Page 82 - Injection Molding Advanced Troubleshooting Guide
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66 8 Machine Performance
If a press does not have good velocity linearity, changes in the fill velocity will not
yield the expected results. In some cases, machines will not be able to achieve the
maximum value that the controller allows. The velocity linearity curve will demon-
strate where the deficiencies in velocity control occur. There will also be cases
where a machine may be tuned to perform well at one end of the velocity range but
not as well at the other end. Figure 8.4 shows an example of a spreadsheet for eval-
uating velocity linearity.
Figure 8.4 Velocity linearity form and graph; note the poor actual velocity response
There will be cases where a process technician is making velocity increases on a
press but the fill time is not changing. This is a good indicator that the press is not
capable of achieving this velocity (always beware of pressure limiting).
Another useful bit of learning from this test is to determine the actual fill velocity
on a press that uses percentage for velocity set values. If a machine only offers
percent settings it is a valuable exercise to document how the percent values corre-
spond to actual velocities or pressures. The values can be documented on a curve
which can be posted on the molding machine to allow a quick visual reference of
