Page 290 - Injection Molding Advanced Troubleshooting Guide
P. 290
30.3 Flash Troubleshooting 283
Figure 30.2 Forces act both parallel and perpendicular to clamp force
30.3.1 Flash Troubleshooting Molding Process Issues
Possible molding process concerns are:
High second-stage pressure
Heavy fill only weight
Clamp tonnage
Melt temperature
Velocity to pressure (V to P) transfer
30.3.1.1 Molding Process: High Second-Stage Pressure
Second-stage set pressure is the pressure used to pack out the part and compen-
sate for the shrinkage that occurs during cooling. If the pressure in the cavity
overcomes the force being applied by the clamp unit flash will occur.
There will be cases where high second-stage pressure is used to either pack out a
sink or try to make a part dimensionally larger. If a mold was cut with the wrong
shrink factor, raising pressure can sometimes help to make the part larger; how-
ever, if the pressure becomes too high a larger tonnage machine may be required.
Cavity pressure will act across the projected area of the mold but will not be uni-
form across the whole mold. Using average cavity pressure can give a prediction of
the amount of force that is acting on the projected area; if this force becomes higher
than the clamp force the clamp will be forced open slightly allowing the plastic
to leak.
