With the advent of the era of 5G and wireless charging, the requirements for signal transmission will be higher, and the defects of the metal back cover for signal shielding will be amplified, becoming a major bottleneck in its development. In the 5G era, metal shells were gradually replaced by non-metal shells. Among the three non-metal materials, glass, ceramics, and plastic, plastic has the worst texture, but the lowest cost; while ceramics are temporarily low in yield and expensive, and only some high-end models are used; glass is the first choice for most brands of flagship phones because of its moderate price and excellent texture.
Glass has become the choice of mainstream flagship mobile phones since 2016. The main applications of glass in the back cover of mobile phones are 2.5D and 3D. 2.5D has become the first choice for flagship mobile phones due to its simple process and high yield. 3D glass (curved glass) has not exploded due to factors such as production capacity, yield, and price.
The current mainstream 3D glass processing processes are as follows:
Figure: Mainstream 3D glass processing processes
Injection molded 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
The yield and cost difference of mobile phone back covers of different materials:
In every stage of mobile phone development, plastic materials play an important role. Because plastic materials have low prices, good plasticity, strong designability and other advantages, and through appropriate surface treatment, the visual vitrification/metalization effect can be produced. For example, the all-metal integrated mobile phone body that was popular in the past few years, the plastic back shell can also achieve a good imitation metal effect after metallization. This is of great significance to the widespread popularity of mid- and low-end mobile phones at that time. Similarly, when glass back covers are popular today, in order to achieve the appearance of glass and reduce costs, the industry has proposed a solution to replace glass with plastic.
First of all, you need to find a plastic material, which can achieve a visual effect close to glass after forming. First of all, this plastic material is transparent. The transparent materials that can be used as mobile phone shells are generally PMMA, ordinary PC, modified PMMA+PC, and high-performance PC. Currently, only high-performance PC is the most suitable. High-performance PC has higher light transmittance and higher surface hardness than ordinary PCs.
The back cover of the mobile phone with high-performance PC transparent injection molding is comparable to 3D glass after surface hardening and rear-section film/spraying. Its advantages include:
. The thickness is benchmarked with 3D glass, and the surface freedom is greater than that of 3D glass;
. It helps to reduce CNC processing time and significantly reduces cost;
. The molding rate is high and it is easy to start;
. High transparency, etc.
Figure: The gradient effect of plastic IMT in the left picture, the gradient effect of Huawei P20 PRO 3D glass back cover in the right picture
Currently, the back cover of the mobile phone with ordinary injection molded transparent PC is generally between 0.8mm and 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 must be as close as possible to 0.5mm to 0.6mm, otherwise it will be limited to its application on thin and light models. Therefore, a new injection molding process is needed to solve the injection molding problem of thin-walled shells.
Injection molding compression process
The so-called injection compression molding process is a molding method in which when the resin injected into the mold cavity shrinks due to cooling, a forced force is added from the outside to reduce the size of the mold cavity, thereby compensating for the shrinking part.
Injection compression molding (ICM) is an extension of traditional injection molding, combining two technologies: injection molding and compression molding . It compacts the polymer material by introducing mold compression action to produce products with high dimensional stability and surface accuracy.
In this process, the set plastic melt is first injected into the incompletely closed mold, and then the mold-closing and compression action is performed, and it continues until the molding process is over.The mold compression action can form a more uniform pressure distribution in the mold cavity, thereby obtaining more uniform physical properties, so that the shrinkage, warpage and molding stress of the product are significantly lower than that of the traditional injection molding process, and are especially suitable for products with small thickness and high transparency requirements.
Figure: Traditional injection molding and injection molding compression process
Figure: Injection molding compression molding process
Disadvantages of traditional injection molding:
Traditional injection molding often fails to work when forming large-area thin-walled products (such as optical lenses, special-shaped light guide strips, decorative strips, etc.), and the result is often a compromise of the designer: increase the product thickness and reduce the product appearance to meet ordinary production. This result is caused by the shortcomings of ordinary injection molding:
) When ordinary injection molding thin-walled products, the injection molding pressure will inevitably be increased to meet the plastic process requirements, otherwise the unsatisfactory cavity will be filled (short injection). High injection molding pressure leads to large internal stress of the product, excessive warping size or stress cracking and deformation after standing, which cannot meet the assembly dimensional accuracy requirements or durability of the product.
) Ordinary injection molded melt glue fills the cavity from a narrow runner system. The injection molding pressure near the gate is high, which can easily cause appearance defects near the gate (especially transparent PC material), and cannot meet the requirements of optical performance and appearance parts.
) Transparent PC phone back cover, the surface of ordinary injection molded products is very likely to appear rainbow patterns, which cannot meet the appearance requirements. Using raw materials that do not produce rainbow patterns will face the problem of dramatic increase in costs.
) The pressure-retaining mode of ordinary injection molding has defects. The melt adhesive surface far away from the gate is the earliest solidified, and it is not sufficient to achieve sufficient pressure-retaining, making it difficult to copy the fine appearance of the mold, resulting in the appearance of thin-walled products that are far away from the gate.
Figure: Comparison of different ICM molding technologies (from Sobert)
Advantages and disadvantages of injection compression molding:
Injection compression molding combines the advantages of ordinary injection molding and compression synthesis, and realizes automated and high-precision repeatable production on a proprietary machine. It has advantages in forming the back cover of thin-walled transparent PC mobile phones. The advantages and disadvantages are as follows:
Advantages:
) Need for smaller injection molding pressure to reduce product appearance defects;
) Need for smaller injection molding pressure to reduce product appearance defects;
) Since there are only slight shear stress in the melt, the internal stress in the product is very small, and the pressure holding pressure is more uniform, which improves the surface quality of the product and reduces the rainbow pattern;
) A larger flow-length ratio can be designed, so a smaller clamping force and injection pressure can be used to reduce the risk of stress deformation and the risk of tiger skin pattern in the part. At the same time, more gates are not required, which reduces the possibility of welding marks, and ensures the dimensional stability and appearance of the part;
) A smaller clamping force is required, and a small machine has the opportunity to achieve multi-mode hole production and improve efficiency;
) It can be modified with a standard machine to reduce equipment investment.
Disadvantages:
. You need to purchase special injection molding equipment.
. Special molds need to be developed.
. It is only suitable for parts with relatively flat shapes, but not for parts with complex structures.
The back cover of the mobile phone after injection compression molding is transparent, and it needs to be treated with subsequent film/spraying and other surface decoration before the final effect can be achieved.
Currently, injection-molded compressed PC mobile phone back cover has the following appearance decoration process routes:
1. In-mold decoration technology IMR process (also known as in-mold transfer):
The main principle is to print the decorative patterns and functional patterns on the film through a high-precision printing machine, and then, like labeling, the patterns on the foil film are transferred to the surface of the plastic product. This is a technology that can realize the integrated molding of decorative patterns and plastic. The IMR process has high production capacity and stable quality, and is suitable for mass production of fewer varieties. However, the product surface is unprotected and not resistant to scratches and corrosion, and is more suitable for flat or 2.5D mobile phone back covers.
2. In-mold insulated injection molding IML process (also known as IMF):
IML The product of the process is a sandwich structure, with a hardened transparent film on the surface, a printing pattern layer in the middle, and a plastic layer on the back. Compared with the IMR process, the IML process prevents the product from scratching and abrasion resistance because the ink is sandwiched in the middle, and can keep the color bright and not faded for a long time. It is more suitable for the back cover of the 3D mobile phone. The disadvantage is that the IML film layer has a risk of peeling off and falling off. The outermost layer of the back cover is a PET or PC IML sheet, which has insufficient visual transparency in appearance and insufficient high-end feeling.
3. In-mold injection molding transfer IMT process:
IMT is a new process formed by the combination and extension of in-mold insert injection molding (IML) technology and in-mold transfer (IMR) technology. The IMT process combines the most popular decorative techniques of 3D optical texture + PVD, which is more beautiful and has a larger curve than traditional ones. In addition to scratch resistance and corrosion resistance, the biggest highlight of the IMT process lies in its double-sided 3D optical texture, which can produce an off-layer optical 3D effect, making the product rich and beautiful colors, and more technological and fashionable. The IMT process combines the advantages and disadvantages of IMR and IML processes, solving problems such as single structure, poor corrosion resistance, single appearance effect, and no independent development rights for molds and diaphragms; and also solving difficulties such as insufficient IML production capacity, volatility, and low yield.
4. Injection molding compression + hardening + film/back spray/coating:
The disadvantage is that it increases the hardening process, the yield decreases and the cost increases. However, after breaking through the bottleneck of hardening and CNC technology, its surface hardness can be comparable to glass, and the rear film decorative process of glass/component board can be reused, with a complete industrial chain and it is difficult to meet the terminal appearance requirements.
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