Rule 1: using a uniform wall thickness wherever possible, indicates a good design because the thickest section will cool last and will shrink more, thus result in a sink mark. The varying lengths of cooling time in a part with sections of varying thickness will also cause internal stresses, warpage or distortion and probably cracking.
Corollary to Rule 1: Using a minimum wall thickness which can result in a satisfactory part will save the material and molding time thus reducing the part cost.
Rule 2: Sharp corners should be avoided because sharp corners add non uniformity to the wall thickness.
Rule 3: Parts should be with the wall sections within the normal molding capability.
Rule 4: When non uniform wall thickness cannot be avoided, then the different wall sizes should be gradually blended. In any case the wall thickness should not vary by more than a ratio of 3:1 if at all possible.
Thumb rule 1: While designing housing or a covering, it can be thermoformed from a sheet of uniform thickness. Also the parts made of impregnated fabrics are with uniform wall thickness.
Thumb rule 2: Wall thickness recommended for heat resistant material like Polycarbonate or Polysulfones can be greater than the conventional material.
Thumb rule 3: Wall thickness for thermoset material depend on the type of resin and molding operation e.g. Epoxies, Alkyds and Polyesters molding powders and compounds are very fluid when heated, hence very thin wall thickness up to 0.01 to 0.015 inch is possible. Phenolics are molded up to 3 inches and more thickness. Epoxies or Polyester fabric reinforced, low pressure molded parts range from 0.005 to 1 inch thickness if and only if the air bubbles are completely removed and the part is thoroughly cured.
Thumb rule 4: In thermoplastics, thickness up to 3 inches is not feasible due to the excessive cooling time required.