In liquid silicone rubber overmolding, the positional accuracy of metal or plastic inserts directly affects the assembly performance and functional reliability of the product. Inserts undergoing positional shift, angular deviation, or deformation from melt impact during the injection process are common challenges in overmolding processes.

Mold Positioning: The Foundation of Precision

The positioning method of inserts in the mold is the fundamental factor determining positional accuracy. In industry practice, a dual positioning structure of "pin + step" is recommended: the fit clearance between the pin and the insert positioning hole should be controlled at 0.003 to 0.005 mm, with a step limit height of 0.1 to 0.2 mm. This dual positioning ensures both radial precision and axial position control.

The support structure in the insert placement area of the mold is equally important. If the insert lacks effective support under injection pressure, displacement may occur during filling even if initial positioning is accurate. For inserts with long cantilever sections, adding auxiliary support points in the mold is recommended.

Injection Process: Restrained Use of Pressure

Excessively high injection pressure is the direct cause of insert displacement. For LSR overmolded products, injection pressure is generally appropriate at around 50 bar, with specific values requiring flexible adjustment based on the structural strength of the insert and support conditions. The more slender the insert and the less support it has, the lower the required injection pressure.

Injection speed also requires control. Excessively fast injection speed increases the impact force of the melt on the insert, particularly in designs where the gate directly faces the insert, where the impact effect is more pronounced. Using a lower injection speed is recommended to allow the melt to flow smoothly around the insert during cavity filling.

Insert Material and Pre-Treatment

The water absorption rate of plastic inserts is a variable requiring attention. Materials with strong hygroscopicity such as polyamide should be dried before injection molding to prevent moisture vaporization at high temperatures from affecting dimensional stability or causing surface defects.

The thermal expansion behavior of inserts in the mold differs from that of LSR. The thermal expansion coefficient of metal inserts is typically much lower than that of LSR, and the fit clearance between the two changes during mold heating. This thermal expansion difference must be considered during mold design to ensure the insert maintains stable positioning at operating temperature.

Process Control Recommendations

For insert placement, positioning fixtures or robotic automated placement are recommended to avoid deviations caused by manual placement. A low-pressure, slow-filling strategy during the filling stage can minimize the risk of insert displacement while ensuring complete filling. During first-article confirmation, the positional and angular deviation of inserts should be measured with priority to ensure they are within tolerance.

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