The design of automotive lamp mold is not as simple as we thought, Every mold designer must know some key points before designing：
1) Special injection molding equipment is required, and injection molding machines specializing in production of BMC plastics are required. Requirements for injection molding equipment are very strict.
2) BMC material is a special hard plastic. In plastic mold design, it is necessary to design a heating system and a parting surface design discharge system. Molded parts must be quenched to improve wear resistance and mold life.
3) Reflector plastic parts are used in car lights to prevent direct light, reflect light and avoid direct light. Light distribution requirements are strict. Plastic parts are the most important appearance parts of automobiles. There are many patterns on surface of plastic parts for purpose of decorative beauty.
4) Plastic mold design of BMC material should be balanced. Specifications of push rod should be designed as large as possible, and number should be as large as possible. Otherwise, mold release will be difficult.
5) Because stripping angle of high-gloss plated part is too small, demoulding is difficult, so draft angle of side wall of mirror plastic part should be designed as large as possible, generally recommended to be 5°~10°. Of course, premise is that it cannot affect function and shape of plastic parts.
6) Plastic parts can not have sharp edges and sharp corners. All corners need to be designed as rounded corners, because molded parts are susceptible to stress cracking after quenching.
7) Note that left and right mirror lamp head holes, plastic part surface pattern are left and right translation, can not be designed as mirror symmetry, because bulb and lamp head are not divided into left and right, and other features are mirror symmetry.
Plastic Part design could become tricky and compromises and trade-offs usually lead to failures. Automotive OEM’s will mainly use plastic material in cosmetic components and some other non-load carrying components.
Main features of plastic part design would include:
Good Knowledge of Material Database – There are many sources of data available for 100’s of plastic material types and grades in the market. But very few would provide complete understanding of material properties. When select the plastic materials, temperature and time variation of strength should be considered, as it could significantly lead to development of creep & fatigue.
Type of Loading & Application – Short term cyclic loading or long term constant load. In both cases, the maximum allowable stresses are going to be significantly lesser than the tensile strength of material. The environmental exposure of the part can also decide the life of the part as it degrades on coming in contact with chemicals, UV rays, sunlight and temperature.
Part Design Features – Many factors like ribs, notches, wall thickness, holes can contribute to stress concentration & residual stresses in the part. Thus careful consideration of part features should be done in order to optimize the stress lines within the part. For example, any abrupt change in wall thickness may lead to high stress concentration and thus reduce strength of the part.
Process Induced issues – process control parameters like cooling time, heating temperature, poor material mixing, poor part design leading to improper material flow can significantly affect the structural characteristics & quality of the part.
Basically, the manufacturer try to cut costs by compromising on either of these factors which should be taken care by the product engineering team of the automotive company.