Page 355 - Injection Molding Advanced Troubleshooting Guide
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36.3 Part Sticking on Ejector Trouble shooting 351
36.3.2.7 Mold: Lack of Cooling
A mold that is lacking in cooling will often lead to problems with ejection. If the
part cannot be adequately cooled it will be difficult to eject from the mold. Typi-
cally, the unfortunate result of lack of cooling is an extended cycle time that allows
ejection of the part without damage or sticking problems.
Cooling must be addressed during the design phase of the mold. Careful evaluation
of water line coverage is important and must be balanced with providing ejection.
It is also important to understand that if the cooling system of the mold is too large
for the available mold temperature control unit it will be impossible to achieve
turbulent flow and cooling will be compromised (this further highlights the im-
portance of the 4M thought process where each of the key items influences the
others).
36.3.3 Part Sticking on Ejector Troubleshooting Machine Issues
Some potential root causes for sticking include:
Machine performance
Ejector plate
Robot handling
36.3.3.1 Machine: Machine Performance
See Chapter 8 on machine performance.
36.3.3.2 Machine: Ejector Plate
If the ejector plate on the machine is not moving in an even fashion it can lead to
the ejector plate of the mold being cocked. If the ejector plate is cocked the ejection
force will not be evenly distributed against the part, which can cause the part to
lock onto the ejector.
Verify that that the knock-out bars are uniform in length. Bars that are different
lengths will lead to a cocked ejector plate. Check the ejector plate on the machine
and check that the bushings are not worn. A wobbling ejector plate is a strong sign
that something is wrong.
Verify that the ejector bars are the proper length. If bars are too short the lifters
may not reach full travel resulting in an undercut that traps the part on the mold.
Proper bar length should always be used when the mold is set up to maximize the
ejection stroke available.
Another factor to check is that the correct number of knock-out bars are being
used. One center knock-out bar on a 55-ton press may work fine but trying to push
