Polyamide Dimensional Shrinkage Differential under Varied Cavity Pressure Conditions
Cavity pressure directly controls polyamide crystallization and volumetric shrink, requiring precise transducer monitoring to hold DIN 16742 tolerances.
H-Pattern Runner
Meaning
Geometrically balanced distribution networks for multi-cavity injection moulds ensure that the polymer melt reaches all cavities at the same moment and pressure. Incorporating an h-pattern runner layout achieves this balance by providing equal flow path lengths and diameters from the sprue to each individual cavity. This layout prevents uneven filling, which can cause part variation in high-cavitation moulds.
The design is widely used in high-volume production where part consistency is required.
Melt Balance
Each flow path in this branching configuration consists of identical run lengths and turns, ensuring that every cavity receives the same shear history. By contrast, unbalanced runner designs cause some cavities to fill earlier, which leads to overpacking and flash. The h-pattern runner prevents this by maintaining equal flow resistance across all branches.
This uniform resistance ensures that the polymer melt viscosity remains consistent as it enters each cavity. This thermal and pressure balance is essential for multi-cavity tools, especially when moulding high-precision parts with tight tolerances.
Part Consistency
Dimensions and weights of parts from different cavities must remain identical to pass quality inspection. If the runner does not provide equal flow, some parts will have sink marks while others will have flash. This defect is avoided by the h-pattern runner because the pressure is distributed evenly at the end of the fill stage.
This balance reduces the internal stress in the moulded parts, which minimizes warpage. Consistent part weight across all cavities simplifies process optimization.
Moulding Cost
Although these balanced layouts require more tool space and increase scrap weight, they reduce the cost of quality control. The higher material usage in the runner is often offset by the reduction in reject parts. Recycling the runner material can further reduce this cost, provided the polymer allows the use of regrind.
This layout is a standard choice for high-precision components.