Carbon fibre reinforced thermoplastic composite moulding process

Traditionally FRPs are based on epoxy resins and reinforced with high performance fibres such as carbon fibres. However, recycling of epoxy-based composites at the end of their service life is quite complicated. Thermoplastics, on the other hand, are cheaper, easier to process and can be easily recycled. With their high strength, low density, high specific modulus small density, high temperature resistance, chemical attack resistance, low current, high thermal conductivity and excellent vibration and noise reduction features, carbon fibre materials are widely used in engineering. In FRP, the matrix polymer acts as the continuous phase, while the reinforcing fibres act as the discontinuous phase.

Thermoplastic resins are one of the most common matrix materials used in carbon fibre materials and possess the properties of softening by heat, melting by heat and stable hardening on cooling, allowing for high heat melting and repeated solidification on cooling. Thermoplastic resins have excellent corrosion stability, fracture toughness, damage resistance and impact resistance, and are small in size.

Traditional moulding process

1. Hot press tank forming

The hot press tank forming process is by far the most common primary forming process. Its forming mechanism is to heat up and pressurise the prepreg material laid down through high temperature extrusion pressure in a heated pressure tank to achieve a solidified forming process. Currently, hot press tank forming technology occupies a large proportion of industry, particularly in aerospace and other areas. The mould lamination unit is placed in a large temperature and pressure controlled vessel.

In practice, there are significant advantages and shortcomings in using traditional hot press tank forming technology.

Advantage:

1) The product is subjected to a uniform distribution of pressure. During moulding, a vacuum bag is used to seal the prepreg into the mould. By compressing the pressurised gas uniformly at each position, the composite material is solidified at the same pressure.
2) The product has a uniform heating capacity and the compressed air is very fast in the container. The heating state of the material is almost identical during the heating and cooling process. The mechanical properties are essentially stable as they are more stable at both pressure and temperature in the same vessel, resulting in a small voidiness and better distribution.
3) The equipment has a large volume, enabling the manufacture of larger, more complex parts with relatively simple moulds. The energy consumption is high and the cost is high. Hot press tank units usually have huge volumes, are complex, costly, consume a lot of energy in the forming process and cause pollution to the environment.

Although the hot press tank forming process is widely used, the forming process is stable and produces products with good overall performance and reliability, it is costly and expensive, which is contrary to the idea of manufacturing high speed, cheap and low pollution composite materials, while providing a direction for the development of new forming processes.

2. Pultrusion moulding

Pultrusion is a process that allows the continuous production of composite parts which can be used to produce parts with a constant cross section. This process involves impregnating carbon fibres with resin and then extruding them by a traction force device. By extrusion, composites with infinite lengths are made and one-way composites with high strength are made.

Pultrusion moulding technology is highly automated, consumes less energy, has smooth quality, low raw material losses and high fibre content. However, the main drawback of pultrusion moulding is the simple shape of the product, which can only be produced in a straight shape and cannot form complex structural parts. In addition, the anisotropic nature of the product and its low strength in the longitudinal direction greatly limit its application. As technology continues to develop, more pultrusion forming techniques will emerge in the future to meet different profile sizes and improve production efficiency.

3. Winding and forming

The winding thermoforming technology, in which continuous fibres impregnated with resin are heated time and wound in a core mould, which is continuously warmed up during winding, and then the pressurised heat is used to melt the prepreg into a kind of, layer by layer bonding and then cooling, to obtain a special kind of formed product.

The use of fibre winding technology allows the carbon fibres to be strengthened to a certain extent, thus achieving automated production. The fibre weaving process has the advantage of being continuous and completed in one pass, with great manufacturing benefits. It is suitable for high volume production, including: cylinders, cylinders, hemispheres, etc., as well as missile casings, rocket motor casings, various tubes, pressure vessels, etc. The shortcomings of this method are that the fibres cannot be combined with the surface of the core mould during winding, making it difficult to form a concave shape, and that the winding machine is more expensive.

New forming process

1. Automatic fibre placement moulding

Automatic Fibre Placement (AFP) is a new rapid prototyping process that has evolved from the winding process. AFP technology is a key technology for manufacturing complex structural parts such as aircraft fuselages and wings, and the AFP and ATL processes are highly efficient, intelligent and easy to use for digital manufacturing. AFP and ATL are highly efficient, intelligent and easy to manufacture digitally.

2. Ultrasonic Rapid Solidification
The ultrasonic rapid consolidation process is a new material forming technology that is shared with the AFP and ATL processes in order to replace traditional heat sources. The frequency range of the ultrasonic waves is usually between 20 and 120 kHz, and the material on which the waves are transmitted is similar to that between the first floors of a building where ultrasound is used. The basic principle is that the ultrasonic waves are propagated between the layers. In the welding area, the local temperature is increased due to its higher acoustic resistance and, at the same time, its thermal conductivity is reduced, which results in the accumulation of heat in the welding area. Immediately upon pressurisation, the contact surfaces of the two resins begin to melt and bond. The corresponding pressure is maintained after the ultrasonic transducer has stopped acting, thus keeping the surface stable.

Ultrasonic fast solidification technology has the advantages of low energy, low cost, fast results and good intelligence. It is suitable for a large number of products that are reinforced with ultrasound. Its function is closely related to the characteristics of the raw material, ultrasonic frequency and amplitude. The organisation of the fibres in the ultrasonic fast curing process is highly dependent on the quality of their consolidation. eds are tiny resin projections formed on the metal surface. eds concentrate the energy of ultrasonic oscillations on the workpiece and are divided into three categories according to their cross-sectional shape: triangular, rectangular and elliptical. the shape has a great influence on the mechanical properties of the metal material.

3. Laser solidification moulding
Laser curing technology is often used in conjunction with AFP, ATL etc. The use of lasers as an alternative high temperature gas heat source allows for reduced energy consumption, less pollution, material savings, faster lay-up of composite materials and increased product automation, particularly in the manufacture of materials such as tail fins and fuselages for aircraft.

4. Electron beam consolidation moulding
Electron beam curing is an advanced curing process that does not require hot pressing. It uses electron beams to make contact with the medium and then transmits electrical energy to the medium in a very short period of time, resulting in physical and chemical changes and cross-linking with polymer molecules to cure the material.

The electron beam curing process, which can be used in combination with technologies such as winding moulding, automatic lay-up moulding, resin transfer moulding and RTM, leads to automated production. The ability to thermally solidify the electron beam at room temperature also reduces the effects of thermal stresses that form during the forming of the material, and the process is characterised by low cost, low pollution and high efficiency.

5. vacuum-assisted forming
Vacuum assisted forming is a new, low cost, high efficiency technology for processing composite components developed from the RTM process. The basic principle of vacuum-assisted forming is to cure carbon fibres under vacuum by extracting the gas from the fibres under negative pressure, allowing the resin to flow and permeate through the air. This method is currently being explored by many scholars and applied to thermoplastic resins or used in the synthesis of multilayer composites.

6. 3D Printed Molding
3D printing technology is a method of creating a three-dimensional material entity by means of a digital control system using layered printing. This control system is automated, intelligent, highly accurate and efficient, and can effectively reduce the cost of manufacturing composite materials.

In recent years, due to the rapid development of 3D printer technology, the use of 3D printer technology to manufacture carbon fibre-reinforced thermoplastic composites has become a focus of interest for researchers abroad, with highly selective laser sintering and melt deposition being two of the more common process technologies used today.

Carbon fibres can enhance the development of 3D printing technology and the expansion of applications. During the production process, high-precision and efficient 3D printing systems are investigated due to factors such as equipment temperature, printing speed, layer height and printing process, as well as the influence of the composite material's own parameters, material diameter and nozzle diameter.