Internal stress and warping

Internal stress and warping

When thick sections are moulded,rapid changes in temperature can cause thermally induced stresses due to differential expansion. If surface layers cool faster than the interior through poor conduction, the contraction of the surface will be greater than the interior thereby setting up stresses which can lead to warping and even failure in service at less than predicted stress levels. Thermal stresses induced during manufacture can be reduced by annealing during moulding or after moulding.

Internal stresses can also arise from flow induced anisotropy. Anisotropy arises from two principal sources: molecular orientation and the alignment of directional fillers such as fibres.

Molecular orientation results from melt flow where the polymer chains are forced to change from their random coil state (ideally) to an elongated coil. The degree of elongation depends primarily upon the nature of the polymer and the shear rate (stress) experienced in flow. Shear rate depends on channel dimensions and increases as the channel cross-section decreases. Typical shear rates in practice are 1000-5000 S in injection

Shear rate depends on channel dimensions and increases as the channel cross-section decreases. Typical shear rates in practice are 1000-5000 S in injection moulding. The degree of coil distortion can be quite marked. On emerging from the channel (e.g.gate), the elongated coil will attempt to revert to the relaxed random coil state and if it can do so,the product will be isotropic. In practice,this ability to revert is hindered by

  • loss of mobility in the polymer due to cooling;
  • continued flow into the mould cavity.

The result is frozen molecular orientation in the general direction of flow. The degree of orientation is low compared with that which is deliberately induced in fibre and film production,but it can nevertheless be significant.

Frozen orientation produces stress which weakens the product and causes failure at lower applied stress levels since cracks can propagate more easily in the flow direction. However,stiffness is increased in the flow direction. Frozen orientation can also lead to dimensional instability. The application of heat induces molecular relaxation which produces warping or even gross distortion.

Molecular anisotropy can be minimised by using generous flow channels, low shear rates and slow cooling. Thin sections should be avoided. Fibres (usually glass) of length 0.3-0.5 mm incorporated into thermoplastics as reinforcements increase the anisotropic effects described above because the fibres tend to orientate in the same flow direction as the polymer chains. Mineral powders of aspect ratio greater than unity (e.g. talc) also contribute to anisotropy but less so than fibres.

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