Micro Molded Part Metrology Standards for Receiving Acceptance Inspections
Establishing rigorous non-contact metrology standards and ISO 14253-1 guardbanding protocols prevents defective micro-molded parts from entering production.
Polymeric Anisotropic Shrinkage
Meaning
Dimensional changes in molded plastics often vary depending on the direction of material flow and fiber orientation. The occurrence of polymeric anisotropic shrinkage means that a part will contract more in one direction than in another as it cools from a molten state. This behavior is especially common in materials reinforced with glass or carbon fibers.
Understanding these directional differences is necessary for designing molds that produce parts with the correct final dimensions.
Vectorial Contraction
Molecules and fibers tend to align themselves in the direction of the flow during the injection phase. Polymeric anisotropic shrinkage is usually higher in the direction of the flow because the aligned chains pull closer together as they cool. In the transverse direction, the reinforcement fibers act as tiny struts that resist this contraction.
Mold Pressure
Holding pressure during the cooling phase can help reduce the overall amount of shrinkage by packing more material into the cavity. However, if the pressure is not uniform, it can aggravate polymeric anisotropic shrinkage and lead to internal stresses. These stresses may eventually cause the part to crack or warp after it is ejected from the tool.
Part Distortion
Uneven contraction across a flat surface or around a corner often leads to bowing or twisting of the finished part. Engineers use simulation software to predict polymeric anisotropic shrinkage and adjust the mold geometry to compensate for the expected movement. Accurate predictions during the prototype phase save time and money by reducing the number of tool modifications required for production.