Design points of injection casing for household appliances

With continuous development of science and technology in my country, in information age, home appliances have become more diversified, at the same time, they are developing towards integration, miniaturization, and intelligence. In context of home appliance market becoming more mature and over-performance, consumers have not only paid attention to performance of home appliances, but also put forward higher requirements for appearance of product design. Shells of home appliance mould are mainly divided into two categories: plastic materials and metal materials. Due to advantages of plastic materials, such as good plasticity, strong melt fluidity, simple manufacturing process, and low cost, materials of parts of household appliances are mostly plastic. Common injection molding materials include polypropylene PP, polycarbonate PC, polystyrene PS, acrylonitrile-butadiene-styrene triblock copolymer resin ABS. Molding process of plastic parts includes blow molding, extrusion, injection molding, etc., among which injection molding is the most widely used molding technology.

Design points of injection casing for household appliances

1 Wall thickness design

Wall thickness of a plastic part is mainly determined by its external dimensions and materials, but when designing plastic products, uniform wall thickness is primary principle. Uneven wall thickness can cause defects in appearance of plastic parts, such as shrinkage, depression, deformation, and even internal stress, resulting in appearance cracks.

If wall thickness is too thin, it will increase flow resistance of molten plastic, hinder flow of material in cavity, finally make it difficult to fill material, and plastic parts will have unfilled and poor strength.

Of course, the thicker wall thickness is not the better, the thicker wall thickness, product is prone to bubbles, shrinkage, depression, deformation and other appearance defects, the thicker product, the heavier its weight, resulting in an increase in cost of entire product. In addition, the thicker wall thickness of product, the longer molding time, mainly because pressure holding and cooling time during molding become longer, resulting in low production efficiency. However, sometimes due to strength of product, local wall thickness will be thicker. At this time, it is necessary to make a gradual transition between thick and thin to avoid sharp changes or sudden changes in thickness.

2 Design of fillet and draft angle

Stress at sharp corners of injection molded case is relatively concentrated, and brittle failure is likely to occur in plastic sharp corners. Eliminating sharp corners of product can reduce stress concentration and increase strength of reinforcing ribs of plastic product, thereby enhancing product's ability to resist external impact.

Therefore, in the appearance and structure design of home appliance mould factory, outer arc corner should be increased as much as possible. Of course, a too large rounded transition will produce a shrinkage effect, especially arc at the corners of raised column roots and ribs. Under normal circumstances, it is more reasonable to control arc size range between 0.3mm-0.8mm.

Drafting is for smoother demolding of plastic parts during mold production. Generally, draft angle of a plastic part is determined by multiple factors such as size of plastic part, wall thickness, shrinkage, and surface roughness of mold cavity. Because plastic parts in mold cavity will shrink continuously, it will be difficult to take out plastic parts if there are design defects in draft angle. Forcible removal will also increase the damage rate, such as napping and scratching. According to different types of plastics and structure of plastic parts, demoulding angle should be controlled between 20'～1°30'.

3 Reinforcing rib design

Reinforcing ribs can increase structural strength of plastic part, reduce probability of warping deformation, plastic consumption, product weight, and cost during production process without increasing wall thickness of plastic part. In design of ribs, position of ribs should be strictly controlled, not too high, not too small, and evenly distributed as much as possible to avoid cold shrinkage of thick rib bottom. Try to keep consistency of rib distribution direction and melt filling direction as much as possible, which is more helpful for opening and closing of plastic molding. Reinforcement rib adopts arc transition method, no parts can be arranged on reinforcement rib, so as to avoid excessive stress concentration. Reinforcing ribs are usually about 50% to 70% of wall thickness of plastic part. For this reason, we usually round corners at the bottom of ribs. Purpose is to improve fluidity of melt and improve stress concentration. Value of R angle is generally 1/8 of wall thickness.

In summary, structural design of plastic shell of home appliances must pay attention to design details. Under conditions of ensuring appearance, mechanical properties, and cost of shell, optimal design plan can be selected, design work can be completed more quickly with the help of computer design software to ensure design quality. At the same time, it is also necessary to adopt corresponding solutions to quality problems of injection molded case to ensure design quality of injection molded case. Of course, in actual design, a reasonable choice must still be made based on specific design schemes and design requirements.