The full name of LFT is long fiber reinforced thermoplastic. LFT is a broad term for plastics. There is an informal but conventional definition in the automotive composite industry, that is, it refers to the products produced by mixing reinforced fibers with thermoplastic polymers with a length of more than 10mm. The long fiber is impregnated with a special resin system through a special mold to obtain a long strip fully soaked by the resin, and then cut into the required length according to the needs. The most used matrix resin is PP, followed by PA, as well as PBT, PPS, San and other resins.
Development
As early as the 1980s, European and American countries put forward the concept of LFT. However, due to the relatively backward fiber impregnation technology, injection molding and molding process and production equipment of long fiber pellets at that time, and the great progress made in the research and application of short fiber reinforcement, filling and other modification technologies of thermoplastic at that time, the research progress of LFT technology was slow. In the mid-1990s, with the rapid development of the automobile industry and the enhancement of people’s awareness of environmental protection and energy conservation, especially the energy crisis and oil price rise, automobile lightweight has become the development trend of new vehicles, and people have turned their attention to the research and development of LFT again.
Processing technology
With the different processing technology, the fiber length of LFT is also different. The impact properties especially depend on the fiber length, so the compression molding process is significantly better than injection molding in terms of mechanical properties. In general, fibers with an average length of about 5 ~ 20 mm in parts are considered to be long fibers.
LFT-G(Long Fiber Reinforced Thermoplastic Granules). The mixed raw materials enter the twin screw extruder for plasticization. After pre dispersion and preheating, the glass fiber is introduced into the specially designed impregnation mold at the same time. Under the action of the special dispersion device in the mold, the glass fiber bundle is evenly impregnated with the polymer, pulled out through the die, cooled by the cooling device, pulled by the traction device, and the composite granules with a fixed length of 6 ~ 15mm are cut by the cutting device.
LFT-D(Long Fiber Reinforced Thermoplastics in Direct Processing). The mixed raw materials enter the twin screw extruder for plasticization, and the molten compound forms a waterfall like polymer film through the film die, which is directly introduced into the opening of the twin screw extruder. The glass fiber roving is introduced to the top of the polymer film through the yarn frame, through preheating, dispersion and other procedures, and enters the twin screw extruder together with the film. The coarse sand is cut by the screw, and they are gently mixed into the pre melted polymer, and then directly sent to the pressing die for molding.
Advantages of LFT compared with short fiber reinforced thermoplastic composites:
The fiber length is longer, which significantly improves the mechanical properties of the products.
It has high specific stiffness and strength and good impact resistance. It is especially suitable for the application of automobile parts.
The creep resistance can be improved, the dimensional stability is good, and the forming accuracy of parts is high.
Excellent fatigue resistance.
Better stability in high temperature and humid environment.
During the molding process, the fiber can move relatively in the molding die, and the fiber damage is small.
|
Short fiber reinforced polymers |
Long fiber reinforced polymers |
||||
|
Fiber length in the part |
＜1 |
1-5 |
5-25 |
5-25 |
＞10-∞ |
|
Raw material |
Short fiber compound |
LFT-granules |
LFT-granules |
Direct-LFT |
GMT |
|
Production process |
Injection moulding |
Compression moulding |
|||
Advantages of LFT compared with metal
Compared with metal materials, LFT materials have very low density and high specific strength, flexible product processing methods, high material recovery rate, low cost compared with materials, and high functionality and integrity of components. Compared with expensive metal impact equipment and molds, LFT component processing equipment such as extruder, injection or press, punching installation equipment and molds, its cost is very low, Especially for models with small production volume, LFT shows excellent economy. Therefore, compared with steel structural parts, LFT reduces the weight of products a lot and has the performance of component consolidation, which can reduce the cost.
Future development
The application of LFT in automobile is increasing rapidly. In addition to their economic appeal, their main advantage is the flexibility in which materials can be mixed. Continuously variable fiber content, different types of fibers, and engineering plastics have opened up potential applications.
