Boeing’s 787
Boeing’s 787 will be the first composites-intensive commercial airliner. Traditionally made from aluminum, carbon fiber composites will work to create a plane that is stronger and lighter with fewer manufactured parts. Carbon Fiber reinforcement with Epoxy resin will be the main construction of these composites, which will make use of an autoclave during processing to control the molding conditions and ensure the quality and durability of the laminate.
Composites will reduce the number of parts for the airplane, and Boeing predicts that the front section alone would normally require using 1,500 sheets of aluminum, which also means drilling between 40,000 and 50,000 holes for the nuts and bolts to attach these sheets together and to the underlying framework. Carbon Fiber composites will allow for the skin and underlying supports to be molded as one large piece. Boeing predicts that assembly line time will be reduced from about three weeks to attach all of this aluminum together to about 3 days to attach the large composites sections together for the entire plane fuselage.
Switching materials has its’ own set of problems to overcome. The customers’ mechanics will need to be trained to repair damage on these composite planes. Damage detection will be important as well. Some will be visible to the naked eye, and other damage will not. Several forms of Non Destructive Testing will be employed to test for damage and wear on the composites body to ensure a safe aircraft.
Composites have been used in aircraft before, but not as extensively in commercial airplane bodies. Existing commercial airplanes have made use of composites in other areas to help make the planes stronger and lighter. Military jets have used carbon fiber composites for many years in their technologies for strength and weight advantages. Private business jets have utilized fiberglass composites for many years in their construction. Homemade kit planes have also made extensive use of fiberglass to make inexpensive craft in personal shops.
The profile of carbon fiber composites will definitely be elevated if Boeing’s 787 becomes as successful as promised.
Repairing the Inner Fender
One of my recent projects involved the repair of a 1993 International Medium-Duty truck hood made from SMC. There were several areas needing attention, and one of them was the driver’s side inner fender. This piece had formerly been attached with button-head pop rivets. This design is common to composites, and allows for easy replacement of the separate fiberglass pieces. The pop rivets had come loose over time, allowed to move around, and cause severe damage to the extent that the riveting flange was broken off. My only solution was to bond the two pieces together.
Material Fatigue in the corner
The loose panel flexed so much and for so long that it fatigued the material and failed in the corner of the inner fender next to the attachment to the rest of the hood. To repair this, I removed the area with the rivets, ground down the surfaces of both pieces on both sides, and reattached them with fiberglass and epoxy resin.
Prepared glass and resin
I wanted to place epoxy and fiberglass on both sides of the repair area to ensure a good, solid bond that would hold very well.
Epoxy Resin and Fiberglass applied
After the area was prepared, I applied epoxy resin to the surface to ensure good adhesion. I had a low spot that was a gap, so I mixed some microfiber and epoxy to make a paste and fill this gap. A stronger bond is produced when the fiberglass is not spanning an open gap between the two pieces. I placed two layers of 3oz Chopped Strand Mat over the paste and worked the air out to make a nice consistent repair. I then ground down the surface to make a nice-looking, consistent repair.
Back end of the inner fender
The rear of the inner fender had similar problems. A hole had emerged in the black SMC piece. I ground down both surfaces and placed some fiberglass across the area to bond it together.
The Green Aspects of SMC
Sheet Molding Compound (SMC) is used to create many composite parts especially for the transportation industry, and contributes heavily to a positive environmental impact. SMC has been developed over the last 25 years to replace steel/sheet metal mostly in transportation applications. It is widely used in many heavy duty semi truck hoods, agricultural equipment, and pickup trucks, SUV’s and muscle cars.
The main goal of this substitution is to reduce weight, which improves fuel efficiency. Other positive side effects include fewer assembly operations, additional design freedom, dent and impact resistance, and the elimination of corrosion. Several “green” resin formulations have been introduced that make use of bio resins, which use much more renewable resources such as soy products. The fillers and reinforcements in this material can also be made from recycled and renewable materials.
SMC has overcome several hurdles in order to get to its present use and application. General acceptance and education had to be proven to the OEM manufacturers and consumers. Paint application and adhesion was one large consideration that had to be proven out. There were issues with popping and blistering from the SMC surface. Making sure the SMC parts held dimensions and aesthetics was also an important milestone.
SMC has become widely used today for many applications, and will find its way into many more. The weight saving aspects are paramount for reducing fuel consumption. The anti-corrosion and dent resistance are loved by consumers.
Curing and Shrinkage
A very important aspect of thermoset resins is their cure cycle. Unsaturated polyester and vinylester, along with epoxy, require time and temperature in order to achieve what we call “Crosslinking.” This is the the “set” part of thermoset, and is the permanent and irreversible chemical bonds in the resin. The amount of time and temperature is dependent upon the formulation of the resin, the ratio of resin-to-hardener, and the presence of additional chemicals used to modify the properties.
Outside of the chemistry, the control of the time and temperature is important to the curing of the resin. If the actual temperature is outside the range of the intended formulated temperature, it will affect the curing reaction. If the part is demolded too early, the resin will continue to cure, but the final shape of the part may not match the mold. The manufacturer of the resin is the very best source for information on the recommended cure time and temperature.
As these resins change from liquid to solid states, there is a certain amount of shrinkage involved. A part made on a female mold will shrink towards the center, and a part made on a male mold will tighten around that mold. This shrinkage factor depends upon the resin chemistry and its additives, but is generally less than 3% by volume. This is why male molds more difficult to demold, and the design of the mold needs to account for part shrinkage and part removal.
Bagging/Infusion On The Cheap
Vacuum Infusion and Vacuum Bagging can be accomplished are not only reserved for industry and can be accomplished in a workshop setting. Relatively inexpensively as well. One of the major variables is the shape of the part trying to be built.
We will look at a flatstock for now. The desired outcome could be either a test panel or a piece needed for flat construction. The mold will be the difference between this and any other more complicated shape. The mold must be able to be sealed from the atmosphere and waxed for release of the finished part, so my favorite “mold” is either a flat sheet of steel or a sheet of plate glass depending upon desired surface finish. This must then be waxed up with some paste wax to enable part release.
I always like to start with creating the perimeter seal. The 1/8 or 3/16 inch thick by 1/2″ wide gray butyl tape is the best. This is basically sticky on both sides and has wax paper on one side. It is sold at infusion suppliers or online as well. My little secret is that it is also sold at building supply stores, as it is used on polebuilding construction projects when sheets of steel siding/roofing are sealed together with this stuff. So this is unrolled and pushed down onto the mold and the wax paper is left on and facing up. Then I like to use cover this with some 2 inch wide masking tape.
Now we can start the part construction by placing our layer of gelcoat if so desired. Once this is cured, it is followed one of two ways. For vacuum bagging, the catalyzed resin-wet layers of reinforcing materials (glass or carbon) are placed down, including any core material. Once this is satisfactory, the peel ply is placed over the laminate, followed by the breather material. This is typically quilting batting, and a couple of good layers will do well to hold the excess resin from the laminate. Then the bag will be installed and a vacuum will be pulled which will pull the resin up into the breather material.
For infusion, the gelcoat is followed by the dry layers of reinforcing glass or carbon and core. Then any resin runner strips are added with a separation from the laminate with peel ply. Two circuits are setup; one for vacuuming the air out, and one for bringing resin into the laminate. The Vacuum circuit is accomplished with spiral wire loom wrap around the perimeter of the part leading to a T connection which will poke thru the bag. A resin inlet T will also pass thru the bag and lead into the runner strip. Now a bag will be placed over the part and sealed down. The hoses need to be vacuum rated, and can be either pvc or polyethylene.
A resin trap will be needed between the vacuum pump and the laminate for both infusion and bagging. This can effectively be accomplished by using a PVC pipe sealed at both ends with endcaps. Then drill and tap plastic or brass fittings and seal well with pipe dope. The fittings need to be at the top so that the resin will drop out and into the pipe. The resin will cure in the pipe and can be disposed when solid and full.
The bagging operation is accomplished by using polyethylene plastic that is free from holes, and in the range of 4 to 6 mils of thickness. Thicker plastic can do the job, but can be difficult to work with. The plastic needs to be cut larger than the area to be covered, typically by around 20 percent. It needs to be placed on squarely, and pushed into the butyl tape without wrinkes to create a good seal. The excess plastic is artfully taken up with extra pieces of butyl tape in these troughs that are connected to the main tape line around the edge. This can be very frustrating and is difficult to describle. But the bag will be sealed to the mold to create an airtight cavity. The resin hose needs to be closed off with vice grips.
Now the vacuum will be turned on. For the vacuum bagged part, once the vacuum is on, the part is basically left to cure. For infusion, the laminate is left to the vacuum to remove air and moisture for 20 mins to 2 hrs depending upon size. Infusion requires at least 27.5 inches of mercury, and 29 would be awesome. Any leaks need to be identified and eliminated. This can be difficult, but is necessary.
Now we are ready to put resin into the laminate. The wet resin is mixed with catalyst in buckets where the resin hose from the part is placed. Resin hoses are opened up and the resin flows from the bucket into the part. Once the part is full, the resin line is clamped off and the part is left to cure under vacuum. I typically wait at least 12 hours before demolding the finished part.
Curing Mechanisms
Temperature plays an important role in the curing process of the resins used in composites. Many of the resins are setup for room-temperature curing. This requires that the ambient room temperature is ideally set between 65 and 75 degrees. And that the resin itself is near this temperature. The old rule of thumb is that a drum of resin takes about 24 hours to get to room temp when moved in from shipping or storage. These room temperature cured resins have windows of open working time before the curing cycle begins to happen. Elevated temperatures in the summer can cause havoc, but can be managed with special mixtures and ingredients.
Some resins cure with time and elevated temperatures, which are achieved with the use of ovens. These allow for nearly unlimited open working time before cure. When things are satisfactorily placed, the temperatures are elevated to start the cure process.
UV Light is another curing mechanism that has special applications and takes the temperature consideration away. This has a big use with the infrastructure restoration industries working onsite and underground. It is much more of a specialized niche application.
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