Automobile R&D: Structural Design Code of Injection Moulding Products

I. What are injection moulding products?

Generally speaking, injection moulding products refer to the injection molded products made by the following process: heat, plasticize and melt the plastic by an injection molding machine, then inject the plastic into the forming mould cavity for molding. After cooling and solidification, demould the plastic.

II. The application of injection moulding products in automotive field

Automobile Plasticization is a developing trend of international automobile manufacturing industry. High-function plastics are increasingly used in automobiles as the automotive market shifts to lighter sport utility vehicles and other light vehicles. The adoption of plastics helps reduce the weight of vehicles. A 10% reduction in vehicle weight can lead to a 5% increase in fuel economy, and carmakers can also reduce costs by replacing traditional steel with plastics. Simple plastic injection molding parts can perform the function of many metal parts that are welded together, and plastics can be molded into more complex shapes than steel assemblies, which reduces the number of parts used in the integration process.

III. Design principles of injection moulding products

What principle should injection molding product abide by in structural design? This section covers them in detail.

1. Thickness design

The problem brought by uneven wall thickness in product design is more serious than any other design problems. These problems include fog marks, jet marks, air marks, scorch marks, shrinkage marks and shrinkage holes, short shots, weld marks, hysteresis marks, stress marks, warping deformation, and long cycle times. These potential defects can be predicted directly or indirectly by CAE.

2. Corner design

Stress concentration occurs at sharp corners. Plastics, such as nylon and polycarbonate, are sensitive to V-shaped notches, while plastics such as ABS and polyethylene are less sensitive to V-shaped notches, therefore, high stress will generate on the inner fillet when moulding.

When the inner fillet radius of the 90° corner is less than 25% of the nominal thickness, there will be a high stress concentration in the corners. When the radius of the inner fillet is increased to 75% of the nominal thickness, the junction of the two walls can be further strengthened. The acceptable average inner fillet radius is 50% of the nominal thickness.

3. Design of demoulding angle

If the demoulding angle is too small, it will lead to the defects such as stress marks, trailing whites or long cycle time. The demoulding angle should not be less than 0.5° and should be greater than 1°. Of course, within the acceptable range of the customer or product structure, the larger the demoulding angle, the better.