Long fiber reinforced thermoplastics (LFTs) have been u […]
Long fiber reinforced thermoplastics (LFTs) have been used in the automotive industry for semi-structural applications. Their main advantages are attractive cost/performance ratios and relatively low densities. The development of this material has been driven by European ELV (end of life vehicle) legislation, which promotes recycling and thus the replacement of thermosets by thermoplastics.
In the automotive industry, LFT is mainly used to make structural and semi-structural components, such as front-end modules, bumper beams, instrument panel frames, battery carriers, spare tire wells, seat frames, footrests, and integral floor panels. Long fiber reinforced polypropylene is used in car hood, dashboard frame, battery bracket, seat frame, car front module, bumper, luggage rack, spare tire, fender, fan blade, engine chassis, roof Liners, etc.; long fiber reinforced PA, which is further extended into the hood, because LFT-PA not only has high hardness and low weight, but also has a high glass fiber content that makes its thermal expansion coefficient almost the same as that of metal, which can withstand the high temperature brought by the engine .
The two main materials of LFT are described below. LFT-G is the result of technological innovation in Short Glass Fiber Thermoplastic Particles (FRTP). Although the length of the early FRTP pellets can reach 5-6mm, after mixing, pelletizing, plasticizing, injection molding and other processes, the final length of the fibers in the product is often less than 1mm, which can only be used as a filler to increase the rigidity of the product. , and the improvement of tensile strength and impact resistance is very limited. Therefore, FRTP was not the mainstream composite material at that time. In order to give full play to the advantages of high production efficiency and low cost of injection molding, and strive to minimize the degree of fiber breakage, LFT-G was born in the early 1980s. The production process of LFT-G products is similar to that of GMT, and it also requires two mature processes, namely the molding of long particles and the injection molding or compression molding of the products.
The diameter of LFT-G pellets is about 3mm, and the lengths are 12mm and 25mm. The pellets with a length of about 12mm are mainly used for injection molding, and the pellets with a length of about 25mm are mainly used for compression molding. In the LFT-G pellet injection molding process, although the injection molding machine has undergone many improvements, due to the limitation of the injection molding process, the fibers in the final finished product can only reach 3.2 to 6.4 mm. Although this length is longer than that of FRTP injection molded fibers, the impact resistance of the product is also significantly improved, but it is shorter than LFT-D injection or compression molded fibers, and its strength and impact resistance are also worse than LFT-D. LFT-D is a process technology for on-line direct production of long fiber reinforced thermoplastic composite materials. The key factor that distinguishes it from GMT and LFT-G is that the semi-finished product steps are omitted, and the choice of materials is more flexible. In LFT-D technology, not only the fiber content and length, but also its matrix polymer can be adjusted directly to the requirements of the final part. The amount of additives can change and affect the mechanical properties of the product and the characteristics of special application materials, such as thermal stability, colorability, UV stability, and the bonding characteristics of fibers and substrates, which also means that each special application All can obtain their unique material formula through LFT-D.
The impact resistance of LFT-D press-molded products is slightly lower than that of GMT, but its impact resistance is significantly higher than that of LFT-G due to much longer fibers than LFT-G after molding. In addition, according to numerous studies, LFT-D injection molding has higher productivity than standard LFT-G pellets because the lower plasticizing requirements of LFT-D improve fiber breakage. For parts with a molding cycle of more than 1min, the LFT-D injection molding equipment can be completed within 30s. Figure 2 is a front-end frame diagram produced using LFT-D.