Multi Cavity Injection Molding Rheological Runner Balancing Principles
Rheological runner balancing uses melt rotation inserts to equalize thermal shear layers across runner branches, preventing filling imbalances across cavities.
Differential Part Shrinkage
Meaning
Variation in the volumetric contraction of a molded polymer across different regions of a single part occurs as it cools from processing temperatures. This variation, or differential part shrinkage, leads to internal stresses that cause warpage and dimensional instability. Highly crystalline resins show a stronger tendency toward this behavior than amorphous polymers.
Melt Orientation
Polymer chains aligned parallel to the flow direction contract differently than those aligned perpendicular to it. In cases of differential part shrinkage, this anisotropy results in non-uniform dimensions. Gates must be positioned to minimize this variation.
Cooling Uniformity
Temperature differences across the mold steel cause uneven rates of solidification in the plastic. This thermal gradient causes differential part shrinkage because the warmer regions continue to contract after the cooler regions have already solidified. Mold cooling channels must be spaced to maintain a uniform temperature across the cavity surface.
Warpage Hazard
Internal residual stresses within the molded part force the structure to twist or bend upon ejection. When differential part shrinkage occurs, the part relaxes into a distorted shape to relieve these internal forces. Correcting this defect often requires changes to the holding pressure or the cooling cycle rather than altering the tool geometry.