A Beginner’s Guide to Chopped Carbon Fiber

 

When it comes to chopped carbon fiber, it is nothing more than short-cut carbon fiber. However, there are many ways to learn about chopped carbon fiber, but there is little information online. In the past two years, the chopped carbon fiber market has grown rapidly, and more and more people need to understand chopped carbon fiber. This article wants to talk about the basic situation of chopped carbon fiber.

 

Chopped carbon fiber, as the name suggests, is short-cut carbon fiber. Here, the carbon fiber has only changed in form, from carbon fiber filament to short filament, but the performance of the chopped carbon fiber itself has not changed. So why should the good filament be cut short for use?

 

First of all, we should start with the molding process of composite materials. Usually, carbon fiber filaments are woven into carbon fiber cloth or made into carbon fiber prepreg, and then various carbon fiber composite products are made through molding, RTM, vacuum bag pressing, autoclave and other processes. Of course, there are also some molding processes that do not require intermediate materials and are directly molded through carbon fiber filaments, such as pultrusion molding, winding molding, etc.

 

When carbon fiber is woven into carbon fiber cloth or made into prepreg, it has a natural disadvantage when it is made into products, that is, it is not easy to stick to the mold. The existence of the mold is to give shape to the composite material. The shape of the mold determines the shape of the composite material. However, if the carbon fiber cloth or prepreg cannot fit the mold well, the shape of the composite material will not match the shape of the mold. In addition, at some corners, the carbon fiber cloth is easy to bridge, forming local cavities, which eventually leads to the decline of the performance of carbon fiber composite products.

 

The carbon fiber filaments in the carbon fiber cloth or prepreg are bound and not easy to move. Especially when pressure is applied, the resin and carbon fiber filaments have poor fluidity, which will eventually cause molding difficulties or performance degradation. Is there any way to increase the fluidity of carbon fiber?

The answer is to cut the carbon fiber filaments into short carbon fibers. The shorter the fiber length, the less bound it is. For example, long-haired beauties often say “you are pressing my hair”, while short-haired beauties don’t have this trouble, you know.

 

Short-cut carbon fiber has good fluidity. The shorter the length, the better the fluidity. Mixing short-cut carbon fiber and resin into granules, and then making various products through injection molding, can achieve mass production. You should know that injection molding is the most widely used and mature technology in plastic molding process. If it is applied to the production of composite materials, it will be a revolutionary change.

 

However, the length of chopped carbon fiber cannot be reduced blindly, because the shorter the length of chopped carbon fiber, the weaker the bond between resin and carbon fiber. Because the bond between resin and carbon fiber is proportional to the contact area between them, if the length is shortened, the contact area will definitely decrease.

 

Then there is a contradiction here, which is the contradiction between the performance and fluidity of chopped fiber. The longer the fiber length, the less likely it is to disperse, and the fibers are easy to knot, but the stronger the bond between the fiber and the resin, the higher the performance of the composite material. The shorter the fiber length, the easier it is to disperse, and it has good fluidity, but the bond between the fiber and the resin is slightly weaker. How to balance this contradiction needs to be studied. Under normal circumstances, the granular material for chopped carbon fiber reinforced plastic needs to be in the range of 1-9mm.

 

Another way to enhance the bonding strength between carbon fiber and resin is sizing agent. Normally, there is a layer of sizing agent on the surface of carbon fiber when it leaves the factory. On the one hand, it is used to protect the carbon fiber from fluffing during packaging, transportation, and operation, and on the other hand, it is used to combine the carbon fiber and resin to enhance the bonding strength.

 

However, this layer of sizing agent is basically for thermosetting resins. The resins used to make granules are mostly thermoplastic resins, so the sizing agent must be readjusted. There are two situations. One is to burn the original sizing agent and make a new layer of sizing agent. The other is to apply a layer of sizing agent on the basis of the original sizing agent, which is called secondary sizing.

 

Normally, manufacturers who do plastic modification buy short-cut fibers without sizing agents, prepare sizing agents by themselves, and then make modified plastics. Or directly buy short-cut fibers that have been sized twice. However, these two types of short-cut fibers are more expensive than short-cut fibers with original sizing agents, after all, there is one more process. At present, the domestic short-cut carbon fibers are basically short-cut carbon fibers with original sizing agents, and there are very few short-cut carbon fibers without sizing agents or with secondary sizing. Some foreign manufacturers will provide them.

 

In addition to making pellets for injection molding, chopped carbon fiber has other uses, such as making carbon fiber felt or carbon fiber paper. The required length of chopped carbon fiber is longer than the length of chopped carbon fiber for pellets.

 

In addition, in addition to the disordered chopped carbon fiber, there are also bundled chopped carbon fiber. This kind of chopped fiber is to pre-shape the carbon fiber tow before chopping, so the chopped fiber has the characteristics of bundles and contains much more resin than other chopped fibers.

 

Today’s chopped carbon fiber is here. If you have any opinions or additions on this topic, please leave a message below for discussion.