Page 266 - Injection Molding Advanced Troubleshooting Guide

258 27 Excessive Cycle Time

See Chapter 14 on mold cooling for advice on optimizing the cooling potential of a
mold.

27.3.1.3„ Molding Process: Slow Fill Speeds
The molding process is made up of a variety of times, one of which is fill time. Fill
time is determined by the fill velocity used on the molding machine. In a normal
Decoupled II molding process the first stage of injection is referred to as fill, in
®
which the mold will be filled to 95–98% full at as fast a speed as possible.
Some molds will require slow fill speeds for optimal product quality; for example,
lenses are a case that often requires a slow fill speed. If there is no product-related
reason that requires a slow fill speed then the part should be filled fast to both
improve cycle time and help maintain consistent melt viscosity.

Do not process around other issues with fill speed, for example:
1. Burns
Burns should not be processed around with fill speed because burning is an
indicator of a lack of venting (see Chapter 7 on venting). Although slowing fill
velocity will in fact usually impact burning, the process window is narrowed
due to a simple tooling fix. Vent the mold and maintain fast fill speeds!
2. Flash
Flash is not driven by velocity because flash is a result of high enough cavity
pressures to overcome the force holding a mold closed. Filling fast does not flash
the mold, but the sudden spike in cavity pressure that occurs if the mold is over-
filled during fill will cause flash. If a machine is transferring well from velocity
control to pressure control and the fill only shot is adequate the mold should not
flash from fast velocities.

27.3.1.4„ Molding Process: Long Cooling Time
Cooling time should be optimized to get the part out of the mold as fast as possible.
Sometimes people will get caught in the “even number” syndrome where 20 sec-
onds of cooling time seems like a better number than 19 seconds. Parts should be
ejected as soon as possible without causing other issues such as warp.
Part out temperature after ejection should be documented during process develop-
ment. Using a thermal imaging camera will provide an accurate picture of any hot
spots and potential cooling problems.
If cooling time is running long due to long screw rotate times consider adjusting
barrel temperature settings to determine if the recovery rate can be improved. Also
review if the shot size is over 75% of the rated barrel shot capacity. Large shot sizes
may limit the cycle time due to long recovery times. The mold may run faster in a
press with a larger barrel capacity. Also watch for screw and barrel wear that can