? It is said that in the 5G era, the proportion of plastic mobile phone cases will gradually increase, and in 20 19 years, plastics may account for 70%. There are several new technologies for plastic mobile phone case, such as composite plate (2.5D/3D), IMT(3D), PC injection glass imitation, etc. Transparent PC imitation glass, the stress line produced by injection molding is particularly obvious after hardening, which makes the terminal cautious in color selection. In order to reduce or eliminate the stress line of transparent PC during injection molding, the injection compression process used in light guide plate industry is used for mobile phone case. So how does injection pressurization eliminate the stress line and what are its advantages? Please see below!
I. Internal stress of plastics
Plastic internal stress is a kind of internal stress, and it is also the stress that exists inside when there is no external force. According to the official, the internal stress of plastic refers to a kind of internal stress caused by the orientation and cooling shrinkage of macromolecular chain during the processing of plastic, especially during injection molding.
Almost all plastics have internal stress, but the degree is different. Plastics such as PC and PPE are easily affected by internal stress.
Causes of internal stress in plastics:
Cooling internal stress: In the process of plastic processing, the plastic needs to be cooled and shaped after molding, and then it will shrink. If the shrinkage is uneven, internal stress will occur.
Orientation internal stress: In the process of plastic processing, it enters the cavity through the gate, which is called mold filling. During the mold filling process, the pressure should be maintained. In the process of mold filling and pressure maintaining overshoot, the macromolecular chain is aligned with the flow direction, resulting in oriented internal stress.
Molecular chain rigidity: the greater the rigidity of the molecular chain, the higher the viscosity, the worse the fluidity and the worse the activity of the molecular chain. Under the action of external force, the recovery performance is poor after elastic deformation. Some polymers with benzene rings in molecular chains, such as PC, PPO, PPS, etc., have large internal stress in their corresponding products, which is determined by nature and cannot be changed like human genes.
Molecular chain polarity: molecular chain polarity, this term may not be understood by many people, so let's put it another way: the greater the attraction between molecules in plastic, the greater the mutual movement between molecules, and the weaker the resilience when elastically deformed.
The concept of compression molding
"Injection compression molding" means that the thermoplastic polymer melt enters a slightly open mold and is compressed at the same time or later by short-stroke closure. This short-stroke clamping compression can be accomplished by machine or die stamping. The pressure established in the mold cavity is evenly distributed in both directions of the product surface, which is usually aimed at improving the accuracy, dimensional stability and repeatability of the product surface production process. Injection compression molding process is a low-pressure molding process, which will not damage inserts, such as decorative films or metal inserts. Because of the low pressure process of injection compression molding, decorative parts or metal inserts are protected.
Figure Injection compression process (from Demag)
In the application of optical engineering, low injection pressure and holding pressure can minimize internal stress, so it has great influence on optical characteristics. Therefore, the injection molding machine must be highly adaptable and have the function of very precise and repetitive motion.
History of injection compression molding
At first, injection compression molding was mostly used in the field of small-size and high-precision optical products such as optical lenses and optical disks. Since the end of last century, injection compression molding has been used to manufacture transparent parts in aviation and automobile fields. In the aviation field, from 65438 to 0996, the United States began to implement the next generation of transparent parts plan, focusing on the development of low-pressure molding technology of transparent parts, including injection compression molding technology; In the field of automobiles, in June 2006, the National Highway Transportation Safety Administration of the United States approved that polycarbonate windows molded by injection compression can be used in automobiles.
Martin and others first put forward the concept of combining injection and compression technology, aiming at improving the filling ability of materials and meeting the molding requirements of thin-walled products with complex structures. Spector and others successfully extended this technology to the development of optical lenses, and then injection compression molding was developed and integrated into injection molding machines by Engel, Demag, KraussMaffei, Mitsubishi Heavy Industries, Battenfield and other manufacturers.
The mobile phone light guide plate with thinness requirement can be pressed by injection molding.
Advantages of injection compression molding
Flexible process sequence, accurate initial motion and control of clamping device in different motion curves and stages
Packing characteristics in two directions (improving dimensional stability and eliminating shrinkage and warping deformation)
Mold filling in mold filling stage >: 100%
Reduce injection pressure and clamping force
Reduce product orientation, fiber product orientation and internal stress.
Reduce product pruning
Improve exhaust
It is convenient to add, cancel and upgrade standard injection molding machines.
1. Low pressure?
Another advantage of injection compression molding is that the shrinkage compensation of the whole surface is more uniform, thus reducing the holding pressure.
Figure Injection compression molding reduces the holding pressure.
2. Lower residual stress
Residual stress refers to the sum of all kinds of stresses remaining in the product after the product is not fully relaxed, mainly including orientation stress and thermal stress. The orientation stress comes from the shear stress and tensile stress caused by the non-isothermal flow of plastic melt in the mold cavity during the mold filling and pressure keeping process of injection molding, which makes the polymer chain stretch and deform along the flow direction. If the polymer is too late to relax during the cooling process, it will be "frozen" in the product, that is, the orientation stress will be formed. Thermal stress mainly comes from the cooling process. In the process of rapid cooling, the temperature distribution inside the product is uneven, and the cooling layer gradually advances from the surface of the mold cavity to the inside. The time for each point in the product to drop from a higher temperature to below the glass transition temperature is different, and the shrinkage deformation experienced is inconsistent, resulting in stress.
Photo of polarized light of transparent PC under parallel plane polarized light field (a) injection molding; (b) Injection compression molding with small residual stress.
Injection compression molding When the injection amount reaches the requirements, the mold cavity is directly compressed by a special compression device to close the mold, and the melt shrinkage is compensated by directly compressing the melt in the whole mold cavity, instead of feeding and maintaining the pressure by the local pressure transmitted by the screw to the gate, so the required pressure is low and uniform, and the product is molded after compression.
Fig. schematic diagram of injection compression molding principle of transparent parts (a) melt filling under the condition of incomplete mold clamping; (b) Compression is completed.
Application example of verb (abbreviation of verb)
Optical products (lenses, diffusers, headlights, displays, etc. )
Long fiber reinforced products (door frames, internal protective layers, etc.). )
Decorative pieces (decorative strips, side plates, mobile phone cases, etc.). )
Foam parts (insulating materials, lightweight structural parts, etc.). )
Parts with difficult-to-machine parts (cavities formed at the fluid end, shrinkage, etc.). )
Digital automobile decorative strip
Figure Transparent PC phone back cover
6. Injection molding machine with injection compression function
The following are several injection compression molding solutions (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 applied Sumitomo and Demag's core all-electric drive technology system, which is superior to competitors, to the development of Sumitomo injection molding machine.
1. Heavy machinery
Sumitomo (SHI) Demag is a global leader in the injection molding machine industry, focusing on auto parts, medical devices, high-speed packaging (lML), precision electronics, silica gel, metal (MLM) & ceramic (ClM) powder and other precision injection molding complete systems and one-click start-up of intelligent industry 4.0 automation equipment. Demag injection molding machine adopts low-pressure injection molding and secondary clamping technology.
Sumitomo injection molding machine with compression function
As early as 1990s, Sumitomo focused on the development and production of electric injection molding machines, and now it has become the undisputed leader in this field. With efficient and advanced injection molding equipment, Sumitomo has gained a good market share in Asia and North America. One out of every four full motors comes from Sumitomo.
After taking over Demag, Sumitomo became stronger and stronger in the European market. With the integration of the two companies, the first truly global company in the injection molding machine industry, as a strong partner, has demonstrated its solid competitiveness to international customers.
2. An Teng molding machine
On CMPE 20185, An Teng AD- 185, an all-electric injection molding machine for 5G mobile phone case, demonstrated the injection proofing of 5G ultra-thin transparent PC plastic glass-like mobile phone back cover. The machine adopts special mold locking operation and special mold structure, which can reduce the filling pressure and improve the product density.
Photo An Teng AD- 185T mobile phone PC transparent back cover injection compression molding machine.
The picture shows the production of injection-molded PC transparent mobile phone back cover by An Teng Plastic Machine.
3.JSW Nippon Steel
Fig. JSW Japan steel injection compression PC transparent mobile phone back cover
Fig. brief introduction of JSW injection compression molding machine
4. Klaus Murphy
Klaus Maffei CX series can also meet the requirements of injection molding to compress the back cover of PC mobile phones.
In addition, TOYO's CS series can also be competent for the injection molding of the back cover of mobile phone PC.
Seven, mold design points
The design requirement of injection stamping die is higher than that of ordinary die, and the die structure is much more complicated because of the existence of secondary clamping. The following are the main points:
1. Exhaust and sealant. Under the condition of smooth exhaust, the sealant design must be accurate, 1-2 thread. The sealant overflow tank can be used for secondary sealant to ensure the post-processing efficiency of products.
2. For runner and gate, low-pressure injection is used for stress problems, so the runner design should be round or tunnel-shaped, and the size ratio should be selected according to the glue content of the product. In short, everything is to smooth the glue. Of course, the pouring gate should be filled with glue from the sector of the large nozzle, and the size of the glue inlet point and the opening of the sector should be determined according to the size and thickness of the product. If pouring glue from the side, it is suggested to choose the golden section as the midpoint of gate design.
3. The design of the movable mold cavity should be based on the capacity of the injection molding machine platform to ensure the smooth opening and closing of the second mold. As far as the die is concerned, the stroke control of the moving die is the key point, and the combination with the outer cylinder must be zero error. In addition, the design of lubrication and water delivery is also very important to ensure that the mold will not get stuck when it is opened and closed at high temperature!
The above points must be carefully explored in combination with the actual situation to form design specifications and standards!
Under the general trend, find an outlet, pigs can fly, let alone people! And the way out for mobile phone modeling is now.