Patent GB-582033-A (GB)
Title: Improvements in or relating to the moulding of plastic substances
IPC: B29C45/72
Filed: 1944-06-29
Abstract: 582,033. Injection-moulding and extrusion machines. GOLDHARD, F. K., and PRENN, D. D. June 29, 1944, No. 12421. [Class 87 (ii)] Relates to the heating of thermosetting or thermoplastic material before it is fed to an injection or extrusion moulding device. According to the invention, the amount of material subjected to such pre-heating, and the amount of heat so supplied are automatically controlled in accordance with the quantity of material used in a preceding moulding operation. The required amount of material, before being preheated, is segregated from a bulk feed supply, and is fed to a heating chamber. In the machine shown in Fig. 1, the injection or extrusion cylinder 1 is connected to a preheating chamber 4 in which during each moulding cycle an automatically-measured quantity or charge of the material to be moulded is heated by highfrequency dielectric heating, to the requisite temperature, before being passed into the cylinder 1. Each charge of material subjected to pre-heating is substantially equal to the amount expelled by a plunger 2 from the cylinder 1 during the preceding moulding operation. The chamber 4, which is of rectangular cross-section, has two opposite walls 5, 6 of conductive material, Fig. 2, connected to the terminals of a highfrequency generator, and the other two walls 7, 8, as well as adjoining parts 9, 10 and a plunger 11 are made of "low loss" insulating material. A chamber 12, communicating with the chamber 4, and of the same rectangular cross-section, is connected to the lower end of a hopper 13 containing the material to be moulded. The amount of material passing from the hopper into the chamber 12, before being fed by the plunger 11 to the heating chamber, depends upon the position to which the said plunger has been retracted. This is determined by the position to which the plunger 2 has moved forward in the cylinder 1 during the immediately preceding moulding operation, the means whereby the plunger 2 controls the plunger 11 consisting of rack and pinion mechanism 14, 15, 16. The rack 16 is moved forward during an operating stroke of the plunger 2, by means of a pawl 17 engaging a dog 18 on the rack, and thereby retracts the plunger 11 to a position corresponding to the forward position of the plunger 2. The dog 18 can be initially set by hand, to determine roughly the quantity of material to be moulded each time. In the forward position of the plunger 2, the pawl is disengaged from the dog, e.g., by means of a solenoid 19, and the rack 16 is retracted by a spring 20. The plunger 11 thereupon feeds the material from the chamber 12 into the heating-chamber 4 and compresses it therein. The high-frequency current is then switched on, and cut off when the required temperature is reached, by a timing or thermostatic device. Subsequently, when the plunger 2 has been retracted, uncovering the connecting passage 3, the preheated material is fed into the cylinder 1, the rack 16 being moved to advance the plunger 11 further for this purpose, e.g., by a solenoid 21 which is energized when a rod 22 closes a switch 23. The temperature to to which the material is heated may be varied by initial adjustment of the voltage &c., or by varying the time factor. For interconnecting the plungers 2, 11, hydraulic means may be used instead of rack and pinion mechanism. In a modification, Figs. 4 and 5 (not shown), the prefilling space 12 and the pre-heating chamber 4 are adjoining sectors of a drum-shaped container, and a rotary plunger therein takes the place of the sliding plunger 11. Fig. 6 shows a further modification, in which the material is fed from a hopper 13 through a pre-heating chamber 4 into a chamber 25 by a conveyer screw 26, driven at a constant low speed. In the chamber 25 is a screw 28, driven synchronously with the screw 26, and movable axially against a spring 29; or a spring-loaded plunger may take the place of the screw 28. Whilst the passage leading from the chamber 25 to the cylinder 1 is closed by the plunger 2, the material fed into the chamber 25 accumulates and compresses the spring 29, until a dog 32 on the spindle 30 of the screw 28 (or the piston rod of the equivalent plunger) operates a switch 31 to cut out the heating current and to stop the conveyer screw or screws. The position of the switch 31, or of the dog 32, is initially set by hand, and is automatically controlled in accordance with the position reached by the plunger 2 on completion of a forward stroke. When the plunger 2 is retracted, opening the passage from the chamber 25, the pre-heated material is fed into the cylinder 1 by the action of the spring 29, the switch 31 is released, and the cycle of operations starts afresh.