DESIGN RULE FOR DECIDING WALL THICKNESS

 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.

Preliminary Product Design

Preliminary design considerations:
Before designing any part, following factors should be considered:
FUNDAMENTAL DESIGN RULES: These are applicable to almost all types of parts (whether made from plastics or not). It includes the avoidance of ribs and fillets, spacing of holes etc.
ENVIRONMENTAL SURVEY: The function of the part and the environment in which it has to operate must be determined. The environment survey includes:
• Thermal conditions i.e. high or low temperature.
• Mechanical stresses on the part i.e. tensile load, compressive load, impact load etc.
• Electrical stresses i.e. high voltage, frequencies, dielectric losses, etc.
• Combined thermal and chemical stress i.e. strong acids, solvents, oils, high humidity etc.
• Operational time i.e. length of service.
USER’S ACCEPTANCE GUIDE: This criterion is difficult to determine, however by proper consideration, the designer can avoid obtaining unacceptable part after the material decision has been taken or after the first part is made. The list includes:
• Colour or lack of color,
• Transparency or opacity,
• Weight (specific gravity) – whether the part should float or it should have heavy fillers,
• Cost
• Hard to define terms like feel of the part, sound it must make when struck etc.
OPTIMUM MATERIAL: While selecting the optimum material, the most important factor is selected as screening guide so that the list of candidate material is shortened. The final choice is then made for a material which gives the optimum performance at lowest cost. Usually a compromise is considered.
FABRICATION PROCESS: The choice depends on:
• The number of parts or the material chosen i.e. when 12 prototypes are made, machining and casting is chosen and when one million parts are required injection molding is chosen.
• The shape of part
• By the equipment available.