Page 387 - Injection Molding Advanced Troubleshooting Guide
P. 387
40.3 Short Shot Troubleshooting 385
Mold surface temperatures should be measured and documented after running on
process. This will provide data of what the running mold surface temperature is.
The set point of the mold temperature controller is not what the mold temperature
will be. Also utilize thermal imaging of part out temperatures after the part has
been ejected.
40.3.2 Short Shot Troubleshooting Mold Issues
There are many mold-related concerns that can lead to short shots including:
Venting
Cavity balance
Plugged gate or hot tip
Stuck plastic
Hot runner temperatures
Leaking hot runner manifold
40.3.2.1 Mold: Venting
One of the biggest mold-related causes of short shots is venting. If venting is lack-
ing in a mold, trapped gas can lead to short shots. Trapped gas can result in either
short shots or burns; both can indicate venting problems.
When troubleshooting short shots always make sure that the mold surface is clean
and the vents are open. A dirty mold surface will restrict venting ability. On mate-
rials that are more prone to plate out, the vents will plug easier and must have
even more focus.
For more details on venting see Chapter 7.
40.3.2.2 Mold: Cavity Balance
When a mold is unbalanced between cavities there will be an opportunity to have
specific cavities short out while others are full. A multicavity mold should be bal-
anced for fill within 3% to provide equal process conditions to all cavities. If a mold
is unbalanced there will be a stop/start effect during filling, and as pressure equal-
izes, the plastic will follow the path of least resistance. When the first cavities fill,
the lagging cavities will continue to fill, which can in turn lead to hesitation be-
tween filling and packing of the various cavities.
Figure 40.6 shows the normal imbalance of an eight-cavity mold caused by shear
rates. Notice how the inner cavities are farther ahead than the outer cavities. For
more details see Chapter 12 on cavity balance.
