Page 115 - Engineering Plastics Handbook

Polyacetals 89

bosses should be generous (not too generous) to avoid internal stress con-
centrations during cooling in the mold. Wall thickness can be from about
0.5 mm (0.02 in) minimum up to 5 mm (0.2 in). Polyacetal resins freeze
rapidly during mold cooling, and the polymers are stiff and strong,
allowing thin walls. The cost advantages of thin walls with polyacetal
resins are faster cycles and less resin consumption than for polymers
that require thicker walls. Thin-wall molding, with judicious process-
ing, can avoid or at least minimize voids and sink marks. High DC
causes greater mold shrinkage during cooling than other engineering
thermoplastics with less DC; and mold shrinkage, based on millimeters
per millimeter (mm/mm) or inches per inch (in/in) or percent of melt
volume, is not exactly the same in thick walls and thin. Uneven mold
shrinkage will cause internal stresses during molding, which leads to
warpage and product malfunction or failure. Mold shrinkage is very
complex, primarily due to the cooling rate of the melt in the mold cavity.
Thicker walls cool slower than thin walls, and thicker walls have greater
mold shrinkage than do thin walls. Mold shrinkage is affected by pack-
ing, the compound recipe, fiber reinforcements, fillers, and additives.
Postmolding shrinkage can continue due to the ambient environment,
especially elevated temperature and humidity. The opportunity to thin-
wall mold polyacetal resins brings with it a critical requirement to strive
for uniform wall thickness in the part, or design gradual tapering from
thick wall to thin wall, and avoid abrupt transition between wall thick-
ness differences. The draft angle for any thermoplastic significantly
affects ease of demolding. For polyacetal resins, a minimum draft angle
of 0.5°, preferably 1°, has been suggested.

Properties
Polyacetals are distinguished from other engineering thermoplastics
by the combined properties listed in Table 5.3.
Typical resins, compounds, and composites are listed in Table 5.4.
ASTM standard specifications are ASTM D-4181 and DIS 99881,2,
“Standard Specification for Acetals (POM) Molding and Extrusion
Materials.” DIS is Draft International Standard, which provides a more
cost-effective electronic system; D-6100 is “Acetal Stock” and D-1855-00
is “Standard Specification for Polyoxymethylene (Acetal) for Medical
Applications—Medical Devices, Instrumentation or Components Thereof,”
developed by ASTM Subcommittee F04.11.
Advances in compounding formulations and reinforcing technologies
narrow the differences in properties between acetal homopolymers and
copolymers. Toughness, for example, is increased by adding an elas-
tomer. Glass fiber reinforcement increases polyacetal tensile strength,
modulus, and hardness, and reduces CTE, creep resistance, and mold
shrinkage.