Page 422 - Injection Molding Advanced Troubleshooting Guide
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42.3 Splay Troubleshooting 421
Lubricants
Cracked mold
Leaking air
Venturi effect
42.3.2.1 Mold: Venting
One of the biggest concerns with tooling issues creating splay is the impact of
venting. Often times by allowing the gas to escape the mold through vents splay
will be eliminated. If wiping the parting lines of the mold seems to improve the
problem for a short period of time look at optimizing venting. All gas that is ge-
nerated in the plastic melt stream as well as the air in the cavity must be allowed
to exit the mold through vents. Many molders underestimate the importance of
proper venting (see Chapter 7 on venting methods and design).
Case Study: Splay Caused by Lack of Venting
The splay was near the start of fill approximately 2–3 in away from the gate
and to one side. The splay did not appear to be coming directly from the
gate. The tool went back and forth to the tool room multiple times to review
the venting; it was eventually vented well beyond the original venting, to the
point where the whole perimeter of the parting line was now being vented.
Feedback from processing was that the issue went away when aluminum
tape was added on the parting line near the gate leading them to believe
there was still an issue with lack of venting. Closer observation of where the
aluminum tape was being placed, which was near the runner, showed this
area was not vented. The mold was pulled from the press and vents were
added to the runner. When the tool was set in the press and restarted the
splay was completely gone! Venting on runners is as important as it is on
the part (see Chapter 7 on venting methods and design).
Case Study: Splay from Screw Boss
In this case the screw boss was 1 in deep and with a shutoff hole in the
center. The splay was occurring downstream from the boss. The mold was
pulled from the press so the tool room could investigate venting options.
The boss had an ejector pin and a sleeve that were not vented. The tool
room added a ring vent to both the pin and the sleeve. When the mold was
run again the splay was gone. If the boss has core detail cut in the solid a
small vent hole can be added in the core detail and vented to the shutoff
(see Chapter 7 on venting).