short shot-injection molding trouble shooting

What is a short shot

short shot in injection molding refers to the phenomenon that the injection material does not fill the mold’s cavity, resulting in incomplete parts. It usually occurs in thin-walled areas or areas away from the gate.

For some internal faces, the short shot does not matter without affecting the beauty; no adjustment is needed. If being adjusted, it may lead to the flash.

short shot

The reasons for the short shot are many, mainly in the following situations:

1. Insufficient material or supplementary not enough

It appropriately adjusts until the parts are filled.

2. The barrel temperature  too low

When the material temperature is low, the viscosity of the molten material is large, and the resistance during filling is also significant.

If the material temperature is appropriately increased, the melt fluidity can be enhanced.

3. Injection pressure or speed too low.

The filling process of the molten material in the cavity lacks sufficient driving force to flow remotely.

Increase the injection pressure so that the molten material in the cavity is always replenished with adequate pressure and material before condensing and hardening.

4. Insufficient injection time.

It takes a certain amount of time to inject a specific weight of the part.

If the time is not enough, the injection volume is insufficient, and the injection time can be increased until the cavity is filled.

5. Improper holding pressure.

The main reason is that the pressure is transferred too soon. The pressure switching point is adjusted too much, and the holding pressure supplements the remaining amount of material.

The weight of the workpiece is inevitably insufficient, and the pressure is insufficient. The pressure conversion position should be readjusted to the optimal point, make the parts complete.

6. The mold temperature too low.

When the shape and thickness of the part change greatly, an excessively low mold temperature will consume too much injection pressure.

The solution is to increase the mold temperature or reset the mold water line.

7. The nozzle and the mold sprue bushing have a misfit.

When the nozzle overflows during injection, part of the material is lost, and the mold is re-adjusted so that sprue bushing fits well with the nozzle.

8. The nozzle is damaged or partially blocked.

When choosing a barrel to retreat production, the nozzle and the mold sprue bushing will continuously make collisions with each other for a long time so that the sprue bushing hole will become smaller and smaller.

 The material flow channel will become smaller, the area-specific volume of the strip will increase, and the cold material will block the nozzle hole.

Alternatively, It will consume too much injection pressure. The nozzle is to be removed for repair or cleaning, and the end position of the injection base is appropriately reset to reduce the impact force to a reasonable value.

9. Poor mold venting

During the injection process, the air in the mold cavity is too late to escape from the parting surface or ejector pins. The last-entered melt is blocked in the mold cavity by the compressed continuously high air pressure, leaving a block at the end of the material flow.

A suitable venting passage is set at the parting plane corresponding to the gas blocking position. If the venting position is not on the parting surface, use the original sleeve or ejector pins to improve the internal venting, or re-select the location of the gate so that the air is discharged at the expected position.

10. The ribs too thin or too deep.

These are the corners where air can be easily stored. It is also a place where the filling is difficult. The ribs should be thickened, or the root arc angle should be increased.

The most thorough method is to add venting measures.

11. The runner or gate channel uneven

Due to continuous improvement of mold manufacturing technology and design level, most of the runner design for single cavity mold is reasonable and meets the molding requirements. The uneven runner refers to the distribution of multi-cavity runners. Often, there are small differences in the gates, resulting in the inhomogeneous flow of components injected into the mold cavities, causing some of the cavities to fill up and some of the cavities to still short shot.

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