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Thermoset plastics, primarily Epoxy, Phenolic
and DAP, have been molded by Rebling Plastics
since 1961. Rebling Plastics uses both the transfer
and plastic injection molding
processes for thermoset moldings.
During the molding cycle, thermoset plastics
undergo an irreversible chemical reaction or cure
brought on by heat, pressure and time. Once cured,
they cannot be reshaped by melting and reprocessing
as is the case for thermoplastics. Cured thermosets
possess a highly cross-linked chemical structure
enabling them to retain their mechanical and electrical
properties as well as dimensional control over
a wide temperature range. All thermosets molded
by Rebling Plastics undergo a "deflashing"
process after molding. This deflashing operation
eliminates the undesirable excess material that
by design is evident on parting lines and vent
passages.
Epoxy molding compounds are characterized by
their long flow length and the ability to flow
through thin cross-sections, filling minute details.
This flow characteristic is particularly beneficial
for encapsulation as there is less pressure required
on the material during the molding cycle and therefore
less pressure on the internal components of the
device to be encapsulated. The outstanding dielectric
properties of epoxy combined with the low pressure
molding requirements are the reasons epoxies are
frequently specified for encapsulation of electrical
devices, semi-conductor devices, transformers
and other electrical/electronic apparatus. Epoxies
have a recommended shelf life as received from
the material supplier. To insure maximum shelf
life, Rebling Plastics stores epoxy molding compounds
in refrigerated storage units. Epoxies are available
formulated with various fillers including glass
and mineral at various concentrations.
Phenolic, the oldest
polymer, is known for its excellent high temperature
resistance. When properly post cured, some heat
resistant phenolics can withstand up to a 550°F
operating environment. Phenolic molded parts exhibit
a glossy finish and a high surface hardness. As
with other plastic materials, phenolics can be
compounded with various fillers to alter the materials
properties. These include wood flour, minerals
and glass fiber reinforcement. Phenolics are used
in applications such as rocket motors, electrical
motor brush holders, handles for cook wear and
wherever plastics need to operate at elevated
temperatures.
Diallyl Phthalate, commonly referred to as DAP,
exhibits outstanding electrical insulating properties
particularly at high humidity and high temperatures
environments. While not as heat resistant as phenolic,
DAP can operate continuously at temperatures as
high as 450°F. DAP is typically available
in flame retardant grades including ratings of
UL 94V-O in cross sections as small as .062 inches.
End uses include coil bobbins, connectors, and
circuit breakers.
Rebling Plastics has a significant amount of
insert molding experience employing epoxy, phenolic
and DAP thermoset resins. Rebling Plastics can
provide a turnkey service including design and
procurement of mold(s), inserts and molding compounds,
as well as post curing, deflashing and secondary
operations. Rebling Plastics also has preforming
equipment to compress the molding compounds into
solid shapes suitable for dielectric preheating.
Mold temperatures for thermosets typically are
in excess of 300°F, so inserts must be able
to withstand this molding temperature and the
increase in temperature caused by the exothermic
reaction that takes place during the cure cycle.
Electrical devices incorporating solder connections
and tin plated metal components are examples of
inserts that could be damaged during the molding
cycle if temperatures are too high.
Rebling Plastics engineers are available for
consultation and recommendations on thermoset
materials, processes and critical molding requirements.
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