Aeroshell Wet Layups for Dummies

Owned by Elizabeth M Rawson
Last updated: May 21, 2024
4 min read


Hi! I'm Elizabeth (Maylin) Rawson, the 2023 Aeroshell lead. I created this to give a general overview of how the layup process works! This is aimed towards new members/people not on aeroshell to get them onboarded to the layup process.

Background

The 2022-2024 aeroshell is made up of 3 major parts: 

  • Top shell
  • Bottom shell
  • Canopy

Materials

Composites are made of two materials: flexible fiber and a hard matrix. These combine to make a strong material.

  • Fiber (fabric)
    • Fiberglass: heavy and flexible
      • Our molds are made of this 
    • Carbon Fiber: relatively strong for how lightweight it is
      • Our car is made of this
    • Kevlar: very strong, shatterproof and fireproof
      • Our canopy is made of this
  • Matrix (epoxy resin)
    • Resin is made from mixing one resin with a hardener according to the ratio specified on the resin packaging (typically a 5:1 ratio of resin to hardener)
      • When the two are mixed a chemical reaction occurs and the viscous liquid turns into a hard solid which reinforces the fabric
      • We have a limited time to work with this (this is called pot life) so work fast
  • We also use foam to reinforce the structure of the composite

Ply is the amount of layers of each fabric. The person in charge of the shift will tell you how much ply to do

  • Topshell is made of 2 ply of carbon fiber, a layer of foam, and 2 ply of carbon fiber
  • Bottom shell is made of 6 ply of carbon fiber
  • Canopy is made of 2 ply of carbon fiber and 2 ply of kevlar on the sides, and fiberglass on the very top dome

Safety Precautions

Roles (Easiest to hardest)

  • Scrapers: scrape out excess epoxy and make sure the carbon fiber is flush to the surface of the mold
    • 6-8 people
  • Bucket duty: mixes an adequate amount of epoxy and distributes it in a timely manner
    • 2-3 people
  • Fabric duty: applies the fabric in the right area so there are no creases, tends to be more experienced members and/or people who cut the fabric
    • 4-5 people
  • Overseers
    • 1-2 people

Step-by-step guide

Pre-Layup

  1. Mold prep: Aeroshell does this step to get the mold smooth. A smoother mold will result in better release of the composite from the mold!
    1. Steps include sanding, filling in gaps, and then PVA/wax or frekote/wax
  2. Cut fabric 
    1. When we cut and lay out the fabric, we want to orient the fabric alternating between 0 and 45 ply. This means that we will rotate it so the checkers are going horizontally and then diagonally. This makes our composite stronger
  3. Apply yellow tacky tape around the very edge of the composite. 
    1. This will help create a vacuum seal for the composite so all of the air can be sucked out of it and it will end up smoothing the composite

Layup: We want to build up fabric and resin around in a loop, going from adjacent area to adjacent area. We want the composite to be as smooth as possible, with no air bubbles.

  1. Use a scale mix resin and hardener according to the ratio specified on the resin
    1. Depends on the brand so read the label
  2. Apply epoxy to the mold with a paintbrush in the size of the carbon fiber piece you are about to lay down
    1. Make sure the entire area is saturated with epoxy
  3. Put the carbon fiber on the epoxied area and press it down very flat until it is flush with the mold
  4. Apply more epoxy until every single bit of fabric is wet
    1. We the fabric to be wet but apply epoxy moderately. We do not want to waste any.
  5. Use a flat scraper to scrape out any excess epoxy from the fabric
    1. Be gentle and firm at the same time.
      1. Try to hold the fabric down taut with one hand so it doesn't move around. 
      2. Scrape with medium pressure and be conscious that you do not alter the weave of the fabric, we want to keep that nice checkerboard pattern.
    2. Move excess epoxy into the next area we plan to add fabric on  
  6. Repeat this process
    1. On all subsequent layers, tack down carbon fiber and let the resin from the prior layer soak in before you add excess resin. This prevents waste

Post-Layup: 

After we finish applying resin and fabric, we need to let the resin cure. We put the composite in a vacuum, and pulling the air out compresses the composite to a smooth finish.

  1. Apply peel ply (thin white opaque material) so that it covers the entire composite but doesn't overlap the tacky tape
    1. Peel ply soaks up excess resin
  2. Apply breather (fuzzy white fabric) so that it covers the entire composite but doesn't overlap the tacky tape
    1. Breather ensures the air is sucked out evenly
  3. Put the vacuum puck on the composite, preferably in a place without fabric or resin
  4. Peel the covering off the tacky tape
  5. Stick the vacuum bag (green transparent plastic) onto the tacky tape. Be very detailed on pressing these two together; we must eliminate every single air bubble and gap otherwise the vacuum is not going to pull.
  6. Poke a hole in the vacuum bag over the puck so you can attach the external vacuum hose to the pump
  7. Turn the vacuum on

Conclusion

Wet layups seem like repetitive manual labor, but it requires you to be conscious of what you are doing every step of the way. If you don't apply the right amount of resin, scrape too hard when scraping out resin, or forget to fully seal the vacuum bag, etc. the part will come out flawed. Please ask questions if you don't understand something! (especially to my members because this will challenge them and further their learning as well 😈)

Most importantly, remember to take breaks, drink water, and eat well. Your health is the number one priority. Layups are a lot of physical labor and they will tire you out

Hope to see you all there!




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