Material Selection Requirements
During injection molding, as the thermoplastic material cools down in the mold cavity, the specific volume decreases, resulting in part shrinkage. Shrinkage continues until the part reaches room temperature and in some materials even longer. Amorphous materials have low shrinkage which is usually isotropic. If the molded parts have high degree of molecular orientation, the shrinkage becomes asymmetric becoming slightly more in the direction of orientation than in the transverse direction. Thick sections cool slowly which may result in higher shrinkage as compared to thin sections. Semi-crystalline materials have higher shrinkage because of additional volume reduction during crystallization. Degree of crystallization also affects the amount of shrinkage. Semi-crystalline materials may show anisotropic shrinkage especially in parts with varying thickness. Thicker sections have higher shrinkage as they cool down slowly creating more crystallization. This shrinkage is in addition to the normal shrinkage difference due to differential cooling. Both amorphous and semi-crystalline polymers with particulate fillers such as minerals and glass spheres; and flake type reinforcements tend to shrink less and have more uniform shrinkage. These materials generally have higher flexural modulus but reduced impact strength. Materials with reinforcements such as glass or carbon fibers can have significantly higher shrinkage in cross-flow direction as compared to in-flow direction. Part distortion or warpage becomes a prime concern with such materials. Warpage is caused by differential shrinkage in different areas of a part. Wall thickness variations, uneven mold temperatures, filling from thin to thick sections and fiber orientation can cause warpage in injection molded parts. |