This weekend I’m making the mould for the tub after many weeks of pattern making.
Here is a summary of the design in picture form
Mould making for the tub
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Category Archives: carbon fibre
Tub Pattern Video
Here’s a brief video I’ve done showing the tub pattern part as it stands now. All the shapes are right and the flanges are on. I wanted to show the flanges, as well as how I support them.
Tub nearly finished
so, here is the tub, and here are some of the gaps. The gaps aren’t going to be a problem because all the in the corners need to have a radiused edge. I will be doing this with plasticine and the radius ball. At the end of this I should have really neat lined edges.
Then it’s ready for the mould making process. Actually it’s not, first I need to put all the flanges on in order to actually pu the mould in this, but it’s nearly there.
I also have some of the polypropylene sheet in the tub which is still a little bit flexible. I’ll be getting in behind that with expanding foam to give it a backing layer to stop it flexing. I will need to cover some of the suspension with release sheet first otherwise the expanding form will stick to it and everything will be an unholy mess. I’m also going to have to take the engine and gearbox out now. I don’t want them to get covered in gelcoat when I start spraying.
Tunnel gets its first coat of wax…
Look at that shine. I did that flaring back with wet and dry. Now the wax goes on.
Testing Carbon Fibre before I commit – Part 2
So, here is the raw results file:
I’ve kept it there in the spirit of honesty, but the following distilled data into the graph is what really matters:
So the numbers that matter are those on top of the line – each of those is the number of KG force required to deflect the part by a given distance. You can correlate the colour to the number to the data in the top chart (click through for details)
Finally, there is the density/deflection ratio, which would show the ideal performing part if absolute strength wasn’t the most desired outcome.
Conclusions
There is a trade-off between core and layers, which is what would be expected. What actually surprised me the most was the difference a core makes.
The top line has two variables set – 4 layers of CF (rather than 2), and a 10mm core, rather than the yellow line, which is 4 layers and a 6mm core. The third highest line is the darker blue line, which is 2 layers and a 10mm core.
So, it’s layers over core but again, there’s a trade-off. The red line is 4 layers over a 3mm core, and you can see it yielded really quickly at 3mm with a very low amount of force (well, 98kg of force). Without doing any statistical analysis, I am observing that each 3mm of core seems to give me an extra 100kg of resistive force before yielding. However, I don’t know how far that scales.
Finally, you can see that there are a selection of flat lines near the bottom, and they are parts made without core. The seem to bend a lot and not yield. For my purposes though, they’re not suitable. Parts 6 and 7 (four and two layers), deflected up to 7mm without yielding, but weren’t much use to me.
As the thicker parts started to yield, we could actually hear them crack (quietly). I’m assuming that’s the fibres snapping. As such, the moment it starts to happen the part is compromised.
Testing Carbon fibre before I commit – Part 1
Abstract
Some carbon fibre flat pieces were made and tested to see how far they bend for a given load. A load cell was used to measure the force.
Introduction
So, before I go mad and put several hundred pounds of material into my CF tub, I thought it best that I do some clever investigation into just how does a composite structure bend, yield and break under various loads. So, I made the pieces, tested the pieces, and did some graphs, and came to some conclusions.
Method
8 trial pieces were made of various thicknesses – different cores and layups. Each piece had a few things in common though:
- single layer of 300 gsm aramid for intrusion protection (if it were on the car)
- this was balanced with a 300 gsm piece on carbon on the opposite layer to the core
- one layer of 600 gsm either side of the core, and one layer of 200 gsm e-glass either side of the core for a little tolerance and flexibility.
I could then vary the thicknesses of the cores and extra layers of 600 gsm carbon I wanted to use. I opted for the following core thicknesses:
- 10mm closed cell foam (CCF)
- 3mm CCF
- 6mm (2x 3 layers of 3mm CCF)
- none
In order to test the bending load, I went to see my good friend Simon at Cornering Force. There are only so many people who like it when you call them at 8pm and say “Hey – do you want to stress test and break some carbon fibre?” After marking up each sample, we then put each on a support, resting on the bed of a mill, and pressed down on it with a load-cell. The load-cell had a 25x50mm contact face. The mill allows us to make vertical adjustments in 0.1mm increments easily, and the load-cell is accurate to the gramme.
Manufacture
The pieces were laid out on one sheet, and infused as one piece, and then cut to size afterwards. The vacuum feed was left on post infusion and for the curing cycle. The resin was degassed. This was to ensure the maximum amount of spare resin was removed until the gel stage. After gelling and the initial cure, they were post-cured at 60C for 12 hours, as per the manufactures instructions. This gave me the maximum strength possible for the sample for the least weight. I decided not to vary curing methods (e.g. length of cure) because a 12 hour, 60C cure is what I can achieve for a part as large as this.
The plan was to work out how much a piece would deflect, how it would yield and fail, and just what were the most significant factors in this – is it core thickness, is it amount of cloth? Did I turn around three times widdershins?
This was also a good opportunity to try a couple of techniques given to me by Vic at Scorpion CDL. I am always astounded at the wealth of technique he has and the quality of work done there.
Lessons Learned
I also had a slight infusion issue, which was the temperature of the resin I infused with. I had it at about 10C which was great for it not going off, but made it too viscous, and meant de-gassing it was a pain. It also meant the infusion was quite slow for such a small part. Further checking says 25C to 30C is a better temperature for this. I should have also wrapped more mesh around the input spiral which would have also alleviated this problem, but that was fixing the wrong problem – get the temperature right – let the resin flow.
Analysis and Conclusions
Coming in the next post.
Here’s the new transmission tunnel part – sides are mdc, and curved surfaces are body-filler. There’s an entire wooden frame in there keeping it all in shape.
I’m using some ultra sticky double sided tape to hold the side panels of the tub in place. You can see the stuff here. It’s incredible.
Here is the polypropylene plastic sheet forming the side of the tub mould. The tape gives me 0.7mm clearance between the sheet and the side, which is a good gap when I apply the epoxy adhesive to put the final tub in place. The adhesive has a maximum structural fill gap distance of 2mm so precision is called for.
Some of the terminology of mould making is quite … interesting
By ‘eck. I’m shoving wax in me flange gaps.
Transmission tunnel top, quantities used
This is probably dull for any reader out there, but I just wanted to keep track of materials used for the tunnel top.
I mixed 300g of resin and 90g of catalyst at 40C (I put the mixing pot in a simple ban-marie), and then put it in the degassing chamber for 20 mins – it hadn’t finished but I was paying attention to the fact that I had the resin quite a bit hotter. It’s definitely like runny soup at this temp. Interestingly when you mix the resin at temperature, it degasses naturally much quicker, so there was a lot less foaming in the chamber – just some polite bubbling.
So, after the infusion, I had 45g of resin left. I’d also left the tap open for 30s after I’d closed the vacuum (a Warren tip) to make the part slightly resin rich to enable a better facing surface, and to have a slightly thicker layer in case I want to do a little polishing.
My rough calculations are that I’ve used are:
So, crudely 550gsm (in two layers, which means more holes) consumes 1.06KG of resin, per msq. The actual weight of the remaining part will be a lot less than that. The larger the part, the smaller the ratio of resin wasted in consumables. Continue reading
An infusion went wrong
So, I was finally ready to infuse the transmission tunnel top, after deciding on a compromise. The compromise was to not spray the part with clear gel-coat first, even though I had some scratches in the mould. [1]. I reasoned that the scratches will leave a positive on the part, and that can be flatted off with some wet-and-dry and a little polishing.
So, I laid up the part (one layer of 350 facing cloth, and one layer of 200 backing), found to my surprise that I got the bag to seal first time, and set about infusing the part.
DISASTER
This is the first time I’ve had a stuck infusion, and as I think about it, it was a culmination of a bunch of factors all adding up together to cause issues. The factors were:
I decided to infuse along the short side and go up the full length of the part, rather than infuse across the shortest distance (I’m running low on infusion spiral)- it was 10C in the shed when I was mixing the resin, and I thought “It’ll be OK – it means I can use quick catalyst and it won’t go off all that fast”. I ignored the gloopy sensations.
- I was feeling impatient, and thought I could get away with quick resin, allowing 10 mins for degassing because it was cold
- my oven is out of action: it’s in pieces while I make it bigger for the transmission tunnel. I thought I had enough kingspan for the job, and I was short by one roof baton length. This is now bought and sitting in the boot of my car. This again was an incentive to go for the quick catalyst
- when I was mixing it, it felt more viscous than normal, which should have been a warning that stuff was about to go wrong.
- when I infused it, it got stuck half way along the part and the resin went off in the pot.
So, I think the resin being too viscous meant it didn’t march along the part quick enough before it started to go off. If I’d used slow resin the infusion could have ran at the snails pace it was going at and it wouldn’t have been a problem. What’s more, I could have warmed the resin up (with the slow catalyst) and it still wouldn’t have been a problem. For the handbrake bracket, I actually had the resin at 40C to ensure it wetted the part out properly. So, this evening I will finish the oven, take the half-part out of the mould, salvage the cloth I can which I’ll keep for backing layers, and have another crack at it.
[1] My compressor has died, and the replacement part (£160 if I can wait for a machine-mart VAT free weekend)