Page 175 - Engineering Plastics Handbook

148 Engineering Plastics

degradation or the breakage of reinforced fillers. It is important to keep
the reground parts clean and to avoid contamination from other materi-
als. Actual reground usage level should be determined for each specific
application.

Extrusion
PBT can be extruded into a number of applications such a film, sheet, pipe,
and tube. It can be achieved with commercial extruders whose screw has
three distinct features, such as feed, transition, and metering zones, with
length-to-diameter ratios (L/D) of at least 24:1. The feed zone at the rear
end of the screw acts as a conveyor of resins from the hopper area through
the heated barrel. The function of the transition zone is to compress the
melt, and its length should be three to four turns between the feed and the
metering section. The metering zone is located at the exit end of the screw
and has a constant flight depth. Three zone screws should have a flight-
depth ratio between 2:1 and 4:1, feed to metering section. The barrel and
the screw should be provided with wear protection to prevent corrosion and
abrasion. When glass-reinforced grades are to be processed, it is recom-
mended to use special wear protection comprising a bimetallic barrel lined
with a wear-resistant coating and a screw with hardened flights. In terms
of the exact flight depths in the feed and metering sections, and the length
of the transition section, the size and screw geometry should be selected
to suit the part size and applications. For extrusion applications, due to
the need of melt strength, a PBT with a high molecular weight is normally
used. Depending on grades and application parts, the melt temperature
of PBT should be between 230 and 260°C.

Injection-molding machinery and equipment
PBT resins can be processed in all standard screw injection-molding
machines. Both toggle and hydraulic clamping machines fitted with either
electrical or hydraulic drives can be used. In general, the classical three-
zone screw injection-molding machines can be employed with adequate
quality for the processing of PBT resins. A single-flighted, reduced flight
depth, three-zone screw having a length-to-diameter ratio of 18 to 22 is rec-
ommended (Fig. 7.8). For the same screw diameter with reduced flight,
screws ensure a shorter residence time of the melt in the cylinder and a
more homogeneous temperature distribution in the melt. If demands on
throughput and melt homogeneity are high, specific screw designs will
afford significant advantages. Melt homogeneity and temperature uni-
formity may also be improved by using higher length-to-diameter screws
and/or additional shear and mixing sections. The screw diameter is selected
on the basis of the shot volume that is required to completely fill the mold