Short Shot
The part does not fill completely. Cushion control, pressure/speed, venting and the runner are examined in that order.
A short shot is the mold cavity solidifying before it is completely filled. The region of the part furthest from the gate, or its thinnest section, remains incomplete. The cause is either that the material is not fluid enough, that the pressure is insufficient, or that air cannot escape and stops the melt. The most practical way to find out which is a partial fill study.
How to Recognize It
- The outer profile is not fully formed; typically at the point furthest from the gate or in thin ribs.
- If it is caused by venting, it can also occur in other areas and a diesel mark is NOT necessarily present.
Physical Causes
- The injected volume is insufficient (the cushion drops to zero).
- Trapped air stops the melt.
- The machine's pressure/speed is insufficient, or a section freezes early.
- A thin section freezes if the melt can flow into a thick section at the same time (a filling problem around ribs).
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- Check whether the screw is fully forward (no cushion): increase the metering; check the non-return valve and the feed.
- If the maximum pressure is being reached, raise the pressure limit; raise the melt and mold temperatures.
- If there is a drop during filling on the cavity pressure curve, DELAY the switchover point.
- Increase the injection speed; check the nozzle opening and temperature.
Mold, Design and Material Solutions
- Improve the venting (end of the flow path, bases of ribs).
- Review the runner system and gate position.
- If necessary, a machine with higher pressure/speed capacity.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Unbalanced Filling | With unbalanced filling some cavities of a multi-cavity mold fill while others remain short; the problem is runner imbalance. With a short shot all cavities are short in the same way. |
| Vacuum Voids | A vacuum void leaves a void inside even though the part looks full. With a short shot the material has physically never reached that area. |
Frequently asked questions
What could be the reason if it does not fill even though I raised the pressure?
Most probably air. If the air in the mold cavity cannot escape, the melt cannot advance however much pressure you apply; a burn mark usually also appears in that area. Check the vents and do not keep the clamping force higher than necessary.
How is a partial fill study done?
By switching off holding and increasing the shot volume step by step, you see how the mold fills, step by step. There is no other way to see in which order the melt advances, where it slows down and where the air gets trapped.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice