Page 300 - Injection Molding Advanced Troubleshooting Guide

30.3 Flash Troubleshooting 293

acts against the clamp tonnage to try to force the mold open. When these factors
are considered the simple tonnage per square area calculations can become more
complex.
To estimate basic clamp tonnage, calculate the projected area of the part. The pro-
jected area of the part should then be multiplied by the tonnage factor of the mate-
rial, which is usually expressed as tons/square area. Normally material suppliers
2
will provide a tonnage range; for example, ABS may need 3–4 tons/in whereas
polycarbonate may require 4–5 tons/in . If running a thin wall with high pres-
2
sures or processes that require high cavity pressure to pack out sink, the required
clamp tonnage will be higher. Using an alternate process like gas assist or MuCell
®
will lead to lower required clamp force.
Another item to watch for is the actual machine tonnage and if that level of tonnage
is being maintained. If a machine is experiencing hydraulic leaks in the clamp
circuit it may not be capable of maintaining clamp tonnage. Some machines have
tonnage settings that allow the clamp to decompress after a time period. Examine
the machine for a clamp decompress setting to determine if tonnage is dropping
prematurely.
Keep in mind that damage to the machine components can lead to a lack of clamp
tonnage. Key areas to verify for damage can include the platen, the tie bars, and
the tie bar nuts. A cracked tie bar or tie bar nut will lead to problems with a
machine being able to build tonnage in a specific quadrant. A test to determine if
the mold or the machine is the root cause is to rotate the position of the mold in the
machine, and if the mold is rotated 180° and the flash does not follow along with
the rotation the cause is most likely machine related.

Case Study: Tonnage Drop
In this case the machine was only reaching about 95% of the peak tonnage
but more importantly it was not maintaining the 95% level. The tonnage
dropped during inject forward until it reached a minimum value that was only
about 70% of the specified tonnage of the machine. This dropping tonnage
led to mold flashing. To resolve the issue a leaking clamp valve had to be
replaced. Look for leaking clamp cylinders when diagnosing this type of
problem.
A 1000-ton machine that only functions as a 700-ton one can create havoc
when trying to establish or troubleshoot a process.
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30.3.3.2„ Machine: Clamp Parallelism
Has the molding machine been checked on a regular basis to ensure that the plat-
ens are square to each other? Has tie bar stretch been verified to ensure that equal