Properties of Molded Phenolic
Phenolic was the first plastic material molded
by Rebling Plastics. Since 1961, Rebling Plastics
has continued to mold this heat resistant thermoset
plastic using both the transfer and injection
molding processes.
The ability of phenolic to maintain its mechanical
and electrical properties at high operating temperatures
is the principal reason for its use today. When
properly molded and post cured, some phenolics
can withstand a 550°F continuous operating
environment. Rebling Plastics utilizes several
industrial ovens for post curing operations.
Prices for phenolic compounds start at $1.00
per pound and are attractively priced compared
to expensive high temperature thermoplastics.
This temperature resistant material is used in
applications such as handles for cookware, car
ashtrays, rocket motor housings, electrical motors
components such as brush holders, and wherever
resistance to high temperatures is required.
Phenolic molding compounds are available in single
and two stage compounds. Single stage compounds
are produced without ammonia and are ideal for
insert molding metal components where metal corrosion
is a concern. Phenolic molding compounds
are available in numerous formulations where both
the filler and the percentage of filler are varied
to alter the properties of the molded part. Mineral
and glass fillers are used to improve heat resistance;
cotton flock fillers are used to improve impact
strength; and glass fillers are used to improve
mechanical strength and stiffness. Specialty fillers
such as carbon fiber and lubricants such as PTFE
are also available.
Phenolic molding compounds are available in limited
colors. Most are available in black and brown,
some also in red and green.
Many grades of phenolic have UL 94V 0 approval
and a UL Relative Thermal Index typically 150°C,
some compounds to 180°C.
While the cost of these compounds is attractive,
processing times are significantly higher than
thermoplastics. Cycle times can range from 30
seconds to several minutes depending on part geometry
and material selection. Unlike thermoplastics
which transform from a viscous liquid and solidify
by cooling down in the mold, phenolic materials
undergo an exothermic chemical reaction or cure.
After molding, the parts must go through a deflashing
operation also adding to the final part cost.
Phenolic materials cannot be reground and reprocessed.
Phenolics have excellent chemical resistance
to organic and halogenated solvents such as carbon
tetrachloride. They are attacked by inorganic
bases.
Design of Molded Phenolic
Parts
Draft and tolerances should be specified on part
drawings indicating maximum and minimum sizes
for sections at each end of the taper. The drawing
should also indicate whether or not tapers are
included in the tolerances and note which end
of a taper has a critically fixed size. Gate locations
should be identified as well as any areas that
are to be free of ejector pin marks.
Maintaining a uniform cross sectional thicknesses
to avoid sink marks by coring is not as critical
for thermoset phenolics, since they do not readily
show sink marks. However, parts with heavy cross
sections require longer cycle times and are subject
to increased warpage.
Rebling Plastics design engineers are prepared
to provide design assistance to insure the successful
outcome of your project.
Secondary operations
Phenolic parts can be bonded or mechanically
assembled. Metal inserts can be molded in or pressed
in after assembly. Decorating is typically limited
to silk screening.
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