Warpage

If the shrinkage throughout the part is uniform, the molding will not deform or warp; it simply becomes smaller.  Warpage can be considered as a distortion where the surfaces of the molded part do not follow the intended shape of the design.

Part warpage results from differential shrinkage in the material of the molded part caused by molded-in residual stresses. Variations in shrinkage can be caused by molecular and fiber orientation, temperature variations within the molded part, and by variable packing, such as over-packing at gates and under-packing at remote locations, or different pressure levels as material solidify across the part thickness. Due to the number of factors present it is a very complicated task to achieve uniform shrinkage. These causes are described more fully below.

For fiber reinforced thermoplastics, reinforcing fibers inhibit shrinkage due to their smaller thermal contraction and higher modulus. Therefore, fiber reinforced materials shrink less along the direction in which fibers align (typically the flow direction) compared to the shrinkage in the transverse direction.

Similarly, particle-filled thermoplastics shrink less than unfilled grades, but exhibit a more isotropic nature.  

For non-reinforced materials warpage is generally influenced by wall thickness and mold temperature.  If wall thickness and mold temperatures are not optimal the molding will most likely warp.

Differential shrinkage for filled and unfilled materials.

For glass reinforced materials totally different characteristics are evident due to fiber orientation.  If a non-reinforced and a fiber reinforced material are compared in the same design it is possible to see contary warpage in the same part.

Unreinforced vs fiber reinforced materials.

Non-uniform cooling in the part and asymmetric cooling across the part thickness from the cavity and core can also induce differential shrinkage. The material cools and shrinks inconsistently from the wall to the center, causing warpage after ejection.

Part warpage due to:
(a) non-uniform cooling in the part
(b) asymmetric cooling across the part thickness

Shrinkage increases as the wall thickness increases. Differential shrinkage due to non-uniform wall thickness is a major cause of part warpage in unreinforced thermoplastics. More specifically, different cooling rates and crystallization levels generally arise within parts with wall sections of varying thickness.  

Larger volumetric shrinkage due to the high crystallization level in the slow cooling areas leads to differential shrinkage and thus part warpage

Diagram of high shrinkage/low cooling vs warped part.

Geometric asymmetry (e.g., a flat plate with a large number of ribs that are aligned in one direction or on one side of the part) will introduce non-uniform cooling and differential shrinkage that can lead to part warpage.  

The poor cooling of the wall on the ribbed side causes a slower cooling of the material on that one side, which can lead to part warpage.

Poor design vs warped part.