Warpage in general terms is referred to the condition of a given material such as wood which can turn or twist out of shape. Warpage is the dimensional distortion developed in a molded product after its ejection from the mold during injection molding process. Because of warpage the molded part does not follow the predetermined design shape. Since the torn part seems like wearing away warpage is also called potato chipping. The main cause of warpage is the difference in internal stresses produced in the material due to differential shrinkage. Warpage is a major problem especially in the case of reinforced materials. It needs to be addressed early on during the molding process as the defects are hardly rectifiable later.
Due to differential rate at which the individual molecules shrink in a material, a bending movement develops leading to distortion. The particular part becomes convex from the mold's cold side and concave from mold's hot side. Warpage is developed due to this inconsistent shrinkage. Factors such as differences in temperature between the sides of the mold, the layers freeze and shrink at varying times leading to generation of internal stresses. The various reasons attributing to warpage are,
The variables affecting warpage are
In case of reinforced materials wall thicknesses need to be uniform and melt accumulations should be avoided. Multipoint gating system helps eliminate warpage problems by achieving a high pressure gradient. In asymmetrical parts warpage is prevented by providing blind cores during mold planning and design. Gate position is also important as every bypass and weld line can be an inherent cause of warping. Provided the molded part, gate and mold are designed properly the molder can keep warpage under control. This is done by maintaining holding pressure and mold temperature. Heat dissipation is optimized using several circuits. Warpage can be minimized by increasing injection pressure or time which allows the cooling of the entire part while it is getting constrained. Study of the shrinkage curve for reinforced materials also helps reduce warpage. Incorporation of flow aids or flow restrictors help in the alteration of the melt flow profile that in turn prevents warping. In case of glass reinforced materials the mold can be stuffed or filled in several stages. The melt front profile is studied and accordingly fiber orientation can be reconstructed.