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The external thread on the workpiece is formed by wedge inserts as shown in Fig.7.32. By exchanging the worm in modern injection machines they are also suitable for the injection of thermosetting plastics. However it is essential to discharge the nozzle completely after each lection, otherwise thermosetting takes place in the nozzle and results in clogging.
The design of suitable air ducts must not be forgotten, because a considerable amount of gases and vapours develop during the thermosetting of plastics, the discharge of which is essential. The distributing runners designed for these dies are generally larger than those used for the injection of thermoplastic materials, and great care must be exercised in the temperature control of the molds or dies. These molds are mounted on the injection machine in such a way as to arrange the cavities horizontally next to each other. From experience, if they are above each other, the upward flowing hot air heats the higher nests, consequently their temperature will be different from those below, leading to wastage.
In this particular case distance pieces threaded on both sides had to be produced for the chemical industry.
Since there is a hexagonal part between the threaded parts-necessary for screwing in the workpiece-the design of a wedge insert mold was the most suitable. For production technological reasons the wedge inserts 2 were assembled of 4 parts, for in this way the threaded parts and the hexagonal intermediate part were easily producible. Of course the parts are fixed to each other with screws and locator pins, and the jointing planes of the truncated pyramidal inserts have to be carefully grounded. The distributing runners are machined to an S-shape for the purpose of symmetry.
The wedge inserts are formed with a dovetail at the bottom, so when the lifting screws pin 12 fixed between ejector plates 10 and II lift the dovetail plate 16 which lifts the wedge inserts 2, they slide in the dovetail guide, since the V-groove, protruding from the inserts and machined into the mold-frame 20, forces the inserts apart. Sleeves 18 fixed between plates 10 and 11 pull the workpieces out of the hole-forming pins 17 fixed between plates 8 and 9. The workpieces suspended from the residual material in the runner and fall into the box of the machine. Spring 6 returns the ejector bar 5, which pulls along the ejector plates 10 and 11 and pins 12, which in tum pull back not only the dovetail plate 16, but also the wedge inserts 2 into the mold. The machine is then ready for the next cycle.
Notes: The vent groove on pins 23 fixed in the stationary part 21 and on the hole-forming pins 17 must be a minimum of 0.02-004mm deep and 0.5-0.6mm wide. If necessary, grooves of the same size must be made between parts 2, possibly at the highest point of the workpieces.
Advantage: the time of thermosetting by injection is much shorter, than by pressing. The material is heated uniformly during inlet, hence the strength and electrical properties of the pieces are better than those molded.
Disadvantage: certain modification of the injection machine is necessary.
