Page 120 - Engineering Plastics Handbook

94 Engineering Plastics

TABLE 5.8 Injection-Molding Parameters of Polyacetals
Homopolymer Copolymer
Injection-molding temperatures 195–245°C 165–195°C
(barrel) (383–473°F) (329–383°F)
Mold temperature 60–120°C 70–90°C
(140–248°F) (158–194°F)
Injection pressure 70–140 MPa 50–115 MPa
(10,185–20,370 psi) (7275–16,730 psi)

®
Rotational molding Celcon acetal copolymer brings the copolymer
advantages to rotomolded shapes, including extremely low gasoline and
alcohol permeation. Low permeation is essential to meet evaporative emis-
sions requirements such as the Environmental Protection Agency (EPA)
and California Air Resources Board (CARB) zero-emissions vehicles (ZEVs)
and near ZEV. Acetal copolymer has thermal stability to 105°C (221°F),
dimensional stability, strength, and stiffness and long-term resistance to
chemicals, fatigue, and abrasion. The rotational molding technology is
aimed at fuel delivery systems for small engines such as outboard (OB),
recreational vehicle (RV), golf cart, and power motors. Research is ongo-
ing on rotomolding acetal copolymer/HDPE and other multilayer systems.
These developments at Ticona open two new avenues for polyacetals:
(1) new rotational molding applications and (2) expansion of the use of
polyacetal resins for small engine fuel delivery systems.
Injection molding and extruding of foam (cellular) polyacetal resins
requires special attention [19]. Processing parameters, machinery, and
equipment recommendations are critical because deviation from recom-
mended processing conditions and the use of blowing agents will result in
rejects. Processing and equipment considerations include the melt tem-
perature, shot size, fill rate, injection pressure, backpressure, holding
pressure, cushion, venting, cooling rate, closed mold time, screw speed,
mold temperature, gate type and locations and size, and runner type and
size [19]. Ashutoff nozzle is necessary. Other factors include letdown ratio
and percent, blowing agent level, and design factors such as percent of den-
sity reduction, wall thickness, and weld lines (if any). By using RTP
Company FCX 27301 foam concentrate for polyacetal resins, the foam
activation temperature is 149 to 177°C (300 to 350°F) [19]. By compari-
son, the foam concentrate activation temperature generally is 238 to 252°C
(460 to 485°F) for other engineering thermoplastics, polycarbonate and
polycarbonate alloys, PBT, PET, and polyamides (except polyamide 11 and
2
12) [19]. Injection-molding clamp force is about 1 to 4 tons/in [19]. The sur-
face of cellular parts has swirls and flow lines, but this attribute can be
turned into an advantage for consumer products by allowing unique