Week 14 Assignment - composites

Goals

Assignment

Results

My process

Testing the epoxy - make a coupon

Our epoxy expired in 2013 so I started with testing it to see if it was still good.

But first I looked up the data sheets and mixing ratios.

Resources:

From the safety sheets:

Resin:

  • Respiratory protection: Not necessary if room is well-ventilated
  • Tightly sealed goggles
  • Protective work clothing
  • Protective gloves: Synthetic rubber gloves

Hardener:

  • Ensure good ventilation/exhaustion at the workplace.
  • Tightly sealed goggles
  • Protective work clothing
  • Protective gloves: PVC gloves

I used polyurethane synthetic rubber gloves AND PVC gloves on the inside of them. As well as long sleeves and wrap around goggles.

The work area is in an big room next to a drafty wall sized door.

Mixing ratio from the data sheet: 100 Resin, 40 hardener.

For the first test I mixed up half a disposable cup of expoy and made a cuopon of four layers. I compressed it with a speedclamp.

I mixed it outside the door to the workshop to avoid generating fumes inside.

Minimum pot life before gelling is 28 min in 25C if you do 500g. Since the test area is fairly cold and I made a much smaller batch I expected it to go much slower.

After one hour the mixture was still very liquid. After two hours the mixture was much stickier.

From the data sheet: Demolding time @ 23°C on laminate (5 plies / glass fabric 290 g/m2) 16 hours

We shall see!
Success!

First coupon worked fine. I demolded it after 24h hours of clamping between plastic sheets and wood for reinforcement

Issues and observations:

The plastic that I was hoping to perforate and use as relese film binds to the epoxy. I don't know the plastic mix of the foil I used but I thought epoxy would not bind to plastic films at all?

After asking around I learned that recycled plastic can stick to expoy due to it having contaminants in it. I'll make more tests with different plastics as release film!

New test - make shapes

New test!

For my second test I choose to try out both convex and concave surfaces. I used fusion360 to design and CAM a simple test form.

It also gave me an opportunity to test out different machining strategies.

CAM strategy

I ended up favouring "morphed sprial" for its bulk material removal with constant tool engagement (adaptive clearing does one depth at a time.) I then used "pencil" to quickly clean up edges and adaptive clearing with 1 mm layers to do the radiuses in all corners.

I used the lasercutter to cut the plys to shape (I first tested the shape for hand with siccors). The diagonal cuts are called "darts", they are critcal to be able to get them into the corners without creasing the burlap.

I also use the lasercutter to perforate plastic to use as a permiable layer so that excess expoxy can bleed through. The lasercutter in the lab has a specific "dot"-mode which made this very easy.

Layers, starting from the bottom.

  1. XPS-foam milled into my mould shapes, convex and concave.
  2. Cling film as mould release
  3. 4 layers of burlap saturated with epoxy
  4. Perforated zip-lock food bag. Laser perforated, 3 mm spacing between holes to let excess epoxy through.
  5. Synthetic white fluffy material, ca 10-15 mm thick. To absorb excess epoxy and lead air to the vaccum port.
  6. Vaccum bag for compaction.

I let it sit like this for 40 min under vaccum. As soon as I turned of the vaccum air leaked into the bag. I think splinters probably pierced it, even if I couldn't find the leakage points.

According to what I have read I would have liked to keep the vaccum pressure until the part has cured. Since my epoxy takes 16h to cure enough for demoulding this wasn't practically possible, attempting that would probably have burnt out the vaccum cleaner.

Issues and observations:

  • Molten mould
    The left cup was made in a concave mould and got so hot during curing that it partially melted the skin of mould, that is the residue on the surface.
  • Vaccum spreading resin
    When I applied the vaccum I could see the epoxy being pressed/sucked into the excess fibers on top of my mold, they were not staturated before.
  • Too late demould
    I let it cure over the weekend before attempting to demould them and by then the excess epoxy on the inside of the concave cup was too hard to be removed. The data sheet specifies 16h before demoulding, I'll stick to that in the future.
  • Sticky relase film
    On the right cup the flower pattern on my release film got transfered to the cup and could not be removed, the paint probably bonded with the expoy. I'll try another plastic when making my box.
  • Application side
    The concave cup mould was MUCH easier to apply the plys into compared to try to wrap them onto the convex cup mould. I will make my next mould concave.
  • IKEA plastic as release film candidate
    I notice that the resin does not stick at all to my workbench, which is covered in plastic from old "FRAKTA" IKEA bags. It is also muchg stronger than the films I have tried so far. I'll try that next.

Mould side after demoulding

Top side after demoulding

Make the product

Armed with the observations I had made from my tests I decided to use the following for my box:

Make the mould

I used Fusion360 to desing the box and mold halves. I suspected that the bottoms would bend in under vaccum so I added 12mm of Oriented Strand Board (OSB) to the bottoms. This also made it easy to make indexing holes and areas to screw the moulds to the milling table. I taped the XPS to the OSB with double stick tape.

CAM strategy

I ended up favouring "morphed sprial" for its bulk material removal with constant tool engagement (adaptive clearing does one depth at a time.) I then used "pencil" to quickly clean up edges and adaptive clearing with 1 mm layers to do the radiuses in all corners.

Too big mould!

I first planned to stick both halves into one vaccum bag but when I test fitted them I realiced the bag would not be big enough to "reach in" and give adequate pressure, so I had to split the moulds from each other. My indexing pins came in very handy!

Burlap compisites using vaccum bagging

Layers, starting from the bottom.

  1. XPS-foam milled into my mould shape.
  2. Cling film as mould release. TIP! Attach it to the bottom of the mould with vaseline to stop it sliding.
  3. 4 layers of burlap saturated with epoxy
  4. "FRAKTA" IKEA bags Laser perforated, 3 mm (tray) and 6mm (lid) spacing between holes to let excess epoxy through.
  5. Synthetic white fluffy material, ca 10-15 mm thick. To absorb excess epoxy and lead air to the vaccum port. (Probably too thick, use a thinner material!)
  6. Vaccum bag for compaction.

I let it sit like this for 40 min under vaccum. As soon as I turned of the vaccum air leaked into the bag. I think splinters probably pierced it, even if I couldn't find the leakage points.

According to what I have read I would have liked to keep the vaccum pressure until the part has cured. Since my epoxy takes 16h to cure enough for demoulding this wasn't practically possible, attempting that would probably have burnt out the vaccum cleaner.

My vaccum setup, I let it sit like this for 40 min under vaccum. As soon as I turned of the vaccum air leaked into the bag

Trim the product

The corners of my mould base plates has indexing holes where I put wood plugs that go into matching milled holes into the sacrificial layer on the mill. This allows easy positioning back into the mill.

I also drilled holes and screwed the composite down as well to prevent it moving.

This allowed me I used the CNC mill to easily trim the excess from the edges.

I first tried milling with the cling film still hanging loose over the sides of the mould, turns out that was a bad idea.

Varning! Do not do this if you are using fibreglass or carbon fiber it would release dangerous micro fibrers into the air and your dust collection bag would become hazardous material.

Video of trimming the edges:

Back to TOP to see the result!