Page 466 - Injection Molding Advanced Troubleshooting Guide
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466 46 Warp
part is sticking when ejected. If sticking is detected the mold should be repaired
rather than trying to process around the problem.
For more details refer to Chapters 35 and 36 on sticking.
Case Study: Warping Parts from Sticking
In this case a PC/ABS was being molded in a part that had to be consistently
flat. If the part had any warp it would cause an assembly and functional con-
cern at the customer. Many experiments were conducted focusing on process
conditions and packaging, all without reliably resolving the problem. During this
time the concerns at the customer continued to amplify.
During a mold sampling run the mold was stopped before the part was ejected.
Examination of the part relative to the ejector half of the mold showed that the
corners of the part were lifted away from the steel. A part was molded with
mold release on the cover side of the mold and the mold was again stopped
prior to ejection. With the mold release the part was tight to the ejector half
steel. Both sample parts were measured for warp and the part molded with the
release agent was perfect while the other part was out of specifi cation.
Closer examination of the cavity showed that there was a slightly rolled parting
line that was causing the part to stick slightly in the cavity. This rolled edge was
cleaned up with a polishing stone and samples were molded. The resulting
samples showed no warp. Checking the parting lines for burrs and rolled edges
was added to the preventative maintenance routine for this mold, and the warp
problem never reoccurred.
Following the STOP method may have led to faster resolution of this problem.
4M troubleshooting would have helped to ensure that the mold was evaluated
for problems rather than trying to rely on the process to solve the problem.
46.3.2.2 Mold: Cooling
Uniform temperature is critical to avoid warp on molded parts. The mold acts as a
heat exchanger and removes the heat energy from the molten plastic. The mold
should be designed with optimized cooling; unfortunately, cooling is often a
tradeoff for ejection and other tooling actions.
See Chapter 14 on mold cooling for more details.
46.3.2.3 Mold: Cavity Imbalance
On a multicavity mold it is critical that all cavities experience the same process
conditions for the plastic. If a multicavity mold has imbalance between the cavities
the process conditions within the cavities will not be the same. The resulting dif-
ferences can impact many process variables including:
Cavity fill rate
Cavity pressure
