Method of producing plastic containers.

In packaging technology, in particular for the packaging of liquid goods, collapsible plastic containers are known which, when empty, can be folded back on themselves by a line of shape symmetry, that is to say they can be turned inside out smoothly on one side.

This has the advantage that a plurality of such containers can be stacked relatively densely in the manner of trays which fit into one another, so that comparatively little space is required for the storage and transport of the empties. Collapsible containers of this type are referred to below as "plastic containers that can be slipped into place".

The z. B. from US-PS 2'950'029 known smooth in-foldable containers form in the filled state cubic, ie cuboid or approximately cubic body and offer in addition to the dense stackability the advantage of a maximum packing density in the filled state.

For the production of smoothly foldable containers, the homogeneity of the wall thickness in the area of ​​the shaped seam is essential, because this generally also defines the fold line during folding, e.g. B. in the form of a parallel to and close to this groove; this fold line or The shaped seam area must be uniform and must not be interrupted by openings or transverse beads if the smooth insertion of one half of the container into the other half is to be ensured.

The known smooth plastic plastic containers were previously generally after the z. B. produced in FR-PS 1'340'312 vacuum molding process; two separate strips or foils produced with parallel slot dies are used to form one half of the container from a foil by sucking into a mold and then to weld these preformed container halves together by bringing the molds together.

As explained in DE-PS 1'178'580, a tubular extrudate is preferred as an intermediate product in thermoplastic processing over strips or foils extruded in pairs because the extrusion from a single ring slot die is much easier to control than the extrusion of two foils from wide slit nozzles.

In spite of this, however, no process for the production of smooth, almost cubic plastic containers has become known which enables the production of such containers from an extrudate tube. In general, blow molding processes with tubular extrudate according to DE-PS 1'178'580 for the production of relatively large containers are considered to be problematic, which is why, according to this prior art, for example for the production of flat-lying hollow bodies, the extruded tube is first pre-stretched with the aid of two spreader bars and is brought between the mold halves or nests of the blow molding tool in this state. The blow molding is carried out only in the closed form by the blow mandrel, the position of which in the form-locking plane also determines the position of the container opening.

DE-PS 1'178'580, but also the blowing process according to GB-PS 1'226'321, which also works with prior expansion of the extrudate tube, is not suitable for the production of smoothly insertable and relatively large-volume containers, because the actual blow molding is only necessary after the hollow molding tool has been closed, namely through the blowing mandrel, ie a nozzle which extends into the mold cavity between the closing surfaces of the tool. This creates an unevenness in the area of ​​the circumferential seam, namely an interruption of the seam in the form-locking plane, usually in the form of the closable opening of the container; this makes it impossible to form the fold line in the area of ​​the seam, which is necessary for the smooth insertion of the finished container.

The object of the invention is a method for producing smooth, almost cubic plastic containers of considerable size, for. B. with 10-40 liter capacity, consisting of two approximately symmetrical parts connected by a circumferential seam in a diagonal plane of the container and with an inverted line lying approximately in the area of ​​the seam by welding two preformed container halves in the diagonal plane that determines the position of the seam which method the two container halves are not made of parallel foils, ie with slot extruders, but together from a single extrudate tube, ie with the help of a ring slot die.

It has been found that the known blow molding process with mechanical expansion of the extrudate tube, which is not suitable for the production of such smoothly insertable cubic containers for the reasons set out, can be modified to achieve the object according to the invention, that the expanded tube is at least partially and preferably predominantly pressed into the cavity halves of the molded parts of the still open molding tool by inflation and this is only closed after the extrudate tube has been inflated to weld the container halves to form a continuous seam in the interlocking plane.

To blow the mechanically spread extrudate hose into the two halves of the open tool, squeeze strips or similar means can be used and the blowing air can be introduced through at least one blowing nozzle extending into the squeezed extrudate hose.

For the mechanical spreading of the extrudate tube, preferably two cooled or approximately vertical, parallel and mutually movable expanding rods, which are used as blowing nozzles, are used in the interior of the extrudate tube, the length of which is generally greater than the height dimension of the closing surface of the molded parts. The blown air can only be introduced through the spreader bars or through an additional nozzle.

If at least one of the spreader bars is used to blow compressed air into the extrudate tube, it is e.g. B. formed as a closed end body, connected to a compressed air line and provided with corresponding jacket openings. Coolable spreader bars can be designed as double tubes and provided with a corresponding coolant connection.

If desired, air can additionally be blown in using the known injection method with hollow needles inserted laterally into the closed hollow mold and / or sucked out of the space between the extrudate hose and the mold wall with suction openings and negative pressure.

The materials that are customary for the production of plastic containers are thermoplastically processable materials, such as polyalkylenes, eg. B. polyethylene, in particular HD-PE, and the extrusion conditions, in particular the choice of the temperature of the extrudate tube, at which it is hot-plastic, depend in a known manner on the type of polymer mass.

The mold halves can be cooled in the usual way and with inserts, for. B. for molding of container openings not lying in the seam or form-fitting plane.

The work cycles can preferably be controlled automatically in a manner known per se; Furthermore, the thickness of the extrudate tube can be adapted to the material requirements of the molds in a manner known per se by controlling the degree of opening of the ring slot nozzle.

Expediently, several spreader bar devices are used for a blow molding system, which are arranged, for example, like a carousel, so that in each case one spreader from the carousel can be inserted into the working position, ie between the opened mold parts of the hollow mold, before the extrudate tube is pressed out; the spreader bars are in the first or initial position, ie their distance is less than the inside width of the extrudate tube. The squeezed extrudate tube is then spread by moving the spreading rods to the second or spreading position, then pinched off and blown into the halves of the open mold; when the mold is closed, the spreader bars of the spreading device in the working position are expediently located in corresponding recesses in the hollow molding tool, for. B. in marginal grooves of the two mold halves.

After the container has been shaped, the spreading device is moved out of the working position together with the container and the attached mold burr parts and passed through a series of post-processing stations on the carousel until the spreading device has passed through all the carousel stations and returns to the working position.

The accompanying drawings serve to further explain the invention. Show it:<ul id="ul0001" list-style="none"><li>1 is a schematic, partially sectioned side view of a two-part hollow molding tool for blow molding extrudate tube, which is pressed out of a ring slot extruder,</li><li>2A and 2B the top view of the spreading device shown in FIG. 1 before or after tensioning the extrudate tube by spreading,</li><li>3 shows the hollow mold of FIG. 1 during the inflation of the extrudate tube after the mechanical tensioning by spreading,</li><li>Fig. 4 shows the hollow mold of Fig. 1 during the welding of the container halves and</li><li>Fig. 5 shows the semi-schematic plan view of a device for mechanically tensioning the extrudate tube by spreading.</li></ul>

1, the extrudate tube 10 is pressed out of the head 100 of a screw press, not shown, through an annular slot nozzle 101, which can be opened or closed by moving the annular slot nozzle core 102 in the direction of the double arrow Z or controlled in intermediate positions.

When the ring slot nozzle 101 is opened, the extrudate tube 10 is pressed out until it has approximately the position shown in FIG. 1; then the ring slot nozzle 101 is closed.

Before the extrudate tube is pressed out, the spreading device 15 has been brought into the working position, ie between the two molded parts 12, 14 of the opened hollow mold 11.

2A, 2B, the diagram of the spreading device 15 is shown in a top view, ie in front of the molded part 14. The two approximately vertical parallel spreader bars 151, 152 can be moved against one another on a rail 157 by sliding shoes 158, 159; to move a (not shown in Fig. 2A, 2B) drive, z. B. a pneumatic or hydraulic cylinder (Fig. 5) can be used.

During the extrusion of the extrudate tube 10 from the ring slot nozzle 101, the expansion rods 151, 152 are in the position shown in FIG. 2A (first position), in which the distance A¹between the expansion rods 151, 152 is smaller than the clear width W of the extrudate tube 10 (indicated in cross-section in broken lines in FIG. 2A) 10. Now the expansion rods 151, 152 on the rail 157 up to the distance A² separated from each other. As a result, the originally approximately round extrudate tube 10 is laid flat (as indicated in broken lines in cross-section in FIG. 2A) and over that of the cavity half 141 in the interlocking plane E (FIG. 1) of the molded part 14 and accordingly also over that of the cavity half 121 of the other molded part 12 spread out in the plane E bounded area. This area has a height dimension H¹ and a width dimension B¹; In order to ensure the essential spreading of the extrudate tube 10 over this area, the length of the expansion rods 151, 152 must be greater than_H¹ and the distance A² in the second position (Fig. 2B) greater than _B¹ be.

In the method according to the invention, the extrudate tube 10 is preferably mechanically stretched or spread so far that it extends practically over the entire closing surfaces 120, 140 of the two molded parts 12, 14, ie A² (Fig. 2B) is at least approximately as large as the width dimension B² of the shaped surfaces 120, 140, and the length of the expansion rods 151, 152 is at least as large as and preferably greater than the height dimension H.² of the molded parts.

After the spreading, the extrudate tube 10 is removed from the strips 128, 129; 148, 149 pinched; such terminal strips are known per se and, as indicated in FIG. 1, with the aid of springs 125, 126; 145, 146 on the carrier plates 124, 144 of the molded parts 12, 14, so that they start at the start of the closing process of the hollow mold 11 when the plates 16, 18 of a press (not shown) run against one another, clamp the extrudate tube above and below the form-fitting surfaces 120, 140.

The terminal strips 128, 129; 148, 149 are each with recesses 128a, b; 129a, b; 148a, b and 149a, b, in which the spreader bars lie in the second position (FIG. 2B). If necessary, an additional recess (not shown) can be provided for an additional short blowing nozzle (not shown) arranged between the expansion rods. For example, at least one expansion rod 151, 152 with at least one blowing nozzle 159 (FIG. 1), which leads via a channel (not shown) inside the spreader rod to a connection 154, 155 (FIG. 2B) for a compressed gas or compressed air line (not shown). The edge recesses 143, 147 serve to receive the expansion rods.

In general, in the method according to the invention, the ratio between the inside width W of the extrudate tube and the distance A² between the spreader bars of the spreader determined by the relationship<maths id="math0001"><img file="EP0087828A1_D0001.tif" /></maths>and it corresponds to a preferred embodiment of the method according to the invention that A² greater than <maths id="math0002"><img file="EP0087828A1_D0002.tif" /></maths> is, e.g. B. by 3-30%, preferably 5-20%, since a certain stretching of the extrudate tube during spreading allows the homogeneity of the spread material to be increased, which can contribute to the uniformity of the wall thickness in the area of ​​the shaped seam. In general, the ratio of wall thickness to edge length of the finished cubic container is less than 1: 100 and is e.g. B. 1: 200 to 1: 400.

After disconnecting the extrudate tube 10 between the terminal strips 128, 129; 148, 149 the position of the hollow mold 11 shown in FIG. 3 has been reached. The form-fit surfaces 120, 140 of the molded parts 12, 14 are not yet in this state, ie the shape is still open; by blowing in compressed gas or Compressed air through the nozzle 159 of a spreader rod 151 or both spreader rods 151, 152 or / and an additional nozzle (not shown), the extrudate tube is pressed into the cavities 121, 141, generally via the intermediate state 10a until it bears against the inner walls of the molded parts 12, 14 in state 10b.

By further moving the pressing plates 16, 18 against one another, the hollow mold 11 is closed and reaches the position shown in FIG. 4.

The air volume enclosed in the expanded extrudate tube 10a or 10b by inflation in the position of FIG. 3 can be sufficient to form the container 40 in FIG. 4 or, if desired, by hollow needle injection (not shown) and / or suction with the aid of (not shown) Suction lines can be added to the molded parts. The wall material of the container 40 is cooled by contact with the adjacent walls of the molded parts 12, 14, which are cooled in a conventional manner (not shown) for solidification.

The shaped seam 41 of the finished container 40 runs in the form-fitting plane defined by the form-fitting surfaces 120, 140 without interruption around the container, so that the latter can be slipped back onto itself in the area of ​​the shaped seam. The inside of the container 40 is also smooth in the region of the molded seam 41 and free of transverse beads which would impede the smooth turning back of the container. The (not shown) opening of the container 40 lies in a known manner outside the plane of the mold seam.

Between the form-fit surfaces 120, 140 of the mold parts 12, 14 which are closed according to FIG. 4, a thin plastic mass layer is usually retained as the mold burr; This offers the advantage that the container 40 remains connected to the expansion bars 151, 152 via the form burr when the tool 11 is opened and can be removed from the reopened tool 11 by the spreading device 15 while hanging on the expansion bars. Deburring or the finished container 40 can then be separated from the spreading device 15 in a subsequent position of the spreading device 15 extended from the tool 11 in the carousel.

FIG. 5 shows a semi-schematic plan view of a spreading device 5 in the working position in front of the one molded part 54 of a hollow mold, which is fed with hose extrudate 50 through the ring slot nozzle 501 of the head 500 of an extruder (not shown).

The two spreading rods 51, 52 shown in full line are in the first position (A¹ &lt;W) so that the extrudate tube 50 can be freely reached to the desired length () _H²) can be squeezed out. The spreader bars 51, 52 with their sliding shoes 511, 521 are now moved on the guide rail 53 by actuating the pneumatically or hydraulically actuatable cylinder 55 into the open positions 51a, 52a, so that the extrudate hose 50 passes over the mold cavity 544 (H¹, B¹) is stretched at least as far by spreading that it extends over almost the entire width dimension B 2 of the form-fit surface 540; the length of the spread extrudate tube is usually somewhat larger than the height dimension H² the form-fitting surface 540 to enable the tool to be clamped between the clamping strips (not shown) before blowing.

In the second broken position 51a, 52a of the spreader bars, these are aligned, for example with recesses 543, 547 which are quarter-circular in cross-section, on the outer edge of the form-fitting surface 540, so that they do not hinder the closing of the tool. However, the spreader bars can also be moved beyond the side edges of the form-fit surface 540, ie the use of corresponding recesses 143, 147 (FIG. 2A) or 543, 547 (FIG. 5) is a preferred but not critical feature of devices according to the invention.

The clamping of the spread extrudate tube, the blowing and the shaping of the container can then be carried out as explained above.

For the preferred use of a plurality of spreading devices on a carousel explained above, each spreading device 5 of the type shown in FIG. 5 can be brought into working position between the molded parts of the hollow mold (only one molded part 54 shown in FIG. 5) by means of displaceable arms 58, 59 are removed from this working position together with the molded container and brought into subsequent processing stations of the carousel.

The compressed air supply lines to the expansion rods 51, 52 of FIG. 5 and the nozzle or blow-out openings in these are not drawn to simplify the illustration in FIG. 5. In general, additional blow nozzles, e.g. B. between the spreader bars 51, 52 may be arranged.

Numerous modifications within the scope of the invention result from the above description for the person skilled in the art.