July 14, 2022

Injection Molding Clamp Pressure

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Two types of injection-compression used for thin walled high aspect ratio parts. The first uses a moving side core or secondary cylinder that leaves the cavity spaced and upon injection clamps forward to compress the part stress free. This usually requires a specific capability of the press. The second method sometimes referred to as coining closes the mold under reduced clamp pressure and the cavity is filled.

Injection profile can be set that as the cavity is filled to peak pressure and the clamp pressure is overcome, the injection has a near zero fill speed and then picked up to 10~20% speed for the crossover to hold. This method usually needs a very repeatable injection and clamping system and dial indicators are used to monitor the clamp breathing stroke. As the material goes in to hold and begins the shrink process, the clamp will follow the melt movement typically not completely closing. I have used this process for over 25 years to yield 0.55 mm thick by 120 mm diameter center gated parts. The flatness, thickness uniformity and stress are significantly reduced, but part to part overall weight can vary depending on your repeatability and press control.

In this design it is also typical to have roughly a 7 degree draft angle on the parting/vent line to avoid flashing during the injection-compression stroke depending on the material and criteria for edge definition. keep in mind other critical control parameters are melt viscosity, plastification, pellet size uniformity as the cushion is typically much lower than with standard straight injection to avoid shot to shot variance.

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