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,
• 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.