Page 375 - Injection Molding Advanced Troubleshooting Guide
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372 39 Scuffs and Scratches
Watch the forward ejection stroke on the machine as well. If someone adjusted the
forward stroke, the robot may be hitting the parts harder than previous runs. The
robot pick position works in concert with the forward ejection stroke, so if one is
changed the other probably needs to be changed as well.
39.3.1.2 Molding Process: Ejection Issues
A common way to run a molding machine is to eject the parts onto a chute or con-
veyor rather than robotically removing the parts from the mold. When parts are
ejected onto a chute or conveyor they can bang into each resulting in handling
damage. Normally this type of ejection should be reserved for noncosmetic parts.
Keep in mind, however, that running without a robot can speed up cycles by reduc-
ing the mold open time. Running without a robot works very well with the right
parts.
Sometimes when a part is ejected from the mold it may contact other areas of the
mold and damage the part. One example of this is when parts are ejected and con-
tact a leader pin, leaving grease on the part. Another example is when the parts
are ejecting fast enough that they hit the cover half of the mold.
If the sprues are still hot enough they can cause a spot of remelted plastic on the
part when they are dropped together in the chute or conveyor. An improvement to
this may be adding a sprue picker to the operation to keep the parts and sprues
separate. Improving the cooling to the sprue may also help the issue, as may down-
sizing the sprue. Adding a hot tip heated sprue bushing to the mold would be an-
other option to eliminate this defect.
Care should be taken in deciding how the part should be removed from the mold.
As mentioned above, cosmetic parts may need a robot to prevent damage. Whether
parts are ejected into a chute, conveyor, or robot, the opportunities for handling
damage must be assessed.
Watch for cases where a part is pulled back onto the core of the mold when the
ejectors return. This will often cause distortion especially on molds with lifters. If
the part is not consistently ejecting with one stroke, the mold should be evaluated
for possible improvements.
39.3.1.3 Molding Process: Packing Parts
For the sake of consistency, packaging will be included in the molding process.
Once the part has been successfully delivered to the machine operator there is
another range of possibilities that can cause damage including:
Operator damaging parts with jewelry or belt buckles
Operator damaging the part with dirty hands
Operator scratching part against work station
