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Technical : 9.1 Oval Shaped Parts

9.1 Oval Shaped Parts

We occasionally receive notice of a discrepancy described as “oval-shaped” for some of the products we manufacture, and have been asked to clarify the situation for inspection departments.

For injection-molded thermoplastic parts the resin is heated to a liquid phase and forced into steel dies. After sufficient time the resin cools to a stable solid phase and is ejected from the dies. During that cooling period the resin is confined to the dimensions of the dies, yet does undergo predictable shrinkage.

Assuming proper molding conditions are present in the molding cycle, i.e. resin melt- temperature, injection pressure, injection location, hold time, back pressure, etc., shrinkage and internal stresses will be at a minimum.

Due to the confinement during the transition from liquid phase to solid phase, any distortion due to shrinkage or internal stresses is minimized. Very high dimensional consistency is easily obtainable.

PTFE (Polytetrafluoroethylene) resins however, are not injection-molded thermoplastics and are fabricated in a compression-sinter process very similar to powdered-metal technology.

The resin is compressed at room temperature to a pre-determined set of dimensions and then removed from the dies. The then fragile part is placed in an oven and undergoes a sintering (baking) cycle, which chemically bonds the resin, causes pre-determined shrinkage to take place, and relaxes all internal stresses. The shrinkage and stress-relief take place because the part is in an unrestricted “free” state during the sintering cycle. As a consequence of this required “free” state, there are no restrictions to inhibit any distortion that takes place during the cycle. It is that distortion which causes un-sintered round parts to become out-of-round. The out-of-round part resembles an ellipse, and is sometimes referred to as “egg-shaped”.

This condition exists in all compression-molded parts to some degree and is normally insignificant due to the plasticity of the sintered resin. Where the distortion is great enough to cause measurement difficulties, the parts are typically placed on an appropriate mandrel which “rounds” out the diameters. The I.D. check becomes a matter of whether the part slips on or off too easily vs too tight a fit. The “rounded” out O.D. is then checked with suitable measuring devices. The remaining difficulty is those circumstances when the cross-sectional wall of the part is so great as to make the mandrel technique difficult. In that case dimensions are taken at several points around the diameters and averaged.

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