CA2168242A1 - A method for the cryogenic production of blow moulded bodies made of plastic - Google Patents

Abstract

The invention relates to a method for the cryogenic production of blow molded bodies made of plastic, in which air is introduced as the blow medium into a parison located in a blow mold, and in which the parison, which is moldable when hot, isinflated with the air via a blow mandrel and is subsequently cooled by means of dried, cooler air; according to the invention, the air used as the blow medium is dried prior to being introduced into the parison, and the dried air is introduced into the inflated parison as deep-cold air with a temperature of between approximately -50°C and approximately -170°C in order to cool the parison; the blow mold and/or the blow mandrel are purged and Rushed through, respectively, with dry, preferably warm air while the blow mold is open.

Description

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Attorney's File: 51 903 X

Air Products GmbH
~itten.~traBe 50 45523 ~ttingen A Method for the Cryogenic Production of Blow Molded Bodies Made of Plastic The invention relates to a method for the cryogenic production of blow molded bodies made of plastic according to the preamble of claim 1.

Blow molded pieces or bodies made of plastic are used particularly in the packaging industry for packaging a wide variety of materials. Such blow molded bodies are produced by means of various techniques according to the prior art.

One method, for instance, known from DE 18 16 771 B2 is to inflate blow molded bodies using the rammed air process, and then simply cool them in the course of mold cooling. This classic method is still highly effective today for very thin-walled blow molded bodies. However, for thick-walled blow molded bodies it has the disadvantage of long cycle times. Also, moisture deposits in or on the blow mold can cause deformations of the blow molded bodies to be produced, impairment of the surface quality of the blow molded bodies (orange peel effect) frequently occurs, as well.

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A method is known from DE 21 60 854 C3 for cooling a hollow body of thermoplastic synthetic material produced by blow molding within a blow mold by quenching a cooling medium consisting of air and water and then injecting it at high pressure into a finished blow molded body, causing an explosion-like adiabatic ex-pansion of the cooling medium inside the hollow body and the formation of fine ice crystals which are deposited on the walls of the blow molded body and cool it. It is obvious that the use of water in the cooling medium results in an increased risk of ice crystal formation in the area of the blow mandrel and/or in the blow mold, causing the aforementioned disadvantages to likewise occur. In this method, too, the cycle times and the quality of the produced blow molded bodies are not convincing.

In ano~er method according to DE 24 42 254 B2, compressed air is temporarily added to a blow gas consisting of nitrogen or argon, the additional supply of com-pressed air being interrupted before the ~mal inflation pressure is reached. The afore-mentioned disadvantages occur in this method, as well. A method is also known from DE ~ 23 580 C3 in which nitrogen or argon is used to produce blow molded bodies, but without the addition of compressed air. Both methods propose external cooling.

Ln the apparatus known from DE 26 36 262 B2, inflation of a blow molded body is effected by introducing carbon dioxide into the molded part via feed pipes in order to inflate the blow molded body. Other feed pipes are used to introduce a cooling medium into cavities in the blow mold in order to cool and/or solidify the inflated blow molded body. The aforementioned problems of long cycle times and/or the risk of moisture deposit, and the disadvantages associated with these problems, occur in this case, as well.

The apparatus according to DE 33 37 651 C2 has a blow mold consisting of two re-latively small-mass blow mold halves which can be enclosed by two larger-mass heating mold halves in order to be able to rapidly cool the blow molded body in the blow mold as a result of the heating-mold-halves' large mass. In addition to theaforementioned disadvantages, this entails additional apparatus complexity, and, also, the cooling temperature cannot be set sufficiently low.

2 1 ~ 2 A method is also known from DE 28 17 472 C2 in which blow molded bodies are inflatefl and/or cooled from inside by means of a mixture of cold air and water, the aforementioned disadvantages arising in this method, as well.

A method of producing blow molded bodies is also known from DE 37 28 208 Al, in which the blow mold is first infl~te~l, following which a cooling medium is in-jected into the blow molded body, with water being proposed as the cooling medium.
However, the method of the sarne type which results from DE 37 28 208 Al has thesame disadvantages as the rest of the prior art.

A method of the same type for producing blow molded bodies made of plastic is known from US 4,091,059. Air is introduced into a parison located in a blow mold.
The parison and/or the blow molded body is infl~ted in hot, moldable condition by means of normal air using a blow mandrel, with the air used subsequently for cool-ing being cooled to no more and no less than -41 ~C.

A method of molding synthetic materials is known from US 3,937,609. In this method, dried air is used for purgin during the molding process in order to prevent moisture from depositing.

In summary, it must be stated of prior art that all known methods, and in particular the aforementioned interior cooling methods, are difficult to reproduce, technologi-cally vulnerable, especially due to ice formation, economically questionable, and have a number of other disadvantages in addition to this.

The mothods according to prior art using cool gases only operate at relatively high temperatures, for instance -41 ~C. The gases cannot be used very effectively for cool-ing at temperatures of this m~ le, since, in particular, the density of the gases at such temperatures results in their relative thermal capacity being low. In order to provide sufficient cooling, large quantities of cool gases must be used, which, essen-tially, is nevertheless only possible with very thin-walled parisons andl/or containers.
Such technologies are referred to as employing the so-called "mechanical coolingtechnique. "

21 ~8~42 In order to remedy the disadvantages of such mechanical cooling techniques as klnown, for instance, from US 4,091,059, technologies prefereably for thick-walled and large-mass parisons and/or containers were developed in which, for instance,water or other cooling media with a higher thermal capacity were used, however, these methods brought forth particular problems with freezing or condensing mois-ture, and cause other problems as well, including the aforementioned ones.

Even the classic rammed-air method with subsequent cooling of the blow molded body through the cooled die and/or blow mold has in any case its limits with respect to the cooling medium temperature, since otherwise the inevitable result is intolerable concle~ tion effects on the cold die or mold.

It is the object of the present invention to provide a cryogenic method for producing blow molded bodies made of plastic; in particular, the invention provides a method with an improved cycle time at reduced blow molded body cost, ~cfelably for the production of large-mass blow molded bodies.

This object is achieved by a method as de~med in claim 1.

Advantageous method variants are shown in the subclaims.

The advantages achieved with the invention are due to air which is dried prior to its being introduced into the parison being used as the blow medium. Furthermore, deep-cold, dry air is introduced into the infl~tetl parison and/or blow molded body in order to cool the infl~terl parison and/or blow molded body. In addition to this, the blow mandrel and/or the blow mold are purged and/or flushed through with dry, warm air while the blow mold is open. The deep-cold air has temperatures of be-tween approximately -50~C and approximately -170~C, preferably between -90~C
and approximately -170~C, which, according to the invention, has the advantage of the deep-cold and dry air being relatively dense, and thus having a high relative ther-mal capacity, so that thick-walled and/or larger-mass blow molded bodies can also be manufactured at a low cycle time in a reproducible manner.

2~
.~ 5 As opposed to the "mechanical cooling technique" employed elsewhere, the invention thus relates to a "cryogenic cooling technique". The cooler temperatures are all the more advantageous in that the gas and/or air becomes all the more dense at lowertemperatures, so that the relative thermal capacity per volume unit becomes larger.

The use of dry air prevents in any case moisture depositing, and deep-cold, dry air also has a higher cooling capacity in kJ/kg than normal compressed air, and is much less problematic in geometrically complex bodies than carbon dioxide, due to the fact that no snow formation occurs, or liquid nitrogen. Also, the purging of the blowmold and/or the blow mandrel when the blow mold is open prevents icing of the in-volved parts. The moisture deposits inside the blow molded bodies is also reliably excluded at every stage of the blow molding process through the use of dry air.

Ln addition to this, when the blow mandrel and/or the needle on the blow mandrelare purged, dry, and in particular warm, air is allowed to act on the interior of the blow mold at the same time, so that the moisture in the roorn air cannot condense on the cold interior contours of the blow mold either, which, on the one hand, is irn-portant for the quality of the molded bodies surface and, on the other, permits lower cooling medium temperatures for cooling the blow molded body and/or the blow mold.

Also, the use of increased pressure in interior cooling makes it possible to avoid shrinkage of the blow molded body, which can signi~lcantly improve the heat transfer from the mold to the molded body.

It is particularly advantageous that it is possible to use the liquid nitrogen to be em-ployed for cooling the dry air for other purposes, for instance as a mixed gas or purge gas, particularly in fluorination processes, where to this end, the pressure of gaseous nitrogen formed during cooling can be adapted to the subsequent processes.

In general, the use of air that is dried by means of drying devices, in particular ad-sorbers and the like, and that is cooled by means of indirect heat exchange with the liquid nitrogen avoids the necessity of blow mandrel cooling with water as customary 21 ~82~2 a~

in the rammed air method. The method according to the invention enables a surpris-ingly short cycle time that is dramatically shortened, namely by 40 % and more, and the method is exemplary with regard to reproducibility.

The fact that, according to an advantageous method variant, the liquid nitrogen em-ployed for generating the necessary low temperature of the dry air is first used as a refrigerating agent and then subsequently used technologically a second time as a warm dry inert gas, results in a sigIuficant improvement in cost efficiency.

According to the method variants of the present invention, the required shortening of the cycle time and/or increase in output of the blow molding m~rlline equipped for the method according to the invention can be achieved by defining the respectively necessary process parameters through control of the difference in the pressures up-st~eam and downstream of the blow mold, control of the temperature of the dry air and control of the time required for purging with dry air. The high improvement rate of the cycle time to be achieved according to the invention requires that a greater ni-trogren consumption in terms of kilograms of nitrogen per kg of plastic mass is ne-cessary, the optimum being a specific function of the blow molded body, blow mold and blow molding machine.

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

Air, as the blow medium, is introduced into a parison located in a blow mold, and the parison, which is moldable when hot, is inll~t~l by means of a blow mandrel.The infl~t~-l parison is cooled, and dried, cool air is introduced into the inflated pari-son to cool it. The air serving as the blow medium is dried prior to its being intro-duced into the parison to be infl~t~rl The dried air is pre-cooled in a recuperator or pre-cooler against deep-cold, gaseous nitrogen coming from a low-temperature cool-er, and then cooled down further to its desired temperature of approximately -50~C
to -170~C in the low-temperature cooler through indirect heat exchange with eva-porating, liquid nitrogen that has a temperature of approx. -180~C to approximately -196~C. Com~lessed air can also be used preferably. The blow mold and the blow 2t 6~42 .~ 7 mandrel are purged and flushed through, respectively, after the one blowing process, and prior to the subsequent blowing process, with dry, warm air while the blow mold is open, the purging of the blow mold also being effected by means of the blow mandrel. The purging of the blow mold by means of the blow mandrel makes it pos-sible to both keep the blow mold free of con-len~te and the blow mandrel free ofcon(le~t~ and ice.

The invention is described in the following in greater detail by means of a preferred method variant with reference to the accompanying figures. These show further ad-vantages and features according to the present invention.

Fig. 1 shows a block diagram of an apparatus designed for the method accord- ing to the invention; and Fig. 2 shows a partial section of detail X in Fig. 1 which in particular shows the design of the blow mandrel in a sectional representation.

Fig. 1 shows an apparatus which is suitable for carrying out the method according to the invention and/or a method variant of the present invention.

An air current, in particular compressed air, is introduced i~to a pressure-controlled drying device 3 and dried there. The drying device 3 can be equipped with two ad-sorbers, in particular bed adsorbers, with one of the two being used for drying,while the other bed adsorber is regenerated. The regeneration can be effected by re-leasing the pressure from the adsorber while at the same time letting dry air flow through it. The switchover intervals depend on the adsorption capacity of the adsorp-tion agent, and are defined in terms of the required time. In principle, any kind of drying device is suitable, however, t~n-lçm bed adsorbers 3 are preferred. Since the various designs of drying device are contained in prior art, there is no need for a more detailed description of the drying device 3.

The air and/or compressed air dried in the drying device 3 is channeled to a recu-perator or pre-cooler 4, where the dried air is pre-cooled. This pre-cooling process is effected through heat exchange with nitrogen coming from a low temperature cool-2 1 682~2 er 5, this nitrogen having been already slightly pre-heated. The pre-cooled and dried compressed air is then cooled down in the low-temperature cooler 5 to the predeter-mined desired temperature of approx. -50~C to approx. -170~C, preferably -90~C to -170~C, with the liquid nitrogen which is located in the low-temperature cooler 5 in indirect heat exchange with the compressed air being evaporated and pre-heated, and subsequently channeled to the recuperator 4 for pre-cooling of the compressed air.
The nitrogen can be heated up in the low-temperature cooler 5 to -180~C to -196~C, or even, if necessary, to a much higher temperature.

At one outlet of the low-temperature cooler 5, where the dried, deep-cold air and/or compressed air is emittefl, a solenoid control valve MV1 is opened or closed with the aid of a temperature measuring point TIC1 as a function of a comparison of desired and actual values. In this procedure, the temperature me~cllred at the temperature measuring point TIC1 and the predetermine-l desired temperature of the dried, deep-cool compressed air are lltili~e~ for controlling the solenoid control valve MV1 by means of an appropriate electronic control system.

The solenoid valve MV1 is connected to a storage tank 1 for liquid nitrogen via a vacuum-insulated pipeline. When the valve MV1 is opened, liquid nitrogen flows into 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 drawing off the heat from the warm or pre-cooled air and/or compressed air. The supply of nitrogen, and consequently the heat exchange, are controlled in such a manner that the desL~ed temperature of the dried compressed air can be m~int~ined.

The evaporated nitrogen passes via the pre-cooler 4 and through the flap trap RKl to a buffer unit 9 which can be connectetl to the works mains. The nitrogen can be stored here for further applications.

In order to make sure that nitrogen that is too cold is never let out of the cooling de-vice, additional monitoring is effected by means of a temperature measuring point TIC2 in the pipeline leading to the "buffer".

21 6~242 ~6, Once a parison 8 has been placed into a blow mold 7 of the blow molding machine,and after the blow mold 7 has been closed, dry air used for infl~ting the parison 8 is supplied by the dryer 3 via the solenoid valve MV2. Valve MV2 is closed again after a brief inflation period. Solenoid valves MV3 and MV4 are opened at the same time.
The deep-cold, dry air now flows from the low-temperature cooler 5 into the still hot parison 8 via the insulated pipeline 6, the solenoid valve MV3, the pipeline 12 and the interior pipe 14. The dry, deep-cold air flows through the parison 8, and re-emerges from the parison via the solenoid valve MV4 and the pressure m~int~iningvalve DMVl.

While ~owing through the blow molded piece 8, the dry, deep-cold air e~ctract~, the heat energy from the blow molded piece by he~ting up. The blow molded piece 8 iscooled down at the same time.

Due to the pressure m~int~ining valve DMVl, it is possible to m~int~in the required interior pressure which ensures that no shrinkage of th-e blow molded piece 8 takes place and that an optimum contact pressure against the blow mold 7 is implemen~
so that the exterior cooling can also be lltili7e~1 better. The air volume can be ad-ditionally determinçd by means of the preset differential pressure which can be in-fluenced by the pressure m~int~ining valve DMVl.

After a cooling period optimi7erl for the method according to the invention, thevalves MV3 and MV4 are closed again. The valve MV5 is opened at the same time in order to release pressure from the air in the infl~te-l blow molded piece 8. Follow-ing this, the blow mold 7 is opened in order to release the cooled blow molded piece 8. When the blow mold 7 is opened, valve MV5 is closed again, and valve MV6 is opened at the same time. Whilst the blow mold 7 is open, dry air flows through the mandrel and over the interior walls of the blow mold 7. The volume can be made back-pressure-dependent by means of the pressure control valve DMV2. The total geometry of detail X of the blow mandrel 15 with the parts 11, 12, 13 and 14 is de-signed in such a manner that there is a dual pipe system consisting of an interior pipe 14 for the supply of the cooling air and an exterior pipe 13 for implementing in-flation and/or purging. The purging of the exterior pipe 13 while the mold is open ~ 0 ~ 2 .

prevents freezing of water from the room air on the interior nozzle of the interior pipe 14 which determines the flow direction of the cold air. In additioin to this, freez-ing of water from the room air in the waste gas pipeline 10, in the exterior pipe 13 and in the connecting pipeline 11 is also prevented. Cross-sectional narrowing is thus no longer possible. The blow mold 7 can also be preferably purged together with the other components by means of the blow mandrel 15 between blow processes in orderto prevent depositing or surface freezing of con~lçn.~

The advantageous dry air flow according to the invention is defined with respect to its volume in such a manner that sufficient security against condensate formation on the interior walls of the blow mold is provided.

Variation of the cold air volume, of the cooling period and of the cold air tempera-ture permits 01JI;111U111 adaption to the capacity reserves of the fusion extruder (not shown). By adapting the vapor pressure of the liquid nitrogen in storage tank 1 it is possible to utilize the high-purity nitrogen for a wide variety of processes.

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for the cryogenic production of blow mold bodies of plastic, comprising the following features:
a) as a blow medium air is passed into a parison (8) located in a blow mold (7);
b) the parison (8) moldable in hot condition is blown up by means of a blow mandrel (15) with air;
c) the blown-up parison (8) is cooled with a dried, deep-cold air;
d) the dried, deep-cold air is cooled in heat exchange with liquid nitrogen; and e) the air to cool the blown-up parison (8) has a temperature between about -50°c and -170°C and is dried;
characterized by the following features;
f) the air passed in as blow medium is at least dried, optionally also cooled, before being passed into the parison (8) to blow this up;
g) the blow mold and/or the blow mandrel (15; 11, 12, 13, 14) are flushed or rinsed with dried and warm air while the blow mold (7) is open; and h) the nitrogen is subsequently fed to a recuperator or a pre-cooler (4) to pre-cool the dried air for the cooling of the blow mold (7).

2. A method according to claim l, characterized in that the liquid nitrogen is heated up against the dry air to temperatures of approx. -170°C and -196°C, preferably between approx. -180°C and approx. -196°C, and/or is evaporated as a result of this.

3. A method according to one of the claims l or 2, characterized in that both the blow mandrel (15; 11, 12, 13, 14) and the blow mold (7) are purged with dry, warm air via the blow mandrel (15; 11, 12, 13, 14).

4. A method according to one of the claims 1 to 3, characterized in that the liquid cryogen used for generating the deep-cold, dry air is subsequently used again after evaporating and heating up and/or after the mold body (8) has cooled, for instance as a mixed gas or purging gas, for other processes.

5. A method according to one of the claims 3 or 4, characterized in that the dry deep-cold air, after it has heated up in the inflated parison (8), flows through this parison under pressure control and/or is channelled out of this parison under pressure control.

6. A method according to one of the claims 1 to 5, characterized in that the cryogen that is evaporated in order to generate the dry, deep-cold air is adapted in its pressure to subsequent processes for further use.

7. A method according to one of the claims 1 to 6, characterized in that the operating parameters are adapted to a fusion extruder connected upstream to the blow mold (7) by using the temperature of the dry, deep-cold air and the air throughput.

8. A method according to one of the claims 1 to 7, characterized in that the air is dried by means of at least one adsorber, preferably by means of at least one bed adsorber.

9. A method according to one of the claims 1 to 8, characterized in that the dried air is pre-cooled in a recuperator or pre-cooler (4) against a cooling medium, preferably nitrogen coming from a low-temperature cooler (5).

10. A method according to one of the claims 1 to 9, characterized in that the dried, pre-cooled air is cooled down to its desired temperature in the low-temperature cooler (5), the low-temperature cooler being an indirect heat exchanger.

11. A method according to claim 10, characterized in that the nitrogen evaporated in the low-temperature cooler (5) is used in the pre-cooler (4).

12. A method according to one of the claims 1 to 11, characterized in that the cooling air escaping under pressure control (10,11) is used for cooling the blow mandrel.

13. A method according to one of the claims 1 to 12, characterized in that the dried air is supplied at the same time via the exterior pipe (13) of the blow mandrel as a purging gas for the blow mandrel and the open blow mold.

14. A method according to one of the claims 1 to 13, characterized in that compressed air is used as the air for drying and/or as dry deep-cold air.