Plastic flow undergoes shear effects during filling. Shear effects are relative to flow rate and pressure drop. With too high shear stress or shear rate, degradation and cracking may happen to the part. The cross section area of the gate is often the smallest over the part, and thus the shear around this area is the highest.
Shear rate at the gate is relative to gate dimension and flow rate. It can be obtained using Moldex3D. Meanwhile, most of material manufacturers provide shear rate limits on the plastics. As shear rate is under the limits, product defects and problems can be reduced.
The below table is the Maximum shear stress limits applying to products and Maximum shear rate limits applying to gates.
Maximum of shear stress and shear rate
|
Maximum of Shear Stress, (MPa) |
Maximum of Shear Rate, (1/sec) |
|---|---|---|
PP |
0.25 |
100,000 |
HDPE |
0.08 |
40,000 |
LDPE |
0.08 |
40,000 |
EVA |
0.30 |
30,000 |
PS |
0.25 |
40,000 |
HIPS |
0.30 |
40,000 |
SAN |
0.30 |
40,000 |
ABS |
0.30 |
50,000 |
ABS (Platting Grade) |
0.30 |
30,000 |
PMMA |
0.40 |
40,000 |
PPS |
0.50 |
50,000 |
PA6 |
0.50 |
60,000 |
PA66 |
0.50 |
60,000 |
PA612 |
0.50 |
60,000 |
PA12 |
0.50 |
60,000 |
POM |
0.45 |
40,000 |
PET |
0.50 |
50,000 |
PBT |
0.40 |
50,000 |
PPO |
0.50 |
35,000 |
PC |
0.50 |
40,000 |
PC/ABS |
0.40 |
40,000 |
TPU |
0.40 |
20,000 |
PC/TPU |
0.40 |
10,000 |
PUR |
0.25 |
40,000 |
s-PVT |
0.30 |
20,000 |
r-PVT |
0.30 |
20,000 |
PSF |
0.50 |
50,000 |
PES |
0.50 |
50,000 |
LCP |
0.50 |
60,000 |
