Page 317 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.109

screw length-to-diameter (L/D) ratio of at least 20:1, and preferably 24:1 to 32:1, is rec-
ommended for most TPEs. The extruder should be capable of operating in the melt tem-
perature range of 170 to 250°C. A variety of screw designs (polyolefin type, flighted
barrier, and pin mixing) have been used. Screw cooling is generally not required. Typical
polyolefin-type metering screws with compression ratios in the range of 2:1 to 4:1 are nor-
mally used, although other thermoplastics screws have also been used. Screen packs of 20
to 60 mesh are used to provide a clean melt stream with even flow. Finer mesh screens can
be used depending on the viscosity of the particular TPE and its sensitivity to the higher
temperatures caused by the finer screen. The polymer melt temperature in the extrusion
process should be about 30 to 70°C above the melting point of the TPE.
Thermoplastic materials generally exhibit a die swell on exiting from an extrusion die.
TPEs can give die swells significantly lower than those of typical rigid thermoplastic poly-
mers.
TPE sheet can be produced (e.g., from TPVs) by extrusion processing. Sheet extrusion
of TPEs can produce thicknesses from about 0.2 to 4 mm and widths of 2 m or more. Very
thin TPE sheeting can be produced by the blown film process used for many thermoplas-
tics. This technique allows hard TPE sheet to be made down to a thickness as low as
0.05 mm, and even soft TPEs can yield sheets 0.2 mm thick.
4.6.4.3 Injection Molding. Injection molding is the most widely used process for fabri-
cating TPE parts. Short molding cycles and elimination of scrap by regrinding quite often
outweigh the generally higher material cost of a TPE compared to that of a thermoset.
Sprues and runners from injection molding can be recycled.
TPEs can be molded in the same type of equipment used for injection molding thermo-
plastics such as PP and PE. Good part definition and integrity are obtained by adequate
mold packing, which gives strong weld lines and minimizes shrinkage. This is achieved by
mold designs with a balanced layout. Runners should be as short as feasible, and small
gates are recommended to provide high shear rates for uniformity of the material as it en-
ters the cavity. Operating conditions should be selected to give melt temperatures in the
range of 20 to 50°C above the melting point of the TPE so as to allow adequate mold pack-
ing.
4.6.4.4 Blow Molding. TPEs can be blow molded (Fig. 4.40) to produce hollow shapes
in the same manner and equipment used for rigid thermoplastics, either via injection or ex-
trusion blow molding techniques. Extrusion blow molding is the simpler process in which
a hollow molten parison is extruded vertically downward into a mold cavity. As the mold
is clamped around the parison, blowing takes place, forcing the molten TPE against the
water-cooled mold. The part is then cooled to give it sufficient structural integrity for re-
moval from the mold, normally by gravity. Injection blow molding is similar except that
very close control of the parison dimensions is obtained by injection molding the parison.
While still hot, the molded parison is transferred into a blow mold for final part shaping.
The need for two sets of tooling makes this process costly but allows for precise wall-
thickness control. Injection blow molding is very capital intensive and thus requires large
production volumes for favorable economics.
Blow molding cannot be used to produce hollow rubber parts from conventional ther-
moset rubbers. These must be injection molded over a manually removable collapsible
core to form thin-walled rubber shapes such as boots, bellows, and covers. Blow molding
is uniquely suited to TPEs as a material for hollow, thin-walled rubber articles.
4.6.4.5 Other Fabrication Methods. Another thermoplastics processing technique that
is suited to some TPEs, but not to thermoset rubbers, is thermoforming. A sheet of TPE is
heated to 10 to 40°C above the melting temperature of the hard crystalline phase (or glass

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