Page 312 - Injection Molding Advanced Troubleshooting Guide
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306 31 Flow Lines
often improve this problem. Remember that on machines with both a pack and
hold volume both may impact the process.
3. Worn or improperly tuned hydraulic system
Pressure can only build as fast as the machine can supply flow, so if the pump
output is lacking the machine will struggle to achieve a quick response.
For more on velocity to pressure response see Chapter 8 on machine perfor-
mance.
31.3.3.2 Machine: Machine Performance
If the machine cannot achieve the set points of a process the result is frequently a
part defect. Evaluate the machine to determine that it is achieving the documented
process.
For details see Chapter 8 on machine performance.
31.3.4 Flow Lines Troubleshooting Material Issues
Some key items to consider from the material side include:
Moisture content
Viscosity
Filler content
31.3.4.1 Material: Moisture Content
Some materials will experience viscosity variation due to moisture content. Nylon
as an example can be molded over a wide range of moisture content (0.05–0.3% or
more). Molding nylon across a wide range of moisture content will lead to notice-
able changes in viscosity with nylon of higher moisture content flowing much
easier (lower viscosity). Due to these moisture-induced viscosity changes a variety
of flow line type of defects may appear. If the material becomes excessively dry the
viscosity may actually become high enough to lead to a pressure-limited situation,
which will in turn result in fill time variation.
Verify that the material being molded is adequately dried. Also examine how much
the moisture content is varying over time. If the process is seeing large swings in
moisture content many odd flow defects may show on the part.
See Chapter 9 on drying for more information.
