Structural Design Analysis of Injection Molded Shells for Home Appliance

Time:2021-01-14 11:57:51 / Popularity: / Source:

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 household appliances 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.

1 Design points of injection casing for household appliances

1.1 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.

1.2 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 appliances, 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'.

1.3 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 injection mold. 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.

2 Design case of injection housing for household appliances

Following uses head shell of soybean milk maker as an example, as shown in Figure 1.
Injection Molded Shells for Home Appliance 

2.1 Design method analysis

(1) Material selection. Commonly used material of soymilk machine shell is generally PP or ABS. ABS has advantages of good fluidity, low shrinkage, impact resistance, heat resistance, etc., its surface has high gloss and wear resistance. Although PP material has good injection performance, its shrinkage rate is very large, product dimensional accuracy is poor, rigidity is poor and easy to deform. Surface of product made of it is prone to shrinkage, poor gloss and easy to scratch, but price of PP will be cheaper than ABS. Surface glossiness of soymilk machine's shell is required to be high, and there are requirements for temperature resistance. Considering comprehensively, it is recommended to choose ABS as material of shell. (2) Screw columns and reinforcing ribs. Due to assembly requirements, head shell of soymilk machine has several screw posts, and bottom of screw posts should be treated with craters to avoid appearance of shrinkage. Each screw column can add some reinforcement ribs to enhance strength of screw column, but thickness of reinforcement ribs should not be too thick, at the same time, root of reinforcement ribs needs to be increased with a small R angle in order to improve fluidity of melt. (3) Gate runner. Surface of machine head shell of soymilk machine is required to be sufficiently smooth, so gate should not be too large, and gate residue of plastic part after demolding should not be too obvious, so glue point method is adopted, gate can fall off by itself, leaving small marks. In order to better carry out follow-up cleaning of parts, use point gates, straight channels, and balanced runners. Cross-section is designed to be circular, so that surface area and volume of runner are minimized, which reduces injection time and helps improve productivity .

2.2 Drafting of main forming process

(1) Structural analysis. Outer contour of plastic part of machine head shell of soybean milk machine designed this time has an inverted groove for locking and locking, mold design must have a lateral parting core pulling or inclined top mechanism. Dimensional accuracy depends on changes in plastic shrinkage and mold processing errors. (2) Surface quality. After shell of machine head of soybean milk machine is injection molded, there must be no defects on inner and outer surfaces, edge position is smooth and tidy, there must be no weld marks and spots. Outer surface is smooth enough, roughness standards of inner and outer surfaces are Ra0.4μm and Ra0.2μm. If surface gloss of plastic part does not meet standard, it may be result of premature cooling of melt, insufficient drying time, insufficient mold temperature or material temperature, and poor polishing of mold working surface. (3) Molding method. Shell of soymilk machine is injection molded with thermoplastic ABS material. In order to control internal stress and reduce weld marks, gate injection process can be appropriately shortened to improve filling efficiency, injection temperature can also be appropriately increased to optimize design of mold exhaust structure. Set gate position scientifically, and produce weld marks on secondary parts of plastic parts as much as possible. During injection molding, injection pressure should not be too high, injection molding process should maintain a high material temperature and mold temperature.

3 Injection molding problems and solutions

3.1 Shrink

Shrinkage problem of injection molded case is directly related to internal structure of injection molding and excessive thickness of ribs, which will cause deviation of solidification degree of plastic case to increase during cooling process of injection molding, resulting in problem of volume shrinkage and surface depression.
Optimize design of injection molding process, formulate a shrinkage prevention plan, and control thickness of reinforcement within a reasonable range. If a material with a low shrinkage rate such as ABS is used, thickness of reinforcement is usually about 1/2 of thickness of main body to avoid overrun design. Shrinkage rate of PP and other materials is higher, thickness of reinforcement should be controlled at about 1/3 of thickness of main rib. Combining actual information, if conditions permit, method of combining internal structure characteristics and appearance structure can be used to avoid shrinkage of injection molded case and improve injection quality of injection molded case.

3.2 Lack of material

Lack of material is mainly caused by poor mold exhaust, excessively deep design structure, excessively thin local wall thickness, and unreasonable gate design.
For this kind of problem, it is necessary to scientifically set injection pressure, gradually increase injection pressure until front surface appears, so as to avoid lack of shell glue. If pressure cannot be fully filled even after pressure increases, adjust model and reset diversion groove, gate size and position of injection molded part, re-adjust injection overpressure parameters for feasibility test until injection molded part can meet molding standard, injection efficiency can be improved on the basis of ensuring quality of injection molding.

3.3 Weld line

Home appliance shell injection weld marks refer to traces on plastic surface. This is caused by excessive aggregates entering mold during injection molding process to split and agglomerate. Under action of external forces, traces are prone to cracking.
Problem of weld marks can be solved by increasing material temperature, mold temperature, injection pressure, and injection speed. However, for invalid weld marks of above solution, you can consider changing gate position, increasing gate size, and changing wall thickness of product.

3.4 Warpage and burrs

There are many factors that cause warpage and deformation of injection molded shell, such as unbalanced product cooling, insufficient holding time, shrinkage, and so on. However, as a very common quality problem, burr drape mainly occurs in joint part of plastic parts. If injection molding parameters are too large, burr drape problem is likely to occur.
In view of problem of warpage deformation, reinforcement ribs can be appropriately added to increase strength of product, injection pressure and speed can be increased to improve problem of warpage deformation. In this process, cooling time of injection molding should be set reasonably, and injection molded part should be shaped as soon as possible after mold is ejected. Rapid cooling through water is used to reduce incidence of warping deformation. For problem of burrs and flare-ups, quality of injection mold must be controlled, parting surface setting of injection molded part shall be considered in an all-round way to reduce impact of parting on the structure of injection shell. Surface of finished product formed on male mold side should be reduced by 0.05-0.1mm, even if finished product has a gap of 0.05-0.1mm, it can better play the role of sealant and achieve purpose of optimizing injection molding process.

4 Conclusion

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.

Go To Top