Short shot


What is a short shot?

A short shot is a molded part that is incomplete because insufficient material was injected into the mold. In some cases, short shots are intentionally produced to determine or visualize the filling pattern. But problematic short shots occur when the polymer melt cannot fill the entire cavity (or cavities)-most commonly at thin sections or extremities.

Causes of short shot
Any factors that increase the resistance of polymer melt to flow or prohibit delivery of sufficient material into the cavity can cause a short shot. These factors include:

Remedies
Several factors influence the polymer's ability to fill the entire cavity. Proper remedial actions can be taken when the cause of a short shot is pinpointed. Here are some suggestions.


Alter the part design
It's important to facilitate the flow of injected polymer melt; doing so can alleviate short shots.



Strategically increase the thickness of certain wall sections (as flow leaders).


Alter the mold design
A properly designed delivery system (sprue, runner, and gate) will facilitate a more balanced filling pattern. If needed, modify your design in the following ways.



Fill the thick areas before filling the thin areas. Doing so will avoid hesitation, which causes early solidification of polymer.


Increase the number and/or size of gates to reduce the flow length.


Increase the size of runner systems to reduce resistance.
Entrapped air inside the mold cavity (Air traps) can also lead to short shots.



Place vents at the proper locations, typically near the areas that fill last.
This should help vent the displaced air.


Increase the size and number of vents.


Adjust the molding conditions
Look closely at the factors that control how material is injected into the mold.



Increase the injection pressure.
Do not exceed the machine's capability. To prevent accidental damage to the machine's hydraulic system, limit the operating injection pressure to 70 to 85 percent of the maximum injection pressure.


Increase the injection speed. Within the machine limits, this will create more viscous heating and reduce the melt viscosity.


Increase the injection volume.


Increase the barrel temperature and/or the mold-wall temperature.
Higher temperatures will promote the flow of material through the cavity. Be careful to avoid material degradation due to prolonged exposure at an elevated temperature.
The molding machine might also be the culprit if you're experiencing problematic short shots.



Check the hopper for sufficient material supply or a clogged feed throat.


Inspect the non-return valve and barrel for excessive wear.
Wear can lead to loss of injection pressure and leakage of injection volume.