Blow molding apparatus and process

Abstract: A process of blow-molding a hollow double wall type article uses a mold incorporated with a heating and cooling system. The mold consists of male and female mold halves, each composed of an outer shell area having a contoured cavity surface part and an inner heat insulating area defined within the outer shell area. The cavity surface part is made of mainly of a metallic material having a high thermal conductivity, and the inner heat insulating area is provided therein with means for reinforcing the outer shell area against a blow molding pressure and a mold clamping pressure. The inner heat insulating area provides a substantial thermal insulation against a transmission of heat thereto from the cavity surface part. The reinforcing means may be formed by a block of a non-metallic material having a low thermal conductivity. Preferably, the block is permeably porous so that it provides heat medium passages for the heating and cooling system.

Description: The present invention relates to an improved apparatus and process for blow molding according to the preambles of claims 1 and 4, respectively, and particularly to an apparatus and process for producing a hollow double wall type plastic article by the blow molding. A blow molding apparatus and process, respectively, of this type and, more specifically, a blow molding tool or mold for such apparatus and process, respectively, are known from DE-A-33 01 337, which discloses a mold wherein the reinforcing means provided in the inner heat insulating area of each mold halve are a block of plastic material comprising channel-like recesses which together with the adjacent interior surface of the inner contoured cavity surface part of the mold halve form closed rectangular channels for a heating and/or cooling medium for the mold. Similar blow molding apparatusses and processes, respectively, are disclosed in US-A-3,784,344, wherein the reinforcing means are separate plates simultaneously defining a fluid channel and in FR-A-21 59 231, wherein said recesses are provided in an intermediate thermally conductive layer surrounded by an external mold body or formed by a correspondingly folded piece of sheet material which is likewise surrounded by an external mold body. Generally, the blow molding process is a well known method of producing hollow plastic articles. Initially, the blow molding process was directed to the production of a hollow article having a solid wall with a generally convex peripheral outer surface. Most of such hollow articles are basically rotation symmetrical or plane symmetrical. Recently, a modified blow molding process of producing a hollow article having a basically concave, convex or bowl shape with a hollow double wall, has been proposed. This modified blow molding process uses a mold consisting of a male or convex mold half and a female or concave mold half, both mold halves defining a mold cavity having a concave shape. The hollow double wall type article has a concave inner surface portion and a convex outer surface portion, both forming the hollow double wall with a substantially uniform small gap between the two surface portions over substantially the entire area thereof. Most of such hollow double wall type articles are designed to have an overall contour showing relatively sharp angled corners, since they are inherently intended for use in place of conventional solid wall type articles produced by a normal injection molding method or a vacuum molding method. In this connection, the modified blow molding process is intended to effect a formation of such convex and concave surface portions, each having substantially a uniform resin thickness over the entire surface area, as far as possible, but it has been realized that the resin thickness at the corners is likely to be smaller than that at the other parts of the hollow double wall, due to a nature of the blow molding. Further, it is noted that a mold for use in the modified blow molding has a larger cavity surface with a smaller cavity volume or space, than those of the mold for use in the original blow molding. Therefore, a considerably higher molding capability is required for the modified blow molding than that for the original blow molding. To cope with this inherent problem involved in the production of a hollow double wall type article, various types of improved heating and cooling systems to be incorporated in a mold have been provided. For example in a prior art blow molding apparatus as shown in Fig. 3, the mold has male and female mold halves 3 and 4, both made entirely of a metallic material. Each mold half 3 and 4 has spaced pipes 5 embedded, or has spaced through-holes formed, in the metallic body in the vicinity of a cavity surface 3a or 4a. The pipes are used for heating the cavity surface, which has been cooled in the last molding circle, with a heating medium such as hot water to a temperature of 20°C to 80°C, necessary before a mold cavity can receive a parison 2 of a hot plasticized resin extruded from an extruder 1. The heating of the cavity surfaces is intended to promote the function of the cavity surfaces, in cooperation with the hot parison forced to expand by blowing air thereinto using an air injector, to cause the expanding parison, as a whole, to be effectively deformed along the cavity surfaces and to fit thereon. The same pipes 5 are used in the same molding cycle to cool the cavity surface of the mold by flowing a cooling medium therethrough just before the blow molding is completed, to thereby cool a molded article in the cavity. These heating and cooling steps are carried out in each molding cycle. In another prior art disclosed in JP-B-63-2769, intended for a production not of a hollow double wall type article but of a normal hollow article such as a plastic bottle, although it may be applied for a production of such a hollow double wall type article, spaced passages formed in mold halves made entirely of a metallic material are similar to those of the first mentioned prior art but are different therefrom in that a laminate film consisting of an electric insulator film and an electric conductor film coated thereon, are used which laminate film is attached to a corresponding cavity surface of each mold half over the entire area thereof. Heating is effected by flowing a current through the attached film, and cooling is effected by flowing a cooling medium through the passages. Another prior art as disclosed in JP-A-59-54524 is directed to the blow molding apparatus as shown in Fig. 4. A male mold half 3 made entirely of a metallic material has spaced passages 5 formed therein at corners of a cavity surface 3a, through which a cooling medium flows, and electric heating coil units 6 embedded therein near the corners, and a female mold half 4 made entirely of a metallic material is provided with the same heating coil units 6 embedded therein near corners of a cavity surface 4a, and with two groups of perforated disks 7 and 8 disposed therein at the corners, the first group of disks 7 being associated with a passages formed in the mold half communicating with a cold gas source and the second group of disks 8 being associated with other passages 10 formed in the mold half for discharging the cold gas fed from the first group disks 7 into a mold cavity. Alternatively, the cooling passages 5 of the male mold half 3 may be replaced by the some first and second group of disks 7 and 8 and passages 9 and 10 associated therewith as those of the female mold half 4, or the first and second group of disks 7 and 8 of the femal mold half 4 may be replaced by the same cooling passages 5 as those of the male mold half 3. The heating coil units 6 are actuated to heat the cavity surfaces before a parison 2 of a hot plasticized resin extruded from an extruder 1 is introduced into a mold cavity. While the parison is in the process of expansion in the cavity, due to a gas blowing into the parison from a gas injector or a blow gas pin, a cold gas or liquid medium is forced to flow through the cooling passages 5 of the male mold half 3 to cool the corner parts of the cavity surface, and a cold gas medium is forced to flow into the cavity from the first group of disks 7 and is discharged from the second group of disks 8 to thereby cool parts of the parison to be molded to form corner parts of a resultant article. The heating and cooling system is intended to effectively prevent a blow molded article from having corner parts with a thinner thickness relative to the other parts, whereby the molded article of a hollow double wall type is provided with a resin thickness with less variation over the entire surface. JP-A-59-54524 does not disclose a cooling of the molded article in the mold cavity just before the mold is opened, but it is assumed, as a matter of nature, that the molded article would be cooled by using the cooling passages 5. Nevertheless a question remains of how effectively the passages 5, originally intended for use in cooling the corner parts of the cavity surfaces, work to cool the mold articles as a whole. The above mentioned prior art solutions are able to attain some improvements for higher quality articles, particularly hollow double wall type articles having a resin thickness with less variations, but - with the exception of the solution according to DE-A-3 301 337 - share the same serious problem in that the adopted heating and cooling system incorporated in the mold causes a blow molding cycle to take a considerably longer time than that of the above mentioned original blow molding for producing solid wall type hollow articles. This is because the original blow molding process does not always use both heating and cooling steps, but may have only a cooling step for cooling a molded article before a removal thereof from a mold. In this connection, the modified blow molding process always requires the time necessary for the heating step, in addition to the time for the cooling step, at every molding cycle. Further, it is most important to note that the modified blow molding process requires alternately and repeatedly heating and cooling the mold, while the molding cycle is repeated. The present inventors recognized that the above repetition causes a substantial operational delay due to the alternate cooling and heating, with the result that the molding cycle time is prolonged by the operational delay. An object of the present invention is to provide a blow molding apparatus comprising an improved mold which solves the above-mentioned problems of the operational delay due to heating and cooling the mold and the variation of the cooling effect at the mold cavity surface. Another object of the present invention is to provide a blow molding process using the above improved mold, wherein the heating and cooling steps are combined so that a high quality hollow double wall type article is blow-molded in a short cycle. The above objects according to the invention are accomplished by means of the apparatus according to claim 1 and by means of the process according to claim 4, respectively. The present inventors first conceived an idea for overcoming the problems of the prior art, wherein if each mold half consisted mainly of a thin metallic layer incorporated with a heating and cooling system for heating and cooling the metallic layer, and the layer per se provided a contoured cavity surface, such an operational delay in the heating and cooling of the mold, and such a variation of cooling effect at the cavity surface as in the prior art mold would be eliminated. This idea would be reasonable and operable so long as such a thin metallic cavity surface layer had been adequately reinforced by some means to an extent enabling resistance against a mold clamping pressure and a blow molding pressure exerted by a blow gas introduced into a parison. This idea is in marked contrast to the prior arts wherein each mold half is formed of a metallic block having a generally cubic shape, wherein one of the block surfaces is contoured to form a cavity surface. It is obvious that the conventional mold consisting of two metallic blocks forming mold halves must have a much greater heat capacity than the above assumed mold having the thin metallic layers. Such a large heat capacity causes the disadvantageous operational delay due to heating and cooling. The present invention embodies the above idea in combination with another critical idea that a substantial thermal insulator per se provides the above mentioned reinforcing means. In connection with this, it should be noted that the present invention is advantageous and reasonable on the ground in that a molding pressure and a mold clamping pressure for use during the blow molding are considerably lower than those used in an injection molding. In this connection, various types reinforcing means can be easily employed in combination with the thin cavity surface layer of a metallic material. The metallic material is preferably a material having a high thermal conductivity. With the mold of the present invention, an improved blow molding process can be provided wherein a parison of a plasticized resin extruded from an extruder is pre-heated as a whole at the outer surface thereof by a U shape radiation heater, before the main heating thereof. The U shape radiation heater heats not only the parison but also the cavity surfaces of the mold halves while the mold is open. Alternatively, in addition to such preheating, the parison is preheated as a whole at the inner surface thereof by using a hot blow gas while it is in the process of expansion due to the blow gas. Further, such a hot blow gas may be alternated with a cold blow gas just before a mold is opened, to thereby cool a molded article at the inner side thereof, while the article is cooled at the outer surface thereof by a cooling system incorporated in the mold. The above heating and cooling are advantageous in producing a hollow double wall type high quality article, and in shortening the mold cycle by a considerable time compared to the prior art, since the mold of the present invention eliminates the operational delay due to heating and cooling. Further, the inner heat insulating area provides a substantial thermal insulation against a transmission of heat thereto from the inner shell contoured part. The above mentioned heat medium passages may be provided as a means for cooling the inner shell contoured part by flowing a cooling medium therethrough. Alternatively, the heat medium passages may be provided as a means for heating and cooling the inner shell contoured part by alternately flowing heating an cooling mediums therethrough or some of the heat medium passages may be provided as means for heating the inner shell contoured part by flowing a heating medium therethrough, while the other heat medium passages are provided as means for cooling the same by flowing a cooling medium therethrough. Further, according to the present invention, there is provided a blow molding process of producing hollow plastic articles using the above mentioned mold having an inner heat insulating area other than that of the permeable porous block. This process comprises the steps of extruding a parison of a plasticized material into a gap between the mold halves while the mold is open, blowing gas into the parison at least while the mold is closed so that the parison is molded to form a hollow article; and cooling the molded article just before the mold is opened. The inner shell contoured parts of the mold halves at the surfaces thereof and the parison at the outer surface thereof, are both heated by using an instantaneous U shape radiation heater disposed in the mold half gap to partially encompass the parison just before the mold is closed. The cooling of the molded article is effected at the outer surface thereof by cooling the mold at the inner shell contoured parts thereof by flowing a cooling medium through the heat medium passages of the mold. The heating of the parison is additionally effected at the inner surface thereof by blowing hot gas into the parison while allowing the blown gas to flow out of the parison, the blown hot gas exerting as a blowing gas an internal pressure on the parison to force the parison against the mold to form the molded article. Further, the cooling of the article is additionally effected at the inner surface thereof by blowing cold gas into the article as a subsequent blowing gas while allowing the blown gas to flow out of the article, the hot blowing gas and the cold blowing gas being made alternately to flow while the mold is closed. Preferably, the inner shell contoured parts of the mold are heated at the surfaces thereof to a temperature of 90° to 200°C. Details of the invention will become apparent from the following description of an embodiment taken in conjunction with the accompanying drawings, wherein: Fig. 1 shows a vertical sectional view showing a blow molding apparatus including a mold incorporated with a heating and cooling system for producing hollow double wall type articles according to the present invention; Fig. 2 is a perspective view of the apparatus of the, present invention; and Figs. 3 and 4 are views corresponding to Fig. 1 and showing conventional molds. An embodiment of the present invention directed to a blow molding apparatus and process for producing a hollow double wall type article will be now described with reference to Figs. 1 and 2. Referring to Fig. 1, the apparatus comprises an extruder 1 with a die for extruding a parison 2 of a plasticized resin, a mold consisting of a male mold half 3 and a female mold half 4, and an instantaneous U-shape radiation heater 60. The instantaneous radiation heater 60 comprises vertically extending tube lamps arranged in a U-shape frame body such that each lamp radiates near infrared rays in the radial direction. Such a lamp per se may be conventional lamp. The die has a gas outlet hole through which air is preblown into the parison after the parison is sealed at the free end thereof, to prevent the parison from being self-adhered before a normal blowing step is carried out. Each mold half 3 or 4 is composed of an outer shell area A of a shell housing and an inner heat insulating area B. The outer shell area A has an inner contoured cavity surface part 20 made of a metallic material having a high thermal conductivity, such as Al alloy or Cu alloy, top, bottom and side frame parts 40, and a backing frame part 50, each made of iron. The inner contoured cavity surface part 20 and the frame parts 40 and 50 are of relatively thin plates and define with the inner contoured cavity surface part 20, a generally cubic mold contour. The inner contoured cavity surface parts 20 have desired male and female contours defining, in combination, a mold cavity. Each inner contoured cavity surface part 20 has many spaced through-holes 21 for flowing a heat medium therethrough as shown in Fig. 1. The inner heat insulating area B is defined by the outer shell area A and is filled with a filler consisting of a non-metallic material such as silica sand, stone, ceramics, epoxy resin, phenol resin, unsaturated polyester resin, polyurethane resin or the like. The filler is a porous block or has a honey comb structure. With respect to the resin material, preferably the filler is formed of a block of a foamed resin. The inner heat insulating area B having the above mentioned filler provides a substantial thermal insulation against a transmission of heat from the inner contoured cavity surface part 20, and further, provides a means for reinforcing the outer shell area A consisting of the inner contoured cavity surface part 20 and the frame parts 40 and 50 against a blow molding pressure and a mold clamping pressure. The female mold half 4 is provided with two conventional movable probes (not shown) disposed in the mold body; one for introducing a pressurized blow gas into the parison and the other for discharging the gas from the parison during a blow molding step. A heating and cooling system is provided in the apparatus as shown in Fig. 2. A partially flexible pipe line 51 is designed so that a heating medium source and a cooling medium source are connected therethrough with the through-holes 21 in Fig. 1, to introduce the individual heat medium into the mold so that the inner contoured cavity surface parts 20 are mainly heated or cooled alternately. The heat medium is discharged from the mold through the holes 21 in Fig. 1 and then through the pipe line 51. The heating and cooling system is also provided with a pipe line 52 extended from a hot blow gas source and a cold blow gas source; both sources being connected alternately with the above mentioned feeding probe. According to the present invention, a parison of a plasticized resin is extruded downward from the die. The free open end of the parison is closed or self-sealed by using a pinch tool, just after the parison appears from the die. Preblow air is fed into the sealed parison from the outlet hole of the die. The U-shape radiation heater 60 is moved to and held at a working position between the mold halves 3 and 4 while the mold is open. The parison extends downward along a normal route provided inside of the U-shaped radiation heater 60. Just after the closed end of the parison passes over the U-shaped heater 60, the heater is retracted horizontally from the working position to an original position out of the mold, and the extrusion of the parison is discontinued until a next cycle. While the U-shaped heater 60 is in the working position, the downward extending parison is heated at the outer surface thereof by the heater, and the mold halves 3 and 4 are heated concurrently at their inner contoured cavity surfaces by the same heater. Since the inner contoured cavity surface parts 20 are very thin and have a small thermal capacity and a high thermal conductivity, these surfaces are effectively heated in a short time by the U-shaped radiation heater 60. The heating by the U-shape radiation heater 60 is effected preferably until the cavity surfaces of the mold are brought to a temperature of 90° to 200°C. In the meantime, the parison is cooled, particularly at the outer surface thereof, to room temperature while extruded from the die, with the result that the parison outer surface becomes harder than the inner part of the parison. Such a hardened surface of the parison causes a blow molded article to have a lower surface quality. Under these circumstances, the above heating by the U-shaped radiation heater causes a hardened surface of the parison to be softened to an extent such that a high molding capability is obtained with a high surface quality. Upon discontinuation of the extrusion, both the mold halves 3 and 4 are moved in the opposite direction toward each other to have the mold closed with the preblown parison therein, while the parison 2 is integrated with the die. A hot air as a heating medium and a subsequent normal blow gas are blown continuously into the parison received in the mold when the mold is closed through one of the blowing probes, i.e., a feeding probe, and the hot air is discharged through the other probe, i.e., a venting probe. These probes are projected to penetrate the softened resin wall of the parison, which has been kept hollow due to the preblowing gas. While the mold is in the process of closing, the parison is forcibly deformed along the line of deeply concaved shape cavity. The hot blow gas exerts a pressure to force the parison against the mold cavity surface, so that the pre-deformed parison is completely deformed along the line of the cavity. The mold is heated at the inner contoured cavity surface parts 20 before the mold is closed, by flowing a hot water or an other liquid through the holes 21 as shown in Fig. 1, with the effect that the expanded parison is heated at the outer surface thereof in contact with the cavity surfaces. The heating of the parison at both the outer and inner surfaces thereof and the cavity surfaces of the mold halves is very advantageous when making such a deep deformation of the parison, and attains a high surface quality of a resultant article and considerably less variation of the resin thickness over the entire surface of the article. The flowing of the hot blow gas is alternated with a flowing of a cold blow gas, using the same probes, to cool the molded article at the inner surface thereof, when the inner contoured cavity surface parts 20 of the mold are cooled by flowing a cooling medium through the holes 21 in Fig. 1. These cooling steps in combination ensure that the molded article has a higher surface quality compared with that of the prior art. Although various kinds of heating steps and cooling steps are adopted as described above, the blow molding cycle is shorter than that of the prior art. This is because the mold of the present invention is designed so that mainly the metallic thin cavity surface parts 20 having a high thermal conductivity and a considerably small thermal capacity are subjected to heating and cooling. Whereas the inner area of the mold is not substantially subjected to heating and cooling, since it is provided in the mold as a substantial thermal insulator. After the molded article is cooled by both the cold blow gas passing through the article and the cooling medium passing through the mold, the mold is opened and the molded article is removed with a top portion of the parison 2 remaining out of the mold torn off from the die. As such, a one blow molding cycle has been completed, and a next cycle starts at a state wherein the mold is cooled. This means that the mold is subjected to repeated heating and cooling by the repeated cycles. With the mold of the present invention, heating the mold, which has been cooled and then cooling the heated mold, can be effected in a shorter time and with considerably less variation over the entire cavity surface than in the prior arts, since only a small part of the mold, i.e, the thin inner contoured cavity surface parts 20, is subjected to the heating and cooling, whereas the entire body of the prior art mold is subjected to the heating and cooling. The cooling medium may be cold water, but liquified carbon dioxide is preferable as the cooling medium if used in a gaseous state. This is advantageous in shortening the cooling time, and thus the blow molding cycle. According to the present invention, it is obvious that a high molding capability for even a hollow double wall type article having a very deep convex, concave, or bowl shape and sharp-angled corners with a substantially uniform resin thickness, a high surface quality and a high dimensional accuracy over the entire surface, can be obtained in a shortened operational cycle.

Claims: A blow molding apparatus comprising an extruder, a mold having formed therein heat medium passages communicating with a heat medium source to be incorporated with the mold, for producing hollow plastic articles, with each of two mold halves (3, 34) which in combination form said mold being composed of: an outer shell area (A) of a shell housing having an inner contoured cavity surface part (20) which is contoured so that the mold halves (3, 4), in combination, define a mold cavity at the inner contoured cavity surface part (20) thereof; and an inner heat insulating area (B) defined within the shell housing, the inner contoured cavity surface part (20) being made of a single thin metallic material haying a high thermal conductivity and the inner heat insulating area (B) being provided therein with a means for reinforcing the shell housing against a blow molding pressure and a mold clamping pressure, said inner heat insulating area (B) providing a substantial thermal insulation against a transmission of heat thereto from the inner contoured cavity surface part (20), and said reinforcing means being made of a non-metallic material having a low thermal conductivity and substantially filling the interior of the shell housing, said apparatus being characterized in that said reinforcing means are formed by a porous block (B) (namely a block of a porous material or a block having a honey comb structure) of the non-metallic material and that said inner contoured cavity surface part (20) has through-holes (21) formed directly with a circular section in the single thin metallic material. A blow molding apparatus according to claim 1, wherein the heat medium passages (21) are arranged in parallel in the inner contoured cavity surface part (20) and extend over the entire width of the mold half (3, 4) while spaced from each other, and that the heat medium passages (21) are provided as means for heating and cooling the inner contoured cavity surface part (20) by alternately flowing heating and cooling mediums. A blow molding apparatus according to claim 1 or 2, wherein said single thin metallic material is Al alloy or Cu alloy. A blow molding process of producing hollow plastic articles comprising the steps of extruding a parison (2) of a plasticized material into a gap between the mold halves (3, 4) while the mold is open, blowing gas into the parison (2) at least while the mold is closed so that the parison (2) is molded to form a hollow article, and cooling the molded article before the mold is opened, characterized in that the blow molding process is carried out using a blow molding apparatus according to claim 2, wherein the inner contoured cavity surface parts (20) of the mold are not only cooled to cool the molded article, but also heated at the surfaces thereof to a temperature of 90 to 200° by alternately forcing cooling and heating mediums to flow through the through-holes (21) thereof in every molding cycle and in that the inner shell contoured parts (20) of the mold at the surfaces thereof and the parison (2) at the outer surface thereof, are both heated by using an instantaneous U shape radiation heater (60), disposed in the mold half gap to partially encompass the parison (2) while the mold is open. A blow molding process according to claim 4, wherein the step of blowing gas into the parison (2) comprises alternately flowing a hot blow gas and a cold blow gas.

Inventors: KANOH YOSHIAKI, YAMADA AKIYOSHI, KANOH, YOSHIAKI, YAMADA, AKIYOSHI
Applicants: UBE INDUSTRIES, UBE INDUSTRIES, LTD.
Application year: 1989
Main classification: B29C49
IPC codes: B29C33, B29C49, B29L22, B29B13, B29C35