Injection Molding Short Shot Troubleshooting: Causes and Solutions
When a molded part is not completely filled, the first step is to observe the screw travel of the injection molding machine during production. Special attention should be paid to the screw position when pressure holding ends and when injection is completed. This helps determine whether the problem is caused by insufficient shot volume, leakage, poor material flow, blocked flow channels, mold design issues, or improper process settings.
Case 1: The screw reaches the set shot position, and the residual material amount is zero
In this situation, the screw reaches the set value and the injection pressure position is zero, which usually means there is no residual melt left in the barrel after injection. Possible causes include the following:
1. Material leakage
First check whether material is leaking from the injection barrel, the nozzle, or the connection between the machine nozzle and the mold sprue bushing. If leakage is found, incomplete filling is a natural result and this issue should be eliminated first. Only after confirming that there is no melt leakage should other causes be investigated.
2. Insufficient shot volume
If there is no leakage but the part is still not fully filled, the shot size may be too small. In this case, increase the melt stroke and observe the screw position, shot volume, and filling condition again. If increasing the melt stroke improves the part, then the original shot volume was insufficient. If the maximum melt stroke is reached and the part is still not fully filled, a larger injection molding machine may be required.
3. Check ring or non-return valve leakage
If increasing the melt stroke does not significantly increase the actual amount of material entering the part, the non-return valve at the front of the screw may be leaking. One way to check this is to keep a part in the cavity during a cycle and use manual injection while gradually increasing pressure. If the screw does not move forward during manual injection, the seal is good. A small amount of screw movement under high pressure may be acceptable and may only indicate melt shrinkage rather than leakage.
When diagnosing a short shot in this condition, first confirm that there is no leakage between the barrel and nozzle, and then confirm that the non-return valve at the front of the screw is sealing properly. Only after these two possibilities are excluded should other factors be evaluated.
Case 2: The screw reaches the set plasticizing stroke but cannot reach the set injection stroke
This means there is still melt inside the barrel, but not enough material is entering the mold cavity. In this case, the problem is usually related to flow resistance, process settings, blocked flow channels, mold defects, or product design.
1. Insufficient melt flowability
If the melt does not flow easily, it may not be able to overcome the resistance of the runner system and cavity. This can be caused by poor raw material flow properties, barrel temperatures that are too low, inaccurate temperature readings, or mold temperatures that are too low. Even if the material flows well inside the barrel, it may cool too quickly after entering the mold, reducing flowability and causing short shots.
2. Improper injection molding parameters
If the injection pressure is too low, the melt cannot overcome the resistance of the cavity. If the injection speed is too low, the melt may cool during filling, which reduces flowability and increases resistance. If the holding time is too short, filling may stop before the part is fully packed. Any of these conditions can result in an incompletely filled product.
3. Blocked flow channels
Cold slug buildup, contaminants, or other obstructions in the nozzle, sprue, runner, or gate can block the flow path and prevent the mold from filling completely. These areas should be checked carefully and cleaned if necessary.
4. Defects in the mold feeding system
Problems in the mold gating system may also cause short shots. Common issues include runners that are too small, too thin, or too long, which increase flow resistance. Rough runner or gate surfaces, sharp corners, poor surface finish, improperly designed cold slug wells, poor gate location, insufficient gate quantity, or poor mold venting can all reduce filling performance and cause incomplete parts.
5. Poor product design
If the product structure is not designed properly, the material may not flow evenly through the part. Areas with very thin wall sections or poor wall thickness transitions can be difficult to fill and may result in local short shots.
Conclusion
Short shots in injection molding should be diagnosed systematically by first checking screw position, shot completion, and residual material condition. From there, the troubleshooting process should focus on material leakage, insufficient shot volume, non-return valve sealing, melt flowability, process settings, flow channel condition, mold gating design, venting, and part structure. A step-by-step analysis helps identify the real cause and improves filling stability during production.