Meaning
Dimensional discrepancies that occur when mold designers assume a molded polymer part contracts equally in all directions lead to out-of-tolerance components and warped geometries. The occurrence of isotropic shrinkage error is most pronounced in fiber-reinforced resins where the orientation of the fibers creates highly directional shrinkage. This error results in incorrect cavity dimensions and costly rework of the mold tool.
Design Assumption
Simplified calculations using a single shrinkage percentage ignore the complex flow patterns and cooling gradients present in the mold cavity. This approach assumes the polymer behaves uniformly, which is rarely the case in complex geometries.
Mold Modification
Correcting the shape of the mold steel is necessary once the parts fail to meet the required tolerances after ejection. Toolmakers must recut the cavity or adjust the cooling lines to compensate for the uneven contraction of the polymer, and this process often requires several iterations of mold trials and measurements.
Material Behavior
Flow-induced molecular orientation causes the resin to shrink much less along the flow direction than across it, especially when glass fibers are present to restrict movement. This difference in contraction is further exacerbated by temperature differences between the cavity sides, forcing the ejected part to twist or warp as it cools to room temperature.
