In the 4G era, mobile phone molding technology has been eroded by glass, ceramics, and hardware! The once prosperous ODM and OEM manufacturers either went bankrupt or laid off employees or changed careers. With the advent of the 5G era, the low cost and high practicality of the molding process have become popular again. Due to the huge increase in 5G signal transmission, metal casings are first eliminated (natural signal shielding problems). The processing properties of ceramics are too low, they can only be used as nobles, cannot be supplied in large quantities, and can only be the simplest design. 2.5-3D glass has a long processing flow and complex technology. The yield and throughput rate are basically unable to guarantee market supply, and the cost remains high. Looking around, only the molding process can meet many requirements such as signal, appearance, cost, supply volume and so on.
? It is said that the proportion of plastic mobile phone casings will gradually increase in the 5G era. Plastic may account for 70% in 2019. There are several new processes for plastic mobile phone casings, composite sheets (2.5D/3D), IMT (3D), and PC injection imitation glass. Transparent PC imitates glass, and the stress lines caused by injection molding are particularly obvious after hardening, which makes the terminal have some concerns about the color selection. In order to reduce or eliminate the stress marks caused by the transparent PC injection molding process, the injection compression process used in the light guide plate industry was borrowed from the mobile phone casing. So how does injection molding compression eliminate stress marks, and what are its advantages? See below!
1. Internal stress of plastics
Internal stress of plastics is one kind of internal stress, and it is also the stress that exists inside without the action of external force. Officially explained, plastic internal stress refers to an internal stress generated during the processing of plastics, especially during the injection molding process. From a microscopic perspective, the orientation of the macromolecular chains within the plastic and cooling shrinkage and other factors are generated.
Almost all plastics have internal stress, but to varying degrees. Plastics such as PC and PPE are easily affected by internal stress.
Causes of internal stress in plastics:
Cooling internal stress: During the plastic processing process, the plastic needs to be cooled and shaped after molding. At this time, shrinkage will occur. If the shrinkage is uneven, it will internal stress.
Orientation internal stress: During the plastic processing process, it enters the cavity through the gate. This is called mold filling. During the mold filling process, pressure must be maintained. During the mold filling and pressure holding overshoot, the macromolecular chain As the flow direction aligns and orients, orientation internal stress is generated.
Molecular chain rigidity: The greater the molecular chain rigidity, the higher the viscosity of the plastic polymer molecules, the worse the fluidity, and the worse the molecular chain mobility. After elastic deformation under the action of external force, The recovery performance is poor. Some high polymers containing benzene rings in their molecular chains, such as PC, PPO, and PPS, have relatively large internal stress in their corresponding products. This is determined innately and, like human genes, cannot be changed.
Molecular chain polarity: Molecular chain polarity. Many people may not understand this term. Let’s put it another way. The greater the mutual attraction force between the molecules inside the plastic, the more molecules move between them. When it is enlarged and subjected to elastic deformation, its ability to recover becomes weaker.
Compression Molding Concept
"Injection compression molding" refers to the thermoplastic polymer melt entering a slightly open mold and undergoing a short-stroke mold closing compression at the same time or subsequently. This short stroke clamping compression can be accomplished by machine or by punching in the mold. The pressure established in the mold cavity is evenly distributed in both directions on the product surface. Usually the purpose is to improve the precision of the fine parts of the product surface, dimensional stability and production process repeatability. Injection compression molding is a low-pressure molding process that does not damage inserts, such as decorative films or metal inserts. Due to the low-pressure process of the injection compression molding process, decorative parts or metal inserts are protected.
Figure injection molding compression process (source: Demag)
In optical engineering applications, low injection pressure and holding pressure can minimize internal stress. Therefore, the impact on optics Characteristics have a significant impact. Injection molding machines must therefore be highly adaptable and also capable of very precise, repetitive movements.
The history of injection compression molding
Injection compression molding was initially mostly used in the field of small-sized, high-precision optical products such as optical lenses and compact discs. Since the end of the last century, the injection compression molding method has been used to manufacture transparent parts in the aerospace and automotive sectors. In the aviation field, the United States began to implement the next-generation transparent parts program in 1996, which focused on the development of low-pressure molding technology for transparent parts including injection compression molding technology; in the automotive field, the U.S. National Highway Transportation Safety Administration approved the Injection compression molded polycarbonate windows can be used in automobiles.
Martin et al. were the first to propose the concept of combining injection and compression processes. The purpose was to improve the filling capacity of materials and meet the needs of molding thin-walled products with complex structures. Spector et al. successfully promoted this technology to In the development of optical lenses, injection compression molding was subsequently developed and integrated into injection molding machines by manufacturers such as Engel, Demag, KraussMaffei, Mitsubishi Heavy Industries, and Battenfield.
As shown in the figure, the light guide plate of mobile phones with thin requirements can be injection molded and compressed
Advantages of injection molding and compression molding
Flexible process sequence, in different motion curves and stages Precise initial movement and control of the mold clamping device
Packing characteristics in two directions (improves dimensional stability, eliminates shrinkage and warping deformation)
Filling the mold during the mold filling stage gt; 100
Reduce injection pressure and mold clamping force
Reduce product orientation, fiber product orientation and internal stress
Reduce product trimming
Improved exhaust
Convenient to install, cancel and upgrade standard injection molding machines
1. Lower holding pressure?
Another aspect of injection compression molding One benefit is that shrinkage compensation is more even across the surface, resulting in lower packing pressures.
Picture of injection molding compression molding to reduce holding pressure
2. Lower residual stress
Residual stress refers to the residual stress in the product that has not completely relaxed after the product is released from the mold. The sum of various internal stresses mainly includes orientation stress and thermal stress. Orientation stress originates from the shear stress and tensile stress caused by the non-isothermal flow of the plastic melt in the mold cavity during the filling and pressure-holding processes of injection molding, causing the macromolecular chains to stretch and deform along the flow direction. During the cooling process If the macromolecules have no time to relax, they are "frozen" in the product, forming orientation stress; Thermal stress mainly comes from the cooling process. During the rapid cooling process of the product, the internal temperature distribution is uneven, and the cooling layer gradually advances from the surface of the mold cavity to the inside. , each point in the product drops from a higher temperature to below the glass transition temperature at different times, and the shrinkage and deformation experienced are inconsistent, thus generating stress.
Figure polarized photos of transparent PC parts under parallel plane polarized light field (a) injection molding; (b) injection compression molding, small residual stress
Injection compression molding when the injection volume When the requirements are met, a special compression device is used to directly compress the mold cavity for mold closing. The melt shrinkage is compensated by directly compressing the melt through the entire mold cavity, rather than through the local pressure transmitted from the screw to the gate to achieve shrinkage and pressure holding. Therefore, the required pressure is low and uniform, and the product is finalized after compression is completed.
Figure schematic diagram of the injection compression molding principle of transparent parts (a) Melt filling under incomplete mold closing; (b) Completion of compression
V. Application examples
Optical products (lenses, diffusers, headlights, monitors, etc.)
Long fiber reinforced products (door frames, internal protective layers, etc.)
Decorative parts (decorative strips, side inlays, etc.) boards, mobile phone casings, etc.)
Foam parts (insulation materials, lightweight structural parts, etc.)
Parts with difficult-to-machine parts (cavitation at the end of the fluid, shrinkage, etc.)
Picture of car decorative strips
Picture of transparent PC mobile phone back cover
6. Injection molding machine with injection compression function
Here are a few examples: Injection compression molding solutions (the information is incomplete, if you have other supplier information, please add it in the message area):
Sumitomo is a global leader in the field of electric injection molding machines. Sumitomo Heavy Industries applies Sumitomo and Demag's core all-electric drive technology systems that are superior to competitors to the development of Sumitomo injection molding machines.
1. Sumitomo (SHI) DEMAG is a global leader in the injection molding machine industry, focusing on automotive parts, medical equipment, high-speed packaging (lML), precision electronics, Precision injection complete systems for silicone, metal (MlM) & ceramic (ClM) powders and one-click smart industry 4.0 automation equipment. Demag injection molding machines apply low-pressure injection molding and secondary mold clamping technology.
Picture of Sumitomo’s injection molding machine with compression function
As early as the 1990s, Sumitomo focused on the development and production of electric injection molding machines, and today it has become the undoubted leader in this field . With efficient and advanced injection molding equipment, Sumitomo has achieved a good market share in Asia and North America. One out of every four all-electric machines comes from Sumitomo.
After taking over Demag, Sumitomo has become increasingly powerful in the European market. With the integration of the two companies, the first truly global company in the injection molding machine industry has demonstrated its solid competitiveness as a strong partner for international customers.
2. Itanium Molding Machine
At CMPE2018, Itanium 5G mobile phone case dedicated all-electric injection molding machine AD-185 demonstrated on-site the 5G ultra-thin transparent PC plastic imitation glass mobile phone back For compression injection molding proofing of caps, this machine uses special mold locking operation, combined with a special mold structure, to reduce filling pressure and increase product density.
Tuanteng AD-185T mobile phone PC transparent back cover injection molding and compression molding machine
Tuanteng molding machine demonstrates the production of injection molded and compressed PC transparent mobile phone back cover
3. JSW Japan Steel
Picture JSW Japan Steel injection molding compression PC transparent mobile phone back cover
Picture JSW injection compression molding machine introduction
4. Klaus Maffei
KraussMaffei CX series can also be used for injection molding and compression of PC mobile phone back covers.
In addition, TOYO’s CS series is also capable of injection molding and compression molding of mobile phone and PC back covers.
7. Key points of mold design
The design requirements of injection stamping molds are higher than those of ordinary molds. Due to the secondary mold closing, the mold structure is also much more complex. The following are the key points. :
1. Exhaust and sealing. While maintaining smooth exhaust, the sealing design must be accurate by 1-2 wires. The overflow groove can be used for secondary sealing to ensure product post-processing efficiency.
2. Due to stress problems, low-pressure injection is used for runners and gates. Therefore, the runner design should be circular or tunnel-shaped as much as possible. The size and proportion are selected according to the glue amount of the product. In short, everything is for Smooth glue transfer. Of course, you should choose a large nozzle fan for pouring glue. Determine the size of the glue feed point and fan opening according to the size and thickness of the product. If it is side feed, it is recommended to choose the golden section point as the midpoint of the gate design.
3. The design of the movable mold cavity should be designed according to the capabilities of the injection molding machine to ensure smooth opening and closing of the secondary mold. As far as the mold is concerned, the stroke control of the movable mold is the key point, and it must be combined with the external cylinder The combination must have zero error. In addition, lubrication and water transport design are also very important to ensure that the mold does not get stuck when opening and closing at high temperatures!
The above points must be combined with the actual situation and carefully explored to form design specifications and standards!
Under the general trend, if you look for an outlet, pigs will fly, let alone people! The trend of mobile phone modeling is now.