Effect of Peak Cavity Pressure on Unreinforced Polyamide Linear Shrinkage Rates
Elevating peak cavity pressure compresses unreinforced polyamide melt, suppressing specific volume and reducing post-moulding linear shrinkage rates.
Cavity Balance
Meaning
Flow uniformity across multi-impression molds represents a critical metric for producing dimensionally identical parts. Achieving cavity balance means that the plastic melt reaches the end of each flow path at the same instant during the injection phase. This condition is determined primarily by runner geometry and melt temperature.
Moulders measure this parameter by performing a short-shot study.
Flow Behavior
Melt viscosity variation inside the hot or cold runner system often disrupts the fill rate between inner and outer positions. Since shear heating occurs unevenly across the flow channels, the material entering different gate locations can exhibit different temperatures. Flow channel design must account for these thermal gradients to prevent unbalanced filling.
If the viscosity remains inconsistent, parts from different impressions will vary in weight and density.
Defect Prevention
Unbalanced filling causes severe warp in the finished parts. Because some impressions overpack, they experience higher holding pressure. This variation causes localized flash or sink marks.
Economic Impact
Tooling optimization lowers the overall scrap rate by establishing a wider processing window. When cavity balance is achieved, a mold can run on shorter cycle times because cooling can be optimized for a uniform part thickness. Using regrind material becomes more viable under balanced conditions because viscosity shifts are handled uniformly across all gates.
Moulders reduce tooling wear and machine tonnage demands when the pressure distribution is uniform across the entire plate. Operating with balanced flow minimizes post-molding sorting costs and increases total yield.