At present, the mainstream non-metallic mobile phone back covers are made of glass, composite boards and ceramics. In 2018, the composite board is used as a back cover for glass phones, thanks to its relatively high cost performance (18-30 yuan/PCS), which is flexible and flexible. Customizable appearance, simpler technological process, fast speed on climbing, etc., adopted by many terminal models, such as OPPO A3, vivo Z1, OPPO realme1, Lenovo K5 PLAY, etc., Tongda, Huicheng, Weibo, A Tesla, Zhong Chen and other companies have benefited from the composite back cover.
After the composite board, the transparent PC back cover is injected as a lower cost, more efficient product form, with better impact strength and durability, which is attracting more and more attention from traditional plastics plants and is expected to become the mainstream mobile phone in 2019. Back cover program. It is understood that BYD, Dongfang Liangcai, Jinsheng, Lianhe, Xinwangda, Tongda, Weike and other manufacturers are accelerating the layout of injection molding transparent PC phone back cover. Mitsubishi Chemical, Sabic and other raw material manufacturers are also Verification of PC injection back cover special materials for mobile phones, can meet the requirements of high hardness, low stress and no rainbow pattern. Currently the industry is developing and verifying ordinary injection molded PC mobile phone back cover, thickness is generally 0.8mm to 1.0mm, compared with mainstream glass rear The cover (0.50mm/0.55mm) is much thicker, and the thickness of the injection molded transparent PC back cover needs to be as close as possible to 0.5mm to 0.6mm. Otherwise, it will limit its application in thin and light models.
Injection molding and compression molding are advanced forms of injection molding, and are more suitable for the formation of thin-walled transparent optical products. They are widely used in optical lenses, ultra-thin light guide plates, and thin automotive decorative plates. Injection-compression molded PC back covers can be used in thicknesses. To achieve the target glass without sacrificing the thickness or appearance of the whole machine, it is being introduced into more and more manufacturers.
Figure injection compression mobile phone back cover (source Bi Sheng)
First, the injection molding transparent PC phone back cover decoration process
At present, the back cover of injection-compressed PC mobile phones has the following appearance decoration process routes:
1) Injection compression + IMR, suitable for flat or 2.5D mobile phone back cover, the disadvantage is the long development cycle of IMR film material, and the freedom of pattern customization is small. The UV layer of IMR decorative film is only a few microns, and the UV layer is damaged due to long-term use. Pattern wear problem.
2) injection compression + IML, suitable for 3D mobile phone back cover, the disadvantage is the risk of peeling off the IML film layer, the back cover is the outer layer of PET or PC IML sheet, the appearance of visual permeability is not enough, lack of high-end sense.
3) Injection compression + hardening + film, the disadvantage is that the hardening process is increased, the yield rate is decreased, and the cost is increased. However, after breaking the hardened and CNC technology bottleneck, the surface hardness can be comparable to glass, and can be reused glass/ After the composite board's film decoration process, the industrial chain is complete, it is difficult to meet the terminal appearance requirements. Xiaobian thinks that this is the most feasible solution.
Figure Composite film appearance (photographed in Jie Rong), injection compression PC back cover can reuse this process map PVD decoration
Currently mainstream 3DGlass processing is as follows:
Figure Mainstream 3D glass processing
Injection molded PC back cover can save the process of red dashed box, the process is greatly simplified, and the cost is greatly reduced.
Figure injection molding PC back cover process
Second, introduction of injection compression technology
Injection compression molding is a high-level form of traditional injection molding. It combines injection molding and compression molding. During injection molding, the mold cavity is not completely closed. At this time, the thickness of the cavity is larger than the thickness set by the product. The glue can be injected into the mold cavity at low pressure. After that, the mold clamping mechanism is operated. The mold cavity of the movable mold is closed to the set thickness of the product. The mold cavity as a whole pushes the melt to fill the entire mold cavity and completes the pressure holding action. The pressure established in the mold cavity is uniform. The two directions distributed on the surface of the product greatly improve the precision of the surface of the product, dimensional stability and repeatability of the production process. Due to the smaller orientation of the injection compressed product, the internal stress is low, and it is particularly suitable for products with small thickness and high transparency. The difference between traditional injection molding and injection compression molding is shown in the following figure:
Traditional injection molding and injection compression process (source network)
Figure injection molding process (source network)
Traditional injection molding is often unsatisfactory when molding large-area thin-walled products (such as optical lenses, profiled light bars, decorative strips, etc.). The result is often the designer's compromise: Increase the thickness of the product and reduce the value of the product to meet the ordinary production. This result is caused by the shortcomings of ordinary injection molding:
1) When the ordinary injection molding thin-walled products, it is inevitable to increase the injection pressure to achieve the plastic flow requirements, and if not fill the cavity (short shot). High injection pressure, resulting in the product of internal stress, product warping size is poor or static After the stress cracking deformation, can not meet the assembly of the product dimensional accuracy requirements or durability.
2) As shown in the figure above, the ordinary injection molding glue fills the cavity from the narrow runner system, and the injection pressure is large near the gate, which can easily cause appearance defects near the gate (especially transparent PC material), and can not meet the optical properties and Appearance of the product requirements.
3) Transparent PC phone back cover, ordinary injection molding products have a large surface appearance of rainbow pattern, can not meet the appearance requirements. The use of raw materials do not produce rainbow lines, and will face the cost of the problem.
4) There is a defect in the pressure maintenance mode of ordinary injection molding. The surface of the melted glue that is far from the gate portion is solidified at the earliest, and the pressure cannot be sufficiently maintained, and it is difficult to reproduce the fine appearance effect of the mold. As a result, the appearance of the thin-walled product away from the gate is not perfect.
Third, the advantages and disadvantages of injection compression molding
Injection molding compression molding combines the advantages of ordinary injection molding and press-synthesizing types, and realizes automation, high-precision repetitive production on an organic platform, and has advantages in forming the back cover of a thin-wall transparent PC handset. Its advantages and disadvantages are as follows:
advantage:
1) The need for smaller injection pressures reduces product appearance defects;
2) Due to the tiny 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 product density, reduce the internal stress, and the chemical resistance of the product is stronger. A higher level of hardener can be used to make the hardness of the product higher;
4) The need for a smaller clamping force, the small machine platform has the opportunity to achieve multi-cavity production, improve efficiency;
5) The machine can be restructured to reduce equipment investment;
Disadvantages:
1) Relative to the traditional injection molding process, the injection molding process is more complicated, the same clamping force tonnage, the equipment cost is higher;
2) Injection compression molds are more complicated and technical requirements are high;