Page 301 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.93

There are certain disadvantages with the use of such multiple-part molds. Much time
can be lost when one removes the mold, and the time required to heat the mold can be con-
siderable. Also, the separation of all three parts of the mold with the extraction of the part
can be difficult. On the other hand, transfer molding simplifies the loading of molds, in
comparison to compression molding.
Injection Molding. The injection molding process has become a mainstay of rubber
part manufacture. It is generally very rapid in comparison to the other types of molding
processes. Schematics for two types of injection molding processes are given by Fig. 4.34.
Injection molding in the thermoset rubber industry is different from what it is in the ther-
moplastic plastics industry. The rubber is only heated to a processing temperature for flow,
but the mold, rather than being cooler than the polymer, is much hotter, at a vulcanization
temperature. The ram, or plunger, injection molding process is a descendent of transfer
molding. In the modern machine, rubber is plasticized and heated by a screw in a separate
plastication cylinder (not shown in the schematic) and transported to the injection cylinder
(shown in the schematic) and then transported to the injection cylinder through a nonre-
turn valve (e.g., in the throat), with the ram in the retracted position. When the required
amount has been accumulated in the cylinder before the ram, the feed screw stops, and the
rubber is injected by means of an injection plunger or ram.
In reciprocating-screw injection molding, the rubber compound is heated by the re-
tractable (reciprocating) screw, in a position where its front end is near the nozzle. Then,
the screw retracts as it “winds” itself (“unscrews”) out, away from the nozzle and the
warm plasticated rubber, which is now up against the nozzle. The screw is then moved for-
ward (toward the mold) and, in so doing, it injects the rubber into the mold.
The higher the temperature of the rubber in the barrel before it is injected, the faster the
rubber can be injected and the faster it will be heated up to vulcanization temperature in
the hot mold. However, if the rubber stock temperature is too high before injection, the
rubber might begin to cure prematurely (scorch). Care must be taken to avoid this.

4.5.6 Example Recipes of Selected Rubber Compounds

Table 4.14 gives example recipes of compounds of various types of elastomer. The recipes
should not be used as a formulary. They are just to give a flavor of the types of compounds
developed by rubber compounders. Since many end-use products have different specifica-
tions, and different rubber-product manufacturing facilities have different types of equip-
ment, compounds must be developed specifically with respect to both the requirement for
the end-use applications and the manufacturing equipment that is available.

4.6 THERMOPLASTIC ELASTOMERS

A thermoplastic elastomer (TPE) is a rubbery material with properties and functional per-
formance very similar to those of a conventional thermoset rubber, yet it can be fabricated
in the molten state as a thermoplastic. ASTM D 1566 defines TPEs as “a diverse family of
rubber-like materials that, unlike conventional vulcanized rubbers, can be processed and
recycled like thermoplastic materials.” Many TPEs meet the standard ASTM definition of
a rubber, since they recover quickly and forcibly from large deformations, they can be
elongated by more than 100 percent, their tension set is less than 50 percent, and they are
sometimes insoluble in boiling organic solvents. Figure 4.35 indicates hardness ranges for
various types of TPEs and conventional elastomers.

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