INJECTION MOLDING PROCESS FOR MOLTEN PLASTIC

Abstract: An injection molding process in which an injection of a molten resin into a cavity is performed under such a condition that an internal resin pressure upon filling of the cavity with the molten resin is lower than a maximum internal resin pressure required upon pressure holding after the filling.

Description: The present invention relates to an injection molding process comprising the steps of: clamping a male die and a female die together with a prescribed clamping force to provide at least one cavity of a prescribed extruded area therebetween; subsequently injecting a molten resin at a prescribed injection pressure into said cavity to fill said cavity with said molten resin; and subsequently holding said molten resin filled in said cavity at a prescribed holding pressure to solidify the resin within the cavity to produce a molded article. A conventional process for injection molding is well known in the art. A plastic pellet is fed into a hopper of an injection molding machine such as an extruder. In the extruder, the plastic pellet is carried in a cylinder having a piston or a screw carrier and heated to produce a molten plastic. The molten plastic is extruded via a nozzle into a die assembly. The molten plastic is then maintained or held in the die assembly to provide a mold product. There are two types of pressures at the cylinder and the die assembly. A plunger pressure, i.e. an injection pressure, is created on the piston or screw in the cylinder while an internal resin pressure for molten plastic occurs in the die assembly. Fig. 1 shows a graph of the plunger pressure of the piston or screw of the conventional extruder vs. the internal resin pressure within a cavity defined in the die assembly when a molding is produced according to the conventional injection process. As shown in Fig. 1, the plunger pressure of the extruder is increased to a maximum ram pressure upon filling the molten resin into the die assembly. Upon holding and cooling the molten plastic the plunger pressure is decreased to the intermediate pressure and then to a minimum ram pressure. It should be noted that the internal resin pressure according to the conventional molding process has a synchronous pressure variation to that of the plunger pressure. The internal resin pressure is therefore increased upon filling the molten resin into the die assembly, substantially following a trace of the plunger pressure. The internal resin pressure then gradually decreases upon holding and cooling the molten resin in the die assembly since the molten resin is gradually cooled and solidified with a slight shrinkage of the resin. Such pressure change is considered to result from various factors such as, for example, the plunger pressure generated by a hydraulic pressure system, the volume or capacity of the cavity in the mold assembly, the flow resistance of molten resin against a passage from the nozzle to the cavity, a diameter of a gate facing to the cavity and the like. The most important factor to be considered is that a volume of the cavity to be extruded does not change upon filling and holding the molten plastic in the die assembly by the predetermined mold clamping force. A filling time is also effective to prevent the mold surface when cooled from flashing. An extruded area EA of the molten resin with an internal resin pressure IRP to a die assembly having a fixed mold clamping force MCF has an upper limit defined by: EA x IRP = MCFmax If the value of the EA x IRP, that is, the MCFmax exceeds the mold clamping force of the die assembly, as shown in Fig. 5, a space or gap 3 appears between surfaces 1a and 2a of respective male and female dies 1 and 2 with an opening amount h . The molten resin M therefore enters the gap 3 to cause a flash Ma to occur on a molded article. This would result in poor products and sometimes in interruption of the molding. On the other hand, in order to increase a filling density of the molded article to a value higher than a predetermined value, it is required to increase the internal resin pressure to a value higher than a predetermined value and, after all, the extruded area of the molten resin is limited by the mold clamping force of the die assembly. An injection molding process as mentioned at the beginning is disclosed in German Offenlegungsschrift DE-OS 25 35 426. In this known injection molding process, after charging the mold, a dwell pressure of controlled variability is applied to the plastic material in the mold and the dwell pressure in a first stage is kept at a low value at which swelling of the mold is avoided and is raised to a higher value in a subsequent stage. It is therefore the object of the present invention to provide an increased output of an injection machine by using the process according to the present invention. This object is achieved by the features of the claim. As the extruded area is at least twice that of a conventional die assembly, the die assembly according to the invention can contain two cavities as compared with the one cavity of a conventional die assembly with the same clamping force. Because the internal resin pressure on the filling is low, the flow rate, i.e., filling velocity of the molten resin within the cavity is decreased and, accordingly, it takes a long time for the molten resin to reach the distal end of the cavity, so that during the movement a thin or shallow layer of the molten resin in contact with the die assembly is well cooled and solidified to form a thin hardened layer. The inside of the thin hardened layer is in a molten condition, and the holding pressure is applied through the interior molten resin to the entire solid resin. At this time, if the current force (EA x IRP) which exceeds the mold clamping force of the die assembly should be applied to the resin to move the male and female dies away from each other to define a gap between them, the molten resin would not enter the gap because of the presence of the thin hardened layer. Accordingly, no flash would occur on a molded product. Subsequently, the interior molten resin is also cooled and solidified. One way of carrying out the invention is described in detail below with reference to drawings which illustrate only one specific embodiment, in which:- Fig. 1 is a graph showing the characteristics of the conventional injection molding method; Fig. 2 is a cross-sectional view showing a die assembly and an injection molding machine, to which the present invention is applicable; Fig. 3 is a fragmentary enlarged cross-sectional view of a portion of a cavity encircled by (a) in Fig. 2, showing a condition of a filling stage; Fig. 4 is a view similar to Fig. 3, but showing a condition immediately after the filling; Fig. 5 is a view similar to Fig. 3, but showing a condition immediately after the filling in accordance with a conventional injection molding method; and Fig. 6 is a graph similar to Fig. 1, but showing the relationship between time and a plunger pressure and an internal resin pressure, in accordance with an embodiment of the present invention. An embodiment of the present invention will now be described with reference to Figs. 2 to 6 of the accompanying drawings. Fig. 2 is a cross-sectional view of a die assembly generally designated by the reference 10, and an injection molding machine such as an extruder generally designated by the reference 50. The die assembly 10 is substantially identical in construction with a conventional one known to those skilled in the art, and will be described briefly. The die assembly 10 comprises a male die 12 and a female die 14 between which a cavity 16 is provided to fill a molten resin. The cavity 16, in this embodiment, is in the form of a shallow cup. The female die 14 therefore is mounted on a fixed bottom plate 18 through a spacer 20. The bottom plate 18 is adjustable in position disposed on a base (not shown), while the male die 12 is mounted on a movable bottom plate 22 through a spacer 24. The movable bottom plate 22 is also accessably disposed on the base. A pair of ejector plates 26 and 28 are movably mounted on the ejector guides (not shown) provided between the die 12 and plate 22, with an ejector pin 30 extending from the ejector plates 26 and 28 away from the bottom plate 22. The movable bottom plate 22 is slidably guided on the base such that the ejector pin 30 causes the mold product to eject from the cavity. The fixed bottom plate 18 has mounted thereon a locating ring 32 and a manifold 34 interfacing the extruder 50 to the die assembly 10. A runner bush 36 is mounted between the female die 14 and the manifold 34, and a torpedo spreader 38 extends into and through the runner bush 36. The female die 14, the runner bush 36 and the manifold 34 have formed therein a runner 40 communicating with a gate 42 which faces the cavity 16. In this embodiment, the die assembly 10 has two cavities 16 each of which has a conventional cavity size with the characteristics shown in Fig. 6. In other words, the die assembly 10 has an extruded area twice that of the conventional die assembly. Molten resin from the extruder 50 is simultaneously injected into each of the cavities 16. The gate 42 facing each cavity 16 has a diameter greater than 1.6 times, for example, that of a gate in the conventional die assembly. The extruder 50 is also substantially identical in construction with a conventional one known to those skilled in the art, and will be described briefly. A plunger 52 which is reciprocally actuated by a hydraulic cylinder 54 is slidably received in a heating barrel 56 which is heated by a heater 58. A hopper 60 is mounted on the heating barrel 56 adjacent to one end thereof, and a nozzle 62 is provided at the other end or tip of the heating barrel 56. Alternatively, a screw rod for carrying the molten resin may be used. The screw rod is rotatably mounted within the heating barrel 56 so as to produce the plunger or ram pressure. An injection molding is performed by the above-described die assembly 10 and the extruder 50. In this case, if the plunger pressure, i.e., the pressure applied by the plunger 52 to the molten resin within the heating barrel 56 is the same as that in the conventional extruder, in other words, if the hydraulic pressure in the hydraulic cylinder 54 is the same as a conventional one, the injection molding is performed in a pattern of pressure change as shown in Fig. 6. Specifically, the internal resin pressure is sufficiently low upon the filling, and the internal resin pressure higher than the pressure upon the filling is maintained upon the pressure holding. The internal resin pressure on the pressure holding should be increased to a level higher than that required for increasing a filling density of a molded article. When the molding is performed in such a pattern of pressure change, the molten resin reaches the distal end of the cavity by means of the filling stage shown in Fig. 3. However, as described above, because the internal resin pressure on the filling is low, the filling rate or velocity of the molten resin is decreased. As shown in Fig. 4, a thin layer of the resin in contact with the male and female dies 1 and 2 is well cooled and solidified to form a thin hardened layer M₁. The inside of the thin hardened layer M₁ is in a molten condition, and the holding pressure is applied to the entire resin through the interior molten layer M₂. In this manner, because the thin hardened layer M₁ is formed, if the split surfaces 1a and 2a of the respective male and female dies 1 and 2 are moved away from each other to form between them a gap 3, the molten resin would not enter the gap 3, so that no flash would occur on the molded article. In addition, since the resin is filled in the entire cavities at a relatively low internal resin pressure, it is possible to obtain molded articles with less possibility that internal stress is developed. Although the illustrated embodiment has been described as being applied to such an injection molding construction and arrangement that the molded articles number twice that or those molded by the conventional molding method and the gate diameter is approximately 1.6 times that used in the conventional molding process, the present invention should not be limited to this embodiment, but may be applied to any injection molding construction and arrangement, provided that as shown in Fig. 6 the molding is performed under the condition that internal resin pressure on filling is lower than the holding pressure. Accordingly, instead of the increase in the number of the molded articles, an extruded area of an article to be molded per se may be increased. In addition, the plunger may be decreased in feed rate, while the extruded area of the molded article per se is maintained as in the past. As described above, in accordance with the present invention, since the injection is performed in such a condition that the internal resin pressure on the filling is lower than the internal resin pressure on the pressure holding, the filling speed or rate of the molten resin is decreased to allow the thin layer of the resin in contact with the die assembly to be formed into the thin hardened layer, so that should the dies be moved away from each other by the force exceeding the mold clamping capacity of the molding machine, no flash would be produced. Accordingly, as compared with the conventional molding method, should a molding machine having the same mold clamping capacity be utilized, it would be possible to mold an article having a greater extruded area. In addition, since the thin hardened layer is formed, no flash would be produced, should the molds be more or less inaccurate as to dimensions so that a gap initially exists between the split surfaces of the respective dies.

Claims: An injection molding process comprising the steps of: (a) clamping a male die (12) and a female die (14) together with a prescribed clamping force (MCF) to provide at least one cavity (16) of a prescribed extruded area (EA) therebetween; (b) subsequently injecting a molten resin (M) at a prescribed injection pressure into said cavity (16) to fill said cavity (16) with said molten resin (M); and (c) subsequently holding said molten resin (M) filled in said cavity (16) at a prescribed holding pressure to solidify the resin (M) within the cavity (16) to produce a molded article; characterized in that: (d) during said filling step (b) the injection pressure is controlled so as to increase an internal resin pressure (IRP) gradually whilst moving said molten resin (M) within said cavity (16) at such a speed that the portion of said molten resin (M) in contact with the surfaces of said cavity (16) is solidified to form during the filling step a thin hardened layer (M₁) on the cavity surfaces, which layer is hard enough to prevent penetration of molten resin into a gap between the dies (12, 14); and (e) said pressure holding step (c) is effected so as to continue increasing the internal resin pressure (IRP) to a maximum at the beginning of said pressure holding step (c) and to maintain said maximum level substantially throughout said pressure holding step (c), whereby the force defined by the extruded area (EA), which is at least twice that of a conventional die assembly with the same clamping force, multiplied by the internal resin pressure (IRP) exceeds said clamping force (MCF) during the pressure holding step (c).

Inventors: TAKEDA, YOSHINOBU, YAMAMOTO, KUNIO
Applicants: MITSUBISHI MATERIALS CORPORATION
Application year: 1985
Main classification: B29C45
IPC codes: B29C45