EP0712350A1

EP0712350A1 - Process for the cryogenic production of blow moulded plastic parts

Info

Publication number: EP0712350A1
Application number: EP94924814A
Authority: EP
Inventors: Olaf Babel; Hans Erler
Original assignee: Air Products and Chemicals Inc
Current assignee: Air Products and Chemicals Inc
Priority date: 1993-07-29
Filing date: 1994-07-28
Publication date: 1996-05-22
Also published as: JPH09503456A; CA2168242A1; WO1995003931A1

Abstract

The invention relates to a process for the cryogenic production of blow moulded plastic parts, whereby air is used as the blowing medium and is fed into a preform (8) in a blow mould, and the preform (8), which is deformable when hot, is blown-up with the air through a blow mandrel (15) before being cooled with dried, cooler air. According to the invention, the air used as a blowing medium is dried before being introduced into the preform, and the dried air is fed into the blown-up preform in a super-cooled state, with a temperature of between approximately -50 °C and -170 °C, in order to cool the preform. While the blow mould is open, it and/or the blow mandrel are flushed with dry and preferably warm air.

Description

Process for the cryogenic production of blow molded parts made of plastic

The invention relates to a method for the cryogenic production of blow molded parts from plastic according to the preamble of claim 1.

Blow molded parts made of plastic are used in particular in the packaging industry for packaging materials of all kinds. Blow molded parts of this type are produced according to the prior art using various techniques.

For example, it is known from DE 18 16 771 B2 to inflate blow molded parts by the ram air method and only to cool them down with tool cooling. This is for very thin-walled blow molded parts classic processes are still very effective today. With thick-walled molded parts, however, this method has the disadvantage of long cycle times. In addition, the precipitation of moisture in or on the blow mold can result in deformations of the blow molded parts to be produced, with the result that the surface quality of the blow molded parts is often adversely affected (orange peel effect).

DE 21 60 854 C3 discloses a method for cooling a hollow body made of thermoplastic material produced by the blow molding process within a blow mold, in which a cooling medium consisting of air and water is quenched and injected under high pressure into a molded blow molded part, whereby an explosive adiabatic expansion of the cooling medium is caused in the hollow body and fine ice crystals form which are deposited on the wall of the blow molded part and cool it. It is clear that when water is used in the cooling medium there is an increased risk of ice crystal formation in the area of the blow mandrel or in the blow mold, with which the disadvantages mentioned above also occur. Here, too, the cycle times and the quality of the blow molded parts produced are not convincing.

In another method according to DE 24 42 254 B2, compressed air is temporarily added to a blowing gas made of nitrogen or argon, and the additional supply of compressed air is interrupted before the final inflation pressure is reached. The disadvantages listed above also occur here. A method is also known from DE 22 23 580 C3, in which nitrogen or argon are used for producing blow molded parts, but without the addition of compressed air. Both methods suggest external cooling.

In the device known from DE 26 36 262 B2, the inflation of a blow molded part is carried out in such a way that carbonic acid is introduced into the molded part via feed lines in order to inflate the blow molded part. A further supply line is used Coolant is led into cavities in the blow mold in order to cool or solidify the inflated blow molded part. The problems of long cycle times or the risk of moisture precipitation and the associated disadvantages also occur here.

The device according to DE 33 37 651 C2 has two blow mold halves with a relatively small mass, which can be enclosed by heat mold halves with a larger mass, in order to be able to cool the blow mold part located in the blow mold quickly due to the large mass of the heat mold halves. In addition to the disadvantages discussed above, there is an additional expenditure on apparatus, the cooling temperature also not being able to be set sufficiently low.

A method is also known from DE 28 17 472 C2, in which blow-molded parts are inflated or cooled from the inside by means of a mixture of cold air and water, the disadvantages listed above arising.

From DE 37 28 208 AI a method for producing blow molded parts is also known, in which the blow mold is first inflated and then a coolant is injected into the blow molded part, water being proposed as the coolant. The generic method emerging from DE 3728 208 AI, however, has the same disadvantages as the rest of the prior art.

From US 4,091,059 a generic method for producing blow molded parts made of plastic is known. Air is passed into a preform in a blow mold. The preform or the blow molded part is inflated in the hot, deformable state by means of normal air with a blowing mandrel, the air subsequently used for cooling being cooled down to a maximum or a minimum of -41 ° C.

From US 3,937,609 is a method for molding plastic materials known. During the molding process, it is flushed with dried air to prevent moisture from being deposited.

The state of the art can be summarized that all known methods or in particular the internal cooling methods listed above are difficult to reproduce, especially due to icing technologically vulnerable, economically doubtful and otherwise have other disadvantages.

The state-of-the-art methods that use cool gases only work at comparatively high temperatures, for example -41 ° C. At temperatures of this order of magnitude, the gases cannot be used particularly effectively for cooling, since in particular the density of the gases at such temperatures leads to their relative heat capacity being low. In order to provide adequate cooling, large amounts of cool gases have to be used, which is essentially only possible with very thin-walled moldings or containers. Such technologies speak of the so-called "mechanical refrigeration technology".

In order to remedy the disadvantages of such mechanical refrigeration techniques, as are known, for example, from US Pat. No. 4,091,059, technologies were preferably developed for thick-walled and bulky moldings or containers, in which, for example, water or other cooling media with a higher heat capacity were used, these processes then, however, cause problems with freezing-out or condensing moisture and other problems that have already been discussed.

The classic ram air process with the exclusive cooling of the blow molding by the cooled tool or blow mold has its limits with regard to the coolant temperature in any case, since otherwise there must be intolerable condensation effects on the cold tool. The invention has for its object to propose a cryogenic process for producing blow mold parts made of plastic; in particular, a method according to the invention with improved cycle time at reduced blow mold part costs should preferably be proposed for the production of bulky blow mold parts.

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

Advantageous process variants result from the subclaims.

The advantages to be achieved with the invention are based on the fact that air is used as the blowing medium and is dried before being introduced into the preform. Furthermore, cryogenic, dry air is introduced into the inflated preform or the blow molding in order to cool the inflated preform or the blow molding. In addition, while the blow mold is open, the blow mold and / or the blow mandrel are flushed with, or flushed with, dry, warm air. The cryogenic air has temperatures between about -50 ° C and about -170 ° C, preferably -90 ° C to about -170 ° C, which has the advantage according to the invention that the cryogenic and dry air is relative is tight and thus has a high relative heat capacity, so that even thick-walled blow molded parts or bulky blow molded parts can be produced reproducibly with a short cycle time.

In contrast to the "mechanical refrigeration technology" otherwise used, the invention is therefore a "cryogenic refrigeration technology". The cooler temperatures are all the more advantageous because the gas or air become denser at lower temperatures, so that the relative heat capacity per unit volume is greater.

The use of dry air prevents the precipitation of moisture in any case, and cryogenic, dry air also has a higher rate Cold capacity in U / kg than normal compressed air and is much less problematic than carbonic acid in the case of geometrically complicated bodies, since no snow formation occurs, or liquid nitrogen. In addition, washing around the blow mold or the blow mandrel when the blow mold is open prevents the parts in question from icing up. The use of dry air at all stages of the blow molding process also reliably precludes the precipitation of moisture within the blow molded parts.

In addition, when the blow mandrel and / or the needle on the blow mandrel is rinsed, the interior of the blow mold is simultaneously subjected to dry, in particular warm, air, so that the humidity of the room air cannot condense even on the cold internal contours of the blow mold, which on the one hand affects the quality the molded part surface is important and on the other hand enables lower coolant temperatures for the blow molding cooling or tool cooling.

In addition, shrinkage of the blow molded part can be prevented by using an increased pressure during internal cooling, which can significantly improve the heat transfer from the tool to the molded part.

In a particularly advantageous manner, it is possible to utilize the liquid nitrogen to be used for cooling the dry air for further uses, for example as a mixed gas or purge gas, in particular in fluorination processes, the pressure of the gaseous nitrogen formed during the cooling being adapted to the subsequent processes can.

By using the air dried by drying devices, in particular adsorbers and the like, and cooled by indirect heat exchange with respect to the liquid nitrogen, blowing mandrel cooling with water, as is customary in the ram air process, is generally avoidable. The method according to the invention enables a surprisingly short cycle time, which is impressive Is shortened, namely about 40% and more, and is exemplary in terms of reproducibility.

Characterized in that, according to an advantageous process variant, the liquid nitrogen for the generation of the necessary low temperature of the dry air is first used technologically as a coolant and then as a warm dry inert gas, a significant improvement in terms of economy is achieved.

According to the process variants of the present invention, the required shortening of the cycle time or the increase in output of the blow molding machine set up for the process according to the invention can be achieved by controlling the difference in pressures before and after the blow mold, controlling the temperature of the dry air and Control of the purge time with the dry air, the required process parameters are determined. The high rate of increase in the cycle time to be achieved according to the invention means that a higher nitrogen consumption in kilograms of nitrogen per kg of plastic mass is required, the optimum being specific to the molding, tool and blow molding machine.

The cryogenic process for producing blow molded parts made of plastic is particularly advantageous, in which the following steps are carried out:

Air is passed as a blowing medium into a preform located in a blow mold and the preform which is deformable in the hot state is inflated via a blow mandrel. The inflated preform is cooled and dried, cool air is passed into the inflated preform to cool it. The air serving as the blowing medium is dried before being introduced into the preform to be inflated. The dried air is in a recuperator or Pre-cooler against pre-cooled gaseous nitrogen coming from a low-temperature cooler, and then by further cooling in the low-temperature cooler to its target temperature of about -50 ° C to

Obtained -170 ° C by indirect heat exchange against evaporating, liquid nitrogen at about -180 ° C to about -196 ° C. Compressed air can preferably also be used. The blow mold and the blow mandrel are flushed or flushed with dry, warm air after the one and before the subsequent blow process, while the blow mold is open, the blow mold also being flushed through the blow mandrel. By rinsing the blow mold through the blow mandrel, it is possible to keep the blow mold free of condensate as well as to keep the blow mandrel free of condensate and ice.

The invention is explained in more detail below on the basis of a preferred method variant with reference to the attached figures. This results in further advantages and features according to the present invention. Show it:

FIG. 1 shows a block diagram of a system designed for the method according to the invention; and

FIG. 2 shows a partial section of the detail X from FIG. 1, which in particular shows the design of the blow dome in a sectional view.

FIG. 1 shows a system which is suitable for carrying out the method according to the invention or a method variant of the present invention.

An air stream, in particular compressed air, is introduced into a pressure-controlled drying device 3 and dried there. The drying device 3 can have two adsorbers, in particular bed adsorbers, one of which is used for drying, while the other bed adsorber is regenerated. The regeneration can be carried out by relaxing the adsorber and simultaneously Flow can be carried out with dry air. The switching intervals are determined by the adsorption capacity of the adsorbent and are fixed in time. In principle, all types of drying devices are suitable, although tandem bed adsorbers 3 are preferred. Since the drying devices are contained in different designs in the prior art, a further detailed explanation of the drying device 3 is unnecessary.

The air or compressed air dried in the drying device 3 is passed into a recuperator or a pre-cooler 4, where the dried air is pre-cooled. This precooling process is carried out in the heat exchange with a nitrogen coming from a low-temperature cooler 5, which is already slightly preheated. The pre-cooled and dried compressed air is then cooled in the low-temperature cooler 5 to the predetermined target temperature of about -50 ° C to about - 170 ° C, preferably -90 ° C to -170 ° C, the in the low-temperature cooler 5 in indirect heat exchange to the compressed air liquid nitrogen is evaporated and preheated, in order to then be passed to the recuperator 4 for precooling the compressed air. In the low-temperature cooler 5, the nitrogen can be warmed up to -180 ° C to -196 ° C, or, if necessary, far beyond.

At an output of the low-temperature cooler 5, at which the dried, cryogenic air or compressed air exits, a magnetic control valve MV1 is opened or closed with the aid of a temperature measuring point TIC1 in accordance with a target / actual value comparison. The temperature measured at the temperature measuring point TIC1 and the predetermined target temperature of the dried, cryogenic compressed air are used by appropriate control electronics for controlling the solenoid control valve MV1.

The solenoid valve MVl is connected to a storage tank via a vacuum-insulated line 1 connected for liquid nitrogen. If the valve MV1 is opened, liquid nitrogen enters the low-temperature cooler 5, in which an indirect heat exchange takes place. The liquid nitrogen evaporates and heats up, while at the same time extracting the heat from the warm or pre-cooled air or compressed air. The nitrogen supply and thus the heat exchange are controlled so that the target temperature of the dried compressed air can be maintained.

The evaporated nitrogen passes through the pre-cooler 4 and via the check valve RK1 to a buffer 9 which can be connected to the works network. The nitrogen can be stored here for further use.

To ensure that nitrogen that is never too cold leaves the cooling device, monitoring is also carried out via a temperature measuring point TTC2 in the line to the "buffer".

After introducing a preform 8 into a blowing tool 7 of the blowing machine, after closing the blowing tool 7, dry air from the dryer 3 is provided via the magnetic valve MV2, which air is used to inflate the preform 8. After a short inflation time, valve MV2 is closed again. The solenoid valves MV3 and MV4 are opened at the same time. The cryogenic, dry air now passes from the cryogenic cooler 5 via the insulated line 6, the solenoid valve MV3 and the line 12 and the inner tube 14 into the still hot preform 8. The dry, cryogenic air rinses through the preform 8 and leaves the preform 8 again Solenoid valve MV4 and the pressure relief valve DMVl.

When flowing through, the dry, cryogenic air extracts its thermal energy from the molded part 8 by the air heating up. At the same time, the molded part 8 is cooled.

The pressure relief valve DMVl enables the necessary internal pressure to be set to maintain, which ensures that no contractions of the molded part 8 occur or an optimal pressure on the blowing tool 7 is accomplished, whereby the external cooling is better to use. The air volume can also be determined using the set differential pressure, which can be influenced by the pressure relief valve DMVl.

After a cooling time optimized for the method according to the invention, the valves MV3 and MV4 are closed again. At the same time, the valve MV5 is opened to relax the air from the inflated molding 8. The blow mold 7 is then opened in order to release the cooled blow molding 8. When the blowing tool 7 is opened, the valve MV5 is closed again, and at the same time the valve MV6 is opened. During the time that the tool 7 is open, dry air now flows through the dome and the inner walls of the blowing tool 7. The quantity can be determined depending on the back pressure via the pressure control valve DMV2. The overall geometry of the detail X of the blower dome 15 with the parts 11, 12, 13, 14 is carried out in such a way that a two-pipe system consisting of an inner pipe 14 for providing the cooling air and an outer pipe 13 for realizing the inflation or purging is present. Flushing the outer tube 13 while the tool is open prevents water from the room air from freezing to the inner nozzle of the inner tube 14, which determines the direction of the cold air. In addition, freezing of water from the room air in the exhaust pipe 10, the outer pipe 13 and the connecting pipe 11 is prevented. A narrowing of the cross-section is no longer possible. The blowing tool 7 can advantageously also be rinsed between the blowing processes via the blowing dome 15 in order to prevent condensation from precipitating or freezing out.

The amount of the advantageous dry air flow according to the invention is determined in such a way that there is sufficient protection against condensation on the inner walls of the blow mold. By varying the amount of cold air, the cooling time and the cold air temperature, it is possible to optimally adapt to the performance reserves of the reflow expander (not shown). By adapting the vapor pressure of the liquid nitrogen in the storage tank 1, it is possible to use the high-purity nitrogen for a wide variety of processes.

Claims

claims

1. A process for the cryogenic production of blow molded parts from plastic with the following features: a) air is passed as a blowing medium into a preform located in a blow mold; b) the preform which is deformable in the hot state is inflated with the air via a blow dome (15); and c) the inflated preform is cooled with dried, cooler air; characterized by the following features: d) the air introduced as a blowing medium is at least dried, possibly also cooled, before being introduced into the preform in order to inflate it; e) the dried air is passed as cryogenic air with a temperature between about -50 ° C and about -170 ° C in the inflated preform (8) to cool it; f) the blow mold and / or the blow dome (15; 11, 12, 13, 14) are rinsed or flushed with dry, preferably warm air while the blow mold (7) is open.

2) Method according to Anspmch 1, characterized in that the dried air is cooled by heat exchange with a liquid cryogen.

3) Method according to Anspmch 2, characterized in that nitrogen is used as the cryogen.

4. The method according to Anspmch 3, characterized in that the liquid nitrogen to temperatures between about

-170 ° C and -196 ° C, preferably about -180 ° C to about

-196 ° C, against which dry air is heated or evaporated.

5. The method according to any one of claims 3 or 4, characterized in that the nitrogen is then passed into a recuperator or precooler (4) for precooling the dried air intended for cooling the blow mold.

6. The method according to any one of claims 1 to 5, characterized in that both the blow dome (15; 11, 12, 13, 14) and the blow mold (7) via the blow dome (15; 11, 12, 13, 14) be flushed with dry warm air.

7. The method according to any one of claims 1 to 6, characterized in that the liquid cryogen used in generating the cryogenic, dry air is used after evaporation and heating or cooling of the molded part (8), for example. As a mixed gas or purge gas for other processes.

8. The method according to any one of claims 1 to 7, characterized in that the dry, cryogenic air after heating in the inflated preform (8) flows through this pressure-controlled, or is led out from this pressure-controlled.

9. The method according to any one of claims I to 8, characterized in that the cryogen evaporated to produce the dry, cryogenic air is adapted to the subsequent processes in its pressure for further use.

10. The method according to any one of claims 1 to 9, characterized in that the operating parameters are adjusted via the temperature of the dry, cryogenic air and the air flow to one of the blow mold (7) upstream melting extruder.

11. The method according to any one of claims 1 to 10, characterized in that the air is dried by means of at least one adsorber, preferably at least one bed adsorber.

12. The method according to any one of claims 1 to 11, characterized in that the dried air is pre-cooled in a recuperator or precooler (4) compared to a coolant, preferably a nitrogen coming from a low-temperature cooler (5).

13. The method according to any one of claims 1 to 12, characterized in that the dried, pre-cooled air in the low-temperature cooler (5) to its target temperature is cooled, the low-temperature cooler being an indirect heat exchanger.

14. The method according to Anspmch 13, characterized in that the evaporated nitrogen in the low-temperature cooler (5) is used in the precooler (4).

15. The method according to any one of claims 1 to 14, characterized in that the pressure-controlled (10, 11) escaping cooling air is used as a bladder cooling.

16. The method according to any one of claims 1 to 15, characterized in that the dried air is simultaneously provided as a flushing gas for the blow dome and the open blow mold via the outer tube (13) of the blow dome.

17. The method according to any one of claims 1 to 16, characterized in that compressed air is used as air for drying and / or as dry, cryogenic air.