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Laminations and Carbon Fiber
By Craig MacKenzie, CP, RTP(c)  Not all carbon fiber braid is created equal.
There are two main suppliers for carbon braid and several different variations within each company. The two main differences in the variations are angle of weave and weight of material. The angle of the weave can change the coverage that you get when the material expands to a larger diameter, and the weight of the carbon braid determines how many layers of carbon a certain lamination requires for the weight and activity level of the amputee (notice the difference between figures 1 and 2). The tighter weave and heavier weight has much better coverage. In this case, two layers of one type of carbon braid equal one of the other. Also, concentration of carbon fiber in key areas, rather than adding another complete layer, can dramatically affect the weight of the socket.
It is difficult to find resources for layups in the industry because of the liability involved on the supplier's side if the socket fails. The layups listed in this article are for reference only, and it is the responsibility of the facility to make sure that the socket provided meets the needs of the amputee. With that said, the layup for an average socket at Velocity Labs is 1¼ carbon braid, 1¼ fiberglass braid, and 1¼ carbon over an 1/8-inch PETG inner plastic. This is the classic carbon finish socket. The 1¼ carbon is one full layer of carbon tied into the plate and reflected two to three inches up from the distal. With this configuration, the "classic" carbon look has two layers of carbon and one layer of fiberglass at the proximal edge of the socket and four layers of carbon and two layers of fiberglass at the attachment plate at the distal. This socket would be for a person weighing 150-200 pounds with an average activity level. This assumes the use of a heavy-weight carbon fiber braid. Some suppliers sell the light- or medium-weight material, in which case additional carbon layers may be required.
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Figure 1: Heavy |
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Figure 2: Regular |
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We have found that the addition of the fiberglass braid helps in edge finishing; without the fiberglass, it is easy to overheat the carbon braid and cause "fuzzy" areas on the trim. However, the fiberglass braid dissipates the heat. Layering fiberglass and carbon is done in structural components in aircraft wings to aid in flexing, but in the prosthetic industry, there is no real structural advantage between 1 carbon-1 fiberglass-1 carbon and 2 carbon-1 fiberglass.
When the socket is a color other than carbon/black, we switch the layup to 1¼ carbon, 1¼ carbon, 2 fiberglass, and 2 nylon. The 2 nylon and fiberglass hide the carbon fiber and provide a good color layer. This style socket is still carbon on the inside. If the socket has to be a different color on the inside, then two layers of fiberglass are added on the inside first, but this only serves to add weight to the socket. Two layers is actually one layer tied into the groove in the plate and reflected. With Velocity lamination plates all of the layers of material are reflected, so there is no cleanup after lamination.
The Velocity Labs lamination chart is shown in table 1 (remember that Velocity uses a specific braid weight and weave). Any time an odd layer is listed, it is one full layer tied into the groove and reflected two to three inches.
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Table 1: Velocity Labs' Lamination Chart |
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All manufacturers of carbon and fiberglass braid sell it by the pound; unfortunately, in the prosthetics industry the majority of it is sold by the foot. To complicate comparisons even further, not all carbon and fiberglass sold in prosthetics is the same weave and weight. It's not a good deal if two layers of one weight carbon is required over one layer of another, when the cost was only 20 percent less.
Craig MacKenzie, CP, RTP(c), is the owner of Velocity Labs Inc., Orlando, Florida, and president of Evolution Liners Inc., Orlando. He may be contacted at craigm@velocity-labs.com 
Table Of Contents - March 2008
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