EP0574472A1

EP0574472A1 - Process and device for producing blow-moulded hollow bodies

Info

Publication number: EP0574472A1
Application number: EP92906020A
Authority: EP
Inventors: Rainer Wild; Michael Mukrowsky
Original assignee: Rudolf Wild GmbH and Co KG
Current assignee: Rudolf Wild GmbH and Co KG
Priority date: 1991-03-05
Filing date: 1992-03-05
Publication date: 1993-12-22
Also published as: HU9302506D0; WO1992015442A1; AU1349092A; DE4107011A1; HUT67122A

Abstract

Il est décrit un procédé et un dispositif pour la fabrication de corps creux (11) formés par soufflage, en matière plastique, notamment en PET. Une préforme creuse coulée (11) est introduite, à l'état chauffé, dans une matrice de soufflage chauffée (14), et est amenée, par étirage du matériau constituant sa paroi, à prendre la forme voulue d'un corps creux, la paroi étant soumise, en fonction de la forme voulue du corps creux, à des degrés d'étirage divers suivant les zones. Afin de modifier un tel procédé et un tel dispositif pour la production de corps creux réutilisables, il est proposé, qu'avant le formage par soufflage, des zones de transition (11c) de la paroi de la préforme soient chauffées complémentairement au moyen d'un chauffage auxiliaire (13), entre des zones de paroi de divers degrés d'étirement, en particulier entre une zone de paroi à ne pas étirer ou à étirer moins fortement, et une zone de paroi à étirer plus fortement.A method and a device are described for the manufacture of hollow bodies (11) formed by blow molding, in plastic material, in particular in PET. A cast hollow preform (11) is introduced, in the heated state, into a heated blowing die (14), and is brought, by stretching the material constituting its wall, to take the desired shape of a hollow body, the wall being subjected, depending on the desired shape of the hollow body, to varying degrees of stretching depending on the zones. In order to modify such a method and such a device for the production of reusable hollow bodies, it is proposed that before the blow forming, transition zones (11c) of the wall of the preform are additionally heated by means of an auxiliary heater (13), between wall regions of varying degrees of stretching, in particular between a wall region not to be stretched or to be stretched less strongly, and a wall region to be stretched more strongly.

Description

Method and device for producing blow molded hollow bodies

description

The invention relates to a method for producing blow-molded hollow bodies of the type explained in the preamble of claim 1, and to an apparatus for carrying out this method according to claim 10.

Such a method is known from EP-A-197 780. This known method is intended to produce screw-top bottles that can be filled with hot liquids. To prevent excessive shrinkage during hot filling in the area of the thread for the screw cap on the top of the bottle neck, that area of the wall of the preform is additionally heated which in the finished bottle is between the upper end of the bottle shoulder and above that extends the top of the bottle neck. This area has a homogeneous degree of stretching. The heating should take place in such a way that the additionally heated area becomes milky.

Although this described method is suitable for producing disposable containers, it is not suitable for reusable containers, since there the problem of possible stress corrosion cracking at transition areas of different degrees of stretching is not solved and, on the other hand, milky areas of the bottle wall could make the user believe that the bottle is defective and no longer suitable for return.

Another method can be found in US Pat. No. 4,039,641. In the known method, obviously in a multi-stage process, a prefabricated preform is first heated to 95 ° C. in a manner not shown and is inserted in this state into a blow mold preheated to 130 ° to 220 ° C. Then the wall of the preform is stretched in the axial and radial direction in the usual way by an inserted stretching mandrel and by compressed gas until the wall rests against the inside of the blow mold, which shows the shape of the desired hollow body. The preform has the shape typical of this method in the manner of a test tube, while the finished hollow body in the known method is, for example, a bottle. With these differences in shape between the preform and the finished hollow body, different degrees of stretching inevitably result in regions in the material of the wall. These different degrees of stretching lead to a different degree of biaxial orientation and ultimately to different degrees of crystallization in the wall with the resulting different mechanical properties and the susceptibility to stress corrosion cracking in transition areas between regions with a different degree of stretching. In the known method, the finished hollow body is cooled in that the liquid to be stored is already cooled down to 0 ° and introduced into the hollow body while it is still in the blow mold. However, this means that the known method only for the production of Disposable containers is designed because multiple filling was not considered.

The invention has for its object to provide a method and an apparatus for producing hollow bodies with which reusable containers can also be produced.

The object is achieved in a method by the features specified in claim 1 and in a device by the features specified in claim 10.

Compared to single-use containers, returnable containers have to meet a number of additional requirements. Reusable containers must have a reduced tendency to stress corrosion cracking in addition to sufficient mechanical strength when filled and when empty. You must survive at least 20 cleaning processes with hot cleaning fluids and, if necessary, disinfectants. They must not shrink too much in the course of multiple use, and must also have an absorption volume that is within the legally prescribed limits even in the last step of use. They must also be able to be repeatedly filled in conventional filling systems, these filling systems, for example, gripping bottles below a bead arranged near the filling opening on the bottle neck and pressing them against a filling lance or the like. Disposable containers cannot meet these requirements.

It has now been found that the additional heating of the preform provided according to the invention is critical before insertion into the blow mold Transitional areas between wall areas, which are stretched less during the subsequent blow molding, and wall areas, which are stretched more during the subsequent blow molding, have an extremely favorable effect on the homogeneity of the biaxial orientation in the wall of the hollow body. The hollow bodies produced by the method according to the invention meet all the requirements placed on reusable containers. For example, in the case of a hollow body produced according to the invention with a volume of 1 liter, a shrinkage of only 0.4% by volume was found even after 30 to 35 cleaning steps at a relatively high cleaning temperature of 65 ° C.-67 ° C. The method according to the invention can also be used to produce thick-walled hollow bodies.

The additional heating of the preform, especially at the critical transition areas between wall areas that are less stretched and those that are stretched more, ensures that it is precisely in these critical areas that the containers do not become milky, but maintain their transparency, as in the other areas . Contrary to the prior art, in which these critical areas can become milky, a bottle produced by the method according to the invention thus remains translucent and does not give the user the impression that it is defective and therefore cannot be recycled.

Claim 2 describes a particularly preferred temperature range for the additional heating. It was found that, in particular in the case of an additional heating arranged in the upper region of the conditioning mold, it is often sufficient to replace the otherwise unavoidable heat losses when preheating the preform. However, the additionally heated area is preferably brought above the temperature of the remaining wall of the preform.

It was also found that the optimization of the cooling of the cast preform for the purpose of its solidification, as described in claims 5 and 6, also makes a significant contribution to optimizing the mechanical and thermal strength or the corrosion resistance. Due to the essentially uniform cooling over the entire wall of the preform, a more uniform temperature profile is achieved and the build-up of stresses inherent in the material is avoided, which could lead to different degrees of orientation and consequently different degrees of crystallization during the subsequent blow molding.

Claim 8 describes a particularly useful temperature for the blow molding and thermosetting.

It has also been found that the method according to the invention is preferably carried out in one step, the preform being continuous from casting to conditioning to blow molding, i. H. without intermediate storage.

The additional and regionally effective heating of the preform is carried out according to claim 10 in the conditioning form, claims 11 and 12 particularly preferred locations for the arrangement of the additional heating are specified. The infrared heater described in claims 13 and 14, which is operated for example at 170 ° C, has proven to be particularly useful.

Claim 15 describes a particularly preferred mold for injection molding the preform, with which an extremely uniform and rapid cooling can be achieved over the wall of the preform.

Claims 16 and 17 describe a form of conditioning with which the wall temperature of the preform can be influenced even further in a targeted manner.

An embodiment of the invention is explained below with reference to the drawings. Show it:

FIG. 1 shows a schematic, partially sectioned illustration of the casting mold modified according to the invention,

Figure 2 is a schematic representation of the conditioning form modified according to the invention and

Figure 3 is a schematic representation of the blow mold used according to the invention.

Before going into the process parameters and the design details of the invention, a brief overview of the most important, partly known manufacturing steps in the manufacture of a blow-molded bottle from polyethylene terephthalate (PET) is to be given. A preform is first injection molded. The injection molding takes place in a cooled injection mold, the inner wall of which is the shape of the The outer wall of the preform is determined, while a cooling lance, which is inserted into the interior of the injection mold, forms the inner surface of the preform. The preform is cooled in the injection mold from the molding temperature to the extent that it solidifies and can be removed from the mold. The preform is then inserted, if necessary after further preheating, into a heated blow mold, the inner wall of which corresponds to the desired outer shape of the bottle. After insertion, a stretching mandrel is first inserted into the interior of the preform, which supports the axial expansion of the preform. Pressurized gas, preferably compressed air, is then introduced into the interior of the preform and presses the wall of the preform radially against the inner wall of the blow mold. This stretching process brings the material of the wall from the amorphous state to a biaxially oriented state. Through contact with the heated blow mold wall or through a subsequent heat treatment, a thermosetting takes place, through which the material of the wall changes into a crystalline state. After cooling, the finished bottle can then be removed from the mold.

The present invention is intended to modify this known method in such a way that it is suitable for producing reusable containers. The injection mold 1 shown in FIG. 1 is used for the method according to the invention. The inside of the injection mold 1 has a mold cavity 2 which is open at the top and is surrounded by a wall 3. The mold 1 is covered on the outside by an insulating material 4. There is a gap 5 between the insulating material 4 and the wall 3, which is divided by a plurality of ribs 6 which extend around the circumference of the wall 3 and are spaced apart in the axial direction are divided into a plurality of cooling compartments 7. In the axial direction extends over the entire height of the wall 3, a longitudinal rib 8, which delimits each cooling compartment 7 in the circumferential direction. In addition to the longitudinal rib 8, passage openings 9 are provided in the circumferential ribs 6, through which the cooling medium can pass from one cooling compartment into the adjacent cooling compartment. The passage openings 9 of adjacent cooling compartments are arranged on opposite sides of the longitudinal rib 8, so that the cooling medium can enter each cooling compartment 7. The coolant enters the cooling compartments 7 through an opening 10 in the lower part of the casting mold 1 and leaves them in the upper region of the casting mold through an opening which is not visible in the drawing.

In contrast to conventional casting molds, the casting mold 1 has relatively thin circumferential ribs 6, which consist of a good heat-conducting material, in particular copper. As a result, the contact area of the coolant with the peripheral wall 3 is greatly increased, which improves the heat transfer into the coolant. In addition, what can not be seen in Figure 1, several inlet or outlet openings 10 can be provided for the coolant over the height of the casting mold 1, so that the cooling effect in the upper region of the casting mold, where the coolant already absorbs heat in conventional casting molds has been further improved. In addition, a cooling lance (not shown) for cooling the inner wall of the preform is introduced into the interior of the injection mold in a known manner.

The coolant is water at 12 to 14 ° C. The cooling time takes 4 to 14 seconds, preferably 12 seconds. During this time, the preform sprayed at 270 ° C is cooled to a temperature of approximately 90 to 95 ° C. With the casting mold described, the preform can be cooled uniformly and very quickly over its entire cross section, as a result of which a shorter cycle time and thus an improved economy of the process is achieved. The particularly uniform cooling is achieved by the special design of the coolant channels described above. At each entry from one cooling compartment to the other, a back pressure is generated which causes turbulence in the flow. This creates a swirl of different temperature ranges and therefore more uniform cooling. This uniform cooling significantly simplifies the manufacturing process of reusable containers in the subsequent blowing process.

From the mold 1, the 90 to 95 ° C. warm preform 11 is inserted into the conditioning mold 12 shown schematically in FIG. 2, which is preferably located on the same machine unit as the mold, so that intermediate storage of the preform is not necessary. As FIG. 2 shows in more detail, the preform 11 is shaped and dimensioned for producing a bottle. The preform 11 has a first region 11a, which forms the bottle neck for screwing on a closure. The area 11a already has the most important dimensions, ie the diameter, the wall thickness and the height, of the finished bottle neck. The area 11a is delimited by an outwardly projecting bead 11b. This bead 11b later serves to grip the finished bottle in a filling system. Below the bead 11b there is a transition area 11c, which in the finished Bottle forms the transition area between the narrow bottle neck, which remains essentially in the amorphous state, and the wide, biaxially oriented bottle body. The preform 11 has a continuously increasing wall thickness in this area 11c. A region 11c adjoins the region 11c, which forms the later bottle body and which has a constant wall thickness in the preform which is enlarged compared to the bottle neck 11a. The area lld is cylindrical and is closed at the bottom by a base area lle. The bottom region III has rounded transitions to the cylindrical region IIId, but has a smaller wall thickness in the preform.

A preferred preform has the following preferred dimensions: area 11a - length about 17mm, wall thickness about 2.5mm, inner diameter about 20mm; Area 11b - outside diameter about 37mm, thickness about 6mm; Area 11c - wall thickness increasing from approximately 3mm to approximately 6mm, inner diameter decreasing from approximately 20mm to approximately 17.5mm with transition radius RIO, outer diameter increasing from approximately 26mm to approximately 29.5mm with transition radius R30, length approximately 23mm; Area lld - wall thickness about 6mm, length about 108mm, outer diameter about 29.5mm; Area lle - wall thickness falling from about 6mm to about 3mm, transition radius outside R9, transition radius inside R5.8, height about 6mm; Overall length of the preform about 160mm.

The preform 11 is clamped in the conditioning mold 12 above the bead 11b in the usual way. The conditioning mold 12 has three heating zones 12a, 12b and 12c, the lower heating zone 12a having a temperature of preferably approximately 70 °, which is directed upwards to the lower one The central heating zone 12b adjoining the heating zone 12a preferably reaches a temperature of approximately 90 ° and the upper heating zone 12c adjoining the heating zone 12b upwards preferably reaches a temperature of approximately 70 °. The heating zones 12a to 12c are located essentially next to the bottom region III and the cylindrical region IIId of the preform 11. The heating zones 12a to 12c are heated in the usual way, for example by circulating, heated oil.

Above the upper heating zone 12c, the conditioning mold 12 contains an additional heater 13, which is preferably designed as an infrared heater. The additional heater irradiates the area 11c below the bead 11b, i. H. the area which, in the finished bottle, lies in the transition between the amorphous neck area 11a, which remains essentially undrawn, and the area lld for the later bottle body, which area has to be drawn biaxially. The auxiliary heater is operated in such a way that a temperature of approx. 170 ° C. is reached in the region 11c without the preform crystallizing in this region.

Subsequent to the conditioning, the preform 11 is inserted from the conditioning mold 12 into a blow mold 14 shown in FIG. The blow mold 14 has in the usual way an inner cavity 15 corresponding to the outline shape of the finished bottle and a wall 17 which can be heated by means of an oil circuit 16 which is only indicated. A stretching mandrel 18 is located in the interior of the preform 11. Furthermore, an inlet line 19 is provided for introducing compressed air into the interior of the preform 11. As can be seen in FIG. 3, the preform 11 is clamped in the blow mold 14 in such a way that the regions 11c, 11d, 11b located below the bead 11b protrude into the mold cavity 15. During blow molding, the stretching mandrel 18 is first moved further into the mold cavity 15 while the preform 11 is stretched. At the same time, compressed air of approx. 30 bar is blown in via the line 19, which also presses the wall of the preform 11 in the radial direction against the wall of the mold cavity 15. From the shape differences between the wall of the mold cavity 15 and the preform 11 it can already be seen that the greatest stretching of the material takes place in the area of the cylindrical wall area lld of the preform 11, while the bottom area 11c, in particular in the vicinity of the vertical central axis, and the clamped neck area 11a are stretched less or not at all. The area 11a in particular remains amorphous. Without the additional heater 13 in the conditioning mold 12, however, the region 11c just below the bead 11b would also remain in an amorphous state. The heat treatment ensures that the entire area 11c is oriented biaxially to such an extent that a sufficient crystalline structure is also formed in this area during subsequent thermosetting due to the contact of the plastic material with the wall of the mold cavity 15 preheated to 140 ° C. without that the shape or the appearance of the bottle so produced is impaired.

After molding, cooling air at 25 bar is passed through the inside of the finished bottle in a continuous process (through small holes in the stretching lance) and then the finished bottle out of the mold taken.

In a modification of the exemplary embodiment described and drawn, the method according to the invention can also be modified for other plastics or other hollow body shapes. Additional heaters can be provided at all critical transition points, especially on the floor. Another heat treatment process can follow blow molding.

Claims

1. A method for producing blow-molded hollow body made of plastic, in particular PET, in which a cast, hollow preform is inserted into a heated blow mold in the heated state and is brought into a desired hollow body shape by stretching the material of its wall, the wall depending on the desired hollow body shape is subjected to different degrees of stretch in some areas, and areas of the wall are additionally heated before blow molding, characterized in that transition areas of the wall of the preform between wall areas with different degrees of stretch, in particular between a wall area that is not or less stretchable and a more stretchable area Wall area, additionally heated.

2. The method according to claim 1, characterized in that the transition regions are brought to a temperature between 70 and 170 ° C.

3. The method according to claim 1 or 2, characterized in that when producing a hollow body with a neck region drawn in relative to the body region of the hollow body, the transition region between the neck region and the body region is additionally heated.

4. The method according to any one of claims 1 to 3, characterized in that the bottom region of the hollow body is additionally heated.

5. The method according to any one of claims 1 to 4, characterized in that the preform is cooled during solidification to a substantially uniform temperature of about 90 to 95 ° C over its entire wall.

6. The method according to claim 5, characterized in that is cooled with water from about 12 to 14 ° C within about 4 to 14 seconds, preferably 12 seconds.

7. The method according to any one of claims 1 to 6, characterized by heating the preform before blow molding in a conditioning mold with a multi-stage temperature range.

8. The method according to any one of claims 1 to 7, characterized in that the blow mold is heated to over 110 ° C, preferably 140 ° C.

9. The method according to any one of claims 1 to 8, characterized by its single-stage implementation without intermediate storage of the preform.

10. Apparatus for carrying out the method according to one of claims 1 to 9, with a cooled mold for the preform, a heated and a regionally acting additional heater having conditioning mold and a heated and provided with a compressed gas connection blow mold, characterized in that the additional heater (13 ) for the production of hollow bodies with a neck region (11a) which is drawn in opposite to a bulged body region (lld) at a transition region (11c) between Neck and body area is arranged.

11. The device according to claim 10, characterized in that the additional heater (13) is arranged on the bottom (lle) of the hollow body.

12. The device according to claims 10 or 11, characterized in that the additional heater (13) is an infrared heater.

13. The apparatus according to claim 12, characterized in that the infrared heater is operated at 170 ° C.

14. Device according to one of claims 10 to 13, characterized in that the casting mold (1) from bottom to top and around its outside, wide coolant channels (7), which by narrow ribs (6) made of heat-conducting material, in particular copper , are separated from each other.

15. The device according to one of claims 10 to 14, characterized in that the conditioning mold (12) contains a plurality of further heating stages (12a, 12b, 12c).

16. The apparatus according to claim 15, characterized in that three further heating stages (12a, 12b, 12c) are provided, the lower heating stage (12a) near the bottom region (III) 50 ° C, the middle heating stage (12b) 70th ° C and the upper heating stage (12c) has 90 ° C.