If you understand how carbon fiber tow and fabric are manufactured, you know that it is a complex and energy-intensive process that requires aligning carbon atoms, subjecting them to high heat, and then coating them with a chemical to help resin stick. But how do those fibers and fabrics become composite tubing?
It is easy to look at a cross section of composite tubing and assume it’s a single-piece construction. The finished product certainly is a single piece, but that is only because the resin and curing process bonds everything together. That section of tubing did not start out as a single piece. It started as multiple layers of fiber or fabric that were combined with resin to create the finished tube.
There are four primary processes for creating composite tubing. These are:
1. ROLL WRAPPING
Roll wrapping is typically done with a prepreg product to ensure consistency. A prepreg is a composite product consisting of fabric or fiber already impregnated with the epoxy resin necessary to hold everything together.
The prepreg material is cut into layers of different fiber orientation. Those layers are then rolled onto a cylindrical rod known as a mandrel. The mandrel and prepreg are then wrapped in a plastic film to contain the epoxy resin and compress the layers during curing. Once curing is complete, the mandrel is removed from the center of the finished tubed.
Roll wrapping results in maximum consistency across both carbon fiber and fiberglass tubing. The process also affords more customization in terms of both fiber/mandrel configuration and production quantities. Roll wrapping is the preferred process for producing small runs.
2. FILAMENT WINDING
The filament winding process begins with a composite tow of either carbon fiber, fiberglass, or a carbon fiber-fiberglass hybrid. The tow is loaded onto multiple spools at the back of the winding machine. At the front is a long mandrel that acts as the form for the tubing.
To create the tubing, a carriage travels up and down the length of the machine while the mandrel spins. The tows are applied to the mandrel in a spiral pattern like the stripe on a candy cane. The angle of the spiral can be adjusted as well as how many spirals it takes to make a layer. Once completely formed, the tube is cured before being separated from the mandrel.
Filament winding is more automated than roll wrapping, and therefore a more cost-effective way to produce large volumes of composite tubing in a short amount of time. Customization is often easier with roll wrapping since the fiber and resin can be mixed and matched at the winding machine instead of during the prepregging process.
The pultrusion process gets its name from its combination of pulling and extrusion principles. Where extrusion forces material through a die by pushing it, pultrusion accomplishes the same thing by pulling the material through the die. Pultruded tubing is created by pulling carbon fiber or fiberglass tow through a heated die as it is being impregnated with epoxy resin. The material is pulled over a mandrel that ensures it holds its shape during the curing process.
The advantage of this process is that it produces a continuous, unidirectional length of tubing that can be cut to size after curing. Since pultrusion is highly automated, it’s a much more cost-effective production process than both roll wrapping or filament winding. Pultrusion makes it easy to produce tubing in various lengths and thicknesses simply by changing up both mandrel and die.
The downside of pultrusion is that all the fibers are oriented along the axis of the tube. Having all the fibers in one direction means the tube is very good in tension but can easily split in compression or torsion. The quest for an automated process that can produce balanced tubes brings us to pullbraiding.
Pullbraiding is an extension of pultrusion. This process is essentially the same as pultrusion with one added feature: the fibers are braided together as they are being pulled through the heated die and onto the mandrel. Layers of different angles can be made by varying the braid, and even unidirectional layers can be inserted.
Both pultrusion and pullbraiding create finished products with high stiffness and strength-to-weight ratios. But the main advantage of the pullbraiding process is that it creates a more balanced tube that performs under a wide range of loads. It also adds an element of aesthetic beauty since the braid is more in line with the traditional “carbon fiber” look. And since this process is highly automated like pultrusion, pullbraided tubes are often less expensive than roll wrapped or filament wound products.