Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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.
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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. "
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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.
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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
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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
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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-
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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".
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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
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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.
