· The tolerances specified for any molded or machined plastic part should be as loose as possible because extremely tight tolerances require more expensive molds, special controls during the molding operation and possibly the use of shrink fixtures during cooling. All these result in a part which is more expensive.
· In many plastic, the dimensional changes due to temperature variation from –550 C to 1200 C is considerably more than the allowable tolerances.
· For thermoplastics injection molding, compression molding generally results in parts with best dimensional tolerances. The dimensional tolerances for thermoplastic parts made by any method vary from part to part and material to material.
· The tolerances for thermoset plastic parts depend on the type of resin, filler, molding condition and the type of mold used.
· Parts made with thermoset plastic can be designed with a tighter tolerances than corresponding thermoplastic parts because thermoset plastic are more stable under thermal condition than the unfilled thermoplastic, when used for the same part.
· Filled thermoplastic depending on the amount and type of filler can approach or be equal to the dimensional stability of thermoset plastic.
· Tolerances vary depending on the location of the parting line and the depth of the flash. The molder should be informed by the critical dimension.
· The flash can affect the dimension in one plane and not in the other.
· Tolerances suggested by the resin manufacturer are given in inches/inch at the ambient temperature and 50%RH.
· Shrinkage of parts made by thermoforming, blow molding, rotational molding etc. (except compression molding and transfer molding) will show higher variation in tolerance than injection molding. This variation is determined empirically.
· Glass reinforced thermoplastic have considerably tighter tolerances than unreinforced material.
· The molder can make slight changes in the finished dimension of a molded part without the cost and trouble of re-matching, re-heat-treating, and re-plating the mold.
· The molder can increase or decrease the dimension or prevent the warpage.