DE3411905A1 - METHOD FOR PRODUCING HOLLOW BODIES FROM BIAXIAL RETURNED POLYPROPYLENE - Google Patents

Description

Description:

The invention is based on a method for producing hollow bodies from biaxially stretched polypropylene with the features specified in the preamble of claim 1.
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Polypropylene is a crystalline synthetic resin, and the pellets, which are usually used as starting material in the manufacture of polypropylene objects, therefore have a crystalline structure. Polypropylene can be molded easily at a temperature between 190 ^° C. and 260 ^° C., and if hollow bodies such as bottles are produced from polypropylene by blow molding or by injection molding of preforms, which are then further processed by blow molding, or by a similar process, then one strives to keep the working temperature as high as possible.

If a hollow body is produced by blow molding or by injection molding and subsequent blow molding, then, when known methods are used, hollow bodies are always obtained, the transparency of which is impaired and which appear milky because the preforms in the known processes are immediately blow molded in Hollow bodies of the desired shape are reshaped.

In order to obtain a milky hollow body made of polypropylene transparent, one could think of the polypropylene reshaped by biaxial stretching and blow molding, a process by which one is transparent

Manufactures hollow bodies made of polyethylene terephthalate.

If, however, the process of biaxial stretching and blow molding is used for the production of hollow bodies from polypropylene and a hot preform produced by injection molding is formed into a hollow body with the desired shape in a continuous operation by stretching and blow molding (a procedure that is used in English also called "hot parison system"), then even at a working temperature very suitable for stretching and blow molding there is a considerable probability that the preform will be damaged by stretching and blow molding; and even if an undamaged hollow body is obtained by this method, the disadvantage remains that this has a very uneven wall thickness and is not uniformly transparent, which is contrary to the marketing of such hollow bodies.

It is believed that the polypropylene hollow body is therefore become milky because the synthetic resin in the course of the manufacturing process as a result of cooling of the molten material partially crystallized, with the occurring Variations in the transparency of the hollow body on the extent of crystallization and on the size of the crystalline Depend areas in the amorphous material.

The invention is based on the object of a method for producing hollow bodies from biaxially stretched material Specify the polypropylene with which it is possible to

to produce transparent, glossy hollow bodies with a uniform wall thickness.

This object is achieved by a method with the features specified in claim 1.

The process proceeds in such a way that a preform is first produced from polypropylene by injection molding. This is based on polypropylene pellets, which pass through the screw in the injection cylinder of an injection molding machine be plasticized; It should be ensured that it is still in the cylinder of the injection molding machine a good mixing of the plasticized Polypropylene takes place before it is injected into the injection mold. In this way, in the injection mold to introduce such molten polypropylene which does not contain any crystalline constituents contains more. The preform is removed from the injection mold at the highest possible temperature and transfers it to a tempering form in which it can be specifically tempered through heat exchange. By the Tempering the preform is intended to set optimal conditions for the subsequent stretching and blow molding will. This process makes it possible to produce transparent hollow bodies made of biaxially stretched material Manufacture polypropylene that have a uniform wall thickness.

When the preform is injection-molded, the polypropylene pellets are initially produced as a result of the rotation of the Screw plasticized in the heated injection cylinder and the material melted in this way is conveyed to the front end of the screw and in the front the screw located space accumulated, from which it is in the machine cycle by a feed movement the screw is displaced and poured into the injection mold. When designing the spraying process care is usually taken to ensure that the pellets are complete melted when the material reaches the front end of the screw, but so far one has no particular attention is paid to specifically influencing the plasticizing process itself "

When processing crystalline synthetic resins such as polypropylene, it depends on the degree of melting in the Injection molding cylinder depends on how large the proportion of crystalline Material is still in the amorphous material at the exit of the injection molding cylinder; by injection molding can A preform can already be produced if, after the plasticizing process, certain proportions are still crystalline Materials are available. The inventor has found that a biaxially oriented hollow body is obtained made of polypropylene, which is transparent and has gloss, not by stretching and blow molding from one preform obtained by injection molding, as long as the molten polypropylene is not completely is amorphous, but still contains crystalline material.

Apparently the plasticized material has which as a result of the screw's rotation, accumulates in front of its front end, even then a non-uniform one Temperature when it is completely melted. There is then a non-uniform temperature distribution also exist in the molten material. after it has been injected into the injection mold, and it is one of the reasons why the resulting preform also has an uneven temperature distribution. This non-uniform temperature distribution in the preform can be partially eliminated by a temperature control process be compensated, which carried out in the method according to the invention before stretching and blow molding will. However, the temperature control process primarily resembles temperature differences on the surface of the preform, which result from an uneven temperature distribution of the cooled injection mold. However, the temperature inside the preform cannot be made uniform so easily, at least not if there is only a short time available for the temperature control process. This uneven Temperature distribution in the interior of the preform is due to a substantial extent to one non-uniform temperature of the plasticized material in the screw cylinder of the injection molding machine.

An uneven temperature distribution in the preform causes local differences in the cooling rate of the preform, and that means that in stretching and blow molding, some areas of the preform are easier to stretch than others

Areas, and that in turn leads not only to uneven wall thicknesses, but also to uneven transparency of the hollow body or even to the production of rejects.
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According to the invention it is therefore provided that a 'preform is only poured when the plasticized polypropylene has been completely mixed. This means that the plasticized (melted) polypropylene becomes so intense when the screw is turned is mixed so that the crystalline components initially still present in the amorphous material disappear and at the same time compensated for the temperature differences that were initially still present in the polypropylene so that the material injected into the injection mold has a uniform temperature has, and this has the consequence that the preform which is formed in the injection mold, has a uniform temperature inside. The influence of intensive mixing of the polypropylene in the screw barrel is clearly shown in the fact that preforms that after intensive mixing of the polypropylene poured have a higher transparency than preforms, which after only moderate mixing, as in the state customary in the art, have been poured and therefore have a milky appearance. The more even temperature distribution in the preform after intensive mixing of the polypropylene prove erpirisch.

According to a further feature of the invention, the injection time is selected when producing the preform and the cooling time as short as possible; the goal is to demold the preforms and as hot as possible they are therefore only cooled down as much as is absolutely necessary in order to be able to carry out the demolding, and the demolding takes place when there is an inner and outer surface in the injection mold of the preform has only formed a thin skin.

The preform, which is thus quickly demolded, is crystallized in the thin skin on its inner and outer surface and only takes on a slightly whitish appearance. Inside the wall of the preform lies However, the polypropylene is still in an amorphous state and at a higher temperature; those from inside the Preforms heat flowing to its inner and outer surface leaves the skins formed there again melt, which can be seen from the fact that the demolded preforms are visibly transparent.

In preparation for the final stretching and blow molding process, they are removed from the injection mold Preforms to a suitable further processing temperature by heat exchange with the wall of a tempering mold brought. How one can carry out such temperature control is, for example, in the American patent application No. 516 768, which goes back to the same inventor described.

A tempering mold is used, its inner surface is larger than the outer surface of the preform, and around the preform with the wall of the To bring the tempering mold into intimate contact, the preform is expanded in the tempering mold. The expansion expediently takes place in such a way that the preform is initially axially in the tempering mold stretches and then pressurized with a pressurized gas. In contact with the inner wall of the tempering mold the preform assumes the working temperature intended for the subsequent stretching and blow molding at. The time required for tempering is n ^ r short, the period of time in which the preform shrinks after it has been exposed to the gas pressure is sufficient has relieved inside him. The inner surface of the tempering mold is larger than that enclosed by it outer surface of the preform in order to be able to stretch and expand it sufficiently. Of the However, the difference between the two should not be too great, so that the preform does not fit in too much the length and diameter is widened, because otherwise the preform could be used in the following Stretching and blow molding process only insufficiently stretched and are stretched and then experience an only inadequate biaxial orientation. It is recommended that the choose the inner surface of the tempering mold approx. 1.4 to 2 times as large as that enclosed by it outer surface of the preform.

The stretching of the preform in the tempering mold is best carried out with a stretching die, which is inserted into the preform through the neck thereof;

The easiest way to effect the subsequent expansion of the preform is to blow in air. The preform consequently hugs the inner surface of the tempering mold and between the two heat is transferred until the preform has been relieved of its internal pressure shrinks and separates from the wall of the tempering mold.

The temperature control takes place as long as the preform is in contact with the inner wall of the temperature control mold. If he after Reaching the intended working temperature for the subsequent stretching and blow molding from its internal pressure is relieved, it shrinks. The shrinkage has a very beneficial effect on the preform. because namely, when shrinking, the wall thickness of the preform increases overall and weir in the course of the change the wall thickness in the preform takes place, the temperature of the preform increases leveled out and at the same time any remaining tensions that occur during injection molding have arisen, eliminated. With the onset of shrinkage, the preform separates from the wall of the Tempering form and thereby the tempering process is ended automatically.

The preforms are tempered in this way immediately transferred to a blow mold and axially stretched therein by means of a stretching punch, then through Blowing air expanded and then rapidly cooled; the hollow bodies obtained in this way have excellent transparency.

Some examples are given below specified. To carry out the procedure was used a machine for injection molding and blow molding, as described in US-PS 41 05 391.

Example 1:

A polypropylene with the designation 6200E, manufactured by Mitsubishi Chemical Industries, Ltd., with a density of 0.897 g / cm ³ , with a melting point of 140 ^° C. and with a melt viscosity of 1.7 g / 10 min was used .

Pellets of this polypropylene were heated, melted and mixed thoroughly by rotating the plasticizing screw in a heated to a temperature of 22O ⁰ C injection molding cylinder, and then poured into a chilled to a temperature between 12 ° C and 15 ° C injection mold and in this way a bottle-shaped parison manufactured with the following dimensions:

Outer diameter 22 mm,

Inner diameter 15 mm,

Wall thickness 3.5 mm,

Overall length 128 mm,

Length below the neck 110 mm.

The preform was as hot as possible from the injection

Casting mold removed from the mold and immediately inserted into a tempering mold. The temperature of the demolded preform in this case was between 100 ° and 103 ⁰ C.

The preform was tempered in a tempering mold with the following features:

Inner diameter of the tempering mold: 36 mm, depth of the tempering mold: 137 mm,

Temperature of the tempering mold: between

120 ° and 130 ⁰ C,

Temperature control time: 6-7 s,

Air pressure: 10 ⁵ to 2MO ⁵ N / m ² corresponding to 1 to 2 bar 10

After the preform was inserted into the tempering mold, it was first passed through a stretching die axially stretched until the bottom of the preform was in contact with the bottom of the tempering mold. Subsequently was Air is blown into the hot preform to expand it and thereby retract its outer surface to bring intimate contact with the inside of the tempering mold.

The temperature control process was completed by the shrinking of the preform, which starts when the stretching ram is withdrawn and the excess pressure inside the preform is removed.

After tempering, the parison a lying between 113 ° C and 120 ⁰ C temperature, an outside diameter of 31 mm and a length had mm below the neck of the 130th

The temperature-controlled preform was immediately transferred to a blow mold and formed therein by stretching and subsequent blow molding into a 600 ml beer bottle, which has an elliptical cross section with a small diameter of 60 mm and a large diameter of 80 mm, a wall thickness of 0.4 mm, had an overall length of 198 mm, a length below the neck of 180 mm and a weight of 22.2 g. The air pressure used for blow molding was between ^{8 #} 10 and 10 N / m ² and the blowing time was between 3 and 5.5 s.

The beer bottle made in this way was transparent, did not show any milky spots, had a gloss and a uniform wall thickness. 15th

Example 2:

In this example, an oil bottle with a volume of 700 ml was formed, the process sequence being that of The sequence in the 1st example corresponds to the following, unless other data are specified below:

Dimensions of the preform:

Outside diameter 28 mm

Inner diameter 20.6 mm wall thickness. 3.7 mm

Overall length 130 mm, length below the neck 118 mm

Temperature of the injection mold: 15 ° C to 22 ° C temperature of the preform after injection molding (outside): 100 ⁰ C.

Data of the tempering form:

Inner diameter: 40 mm depth of the mold: 122 mm Temperature of tempering: between 90 ° and 100 ⁰ C air pressure: 8 * 10 ⁵ N / m ^z

Tempering time: between 0.5 and 1 second.

Data of the tempered preform: 15 Outside diameter: 33 mm Length below the neck: 119 mm

Temperature: 115 ° C

(The temperature of the tempered preform is higher than that of the tempering mold, because the preform after the Demolding from the injection mold initially has a higher internal temperature than the external temperature, which means that the Outside temperature rises later.)

finished oil bottle:

Outer diameter ser: 70 mm (max. 77 mm) Length over al les: 225 mm Wall thickness: 0.4 mm Length under Ib des Neck: 213 mm Weight: 29.6 G

The oil bottle thus prepared was excellent Transparency, gloss and a uniform wall thickness.

In conclusion, it can be stated that an advantage of the invention is that to carry out the Process a machine can be used for the production of hollow bodies from polyethylene terephthalate or the like by injection molding, stretching and blow molding is already known. Already by that the injection molding and the temperature control in the specified manner in the shortest possible time performs hollow bodies with the desired high transparency.

Claims (2)

DR. RUDOLF BAUER DIPL.-ING. HELMUT HUBBUCH DIPL.-PHYS. ULRICH TWELMEIER WESTERN? 9 - 31 (AM LEOPOLOPLATZI D-7530 PFOHZHEIM iwest-Germany) ff 10 72 3 1) 1 0 22 90/70 TELEGRAMS PATMARK March 27, 1984 III / Q Katashi Aoki, Japan Process for the production of hollow bodies from biaxially stretched polypropylene Patent claims: Process for the production of hollow bodies from biaxially stretched polypropylene by injection molding of a preform and its further processing by stretching and blow molding through the following sequence of work steps: - Plasticizing and complete mixing of the plasticized polypropylene in an injection cylinder as a result of the rotation of an injection screw in this; - Injection of the molten polypropylene, which no longer contains any crystalline components, into a Injection mold for molding a preform; - Demolding the preform as soon as it is Has formed a thin skin on the surface and as long as it is still hot and amorphous inside; - Placing the preform in a tempering mold, the inner surface of which is greater than the outer surface of the preform, which is enclosed by it; - Tempering the preform by nestling its outer surface on the inner surface of the tempering mold by stretching and expanding the preform; - Relief of the preform from its internal pressure so that it can shrink by itself; - Reshaping of the preform in a blow mold by a combined stretching and blow molding process, whereby the desired hollow body made of biaxially oriented polypropylene is obtained. 2. The method according to claim 1, characterized in that the preform is ^{removed from the injection mold at a temperature between 100 0} C and 103 ⁰ C and in a temperature control ^{mold between 90 0} C and 130 ⁰ C for a period of 0.5 s to 7 s is tempered, the inner surface of which is approximately 1.4 to 2 times as large as the outer surface of the preform that has just been removed from the mold, which is enclosed by it.