Page 325 - Engineering Plastics Handbook

Polyamide-imide (PAI) 283

10 8

Torlon 4203L (645°F, 340°C)
7
10
Apparent viscosity, P 10 6 ABS (390°F, 199°C)

5
10
Polycarbonate (535°F, 279°C)

10 −1 1 10 10 2 10 3 10 4
Shear rate, s −1
Figure 12.22 Apparent viscosity versus shear rate.

Polyamide-imides show typical shear thinning behavior. Due to the
change in viscosity at different shear rates, it is important to injection-
mold at proper speed and pressure. Figure 12.22 shows the viscos-
ity–shear rate relationship between Torlon and other common polymers.
Thin wall sections and parts may require high injection speeds; how-
ever, care must be taken to avoid blistering at the gate, color change or
degradation, splay or surface delamination, and gas burning at the knit
lines or vents. High injection pressures are required to fill the entire mold;
then during the hold period the pressure should drop from a high set-
ting of 6 to 8 kpsi (41 to 55 MPa) for several seconds, to 3 to 5 kpsi (21
to 34 MPa) for the duration of the hold pressure sequence. This is
intended to minimize or eliminate internal porosity or sink. Moderate
backpressure, approximately 1 kpsi (6.9 MPa), and lower screw recov-
ery speeds (50 to 100 rpm) are recommended in order to match the mate-
rial’s rheological behavior. The recommended barrel temperatures are
shown in Table 12.6.
The total cycle time should be as short as possible, to reduce residence
time in the barrel and mold. Excessive residence time will cause the poly-
mer to increase in viscosity due to molecular weight growth, reducing
flow. However, if the cycle time is too short, sprue breakage, sticking,
warpage of the part, foaming, or blistering in thick sections may occur.
Automatic operation is recommended to maintain a consistent cycle
time. The recommended range for the surface temperature of the mold
is 163 to 218°C. The mold should be insulated from the platen to mini-
mize heat loss to the molding press.