Mainly due to the injection molding process to produce a polymer bond warping is due to internal displacement occurs under stress due. Deformation depends on the product stiffness and the degree of uneven shrinkage. Reasons imbalance causes contraction can be analyzed from the following four aspects:

(1) non-uniform mold temperature distribution
The cooling rate on both sides of the article surface in the thickness direction of the article leads to different shrinkage asymmetric, resulting in a surface of the article to the low cooling rate (higher mold temperature) of the side of the net bending moment. Corner effect which is a typical example: the product due to uneven shrinkage of the corner of a small warpage, will be extended to products and the longitudinal direction of the bottom wall and produce significant warpage in the middle of the wall.

(2) non-uniform wall thickness distribution
Other conditions being equal, including the thickness of thin sections of the cooling rate of the same, the thickness of thin sections of frozen proportion than large walled area, the shrinkage rate lower than the walled area.

(3) The uneven pressure distribution uniform
Filling pressure and holding pressure caused by uneven distribution, near the gate area freezes at a higher pressure, resulting in lower shrinkage, the gate area due to the low pressure generated far higher shrinkage.

(4) the orientation effect produced uneven shrinkage
Polymer chains oriented by stretching along the flow direction in the flow direction will result in products and differential shrinkage occurs perpendicular to the flow direction. Close to the die wall melt rapidly frozen molecules remain stretched state; melt away from the mold wall layer is frozen spacer layer, and thus a higher temperature, so that the molecular chain is composed of inner tension turn into a relaxed state, resulting in a greater orientation to the outer core almost no orientation gradient, leading to the surface and pull the core by compression state. Net shrinkage cross section is the sum of the shrinkage of frozen layers. Flow direction shrinkage generally larger. Glass fiber-containing material, because the orientation in the flow direction shrinkage of less than the shrinkage perpendicular to the flow direction, is more prone to deformation.

When the product has a deformed, from the above four aspects, basically you can find the distortion / warping why.