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In this process, a thermosetting
charge (preform) is loaded into a chamber immediately ahead of the mold
cavity, where it is heated; pressure is then applied to force the
softened polymer to flow into the heated mold where curing occurs. There
are two variants of the process: (a) pot transfer molding, in which the
charge is injected from a "pot" through a vertical sprue channel into
the cavity; and (b) plunger transfer molding, in which the charge is
injected by means of a plunger from a heated well through lateral
channels into the mold cavity. In both cases, scrap is produced each
cycle in the form of the leftover material in the base of the well and
lateral channels, called the cull. In addition, the sprue in pot
transfer is scrap material. Because the polymers are thermosetting, the
scrap cannot be recovered.
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Step #1 - A piece
of uncured rubber is placed into a portion of the mold
called the "pot." The plunger (on the top-most part of the
mold) fits snugly into the "pot." |
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Step #2 - The
mold is closed up and under hydraulic pressure the rubber
is forced through the small hole (the "gate") into the
cavity. The mold is held closed while the rubber cures. |
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Step #3 - The
plunger is raised up and the "transfer pad" material may
be removed and thrown away. |
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Step #4 - Mold is
opened and the part is removed. The flash and the gate may
need to be trimmed. |
Advantages:
- Loading a preform into the pot takesless time than loading
preforms intoeach mold cavity.
- Tool maintenance is generally low, although gates and runners
aresusceptible to normal wear.
- Longer core pins can be used and canbe supported on both ends,
allowingsmaller diameters.
- Because the mold is closed before theprocess begins, delicate
inserts andsections can be molded.
- Higher tensile and flexural strengths areeasier to obtain with
transfer molding.
- Automatic de-gating of the mold's tunnelgates provides cosmetic
advantages.
Disadvantages:
- Molded parts may contain knit lines in back of pins and
inserts.
- The cull and runner system of transfer molding leaves waste
material, but this scrap can be greatly reduced by injection
molding with live sprues and Runnerless Injection Compression (RIC).
- Fiber degradation of orientation occurring in the gate and
runner system reduces the molded part's impact strength.
- Compared to compression molding, high molding pressures are
required for the transfer process, so fewer cavities can be put
into a press of the same tonnage
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