This foaming agent is applicable with plastics having a melting temperature above 180° C, such as PP, ABS and PS. To the plastic mass, 1.2% of foaming agent was added, based on the polymer. The container was injection molded with an initial wall thickness of approximately 0.3 mm, with a massive wall structure, in a mold as shown in Fig. 6. The undercuts in the mold walls were provided on opposite sides, adjacent the places where foaming was desired, such that the wall thickness in situ was slightly thicker (approximately 2/10 mm in the embodiment described) than the parts situated immediately adjacent thereto. After the mold had been filled at conventional filling pressure and shot weight, the plastic was allowed to cool to some extent. This gave rise to gas evolution, at least foaming, in the wall of the container, so that a pressure increase was obtained. Thereafter the mold was opened at least partly. In the parts of the wall having the slightly greater wall thickness, there arose, as a result of the greater mass present, the higher local pressure and the lower material stress, at least adjacent the skin of the wall, further foaming as a result of the sudden pressure drop on the outside of the respective wall parts. As a result, the skin of the respective parts having a slightly greater wall thickness was integrally pushed outwards relative to the adjacent parts, which substantially retained a massive wall structure. The skin of the wall parts, that is, the outer side of the product, remained substantially smooth, at least substantially retained the texture which it had obtained in the substantially massive form. It will be clear that the extent of massiveness of the parts not (further) foamed depends to a considerable extent on inter alia the filling pressure and the amount of foaming agent in the mass, the after- pressure used and the moment and the extent of opening the mold.

'Massive' and 'foamed' in this description should therefore be understood substantially in relative terms with respect to each other. The wall thickness of the relevant parts in the example described increased to approximately 2 mm, while the thickness of the other parts remained approximately 0.3 mm. The density of the respective parts decreased by approximately 30%, while the thermally insulating value increased accordingly. It will be clear that with respect to the conventional application of the foaming agents, preferably a relatively high filling pressure and/or shot weight are used, so that in the massive parts relatively little or no foaming occurs, in particular also prior to the opening of the mold.

In an alternative embodiment, for those parts of the product in which the (additional) foaming is desired, a surface tension reducing agent is added or material stress reducing means are provided, for instance notch effect enhancing ridges or the like. Such means too can further enhance foaming and hence the extent of foaming.

It will be clear that the invention is not limited to the exemplary embodiments shown in the description by way of example. Many variations thereon are possible within the scope of the invention outlined by the claims. Thus, other plastics than those mentioned can be used, while moreover other foam forming means can be applied, for instance means leading to foaming in the respective plastic without gas formation. Also, other propellants than C0₂ or C0₂ containing gases can be used. Other means may be provided for enlarging the available space in the mold cavity. Enlargement of the respective mold cavity should be understood to include moving the mold parts completely apart. It will be clear, incidentally, that other molds can be used for a method according to the present invention, for instance multipart molds with or without moving parts. Moreover, other fastening means for, for instance, covers and the like can be used, for instance integrally injection molded screw thread, in particular in those parts of products that have a substantially massive wall structure. Also, products can be manufactured having different wall parts with a slightly foamed, at least less massive wall structure, whilst these different parts can have mutually different densities.

These and many comparable variations are understood to fall within the scope of the invention outlined by the claims.

Claims

1. A method for manufacturing plastic products, wherein in a mold products are injection molded from plastic, wherein in the plastic mass from which the products are injection molded propellant or propellant evolving agents are included, which method comprises the following steps: - closing a mold, such that at least one mold cavity is formed; introducing the heated plastic mass under pressure; during cooling of the product in the mold, enlarging the space for the mass and/or lowering the pressure in the mold, such that in the partially cooled product a propellant is released so that at least locally in the walls of the product formed cavities are blown and at least locally a slightly foamy wall structure is obtained; and further cooling the product formed, the procedure being such that at least a portion of the product has a substantially massive wall structure and at least one other portion of the product has a foamed wall structure.

2. A method according to claim 1, wherein during the introduction of the plastic mass into the mold, the propellant, the propellant evolving agents and/or propellant evolution promoting means are added to the plastic mass.

3. A method according to any one of the preceding claims, wherein in the mold material stress reducing means are included in or adjacent at least a portion of the mold walls, such that upon partial opening of the mold, at least increasing the space propellant forming within the mass or being released therein under pressure, this propellant will expand at least adjacent said stress reducing means, at least will bring about foaming.

4. A method according to any one of the preceding claims, wherein the mold is slightly opened for increasing the space for the mass, such that the desired foaming can be obtained.

5. A method according to any one of the preceding claims, wherein a mold with movable parts is used, wherein through partial retraction of at least one of the movable parts the space for the mass is slightly enlarged, such that the desired foaming can be obtained.

6. A method according to any one of the preceding claims, wherein containers are formed, while in wall faces, bottom faces and/or cover faces of the boxlike products, said foamy wall structure is induced, while free longitudinal edges of and transitions between said faces are formed to be relatively massive.

7. A method according to any one of claims 1-5, wherein plastic construction parts are formed, provided with shell parts and assembly parts, while at least the assembly parts are formed to be substantially massive and the shell parts are formed to have substantially a foamed wall structure.

8. A method according to claim 6 or.7, wherein at least the wall faces and/or shell parts are formed with a ribbed structure by alternately causing more and less foaming.

9. A method according to any one of the preceding claims, wherein products are manufactured having wall parts foamed such that they have an increased thermally insulating action.

10. A method according to any one of the preceding claims, wherein as propellant CO2 or a CO2 containing gas is used.

11. A method according to any one of the preceding claims, wherein as propellant evolving agents at least one chemical component (foaming agent) is added which, under the influence of pressure change and/or temperature, provides for gas evolution .

12. A method according to any one of the preceding claims, wherein the mold is controlled such that the foamed wall parts acquire a thickness at least 1.5, preferably 2 and more particularly more than 2.5 times the thickness of a wall injection molded from the same amount of plastic in the mold in closed condition.

13. A method according to any one of the preceding claims, wherein the products are formed such that at least 50%, more particularly more than 60% and preferably more than 75% of each product has a foamed wall structure.

14. A method according to claim 13, wherein the products are formed such that at least 85% of each product has a foamed wall structure, based on the external surface of the product.

15. A method according to any one of the preceding claims, wherein a mold is used with springing means in the or each mold cavity, which springing means are held in a first position during injection of the plastic mass into the respective mold cavity, and during gas evolution in the plastic mass as a result of the elevated pressure thereby obtained are pressed away to a second position, for providing said space for expansion of the plastic mass and formation of the foamed wall structure.

16. A plastic product, substantially manufactured by injection molding, wherein at least a portion of the product has a substantially massive wall structure and at least one other portion of the product has a foamed wall structure.

17. A plastic product according to claim 16, wherein in and/or on at least one of the massive parts of the product fastening means are provided.

18. A plastic product according to claim 16 or 17, wherein at least a number of the portions of the product having a foamed structure are shell parts.

19. A plastic product according to any one of claims 16-18, wherein the product is container-shaped, provided with a bottom and a longitudinal wall extending upright from the bottom, while at least one free longitudinal edge of the container has substantially a massive wall structure, while the longitudinal wall has substantially a foamed wall structure.

20. A plastic product according to claim 19, wherein a cover is provided, which cover comprises a closing edge with which the cover is securable to the longitudinal edge of the longitudinal wall of the container-shaped part, which closing edge has a substantially massive wall structure, while the cover has a closing face, within the closing edge, which closing face has substantially a foamed wall structure,

21. A plastic product according to any one of claims 16-20, wherein in the portions of the product having a foamed wall structure, gas inclusions are included, as a result of which the respective parts have an increased thermally insulating action.

22. A plastic product according to any one of claims 16-21, wherein the product comprises springing parts which have substantially a massive wall structure, while the product additionally comprises clamping parts, attached to the springing parts and having a foamed wall structure, the clamping parts being preferably slightly elastically compressible.

23. A plastic product according to any one of claims 16-22, wherein in at least some of the portions having a substantially massive wall structure, at least one insert is included.

24. A method for adapting molds, wherein in an existing injection mold in at least one portion of the walls of a mold cavity a lowered portion is provided, such that at that location the distance between two opposite wall parts of the respective cavity is enlarged; wherein means are provided by which, in use, during the cooling of a product formed in the respective cavity, at least those parts of the mold can be pulled away to some extent where said at least one lowered portion is provided, for increasing said distance.

25. An injection mold for use in a method according to any one of claims 1-15.

26. An injection mold according to claim 25, wherein in at least one mold cavity springing means are included which upon gas evolution in the plastic mass can be pressed away for enlarging said space, the springing means having a spring characteristic such that they are in a first position during injection of the plastic mass and are brought to a pressed-away second position upon increase of the pressure in the cavity to above the injection pressure, for providing said space.

27. An injection mold provided with at least one mold cavity, wherein at least one wall part of the mold cavity is movable in a direction of the interior of the cavity, such that in the mold a product can be injection molded with said at least one wall part in a retracted position, while during cooling of the product in the mold cavity said at least one wall part can be moved in the direction of the interior of the mold cavity, for compressing at least one wall part of the product, to increase the density thereof.