Wednesday, March 3, 2010

Troubleshooting short shot problems on injection mold

The term "short shot" or "short mold" is used to refer to a phenomenon where plastic being injection molded does not reach certain portions of the inside of the die before solidifying. or it means that not enough resin has flowed into the mold to fill it adequately. The main causes for short shot are the mold condition or flowability problems with the resin. The problem is alleviated when a greater volume of resin flows more easily.
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?""


Some of the typical factors affecting this type of problem are as follows:
- Fluidity and viscosity characteristics of the plastic
- mold design (Flow restrictions. Due to channels freezing or inadequate runner design. gate design, bushing construction for ribs and bosses, the presence of venting, .)
- Hesitation and long or complex flow paths.
- Inadequate venting. Back pressure due to unvented air traps can cause a short shot.
- Molding machine performance (Including an empty hopper, blocked feed throat, or a worn non-return (check) valve that causes loss of pressure or volume leakage.)


picture above i copy from UMgabs.com, those check point is very important for troubleshooting short shot problems

Solution
- 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.
- if Resin flows too slowly, Increase temperature in the cylinder. Increase injection speed. If no change in the filling time is noticed after setting of a high injection rate, there is a possibility that the performance of the molding machine is insufficient or that there is excessive pressure loss in the sprues, runners, and gates
- check resin viscosity, Increase the mold 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.
- Increase the sectional area of the gate, sprue and runner, and shorten the length. Make the sprue, runner and gate surfaces more slippery.
- Change the gate position, increase the thickness of the mold cavity, modify design to improve flow.
- 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.

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