Blow moulding method and apparatus

Abstract: An article is formed in a mould by blowing a parison hot plastics material with a blowing gas. A cooling fluid is introduced through either blow pin 26 or blow pin 28 into the article to cool the article and thus allow its removal from the mould. The cooling fluid is formed by mixing liquid nitrogen and relatively warm nitrogen gas in a jet pump 22. The nitrogen gas is supplied from a storage vessel 24, having been recovered from a previous cooling step. The cooling fluid flows through the article to the other of the pins 26 and 28, is withdrawn from such pin and flows to the vessel 24 for use in cooling a subsequently formed article. Valves may be provided to switch the roles of the pins 26 and 28 and thereby reverse the direction of flow in the article being cooled.

Description: The present invention relates to a blow moulding method and apparatus in which a parison composed of a hot plastics material is inflated within a mould with a pressurised blowing gas to form an article and thereafter the article is cooled by introducing a cryogenic cooling fluid into the article. More particularly, the present invention relates to such a method and apparatus in which the cryogenic cooling fluid and optionally the blowing gas is formed by combining liquid and gaseous forms of a cryogen and recovering the warmed mixture from the article in order to supply the gaseous form of the cryogen. In the production of blow-moulded articles by a variety of techniques (notably extrusion blow moulding where a hot parison formed of a plastic material is blow within a mould), a significant portion of the time required to mould the article is spent in cooling the article. Since the total time required to mould each article will affect production capacity, a reduction in the time spent in cooling the article will increase production capacity. The prior art has thus supplied various apparatus and methodology involved using a cryogenic gas or liquid as a cryogenic cooling fluid for cooling the article. For example, US-A-3 505 422 provides a method and apparatus for cooling a blow moulded article in which a cooling fluid, such as carbon dioxide at as low a temperature as -32°C (-25°F) is injected into the article, once moulded. After removal of the article from the mould, the carbon dioxide is allowed to escape from the article. In order to prevent such wastage of the cooling fluid, US-A-3 065 501 provides a method and apparatus in which the blow moulded article is cooled with liquid carbon dioxide, and the resultant carbon dioxide vapour produced from the vaporisation of the liquid carbon dioxide is reclaimed and reliquefied for use in subsequently cooling the article within the mould. US-A-4 367 187 provides yet another method and apparatus for cooling a blow moulded article. In this patent, after the article is partially blown, liquid nitrogen is injected into the article. This causes the liquid nitrogen to expand into a gas to complete the blowing of the article. After the article is blown, the nitrogen gas is reclaimed and stored for use in partially blowing an article to be subsequently formed within the mould. The disadvantage of these last two mentioned patents relates to their use of a liquid cryogen as a cooling fluid. When liquid cryogen is used in this manner, the cooling of the article may not be uniform throughout the article. Methods and apparatus for utilising liquid carbon dioxide are disclosed in US-A-3 520 020, US-A-3 450 805 and US-A-3 666 849. In accordance with these last mentioned patents, liquid carbon dioxide is mixed with air to form a gaseous cooling fluid that is used in cooling the blow moulded article. The advantage of such a gaseous cooling fluid is that the blow moulded article can be more uniformly cooled in the mould as compared with the use of the pure liquid cryogen. Additionally, since the mixture of air and vaporised liquid cryogen is being injected into the moulded article, none of the cooling potential of the liquid is lost, such as would be the case if the liquid cryogen were vaporised with an exterior heat source. The drawback of the process and methodology is that air contains moisture, and as such, expensive air driers typically must be used to dry the air sufficiently so that ice formation and build up is prevented. As will be discussed, the present invention provides a method and apparatus in which the blow moulded articles are cooled with a cryogenic cooling fluid in the form of a gas or a cryogenic mist carried within a cryogenic gas in order to produce more uniform cooling of the article. As will be further discussed, the present invention has the desirable feature of utilising the full cooling potential of the liquid cryogen. However, since air is not used to vaporise a liquid cryogen, cooling temperatures can be achieved that are as low as the liquid cryogen without requiring the use of air driers. According to the present invention there is provided a method of making blow moulded plastics articles comprising forming a plastics article by introducing a pressurised blowing gas into a parison of hot plastics material located within a mould until the parison is formed to the shape of the mould, whereby the parson is cooled and the cooling fluid is warmed, and then introducing cooling fluid into the shaped parson, characterised in that the cooling fluid is formed by mixing pressurised liquid cryogen with warmed gaseous cryogen recovered from a previously inflated plastics parison during the cooling thereof, whereby the liquid cryogen at least partially vaporises as a result of said mixing, the composition of said gaseous cryogen being essentially the same as that of the liquid cryogen. The invention also provides a blow moulding apparatus comprising a mould, blowing means for blowing a parison of hot plastics material located within the mould with a pressurised blowing gas until the parison fits the mould and thereby forms an article, and at least one inlet for introducing a cooling fluid into the article so that the article cools and the cooling fluid warms, characterised in that the apparatus additionally includes means for mixing pressurised liquid cryogen with warmed gaseous cryogen recovered from a previously cooled article, and means for conducting the resulting mixture, including at least partially vaporised liquid cryogen, to the inlet. Thus the present invention advantageously utilises essentially a gaseous cooling fluid to promote uniform cooling of the article. Since no external vaporiser is used, the full cooling potential of the liquid cryogen is expended in cooling the article. Lastly, since air is not used in vaporising the liquid cryogen, expensive air driers do not have to be supplied. Moreover, since air driers have limitations in the amount of moisture that they can remove from air, very low cryogen cooling temperatures cannot be reached when air is used in vaporising a liquid cryogen. The invention will now be described by way of example with reference to the accompanying drawing which is a schematic diagram of a blow moulding apparatus 10 in accordance with the present invention. All valves illustrated in the drawing are normally closed except for an exhaust valve 35 which for safety reasons is normally open. Blow moulding apparatus 10 includes a mould 12 having two mould halves 14 and 16. Blow moulding apparatus 10 is of a kind in which an extruded parson formed from a hot plastics material descends between the halves 14 and 16 of the mould 12. The article is formed within the mould 12 during a blowing stage by inflating the parison with a pressurised blowing gas until the plastic material fits the contours of mould 12. During a subsequent cooling stage, the article is cooled to solidify the plastics material and to allow separation of mould halves 14 and 16 and removal of the moulded article. In such cooling stage a cooling fluid is introduced into the moulded article. It should be mentioned that cooling is also accomplished through heat transfer from the moulded article to the cooler mould metal. Since the apparatus for extruding the parison, the apparatus for forming the article within mould 12, and the associated apparatus for opening and closing mould halves 14 and 16 are each well known in the art, they are not illustrated herein. The cooling fluid is cryogenic and is formed by adding warmed gas (which as will be described is formed of warmed cooling fluid recovered from the article) to pressurised liquid cryogen. In apparatus 10, liquid nitrogen from a pressurised liquid nitrogen supply 18 is introduced into a high pressure inlet nozzle 19 of a jet pump 22 by opening a valve 20 and gaseous cryogen stored in storage tank 24 is introduced into low pressure inlet 25 of jet pump 22 by opening a valve 50 which will also be discussed hereinafter. It is understood that other possible substances could be used as the pressured liquid form of the cryogen, for instance, carbon dioxide. The jet pump 22 accomplishes two functions. First, it combines the high pressure liquid cryogen entering through nozzle 19 with the lower pressure gaseous cryogen entering through inlet 25 from storage tank 24 and produces a mixture that exits through a diffuser 23 at a pressure greater than the pressure within storage tank 24. This pressure building is accomplished through the well known venturi effect produced within a jet pump, and provides the necessary pressure differential required to effect eventual recovery of a portion of the mixture to replenish storage tank 24. Second, jet pump 22 provides efficient and uniform thermal mixing of the liquid and gaseous forms of the cryogen. Depending upon the rate at which the gaseous cryogen is added to the liquid cryogen, the liquid cryogen totally vaporises; or partially vaporises into a fine mist of the liquid cryogen which is carried along within a stream of the gaseous cryogen. The cooling fluid is introduced into the article by a blow pin or needle 26. It may be appreciated, depending upon the type of part, there may be many blow pins, namely anywhere in the article where a hole is allowable. As is known in the art, blow pin 26 is designed to penetrate the parison upon closure of mould 12. Thus, once the article is blown, blow pin 26 will continue to extend into the article. As a result of the cooling fluid contacting the hot article, the cooling fluid heats up while the hot article cools. Additionally, when a fine mist of liquid cryogen is present within the cooling fluid, such liquid cryogen is vaporised. The cooling gas circulates through the article by being discharged from a blow pin 28. In this regard, blow pins 26 and 28 are preferably spaced apart from one another in order to ensure circulation of the cooling fluid within the article. The present invention could also function with the use of a single blow pin. For instance, a single blow pin could comprise a central opening through which the cooling fluid enters the article, and an annular space surrounding the central opening through which the warmed cooling fluid vents from the article. Some of the cooling fluid, after circulation through the article, is recovered in storage tank 24 for recycling back to jet pump 22 during a subsequent cooling cycle. A storage tank feed line 30 is provided with a pressure relief valve 32 that is set at the desired pressure of storage tank 24. When storage tank 24 contains a sufficient supply of the gaseous cryogen, pressure relief valve 32 will open to ensure that the cooling fluid will continue to circulate through the mould. As will be discussed, storage tank feed line 30 is also provided with a check valve 33 and a delay valve 34. Once the article has been fully cooled, it is under pressure due to the pressurised gaseous cryogen that has served as the cooling fluid, or as will also be discussed, possibly the blowing gas. In order to allow mould 12 to open, these gases within the article are allowed to purge by opening the exhaust valve 35. As may be appreciated, as cooling fluid flows from blow pin 26 to blow pin 28, it increases in temperature. Therefore, there exists some degree of non-uniformity in the cooling of an article within mould 12. In order to promote uniform cooling, which can be particularly important for a large plastics part, a change-over circuit 36 is provided. Change-over circuit 36 has two valves 38 and 40 in parallel connected to the outlet of jet pump 22, and directly to blow pins 26 and 28. In addition, two valves 42 and 44 in parallel are connected to a storage tank feed line 30. When valves 38 and 44 are opened, and valves 40 and 42 are closed, cooling fluid flows from jet pump 22 into blow pin 26 and out of blow pin 28 to storage tank feed line 30, and finally into storage tank 24. Valves 38 and 44 can be closed and then valves 40 and 42 can be opened. At such time, the flow reverses direction and now cooling fluid from jet pump 22 flows into blow pin 28, circulates through the article within mould 12, and is discharged through blow pin 26 into storage tank feed line 30. The succession of opening and closing the valves in the manner described above will cause the circulation of cooling fluid within the article contained within mould 12 to successively reverse itself, and promote the uniform cooling of the article contained within mould 12. As may be appreciated, it is possible to have a succession of blow pins and change-over circuits to allow multidirectional circulation of the cooling fluid within the mould. The article to be moulded within mould 12 can either be blown with a blowing gas that comprises air or is formed in the same manner as the cooling fluid, namely, by mixing liquid cryogen from pressurised liquid cryogen supply 18 and gaseous cryogen stored within storage tank 24. If air is the blowing gas, an air valve 46 is provided, which during the cooling cycle described above, is closed. When the article is to be blown with air, air valve 46 is set in an open position. After the article is blown, delay valve 34 is maintained in the closed position for a preset period of time following the start of the cooling stage. Air will then vent from pressure relief valve 32 to purge the system. In fact delay valve 34 would normally remain closed at the start of the cooling stage because the gaseous cryogen will be at an elevated temperature. After a suitable time delay to allow the temperature of the recovered gaseous cryogen to fall to a more desirable level, delay valve 34 opens to allow the filling of storage tank 24 with warmed cooling fluid after its circulation within the article contained within mould 12. As mentioned previously, the blowing gas can be formed in the same manner as the cooling fluid. In such an operation, at the start of the blowing stage valve 20 is opened to introduce pressurised liquid cryogen into jet pump 22 and to draw previously recovered gaseous cryogen from supply tank 24, after opening of valve 50, into a mixture so that the liquid cryogen is either partially or fully vaporised, depending upon whether there are any constraints on the temperature of the blowing gas. Valve 38 would be set in the open position to allow the thus formed blowing gas to enter the parison through blow pin 26. Valve 35 would be set in the closed position. Valve 40 could also be opened so as to allow the article to be blown from plural points. Valve 34, after a delay to allow the gas to be recovered to undergo a reduction in temperature, would open to allow some of the formed blowing gas to be recovered in supply tank 24. At the conclusion of the blowing stage, the valves would be reset as outlined above for the cooling stage. Blowing gas could also be formed by simply supplying recovered gaseous cryogen from storage tank 24 by opening valve 50. In such case, valve 20 would be closed and the blowing gas would be supplied as outlined above. As may be appreciated, the blowing and cooling processes outlined above rely on a pressure differential existing between the discharge of jet pump 22 and storage tank 24 (whose pressure is nominally maintained at the setting of pressure relief valve 32). Jet pump 22 cannot provide an arbitrarily large pressure boost to the gaseous cryogen stored in tank 24. In order to provide additional pressure boost, the gas pressure within storage tank 24 can be boosted by evaporation of liquid cryogen from supply 18 (through heat transfer with the recovered gaseous cryogen). This direct pressurisation of storage tank 24 would occur during the time between the end of the previous blowing stage and the start of the next blowing stage (or, if the blowing gas is provided by the cryogen, the start of the next cooling stage). This would preferably be accomplished by setting valves 34, 38, and 40 in the closed position, and setting valves 20, 48, and 50 in the open position. In this manner, gaseous cryogen from storage tank 24 would vaporise liquid cryogen from liquid cryogen supply 18, and the mixture would return to storage tank 24 through valve 48. It may be appreciated that when this direct pressurisation scheme is employed, the pressure within tank 24 may be above the setting of pressure relief valve 32 during a portion of the blowing and cooling stages. To prevent direct loss of the stored gaseous cryogen in storage tank through pressure relief valve 32, check valve 33 is placed within storage feed line 30. If several blow moulding stations are to be supplied, it is possible that instead of a storage tank 24, a manifold might be used that is connected to multiple blow moulding apparatus. Hence, one blow moulding apparatus might produce cooling fluid or blowing gas using gaseous cryogen being recovered from another blow moulding apparatus. The operation of valves can be controlled by sequential timers well known in the art. Alternatively, the operation of the valves can be controlled by a programmable logic computer. In either type of control, the logic would be the same. Since, the computer or timers are so well known in the art as would be their connections to the operators of the valves, the same are omitted from the drawing for the sake of simplicity of illustration. The signal used by most if not all blow moulding machines to open air valve 46 can be used to initiate the timers or the programmable logic computer. As mentioned above, all valves mentioned herein (except exhaust valve 35) are of the normally closed type. They are preferably solenoid actuated and have a two-way action. An example of the operation mentioned above in which air is being used as the blowing gas would function as follows. Valve 46 would be commanded to be open for a time interval preset as the blowing time for the article and exhaust valve 35 would be commanded to close. At the same time, independent delays would be initiated for valves 20, 50, and 34. At the conclusion of these independent delays, these aforementioned valves would be triggered to open for preset time intervals that would conclude so that all these valves shut at the same time. Thus, valves 20 and 50 would be set open to form the cooling fluid and then delay valve 34 would be set open so that the gaseous form of the cryogen could be recovered. The closing of these valves would in turn trigger opening of exhaust valve 35 after a time delay. The opening of valve 20 would also initiate a recycle timer to trigger valves 38 and 42 as a pair and valves 40 and 44 as another pair. In each pair, when valve 38 were open, valve 42 would be closed and vice versa and when valve 40 were open, valve 44 would be closed and vice versa. This would allow the cooling fluid to be alternately introduced into the mould from the two spaced location or blow pins 26 and 28. If the blowing gas is to be formed in the same manner as the cooling fluid, the preset time interval for air valve 46 would be set at "0.0" seconds. Valves 20 and 50 would be triggered to be set in the open position and delay valve 34 would be set to open after a delay. Valves 20, 50, and valve 34 would be set to remain open so that the article was blown and then cooled by the cooling gas being formed and would then all close at the same time. Exhaust valve 35 would then open after a time delay to allow the removal of the article from the mould 12.

Claims: A method of making blow moulded plastics articles comprising forming a plastics article by introducing a pressurised blowing gas into a parison of hot plastics material located within a mould until the parison is formed to the shape of the mould, whereby the parison is cooled and the cooling fluid is warmed, and then introducing cooling fluid into the shaped parison, characterised in that the cooling fluid is formed by mixing pressurised liquid cryogen with warmed gaseous cryogen recovered from a previously inflated plastics parison during the cooling thereof, whereby the liquid cryogen at least partially vaporises as a result of said mixing, the composition of said gaseous cryogen being essentially the same as that of the liquid cryogen. A method according to claim 1, wherein during blowing, the cooling fluid is also introduced into the parison to function as the blowing gas. A method according to claim 1, wherein gas recovered from a previously made article is used as the blowing gas. A method according to any one of the preceding claims, wherein the cryogenic cooling fluid is alternately introduced into and discharged from each of at least two spaced apart locations of the article so that the cooling fluid circulates within the article first in one end and then in the opposite direction to promote even cooling of the article. A method according to any one of the preceding claims, wherein the recovered gas is mixed with the pressurised liquid cryogen in a jet pump that draws the gas into the liquid cryogen. A method according to any one of the preceding claims, wherein the recovered gas is introduced into a storage tank, and is supplied from the storage tank to be mixed with the pressurised liquid cryogen. A blow moulding apparatus comprising a mould (12), blowing means (26) for blowing a parison of hot plastics material located within the mould (12) with a pressurised blowing gas until the parison fits the mould (12) and thereby forms an article, and at least one inlet (26) for introducing a cooling fluid into the article so that the article cools and the cooling fluid warms, characterised in that the apparatus additionally includes means (22) for mixing pressurised liquid cryogen with warmed gaseous cryogen recovered from a previously cooled article, and means for conducting the resulting mixture, including at least partially vaporised liquid cryogen, to the inlet (26). Apparatus according to claim 7, wherein the mixing means (22) comprises a jet pump (22) having a high pressure inlet (19) for receiving the pressurised liquid cryogen; a low pressure inlet (25) in communication with recovered gas supply means; and a diffuser (23) communicating on its outlet side with said cooling fluid inlet (26) and on its inlet side with said low pressure and high pressure inlet (25 and 19, respectively). Apparatus according to claim 7 or claim 8, including first and second blow pins (26 and 28 respectively) spaced from one another, said cooling fluid inlet (26) comprising the first blow pin (26), and the second blow pin (28) able to be placed in communication with a reservoir (24) for said recovered gas, whereby in operation cooling fluid is able to flow through said article in a direction from said first blow pin (26) to said second blow pin (28). Apparatus according to claim 9, additionally including valve means (38, 40, 42, 44) operable to place the second blow pin (28) in communication with said mixing means (22) and the first blow pin (26) in communication with the reservoir (24) whereby to reverse said direction of flow.

Inventors: LEE RON CLARK, LEE, RON CLARK
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1993
Main classification: B29C49
IPC codes: B29C35, B29C49, B29L22