Cracking of injection molded chair base

Background: Bright color chairs made for kindergarten with metal legs fastened via screws and bolts were failing during shipment or use. Chair Bases were made of HDPE with yellow, blue, red and green colors.   Yellow chairs showed no failure but the others did.

Approach:

  • Visual observation indicated chairs were breaking close to where legs were fastened.
  • The creaks were straight, not radial and the edges of cracks were clean. There was no sign of crazing or stress whitening.
  • Density measurements indicated the resin to be HDPE with specified density.
  • MFR was close to specified indicating no process induced degradation.
  • DSC test of samples taken from crack areas indicated that the HDPE in red and green color chairs had higher Tc and % crystallinity than the HDPE of yellow chairs. Color pigments used in red and blue were acting as nucleating agents causing material to crystallize faster.
  • Further measurements indicated higher post mold shrinkage in red and blue chairs.
  • It was found that the metal legs were assembled as chairs. Molded and fully assembled chairs were stored in stacks of 50.
  • The post assembly shrinkage caused high level of stress in highly crystalined areas and caused chair bases to crack.

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