Transparent injection molded back cover of mobile phone is considered to be the next mobile phone shell technology that may erupt. It is generally believed that injection compression process must be adopted. Ordinary injection molding can not solve the stress rainbow pattern produced by ultra-thin transparent products, but in fact, the benefits of injection compression process are not only to reduce the rainbow pattern.
"Injection compression molding" means that the thermoplastic polymer melt enters into a slightly open mold, and at the same time or subsequently carries out short stroke mold closing compression. This kind of short stroke die closing compression can be completed by machine or by stamping in the die. The pressure established in the cavity is evenly distributed in the two directions of the product surface, usually to improve the precision of the product surface, dimensional stability and the repeatability of the production process. The injection compression molding process is a low-pressure molding process that does not damage inserts, such as decorative films or metal inserts. Trim or metal inserts are protected due to the low pressure process of the injection compression molding process.
Fig. injection compression process (source: 3 scenes)
In the application of optical engineering, low injection pressure and holding pressure can minimize the internal stress, so it has a significant impact on the optical characteristics. Therefore, the injection molding machine must have a high degree of adaptability, but also a very accurate, repetitive movement function. So why injection molding compression should be used for the injection molding of transparent mobile phone shell? It's not just rainbow lines.
Figure: action diagram of compression mold (source: 3 scenes)
Reduce the rainbow pattern after hardening: This is just a small problem. After the back color and film are added, the rainbow pattern phenomenon is no longer obvious. It really affects the follow-up of injection molding. Only after the products are processed in batches can we summarize why compression injection molding is used.
Solve the problem of hard crack and local color difference in injection molding: the stress is too concentrated to improve the hardness. At present, the hardness hardening method generally adopts special hardening liquid, which has strong corrosion to batch color. If the stress can't be released, it will be hard to crack. The ball dropping experiment and roller experiment of the finished product are also very sad.
To solve the problem of partial thin filling: at present, transparent injection molding needs to be spray coated on the front and hard, with arc edge, otherwise the spray liquid will enter the back. In this way, the corresponding structural design will be extended. At present, all the back covers of mobile phones are assembled with an angle. The wall thickness at the end is thinner than that at the front. Generally, the way of dispensing and pasting is used. In this way, the local thick wall is very thin, some of which are 0.2 and 0.3 thick, which can be completely filled when compression injection molding is used.
Solve product Flatness: the flatness effect of compression injection molding is more than 50% higher than that of conventional non compression injection molding.
Solve the heating deformation in the later process: in the later process, hardening, coating, back printing, back spraying and so on, all need temperature, and release the stress, which is equivalent to annealing once. For example, the mirror silver may be baked for 1-2 hours, which is also the process of stress release of the material itself during the heating process. If compression injection molding is not used, the stress is relatively concentrated, and deformation will occur during the release process. It is difficult to control the deformation, flatness and size of the whole finished product in the later stage.
