Page 120 - Injection Molding Advanced Troubleshooting Guide

106 12 Cavity Balance

mold is watered the same on each run, which is most easily done by hard plumbing
jumpers and limiting the ins and outs. A water diagram should be documented for
every mold and must be the standard for every mold run. Also beware of swapping
a lower flow capacity thermolator; this could lead to insufficient water flow to pro-
vide adequate cooling.
Variation in cooling of cavities can be detected by checking the surface tempera-
ture with a pyrometer and surface probe. Another means of verifying consistent
cooling is to use a thermal imaging camera to take a picture indicating the tem-
perature of the part after ejection.
Some rules of thumb for cooling are:
1. Start the inlet water at the center of the mold where the most thermal load
occurs from the hot plastic entering through the sprue.
2. Look for a mold surface temperature variation of 10 °F or less. Document a sur-
face temperature map during process development for future troubleshooting.
3. The difference between the water inlet temperature and the outlet temperature
of any circuit should be a maximum of 4 °F [3]. If the difference is larger than
4 °F the circuit should be split into shorter flow lengths or the water flow rate
should be increased with a pump of higher capacity.

„ „12.5„ Venting

Venting in all cavities must be equal to ensure that they experience the same
conditions. If a given cavity is poorly vented compared to other cavities it will tend
to burn and/or short. Also examine the runner venting to ensure that the runners
are all equally vented; all cavities must be vented the same.
For venting specific details see Chapter 7.

„ „12.6„ Clamp Pressure

If the pressure of the clamp is not applied equally across a mold there will be a risk
of creating non-uniform venting. This uneven clamp force can come from a number
of problems including lack of support in a mold, platen deflection, lack of platen
parallelism, or even something as simple as rust buildup on platens. Many of these
issues need to be addressed with robust mold design and adequate machine main-
tenance.