Page 119 - Engineering Plastics Handbook

Polyacetals 93

TABLE 5.7 Viscosity and Melt Flow Index
of Polyacetals
Typical grades

General-purpose
High MW
High, medium, low viscosity
Low viscosity for injection molding
Higher viscosity for extrusion and high toughness
High, medium, low flow (MFI)
Wear and friction resistance
UV, light-stabilized, black, TIO 2
Low-warp (20%, 30% mineral)
Low plate-out
Conductive, ESD, EMI/RFI
Laser-markable
Medical grades
Enhanced crystallization

Polyacetals can be classified by viscosity or melt flow index (MFI). See
Table 5.7. Higher viscosities are used for extrusion and blow-molded;
higher-melt-strength, low-MFI terpolymers are thermoformed.
Polyacetal high-flow grades are fast-cycle injection-molded into thin-
wall and complex parts in multicavity molds. High-viscosity grades are
extruded into profiles, rods, slab, shapes, tube, and sheeting. Their
higher melt strength makes these grades useful for thermoforming and
parison forming for blow molding. High-viscosity polyacetal composites
with higher toughness, rigidity, and creep resistance are molded into
heavy-duty gears. Acetal terpolymer resins with higher melt strength
for blow-molded parison forming are produced by Ticona.
®
Formosa Plastics Corp. of Taiwan supplies Formosacon acetal copoly-
®
mers in four principal grades: Formosacon FM 024 extrusion, FM 270
high-flow, FM 450 super-high-flow, and FM 090 standard.

Processes
Polyacetals are typically injection-molded, extruded, blow-molded, com-
pression-molded, rotation-molded, and thermoformed. Primary resin
producers and compounders abound with information on conventional
processing of their polyacetals. Different polyacetal resin, compound, and
composite sources and grades have varying ranges for injection-molding
parameters. Typical ranges for injection-molding temperatures, mold
temperatures, and injection pressures are higher for acetal homopoly-
mers. See Table 5.8.
In this chapter, we look at processes that are especially interesting for
the coming years.