General Layup Process, 16.821 Style!

16.821 is a capstone class centered on building an aircraft. Many students learn a technique for layups through this class. Here, we'll break down that technique into several steps.

1. Make a Mold
2. Prep for the Layup
3. Do the Layup

Make a Mold

Not every layup requires a mold, but for some (like a nosecone layup or bulkhead layup), molds are very useful. They ensure that your part comes out the right size and shape, and also gives the part a beautiful surface finish if done properly! To make a mold:

1. Buy some RenShape.
   1. RenShape is a dense, sturdy foam that is very easy to work with. Aluminum is a preferred mold material because it is harder and less porous than the foam. But, Renshape is cheaper, lighter, and easier to manufacture, especially when the mold becomes very large.
   2. 16.821 uses this type: https://www.freemansupply.com/products/machinable-media/renshape-modeling-and-styling-boards/renshape-440-styling-board
2. Create a CAD model of what you want the mold to look like.
3. Using the CAD model, route the shape into the RenShape.
4. Sand the RenShape mold.
   1. It is important to be detail oriented in the next couple of steps, because it will determine the surface finish of your part.
   2. Sand the mold to remove any imperfections or router marks.
   3. Use progressively finer grit, start at 150 and work your way up to 1500. (150, 300, 500, 1500 is usually adequate). But, depending on how rough it is off the router, the mold may not need that much sanding. 
   4. Vacuum out the mold and wipe it down with a damp paper towel. When it dries, wipe it with a dry paper towel. This removes all the sanding dust from the mold.
5. Epoxy the mold.
   1. Using epoxy, put a light coat on every surface of the mold that will come in contact with the composite materials during a layup. This includes overlapping a bit on the sides of the mold! 
      1. Coating the mold with epoxy seals the pores in the foam so that the mold does not suck up the epoxy from the layup or stick to the layup after it cures. 
      2. It's easy to apply the epoxy with a paper towel, or just smear it around with a (GLOVED) hand.
      3. Note: 16.821 uses MGS epoxy, 285 cure resin for this process (12 hour cure time). Just use something more substantial than a 5-minute epoxy. 
   2. Wait a couple of minutes, then wipe off any extra epoxy with paper towels and squeegees. 
      1. You do NOT want any pools or thick spots of epoxy; this will change the shape of the mold.
      2. You DO want a paper-thin layer of epoxy on everything.
   3. Let the epoxy cure. This will make the RenShape a darker color, but that's good. Means the epoxy soaked in like it was supposed to.
   4. Once cured, wet-sand the mold (put water on it before you sand it) one more time with very fine grit sandpaper (1500 grit or so).
      1. after wet-sanding, wipe the mold down with acetone to clean it up.

You did it! That's how you make a mold.

Prep for the Layup

Now say you want to use your spiffy new mold to make a composite part! It's time to prep for a layup. This is usually the more time intensive part of a layup.

1. Cut out materials for the layup.
   1. Breather, peel ply, vacuum bag, mylar.
      1. Breather should be smaller than the peel ply so it does not get in contact with the epoxy at any point, or it will give your part a fluffy cotton beard once cured.
      2. Cut these out to fit loosely in the mold and around your part. 
      3. If unsure, cut larger pieces. It's easier to trim them down to size during the layup than it is to cut entirely new pieces during the layup.
      4. The mylar should be cut out precisely to the shape of the piece as possible. Mylar gives things a nice surface finish.
         1. Surface prep the mylar!
            1. Wipe it down with IPA or ethanol, then give it several coats of Frekote (770 Release Agent).
2. Cut out the composite fabric for the layup.
   1. Lay out the fabric neatly on a table, make sure it's straight and flat.
   2. Cut out a sheet of wax paper to the size you want the fabric to be. If you want to end up with a strip of fiberglass that's 10"x5", cut out wax paper that's 10"x5". 
   3. Spray the wax paper with 77 (a light spray adhesive)
      1. You'll need some extra hands here: two people hold the wax paper horizontal in the air. You spray a light layer of 77 over the wax paper, and the two people holding the wax paper swoosh the paper upwards into the cloud of 77. 
      2. Try not to spray 77 on people or important stuff. It sucks.
   4. Put the wax paper on the fabric you laid out, sticky side down. 
      1. Smooth out the wax paper, gently sticking it to the fabric. Be careful not to put wrinkles into the wax paper, but it doesn't have to be perfect.
   5. Once stuck, cut the fabric with the wax paper backing out of the larger roll of fabric.
      1. You should have the nice piece of fabric that you wanted with a nice wax paper backing on it.
   6. Note: The wax paper backing serves several purposes. It makes it easy to work with the fabric composite (move it, carry it, etc.) without disrupting the fibers. It also makes it easy to wet the fabric with epoxy (you can tell when it is wet, without over-wetting it). If you overwet the fabric, it will make your part much heavier.
3. Prep the mold.
   1. Yup, back to this guy.
   2. Clean the mold
      1. Wipe off the mold with IPA or ethanol. If there's dried epoxy on the mold from a previous layup, scrape it off.
   3. Wax the mold.
      1. put a light layer of wax on the mold, then buff it out until the mold is shiny.
      2. wait for the mold to dry.
      3. Note: Waxing the mold serves as both a nice surface finish and a mold release agent.
   4. Vaseline the mold.
      1. put a light layer of vaseline on the mold.
      2. Note: Vaseline-ing the mold serves as both a nice surface finish and a mold release agent.

Do the Layup

Using this process, you do NOT need to do the layup prep and the layup itself on the same day! That's nice if you have really busy people that can't dedicate a solid 7 hr block to a layup.

1. Make sure all your stuff is ready to go. Lay it out neatly on tables so it's easy to work with. Put a layer of painter's plastic over the table so you don't destroy it with epoxy.
2. Wet the fabric with epoxy.
   1. This is where the backing comes into play. To check if the fibers are appropriately wet, check the backside and look on the wax paper. If the wax paper looks wet, you're good to go. If it's dry, add more epoxy.
   2. Once wet, blot the fabric with a paper towel to remove excess epoxy (and weight!)
3. Put the fabric into the mold. 
   1. This process will depend a lot on your part and it's geometry.
   2. Take your time, make sure things are smooth and pretty and there aren't bubbles in the fabric.
4. Bag the layup.
   1. As applicable, put the mylar (Frekote side DOWN) onto your composite. Smooth it out nicely to give your part a gorgeous surface finish. 
      1. Only put mylar where you want the layup to be smooth and it's not already touching the mold.
   2. Put peel ply over the mylar on everything that has epoxy on it.
      1. Peel ply helps you remove the breather, so it doesn't stick to your layup like a beard of cotton.
   3. Put breather over the peel ply.
      1. Don't let the breather touch epoxy. Trust me on this one.
      2. Breather is important because it helps "distribute the vacuum" over the whole layup. Without breather, the vacuum tube would only suck down in one spot of the vacuum bag, and that would be sad. Breather helps distribute that pressure evenly over the layup.
   4. Line the mold with vacuum tape, and put the vacuum bag over the mold.
      1. first, wipe off the mold with IPA where the tape is going so the tape can stick to a clean surface.
      2. Tack the corners of the bag down first, then slowly seal the rest of the edges. It's important to make sure that all the edges are sealed or the vacuum pump won't actually pull a vacuum.
      3. Do not, I repeat, do not get epoxy on the vacuum tape. It will never stick. Never. 
      4. Make sure to leave room for the vacuum hose.
   5. Seal everything up, check it, then check it again.
   6. Turn on the pump!
      1. watch with satisfaction as the vacuum bag sucks down around your beautiful composite part.
5. Let it cure. 
   1. Patience, young padawan.
6. Unbag it! 
   1. Careful not to stab yourself on sharp composite edges.
   2. Spend an appropriate amount of time admiring your layup, then sand it and finish it if needed.