How to Spot the Difference Between Good Quality and Poor Quality Carbon Fiber
As with any market, there are carbon fiber vendors out there who are more interested in making a buck than in selling you a quality product. When selecting carbon fiber parts, it’s important to know whether you’re purchasing a quality carbon fiber product or a product with a thin layer of carbon fiber bonded over fiberglass.
A true carbon fiber part will contain carbon fiber in every laminate layer and use only quality core materials. A cheaper product might be made up of only a thin layer of carbon fiber over a fiberglass core. This can be accomplished with a simple wet-lay technique. It’s important to be able to spot the difference.
The Backside of Carbon Fiber vs The Backside of Fiberglass with a Carbon Fiber Overlay
One simple way to tell the difference between a solid carbon fiber part and a carbon fiber/fiberglass part is to look at the back or inside of the part. A true carbon fiber part will still look like carbon fiber from the reverse side. In other words, you will be able to see the actual fibers of carbon. Depending on the carbon fiber finish you selected, it might be matte or shiny, but the sheen will be fairly even.
A fiberglass part, by contrast, will look like a piece of fiberglass cloth instead. As you move it around in the light, there will often be some areas that are shinier than others. This indicates that the part was molded using a “wet lay” process wherein one layer of carbon fiber, followed by several layers of fiberglass are fitted into the mold, and then resin is poured, brushed, and left to cure. Furthermore, a single layer of carbon over fiberglass will sometimes allow light to shine through.
Weight of Carbon Fiber vs Fiberglass with Carbon Fiber Overlay
A fiberglass part with carbon fiber skin can be up to 70% heavier than a true carbon fiber part. This amount can vary based on details of the core material used among other factors, but the fiberglass will always be significantly heavier. There are several reasons for this:
Fiberglass simply weighs more than carbon fiber. Below are the density ratings for both.
Fiberglass 𝛒 = 2.6 g/cm³
Carbon Fiber 𝛒 = 1.55 g/cm³
The wet lay process adds extra weight since excess epoxy is not drawn out of the material via the vacuum as it is with vacuum bagging or epoxy resin infusion typically used with higher quality carbon fiber parts.
That extra weight does not equal strength. The modulus of elasticity of carbon fiber is four times that of fiberglass.
Fiberglass E=30 GPa (gigapascals)
Carbon Fiber E=125 GPa
Edges of Carbon Fiber vs Fiberglass with Carbon Fiber Overlay
When an outer skin of carbon fiber is bonded to an inner layer of fiberglass, a white line shows at the edge due to the white color of the fiberglass. Often that line is masked by painting a black trim line around the edges. A solid carbon fiber part will have smooth edges with a uniform black color.
Bottom Line
Sometimes, the cost is a driving factor, and fiberglass with carbon fiber overlay will certainly be less expensive than solid carbon fiber parts. Being aware of what you have purchased and understanding the ramifications can save you a lot of headaches later in your project. A solid carbon fiber part will be lighter, stronger, and stiffer than a fiberglass part with a carbon fiber overlay. If those characteristics are important to the integrity of the project then you will want to be sure you’ve gotten true carbon fiber.
Any time you are purchasing parts you want to be sure you know what you are getting. Everyone hates thinking they are getting a real deal and finding out they got exactly what they paid for. When making an investment in carbon fiber parts it is crucial to know exactly what to look for, and to be sure you’re getting what you wanted. Using these techniques to check your parts will ensure you have purchased a quality product that will work as specified for years to come.