Page 173 - Engineering Plastics Handbook

146 Engineering Plastics

TABLE 7.8 Recommended Injection-Molding Conditions for Various PBT and Blends
Division Unit Unreinforced Reinforced PBT/PC blend
Cylinder set
Rear zone °C 225–240 230–245 230–250
Middle zone °C 230–245 235–250 240–260
Front zone °C 235–250 240–260 250–270
Nozzle temperature °C 235–255 240–260 250–270
Melt temperature °C 235–255 240–260 250–270
Mold temperature °C 40–80 50–100 40–80
Injection pressure
First stage MPa 40–70 60–110 50–80
Second stage MPa 35–55 40–85 45–65
Backpressure MPa 0–4 0–4 0–4
Screw speed rpm 60–100 40–80 70–100
Drying conditions °C, h 120, 4 120, 4 100, 4
SOURCE: Lupox PBT brochure and data sheets [20].

Flame-retardant grades are more sensitive to thermal degradation and
should not be processed at temperatures over 260°C.
For economic reasons, fast cycle time and mold temperatures with the
lowest possible values are preferred. If the mold temperature is too low,
there will be a danger of an inadequate surface finish and part perform-
ance, which can be related to insufficient crystallization during the cool-
ing time. Crystallinity should be controlled by the cooling rate and
duration, which is in turn controlled by mold temperature. With rein-
forced PBT materials, a good surface finish can only be achieved with
high mold temperature. If there is insufficient crystallization, part dimen-
sion is also affected by postshrinkage (see Fig. 7.7). Lower postcrystal-
lization and postshrinkage can be obtained as the mold temperature is
increased. For sufficient crystallinity of PBT, mold temperatures usually
are set between 60 and 100°C.
The injection speed and pressure depend on the types of materials and
the nature of the molding part. High injection speed is preferred to avoid
premature freezing due to crystallization in the mold, particularly when
parts are thin and complicated. High injection speeds and mold tempera-
tures generally yield a better surface appearance, especially for reinforced
PBT grades. In many cases, surface defects such as flow marks, jetting,
streaks, and weld lines are mainly related to injection speed. Therefore,
the optimum velocity profile of the molding process should be determined
through optimization for specific parts. It is important to switch over to
holding pressure stage at the right moment, to prevent overpacking in the
molding part. Holding pressure serves to offset the volume shrinkage
when the molded part cools in the mold cavity. In general, 40 to 80% of
the maximum injection pressure is recommended as the proper level of