Quality

GUIDE TO CARBON COMPOSITES

THE RAW MATERIAL:
Carbon composite structures are manufactured from millions of carbon fibers held together by epoxy resin. Carbon fiber itself is an old material. Thomas Edison made and used this material as a filament in his first light bulbs back in 1880. In the last 30 years, production techniques have been discovered that allow this material to be made strong, stiff and relatively inexpensive.

The carbon fiber that we use is made from PAN (polyacrylonitrile). This is a polymer consisting of carbon, hydrogen and nitrogen molecules. The process by which PAN is made into carbon fiber involves four steps. First the polymer is extruded through a spinneret. The tread that comes out is stretched as it dries. The tread is then heated to 220 C under tension. This process causes the molecules in the thread to align. With the strong axis of the molecules aligned with the length of the thread, the next process locks this structure in. In this third step, the tread is carbonized by heating it to 1500 C in a nitrogen atmosphere. The heat drives off all the non-carbon elements from the thread. The temperature is then raised to graphitize the thread. This last step improves the strength and stiffness of the final thread. The carbon fiber manufacturer takes these threads which are much thinner than a single hair on your head and groups then into ropes of 12000 threads called tows. The tow is wound on a bobbin and sent off to another manufacturer to be made into cloth or uni-directional material.

HOW PARTS ARE MADE:
Most of the material that we use at GMT is uni-directional carbon fiber that has been pre-impregnated with epoxy. Uni-directional material is a material where all the fibers are aligned and parallel. This material is inherently stronger and stiffer than woven cloths since in a cloth, the fibers pass over and under each other. When a woven material is pulled, the weave flattens out as the fibers press down on each other. The kinks in the fiber cause weakness and additional stretch.

Pre-preging is the sophisticated process of adding a high temperature cure epoxy to carbon fiber. It is a more expensive but far superior alternative to traditional wet lay-up, filament wound or wet-preg materials. In a pre-preg, the epoxy is first cast onto a roll of backing paper as a thin, uniform sheet. The carbon fibers (either as unidirectional material or woven cloth) are then laid on top of the epoxy. The sandwich is passed through heated rolls that melt the epoxy and allow it to completely coat the carbon. As the material comes out of the rolls, it is cooled and re-wound onto a roll. The resin is now at a B-Stage cure. It is stored and shipped frozen. In this condition, it will last for many months without being damaged. At room temperature, the pre-preg has a slight tack. It sticks lightly to itself when pressed down. The material is stable, clean and easy to work with.

The use of pre-pregs results in composite parts that are dramatically better than parts made with less expensive materials. In a pre-preg, the ratio of fiber to resin is tightly controlled and very consistent throughout the part. In a wet lay-up part, the resin distribution is un-even. This results in pockets of resin and areas where the fibers are too dry. Strength goes down and weight , due to excess resin, goes up as the test results show. The quality of a pre-preg part is also enhanced because the material is easy to work with. At room temperature, we have 30 days to work with the pre-preg before it goes bad. The technicians don’t have to rush. They can take their time to accurately position each ply and reinforcement patch. After a portion of the plies are put down, the part is put under vacuum. This compacts the laminate and removes air that is trapped between the layers. In contrast, with a wet lay-up, you are always rushed because the epoxy cures within a couple of hours. The laminating team is fighting with slippery material against a tight time schedule to get the part made and under vacuum before it is ruined.

THE CURE CYCLE:
Once the laminating portion of the pre-preg construction process is finished, the part is ready for final curing. The part is wrapped in various film layers and placed in a special flexible tube. All air is evacuated from the tube to compact the laminate. The part is placed into our 65’ long convection oven. The temperature is ramped up according to a carefully controlled schedule. As the temperature of the part rises, the epoxy in the pre-preg gets less viscous. It flows and coats all fibers uniformly. The pressure from the vacuum bag presses down and drives any remaining air out of the part. As the temperature gets up to 250 F, the resin begins to polymerize. After about 60 minutes, the cure is complete. The part is then cooled, removed and is ready for further assembly.

There are a number of benefits to this elevated cure temperature system. The resin system that we use has been specifically developed for use in vacuum bagging applications. The viscosity and flow characteristics of the resin are such that higher pressures, such as produced in an autoclave, are counterproductive. An oven cured part made with this resin system has the same high strength and low voids as an autoclaved part. This is achieved without the danger of locally thinning out the laminate in the corners or driving out too much resin as sometimes happens with an autoclave. A pre-preg part is also much stronger at elevated temperatures. A composite part sitting out in the sun on a hot day can have a surface temperature of over 150 F. At these temperatures, room temperature cured parts (wet lay-up, wet preg, etc) loose most of their strength. Deformation and failure can occur. A pre-preg part can be painted a dark color without the danger of it failing in a hot climate.

CONCLUSION:
Carbon fiber is a wonderful material for many applications. It is light, stiff and strong. Pound for pound, it is 9 times stronger than aluminum and 30 times stronger than steel. It doesn’t corrode. It doesn’t fatigue. A highly engineered, well made structure will last longer than the metal part it replaces. At GMT, we have been designing and building with this material since 1984. We have helped our customers with parts ranging in size from small robotic paddles weighing under a pound to 40 meter masts weighing over 1000 pounds. Put our experience to use. Contact us to discuss your applications.