Is the length of chopped carbon fiber always better the longer it is? Clearing up the selection once and for all 

When purchasing short-cut carbon fibers, many customers tend to believe that "the longer the length, the better the reinforcing effect". This statement has some validity, but it is not absolute. If the length is chosen incorrectly, it may lead to uneven distribution, rough surface of the product, and even performance degradation. Let's start from two common misunderstandings to help you clarify your selection. 

One, Misconception 1: The longer the length, the higher the strength. 

The length of the chopped carbon fibers does indeed affect the "framework" role of the fibers in the composite material, but this effect is not linear. 

The relationship between length and enhancement effect: Generally speaking, the longer the fiber length, the stronger the anchoring effect in the matrix, and the more significant the improvement in tensile strength and modulus. However, there is a "critical length" for this relationship - beyond this length, the increase in strength will slow down, while the processing difficulty will significantly increase. 

Risk of excessive length: In the injection or extrusion processes, overly long carbon fibers are prone to breaking in the screw and mold, instead becoming shorter fragmented fibers, thereby undermining the original "long fiber reinforcement" advantage. Additionally, overly long fibers are likely to aggregate within the matrix, resulting in uneven dispersion and causing floating fibers or roughness on the surface of the product. 

II. Misconception 2: One length is suitable for all processes 

The adaptability range of carbon fiber length to different processing methods varies greatly. There is no single length that can accommodate all processes. 

Injection molding process: Generally, 6-12mm chopped carbon fibers are recommended. With a length within this range, the carbon fibers can be uniformly mixed with plastic particles in the twin-screw extruder. When passing through the screw of the injection molding machine, they are less likely to be excessively broken, and they can retain a certain length in the final product. This length range has been widely verified when used to reinforce thermoplastic materials such as nylon and polypropylene. 

Compression molding or extrusion process: This type of process has a relatively weak shear effect on fibers. Larger specifications of 12mm or above can be selected, resulting in longer fiber retention and more significant enhancement effect. It is suitable for sheet molding compounds or continuous extrusion products. 

Powder coatings or fine fillers: For these applications, shorter fibers (typically 3-6mm) are required or carbon fiber powder is used directly to ensure uniform dispersion and no settling within the coating or slurry. 

Summary and Selection Suggestions 

When purchasing short-cut carbon fibers, the following principles are recommended for selection: 

First, consider the process, then determine the length: For injection molding, the thickness can be 6-12mm; for compression molding or extrusion, it can be 12mm or more; for powder coatings or fine fillings, the thickness can be 3-6mm or powder form. 

Clarify the substrate and application: Inform the supplier about the type of resin you are using (nylon, polypropylene, epoxy, etc.) and the requirements for the final product, so that the sizing agent type and length specifications can be matched. 

Actual measurement on the sample machine: Conduct a small-scale trial on your production line to observe whether the uniformity of the mixture, the surface quality of the product, and the mechanical properties meet the standards. 

Yancheng Xiangsheng Carbon Fiber Technology Co., Ltd. specializes in the production and research of short-cut carbon fibers and carbon fiber powders. We can customize the length specifications and sizing agents according to your process requirements. Please feel free to request samples for testing and communicate technical requirements. 

Official website: http://www.xlfibre.com/