Short shots, or insufficient filling, refer to defects where the end of the cavity or thin-wall areas are not completely filled with material. In liquid silicone rubber injection molding, short shots often result from the combined effects of material characteristics, mold design, and process parameters.

Material-Level Causes

The viscosity of LSR directly affects fluidity. If the material storage temperature is too low, viscosity increases significantly, and flow resistance rises. For example, an LSR with a viscosity of 300 Pa·s at room temperature may double in viscosity when the temperature drops to 10°C.

Uneven mixing of A/B components can also cause localized viscosity abnormalities. When the mixing ratio deviates from 1:1, crosslink density and curing speed change, affecting filling behavior.

Mold Design and Venting

Excessively long or narrow runners are common mold-related factors causing short shots. When runner length exceeds 200 mm or diameter is less than 3 mm, pressure loss can reach over 30%, making it difficult for the cavity end to receive sufficient filling pressure.

Venting system failure also warrants attention. LSR requires extremely shallow vent groove designs, with typical depths of 0.01–0.03 mm. If vent groove depth is insufficient, air cannot escape, forming air resistance that impedes melt advancement. For complex-structure molds, vacuum-assisted venting can minimize residual air in the cavity.

Coordinated Control of Process Parameters

Injection speed significantly affects the filling behavior of LSR. LSR is a shear-thinning material; when injection speed is too slow, shear rate is insufficient, viscosity remains high, and filling lags. Industry practice recommends a three-stage injection strategy of "low speed–high speed–low speed": initially at low speed to eliminate turbulence, at high speed in the middle to fill the main body, and at low speed at the end to ensure terminal filling.

Mold temperature control is equally critical. Vulcanization temperature must be maintained within the material's recommended range; excessively low mold temperature accelerates cooling of the melt front and reduces fluidity. Holding pressure is recommended to be maintained at 50%–70% of injection pressure, with holding time covering the stage before gate sealing.

Troubleshooting Recommendations

When short shot problems occur, it is recommended to first confirm whether the unfilled location is consistent. If concentrated at the end or in thin-wall areas, prioritize checking venting and injection speed; if the distribution is irregular, check metering accuracy and material mixing status.

More information, please click here.