CA2145208A1

CA2145208A1 - Process and device for producing blow-moulded plastic parts

Info

Publication number: CA2145208A1
Application number: CA002145208A
Authority: CA
Inventors: Olaf Babel; Hans Erler
Original assignee: Individual
Current assignee: Air Products and Chemicals Inc
Priority date: 1993-07-29
Filing date: 1994-07-28
Publication date: 1995-02-09
Also published as: WO1995003932A1; DE4325559C2; DE4325559C1; DE59405789D1; JPH08503431A; EP0664742B1; EP0664742A1

Abstract

The invention relates to a process for producing blow moulded plastic parts. The blowing medium used is an inert gas such as nitrogen, which is fed into a deformable preform in a blow mould in order to blow it up and subsequently cool it from within. According to the invention, the nitrogen introduced as a blowing medium consists substantially, and preferably exclusively, of warm, dry nitrogen which has previously been vaporized during the internal cooling of a blown-up preform, the blow mandrel and blow mould being flushed with warm, dry nitrogen when the latter is open. The internal cooling is carried out at a higher inlet pressure than that of the blowing-up processes.
The invention also relates to a device for implementing the process.

Description

211~208 Proces~ and Device for Producing Blow-Noulded Pla~tic Partq The invention relates to a process for producing blow-moulded plastic parts according to the preamble of claim 1 and to a device for producing blow-moulded plastic parts according to the preamble of claim 10.

Blow-moulded plastic parts are used for instance in the packaging industry for packaging materials of the most varied kinds, e.g. liquid soap. Such blow-moulded parts are produced by a variety of processes and devices.
.
For instance according to DE 18 16 771 B2 the plastics material of blow-moulded parts is blown in the form of hot preforms or parisons and then brought to crystallization simply by cooling the blow mould and thus by cooling the mould or form tool. In the production of extremely thinwalled blow-moulded parts this process which has become known as the "ram-air process" is still very effective even today. For thick-walled mouldings this process has the disadvantage of long cycle times. In addition there is a danger of room air moisture depositing on the interior contour of the blow mould when the blow mould is colder than room air so that in a subsequent blowing process the resulting blow-moulded part has a reduced surface quality (orange skin effect).

From DE 21 60 854 C3 a process is known for cooling within a blow mould a thermoplastic hollow body produced by the blowing process in which a cooling medium consisting of air and water is chilled and injected under high pressure into a finish-moulded blow-moulded part in a blow mould. In the blow-moulded part a sudden adiabatic expansion of the cooling 21~5208 medium takes place, causing fine crystals of ice to form which deposit on the wall of the blow-moulded part, cooling it. However, when water is employed as the coolant there is an added risk of ice crystals forming in the region of the blow mandrel or in the blow mould. It will readily be appreciated that the afore-mentioned disadvantages occur to a higher degree in the process or device according to this publication. Here too, the cycle times and the quality of the blow-moulded parts produced fail to be convincing.

From DE 22 23 580 C3 a process is known in which nitrogen or argon is used for producing the blow-moulded parts.

In a further process according to DE 24 42 254 B2 as ac-knowledged in the preambles of claims 1 and 10, compressed air is at times admixed with a blowing gas of nitrogen or argon, and before the final blow-up pressure is reached the additional supply of compressed air is discontinued. Cooling is done with liquid nitrogen.

Both of the latter processes propose an external cooling, i.e. cooling of the blow mould.

In the process and/or device according to DE 26 36 262 B2 the procedure is such that carbon dioxide is introduced by feeder conduits into the blow-moulded part to blow up the blow-moulded part. Via further feeder conduits a coolant is introduced into cavities in the blow mould to cause the plastics material of the blown-up blow-moulded part to crystallize and thus to cool and solidify. Long cycles times, the risk of moisture being deposited and the attendant disadvantages as already indicated above thus exist here too.

3 21~5208 From DE 28 17 472 C2 a method is known in which the blow-moulded parts are blown-up or cooled from within by means of a mixture of cold air and water, the problems as already given above again resulting.

The device according to DE 33 37 651 C2 comprises as the blow mould two blow mould halves of relatively low mass which may be enveloped by heating mould halves of greater mass. As soon as a plastic blow-moulded part located in the blow mould halves is blown-up, the heating mould halves are brought into place and placed externally on the blow mould halves to cool the blow mould halves and the moulded preform or parison located therein and to crystallize the material thereof. The apparatus expense in this system is relatively high, however the cooling temperature cannot be adjusted to a sufficiently low level.

From DE 37 28 208 A1 yet a further process for the production of plastic blow-moulded parts is known in which the preform is first blown-up before a coolant is injected into the blow-moulded part, water being proposed as the coolant. The process disclosed by DE 37 28 208 A1 has more or less the same drawbacks as those of the remaining prior art.

Prior art as described above and particularly the internal cooling systems involved fail to result in reproducible products. Apart from this, the processes are technologically weak due to icing, lack cost-effectiveness and are otherwise thwart with further drawbacks.

The classical ram-air process currently very much in use with subsequent cooling of the blow-moulded part through the cooled mould or the cooled blow mould respectively is also at 4 21~5 20 8 its limits in any case as regards the coolant temperature, since otherwise intolerable condensation effects occur on the mould when cold.

The object of the present invention is to define a process and a device which eliminates at least substantially the drawbacks of the prior art as discussed above; particularly, a process and a device are to be defined permitting shorter cycle times and an improved and reliably reproducible quality of the blow-moulded parts produced in faster cycling.

This object is achieved by a process having the features given in claim 1 and by a device having the features given in claim 10.

Advantageous variants of the process and embodiments thereof are disclosed by the subclaims.

By employing liquid nitrogen for internal cooling of the mouldings, by making multiple use of the evaporated nitrogen for blowing up subsequent blow-moulded parts and/or preform (parison) and flushing the blow mandrel and/or the blow mould with warm, dry nitrogen resulting from the liquid dry nitrogen used for cooling, very short cycle times are achievable which, as compared to the cycle times of known systems, are reduced by half or even more.

By means of the process according to the invention condensa-tion of room air moisture on the cold mould (orange skin effect) can be reliably prevented as compared to the classi-cal ram-air process with the exclusive cooling of the moulding by the cooled form tool or the cooled blow mould. In addition, the limits as to the coolant temperature imposed by the known classical ram-air process do not apply to the process according to the invention.

By employing gaseous dry and warm nitrogen, obtained from the originally dry liquified nitrogen, the danger of icing during blowing up of the preforms is avoided as compared to prior art, thus ensuring a reliable protection from icing during blowing up.

To protect the blow mandrel which may be cooled down to -160C from icing also when the blow mould is open (in-troducing the parison or preform, removing the moulding) the blow mandrel is flushed according to the invention on all sides by the dry, warm, gaseous nitrogen which acts as a buffer gas against room air. Thus a formation of ice on the blow mandrel is also reliably prevented in this phase of the blowing process.

Advantageously the dry liquid nitrogen which evaporates during internal cooling in the blow-moulded part is dis-charged pressure-regulated from the blow-moulded part, re-heated, held in an intermediate storage means, and then made use of for blowing up and/or also - following pressure reduction - for flushing out the blow mandrel and the blow mould and thus also utilized as a secondary gaseous compo-nent. This system not only boosts efficiency and reduces the cycle time respectively, it also further adds to cost--effectiveness, since the evaporated, formerly liquid, dry nitrogen, may be utilized multiple times.

A major advantage is also to be had from being able to reheat the liquid nitrogen evaporated during internal cooling by means of a heat exchanger, particularly by a recirculation 6 214520~

air heat exchanger so as, on the one hand, to provide a cooling capacity for other purposes and, on the other, to control the pressure and to ensure that a blowing gas or flushing gas is available having optimized parameters.
Advantageously a cooling-water-operated plate-type or shell-and-tube-type heat exchanger may also be employed as the heat exchanger.

If, before it reaches the mouldings for the purpose of cooling them from within, the liquid nitrogen is passed through a phase separator, it may be assured to advantage that exclusively liquid nitrogen enters the preforms for internal cooling, thus ensuring improved reproducibility of the process parameters. In this arrangement the gaseous nitrogen separated out in the phase separator may also be made use of in a secondary step for blowing or flushing, especially when, as afore-mentioned, it has been reheated.
The storage may then be done in an intermediate storage means or in the intermediate storage means for the evaporated, formerly liquid, dry nitrogen.

If the difference in pressure between that of the internal cooling and that of blowing up is adjusted to approx. 0.5 to 10 bar, shrinkage of the moulding during cooling or solidifi-cation of the moulding material may be effectively counter-acted and the efficiency of the cooling of the moulding may be enhanced by cooling the form tool and/or the blow mould.
Due to a high internal pressure the cooling of the moulding part is supported by the mould. Improved efficiency of the moulding-part cooling by the mould paired with a high-calory internal cooling according to the invention results in a combination of advantages as to quality and quantity from 21452~8 evenly cooled mouldings produced in a substantially shorter cycle time.

As already indicated, by flushing the blow mandrel(s) and/or needles out and/or on all sides as well as, where necessary, flushing out the opened blow mould with dry, warm, gaseous nitrogen, condensation of moisture from room air on the corresponding parts and particularly also on the cold internal contour of the opened blow mould is prevented so that the surface quality of the produced blow-moulded parts and the cycle time may be improved at least on an average.
Furthermore, it is also possible due to the resulting condensation barrier to employ lower cooling temperatures in cooling the mould.

In addition, due to internal cooling of the blown-up preform with liquid, dry nitrogen there is no need for water-cooling the blow mandrel in the majority of applications as is usual in the ram-air process.

By advantageously setting the operating parameters such as nitrogen pressure, flushing time and thus the gas throughput, chiller capacity etc., the process according to the invention and/or the device according to the invention may be adapted at least over a broad range to the parameters of the melt extruder of the extrusion blowing machine.

By its employment of an intermediate storage means for the evaporated, formerly liquid, dry nitrogen, the device according to the invention provides the advantages as already discussed, at least in part, for the further use thereof as blow and/or flushing gas.

8 21~2~8 The invention will now be discussed in more detail in the following description with reference to the attached FIGS, disclosing further advantages or features according to the present invention, in which:
IG. 1 is a single-line drawing showing the device accord-ing to the invention, said device being suitable for implementing the process according to the invention;
and IG. 2 is a section view of a blow mandrel and an upper section of a blow mould indicated as detail X in FIG. 1.

In FIG. 1 a device configured according to the invention is shown having a tank 1 for liquid nitrogen (LIN) which is connected via conduits, preferably a vacuum-insulated conduit

2 to a phase separator 3. In the phase separator 3 the liquid, dry nitrogen is separated from any gaseous components existing. These gaseous components may be discharged via the valve RV from the phase separator 3. The phase separator 3 is located to advantage on the blow device 5, 6, 7, preferably at the highest point of the conduit in the direction of the blow mandrel 5 (11, 12, 13, 14 in FIG. 2).

The liquid nitrogen from the phase separator 3 is introduced via a preferably short and well insulated conduit 4 and a controllable metering valve MV2 into the blow mandrel 5 and then into the preform (parison) which is already partly or fully blown-up to cool the latter from within and to cause crystallization of the plastics material, of which the moulded part 7 consists. The preform itself is introduced in ~, 9 21q~208 -hot condition into the blow mould 6 after the preform has been extruded from a melt extruder and is placed on the blow mandrel 5. The blow mould 6 is comprised of at least two parts or halves which are brought together to form a closed mould as soon as the hot preform is disposed on the blow mandrel within the space which is surrounded by the compo-nents of the blow mould 6.

The dry, warm nitrogen for blowing up the preform gains access to the preform via the blow mandrel 5 or the feeder conduit 11 (see FIG. 2) in the outer conduit 13 for blowing up the preform until it is in contact with the inner walls of the blow mould 6. The blow pressure is controlled by valve means MV1 and DMV3 respectively, a measuring point being possibly located in the conduit and/or in the blow mandrel 5 itself for precisely measuring or controlling the blow pressure.

The blow mandrel 5 is connected via a conduit (indicated corrugated) to an intermediate storage means 8 into which the nitrogen formerly dry and liquid, now evaporated by the internal cooling of a blown-up preform 7 is introduced and intermediately stored. Upstream of the intermediate storage means 8 a heat exchanger 9 is provided which, as long as the blow mould 6 is open, adjusts the gaseous nitrogen, which may still be cool if necessary, to a desired temperature for the blow or flushing gas required for the blowing and/or flushing procedure.

The intermediate storage means 8 is connected to both the plant network and to the blow mandrel 5 (11, 12, 13, 14 respectively). Should not as much dry, warm nitrogen be required for the flushing procedure and/or the blowing ~, 21~208 procedure as previously evaporated as liquid, dry nitrogen in cooling the moulded parts, nitrogen may be fed from the intermediate storage means 8 into the plant network for further use. Otherwise the nitrogen is directed from the intermediate storage means 8 into the blow mandrel for use either for blowing or for flushing. Gaseous, dry nitrogen may also be added from the plant network.

The nitrogen, formerly dry and liquid, now evaporated and heated by the internal cooling of the blown-up preform, gains access to the intermediate storage means 8 via a heat exchanger or a recirculation air evaporator 9. From the intermediate storage means 8 the blowing gas having prefera-bly a pressure of 8 bar is tapped to blow up the preform in the closed blow mould 6 sufficiently so that it comes into contact with the inner walls of the blow mould 6.

Whilst the blow mould 6 is open, a flow of gaseous nitrogen having a pressure of approx. 100 mm WC above atmosphere passes through the blow mandrel via an additional pressure reduction provided by DMV2 and MV4, the opened blow mould 6 also being able to be blown through at the same time. This prevents the condensation of room air.

In the production of a moulded part 7 dry, warm nitrogen having resulted from the internal cooling of the blow-moulded parts 7 produced before is tapped from the intermediate storage means 8 and introduced via the valve MV1 and the blow mandrel 5 into a hot preform located in the closed blow mould 6. Thus the preform is blown-up with controlled pressure until a final pressure is reached or the walls of the blown-up preform are in contact with the inner walls of the blow mould 6.

214520~

-Subsequently, liquid nitrogen is introduced via the valve MV2 and the blow mandrel 5 into the blown-up preform so that it is cooled and the wall material of the blown-up preform crystallizes, i.e. hardens. The formerly liquid, dry nitrogen now evaporated is thereby introduced via the heat exchanger 9 into the intermediate storage means 8 where it is stored for subsequent blowing and/or flushing operations.

Once the walls of the now finished blow-moulded part 7 have solidified, the blow mould 6 is opened and the blow-moulded part 7 removed from the blow mandrel 5. Whilst the blow mould 6 is open, dry, warm nitrogen tapped from the intermediate storage means B is blown out via the blow mandrel 5. This flow of nitrogen is also directed at the internal walls of the blow mould 6 and prevents icing and/or room moisture condensating on the blow mandrel 5 and on the inner surfaces of the open blow mould 6.

In FIG. 2 the blow mandrel 5 as shown in FIG. 1 is depicted.
This features a supply conduit 12 and the internal conduit 14 for the liquid nitrogen respectively, the supply flow of liquid, dry nitrogen being controlled via the valve MV2. Dry, gaseous, warm nitrogen for blowing up and for flushing may be supplied via the conduit 11 and 13. The gaseous nitrogen produced by the internal cooling and resulting from the formerly dry, liquid nitrogen from conduit 12 and 14 may be tapped from the blow-moulded part 7 via conduit 10 and 11 and introduced into the intermediate storage means 8 (see FIG.
1) ~.

Before the blow mould is opened for the purpose of removing the moulded part, the moulded part 7 is pressure-relieved by the valve MV3 being opened.

Claims

P a t e n t C l a i m s

1. A process for producing blow-moulded plastic parts having the following features:

a) as the blow medium, nitrogen is introduced into a preform or parison located in a blow mould;
b) the preform mouldable in its hot state is blown-up via a blow mandrel; and c) the blown-up preform is cooled from within by liquid nitrogen;
characterized by the following features:
d) the nitrogen introduced as the blow medium consists sub-stantially, preferably exclusively, of a dry and warm nitrogen which was evaporated beforehand during the internal cooling of a blown-up preform (7);
e) the blow mandrel (5; 11, 12, 13, 14) and/or the blow mould (6) are flushed through and/or surroundingly by the warm and dry nitrogen whilst the blow mould (6) is open; and f) during internal cooling of a blown-up preform (7) the inlet pressure is set higher than during blowing up of a hot preform.

2. A process according to claim 1, characterized in that the dry liquid nitrogen which evaporates during internal cooling in said moulded part (7) is discharged pressure-regulated from said moulded part (7).

'

3. A process according to one of the claims 1 or 2, characterized in that for blowing up the preform the nitrogen is reheated, held in an intermediate container (8) and subsequently used for blowing up or, pressure-reduced, for flushing through and/or around said blow mandrel (X; 11, 12,13,14) and/or said blow mould (6).

4. A process according to claim 3, characterized in that the liquid nitrogen which is evaporated during the internal cooling is reheated by means of a heat exchanger, particular-ly by means of a recirculation air heat exchanger.

5. A process according to one of the claims 1 to 3, characterized in that the liquid nitrogen which is evaporated during the internal cooling is reheated by means of a cooling-water-operated plate-type or shell-and-tube-type heat exchanger.

6. A process according to one of the claims 1 to 5, characterized in that in a phase separator (3) gaseous nitrogen is separated from the liquid nitrogen which is used for the internal cooling of said moulded part (7).

7. A process according to claim 6, characterized in that the gaseous nitrogen separated out by said phase separator (3) is reheated and intermediately stored (8).

8. A process according to one of the claims 1 to 7, characterized in that the pressure difference between the inlet pressures for the internal cooling of the moulded part (7) and for blowing up the preform amounts to approx. 0.5 to 10 bar.

9. A process according to one of the claims 1 to 8, characterized in that the process parameters such as e.g.
nitrogen pressure, flushing time, gas throughput or cooling capacity, etc., are adapted to an upstream-located melt extruder for the plastics material of a respective moulded part (7).

10. A device for producing blow-moulded plastic parts having the following features:
a) an storage means for a coolant and/or a means for generating a coolant;
b) a blow mould;
c) a blow mandrel having a connection to said storage means and/or said means for generating a coolant;
d) a conduit for removing the evaporated coolant from a blown-up preform located in said blow mould; and e) a conduit which is connected to said blow mandrel for blowing up a hot preform by means of a blowing gas;
characterized by the following features:
f) connected to said conduit for removing the evaporated gas (10) is an intermediate storage means (8) for the intermediate storage of the gas evaporated during the internal cooling; and g) the intermediate storage means (8) is connected to said conduit (11) for supplying said gas and/or said blowing gas.

11. A device according to claim 10, characterized in that upstream and/or downstream of a said intermediate storage means (8) a heat exchanger (9), particularly a recirculation air evaporator or a cooling-water-operated plate-type or shelland-tube type heat exchanger, is disposed for reheating the evaporated nitrogen.

12. A device according to one of the claims 11 or 12, characterized in that between said storage means (1) for the coolant and/or said means for generating the coolant and said blow mandrel (5) a phase separator (3) is disposed.

13. A device according to claim 12, characterized in that said phase separator is connected to said intermediate storage means (8) and/or said heat exchanger (9) by a conduit.

14. A device according to one of the claims 10 to 13, characterized in that control means and/or valves are provided for setting the inlet pressure of the coolant higher than the pressure of the blow medium.