Page 253 - Engineering Plastics Handbook

Polyphenylene Ether (PPE) Blends and Alloys 215

specific grades should always be considered in order to minimize molded-
in stress and avoid resin degradation. Modified PPE resins, like other
engineered thermoplastics, should not be left at elevated temperatures
for prolonged periods without occasional purging.
Key molding parameters of modified PPE include the following:

Drying. Although modified PPE resin has the lowest moisture absorption
of any engineering plastic and may be molded as received in many appli-
cations, it is a good practice, particularly where surface appearance is
critical, to dry modified PPE resin before molding. Modified PPE resins
should not be dried longer than 8 h. Excessive drying may result in loss
of physical properties, color shift, loss of processability, or a combination
of the three.

Melt temperature. Since modified PPE resins cover a wide range of com-
positions and melt viscosities, the manufacturers’recommended melt tem-
peratures should be consulted. The majority of thermoplastic molding
materials are sensitive to prolonged exposure to heat. Long residence
times and excessive melt temperatures should be avoided to minimize
material degradation. It is suggested that melt temperatures be measured
using handheld pyrometers. These measures should be taken on the ther-
moplastic melts after the machine is on cycle.

Mold temperature. The usual range for processing unreinforced modi-
fied PPE grades is from 150 to 200°F (66 to 93°C). Operating molds in
this temperature range can also be used to improve flow, knit-line
strength, and surface finish.

Screw speed. Screw speeds (rpm) may be adjusted to permit screw rota-
tion during the entire cooling cycle without delaying the overall cycle. Low
screw speeds can help reduce glass fiber damage when molding reinforced
grades. Suggested screw speed is dependent on screw diameter. Optimum
linear velocity of screw outside diameter (OD) is about 8 in (202.4 mm) per
second. Thus the rpm would be calculated from the screw diameter ×π
divided into the optimum linear velocity [8 in/s (202.4 mm/s) × 60 s/min].

Backpressure. A backpressure of 50 to 100 psi (0.3 to 0.7 MPa) is gener-
ally suggested to promote a homogeneous melt and helps maintain con-
sistent shot size. Higher backpressures used to improve melt mixing can
result in higher melt temperatures. When molding reinforced grades, low
backpressure will help reduce glass fiber damage during plastication.

Shot size. The shot size should at least in part dictate the size of
machine used. It is generally suggested that the optimum shot be 40 to