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At present, the mainstream non-metal back cover of mobile phones is made of glass, composite board and ceramics. In 2018, the composite board is used as a glass-type mobile phone back cover solution, which is beneficial to its relatively high cost performance (18-30 yuan/PCS) relative to glass. Customizable appearance, simple process flow, fast climb and other advantages, adopted by a variety of terminal models, such as OPPO A3, vivo Z1, OPPO realme1, Lenovo K5 PLAY, etc., Tongda, Huicheng, Weibo, A Tes, Zhongchen and other companies have benefited a lot from the back of the composite board.
After the composite board, the injection-molded transparent PC back cover as a lower cost and high-efficiency product form, with better impact strength and durability, is attracting more and more attention from traditional plastics factories, and is expected to become the mainstream mobile phone in 2019. Back cover solution. It is understood that BYD, Oriental Bright, Jinsheng, Liansheng, Xinwangda, Tongda, Huike and other manufacturers are accelerating the layout of transparent PC mobile phone back cover. Mitsubishi Chemical, Sabic and other raw material manufacturers are also verifying the special materials for PC injection mobile phone back cover, which can meet the requirements of high hardness, low stress and no rainbow pattern. At present, the industry is developing and verifying the normal injection PC mobile phone back cover, the thickness is generally 0.8mm to 1.0mm, which is much thicker than the mainstream glass back cover (0.50mm/0.55mm). The thickness of the injection molded transparent PC back cover should be as much as possible. 0.5mm to 0.6mm close, otherwise it will limit its application on thin and light models.
As an advanced form of injection molding, injection compression molding is more suitable for the molding of thin-walled transparent optical products, and is widely used in optical lenses, ultra-thin light guide plates, and thin automotive decorative panels. The injection-molded PC back cover can achieve the benchmark glass in thickness without compromising the thickness or appearance of the whole machine. It is being imported and developed by more and more manufacturers.
Figure Injection compression mobile phone back cover (source Bi Sheng)
First, the decoration process of injection molding transparent PC mobile phone back cover
At present, the injection molded compressed PC mobile phone back cover has the following appearance decoration craft routes:
1) Injection compression + IMR, suitable for flat or 2.5D mobile phone back cover, the disadvantage is that the IMR film development cycle is long, the pattern customization freedom is small. The UV layer of the IMR decorative film is only a few micrometers, and the long-term use of the UV layer is destroyed to cause pattern wear problems.
2) Injection compression + IML, suitable for 3D mobile phone back cover, the disadvantage is that the IML film layer has the risk of peeling off. The outermost layer of the back cover is PET or PC IML sheet, the visual permeability is not enough, and the high-grade feeling is insufficient.
3) Injection compression + hardening + film, the disadvantage 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, the surface hardness can be comparable to that of glass, and the rear-end film decoration process of glass/composite board can be reused, the industrial chain is complete, and it is difficult to meet the appearance requirements of the terminal. Xiao Bian believes that this is currently the most feasible solution.
Figure Composite film film appearance effect (photographed in Jierong), injection compression PC back cover can reuse this process map PVD decoration
The injection molding PC back cover can save the process in the red dotted frame, the process is greatly simplified, and the cost is greatly reduced.
Figure Injection molding PC back cover process
Second, the introduction of injection compression technology
Injection compression molding is an advanced form of traditional injection molding that combines both injection molding and compression molding. In the filling stage of the injection rubber, the cavity of the mold is not completely closed. At this time, the thickness of the cavity is larger than the thickness set by the product, and the melt can be injected into the cavity at a low pressure state, after which the clamping mechanism is operated, and the cavity of the movable mold is closed to the product. The thickness is set, and the entire mold cavity pushes the melt to fill the entire cavity and completes the pressure holding action. The pressure established in the cavity is evenly distributed in the two directions of the product surface, which greatly improves the precision of the surface of the product, dimensional stability and repeatability of the production process. Due to the small orientation of the injection molded product and low internal stress, it is particularly suitable for products with low thickness and high transparency. The difference between traditional injection molding and injection compression molding is shown in the following figure:
Figure Traditional Injection and Injection Compression Process (Source Network)
Figure Injection molding process (source network)
Conventional injection molding is often incapable of forming large-area thin-walled products (such as optical lenses, shaped light guides, decorative strips, etc.), and the result is often the designers compromise: increasing product thickness and reducing product color values to meet common production. This result is caused by the inadequacy of ordinary injection molding:
1) When ordinary injection molding of thin-walled products, it is necessary to increase the injection pressure to meet the requirements of the plastic process, or to fill the cavity (short shot). The high injection pressure leads to high internal stress, excessive warpage of the product or stress cracking after standing, which cannot meet the assembly dimensional accuracy requirements or durable use of the product.
2) As shown in the above figure, the ordinary injection molding melt fills the cavity from the narrow flow channel system, and the injection pressure is high near the gate, which easily causes appearance defects (especially transparent PC material) near the gate, which cannot satisfy the optical performance and Requirements for appearance products.
3) Transparent PC mobile phone back cover, the surface of ordinary injection molded products has a great chance of rainbow pattern, which can not meet the appearance requirements. The use of raw materials that do not produce rainbow stripes will face the problem of a sharp increase in costs.
4) There is a defect in the pressure-preserving mode of ordinary injection molding. The surface of the molten rubber away from the gate portion is solidified at the earliest time, and sufficient pressure is not obtained. It is difficult to duplicate the fine appearance effect of the mold, and the appearance of the thin-walled product away from the gate position is not perfect.
Third, the advantages and disadvantages of injection compression molding
Injection compression molding combines the advantages of ordinary injection molding and compression molding, and realizes automatic, high-precision and repetitive production on a dedicated organic table. It has advantages in thin-walled transparent PC mobile phone back cover molding, and its advantages and disadvantages are as follows:
Advantage:
1) Requires less injection pressure and reduces product appearance defects;
2) Due to the slight shear stress in the melt, the internal stress of the product is small, the pressure of the pressure is more uniform, the surface quality of the product is improved, and the rainbow pattern is lowered;
3) increase the product density, while the internal stress is reduced, the chemical resistance of the product is stronger, and a higher grade hardening liquid can be used to make the hardness of the product higher;
4) A smaller clamping force is required, and the small machine has the opportunity to realize multi-cavity production and improve efficiency;
5) It can be modified by standard machine to reduce equipment investment;
Disadvantages:
1) Compared with the traditional injection molding process, the injection molding process is more complicated, and the equipment cost is higher under the same clamping force tonnage;
2) Injection compression mold is more complicated and technically demanding;
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