Page 425 - Injection Molding Advanced Troubleshooting Guide

424 42 Splay

42.3.2.5„ Mold: Sharp Corners
Another issue with tooling that will lead to splay is sharp corners. Sometimes as
the material flows across a sharp area splay will appear downstream. This can
frequently be improved by softening the sharp corner and adding some radius.

42.3.2.6„ Mold: Gate Flaking
Improper sub gate or cashew gate design can contribute to flow lines or defects
with a splay-like appearance; this is due to plastic flaking from the gates as they
separate from the part. If this is the suspected root cause you may be able to detect
flakes of plastic on the mold near the gate. A useful trick to help diagnose this
problem is to manually blow air through the gate between cycles. See Chapter 4 on
gate and runner design.
If flaking is the root cause magnification of the defect will show that the starting
point of the splay has a visible speck.

42.3.2.7„ Mold: Lubricants
Sometimes splay will come from lubricants used on the mold. There are cases
where grease from lifters or ejector pins can bleed onto the core surface. This
grease can be pulled onto the surface leaving behind a splay streak. Also lubri-
cants and metal protectants can be trapped in inserts and bleed out onto the mold
surface, which much like grease can be pulled onto the surface of the mold result-
ing in splay. If the splay shows up immediately after the mold has come back from
the mold shop you may have an issue with too much lubricant resulting in splay.
Avoid over-lubricating molds, because this is not a case where “if a little is good,
a lot is better”. If you have a mold that is over-lubricated you may need to wipe
the excess lubricant out, and may even have to disassemble the mold to solve the
problem.

42.3.2.8„ Mold: Cracked Mold
Another tooling issue that can cause splay is a crack in the mold. If a mold cracks
to a water line the water can wick to the surface of the mold resulting in splay.
Cracks in the mold can be obvious and with water drips showing on the mold sur-
face. However, there will be cases where the water only shows up after the mold
has been closed and clamped up, so when checking for suspected cracks always
close the mold to see if the clamp pressure opens up the crack. A short-term fix for
a cracked mold is to use a reverse-flow thermolator; but this will affect the cooling
capability of the mold. The ultimate fix will be to repair the crack. Be aware during
tool design that sharp radii will be more prone to crack.