Understanding the molding process of fiber-reinforced thermoplastic composites

Fiber-reinforced thermoplastic composites are an important part of composite materials. Various thermoplastic resins are reinforced with glass fibers (GF), carbon fibers (CF), aramid fibers (AF) and other fiber materials, which are called FRTP ( Fiber Reinforced ThermoPlastics). Advanced fiber-reinforced thermoplastic composites have excellent properties such as high toughness, corrosion resistance and fatigue resistance, simple molding process, short cycle, high material utilization rate (no waste), and no need for low-temperature storage, and have become a research hotspot in the materials industry.

Typical performance benefits of FRTP

Thermoplastic composite FRTP has been used for decades. Compared with thermosetting composites such as phenolic resin, urea-formaldehyde resin, epoxy resin, and polyurethane, thermoplastic composites have some special properties:

◆ Low density and high strength: The density of FRTP is 1.1-1.6g/cm3, which is only 1/5-1/7 of that of steel, and 1/3-1/4 lighter than that of thermosetting FRP, which can be obtained with a smaller unit mass. Higher mechanical strength and application grade.

◆ Large degree of freedom in performance designability: The physical properties, chemical properties and mechanical properties of FRTP are designed through the reasonable selection of raw material types, proportions, processing methods, fiber content and layup methods. There are many materials, the most important ones include polyetherketoneketone (PEKK), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), nylon (PA), polyetherimide (PEI), etc. Therefore, its The degree of freedom in material selection and design is also much greater.

◆ Thermal properties: The general use temperature of plastic is 50-100℃, and it can be increased to above 100℃ after being reinforced with glass fiber. The heat distortion temperature of PA6 is 65°C, and after reinforced with 30% glass fiber, the heat distortion temperature can be increased to 190°C. The heat resistance of PEEK reaches 220°C. After reinforced with 30% glass fiber, the operating temperature can be increased to 310°C. Thermosetting composite materials cannot achieve such high heat resistance.

◆ Chemical resistance: It is mainly determined by the performance of the matrix material. There are many types of thermoplastic resins, and each resin has its own anti-corrosion characteristics. Therefore, the matrix resin can be optimized according to the use environment and medium conditions of the composite material. Generally, can meet the requirements of use. The water resistance of FRTP is also better than that of thermoset composites.

◆ Electrical properties: FRTP generally has good dielectric properties, does not reflect radio waves, and transmits microwaves well. Because the water absorption rate of FRTP is smaller than that of thermosetting FRP, its electrical properties are better than the latter. After adding conductive material to FRTP, it can improve its conductivity and prevent the generation of static electricity.

◆ Waste can be recycled: FRTP can be re-processed and formed, waste and leftovers can be recycled, and the physical and mechanical properties do not change significantly, and it will not cause environmental pollution. environmental requirements.

Classification of FRTP

There are many kinds of FRTP, and this industry is also full of too many terms and English abbreviations. According to the fiber retention size (L) in the product, it can be divided into: short fiber reinforced thermoplastic (SFRT, L<1.0 mm), long fiber reinforced thermoplastic (LFT or LFRT, generally L>10 mm), continuous fiber reinforced thermoplastic composite material Plastics (CFRT, generally fiber continuous without cutting).

Compared with SFRT, LFT has the characteristics of low density, high specific strength, high specific modulus and strong impact resistance, which is suitable for harsh application conditions, which has become one of the main reasons why LFT is favored by the downstream application industry. The widely used LFT materials can be roughly classified into three categories: Glass Mat Reinforced Thermoplastic GMT (Glass Mat Reinforced Thermoplastics), Long Fiber Reinforced Thermoplastic Granules LFT-G (Long-Fiber Reinforced Thermoplastic Granules) and Long Fiber Reinforced Thermoplastic Granules Thermoplastic direct in-line molding LFT-D (Long-Fiber Reinforced Thermoplastic Direct). CFRT can be recycled, has high specific strength and specific stiffness, good corrosion resistance, impact resistance, heat resistance, low cost and design flexibility, and has great application potential in automotive lightweight design, which can replace parts Metal materials and high-end polymer materials.

Applications of FRTP

With the emergence of aromatic thermoplastic resin matrix (such as PEEK, PPS) with excellent rigidity, heat resistance and medium resistance, as well as carbon fiber and aramid fiber with excellent properties such as high strength, high modulus, high temperature resistance and corrosion resistance , the development of high-performance fibers such as silicon carbide fibers, so that advanced FRTP is used in more and more industrial fields, such as: rail transit, automobiles, aerospace, home appliances, electric power and other industries.

◆ Aerospace

FRTP's high stiffness, low processing costs and reworkability, good flame retardancy, low smoke and non-toxic properties, and cure cycles in minutes make it an ideal material for lightweight, low-cost aerospace structures.

In the structural parts of the aircraft body, FRTP is mainly used in the floor, leading edge, control surface and tail parts, which are secondary load-bearing components with relatively simple shapes. The Airbus A380 airliner, Airbus A350 airliner, Gulfstream G650 business jet and AgustaWestland AW169 helicopter are all major applications of thermoplastic airframe structural parts. The most important FRTP structure on the Airbus A380 is the fixed leading edge of the wing made of fiberglass/PPS material. The FRTP of the Airbus A350 airframe is mainly distributed on the movable spars and ribs and the fuselage connections. The Gulfstream G650 business jet is a milestone in FRTP applications with carbon fiber/PEI for pressure bulkhead ribs and carbon fiber/PPS for rudders and elevators

◆ Cars

The development of low-cost, short-cycle and high-quality composite materials technology has become one of the key elements in promoting vehicle lightweighting. Many domestic car companies have already cooperated with injection molding equipment companies with advanced composite material technology. Applications in passenger cars are: seats and their frames, window guides, door inner panels, bumper brackets, hoods, front brackets, footrests, instrument panel frames, air deflectors, compartment floor, spare parts Tire box, battery carrier, automobile intake manifold. In domestic cars, Passat, POLO, Bora, Audi A6, Golf, Buick Excelle, Buick GL8 commercial vehicle and other models have adopted a large number of high-performance FRTP parts, most of which use GMT or LFT.

In the application of box trucks, it is mainly PP honeycomb composite board, which replaces the steel frame outer aluminum alloy small corrugated board and steel corrugated board in the current box truck.

◆ Rail Transit

According to the load-bearing characteristics, it can be roughly divided into two categories: the main load-bearing parts of composite materials and the non-main load-bearing parts of composite materials. The main load-bearing parts of composite materials mainly refer to the large load-bearing components of trains such as train body, driver's cab and bogie frame. The non-main load-bearing parts of composite materials can be divided into non-main load-bearing parts (such as body, floor and seat and other non-main load-bearing parts) and auxiliary parts (auxiliary parts such as toilets, toilets and water tanks).

Common FRTP molding process

The key process step for converting raw materials into FRTP structural products is the molding process, which is the basis and condition for the development of this industry. With the widening of the application field of composite materials, the composite material industry has developed rapidly, some molding processes have been improved day by day, and new molding methods have been emerging. At present, there are more than 20 molding methods for FRTP, which have been successfully used in industrial production.

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