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Solving molding 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 all of the flow paths have 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, 'Can a good quality part be made?'
Due to channels freezing or inadequate runner design.
Hesitation and long or complex flow paths
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.
Including an empty hopper, blocked feed throat, or a worn non-return (check) valve that causes loss of pressure or volume leakage.
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.
Eliminate air traps
If air traps do exist, they should be positioned in regions that can be easily vented or ejection pins added so that air can be removed.
Increase mold & melt temperature
This will decrease the viscosity of the melt, making it easier to flow through the part.
Increase ram speed
This can cause greater shear heating, which can decrease the viscosity of the melt, making it easier to flow through the part.
Change 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.
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