Why is PC injection moulding imitating glass becoming the key technology of mobile phone back cover in 2019?

Throughout the development of mobile phone structural parts, the industry has already moved from the aluminum alloy and glass back cover dominated by Apple mobile phone around 2010 to a diversified material solution, and has introduced a variety of mobile phone material solutions under the technical route of ceramics, composite panels, injection moulding PC and so on. For example, China's BYD and other powerful suppliers have completed the whole industrial chain layout of ceramics, glass, composite panels, plastics, CNC, and provide a variety of packaging solutions. BYD Electronics has successfully transformed into a diversified customer and diversified technical service line after experiencing the experience and lessons of the decline of functional machines, the sharp decline of Nokia orders, and the near stagnation of the business of institutional components. Today, glass, ceramics, plastics, hardware and other businesses are blooming all over the line, with high risk resistance.

At present, the mainstream mobile phone non-metallic back cover materials in China market are glass, composite board and ceramic materials. In 2018, as the back cover of glass-like mobile phone, the composite board has been adopted by many terminal models, such as OPPO A3, vivo Z1, OPPO realme1, Lenovo K5 PLAY, due to its high cost performance ratio (18-30 yuan/PCS), flexible customizable appearance, simple process and fast climbing. Chengcheng, Weibo, Artes, Zhongchen and other enterprises have benefited a lot from the composite backing.

After the composite board, injection-moulded transparent PC back cover as a lower cost and efficient product form, with better impact strength and durability, is attracting more and more attention from traditional plastic factories, and is expected to become the mainstream mobile phone back cover scheme in 2019. Injection glass imitation process has been further developed and matured. Its core technology is compression injection mold. The former large injection machine parts factory has the ability to develop, and the retained precision electric injection machine can meet the production demand of injection compression after adding ejector mechanism and refitting control software. This time, without introducing new equipment, the traditional injection moulding factory can benefit from the market dividend of plastic back cover of mobile phone in 5G era, and recover the lost voice right for 10 years in smartphone market. It is understood that BYD, Oriental Brilliant, Jinsheng, LianMao, Xinwangda, Tongda, Huike and other manufacturers are accelerating the layout of injection-moulded transparent PC mobile phone back cover. Mitsubishi Chemical, Sabic, KCI and other raw material manufacturers are also verifying that PC injection moulding mobile phone back cover special materials can meet the requirements of high hardness, low stress, rainbow-free and so on. At present, the industry is developing and validating the back cover of ordinary injection-moulded PC mobile phone, which is generally 0.8mm to 1.0mm thick. Compared with the mainstream glass back cover (0.50mm/0.55mm), the thickness of injection-moulded transparent PC back cover should be as close as possible to 0.5mm to 0.6mm, which will limit its application in light and thin machine.

Picture：OPPO Realme (Injection Moulded Glass Technology)

As an advanced form of injection moulding, injection compression moulding is more suitable for thin-walled transparent optical products. It is widely used in optical lens, ultra-thin light guide plate, thin Automobile decorative plate and other products. Injection moulding and compression moulding of PC back cover can achieve standard glass in thickness without compromising the whole thickness or appearance. It is being imported and developed by more and more manufacturers.

 
Figure: Injection-moulded compressed mobile phone back cover (source Bisheng)

I. Decoration Technology of Transparent PC Mobile Phone Back Cover by Injection Molding

At present, the back cover of injection-moulded compressed PC mobile phone has the following appearance decoration process routes:

1) Injection compression + IMR, suitable for flat or 2.5D mobile phone back cover, the disadvantage is that the development cycle of IMR film is long, and the freedom of pattern customization is small. The UV layer of IMR decorative film is only a few microns. There is a problem of pattern wear caused by the destruction of the UV layer in long-term use.

2) Injection compression + IML is suitable for the back cover of 3D mobile phone. The disadvantage is that the IML film has the risk of peeling off. The outer layer of the back cover is PET or PC IML sheet, which has insufficient visual permeability and high-grade sense.

3) Injection compression + hardening + film sticking, the shortcoming is that the hardening process is increased, the yield is reduced, and the cost is increased. However, after breaking through the bottleneck of hardening and CNC technology, its surface hardness can be comparable to that of glass, and it can reuse the decoration technology of glass/composite board posterior film. The industry chain is complete, and it is difficult to meet the terminal appearance requirements. Xiaobian believes that this is the most feasible solution at present.

Figure: Appearance effect of laminated sheet film (taken in Jerong). Injection-moulded compressed PC back cover can reuse this process drawing PVD decoration.

At present, the main processing procedures of 3D glass are as follows:

Figure: Mainstream 3D Glass Processing Processes

Injection moulding of PC back cover can save the process in red dotted wire frame, greatly simplify the process and reduce the cost.

Figure: Injection Molding Process for PC Back Cover

Introduction of Injection Compression Technology

Injection compression moulding is an advanced form of traditional injection moulding, which combines injection moulding and compression moulding. In the filling stage, the die cavity is not fully closed. At this time, the thickness of the die cavity is larger than the thickness set by the product. The melt can be injected into the die cavity at low pressure. Thereafter, the clamping mechanism runs. The dynamic die cavity closes to the thickness set by the product. The dynamic model cavity promotes the melt to fill the whole die cavity and complete the holding action. The pressure established in the die cavity distributes evenly in the two directions of the product surface, which greatly improves the product surface fine precision, dimensional stability and production process repeatability. Because of the small orientation and low internal stress of injection-moulded compression products, it is especially suitable for products with low thickness and high transparency. The difference between traditional injection moulding and injection compression moulding is as follows:

 
Figure: Traditional Injection Molding and Injection Molding Compression Processes (Source Network) 

Figure: Injection Molding Process (Source Network)

Traditional injection moulding often fails to produce large-area thin-walled products (such as optical lenses, special-shaped light guide strips, decorative strips, etc.). The result is often a compromise of designers: increase product thickness, reduce product appearance to meet the needs of ordinary production. Such a result is due to the shortcomings of ordinary injection moulding:

1) When ordinary injection moulding thin-walled products, the injection pressure must be increased to meet the requirements of plastic process, or filling the cavity (short shot). High injection pressure leads to high internal stress, warping dimension oversize or stress cracking deformation after stationary, which can not meet the requirements of assembly dimension accuracy or durability of products.

2) As shown in the figure above, the melt of ordinary injection moulding fills the cavity from the narrow runner system, and the injection pressure near the gate is high. It is easy to produce appearance defects near the gate (especially transparent PC material), which can not meet the requirements of optical properties and appearance products.

3) Transparent PC mobile phone back cover, the surface of ordinary injection moulded products is very likely to appear rainbow lines, which can not meet the appearance requirements. The use of raw materials that do not produce rainbow pattern will face the problem of soaring costs.

4) There are defects in the pressure-retaining mode of ordinary injection moulding. The surface of melt glue far from the gate is solidified earliest and can not be fully preserved. It is difficult to replicate the fine appearance of the mould, resulting in the appearance of thin-walled products far from the gate position is not perfect.

Advantages and Disadvantages of Injection Compression Molding

Injection compression molding combines the advantages of ordinary injection molding and compression synthesis, and realizes automatic and high precision repetitive production on a dedicated platform. It has advantages in thin-walled transparent PC mobile phone back cover forming. Its advantages and disadvantages are as follows:

Advantage:

1) Need less injection pressure to reduce product appearance defects;

2) Because there is only a small shear stress in the melt, the internal stress of the product is very small, the holding pressure is more uniform, the surface quality of the product is improved, and the rainbow pattern is reduced.

3) Increase the density of the product, reduce the internal stress, and make the chemical resistance of the product stronger. The hardening liquid of higher grade can be used to make the hardness of the product higher.

4) Need smaller clamping force, small machine has the opportunity to achieve multi-mode hole production, improve efficiency;

5) It can be restructured with benchmarking machines to reduce equipment investment.

Disadvantages:

1) Compared with the traditional injection moulding process, the injection moulding process is more complex. Under the same tonnage of clamping force, the equipment cost is higher.

2) Injection compression moulds are complex and require high technology.

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