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Molding Defects


Warping or Twisting

Factors influencing warping are as follows:
・Differences in shrinkage and cooling time as dependent on the differences in both surface contraction and component thickness which result from differences in die temperature distribution.
・ In fiber-reinforced materials, there are large differences in the degree of shrinkage in the flow direction and the perpendicular direction, and for this reason, special consideration must be given to gate design (ie, quantity and location) at the die design stage.

 For example, if the degrees (or rates) of shrinkage at the various points in a molded component are theoretically identical, this will simply lead to the generation of small, similar-looking cavities, and regardless of the size of the material's shrinkage rate, there will be no occurrence of warping.

  Nevertheless, a complex mix of the above-mentioned factors will exist during actual molding, and after release of die clamping pressure and removal, the internal strain will try to fall to the minimum level (ie, to reduce energy to the minimum), thus resulting in the occurrence of molded-component warping.

 Furthermore, consideration must also be given to insufficient cooling of the molded component and to deformation as a result of defective push-out mechanisms when dies are being designed.



Warping and Twisting Checkpoints


Cause(1)
Molded component shape
  • Distribution of wall thickness : Areas with non-uniform distribution are included.
  • Insufficient structural stiffness : The structural stiffness of ribs and the like is insufficient. Ribs can actually contribute to warping, and therefore, a detailed examination of thickness and height factors must be undertaken.
Cause(2)
Die
  • Cooling circuit : Die temperature distribution is non-uniform, the cooling circuit is too long (i.e., large temperature difference between in and out points), control is inadequate, or the cooling method is not suitable.
  • Die material : Low thermal conductivity (i.e., low cooling efficiency)
  • Gate and runner : Non-uniform distribution of pressure in the dwelling process due to an insufficient number of gates or poor positioning.
  • Push-out mechanism : Poor push-out balance or excessive ejection load with respect to the pin surface area.
  • Parting : Insufficient polishing in the core extraction direction, inadequate extraction angle.
Cause(3)
Molding machine and accessories
  • Insufficient die clamping force : Inability to setup suitable clamping conditions (i.e., pressure and time)
  • Die temperature regulator : Flow volume of cooling agent is insufficient (i.e., Reynolds number is not large enough for turbulent flow), insufficient performance in terms of die thermal capacity.
Cause(4)
Molding conditions
  • Resin temperature : Pressure transmissibility drops when the viscosity is high; consequently, uniformity in the degree of shrinkage is not possible in the dwelling process.
  • Die temperature : When excessively low, the viscosity increases and the pressure transmissibility drops; consequently, uniformity in the degree of shrinkage is not possible in the dwelling process. Crystallization (or solidification) takes place before directional or stress relaxation can take place, and anisotropic residual stress remain.
  • Injection pressure : Either too high or low (i.e., flow length in excess of the plastic's flow characteristic)
  • Dwelling pressure : Either too high or low (Over-packing in the vicinity of the gates, or back-flow as a result of poor gate sealing)
  • Dwelling time : Either too long or short (Over-packing in the vicinity of the gates, or back-flow as a result of poor gate sealing)
  • Cooling time : Too short (dependence of material strength on temperature)

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