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Technical Progress Of Fiber-reinforced Thermoplastics For Vehicles

Aug 11, 2020

With the development of the automobile industry, the demand for fiber-reinforced thermoplastics has grown substantially, and its manufacturing process technology has also entered a period of rapid development. Such as direct compounding of long fiber reinforced thermoplastics, mold cavity induction heating used to improve the surface quality of long fiber reinforced thermoplastics molded products, LFT masterbatch, fiber reinforced composite materials based on in-situ polymerization of cyclic polyester, LFT special New technologies such as glass fiber and surface wetting agents are constantly emerging. A review of these latest technological advances will provide a reference for the development of fiber-reinforced thermoplastics for vehicles.

  Fiber-reinforced plastics have developed rapidly with the development of the automobile industry. In particular, fiber-reinforced thermoplastics have become the focus and direction of the development of automotive plastics because of their good reproducibility, ease of recycling and development of circular economy. Among fiber-reinforced thermoplastics, the development of long-fiber-reinforced thermoplastics has attracted the most attention. While the demand for LFT is growing rapidly, the process technology of fiber-reinforced thermoplastics for vehicles has also made great progress.

From the already dominant fiber felt reinforced thermoplastic composite sheet and LFT, it has further developed the direct batching LFT process technology, long fiber reinforced masterbatch technology, and the rapid change of mold surface temperature technology to improve the long fiber reinforced thermoplastic molding process. In-situ polymerization technology of polyester fiber reinforced thermoplastic matrix, special glass fiber and surface treatment technology for long fiber reinforced thermoplastics, etc.

  1 D-LFT process technology

   Compared with the use of long fiber reinforced thermoplastic pellets, the D-LFT process has several outstanding advantages:

   integrates the ingredients and molding, eliminating the need for prepreg preparation, and greatly saving manpower;

   less heating times, no secondary melting and plasticization, energy saving and consumption reduction;

Avoid the influence of the secondary melting and plasticization process on the resin and long fibers, and not excessively damage the added fibers. The performance and fiber length of the resin matrix can have a higher retention rate than LFT-G molding, and the average fiber length is higher than that of LFT-G. The molding is 15%-20% higher, and it has high-quality product performance. Through the optimization of product and mold design and process conditions, the mechanical properties of the product can be close to GMT products;

  The process and production cycle are shortened, and the production efficiency is improved. The production cost is low. Compared with the long fiber pellets, the product cost can be reduced by about 40%.

  D-LFT process was pioneered by Woodshed Technologies. Plastic Comp obtained its Pushtrusion production technology license in 2005. In recent years, Pushtrusion has been continuously developed and improved.

The basic process principle is: the thermoplastic resin melt is supplied to the coating impregnation die through an extruder, and then the long fibers are coated with the high-pressure melt and dragged to the cutter at the end of the impregnation die. The cutter is based on the preset The length cuts the long fibers covered in the melt, and the melt is injected into the injection molding machine for injection molding.

On the basis of Pushtrusion D-LFT, Plastic Comp has also developed a direct online batching and compression molding process. The obtained long fiber and molten resin composite are directly moved into the compression molding machine and pressed into the final product without using prefabricated GMT sheets. It is also called Pushtrusion D-GMT. Pushtrusion D-GMT provides processors with great flexibility in producing long-fiber reinforced plastic products, especially in the formulation of resin and fiber varieties, pigments and other additives, and is no longer restricted by suppliers.

   Engel and Krauss Maffei use a combination of a co-rotating twin-screw extruder and an injection molding machine to combine continuous extrusion and injection cycles to form a highly efficient long fiber reinforced thermoplastic online batching injection molding system. When necessary, the systems can also operate independently.

  Husky uses a special-shaped single screw compounding rod extruder and injection molding machine to form an online batching injection molding system. At the low-pressure exhaust zone of the extruder and the fiber entrance, the pinch zone is equipped with scrapers. The scraping motion of the screw improves the melt delivery in these low pressure areas in the extruder.

Dow company uses twin-screw extruder for online batching. The thermoplastic resin and additives are melted, mixed and plasticized by the twin-screw extruder. The continuous fiber yarn is introduced downstream of the twin-screw extruder, and the compound is formed by the extruder head. The continuous extrudate of the shape is cut by the cutter to form the blank, and then directly sent to the forming machine to form the product.

  2 Rapid change technology of mold cavity surface temperature

In order to expand the application range of fiber-reinforced thermoplastics and adapt them to automotive exterior parts with high surface quality requirements, during injection, the temperature of the mold surface in contact with the melt is kept above the glass transition temperature of the matrix resin or even close to the melt Temperature, obviously, it is easy to obtain products with high surface quality.

   However, thermoplastic products must be cooled to below the heat distortion temperature for demolding, and the usual mold cooling is still necessary. In order to improve the molding process of fiber-reinforced thermoplastics, rapid change of the mold cavity surface temperature has become the key to solving the problem. The latest mold induction heating technology meets this rapid change requirement.

  In order to realize the rapid change of the surface temperature of the mold cavity, Roc-tool company proposed a solution for mold induction heating. With electromagnetic induction technology, the ideal melt rheological conditions can be obtained by rapidly heating the mold surface to the temperature close to the injection plastic melt. The resin and the mold surface are at a similar temperature, and the flow is very easy.

In order to be more widely used in a variety of different occasions, Roc-tool combines its partner Wittmann Battenfeld's Indumold technology in its mold cavity induction heating technology to launch a two-type mold induction heating system, namely cage Type system and built-in sensor integrated in the mold. Successfully used induction heating technology in LFT plastic injection and compression molding molds to produce large-scale automotive parts with A-level surfaces. The patented method adopted by Roc-tool only heats and cools the surface of the mold cavity, and its cycle time is much faster than traditional methods. It only takes a few seconds to tens of seconds to heat up the inductor. The depth of heating is only 0.2mm below the surface of the mold, and 99.9% of the mold is not heated.

   Roc-tool can also design materials with different conductivity as the surface layer of the mold, so that the temperature of some parts of the mold is higher than other parts, so that the product can only meet the A-level surface requirements in a certain area.

  3 long fiber masterbatch

  In order to produce LFT products more conveniently and economically, and to effectively improve the performance of LFT products. RTP Company, Dow Company, etc. have successively developed long-fiber masterbatch, so that product manufacturers can flexibly adapt to the production needs of LFT products.

   RTP company's long fiber masterbatch, the mass fraction of glass fiber is 60%-75%, and it can be used after being diluted with pure resin in the proportion required during molding. The masterbatch fiber length is 12mm, and it can also be customized within the range of 8-25mm.

The long-fiber masterbatch developed by    Dow Company is completed in two steps. That is, the additive masterbatch is prepared first, and its composition is 70 parts of polypropylene, 15 parts of carbon black, 6.7 parts of 168 antioxidant, 3.3 parts of 1010 antioxidant, and 5.0 parts of PS802 antioxidant. Then, the additive masterbatch is extruded together with the polypropylene carrier, dispersant, modifier, etc. through an extruder, and the continuous glass fiber is impregnated through a dipping die, and the long fiber masterbatch is obtained through pultrusion, cooling and pelletizing.

   The composition of the masterbatch is 32.4 parts of polypropylene, 4.0 parts of polypropylene, 2.1 parts of additive masterbatch, 60.0 parts of glass fiber, and 1.5 parts of polymer coupling agent.

   Here, the melt flow rate of polypropylene under the brand name of Borflow HL504FB is 450 g/10 min, and the melt flow rate of polypropylene under the brand name of H734-52RNA is 52 g/10 min. The latter is an injection-grade material with high fluidity and a good balance of physical properties, while the former has a very low viscosity, which is conducive to the good infiltration of the glass fiber by the matrix resin.

  4 new raw material technology

  In terms of automotive fiber-reinforced thermoplastics, the most revolutionary is fiber-reinforced thermoplastics based on in-situ polymerization of cyclic polyester.

   CBT is a mixture of different low molecular weight oligoesters with macrocyclic structure and produced by Cyclics patent in the United States. CBT is added with tin catalyst in the process of processing, and it is ring-opened and polymerized to linear polybutylene terephthalate at an appropriate temperature. The relative molecular weight is twice that of general PBT resin. It has outstanding processing advantages. The extremely low viscosity allows it to quickly and excellently wet the fiber reinforced material, and can be molded under very low pressure, and can be quickly polymerized; almost all thermoplastic and thermosetting resin fiber composites can be used Material method for molding processing. In addition, the polymerized material has the reproducible thermoplastic characteristics of the usual PBT resin.

   In addition, in order to meet the requirements of LFT glass fiber yarn use technology and product performance, as well as to solve the problem of long fiber surface wettability, American OCV company, PPG industry company, France's Saint-Gobain company, etc. are engaged in the development of special sizing agents. PPG Industrial Company uses maleic anhydride, polyethylene glycol or polypropylene glycol, fatty acid glycidyl ester and rosin resin to react step by step to prepare modified rosin, and then with A174, A187, A1100 coupling agent, and PH regulator, Lubricant, defoamer, water, etc. are made into sizing agent. The outstanding advantages of the sizing agent are: it shows good strand bundling in fiber production, improves the friction performance of the fiber, reduces the single filament breakage during the production of glass fiber strands, helps maintain the consistency of the product, and Reduce the production cost of fiber precursor, improve the scratch resistance in the subsequent processing, reduce the break of the monofilament in the subsequent process, and improve the production efficiency of the subsequent process; it has good adaptability to a variety of resins and a variety of reinforced plastic manufacturing processes Wait.

  5 Conclusion

Fiber-reinforced thermoplastics are being used more and more widely in the automotive industry, and the development of its process technology is changing with each passing day, especially LFT and D-LFT. Foreign countries have formed a complete industry from raw materials, equipment, molds and many other aspects. Chain, technology is becoming more mature.

   But the domestic work in these areas has just started, and there is still a lack of in-depth research and development. It is hoped that it can attract the attention of domestic counterparts, strengthen the process technology research and development of automotive fiber-reinforced thermoplastics, and promote the common development of the automotive industry and the plastics industry.