Page 121 - Engineering Plastics Handbook

Polyacetals 95

decorative effects. Cellular products include housings, bezels, shrouds,
seat belt buckles, tennis racket handles, consumer products such as toys
and kitchenwares, profiles, and stock shapes—rods, tubes, sheet, plates,
and cones. Cellular polyacetal parts produced with foam concentrates can
have up to 20% lower weight than solid parts; and for consumer applica-
tions the sink marks are eliminated.
Polyacetals are overmolded, insert-molded, and coextruded by using new
adhesion technology. Overmolding, a multicomponent molding process,
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extends product performance properties and applications. Celcon and
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Hostaform acetal copolymer two-step overmolding was codeveloped by
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Ticona and Kraiburg TPE, using adhesion modified Thermolast K styrene
block copolymers (SBCs) such as the styrene-ethylene-butylene-styrene
(SEBS). It can be used for nonslip, energy-absorbing, sound-dampening;
molded seals; and complex molded-in-place gaskets for fluid handling sys-
tems, according to Celanese [21]. Advantages of overmolding and insert
molding polyacetals and other engineering thermoplastics are that (1) this
reduces assembly steps, assembly time, and assembly costs; (2) there is
design versatility such as combining surface appearance (e.g., textured
surfaces) with functionality; (3) this produces multifunctional products;
(4) it produces integrated parts and benefits of parts unitizing such as
seamless, leakproof construction; (5) it allows in-mold decorating (IMD) and
in-mold assembly (IMA); and (6) it produces soft-touch products.
Thermoforming acetal terpolymers, which have high melt strength
and low MFI, provide the following advantages: (1) shorter lead times
for tooling and processing typically than for injection molding; (2) lower
cost and faster production of prototypes; (3) greater cost efficiency for
lower-volume housings and other enclosures than injection molding;
and (4) textured, colored, and printed surfaces for decorated thermo-
formed parts and in-mold decorating films.
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Delrin FS (Forming Solutions), the first polyacetal for thermoforming,
was jointly announced by DuPont Engineering Polymers and Ensinger/
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PennFibre Plastics in June 2003 [8]. Delrin FS was run on Adolph Illig
Maschinenbau Model UA-100 4G sheet processing machines, using 3-mm
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sheet on a Porsche tool 24 in long, 12 in wide, and 6 in deep. Delrin FS
is supplied as coil and sheet. The transforming temperature is 215 to
235°C (419 to 455°F). The lower and upper heat ranges are set to 500 and
700°C (932 and 1292°F). Heat cycle for 3-mm sheet is 70 s; mold temper-
ature is 100°C (212°F). There is homogeneous heating of the sheet, with
little heat sag; and overall shrinkage is 1.6 to 2.3% according to the com-
pany [8]. The acetal homopolymer has uniform processing parameters, and
it handles topographical decorating and printing and sublimation print-
ing. The process targets equipment and decorated housings, panels,
trays, handles, conveyor chutes, medical/dental equipment, biochemi-
cal research products, and other products that the polyacetal properties