Groover/Introduction to Manufacturing Processes
Composites Processing Case Study: Resin Transfer Molding in the Northeast
Luke works for a Northeast company that produces structural components for the internal cockpit and passenger areas on luxury and corporate jets. The components are all fiber-reinforced polymer-matrix composites produced using a vacuum-assisted resin transfer molding (RTM) process. They typically use a polyester resin as the matrix material, though certain parts that need additional strength and high-temperature protection use an epoxy resin. The preform mats that sit in the mold and give the structures their shape are made of long strands of S-glass fiber. Occasionally, the company will make carbon fiber reinforced moldings, but orders for those expensive parts are usually pretty small.
The RTM process is fairly simple. The bottom half of a mold is typically machined out of aluminum or a dense particulate composite material to match the desired shape and critical features of each component. A more flexible top half to the mold (no critical features) is molded out of fiberglass and assembled onto the bottom half. Mats or a preform of the fiber reinforcement are placed into the mold. The top mold has a hole in the middle for liquid resin to be drawn into the mold cavity. A sealed plastic bag encloses the mold and a vacuum is pulled to draw in the resin. After the mold is filled, it is moved to an autoclave to speed up curing. Luke is the engineer responsible for quality control in the RTM process, so he keeps track of vacuum pressure, mold quality, curing time for the resin, and final part density, strength, and surface finish.
Luke has gotten pretty good at diagnosing problems with the RTM process over the two and a half years since he’s been at his current job. One of the biggest problems he had to solve dealt with parts that were cracking during the aircraft assembly process. It turned out that variation in the power source for the vacuum was leaving some parts with bubbles (weak spots) randomly hidden in the cavity during the process. Another time he discovered that an error in the mold size was leaving the part with too much resin, over 55% for a part that specified 40% resin. Those parts had the wrong stiffness and caused discomfort for some of the passengers during flight. Luke had actually started learning about composites processes years earlier when he worked summers for a large theme park in Florida. There they made stage props and costume accessories out of fiberglass by putting down layers of glass weave and resin into plaster molds. They even had an automatic spray gun for chopping and spraying fibers onto part molds that needed quick production and repair for parades and other special events. Of course the RTM process at Luke’s current job involves a lot more investment and is able to make higher quality parts efficiently.
WATCH THE VIDEO: Composite Materials and Manufacturing
1. What is a “gel coat” in the manual lay-up process?
2. What can be done manually during lay-up and spray-up processes to ensure that no air bubbles are left in a composite part?
3. Prepreg composite tapes and mats are more expensive than plain fibers and resin, and prepregs require refrigeration storage so they won’t cure before use. Why would any company use prepregs? List advantages.
GO TO THE TEXT: Chapters 2 and 9
4. Why are composite materials so frequently associated with aircraft applications and high-efficiency vehicles? See Section 2.4.
5. Kevlar is a trade name for a type of polyamide thermoplastic polymer often used as a fiber in composites. Compare the strength-to-weight ratio of Kevlar to that of steel. See Section 2.3.
6. What happens chemically during curing? See Section 2.3.
7. How is an autoclave different from a standard oven? See Section 9.4.
8. The spray-up process can be used to make a laminated composite part quickly. What are its limitations compared to hand lay-up? See Section 9.4.
9. What shape parts are made by filament winding? See Section 9.6.
SOLVE
10. The mechanical properties of composites can often be estimated based on a weighted average (by volume) of the constituent materials’ properties. If the case study part that was supposed to get 40% (by volume) resin was made of polyester and S-glass, compute its expected stiffness and the stiffness of the defective parts with 55% resin. Use 1,015,000 psi as the modulus of elasticity (stiffness) for polyester and 12,000,000 psi for the stiffness of S-glass fibers.