Page 302 - Injection Molding Advanced Troubleshooting Guide

30.3 Flash Troubleshooting 295

Comparison of the cavity pressure acting on the square area of a core can be made
with the square area of the cylinder and the applied core pressure. See the calcula-
tion in Figure 30.7 for specifics on calculating required pressure and cylinder size.

30.3.3.4„ Machine: Mold Size
The obvious problem with mold size is trying to run a mold that is too large for the
given machine size. There are cases where even though the mold fits between the
tie bars of the press the projected area and cavity pressure required are too high
for the clamp to keep the mold closed. It is important to understand that every
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material has a range of required clamp force expressed in tons/in . This range is
based on the normal cavity pressures that a material will need to pack out a part.
Understand, however, that these are average values that can be affected by things
like wall stock, number of gates, flow distance, and dimensional requirements.
The opposite end of the spectrum is when a mold is too small for a given machine
size. Normal recommendations are that a mold should cover approximately 2/3 of
the platen surface area. If a mold smaller than the recommended minimum is run
in a machine, platen wrap can occur. Platen wrap occurs when the oversized platen
is literally distorting around the small mold. In a case where platen wrap occurs
the clamp force applied near the center of the mold will be reduced, which in turn
can lead to flash in the center of the mold.
Always verify what the minimum mold size is for every machine in the shop. This
issue not only leads to flash but can also cause mold damage and could also cause
machine damage. If a mold is to be run in an oversized press use of outrigger
support pillars will eliminate mold damage.

30.3.3.5„ Machine: Toggle Wear

Any mechanical item will experience wear over time. Clamp components on mold-
ing machines are no exception. As the toggle links and link pins become worn over
time it will become more difficult for a machine to achieve a solid clamp lockup. In
some cases the toggle becomes so worn that it is visibly sloppy as the clamp locks
up. This slop can lead to non-uniform dispersion of clamp forces.

Wear on toggle components should be evaluated over time. A clamp that is running
slower than normal or making unusual noises may indicate excessive wear. Clamp
rebuilds are time-consuming and expensive but pay off in the long run by reducing
mold damage, yielding faster cycle times, and producing better quality parts.