Page 239 - Injection Molding Advanced Troubleshooting Guide
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230 24 Cracking
degrade the plastic, which in turn can lead to problems with cracking or breaking.
Verify that the actual back pressure and screw rotation speeds are equal to the
required process values.
Another point of impact for machine control is second-stage pressure. If the sec-
ond-stage pressure is not controlled the mold may be over-packed, which could
lead to sticking. If a part sticks either during mold opening or during ejection,
damage to the part may occur. Frequently when sticking is occurring cracking
noises can be detected either at mold open or during ejection. Verification that the
machine is providing an actual second-stage pressure that matches the required
value is critical. To effectively troubleshoot it is critical to know what is actually
occurring as the machine sequences through the process.
For further details see Chapter 8 on machine performance.
24.3.4 Cracking Troubleshooting Material Issues
Some potential material-related issues for cracking include:
Moisture content
Contamination
Regrind
Manufacturing issues
Molecular weight
Wrong material
24.3.4.1 Material: Moisture Content
Many materials will undergo hydrolysis when processed with excess moisture con-
tent. During hydrolysis the molecular chain length of the plastic will be broken
down and shortened. As a result of this breakdown of the material the physical
properties will experience a large drop off. This drop in physical properties will
often lead to cracking of the part during ejection or post-mold handling. If cracking
suddenly shows up verify that the material has been adequately dried.
As with many other defects it is critical to follow the recommended drying proce-
dure for all materials. See Chapter 9 on drying for troubleshooting the drying pro-
cess.
It is important to understand that not all materials will develop splay as soon as
they are molded with excess moisture. Polyesters are a good example as they can
be degraded to the point where the part will shatter if dropped after molding but
they may not have splay. Sometimes the first indicator that the polyester is wet is
when a part sticks and the ejector pins push right through the part.
