DE3411905C2 - - Google Patents

Description

The invention is based on a method for manufacturing of hollow bodies made of a biaxially stretched polymer with those specified in the preamble of the claim Activities.

Such a process is for hollow bodies made of polyethylene terephthalate known from DE-OS 28 19 813. The present The invention is concerned with production of hollow bodies made of polypropylene.

Polypropylene is a crystalline resin, and the pellets, of which one is used in the manufacture of polypropylene articles usually comes out as primary material, therefore crystalline structure. At a temperature between 190 ° and 260 ° C you can mold polypropylene well, and if you use hollow bodies, e.g. B. bottles made of polypropylene by blow molding or by injection molding preforms, which are then further processed by blow molding, or by produces a similar process, then one tries to get the To keep the working temperature as high as possible.

If you blow mold a polypropylene hollow body by injection molding and then blow molding, then you get using known methods always hollow bodies whose transparency is impaired and that appear milky because the preforms go through immediately Blow molds can be formed into hollow bodies of the desired shape.  

It is believed that the polypropylene hollow body therefore become milky because of the resin in the course of the manufacturing process due to cooling of the melted Material partially crystallized, the occurring Fluctuations in the transparency of the hollow body from Extent of crystallization and the size of the crystalline Suspend areas in the amorphous material.

For the manufacture of hollow bodies made of polyethylene terephthalate known methods are the by Injection molded preforms are only removed from the mold, after maintaining on a around 60 ° C or 70 ° C Crystallization temperature cooled are. It is also known to preform before final stretching and blow molding in a pre-blow mold expand slightly by swelling. Primarily its purpose is to detach the preform from the injection molding core, so that the inflation is preferably without using a Pre-blowing takes place in the open air.

The object of the invention is a method for the production of hollow bodies from biaxially stretched To specify polypropylene with which it is possible to produce transparent, to produce shiny hollow bodies with uniform wall thickness.

This problem is solved by a method with the Features specified claim.

The process is such that you first by injection molding produces a preform from polypropylene. To one assumes polypropylene pellets that go through the snail  plasticized in the injection cylinder of an injection molding machine will. The melted polypropylene becomes the front end the snail and in the in front of the snail Space accumulated in the injection cylinder, from which it in the machine cycle by a feed movement of the screw is displaced and poured into the injection mold. One biaxially oriented hollow body made of polypropylene, the transparent and shine, you cannot stretch it and blow molding from a preform obtained by injection molding manufacture as long as the molten polypropylene is not completely amorphous, but still crystalline material contains. Therefore it should be ensured that Cylinder of the injection molding machine a good mixing of the plasticized polypropylene takes place before it enters the Injection mold is injected. In this way, such molten polypropylene in the injection mold to bring in, which no longer any crystalline components contains. The preform is removed from the injection mold with the highest possible temperature and transfers it into a pre-blow mold, in which it is specifically tempered by heat exchange can be. By tempering the preform should be optimal Conditions for subsequent stretching and blow molding can be set.

The plasticized polypropylene, which is due to the Rotation of the screw in front of its front end in the injection cylinder accumulates, still shows a non-uniform Temperature when it is completely melted.  

Then there is a non-uniform temperature distribution also in the molten polypropylene, which was injected into the injection mold, and it is one of the reasons why the resulting one Preform an uneven temperature distribution owns. This non-uniform temperature distribution in the preform is in the pre-blow mold partially compensated for a temperature control process, which in the method according to the invention before the final Stretching and blow molding is carried out. The The tempering process is primarily the same as temperature differences on the surface of the preform, which from an uneven temperature distribution of the cooled Injection mold. The temperature in The interior of the preform can be retrofitted Tempering is not so easy, at least then not if only for a short time for the temperature control process is available. Because of an uneven temperature distribution essentially inside the preform from a non-uniform temperature of the plasticized Polypropylene in the injection cylinder of the injection molding machine originates and not only to uneven wall thicknesses, but also to uneven transparency of the hollow body can lead should still in the injection cylinder Temperature in the melted polypropylene through intense Mixing can be evened out. As a result, that also the preform, which is in the injection mold  is formed, an even inside Temperature. The influence of intensive mixing of polypropylene in the injection cylinder, is clearly shown in the fact that preforms that follow poured intensive mixing of the polypropylene have a higher transparency than preforms, which were poured after only moderate mixing and therefore have a milky appearance. The more even Temperature distribution in the preform according to the previous one intensive mixing of the polypropylene can be demonstrated empirically.  

The injection time is selected when producing the preform and the cooling time as short as possible; The aim is, demoulding the preforms as hot as possible, and you only cool them down as much as absolutely necessary is necessary to be able to carry out the demolding, and the demoulding already happens when in the injection mold on the inner and outer surface only a thin skin is formed in the preform Has.

The rapidly demolded preform is thin Skin crystallizes on its inner and outer surface and takes on a slightly whitish look at. Is inside the wall of the preform the polypropylene, however, is still in an amorphous state and with higher temperature before; from inside the Preform to its inner and outer surface flowing heat leaves the skins formed there again melt what you can see from the fact that the demolded Preforms are becoming increasingly transparent.

To prepare for the final stretch and Blow molding process are those taken from the injection mold Preforms by heat exchange with the Wall of a pre-blow mold to a suitable processing temperature brought. How to do one Tempering can, for example, be in U.S. Patent Application No. 516,786, which goes back to the same inventor.  

A pre-blow mold is used, the inner surface of which is larger than the outer surface of the preform, and around the preform with the wall of the He will bring the pre-blow mold into intimate contact expanded in the pre-blow mold. The expansion expediently takes place in such a way that the preform is initially axially in the pre-blow mold stretches and then pressurized with a gas. In contact with the inner wall of the pre-blow mold the preform takes the one for the next one Stretching and blow molding provided working temperature at. The time required for tempering is only small, that period of time in which the preform is sufficient is sufficient shrinks after you take it off the gas pressure relieved inside. The inner surface the pre-blow mold is larger than the one it encloses outer surface of the preform to this to be able to stretch and expand sufficiently. The Difference between the two shouldn't, however be too large so that the preform is not too strong the length and diameter is expanded, because otherwise, the following could The stretching and blow molding process is insufficiently stretched and be stretched and then just do one insufficient biaxial orientation. The inner surface of the pre-blow mold should be approx. 1.4 to 2 times the size of the one it encloses outer surface of the preform.

Stretching the preform in the pre-blow mold is best done with a stretch stamp, which one enters through the neck of the preform introduces;  the subsequent expansion of the preform the easiest way is by blowing air. The preform consequently hugs the inner surface the pre-blow mold, and between the two there is a heat transfer until after relieving the Preform from its internal pressure the preform shrinks and comes off the wall of the pre-blow mold solves.

The tempering takes place as long as the preform of the Inner wall of the pre-blow mold is present. If he's after Reaching the intended working temperature for the subsequent stretching and blow molding from its internal pressure is relieved, it shrinks. The shrinking affects the preform quite advantageous. Because namely when shrinking the wall thickness of the preform overall increases and because in the course of the change heat balance in the preform takes place, the temperature of the preform continues equalized, and at the same time, any tensions still present, which occur during injection molding have arisen, eliminated. With the onset Shrinking releases the preform from the wall of the Pre-blow mold, and this is the temperature control process automatically ended.

The intermediate moldings formed in this way are immediately transferred into a blow mold and in it by means of of a stretching die axially stretched, then through Air blowing expands and then rapidly cooled down; the hollow bodies obtained in this way have excellent transparency.  

The following are two exemplary embodiments specified. It was used to carry out the procedure a machine for injection molding and blow molding, as described in US-PS 41 05 391.

Example 1

A polypropylene with the designation was used 6200E, manufactured by Mitsubishi Chemical Industries, Ltd., with a density of 0.897 g / cm³, with a melting point of 140 ° C, and with a melt viscosity of 1.7 g / 10 min.

Pellets of this polypropylene were heated, melted and by rotating the injection screw in one Injection cylinder heated to a temperature of 220 ° C mixed thoroughly, then into one on one Temperature between 12 ° C and 15 ° C cooled injection mold poured and in this way a bottle-shaped Preform made 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 became as hot as possible from the injection mold removed from the mold and immediately into a pre-blow mold used. The temperature of the unmolded preform in this case was between 100 ° C and 103 ° C.  

The temperature of the preform was done in a pre-blow mold with the following characteristics:

Inner diameter of the pre-blow mold: 36 mm,
Pre-blow mold depth: 137 mm,
Pre-blow mold temperature: between 120 ° and 130 ° C,
Tempering time: 6-7 s,
Air pressure: 10⁵ to 2 · 10⁵ N / m² corresponding to 1 to 2 bar.

After inserting the preform into the pre-blow mold this was initially done with a stretch stamp axially stretched until the bottom of the preform on Because of the pre-blow mold. Then was Air is blown into the hot preform to it expand and thereby its outer surface in intimate contact with the inside of the pre-blow mold bring to.

The temperature control process was completed by the Shrinking of the preform that starts when the stretching stamp is withdrawn and the overpressure in the Is removed inside the preform.

After the tempering process, the intermediate molding thus formed had one temperature between 113 ° C and 120 ° C, a Outside diameter of 31 mm and a length below of the neck of 130 mm.  

The tempered intermediate molding was immediately in a Transfer blow mold and therein by stretching and subsequent Blow molding into a 600 ml beer bottle reshaped 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, one 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 · 10⁵ and 10⁶ N / m², and the blowing time was between 3 and 5.5 s.

The beer bottle made in this way was transparent, showed no milky spots, had shine and a uniform wall thickness.

Example 2

In this example, an oil bottle with 700 ml Shaped volume, the process flow the The procedure in the first example corresponds to the following other data are not mentioned:

Preform dimensions:

Outer diameter: 28 mm,
Inner diameter: 20.6 mm,
Wall thickness: 3.7 mm,
Overall length: 130 mm,
Length below the neck: 118 mm,
Injection mold temperature: 15 ° C to 22 ° C,
Preform temperature after injection molding
(outside): 100 ° C.

Pre-blow mold data:

Inner diameter: 40 mm,
Depth of the mold: 122 mm,
Pre-blow mold temperature: between 90 ° C and 100 ° C,
Air pressure: 8 · 10⁵ N / m²,
Tempering time: between 0.5 and 1 sec.

Data of the tempered intermediate molding:

Outer diameter: 33 mm,
Length below the neck: 119 mm,
Temperature: 115 ° C.

(The temperature of the tempered intermediate molding is higher than that of the pre-blow mold, since the preform after the First remove a higher one from the injection mold Has inside temperature as outside temperature, which makes the Outside temperature rises subsequently.)

Finished oil bottle:

Outside diameter: 70 mm (max. 77 mm),
Overall length: 225 mm,
Wall thickness: 0.4 mm,
Length below the neck: 213 mm,
Weight: 29.6 g.

The oil bottle made in this way was excellent Transparency, shine and a uniform Wall thickness.

In conclusion, it can be said that an advantage the invention is that to carry out the Procedure a machine can be used that for the production of hollow bodies made of polyethylene terephthalate or the like by injection molding, stretching and blow molding is already known.

Claims (2)

Application of a process for the production of hollow bodies from a biaxially oriented polymer

• Plasticizing and mixing of the polymer in an injection cylinder due to the rotation of its injection screw,
• Injecting the molten polymer into an injection mold to form a preform,
• - demolding of the preform after a skin has formed on it and as long as it is still hot and amorphous inside,
• Introduction of the preform into a pre-blow mold, the inner surface of which is larger than the outer surface of the preform enclosed by it,
• Nestling of the preform to the inner surface of the pre-blow mold by swelling into an intermediate preform,
• - Relief of the intermediate form from its internal pressure and
• - Forming the intermediate molding in a blow mold by a combined stretching and blow molding process,

on hollow bodies made of polypropylene with the proviso

• that the preform is removed from the mold as hot as possible, namely at a temperature between 100 and 103 ° C,
• - That the preform is stretched axially in the pre-blow mold before inflation and
• - That the intermediate preform is heated for a period of 0.5 to 7 s in the pre-blow mold, which is between 90 and 130 ° C., the inner surface of which is approximately 1.4 to 2 times as large as the enclosed surface of the preform.