Well, being an engineering student I had the opportunity to go on a tour of a composite materials plant, as a part of a course I was taking in manufacturing. Now, it's been a few months since I've gone on the tour, and I haven't had gotten around to posting this until now, but I thought maybe some people on this forum would be interested. Anyways, here goes. Also, note that some of the descriptions I'll use are not really technical, I don't know all the technical descriptions and for those I do know, well, I'm trying to communicate an image of the processes involved.
In late November I had the opportunity to visit the Boeing Canada plant here in Winnipeg. Unlike most industries I visited as part of my course, Boeing had a more rigid security arrangement in their plant. In the front lobby, we were greeted by name tags already made up with our names on them, and instructions that our visitor passes strictly had to be returned at the end of the tour. At the entrance there was a display case with models of all the Boeing commercial aircraft from the 707 forward, which looked really nice. Anyways, we were taken upstairs to a training room, where we all gathered and were given a brief introduction by a Boeing communications department employee. Boeing Winnipeg makes mostly composite panels for Boeing commercial aircraft, but also some other types of parts are made or assembled there, also for their commercial aircraft. Anyways, after a little discussion on that we were being split into more manageable groups of around half a dozen students per tour guide, we were taken and shown the plant.
The first step in manufacturing composite panels is to cut the core material. This is what is also known as the honeycomb, because of the hexagonal cells which it is formed out of. The core material arrives at the plant in large blocks of roughly 4 feet by 4 feet by 8 feet (just guessing the size, roughly). The first step is to cut these large blocks into sheets of roughly an inch or two thick. A special saw cuts them into the correct thickness sheets, and various thicknesses are used for different parts. At this point the core material looks like a large sheet of plywood in size, but it has hexagonal holes very closely spaced in that honeycomb pattern so that most of the space is empty in the sheet. If you can imagine stacking many pieces of corrugated cardboard together, and at the wavy end part as the main surface, this is what the sheet of core material looks like, other than the shape of the holes. The real core material is basically a sheet full of hexagon shaped cells. I'm not totally sure what the material the honeycomb is made out of, but just generally it was described as a kind of a plastic-paper hybrid. The sample pieces we were given to look at felt slightly flexible, but quite strong, and had almost a waxy sort of surface texture, if I can describe it as that.
The next thing to occur when making composite panels is for these sheets of core material to be cut into the proper shapes. There are machines used to cut the proper thickness sheets into many various shapes (some machines are computer controlled, some aren't), and the edges of these pieces are sometimes made at an angle, or with a rounded edge, or whatever is required. Rough edges of cut material are cleaned up, and the core pieces are then sent over to be assembled. Now just hold onto the image of what this honeycomb panel is for a moment, because it will be needed in a while, and we'll go to another part of the plant.
The next part of the composite manufacturing process is to cut out the fibre material. This is like a fabric, woven in various patterns out of carbon fibres, or fibreglass. This material comes in a roll, and has to be stored in a freezer until use. This keeps the resins woven into the fabric from breaking down before they are used, but I'll get to that in a moment. Also, there are several different materials used, which have a colour coded plastic film on them to keep track of them. Anyways, this fabric is first thawed out and rolled out onto a large cutting table, to be cut to the correct shape. These tables are probably about 50 feet long, 6 feet wide, and there are several of them in the plant. Then, a computer controlled cutting machine moves down the table, and cuts the fibre fabric into a pattern, and prints an identification number onto the pattern, to identify which part is supposed to be made out of the fabric piece. After the fabric material is cut, it is picked up by workers and placed on trays, or on long trailers, which are taken over to the lay-up area by forklifts. The scrap material is discarded, but the computer process used for cutting the material minimized the waste by using a nesting function to use as much material in the patterns as possible.
Now the lay-up. Various jigs and fixtures in many different shapes are used in this area. What happens is a layer of the fabric material is placed on the shaped jig, and is carefully worked into place with certain tools, and the coloured film is peeled away from the woven fibre fabric. Many layers are put on, some of the parts we saw had over 10 layers of the fabric put on. The jig which is used has markings on it to show where the fabric material has to be placed, and it has to be placed there very smoothly. As you would probably guess, cleanliness is important, and we weren't allowed to touch the lay-up, and all the workers handling the material wore gloves. Then, after all of the fabric layers are in place, a piece of the honeycomb material is placed on the correct spot of the material already on the jig. This honeycomb was previously shaped to fit properly, and after being placed on the lay-up about 10 more layers of fabric material are placed on top of the honeycomb material. This has to be carefully worked into place as well, to make sure the core is competely sealed in between the two sides of the fabric material. Effectively the honeycomb is sandwiched in between the layers of fabric. After this the entire panel lay-up and jig is sealed in a vacuum bag.
The next procedure is to cure the part. Do you remember the resin I referred to being in the fabric? Well, the fabric material has a special polymer embedded inside it. The part is taken in its vacuum bag over to an autoclave area, and the vacuum lines are connected. All of the air is removed from inside the bag so, and the autoclave is shut. What the autoclave does is control the temperature and pressure around the material. It is really just a huge pressurized and heated tube. The combination of pressure and temperature applied to the formed composite part causes the resin to cure. In essence, the resin hardens like a two-part glue, except that it is the pressure and temperature that causes it to solidify, not a chemical reaction between two components. The absence of air inside the vacuum bag prevents any air from interfering with the process. Boeing Winnipeg has a large area full of autoclaves for this process. When the part is removed from the autoclave, which may be up to something like 22 hours later, the vacuum lines are disconnected and the vacuum bag is removed. The panel is not, however, complete yet.
After removal from the autoclave the panel can be taken off its appropriate jig, and the jig is sent back for another part to be laid up on it. The panel is sent for finishing then. The edges of the panels just out of the autoclave are quite sharp, as a single piece of fabric material is quite thin but is now hardened like a knife edge, so it has to be trimmed. A layer of white, light reflective material may be applied to the inside of the panel make it easier for mechanics to see inside of the aircraft with a minimal light source, if the panel is destined for an area where this feature would be useful. Also, the lay-up process is not perfect, so the edges have to be trimmed to the exact final shape and size. After trimming and smoothing the edges of the panel, it is sent for finishing. A light sanding or cleaning may be required before painting, and some hardware may have to be installed on the panel. Such items added to the panel would include brackets to mount it on the aircraft, or other such metal parts needed for the function. Once the panel is completed it is shipped out of the plant to either Seattle or to Wichita for installation.
There are some other notes I wanted to add on to my trip to the Boeing plant. The composite panels I described above are used all over the aircraft. Mostly those were smaller than 6 feet by 4 feet. However, it can be noted that some larger parts are made in the plant. I didn't get a chance to look at the processes used, but a large panel is made for under the belly of the 747, the jig for which is 2 stories high. There are also parts made such as the thrust reverser blocker doors for the 737, the 757, and the 777. There is also a sheet metal area. We observed various components being built such as the structure containing the heat shield, just behind the engine and on the bottom of the engine pylon, for the 737 and 777 aircraft. Needless to say, the 777's assembly was much larger than the 737's. Various other assemblies are built at the plant, I remember looking at a fairly large jig for the production of a part that would have been of specific interest to people on this forum, but I can't remember what that part was.
I think I'll stop there because I can't remember all the specifics of other parts built at this facility, but parts are built for basically all of the Boeing commercial aircraft family at this plant, other than the 717 as I believe it uses a different supply line. I was very impressed by the the plant, and the record keeping of the parts built. In the aerospace industry a log of manufacturing has to be kept for every part, but it is still incredible to see. I found it very fascinating to visit, I learned a lot and I hope you guys all enjoyed learning a little about this, too. I hope I've been clear, if not please just post whatever questions you have and I'll try to answer them, if I can.
Also of interest is some of the other composites facilities in the Winnipeg area. Bristol Aerospace also builds structures for Boeing aircraft, and Air Canada's heavy maintenance base does composite repairs and even has its own autoclave. There is also some research being done by a company just outside of Winnipeg on using an electon beam instead of an autoclave to cure composites. This method allows different things to be done, and cures parts almost instantly instead of a long cure time in an autoclave. It is indeed an interesting time for composite materials use, and it appears they will only be used more in the future, both in the Boeing family of aircraft as well as in almost any other aircraft being developed.