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.
- Make a Mold
- Prep the Layup
- 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:
- Buy some RenShape.
- 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.
- 16.821 uses this type: https://www.freemansupply.com/products/machinable-media/renshape-modeling-and-styling-boards/renshape-440-styling-board
- Create a CAD model of what you want the mold to look like.
- Using the CAD model, route the shape into the RenShape.
- Sand the RenShape mold.
- It is important to be detail oriented in the next couple of steps, because it will determine the surface finish of your part.
- Sand the mold to remove any imperfections or router marks.
- Use progressively finer grit, start at 150 and work your way up to 1500. (150, 300, 500, 1500 is usually adequate)
- 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.
- Epoxy the mold.
- 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! 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.