Composites are becoming more and more common in everyday products; but what are they and how are parts made?
Here's our guide to material selection, finishing, and everything in-between.
Before we jump into the advanced topics of automated fiber placement, or even additive manufacturing of composites, we need to create a strong foundation of what composites really are.
In short, composites are a combination of at least two unique materials with different properties, that when combined, compliment each other to create a superior result. The main materials contributing to a composite part are reinforcements, or fibers, held together by a polymer glue, called a matrix.
Let's think of the fibers as individual ropes. Everyone knows ropes are ideal for pulling heavy loads, and not so good for pushing them. This means that ropes have high tensile strength, and low compressive strength. When the polymer matrix is introduced, this creates a stiff backbone for the fibers which transfers loads between the fibers. Where one fiber would be in compression, the load gets transferred via the matrix to a different fiber that is in tension. This constant load transfer between fibers, and knowing when to orient the fibers to achieve different stiffnesses, is what makes composites so unique for many different applications. There are two types of polymers used as matrices, and this is how composites are commonly categorized; thermosets or thermoplastics. Learn more about the basics of composites. Thermoset composites are the more traditional method, so we will focus on that for now.
Composites don't necessarily have to consist of simply carbon fiber and an epoxy resin; there are many different types of reinforcements, and even combinations of reinforcements, available today. For more information on the different classifications of composites, read this blog.
Ok, so now that we know we need to combine these two unique materials somehow to create a composite part, but how? The most common manufacturing method of composites is done through a manual process called hand layup. In this process, there is an operator placing fiber in specific orientations, mixing the epoxy resin, and then manually soaking the fibers with the epoxy, called impregnating, using extreme caution to not move the fibers. In order to get a solid part without any defects, vacuum must be applied to press the wet fibers into all corners and crevices of the mold, creating the correct shape with high quality.
Image of wetting fiber
The next level of manual layup is called Resin Transfer Molding, or RTM. RTM begins the same way of manually placing fibers, but skips the wetting phase and goes straight to applying vacuum. Once vacuum is applied, a tube is placed into a bucket of wet resin, and the vacuum pulls the resin through the tube and infuses the fibers with resin.
Image of RTM
This process is quite tedious and arduous, and production rates are rather slow. A skilled layup technician could create parts at a rate of about 1 kg per hour. As a result, it was soon realized there was a way to quicker create parts from composites - instead of placing the fiber and matrix individually, companies preimpregnated the fibers with the matrix at the point of creation, developing a material widely used today. These materials are known as prepregs. While they changed the industry forever, they come with their own processing challenges. Learn more about prepreg materials.