What are the Causes of Warpage?

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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.

 

What are the Causes of Warpage during Molding?

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,

  • If the injection pressure is not enough, the indivdual molecules do not pack properly which can result in a difference in the way they cool and solidify. Since enough pressure hasn't been provided there is too much space while the material is cooling down and the molecules move while cooling resulting in warpage.
  • Shape factors like Molecular and fiber orientation
  • Process conditions like molding temperatures can cause warpage. If enough temperature is not provided to the material, the molecules do not get ambient heat to melt and flow thus causing a differential in both expansion and cooling and can result in a bad warped product. Therefore always maintain the right molding heat temperature.
  • Inconsistent process cycle : If the product is ejected out by opening the gate prematurely before the material had time to cool down, it can result in warpage as the material could not get enough time to cool down evenly.
  • Inadequate residence time : This is the time the material is exposed to heat. If the exposed time is not sufficient for all the ,molecules to get heated uniformly then during the process of cooling the individual molecules due to the difference in the heat absorbed cool differently thus causing the warpage. Therefore the proper residence time has to be given for the individual molecules to absorb heat uniformly through out the material.
  • Factors such as inadequate time given to heat and coll can cause a stress in the material thus causing warpage

     

 

What are the Factors affecting Warpage?

The variables affecting warpage are

  • Machine variations like unstable controller
  • wall thickness
  • mold temperature
  • Wall thickness
  • Gate location
  • Flow restrictions and bypasses
  • Inherent rigidity of the molded part

 

In what ways can warpage be minimized or tackled?

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.

 

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