Parting Line Steel Preload Calculation for Multi Cavity Injection Moulds
Calculate multi cavity parting line preload by balancing machine clamp force against internal hydraulic separation force to maintain positive shut-off land sealing stress.
Parting Line Preload
Meaning
Compressive force applied to the mating surfaces of a mould before the injection of plastic begins. Flash-free moulding and consistent part thickness depend on the integrity of this initial squeeze. Parting line preload ensures that the tool halves remain in absolute contact when the internal pressure of the melt tries to force them apart.
Mechanical Setup
Clamping units generate this force by stretching the tie bars of the machine or by moving a toggle mechanism past its center point. The amount of preload must exceed the expected separation force generated by the injection pressure. If the preload is insufficient, the parting line will open slightly, leading to dimensional errors in the finished components.
Pressure Distribution
Strategic placement of support pillars and the use of thick mould plates help to distribute the preload evenly across the entire shut off area. Uneven compression can cause the tool to warp or lead to localized wear on the cavity edges. Proper alignment of the machine platens is also necessary to ensure that the preload is square to the tool face.
Preload Limits
Excessive preload can crush the mould vents and prevent the escape of air, leading to burn marks on the parts. It can also exceed the tool steel yield strength, causing permanent deformation of the shut off surfaces. Balancing the preload against the injection pressure is an essential task for the process engineer during the initial tool setup.