DE1479116B2 - METHOD OF BLOW MOLDING A HOLLOW BODY FROM PLASTIC - Google Patents

Description

The invention relates to a method for blow molding a hollow body from a stress crack-sensitive, thermoplastic material, in which a preform is produced and then to the hollow body blow molded and cooled.

A method of the type mentioned at the outset is already known in which the preform is first produced by injection molding and then blow molded in a blow mold (DT-AS 10 46 867). The on have hollow bodies produced in this way if they are made of a plastic that is sensitive to stress cracking, do not provide satisfactory strength, but tend to crack in the direction of the flow of the Plastic in injection molding, as they have a predominantly molecular orientation in this direction. Such a plastic also has insufficient resistance to stress cracking. Consequently the use of these plastics for blow molded hollow bodies for packaging is limited.

It is also already known that the stress cracking resistance of plastics which are sensitive to stress cracking to increase by the fact that the molding gives a pronounced molecular orientation in several directions becomes (FR-PS 12 88 393). This process was previously more or less based on rotationally symmetrical or axially symmetrical bodies limited. The desired molecular orientation was achieved, for example, that when injection molding the plastic in the shape of the mold core opposite the outer shape was twisted. This torsion treatment was continued until the molding approximately solidified would have.

For bodies of any shape, that is to say non-rotationally symmetrical, there has been an improvement up to now stress cracking resistance by producing a pronounced molecular orientation in several directions not possible.

It is the object of the present invention to provide a method of the type mentioned at the outset, with which it is possible, also blow-molded hollow bodies of any shape to be susceptible to stress cracking diminishing molecular orientation in several directions and thus a considerable one To give strength increase.

This object is achieved in that the production and cooling of the preform for Creating an orientation of the molecules of the art; fabric in many directions during torsionsbear judgment takes place.

The process is carried out in a known manner in zwt Stages, namely the production of the preform and the subsequent blow molding. In the first stage the preform can be given the desired molecular orientation, since the preform is without can also have a rotationally symmetrical shape

ίο the subsequent blow molding is done so that the beir Molecular orientation achieved preforms remain It has been shown that the application of both de Production of rotationally symmetrical bodies already known effect, namely the body by:

To give a certain molecular orientation to torsion treatment, which is also known per se, Two-stage blow molding process allows the strength of a blow molded hollow body from one of them: Tension crack-sensitive plastic significantly increase zi. In connection with this known per se: Process steps is seen the invention.

According to a feature of the invention it is provided that the blow molding takes place after re-heating de preform. According to the invention is easily seen that the reheating takes place on a tempera between 110 ⁰ C and 140 ⁰ C. The special technique of reheating that was used surprisingly caused no or no substantial loss of the molecular orientation, which in the foregoing:

Process step was granted to the plastic molecules of the preform. During the investigation as well as be the actual use of the finished: hollow body it follows that even under heavy: Conditions shows great strength in all directions. It shows a high module and has a excellent resistance to stress cracking.

Reheating is preferred to preforms that are still warm because it enables the process step de preforming from the process step of blow molding zi separate, which has some advantages. When rewarming, the temperature of the vor molded quickly and preferably from both the inside and the outside sufficiently high temperature increased.

The upper limit of the temperature should, however, be low enough to allow the preforming process to be released to avoid achieved molecular orientation. Sections of the preform can be reheated be shielded to allow blow molding of special shapes without loss of de Molecular orientation occurs. Shielding can also be used to create thicker edges Soften wall sections of the preform, so that the wall thickness of the finished hollow body is regulated is possible.

For the process, a preform is used to orient the molecules of the plastic in many directions (hereinafter referred to as multiaxial orientation). It can be manufactured by injection molding, whereby the preform is given multiaxial orientation by rotating one of the mold elements after the mold cavity has been filled, applying tension to the molecules as the plastic solidifies. The preform can also be produced by impact deformation. The orientation w ^: xd then the molecules di

Plastic issued by an element of the stroke shape during the stroke movement and during one a short period of time thereafter, when the plastic solidifies, is rotated.

The injection molding conditions for general-purpose polystyrene preforms can be:

Table I.

Rotation resistance
of the rotating
Mold core in kg

number of
Revolutions of the
Mold core

Plastic temperature during injection molding
in ⁰ C

Wall thickness of the preform 0.089 cm

0 0 237.8

165 0.1 235.0

331 0.34 0 232.2 494.4 0.87 0.96 229.4 662.3 1.06 1.80 229.4 825.5 1.75. 1.90 226.7 988.8 2.28 2.72 226.7 1156.7 3.03 3.20 226.7 1324.5 3.47 4.37 226.7 Wall thickness of the preform 8.30 0.114 cm 0 13.00 229.4 331 21.00 224.4 494.4 221.1 662.3 221.1 825.5 221.1 993.3 221.1 1161.2 221.1 1324.5 221.1 1487.8 221.1 1651.1 221.1

IO

15th

40

45

30th

35

A number of the hollow bodies blow molded by the method were used to determine the extent and the type of orientation remaining after blow molding was investigated

The degree of orientation was determined by a subjective test, with the test person the Specimen knocked, stabbed, bent, bruised, and generally destroyed, taking the appearance of each flaw noted became. A typical result as obtained is shown in FIG. Obviously he owned Hollow bodies have a high degree of orientation in several directions so that no perpendicular cracking occurs happened.

In another attempt, strips were removed from the side walls of a blow molded hollow body and cut from the side walls of a hollow body blow molded according to previous methods. These strips were heat shrunk by placing them in an oven at 115 ° C became. All of the strips shrank, but the shrinkage was that of the first strip cut snail-like, while that from the second hollow body shrank in all directions. One Helical shrinkage is the shrinkage of different layers in different directions traced back. This layered shrinkage shows that the orientation is indeed laminar and is typical of the laminar structure of the preform used.

Birefringence measurements were also carried out on hollow bodies. The obtained double

55

Sixty refraction patterns were compared to the birefringence pattern of the preform. Apparently the size of the orientation in the preform was maintained in the blow molded hollow body. It was also found that the shape of the birefringence profile was similar to that of the preform. Indeed, the blow molded hollow body exhibited greater birefringence, presumably due to the additional orientation created by blow molding the walls of the preform.

Stress cracking tests were also carried out using stress cracking oil, including circular ones Specimens were cut from the side walls of numerous hollow bodies. Four specimens were examined from each hollow body, and the mean time to failure was compared to the Rotational resistance applied during injection molding of the preform is plotted each Test piece was loaded with a hydrostatic pressure of 0.68 atmospheres against one side while vegetable oil touched the other side. Reference is made to FIG. 6, in which the results of such Experiments are shown.

As can be seen, with a rotational resistance of 0 for the preform, only flow orientation is established in the hollow body obtained and the test specimen failed within 1 to 2 minutes; however, was at Use of a preform with torque applied during injection molding provided an improved method Preserved crack resistance. The test specimens lasted a few thousand minutes before they failed.

In Fig. 7 is a graph in which the time to crack formation as a function of the ratio of Average birefringence of the tire versus birefringence in the direction of flow is. It can be seen from the curve that optimum biaxial stress cracking resistance is obtained when the Orientation is high and reasonably between the flow direction and the tire direction is balanced. Such an optimal orientation is obtained when a preform with multiaxial Orientation is used and when blow molding that originally contained in the preform Orientation is not blurred or canceled.

From these values it is evident to the person skilled in the art that a hollow body has been formed which is an improvement in terms of stress cracking resistance.

The subject of the invention is described below with reference to the drawing of a device for implementation of the method according to the invention described in more detail. It shows

F i g. 1 a side view of the device,

F i g. FIG. 2 is a plan view of the device according to FIG. 1, partially in section along the line II-II in F i g. 1,

F i g. 3 is a side sectional view of the device according to FIG. 1 along line IH-III in F i g. 2,

4 is a side view of a spindle and a Furnace of the device according to FIG. 1, partly in section,

5 shows a blow-molded hollow body after application drastic experimental conditions or drastic conditions of use,

F i g. 6 is a graph based on semi-logarithmic Scale the importance of the torque applied during molding and its influence on errors in the hollow body during application of vegetable oil under pressure, with the applied rotational resistance in kg on the abscissa and

the time to failure expressed in minutes, indicated on the ordinate,

F i g. 7 is a graph which is in semi-logarithmic Scale the importance of birefringence both in the direction of the ring and in the direction of flow, shown as the ratio to the strength of the hollow body, represents the ratio of the average Ring birefringence for flow direction birefringence along the abscissa and the Zeit'bis to the error is given in minutes on the ordinate. ι ο

If desired, blow molded hollow bodies of high multiaxial orientation in mass production produce, so the in the F i g. 1 to 4 shown device can be used. This is for treatment of preforms 10 with multiaxial orientation possible at high speed.

The apparatus comprises a vertically movable table 50 with six heating ovens 52 circularly on it are attached. The exact number of heating ovens 52 is unimportant as long as the preform 10 treated therein is heated sufficiently for blow molding by the time it reaches blow molding station 54.

The table 50 also contains an input point 56 for the preforms 10. It consists of a vertical attached feed pipe 58, which is open at the top. The supply pipe 58 has a spring 59 which is a stacked row of preforms 10, each of which has a multiaxial orientation, upward pretensioned. The preforms 10 are through the upper open end of the feed tube 58 through the Action of a spindle 60 which engages the cavity of each preform 10 when the table 50 rises, removed. The blow molding station 54 is also mounted on the table 50

• Each heating furnace 52 consists of a radiant heat source covering the outer surface of the preform 10 heated when lowered into the heating furnace 52. The inner surface of the preform 10 becomes heated at the same time by the spindle 60 in that heating fluid is passed through its body. These external and internal application of heat results in an even more uniform and rapid rise in temperature in the wall of the preform 10.

As in Fig. As shown in FIG. 4, each heating furnace 52 is composed of a vertically attached outer tubular furnace wall 62 which is attached to a base plate 64, which is fastened to the table 50 via bolts 66. Within the outer furnace wall 62 there is a concentric arranged inner furnace tube 68 made of steel with a resistance wire 70, which around the outside of the Stovepipe 68 is wound. When an electrical current passes through the resistance wire 70 heat is generated therein and the stovepipe 68 becomes red heat. This red warmth represents the prominent listed radiant heating source.

A cover plate 72 with a preform receiving opening 74 axially arranged therein comprises the upper side the furnace wall 62 and the inner furnace tube 68. The preform receiving opening 74 is sized so that they the preforms 10 with a relatively tight fit between the upper preform wall and the Receives opening wall. The top plate 72 and the base plate 64 are not built into them shown resistance wire constructed.

Together with the heating furnace 52, the spindle 60 on which the preform 10 is attached leads to the Preform 10 heat to. As shown in cross section, the spindle 60 includes helical shapes Heat coils 76 positioned near the surface of the spindle 60. The heat coil 76 is fed through inlet pipe 78 and return pipe 8G. Both tubes 78 and 80 are with one no constant temperature oil bath shown.

The spindle 60 also has a mandrel 82 for handling the preform 10 during blow molding. Of the Pick-up mandrel 82 is attached to the end of a passage 84 and is axially within spindle 60 arranged. A flexible hose 86 connects the passage 84 to a four-way control valve, not shown, which is the application of vacuum to the mandrel 82 to receive a preform 10 from the feed tube 58 and for holding the preform 10 in place as it passes through the heating ovens 52 is performed, and for applying air pressure to blow molding the preform 10 into a hollow body and then regulated to release the finished hollow body.

Each spindle 60 is connected to a turntable 90 by means of bolts 88 and a retaining ring 89, which made of a heat insulating material to avoid heat conduction from the spindle 60 to the Turntable 90 is made. A spring 91 holds the spindle 60 under tension in the downward direction and allows a Side play thereof, so that an accurate adjustment of each spindle 60 and each preform receiving opening 74 is made possible during operation of the device.

The operation of the device results from the above description. This includes a gradual Rotational movement of the turntable 90 with a related perpendicular reciprocating movement of the table 50 when each preform 10 is exactly above either a heating furnace 52 or the blow molding station 54 or the input point 56 is. Starting from the input point 56, the spindle 60 moves, which is above the Input point 56 protrudes into the cavity of the uppermost preform 10 within the feed pipe 58 the vertical upward movement of the table 50. The four-way control valve of the spindle 60 applies a vacuum the receiving mandrel 82 located therein, whereby a gripping of the uppermost preform 10 in the Feed pipe 58 takes place. The table 50 then descends and, after it has been fully lowered, it rotates Turntable 90 so that the preform 10 is located above the first heating furnace 52 adjacent to the feed pipe 58 stands. Then the table 50 rises, whereby the preform 10 enters the heating furnace 52. Of the Preform 10 is then initially by the radiant heat of the heating furnace 52 and by the Heating fluid heated, which begins to flow through the spindle 60. Meanwhile, the next spindle takes 60 in the row on the turntable 90 on a second preform 10. Then table 50 lowers again, and the turntable 90 rotates one part so that the first preform 10 is above a second heating furnace 52 stands while the second preform 10 stands above the first heating furnace 52. Then table 50 rises again to repeat the heating of the two preforms 10 and a third preform 10 until the first preform 10 reaches the blow molding station 54. At this point the Preform 10 heated sufficiently to allow blow molding. The blow molding sections 92 and 94 of the blow molding station 54 close around the preform 10. Then the four-way control valve switches on

spindle 60 for the supply of air into the interior of the preform 10. As a result, the preform 10 is blow molded in a jswward direction corresponding to the inner wall of the closed blow molding sections 92 and 94
After the blow molding of the hollow body, the air pressure within it is blown off to the atmosphere and the blow molding sections 92 ad 94 are opened. An additional air pressure entered by the mandrel 2 causes the ejection of the hollow after lowering the table 50.

body from the spindle 60. A conveyor device, not shown, removes the hollow body and promotes it to a subsequent operation, for example for use in a filling machine.

It goes without saying that the blow molding process described above allows a rapid operation. In fact, it is possible to add a blow molded, multiaxially oriented hollow body every 6 seconds produce.

For this purpose 4 sheets of drawings

709 520/330

Claims (3)

Patent claims: 1. A method for blow molding a hollow body made of a stress-crack sensitive thermoplastic Plastic in which a preform is produced and then blow-molded to form the hollow body and is cooled, characterized in that the manufacture and cooling of the Preform (10) for creating an orientation of the molecules of the plastic in many directions while torsional stress occurs. 2. The method according to claim 1, characterized in that the blow molding after reheating of the preform (10) takes place. 3. The method according to claim 2, characterized in that the reheating to a temperature between 110 ⁰ C and 140 ⁰ C takes place.