DE3326902A1 - METHOD FOR ADJUSTING THE TEMPERATURE OF HOLLOW PREFORMS - Google Patents

Description

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Descriptio nce:

The invention relates to a method for adjusting the temperature of hollow preforms which have a base have, produced by injection molding and processed into hollow bodies by subsequent stretching and blow molding will.

If you have a preform produced by injection molding immediately after demolding, one for the next Stretching and blow molding sets a suitable temperature and then just by stretching and blow molding processed further to the finished hollow body, then this has the advantage that the production of the hollow body from Injection molding to blow molding runs continuously. Unfortunately, the one out of the injection mold upcoming preform has an uneven temperature distribution; this makes it necessary to take hiring measures the temperature of the preform to take.

There are several reasons why the preforms one get uneven temperature distribution:

A “In contrast to an extruder, an injection molding machine hits the molten material in individual steps and therefore forms in the too splashing material in the heating cylinder of the machine an uneven temperature distribution.

B. While the molten material through the nozzles, the runner and through the inlet opening of the Injection mold flows, forms in the flow of material a turbulence which contributes to the non-uniformity of the temperature of the preform.

C. A temperature difference also arises in dependence on the amount injected into the injection mold Materials.

D. If the cavity of the injection mold is eccentric or the wall thickness is one-sided, then this leads to an uneven wall thickness of the preform, which an uneven Temperature distribution entails.

When a preform with an uneven temperature distribution is further processed by stretching and blow molding then the areas with a higher temperature are stretched more than the areas with a lower temperature Temperature, and as a result, the finished hollow body has an uneven wall thickness. It is there possible that a hotter area of the preform is stretched so much that it tears open. But even then if the stretching is not so strong that tearing occurs, it is still the case that the hollow body with uneven Wall thickness is of poor quality; it may be that the unevenness of the wall

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strength does not look at the hollow body from the outside, but the strength of the hollow body will sometimes suffer drastically and make it unusable.
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The adjustment of the temperature of the preform with the aim of equalizing the temperature distribution is therefore of considerable importance in the process described for the production of hollow bodies to.

The temperature setting is generally done by Heating the preform indirectly by means of radiant heat; with this approach However, the heat transfer takes place with low efficiency and it requires a lot of skill and experience To equalize the temperature of the preform and to a value suitable for further processing bring to.

From US Pat. No. 4,145,392 it is known to adjust the temperature to make the preform by cooling, in that the high temperature from the injection mold removed preform is pressurized from the inside, so that he is with its outside creates the surface of a temperature control body. Due to the intimate contact with the temperature control element In this process, the efficiency of the heat transfer is better than in the process with indirect Heating up, and therefore the temperature setting

of the preform done in a short time. However, it is technically difficult to use the temperature body partial temperature change to supply a heat transfer medium with a different temperature, like it in the case of temperature setting by heating with electrically generated heat can take place, rather the temperature setting of the preform as a whole made at the same temperature. In this case you can not even if the temperature of the preform coming out of the injection mold is high, eliminate the differences in the temperature distribution of the preform because it is against everywhere the same temperature is cooled. It is true that the preform is cooled by means of a temperature control element the temperature control time is reduced, but a completely uniform temperature distribution of the preform and a uniform wall thickness is not achieved in the hollow body made from it.

The invention is based on the object of creating an improved method for setting the temperature of hollow preforms produced by injection molding, which are intended for further processing by stretching and blow molding ₍ which leads to a very uniform temperature distribution in the preform at a temperature level suitable for stretching and blow molding.

This object is achieved by a method with the features specified in claim 1. Beneficial Developments of the invention are the subject of Subclaims. ■

While in the prior art the equalization of the temperature distribution of the preform and the Adjusting to a temperature suitable for stretching and blow molding together by a unit Tempering process take place, the inventor has recognized that a significant advance can be achieved is when you first put the preform in a state offset, in which existing irregularities in its temperature distribution already exist its targeted temperature setting through cooling dismantle.

As is well known, a preform tends to have an uneven temperature distribution, even if it is only weak Temperature differences mean that an area with a higher heat content stretches more easily than an area with lower heat content. The stretching that occurs goes hand in hand with an increase in the surface area and with a reduction in wall thickness, and therefore a hotter area cools down significantly more quickly as a less hot one and therefore less enlarged in its surface and reduced in its wall thickness Area.

The invention makes use of this in order to even out the temperature of the preform even before

it is targeted by contact with the tempering mold on the for further processing by stretching and Blow molding required value is lowered.

In the method according to the invention, the temperature is set by cooling the preform. Therefore, the starting temperature of the preform, with which it is removed from the injection mold, must be above the set temperature. The preform coming from the injection mold is brought into a temperature control mold by means of a transfer device known per se, which is preset to a predetermined lower temperature. In the case of the known temperature control method mentioned at the beginning, which works with cooling, only air is blown into the preform until it comes into close contact with the inner wall of the temperature control mold. According to the invention, on the other hand, the preform is axially stretched before air is blown into it and it also expands radially and rests with its outer surface under pressure on the inside of the tempering mold.

In the previously known method, in which the equalization and the lowering of the temperature are effected by a uniform cooling process in contact with the inside of the tempering mold, the tempering mold is of course only slightly larger than the preform. In the prior art, the preform becomes small due to the inflation with air

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little stretched, but the extent of the stretching is too small to noticeably equalize the temperature distribution to contribute; rather are also performed after Temperature setting there are still temperature differences.

In the method according to the invention, however, one uses a tempering mold which is significantly larger than the preform, so that the preform can be stretched axially to a sufficient extent and then expanded radially. The temperature control form should not be chosen so large inside that the process of temperature setting of the preform permanently increase its length and diameter to a considerable extent, because that would add to it lead to the fact that in the subsequent combined stretching and blow molding process, the stretching factor and the expansion factor become smaller, which can lead to insufficient biaxial orientation.

Therefore is preferred in the method according to the invention uses a tempering mold whose inner surface is 1.5 to 3 times the size of the outer surface of the Is preform, and the elongation of the preform in the axial direction is preferably 10% to 20% of its original length; the neck remains with this information of the preform is disregarded insofar as it is in the shape that it was used in during the various phases of manufacture of the hollow body encloses and holds, sticks and does not take part in the stretching or stretching.

The stretching of the preform is best done by putting it in it while it is in the neckline Shape hangs, introduces a stretching punch,

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and the (radial) expansion is best done by blowing in air. Only after stretching and the following Injection of air comes into intimate contact with the outer surface of the preform Inside of the tempering mold, and this contact remains as long as the internal pressure in the preform is maintained, and meanwhile a rapid heat exchange takes place between the preform and the tempering form.

The cooling should proceed so quickly that the preform reaches its target temperature suitable for the subsequent stretching and blow molding process, as long as the preform is still capable of automatic shrinkage after it has been relieved of the internal pressure. This automatic shrinkage has a very positive effect: the overall wall thickness of the preform increases due to the shrinkage, the heat distribution in the preform becomes even more even during uneven shrinkage, the wall thickness is also evened out and internal stresses resulting from injection molding are evened out. The cooling process ends as soon as the preform detaches itself from the inside of the tempera rform after releasing the air pressure in its interior as a result of shrinkage.

The temperature of the tempering form is also determined by the wall thickness of the preform and its temperature.

It is not expedient to stretch the preform takes place only inside the tempering form, but also outside it. If you have the preform before it is introduced stretches into the tempering form, then the cooling and equalization of the temperature distribution initially take place at room temperature.

The method according to the invention is illustrated below with the aid of the following of the accompanying drawings will be described in detail. 10

FIG. 1 shows a longitudinal section through an injection mold with the preform that has already been injected in this,

Figure 2 shows a longitudinal section through a tempering mold with inserted preform during the Tempering,

FIG. 3 shows a longitudinal section through the tempering mold from FIG. 2 at the end of the tempering process,

FIG. 4 shows a longitudinal section through a blow mold in which a preform is just passing through Stretching and blow molding is further processed,

Figure 5 shows a longitudinally divided tempering mold together with a stretching device for use in a process in which the preform already stretched outside the tempering mold

will,

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FIG. 6 shows, in longitudinal section, the tempering mold from FIG. 5 in the closed state and with the preform inserted during the tempering process, and 5

FIG. 7 shows the removal of a temperature-controlled preform from the tempering form shown in FIGS.

To produce a hollow preform 4 for a Bottle or the like hollow body made of plastic is used an injection mold 3, which at her one end is closed by a mold 1, which only encloses the neck of the preform 4 and through which a core 2 leads through it into the injection mold 3. The injection port is located on the the end of the injection mold opposite the mold 1 3, i.e. where the bottom of the preform is formed. The manufacture of the preform 4 is not open limited to a specific resin. After the required curing of the preform 4 is the The core 2 is pulled out and then the preform is removed from the injection mold 3 and over the inlet opening conveyed a tempering mold 5; while· the preform 4 remains enclosed by the mold at the neck, i.e. the mold 1 holds it during the Transfer to tempering form 5, then also during the temperature control and finally also during the transfer from the temperature control mold 5 to the blow mold 7.

The tempering mold 5 is a substantially cylindrical body with a substantially cylindrical cavity which, in the example shown, is closed by a head part 5b with a narrowed passage. Such a head part 5b is only required in the case of an axially indivisible tempering mold 5, as shown in FIG. 2; with an axially divisible tempering mold 5 ^; As shown in FIG. 6, one can do without such a head part. The tempering mold 5 contains cooling channels 5a around the mold cavity.

The preform 4 is inserted into the tempering mold 3 by being lowered through the head part 5b into the latter will; in the process, the mold 1 fits into the passage opening of the head part 5b and closes it. Afterward a stretching die 6 is inserted through the mold 1 and the head part 5b into the preform 4 and stretch it; the stretching die 6 is inserted up to the stop at the bottom of the tempering mold 3. Then - also through the mold 1 - compressed air is blown into the preform, sodu ^ this also expands radially until it is with its outside resting on the inner surface of the tempering mold. The preform expanded in this way is designated in Fig. 2 by the reference number 4a. The elongation of the preform 4 includes its hotter ones Areas stronger than its cooler areas. As a result, the wall thickness is determined by the different expansion of the hotter areas of the preform 4 is smaller than that

Wall thickness of its less hot areas, so that the hotter areas are in contact with the tempering mold 5 cool down more than the less hot areas.

During the tempering process, the pressure in the interior of the preform 4a is generated by the stretching die 6 and the compressed air blown in. Meanwhile, there is an exchange of heat between the preform 4a and the tempering mold 5, the inside of which it rests. As a result of the drop in temperature the preform 4a would like to shrink on its outside, but cannot because of the persistent internal pressure, so that the cooling can also proceed into the interior of the thicker areas of the preform 4a. the hotter areas of the preform, which through the Elongation increased their surface area more and decreased their wall thickness more than the cooler ones Areas are therefore cooled more rapidly than the cooler areas, and this favors the cooling the hotter areas leads according to the invention to an equalization of the temperature of the preform, during its temperature at the same time to one suitable for the subsequent combined stretching and blow molding Temperature (hereinafter also referred to as its target temperature) is lowered.

As soon as the preform 4a reaches its target temperature the stretching punch 6 is withdrawn and the preform is thereby vented. By dismantling of the internal pressure in the preform 4a, this can now - in accordance with the temperature reduction that has taken place -

shrink, whereby it detaches itself from the cooling inner surface of the tempering mold 5 and the cooling ended automatically. With a divisible tempering form but you can also open it first and then reduce the internal pressure in the preform 4. Through the Shrinkage of the preform 4a increases its wall thickness overall, the increase of course from The amount of shrinkage depends, and with the change the wall thickness is linked to a heat transfer, which leads to a further equalization of the temperature of the Preform contributes. Therefore, cooling of the preform should be completed when it is in a State in which it tends to shrink automatically.

FIG. 2 shows the temperature-controlled, shrunk preform, which in this state is designated by the reference number 4b became. 'It is immediately - with its neck hanging in the mold 1 - transferred to the blow mold 7 and received therein by a stretching and blow molding process its final Shape 8.

Figures 5 to 7 illustrate another embodiment of the method according to the invention, namely νύrd in this case the preform already outside the Tempering mold 5 'stretched. Fig. 5 shows the preform in the stretched state, in which it has been designated by the reference number 4c.

In this second procedure, too, the preform 4 is first produced by injection molding (FIG. 1), removed from the mold and immediately transferred to a position above the open, axially divided temperature control mold 5 '. Here, a stretching punch 6 is first inserted through the mold 1 holding the preform 4 on the neck into the preform and stretches it axially to a predetermined extent. During stretching, the overall wall thickness of the preform is reduced, more specifically in hotter areas than in cooler areas. At the same time, the surface area is correspondingly enlarged and cooling is thereby promoted, again to a great extent in the hotter areas of the preform 4c, because the increase in surface area is greatest in them. Even by stretching the preform outside of the tempering mold 5 ¹ while it is cooling down in ambient air from room temperature, a contribution to equalizing the temperature of the preform 4c is already made.

Immediately after the end of the stretching process , the preform 4c is introduced together with the stretching punch 6 in the stretched state into the tempering mold 5 ', the inner surface of which is larger than the outer surface of the unstretched preform 4 and is also larger than the outer surface of the axially stretched preform 4c by whose two halves each other

move upwards and then join together enclosing the preform 4c and the lower end of the mold 1. Immediately after closing the tempering mold 5 ¹ , compressed air is blown into the preform 4c through the root of the stretching ram 6 (that is a section of the stretching ram below the form 1) in order to expand it radially as well, until it is completely on the inner surface of the Tempering form 5 ¹ creates. It assumes an expanded shape in which it is designated (as in FIG. 2) with the reference number 4a.

This expansion of the stretched preform 4c causes a further increase in the surface area and Wall thickness reduction of the preform and brings it into intimate contact with the tempering mold, so that between both a heat transfer begins. The cooling sets in on the outside of the preform 4a and this strives therefore, after shrinking, it cannot, however, as long as the internal pressure is maintained in the preform remain. Rather, the cooling continues into the interior of the wall of the preform. There too with radial expansion by compressed air, the hotter parts of the preform were stretched more than that As already explained above, they are cooled more rapidly than the cooler parts of the preform and thereby an equalization takes place in the temperature distribution of the preform 4a.

As soon as the preform 4a has reached its target temperature, it is ventilated, ie its internal pressure drops; consequently, the preform 4a experiences a radial shrinkage, as a result of which it separates from the inner surface of the tempering mold 5 'and ends the cooling. Then the two halves of the tempering mold 5 ^{1 are} separated and then the stretching die 6 is brought back; now the preform can and does shrink axially. As a result of the radial and axial shrinkage, the wall thickness of the preform increases overall to an extent that is dependent on the amount of shrinkage. A heat flow is associated with the change in the wall thickness, which also contributes to the equalization of the temperature distribution.

In addition, internal stresses resulting from injection molding are relieved. The temperature control should therefore be with Advantage can be carried out when the preform is in a state in which it able to shrink automatically.

FIG. 7 shows the preform in the shape it has assumed after shrinking. In this form it is - as in Fig. 3 - denoted by the reference number 4b.

After completion of the tempering and removal from the tempering mold 5 ¹ , the preform 4b - with its neck still hanging in the mold 1 - is transferred to a blow mold 7 (FIG. 4) and is preserved therein

Stretching and blow molding its final shape 8.

A few numerical exemplary embodiments are given below. The stretching and The blow molding machine has the structure described in US Pat. No. 4,105,391.

example 1

A preform is produced from a vinyl chloride resin by injection molding, the outer diameter of which is 38 ₅ 5 mm, the wall thickness of which is 2.85 mm and the length of which - with the exception of its neck - is 122 mm. The preform is cured for 20 s and has a temperature of 130 ^° C. after demolding. It is tempered after demolding, specifically using an axially divided tempering mold (cf. FIGS. 5-7) whose i "w. cylindrical mold cavity has a diameter of 43 mm and a length (without neck) of 138 mm. The ^T emperierform has cooling channels through which 80 ° C hot water flows.

The preform is first outside of the tempering mold axially stretched, then inserted into the tempering mold and into it by blowing air with a pressure of 4 bar stretched, whereby it rests with its outside against the wall of the mold cavity. After leaving

The pressure is released for a cooling time of 2 seconds, whereupon the preform shrinks radially away from the wall the tempering mold dissolves and the cooling process ends. Then the tempering mold is opened and the stretching die withdrawn, whereupon the preform also shrinks axially. After shrinking, the preform has an outside diameter of 40.5 mm, a wall thickness of 2.6 mm and a length of 127 mm, so opposite its dimensions did not show any major changes prior to temperature control.

The tempered preform is then stretched and blow molded in a blow mold in a known manner in an i.w. cylindrical bottle with a Formed an outer diameter of 80 mm and a length of 240 mm.

The finished bottle has a uniformly thick and homogeneous wall and shows excellent transparency compared to bottles which were manufactured according to previously known methods.

Three other examples are i.w. given in a table. The specified temperatures are surface temperatures. After the reference to Figs. 1-4 The process described were the preforms in the example 2 temperature controlled, according to the method described with reference to FIGS. 5 to 7 in Examples 3 and 4.

material

Tabel

A. Preform

Length (mm)

Example no.
12 3 4

PVC PVC PVC PP

122 107 101 118

Outside diameter (mm) 38.5 29 19 28

Wall thickness (mm) 2.85 2.6 2.4 3.5

Demould temperature ("C) 130 130 125 103

B. Tempering mold (mold cavity) depth (mm)

138 124 101 122

Inner diameter (mm) '^ mperature ( ⁰ C) Air pressure (bar) Cooling time in form (s) 43 39 19 40 80 100 90 100

Continuation of the table

C. Tempered Preform Length (mm)

Outside diameter (mm) example no.
12 3 4

Temperature ( ⁰ C) 127 114 107 119 40.5 31 22 33

100 98 115

U. Finished hollow body (bottle) Length (mm) 240 240 117 213

Outside diameter 80

70

Wall thickness (mm) 0.35 0.40 0.40

Volume (cm ³ ) 1000 250 700

All examples resulted in bottles with very uniform wall thickness, and apart from example 4 the transparency was better than in the case of bottles which were manufactured according to previously known processes became.

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Claims (3)

ΡΑ.ΪΕ W TÄNWAI "T | *"., ** .. * DR. RUDOLF BAUER DIPL-ING. HELMUT HUBBUCH DIPL.-PHYS. ULRICH TWELMEIER Wf * 1 1 I 'III , ·' -! J 1 | ΛΜ tt I »I'M IIH ¹ I ■■ ϊ. ' t) /►ΗΠΙΊΟΗ/ΗΕίΜ.νΜΜ ι.ι ιιπ-.ι .. = ff ι! ''«' I 1 JI f) J * ¹ JO / / I) ItI [IiMAMMl 1 ¹ AIMAHK July 25, 1983 III / Be Katashi AOKI, 6037 Ohazaminamijo, Sakakimachi, Hanishina-gun, Nagano-ken, Japan "Procedure for adjusting the temperature of hollow preforms" Patent claims: M.) Procedure for setting the temperature of V ■ hollow preforms made by injection molding are generated and, after the temperature has been set, are further processed into hollow bodies by stretching and blow molding, the hot preform being held in a tempering mold by a mold that surrounds its neck inner surface is larger than the outer surface of the preform, inserted and internally pressurized therein is applied and expanded until it is in intimate Contact rc it is the inner surface of the tempering mold, which is cooler than the preform and this cools down on a ζ aiii subsequent stretches and Blow molding suitable temperature, characterized in that that the preform is first stretched in the axial direction and only then internally pressurized, ÜJZ03UZ and that the cooling of the preform only within one Time span takes place in which it - after relieving the internal pressure - is capable of automatic shrinkage and terminated by the shrinkage of the preform will. 2. The method according to claim 1, characterized in that the preform is already outside the tempering mold axially stretched and then inserted into the tempering mold and then by applying a Internal pressure is expanded radially until it rests on the inner surface of the tempering mold. 3. The method according to claim 1 or 2, characterized in that the preform for temperature control is axially stretched by 10% to 20% of its length and, moreover, is stretched so much that its surface area is increased by a factor of 1.5 to 3.