GB629228A - Improvements in or relating to the injection moulding of plastic substances - Google Patents

Abstract

629,228. Injection-moulding. GOLDHARD, F. C., and PRENN, D. D. Aug. 8, 1945, Nos. 20323/45 and 3045/46. [Class 87 (ii)] [Also in Group XI] A quantity of thermosetting moulding material that has been heated to its softening point and is slightly in excess of the quantity required for one moulding operation is placed upon a movable conveyer and transferred by the conveyer to a position adjacent the injection chamber, whereupon it is immediately discharged from the conveyer through the injection chamber and through a feed passage to fill the mould cavity. Pressure is maintained on the material in the cavity until at least the excess portion remaining in the injection chamber and/or the feed passage is hardened and then the whole of the excess material is removed before the next snjection operation is commenced. In the machine shown in Figs. 1, 4, 5 and 7 material drops from a hopper 19a into a chamber 19 whence it is pushed by a hydraulically-operated plunger 20 of electricallyinsulating material through a high-frequency heating chamber 18 and compressed into block form against a plate 14f of fused silica or other low-loss insulating material. This plate is carried on one arm 141 of a three-armed lever 14<SP>1</SP> which is keyed by a second arm 142 to a shaft 14a. The third arm 143 is in the form of a jaw and co-operates with a jaw 14c which is pivoted on the lever 14d. A spring 14e normally urges the jaws apart. In the Fig. 5 position the plate 14f closes the end of the chamber 18, as shown in Fig. 7, and the jaws are held closed by a stop 17. When material has been heated and compressed against the plate 14f, the lever is rocked from the Fig. 5 position to the Fig. 4 position by a hydraulicallyoperated rack 16 which engages a pinion 14b fixed on the shaft 14a, and the jaws open. The plunger 20, which has been halted, then moves further to the left and pushes the block of heated material into a space between semi-circular recesses 14g, 14h in the jaws 143, 14c. The lever then rocks back to the Fig. 5 position, in which the recesses are aligned with the injection cylinder 13a, the jaws being thus closed by the stop 17 and the block being compressed from rectangular cross-section to circular crosssection. The injection plunger 10 then moves leftward to the Fig. 1 position, pushing the block out of the jaws, through the cylinder and into the mould space x; during this movement, the jaws are held in precise alignment with the cylinder by reciprocable pins 15b. Pressure is maintained by the plunger 10 at least until any excess portion of the material remaining in the bore 8b of the mould part 8 has hardened. When the mould section 9 opens, the whole of the material, including a small excess present in the mould end of the cylinder is ejected from the mould, leaving the cylinder empty. The amount of material discharged from the space 19 by the plunger 20 is determined by the axial position of a stop pin 22a, against which the plunger abuts in its retracted position. If the injection cylinder has been fed with less than the optimum amount of material, the stop-pin is withdrawn automatically to the right to increase the amount of material fed by plunger 20. If the cylinder has been over-fed, the stop-pin is advanced automatically to the left to decrease the feed. The feed may also be adjusted by hand by making the piston rod P in two relatively-adjustable parts. The (electrode) walls of the heating chamber 18 may be perforated to allow escape of vapour produced during heating, or the heating chamber may be circular in cross-section (see Group XI). Fig. 12 shows a modified machine in which the material is pressed by electrode 18a against an electrode 18i attached to a wheel 25. The wheel then turns to present a hole 25b to the heated material, which is pushed through the hole into a cavity 14j in a drum 14". The drum then turns through 180 degrees to align the charged cavity with the injection cylinder 13, and sprue channel 8b, through which the material is pushed by the plunger 10. The excess material is retained in a dovetail slot 10g in the plunger head, and is withdrawn through the cylinder and cavity 14j, being subsequently removed from the slot. The mould section 9 comprises an insert 28a which is heated by electrical means 28b during the injection stroke of the plunger 10. The sprue channel 8b is heated from the inside by this insert 28a, or may be heated from the outside. The insert 28a may be overheated to accelerate the curing process in the mould, in which case it is separated by an air gap or otherwise insulated from the rest of the mould to prevent heat transfer to the mould. Close control of the temperature of this insert may be obtained by making use of latent heat at the change of condition of a solid or liquid, e.g. lead, sulphur, water or metallic alloys, this solid or liquid being used as a heat transfer medium. Fig. 17 shows a machine in which the plunger 10 traverses the whole of the cylinder, filling of the mould being completed by an auxiliary plunger 40, described in Specification 592,056. Figs. 18, and 19 (not shown) illustrate a conveyer sliding horizontally between the heating device and the injection cylinder. In Figs. 21 and 22 material is extruded by a screw 32 through a high frequency heating chamber 18 and a die- 34 on to a carriage 14. A rotating eccentrically-mounted knife 35 cuts pieces off the extruded bar, and the carriage conveys the pieces into a temperature-controlling device 38 where they are heated or cooled as required. The carriage then aligns the pieces with the injection plunger 10. The carriage may have several recesses 140, each of which receives a cut piece. In Fig. 23 the material is heated by steam, preferably superheated, or humidified hot air passing through pipe 20j, bore 20f in the feeding plunger 20, and holes 20e in the forward end of the plunger. Otherwise the arrangement is similar to that of Fig. 12. Figs. 24 and 25 (not shown) show alternative ways of heating the material with steam or humidified hot air. One injection device may feed more than one mould, the conveyer having a number of spaces which are moved into the appropriate injecting positions. Or more than one high frequency heating device can be used in conjunction with one revolving drum to feed two or more injection devices. Or one high frequency heating device may feed two or more conveyers which feed several injection devices. Specification 582,033 also is referred to.