Page 426 - Injection Molding Advanced Troubleshooting Guide
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42.3 Splay Troubleshooting 425
42.3.2.9 Mold: Leaking Air
If the mold is relying on air poppets for ejection, splay may appear from the poppet
leaking as the material is injected into the mold. Poppets should only be energized
when the part is to be ejected from the mold. Poppets that are leaking air will intro-
duce air into the melt stream potentially forming splay.
If running a gas-assisted part, a leaking gas pin can allow nitrogen to enter the
melt stream resulting in splay. Also if the gas is turned on too early splay could be
evidenced in the part. Running gas assist on parts creates another level of com-
plexity and must be considered during the troubleshooting process.
42.3.2.10 Mold: Venturi Effect
There will be cases where the plastic flowing in the mold will pass over a rib rather
than filling into the rib. The plastic may pull some of the air trapped in the rib up
into the melt stream producing splay on the part. The key to avoiding this issue is
to make sure that ribs are vented whenever possible. Fill velocity may also impact
this issue but does not address the root cause and will narrow the process window.
Case Study: Splay from Ribs at End of Fill
This case was a polypropylene part with ribs approximately 2–3 in from the
end of fill; there was a splay issue near the ribs that could not be eliminated.
The mold was worked on to add radii to sharp corners at the ribs as well as
adding ejector pinsin the ribs to help vent the gases out, but these changes
did not impact the splay problem. To take all possible root causes off the list
the decision was made to add perimeter venting to the whole end of the
part. A vent track was added with a 0.060-in land and the vent was ground
open to 0.001 in. When the mold was rerun the splay was gone; however, it
came back a couple of days later. When the mold was reviewed again it was
observed that the vents were already closed off. The mold had very little
parting line bearing surface with too much relief, which allowed the parting
line to hob in and close off the vents. When the vents were opened the splay
again disappeared!
42.3.3 Machine Issues
The root cause of splay can sometimes come from issues with the machine itself.
These issues tend to be rarer but can be the case where you try “everything” but
cannot find a solution. Below are some potential machine-related causes to splay:
Temperature control
Screw design
