Production Method (8) – Compression Molding
Carbon fiber is widely used in aerospace, infrastructure, electromagnetic shielding, automobile manufacturing, oil exploitation and other fields due to its advantages of fatigue resistance, high temperature resistance, high strength, high modulus, conductivity, radiation resistance, small coefficient of thermal expansion, light weight, etc. It has high flexibility in design and can be produced by various production method, including winding, extrusion, injection, hand layup, and molding. Below is going to show you more details regarding one of the most common production method of carbon fiber product – Compression Molding.
The steps of compression molding production method:
1. Embedded Inserts Placement (if any)
Embedded parts are generally made of metal and can enhance the mechanical properties of products, such as the conductivity, thermal conductivity, or other functional characteristics.
It is best to preheat the insert before placing it and the insert should be placed accurately and stably to prevent displacement or detachment. Otherwise, the purpose of using inserts cannot be achieved, and it may lead to product scrapping and even damage to the mold.
2. Material laying
Material laying affects the size and density of the product, and hence the material should be strictly measured before putting into the mold by weight method, volumetric method and counting method.
The weight method is accurate but clumsy and is often used for fragmented and fibrous materials. The volumetric method is not as accurate as the weight method, but it is easy to operate and is generally used for powder measurement. The counting method is only used for prepreg laying.
3. Mold closing
Mold closing consists of 2 steps:
1) before the male mold contacting the materials: low pressure (1-5-3.0MPa) is used to shorten the cycle and avoid any material change
2) after the male mold contacting the materials: the mold closing speed should be slowed down, and high pressure (15-30MPa) should be used slowly to avoid damaging the insert and causing air to be discharged from the mold
4. Curing
The process of a material transitioning from a fluid state to a hard, insoluble state is called the solidification of thermosetting resins. The rate of curing depends on the rate at which the low molecular weight components in the resin transform into high molecular weight products, that is, the curing rate is related to the molecular structure of the resin.
5. Pressure Maintaining
The process of resin curing in the mold is always under high temperature and high pressure, and the pressure maintaining time is essentially the time to maintain temperature and pressure, which is completely consistent with the curing rate. If the pressure maintaining time is too short, meaning premature cooling and pressure reduction that can lead to incomplete resin curing, reducing the mechanical and electrical properties as well as heat resistance of the product. At the same time, the product will continue to shrink and experience warping after demolding.
6. Demolding
Products with forming rods or certain embedded parts should first be unscrewed with specialized tools before demolding.
8. Mold Cleaning
It is necessary to clean the mold after each production to avoid any residual materials in the mold. After cleaning, release agent can be applied for the next molding.
The main advantages of adopting compression molding for carbon fiber composite material:
1. High production efficiency, easy to achieve specialization and automated production
2. The product has high dimensional accuracy and good repeatability
3. The surface is smooth and does not require much post-process
4. Capable of forming products with complex structures in one go
5. Price is low for mass production
Disadvantages of carbon fiber composite material molding:
1. Mold making is complex and the investment is high
2. Only suitable for small and medium sized composite products due to the heavy mold weight
With the continuous improvement and development of metal processing technology, press manufacturing level, and synthetic resin process performance, the tonnage and table size of the press continue to increase, and the molding temperature and pressure of the molding material are also relatively reduced, leading to the gradual development of large-sized molded products.
During the molding process, special attention should be paid to the three factors of temperature, pressure, and time. Temperature has a decisive impact on the melting, flow, and solidification of materials; Pressure can make layers of carbon fiber prepreg bond more tightly; Time can allow sufficient time for the product in the mold cavity to solidify.
