Troubleshooting short shot problems
A short shot is the incomplete filling of a mold cavity which results in the production of an incomplete part.
If a part short shots, the plastic does not fill the cavity. The flow freezes off before the flow paths have completely filled.
To ensure the finished part is of good quality, the part must also be adequately packed with plastic. Therefore the question to ask is not only, "Will the part fill?" but also, "Can a good quality part be made?"
Causes
Flow restrictions. Due to channels freezing or inadequate runner design.
Hesitation and long or complex flow paths.
Inadequate venting. Back pressure due to unvented air traps can cause a short shot.
Low melt and/or mold temperatures.
Insufficient material entering the cavity. An undersized machine, low shot volume, or inadequate ram speed.
Machine defects. Including an empty hopper, blocked feed throat, or a worn non-return (check) valve that causes loss of pressure or volume leakage.
Remedies
Before you try one of the methods listed below, check all of the other results, so that you know the exact cause of the short shot.
- Avoid hesitation.
- Eliminate air traps. If air traps do exist, they should be positioned in areas that can be easily vented or ejection pins added so that air can be removed.
- Increase mold and melt temperature. This will decrease the viscosity of the melt, making it easier for the plastic to flow through the part.
- Increase ram speed. This can cause greater shear heating, which decreases the viscosity of the melt, making it easier for the plastic to flow through the part.
- Change the part geometry. Balance flow paths so they fill in an equal time and an equal pressure. You may need to thicken thin sections, or reduce the complexity of a flow path.
- Use a different material. Select a less viscous material (higher melt flow rate). By choosing a material with a higher melt flow rate, less injection pressure will be required to fill the part.
- Increase the maximum injection pressure for this part.
Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.