So, here is everything laid up as a reminder of where this started. I think it’s a good reminder of how far the tub has come, but at the same time it’s also making me think the duratec is photobombing everything in my garage.
Now, here it is in the car:
It’s pretty close to where I want it now, and I’m doing the final fettling. I made this with a positive mould, which has meant the outer surface (that fits to the chassis) isn’t perfectly flat. I’ve had to take it back a little at a time with the flap disk in the appropriate areas to be sure it fits. I developed a methodology to do this after I’d removed the obvious obstructions.
I go around the gap with a feeler gauge set to 0.7m, and look for areas it traps. When I find a trapped area, I set the gauge to 0.05 mm and see if that still sticks, and anywhere it does, I mark with tape (hence the packing tape on the tub at the back top) and take it back about 1mm with a flap disk. I deliberately only mark the jamming points rather than the tight points so I can avoid making gaps unnecessarily larger than they need to be.
Once everything is marked, I lift the tub out (it’s only 13kg, so two adults can easily lift it with fingertips) and do the sanding. When it goes back in it fits a little better. Everywhere is pretty much where I want it, apart from the back where it’s still a little high. Everytime I advance the fit, the back lowers down a bit. I have about 3-4mm left before it’s flush and I’m happy.
This one is a little more difficult to wrap your head around, but it’s the mating surface between the tub (carbon on the right, starting in the top right corner) and the chassis rail. As you can see, the gap is pretty uniform now, and I’m aiming for between 1 and 2mm.
It unfortunately means I’m doing to have to remove some of the powder coat with a flap disk.
Here is the birds eye view. You can just see the lip at the back. I don’t want to just sand it off – I think I can make the tub a better fir before I have to do that.
Then the final act before bonding it in is to have it lacquered and cured. Then I will have an awesome finished product.
So, in feeling around the part, I’ve found a few small undercuts where I’d repaired the gel-coat with more gel-coat. I went around it with my fingers to look for issues, and found a few where the repair I’d done accidentally still went in a little, like a dimple. I’ve started using Dolphin Glaze as filler – it’s really thin and spreads well. It also comes in a squeezy tube so application is easy. I also bought a pack of onion paper for mixing on – wish I’d done it much sooner.
It’s styrene based though, but because I’m spraying in a barrier gel-coat first, I’m not concerned.
So, next is to make a 2×2 frame and apply plastic sheet to keep the dust down, and then I’m making cloth templates.
So, I’ve been doing the tidy-up on the mould before I infuse it, and I’ve had a bunch of problems with bubbles behind the gel-coat. These are down to a mistake I made whilst building the damn thing. One puts the gel coat on the mould, or it gets the hose again. Well, I put the gel coat into the mould (spraying), and let it set. Then you put a coupling coat of glass on the back. This is pretty thin (tissue) glass – 100gsm chopped strand. Or you do the wrong thing, and try a new product. I tried a coupling veil, which is like breather fabric. I found it was very very hard to wet out, and hard to make it stick to the gel. What’s more, I also started running out of resin. So, half way down the tub I stopped using it and went back to tissue. When you look at the mould now, you can many more bubbles on one side than the other where I swapped from veil back to tissue.
What I’ve found is that I’ve ended up with a lot of repairs to do to the mould. Here’s my technique – it may work for you.
Step 1 – Find out where the hole is
Once you’ve found it, dig it out with a screwdriver – be sure to probe around the edges – it’s quite surprising how far a run is from a simple small blob.
Step 2 – make it concave
One of the things I’ve worked out is that when getting the gel in the hole for the repair, you can’t get it into every pocket a the edge of the gel .If you’re digging out with a screwdriver you won’t get under every edge, and if you’re coming in down from the top with the screwdriver, you may not lever out the weak points as well where adhesion to material and gel isn’t perfect. So, what I do is put a grinding stone on the Dremmel and go around all the edges until the edge of the hole is convex. Then there’s two advantages – one: no overhanging void that the gel can’t get under and leave a small air pocket you’ll be repairing later; two: I’ve found that it occasionally chips out another void that wasn’t visible at all. It only takes a few seconds to do this. Of course kiddies – safety first. This kicks out a lot of dust and is prone to kicking up gel-flakes, so wear a mask and goggles. It’s also loud so i go for ear defenders. I look like the human fly.
Here it is, all dug out. It’s interesting that I found a lot more void when digging it out.
Step 3 – Apply the gel
Gel-coat is weird stuff, and doesn’t fully set in air – it needs a barrier or it remains tacky. There are two ways to do this – first is add a liquid wax solution to the gel coat (typically at 2%). As it sets, the wax migrates to the surface and forms an air-tight seal. It works if you are doing your repair as a one-off or is perfect if you’re spraying a repair. However, I think there are downsides to this for spot repairs:
- If you have to build up a repair, you will have to take off the top layer to get rid of the wax (mould cleaner disolves it away, but you’re using expensive chemicals).
- If you are building up a repair, it will leave a relatively smooth surface, so not much of a key
- The wax is dissolved in styrene (which should totally gas out though) and epoxy and styrene don’t get along well
- It is extra faff adding 2% wax to, say, 30g of gel-coat.
So what I do is make up the repair, and seal it with flash tape. It’s specifically designed for resins not to stick to it, as well as being stable at high temperatures. This works quite well for making deeper spot repairs, and the tape gives the gel-coat lots of support if I’m repairing a vertical surface. If you’re using wax, you have to build it up in multiple layers or else it will run. This method is faster (for me).
Step 4 – Flat it off
I don’t have a picture of the large wound I repaired half-way through, so I’m going with this one. If you run your fingers over the repaired gel, quite frequently you can feel it’s slightly proud. If you can feel it, you certainly will see it on a cosmetic part. Worse, if it’s half a millimetre proud or more, it may give you release issues as well. You can see around the repair that I’ve started to flat it off. I find a 120 grade paper on a small random orbital sander such as this Ryobi. It’s great because it also has a extensible pointy nose thing that gets into the corners. Like a wasps ovipositor. Kind of. Maybe. I just like saying ovipositor.
The white speckles you can see are where the gray top coat has been flatted back to the white undercoat. Once you start seeing white, you stop. The white line at the top of the repair is a slight highlight reflecting the light from the spotlight I’m using. It certainly shows there’s a lip there. What’s more, this was two holes so I knew to keep flitting back until I’d seen the two holes again. It takes about 10 mins and some patience to do it with the Ryobi.
I didn’t want to go any more coarse than 120 grit because I’d be digging deeply into the part. 120 makes good progress, but then flats off well. To flat the part, I used an 800 grit disk on a sander like this one which did it quickly and safely . No gouging. From 800 I went straight to 1500, then onto the polishing compounds.
Here’s the finished version of the holes above, or one similar. What you end up seeing is the original holes filled in. once they’re polished (800 -> 1200 -> 1500 polish ->2000 polish -> anti-swirl polish) the new black gel-coat comes out as shiny as the original.
So, here’s the final result, polished as well. You can even see the little loop in the picture in black where i accidentally scratched the gel coat with the Dremmel. it’s in the third photo from the top. This is a very solid repair and will take multiple pulls if necessary
3D Printing – Can I Get That in Carbon Fibre?
Yes, but only by printing the part, taking a mould, then infusing the part. But hey – this is the future right? Food is blue, Boris is Foreign Secretary, and I can buy things and my watch pays for them.
The Target Pieces
Here we have the two target pieces, 60mm * 40mm * 6mm thick. I was experimenting with quality settings to see what is useable for moulding.
Lower Quality Settings (the default)
- Support Matrix: 10%
- Wall Thickness 0.7mm
- Speed (max) 48mm/s
This is the top layer – you can see plenty of holes
This is the top layer, and you can see plenty of holes – If I was to take a mould off this I’d struggle – it’s an excellent keying surface. I may get away with a lot of brushed on thick PVA release agent, but if I was aiming for fine surface tolerances I’d not get it. Best bet would be to cover with gel-coat, polish then mould.
Higher Quality Settings
- Support Matrix: 20%
- Wall Thickness 0.1mm
- Speed (max) 43mm/s
This is the top surface with the number of supports in the hollow space doubled, and a thicker shell (1mm) added. If you look closely in the top picture, you can see the cross-hatching that is the support matrix, and the differences between them.
Increasing the shell thickness has also laid down a lot more plastic, making for a virtually sealed surface, but still with some tiny holes.
The Base here isn’t much better – I didn’t make any significant changes to the base, and I don’t think I would – If I design parts that are assemblies, I’d just use the base as my adhesive point. I’d use epoxy as well.
This organic looking beauty is a 3mm hole with a 1/2mm counter-sink. I wanted to see what I can do to leave dimples in the part. Dimples are important because I’d have dimples in the final part, which would make for an accurate drilling point if it needs to be demountable (such as the fuel-pump hatch in a petrol tank).
Praise for The Electronics Shop and Velleman
I can’t recommend the team enough here, and Anthony, with whom I dealt. They have the printer in stock, and all the consumables, spares and upgrades as well. I can’t emphasise the quality of service enough with these guys. I had a question about alignments whilst I was building the printer, and got an immediate helpful response. When I tried the printer for the first time, it printed exactly like it was meant to do, like a piece of consumer electronics, not a cutting-edge piece of manufacturing equipment.
Here’s the tub finally released before it was lifted out. It took quite an effort to release it, with multiple use of wedges, squirting water, and at one point, the 2lb percussive force transduction instrument to knock out the internal bracing in the transmission tunnel out. There was a large amount of plasticine in one part when I was filling in an undercut. At one point I was lying on the ground and trying to push the mould up and out with my feet. There was also some action with the trolley-jack as well to free it up.
Here it is out of the tub. It hasn’t been trimmed yet and it didn’t really change colour as I could see after I post-cured it, but it feels as solid as a rock. It’s pretty heavy as well, and I can’t move it around on my own. You can see where you sit, and it’s not symmetrical – it was not designed to be – the Fury has a bend in the transmission tunnel to allow the engine to be offset to the passenger side a little bit to set the weight balance more evenly 50/50 down the centre-line.
Now we’re looking down it as you sit in it. It is worth noting that the footwells are only in the mould to make it a closed container. I will lay a couple of layers of e-glass in there just to keep the part dimensionally stable, but they will be cut out of the final part. This then gives access to your feet for the CF footwell and steel footwell I have already made. I will need to cast up some jointing strips between the tub and the footwells, just to make extra sure everything passes force to everything else. This process will be a really easy moulding process – just put some gel-coat down, and then slap some shredded glass putty (like isopon p40) behind it. I may use glass and epoxy paste though, to ensure better mould compatabity. If I use this, I will need to bake it for a while to be sure all the styrene is out, else I won’t get a good epoxy part.
Here is the mould now fully out of the car. It’s a positive mould (if you hadn’t guessed) and needs trimming and polishing. There are one or two bubbles behind the coupling coat where the gel-coat will come off, so these need either digging out, or if there’s a small break, in a larger bubble, I can inject repair gel-coat in behind it which will bridge the gap and make a solid plug with the minimum of sanding and polishing. Most of the tub gel-coat is really solid though.
Here it is from the other side. The white stuff you can see all over the place is the plasticine I used to form the inner radii. I will scrape it off, and then clean it off (hopefully acetone will shift it), then it’s polish, polish, polish.
I have a machine polisher so it’s actually not much of a chore, and there are two coats of gel down, so if I start seeing white behind the gray, I know I’ve gone as far as I dare.
Here we have an inner radius with the plasticine scraped off. There’s a small ridge there (looks a lot worse in the photo) which will sand off with a bit of wet-and-dry. I will start with a fine grade (say 800) and see how that does, before finishing off with 1200, then 1500, then polish, then wax.
The inner radius transferred brilliantly from the part, but it will need some rubbing to bring up to a good polish. I did form the shape with body-filler and then waxed it, so it was always going to be an OK finish. Because I sprayed PVA release over the poly board (which is meant to have a good inherent release), I didn’t get the full shine from it, but there is a bit of a shine there already. It will be trivial to polish up.
Look at the bloody mess the demoulding process made. This was after I gave the garage a bit of a tidy and did a tip run. Sigh. Guess I’ve got to do it all again.
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.
So, I was taking a wander around East-Coast Fibreglass because I needed more 100gsm chopped strand map to act as the first layer when laying up to prevent print-through in a mould. I came across something called Finishline Polyester Veil. What you’d normally do is put a layer of 100gsm chopped strand mat down first, and let that go off. Then put your heavy 450 down afterwards.
This stuff, being a felt doesn’t need a binder (to hold the chopped strands together) which means (apparently) it has a better chemical grip on the gel-coat. Then, once it’s set, you can put the final 4 layers of 450gsm down. Hopefully this will give me a great finish and is really cheap. I have had print-through before, so I know not to let this happen.
Then I discovered that Marbocoat do a release agent called Fastcote, which you wipe on and it leaves a shiny release surface. I called them today and it is good for epoxy. Fantastic. Everything that contributes to a nice surface finish is welcome.
I will test it first before I use it, I think.
Here is the first attempt at a fillet. I appreciate that it doesn’t look much from this distance (so there’s a close-up to follow). I’ve been using a 1″ ball and liquid wax dissolved in styrene as a lubricant. It helps if the plasticine is nice and warm and soft before one rolls it into a sausage and shoves it in.
However, I’ve got to stop now for a couple of reasons:
- the wooden frame supporting the polypropylene has come away from the chassis – the adhesive holding it together came away today. This is a bit frustrating. What it means is that the base isn’t as accurate as it should be, and isn’t as supported as it should be, meaning things may shift under the plasticine.
- I realised there’s still a little sanding to be done on the tunnel part to make it fit perfectly to the chassis. This is going to throw a load of dust about which will settle into the plasticine and contaminate the finish.
This is the filleted edge up close and again, it’s not easy to judge the finish. However, this is at the bottom of the tub and most of it will be covered by seat, so whilst I want the best finish I can make, I’m not going to get het up on making it perfect. I can always do a little post-finishing on the mould or even the part if the urge gets me.
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.