DE3221273A1 - Process and device for producing a hollow body from thermoplastics - Google Patents

Abstract

Process for producing a hollow body with non-circular cross-section from a thermoplastic by extruding a tube and subsequently inflating it in an intermediate mould to give a parison (intermediate moulding) which has a circular cross-section and is reshaped to form the hollow body in a final blow mould. For reasons of material economy it is desirable to achieve a substantially uniform wall thickness of the hollow body with, at the same time, the least possible amount of material to be pinched off. To this end the tube is extruded from the extruder nozzle directly into the intermediate mould, it being sealed at an early stage by pinching its lower end, preblown to a slightly pear-shaped form approximately when it reaches a third of its length and blown to give the parison after completion of extrusion, said parison being designed such that its circumference is the same as, or if necessary a few per cent smaller than, the circumference of the hollow body that is to be produced, said hollow body being then subsequently produced in the final blow mould without significant further stretching of the parison. The device for carrying out the process comprises a blow moulding machine with reciprocating screw extruder and an extruder nozzle having a channel for inflation air, a split intermediate mould being disposed below said extruder nozzle, whose cylindrical mould cavity has a circumference the same as or at the most a few per cent less than the circumference of the hollow body that is to be produced. The unsplit base of the intermediate mould is height-adjustable with respect to the mould halves so that the neck shape and wall thickness in the neck area can be influenced. An analog-controllable compressed air solenoid valve is provided in the compressed air line to the extruder nozzle, since the blowing operation must be controlled very precisely.

Description

Method and device for producing a hollow body from

thermoplastic plastic A major problem in manufacturing of plastic hollow bodies, e.g. bottles, with a cross-section that deviates from the circular cross-section Cross-section consists of at least one over the actual bottle body (So apart from the floor, shoulder and neck area) constant wall thickness to be observed.

The blow molding process is common today, in which from an extruded tube is assumed, the circumference of which is almost equal to the largest The scope of the hollow body to be produced is.

For this purpose, a hose of the appropriate diameter and with the desired Wall thickness freely hanging continuously extruded and after reaching one of the height of the hollow body corresponding length taken from a final blow mold, in which the tubular preform is now made into the finished hollow body by means of a blow quill is blown out, the quill at the same time the neck and mouth of the hollow body calibrated, while the final blow mold-already on closing the considerable remaining Has squeezed excess material, especially in the shoulder and neck area. There The amount of material squeezed off, depending on the shape of the hollow body, ranges from 10% to 25 % and more of the weight of the finished hollow body is this type the production of hollow bodies relatively uneconomical, because not only the squeezed-off portion of the material must be removed again, but the apparatus-related one Effort, but above all the power requirements for drive, heating and cooling of the blow molding machine is significantly higher than for the manufacture of the actual hollow body necessary.

In the case of hollow bodies with rotationally symmetrical over their entire length or their entire height In contrast, the cross section of a hose with a considerably smaller diameter, for example equal to the diameter of the neck of the hollow body to be produced, However, greater wall thickness can be assumed, with the tube then in the final blow mold is blown out to the finished hollow body without resulting in a non-uniform Wall thickness distribution is coming.

For hollow bodies with a cross-section deviating from the circular cross-section this type of preparation is detailed from those described in US Pat. No. 3,892,829 given reasons not feasible. The aforementioned US-PS therefore has its own Intermediate molding station provided in which an intermediate molding is produced, the has a circumference or a surface while maintaining the circular cross-section, the or the approximately 25 g under the circumference or the surface of the to be produced Hollow body lie. A uniform wall thickness over the circumference of the hollow body cannot be achieved in this way, however, because the stretching that is still necessary of the intermediate molding in the final blow mold takes place exclusively in the surface areas of the intermediate molding, which does not cook on the cooled inner wall of the mold abut, in the case of a hollow body with an approximately rectangular or elliptical Cross-section in the area of the short side of the rectangle or the long axis of the ellipse. In order to obtain a sufficient wall thickness in these areas, it is inevitable be worked with an excess of material. So the finished hollow body has over the greater part of its surface, namely in the area of its two large side surfaces, an unnecessarily large wall thickness. Furthermore, the additional effort also turns out to be for a complete intermediate blow station, the not just your own Blow quill but also own transfer devices and the associated drives required as a disadvantage, so that this known method does not enter into the Has found practice.

A process for the production of angularly delimited hollow bodies with more uniform The wall thickness of a plastic tube is known from DE-OS 14 54 926. Here the tube section transferred into the final blow mold opens when the mold is open the circumference of the hollow body to be produced is inflated and then the shape for upsetting the wall parts that assume a comparatively thinner wall thickness during the blow molding process closed in the area of the edges and corners. By upsetting the blank in the form it is achieved that the finished hollow body has a constant wall thickness everywhere having. Apart from the fact that to carry out this procedure a ten movable Parts comprehensive final blow mold is required, but here is always the Danger that parts of the blank in the corner areas of the closing blow mold be squeezed. In addition, the process is only possible with relatively thick-walled hollow bodies, such as canisters, applicable, but not for so-called flat bottles, especially those where the bottle neck in the bottle body 'in the area the narrow side merges into two relatively large radii of curvature.

Another method for producing hollow plastic bodies with if necessary, a cross section deviating from the circular cross section is from DE-OS 21 61 066 known. Here, too, the tube-shaped extruded preform is in transferred to an intermediate blow station, although the actual intermediate blow mold has smaller dimensions than the final hollow body, but its shape already is largely approximated. However, the intermediate blowing step serves here primarily Line to shorten the cycle time of the blow molding machine on the one hand and on the other hand the intermediate molding on the for a subsequent biaxial stretching in the To bring the final blow mold the most favorable temperature. That is definitely a disadvantage with hollow bodies with clearly deviating from the circular cross-section Cross-section of the intermediate molding and even more of the finished blown hollow body Significantly different wall thicknesses in the area of the broad sides on the one hand and the narrow sides have on the other hand. The effort for a complete intermediate blow station with a non-rotationally symmetrical and therefore expensive intermediate blow mold from the point of view of uniform wall thickness and only one possible Optimization of the use of plastic raw material no discernible success compared to.

The invention is based on the object of a method and a device for the production of plastic hollow bodies with a non-rotationally symmetrical cross-section, in particular so-called flat bottles with approximately parallel wall surfaces to create at least over most of their height, namely between the Bottom and shoulder and neck area almost uniform over the circumference Have wall thickness.

This object is achieved in that the extruded Hose, closed at its lower end, freely extruded into an intermediate shape is, here already during the extrusion until a slightly pear-shaped Pre-blown shape and after extrusion of the given length in the intermediate form is blown out to an intermediate molding, which has a spherical cap-shaped bottom, a cylindrical body with a circumference equal to the circumference of the one to be manufactured Hollow body or at most a few percent less than this and a neck, which has already approximated the shape of the neck of the hollow body to be produced.

Although it is well known, the extruded tube when closed Unterende already during the extrusion over the extrusion nozzle with so-called To apply supporting air. However, this support air is not used to expand the The only purpose of the hose is to prevent the hose from collapsing and during its takeover by or into the final blow mold to impede. It is also from OE-PS 272 627 and FR-PS 1 450 395 already known to use the hose with the lower end closed during extrusion To apply blown air from the extrusion nozzle in such a way that the hose is expanded will.

In both cases there is an open one under the extrusion nozzle Final blow mold. While in the former case from the extruded tube a plurality made of ampoules with a longitudinal axis perpendicular to the tube axis and the dividers between the individual mold cavities for the necessary support of the hose, without which an approximately cylindrical widening would not be possible is achievable, a rotationally symmetrical hollow body is directly in the latter case extruded into the final blow mold, whereby by the pre-blowing over the extrusion nozzle prevents undesired accumulation of material in the bottom area of the finished hollow body which would otherwise result from the fact that a finished extruded and then inflated For known reasons, the tube initially widens in its middle section, until it comes to rest against the inner wall of the final blow mold.

In contrast, the purpose of the pre-blowing step in the method according to The invention does not primarily affect the wall thickness in the floor area of the finished hollow body, This can be done through other steps to be explained later to be influenced.

Rather, what is achieved above all by the pre-blowing is that the intermediate molding forms progressively from bottom to top with uniform wall thickness.

The intermediate molded article is preferably inflated with a program-controlled method Blown air pressure, with a first pressure peak during the exit of the first third of the hose and a second pressure peak at or immediately after termination of the extrusion process is achieved.

The cylindrical intermediate molding produced in this way is to the hollow body, preferably in a final blow mold known from DE-PS 2 720 448 reshaped.

The device for carrying out the process comprises a screw extruder, under the extrusion nozzle an intermediate mold is arranged, which consists of two symmetrical, there is mold halves movable towards and away from each other and their mold cavity bounded by a spherical cap-shaped bottom and a cylindrical wall is, the scope of which is almost equal to the scope of the molded body to be produced.

The spherical cap-shaped bottom is advantageous along one with the Hose central axis coincident axis can be moved. With the same extruded As a result, the length of the tube can determine the shape and the wall thickness, in particular of the neck area affects the finished blown intermediate molding and thus the later hollow body will.

Furthermore, the intermediate shape is overall opposite the extrusion nozzle height adjustable. This and in connection with a suitable pressure control the blown air can determine the shape and the wall thickness of the shoulder area of the intermediate molding according to the desired wall thickness and shoulder shape of the later hollow body to be influenced.

Another advantage is the blowing air supply into the extrusion nozzle an analog or quasi-analog controllable compressed air valve is provided which its control signal received from a program control device that has a freely programmable pressure control curve runs in constant time steps within a preselectable cycle time.

In the drawing, the method according to the invention is based on a Device for its implementation, schematically simplified in one embodiment shown. It shows: FIG. 1 the parts of a blow molding machine that are essential to the invention, Fig. 2a shows the various steps for production - 2d one Intermediate moldings for a flat bottle with a relatively slim neck, Fig. 3a shows the various steps for producing 3d an intermediate molding for a Flat bottle with a pronounced, almost rectangular shoulder area.

Figure 1 shows a schematically simplified in an end view Screw extruder 1, preferably a screw extruder known per se Design with a head 2 which ends in an extrusion nozzle 3 with a central bore is provided, which can be acted upon via a line 4 with compressed air. The administration 4 contains a compressed air solenoid valve 5, which is electrical as analog or quasi-analog adjustable throttle valve is formed and its electrical control signal of a control unit 6 receives.

Below the known extrusion nozzle designed as an annular gap nozzle 3 is coaxially a split intermediate mold with the mold halves 7a, 7b and one spherical cap-shaped bottom 8 is arranged. The mold halves 7a and 7b are in the closed position the intermediate form drawn and can in a known manner in the horizontal direction be pulled apart. In addition, the intermediate form is made out later for reasons to be explained, however, also with reference to the lower surface of the extrusion nozzle 3 height adjustable. The bottom 8 of the intermediate form in turn sits at the end of a Piston rod 9 of a piston / cylinder unit 10 and has a number of different Functions. It is initially used to form a spherical cap-shaped floor area on the intermediate molding This bottom area is in the final blow mold by an in The cutting edge lying at the height of the curvature of the floor is squeezed off. The ground continues to provide support 8 the still plastically deformable intermediate molding after opening the mold halves 7a, 7b until shortly before the takeover in the final blow mold. After all, the ground plays 8 plays an essential role in the formation of the shoulder to be described later and especially of the neck area in terms of shape and wall thickness.

The mold halves 7a and 7b and the bottom 8 of the intermediate mold limit a mold cavity or a mold cavity, the bottom of which is spherical and the otherwise has a circular cross-section, the circumference being equal to the circumference of the Flat bottle or at most a few percent smaller than this. This latter Dimensioning of the circumference is especially necessary when specially shaped flat bottles are to be produced in which a certain post-stretching in the here is not described final blow mold is inevitable. This is for example with flat bottles the case in which the circumference of the floor and shoulder is a few percent greater than the circumference of the bottle body. In other cases, the Perimeter of the intermediate molding to that of the hollow body to be produced possibly also instead of an intermediate mold with a cylindrical mold cavity slightly conical Mold cavity can be used.

The nozzle 3 extrudes a thermoplastic material Schiauch, which is directly attached to its lower end by a pair of pliers, not shown is squeezed together after its exit from the nozzle 3 and thus closed. After about a third of the desired hose length emerges, it opens electronic control unit 6> that synchronizes with the work cycle of the machine, is, time t1 the valve 5 to a first value a during a predetermined Time without the extrusion being interrupted. The time ti that Duration. and the aperture value a are chosen so that d is easy to achieve pear-shaped expansion of the tube results, which is thus roughly the one drawn Takes shape.

The extrusion process is ended at time t2 when the hose section 11 has reached its predetermined length, either on the bottom 8 of the intermediate form sits or stands opposite this bottom 8 with its lower, closed end. During the greater part of those between t1 and t2; Time of the # further When the hose is extruded, the control unit 6 has the air pressure via the valve 5 lowered to a value b which either corresponds to the usual support air that a Collapse of the tube is prevented or, at most, a slight progression causes the pear-shaped expansion of the tube upwards. Pretty much in the Time t2, that is, after the extrusion process has been completed, the control device now opens 6 the valve to a value c, which is so determined by height and duration that the The hose in the intermediate mold is blown out to form the desired intermediate molding, wherein the expansion as a result of the pre-blowing at time t1 in the intermediate form from bottom to top progresses rapidly and evenly, whatever for the achievement a wall thickness of the intermediate molding that is uniform over the height of the intermediate mold is of vital importance.

The control unit 6 is preferably freely programmable so that in the diagram drawn in, its abscissa the time and its ordinate the air pressure symbolizes after the valve 5, the curve drawn schematically to the respective Application can be easily adapted.

Figures 2a to 2d illustrate the production of an intermediate molding for the production of a flat bottle, the body of which can be converted into a slim and relatively long neck. For this purpose, the height-adjustable intermediate form 7a, 7b adjusted so that between its upper edge and the lower edge of the nozzle there is a relatively large distance.

In Figure 2a, the intermediate molding 13 is finished in the intermediate form blown out.

In Figure 2b, the mold halves 7a, 7b are moved apart.

In Figure 2c, the bottom 8 of the intermediate form is by means of the piston / cylinder unit 10 moved so far down that the initially approximately elliptical neck area of the intermediate molding 13 in an almost conical neck area reshaped. Depending on the ratio of the dead weight of the intermediate molding 13 for the deformability of the thermoplastic material can be carried out by this deformation simply lowering the bottom 8 obtained or additionally by one in the mold bottom included pliers 12, whose closing movement is controllable, are supported.

In Figure 2d was with simultaneous, further lowering of the floor 8 again a short section of hose is extruded around a cylindrical mouth area to obtain. The screw extruder is special for this post-extrusion suitable.

The finished intermediate molding is made after lowering the floor again 8 separated by known devices from the nozzle 3 and into the final blow mold accepted.

The further treatment of the intermediate molding according to the figures 2c and 2d are of course based on the shape and the desired wall thickness distribution the flat bottle to be produced. As can be seen without further ado and otherwise known, can be achieved if necessary by cooling and / or heating the intermediate mold, that in the train. the stretching according to the illustration in Figure 3c, the wall thickness remains practically the same in the cylindrical part of the intermediate molding, but optionally decreases in the neck area and / or in the bottom area. The wall thickness, especially in the floor area can, however, in particular by changing the length of the extruded preform 11 (Fig. 1) can be influenced, since a thick-walled floor area then results, when the extruded parison rests with its lower end on the bottom 8 of the mold, while a thin-walled bottom area of the intermediate molding results when the extruded length is adjusted so that the lower end of the preform 11 from the bottom 8 of the intermediate mold at the end of the extrusion still a certain amount Distance.

Figures 3a to 3d illustrate the production of a Intermediate molds 14 for a flat bottle with a pronounced shoulder area, i.e. the side view approximately rectangular format. A nozzle 3a is preferably used here, their mouth area compared to the nozzle head by a small amount downwards protrudes. This prevents the plastic from sticking to the nozzle head after Avoid blowing out the intermediate molding. In this embodiment it could such sticking occurs because the intermediate shape is relative to the nozzle 3 is set so that between its upper edge and the nozzle orifice only one small distance remains.

In Figure 3a, the intermediate molding 13a is completely blown out.

In Figure 3b, the intermediate mold halves 7a and 7b are open.

In Figure 3c is for the formation of a cylindrical neck in connection a short piece of hose on the shoulder area while the floor is lowered synchronously 8 post-extruded from the intermediate form.

In Figure 3d is for severing and transferring the intermediate molding 13a in the final blow mold, the intermediate mold 7a, 7b is pivoted laterally away and the bottom 8 of the intermediate form moved completely down.

Of course, in the case of FIG. 3, an additional axial stretching of the neck area can be provided as they are related is mentioned with FIG. The tongs that may be required for this in the bottom of the mold 8 is omitted for the sake of simplicity.

Blank page

Claims (7)

Claims: W method for producing a hollow body with non-circular cross-section made of thermoplastic material by extrusion a tube and then inflating it in an intermediate shape to form a circular cross-section having intermediate molding, which is deformed in a final blow mold to form the raw body is, characterized in that the intermediate molding with a circumference of at least is almost equal to the circumference of the hollow body to be produced, it is generated that for this purpose, the tube is extruded directly into the intermediate shape, early on in his Exit from the extrusion nozzle closed by squeezing its end and about when reaching a third of its length it is pre-blown until it is slightly pear-shaped Assumes shape that during further extrusion by further supply of compressed air at least the achieved shape of the tube is maintained, and that after Reach the specified length and stop extruding the hose through further compressed air supply is inflated to the intermediate molding. 2. The method according to claim 1, characterized in that the blowing air pressure is programmatic. 3. The method according to claim 1 or 2, characterized in that the Intermediate molding after opening the intermediate mold and before separating it from the Extrusion nozzle for influencing the shape and wall thickness of the neck area a predetermined distance is moved down. 4. The method according to claim 3, characterized in that for formation A piece of tube is post-extruded into a cylindrical neck or neck section and the intermediate molding is moved downwards synchronously with the post-extrusion. 5. Device for performing the method according to one of the An. Proverbs 1 to 4, with a blow molding machine with a screw extruder and an annular gap extrusion nozzle having a blown air duct, as well as with a split intermediate form, characterized in that the intermediate form (7a, 7b) under the extrusion nozzle (3a) is arranged and an at least approximately cylindrical Mold cavity has, the extent of which is equal to or at most a few percent smaller than the circumference of the hollow body to be produced is that the bottom (8) of the intermediate mold (7a, 7b) is adjustable in height by means of a piston / cylinder unit (10), and that in the compressed air supply line (#) to the extrusion nozzle (3> 3a) via a program control device (6) analog or quasi-analog controllable compressed air solenoid valve (5) is provided. 6. Apparatus according to claim 5, characterized in that the intermediate form (7a, 7b) is adjustable in height in relation to the extrusion nozzle (#> 3a). 7. Apparatus according to claim 5 or 6, characterized in that the program control device a freely programmable pressure control curve in constant Time steps within a preselectable cycle time.