| TYPES
OF RUBBER MOLDING
LIQUID
INJECTION MOLDING: (LIM™)
The custom injection
molding of Liquid
Silicone Rubber (LSR) uses much of the same process
and equipment as does injection molding. In both processes,
the mold is
heated and closed before the uncured rubber is injected. Unlike high
consistency rubber (HCR), also known as gum stock, liquid silicone
is just that, a "high viscosity liquid" that comes in two
parts, Part A and Part B.
Both parts are liquid but one part contains the catalyst which is
platinum. Liquid silicone has the consistency of cold molasses, is
naturally translucent and comes in either pail kits (approx. 80 lbs)
or drum kits (approx. 800 lbs) The two parts are mixed and delivered
to the barrel of the injection press via a mixing/metering pumping
system. If desired, pigment
may be added via a pumping system to color the liquid silicone. The
mixed liquid silicone is then injected into the mold much like gum
stock, but the runner,
gate
and sprue
are generally smaller due to its lower viscosity.
Once in the mold, the liquid silicone cures at a much faster rate
than typical high consistency silicone due to its platinum cure system.
As a result of this rapid cure cycle, the part price is frequently
lower than parts made using conventional molding processes such as
Compression,
Transfer
and Injection
Molding.
LIM™ is ideal
for applications where contamination is a problem or where high
volumes
are required. The Medical Industry prefers LIM™
because it is a closed system of molding, meaning, the pail or drum
kits of uncured liquid
silicone are loaded into the pumping system, injected and
cured without human contact. Liquid Injection Silicone Molding
is also ideal because of its ability
to manufacture Silicone Products at a low cost thus ensuring competitiveness
in the Medical Industry.
INJECTION
MOLDING
In the case of
Injection Molding, the rubber is supplied to the injection machines
barrel & nozzle assembly on a continual basis. In this case, the
rubber used is a high consistency silicone
rubber (HCR) known as "gum stock".
The gum stock
is supplied to the injection barrel via a pressurized device called
a "stuffing box".
The injection
press closes the heated mold and applies pressure to keep the mold
closed during the injection and curing cycles. The screw then injects
the rubber through the nozzle into the mold. The rubber travels through
the mold via a system of runners and sprues and finally enters the
mold cavities through a gate
or gates. It
then fills and packs the cavities to form the desired part. The rubber
is heated by the mold and the friction of the screw,
runner,
sprue
and gate.
The rubber then completes its cure cycle and is removed from the mold.
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Injection molding
may be used to mold such materials as Silicone, Viton™, and
Fluorosilicone, Nitrile, Neoprene, and Ethylene
Propylene. Molded rubber products may consist of Medical
Rubber Parts, Viton™ Gaskets, Viton™ Seals or other
custom rubber molded products.
Are you ready for us to assist you? Click
here for a direct
contact to KDL Precision Molding.
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COMPRESSION
MOLDING
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Compression
molding is the simplest form of thermal set molding. Compression
molds vary considerably in size, shape, and complexity and also
contain from one to a very high number of cavities. The compression
molding process is as follows: An uncured rubber preform, which
is often shaped to the approximate size & configuration of
the desired finished part, is placed directly in the mold cavity
prior to mold closure.
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During
the molding process, rubber is compressed (squeezed) between
the
Top and Bottom plates. The uncured pre-form must weigh more than
the finished part in order for air to be driven out of the
mold
cavity during mold closure. If the preform weighs the same or
less than the finished part; voids, air traps or nonfills will
appear on the finished product. The excess rubber will flow into
the overflow or flash groove. Two, three or more mold plates
may
be used depending upon part configuration and complexity. The
rubber is then vulcanized (cured) utilizing both heat and pressure.
The end result is a Rubber Product that will meet your specifications.
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Are
you ready for us to assist you? Click
here for a direct contact
to KDL Precision Molding.
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TRANSFER
MOLDING
Transfer molds
also vary considerably in size, shape and complexity and may contain
from one to a high number of cavities.
In the Transfer
Molding Process the uncured rubber is not placed directly in the
mold
as in compression molding. Instead, it is placed in a Transfer Pot
located above the cavity area. It is then pushed or transferred into
the cavity by a Piston through a runner,
sprue
and gate
system. The rubber is then vulcanized (cured) utilizing heat &
pressure. Transfer molding is similar to Injection
Molding because rubber enters the cavity after the
mold is closed. A rubber part that is transfer molded may be less
expensive than a part that is compression molded.
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