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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)
    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.
    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, sand the mold one more time with very fine grit sandpaper (1500 grit or so).

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, and vacuum bag.
      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.
  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.

 

 

 

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