The current popular material for the smart phone back cover market is composite panels and glass, both of which require a similar processing process: hot press forming, this process is currently slowing the yield rate, leading to a high cost of the back cover unit price.
The back cover of the composite plate is generally a 2.5D back cover and a 3D back cover. The 2.5D back cover has a planar structure, and the edge is processed by a CNC to form a circular arc, the process flow is simple, and the yield rate is also high, but the planar structure limits the appearance of the entire machine. 3D back cover must be thermoformed at high pressure, can make two or four sides arc to match the shape of the whole machine, the degree of freedom of the shape is large, but there are problems such as large processing technology, low material utilization rate, large CNC machining volume, low yield rate, etc. .
Figure 2.5D composite cover plate: The processing technology is mature, but the planar structure limits the overall shape
Figure 3D composite cover plate: There are problems such as large processing technology, low yield rate, and high costs.
The glass back cover is generally divided into a 2.5D back cover and a 3D back cover. Glass processing has problems such as long CNC time, long grinding and polishing time, and low hot-pressing yield rate. The 2.5D glass back cover unit price is around 50 yuan, and 3D glass. Back cover unit price is about 100 yuan, due to the cost of 3D glass is generally only used in the high-end or flagship models of the terminal.
As a long-applied mobile phone material, plastic can be injection molded and applied to most structural parts of mobile phones such as panels, middle frames, and back covers. However, as metal, glass, and composite board become mainstream materials in the intelligent machine age, plastic The application on mobile phones has been greatly reduced. The reduction in plastic applications, on the one hand, is due to the user's aesthetic fatigue, terminal manufacturers need to use new materials, new processes to enhance consumer desires and boost consumption levels, where the iPhone is a leader; the other is The limitations of plastic materials, such as low surface hardness and poor wear resistance, thin-walled products, poor rigidity, plastic injection molded parts can not meet the strict space and thickness requirements of smart phone terminals.
Plastic material suppliers have long been aware of the shortcomings of plastic injection molding, and have been working hard to develop materials with improved hardness, wear resistance, transparency, formability, etc., and are actively looking for application opportunities. The industry believes that short-time glass and 3D composite The board cost is difficult to reduce. The plastic injection phone back cover may be an application breakthrough for injection molding. The injection molding back cover is an efficient and low-cost alternative. There are several aspects that need to be focused on:
First, the material's wear resistance
Plastic back cover is made of PC material. Generally, the surface hardness of PC is lower than HB, hardened no higher than 2HB, lower than 4H of PMMA/PC composite plate, 7H of glass, hardness does not meet the wear resistance requirements of mobile phones. , Many raw material manufacturers continue to research and develop. Mitsubishi Chemical promoted in 2017 the new bio-engineered plastic DURABIO material for mobile phone back cover to shape the mobile phone back cover, and improved the glossiness and texture of the back cover through UV coating light curing treatment. Forms a wear-resistant, scratch-resistant protective layer (5H hardness), which gives the product a glass gloss and abrasion resistance, as well as high impact strength requirements.
Figure DURABIO material injection phone back cover
Second, ordinary injection molding process
Due to the thin wall design of the back cover of the mobile phone, general PC materials are prone to short shot and poor rainbow pattern under normal injection molding process conditions. Xiao Bian learned that Mitsubishi can provide support in this regard and solve the problems of ordinary injection molding of the rainbow pattern. Mitsubishi DURABIO bio-engineered plastic has ultra-high light transmittance (3MM92%), high surface hardness, excellent yellowing resistance and impact resistance make it more perfect in the injection phone back cover. At the same time, due to the DURABIO has a good flow The excellent performance on the nature and in the injection rainbow pattern makes DURABIO can use the ordinary injection molding process to produce the mobile phone back cover, and the ordinary injection DURABIO has no rainbow pattern, so that the customer can use the ordinary injection molding machine to produce without repetitive investment.
Figure Ordinary injection molding mobile phone back cover (source Mitsubishi)
Third, the injection compression process route
At present, the thin and light design of smart phones, the thickness of the composite plate or glass back cover is generally about 0.5mm, this thickness is very difficult for molding 5 inch and above mobile phone injection backboard. Injection backplane requires high transparency, high brightness, thin Small deformation of the wall, excellent optical performance, difficult to meet the general requirements of the injection molding process. According to the Xiaobian, an injection molding compression molding technology that was applied to the front transparent panel injection as early as Nokia's functional machine era is expected to solve the molding problem of the plastic back cover. .
The significant feature of injection compression molding compared with traditional injection molding process is that the mold cavity space can be automatically adjusted according to different requirements. For example, it can make the mold guide part closed before the material is injected into the cavity, and the cavity space It can be enlarged to twice the wall thickness of the finished part. In addition, the size of the cavity space can be controlled during the injection of the material or after the injection, depending on the operation method, so that it can be matched with the injection process to keep the polymer. Appropriate pressure state, and to achieve the effect of compensating material shrinkage. Injection compression generally has three ways to achieve:
1, parallel injection compression molding: In this method, the melt is first injected into a larger cavity, the two mold halves together to achieve the target thickness.
2. Extended Injection Compression Molding: In this method, the cavity pressure generated during the melt injection stage or controlling the movement of the closed end causes the cavity to expand first and then decrease to the target thickness.
3, wedge-shaped injection compression molding: In this method, the compression gap starts with a wedge-shaped opening, and then with the injection of the melt, while 'collapsed' to the target thickness.
Figure injection molding compression mobile phone back cover (source Mitsubishi)
The following are some of the manufacturers Xiaobian understands that can provide injection molding compression molding services:
Engel Austria GmbH
Demag Plastics Group
Sumitomo
Arburg
Toshiba Machine Group
Mitsubishi Chemical
Haitian Plastic Machine
Zhenxiong presses
Buston Machinery Technology Co., Ltd.
Fourth, the injection molding mobile phone back cover decoration process
Injection back covers can take two mature high-yield exterior decoration processes:
1, IMR + injection back cover, after molding the product does not need to increase UV coating hardened, only a small amount of shape CNC machining;
2, paste explosion-proof membrane + injection back cover, after injection molding products and then UV coating + AF, and then paste the back of the explosion-proof membrane, explosion-proof membrane technology and the same composite board or glass, without adding new equipment investment.
It is understood that industry players such as BYD, Dongfang Liangcai, Jierong, Tongda, Jinsheng, Lianyu, Xinwangda, and Waikiki are all investing resources in the research and development of injection smart phone back covers, millet, Huawei, OPPO, vivo, Lenovo , Transmission, Tianlong, ZTE and other terminal manufacturers are expected to apply this technology to launch related terminal products.