So, as you can see in this photo below, I need to make an access hatch for the transmission tunnel. I want it to be flush fitting and reasonably strong.
I’ve taken a mould from this, and made the flat sheet for the top plate (2 layers of 300gsm, one layer of 200gsm gives 1.05mm thickness).
I then made the adhesive to bond the two – infusion resin mixed with glass microbeads to make a paste. All good.
I filled the bottom panel with adhesive, clagged the two together and clamped them. After that, I popped it in the oven at 60C for about 30 mins to kick the exotherm off (with fast resin) and I left it alone for a while. I even remembered to cover the facing side with bagging film to stop it getting mucked up when the adhesive ran out as the parts were compressed.
Disaster!
It’s not easy to see from this, but what I think has happened is that the heat from the oven has started to set the adhesive from the outside in, sealing it. Then this has kicked off the exotherm inside which has caused the adhesive to expand, but because the outside edges are setting and gripping, and the adhesive wants to expand and can’t. So, it warps.
I now need a different and better method, which I’m cooking up right now. Either a much slower cure, or cross drill it (where the hatch needs to be drilled anyway, and leave a few extra drill holes in the base only in the middle. Then bolt it through with a couple of plates on the outside help reinforce the shape and a tube inset at each bolt through hole to stop over clamping. I will then inject the adhesive in through the spare holes and let it go off. The open holes will give the expanding adhesive somewhere to go.
Current Plan
I have bunch of stuff to do, and am feeling a little overwhelmed about which job to tackle next (at 19:30 on a Sunday night). I’m now getting over the disappointment, time and materials lost, and have a plan. The plan is:
- buy beer
- make savoury bacon cupcakes with my daughter
- watch The Blues Brothers with the kids.
- Sleep.
sounds like a good plan, you need to step back and then with some new eyes have another go. Cake and beer sounds very good
cakes and beer, red wine and black books. I can’t say my head was brilliant this morning.
Why did you make to parts that needed bonding togethere? I would have asumed that you would lay it up as one part.
If it needs drilling anyway just get someone to laser cut you a bit of duralium sheet?
Anodized if you want a spangly bit.
The entire transmission tunnel is going to be a single, structural CF piece, and I have the moulds all made. This is why I need to do the access panels in CF. They need to fill the 5mm recess in the photo, and fit very snugly.
I think my initial approach was wrong (bummer) and I should do it differently. The mould I took from the tunnel is now trashed (it was only a temporary mould made with normal, not tooling gel-coat) and it was only really good for about 6 pulls before the gel started coming off in chunks. It’s a painless job to make another one.
I’m thinking aloud now about my stack, and operating on the principle that 800gsm of cloth (more or less = 1mm thickness, and my panel is 5mm deep)
Next time I think what I need to do is actually make up the depth with core instead of adhesive. So, it will be 2 layers of (350gsm + 200gsm) carbon, one of 3mm closed-cell core, and the same outer layer (350 + 200). This will give me a thickness of just over 4mm. Then I relieve the core back to the base where I want the bolts to go through, and for each hole, bond in a washer using araldyte, clamped down. This will give me a good hard-point which is 1mm thick. Then I will remake the top as 350+250, which will be a bit thinner. The top is then bonded to the base with a lot less adhesive and hopefully no warping.
Other options are to make a 1mm thick base, and core without a second sandwich on the other side of the core. I can then bond on a 1mm top and still get my accurate 5mm thickness. It’s common practice to infuse up to the core, and then infuse the second later on afterwards. I wouldn’t be infusing the second layer, but bonding it with infusion resin, under vacuum to ensure good consolidation.
I’ve ordered some low-headed cap screws which will sit flush with the top after sitting on a 0.8mm base and a 1mm washer.