Bodywork Mold Manufacture

[SamB]

Checked all the old topics and they're just about materials, so a bit of help here would be much appreciated.

We have sent some polyurethane off the be CNC'd for our bodywork mold, once it comes back Im coating it in a layer of bodyfiller then painting it in some way to take a glass fibre mould off. Does anyone know whether its better to high build primer-it and then sand that down or just go for a straight spray of paint and take the mold off that? Any tips for taking the mold would be great too.

Cheers guys

Sam

[MalcolmG]

if you're coating it with body filler I'd strongly suggest you primer it before putting your final coat on. The porosity of filler tends to give a really crap surface finish. If you use a buff-able primer (like Duratech) then you should get away with giving it a layer of that, sanding it back and buffing it - provided you don't sand through too much (a little can be tolerated if the filler remains sealed).

[Matthew Bell]

Make sure you test a small portion of the foam with the body filler first to make sure it won't eat through it. We CNC our molds, sand them as smooth as possible with 220 grit, then put a layer of fiberglass with epoxy to provide a firmer surface(important to not use vinylester resin b/c it might eat through the foam, again test a small section to make sure whatever you choose doesn't react with the foam!). Overzealous sanders can destroy contours in seconds if they push too hard on the foam surface. We've found that this layer of fiberglass helps keep that from happening. Once the epoxy sets up, you can put body filler to fill the pin-holes and low spots. Then spray with an automotive primer and sand, sand, sand, sand, and sand some more to get as smooth a surface as possible. If you try to skip steps in sand paper, it ends up becoming very hard to pull the part off the mold when you're finished. A little bit of mold-release helps too, either wax or fre-kote.

[RiNaZ]

hey sam, have you check this thread?

http://fsae.com/eve/forums/a/t...10888641#42810888641 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/46710775641?r=42810888641#42810888641)

in case it doesnt answer your question, i'll see if i can answer it better right here.

When you get your poly foam back, you want to cover it with epoxy. On the side note, i like to dye my epoxy with red color just so they guys know when to stop sanding, when they see that red color.

Cover the whole foam with layers of epoxy. Brush one layer of epoxy, let it cure, and then start another layer. I usually do 3 layers, but use your judgment. If you see big gaps, make sure you fill in those gaps with something, before you seal it with epoxy. Epoxy is just a sealer, not a filler.

The reason you put epoxy on the foam is so that you can spray gelcoat/primer/resin on top of that foam. Matthew suggested that you put a layer of fiberglass/epoxy on the foam. It's not a bad idea, provided that you lay the glass right, and use the right material. If you wanna put glass on the foam to provide firm surface, i suggest using the surfboard fiberglass. It's a 6oz woven fiberglass. If your body isnt curvy, i suggest using one big piece of fiberglass instead of patching small sections.

the explanation above is for the base. Once you got that, spray the whole body with some type of primer. Duratec is a very good brand. They dont sell this brand at a regular paint shop here in the US, so i dont know what the deal is over there. But worth checking it out.

I usually spray primer, sand it down, and then fill it low and highs with Bondo. And then smooth it all out. And then, i spray another layer of primer. And then fill in with bondo. I usually have 3 layers of primer, and by the 3rd, i shouldnt have to fill in with bondo anymore.

I spray my primer no less than 15mil and no more than 20mil.

Finally, i'll spray a coat of surface coat, which is another layer of primer, but has more strength in it. The 3 layers before this is a sandable primer, so it will still have some pinholes, so the final surface coat, is usually of a different type of primer. Or if you could careless, you can just use gelcoat as the final surface coat, and sand it smooth. But make sure you put wax additive, becoz gelcoat will stay sticky if it doesnt have a wax additive.

These are as far as tool making go. After this, you just mold sealer/frekote/wax the plug, and spray gelcoat, and lay up glass etc.

[ZAMR]

Hey all,

Another method you may want to explore is using a latex based paint instead of epoxy to coat your foam. Bondo eats through foam and turns into a muushy mess, but epoxy is expensive, unless you have a sponsor that gives it to you for free.

We sacrifice machining time and use MDF for our molds, so that we don't oversand or have to worry about the bondo issue.

But yeah, middle layers can be made of primer and body filler, the lightweight filler works the best if you can afford it.

Anyways, your final coat could also include a few layers of wax, buff it, and spray a PVA film over the wax. The PVA works like magic in mold releasing. We use carbon though, so I don't know how it works with glass.

Zach Moorhead
Sooner Racing
Engine & Body

[RiNaZ]

PVA works with both carbon and glass. If you're going with PVA for the mold release, try not to spray it like you would with paint. PVA is mist on the surface. You let it dry, and then mist some more. If you've never used PVA or spray anything in your life, i suggest you do it a couple of layers (after each layer dry up).

It's been a while since i've bought epoxy, but i think it cost about USD 100+ for a gallon of it. If you're sending your foam to be machined, USD 100+ isnt too bad.

Ive never used latex paint on foam before, so that might work too. MDF is good idea too, but cost twice of regular poly foam. And it might be very heavy for you to move the tooling if you have to.

btw, i personally think no one should spend more than a month building the body/nosecone. There are better things to be worked on.

in case you're one of those ppl who are short on time, i suggest that after you machined out your foam, cover the whole foam with clear tape. Wax the hell out of that tape, and splash yourself a mold from that master. All in 2 easy steps, tape and wax.

[Pennyman]

Originally posted by ZAMR:
Hey all,

Another method you may want to explore is using a latex based paint instead of epoxy to coat your foam. Bondo eats through foam and turns into a muushy mess, but epoxy is expensive, unless you have a sponsor that gives it to you for free.

We sacrifice machining time and use MDF for our molds, so that we don't oversand or have to worry about the bondo issue.

But yeah, middle layers can be made of primer and body filler, the lightweight filler works the best if you can afford it.

Anyways, your final coat could also include a few layers of wax, buff it, and spray a PVA film over the wax. The PVA works like magic in mold releasing. We use carbon though, so I don't know how it works with glass.

Zach Moorhead
Sooner Racing
Engine & Body

PVA works fine on glass too. Just used some yesterday for our radiator duct. It tends to be kinda thick so I just painted a couple layers on with a brush.

[SamB]

Thanks for the help guys!

RiNaZ I didnt find that thread no, cheers for explaining it. Do you all find that the cost to CNC the mold is worth the time saved messing around with MDF?

[ZAMR]

Oh yeah, it took us several painful days to just machine our parts on the router (I mean 24 hour a day operation). We make the tools hollow to save weight, which adds even more machining time. I haven't messed with foam before, but we definitely will next year because MDF is a pain in the arse.

Zach Moorhead
Sooner Racing
Engine & Body

[Auerbach]

Another mold release agent I've used is FibRelease from Fiberglast.
http://www.fibreglast.com/show...s+and+Clips-152.html (http://www.fibreglast.com/showproducts-category-Wedges,+Mold+Releases+and+Clips-152.html)

It has the consistency of water and just wipes on. I used it and a couple coats of good ol' turtle wax and the full two-seat tub for my car popped right out with a couple swings of a 2x4.
The mold is a combination of MDF, hand shaped foam, and gallons of bondo.

[Thrainer]

I'm not sure if I understood correctly, but it sounds like "CNC or MDF". We're combining them, our monocoque plugs are CNC machined MDF: http://www.amzracing.ch/amz/ga...il.ort?folder_id=738 (http://www.amzracing.ch/amz/galleryDetail.ort?folder_id=738)
I believe our bodywork moulds are MDF as well.

http://www.amzracing.ch/amz/files/amz802.jpg

Thomas

[RiNaZ]

that's beautiful thomas. MDF is great for tooling, but it's heavy, and you have to build a table or some type of cart to roll it around. And it cost a bit more too. But for a monocoque chassis, yeah, MDF is good material (especially when you have access to a machine that can compress layers of MDF). Either ways, both MDF and foam are prone to warping, and it's best to put it under weight for a day or two.

btw, i think you meant to say plug/buck/master and not mold being made out of MDF. Unless of course, you did machine out the the bodywork mold using MDF.

[Auerbach]

I meant plug when I said mold being made of MDF. My mold is actually glass and the part is carbon fiber.

[Thrainer]

Thanks, RiNaz.

For the monocoque, there's the MDF plug, then a CFRP mould, and the CFRP monocoque.
For the bodywork, CFRP is laid directly into an MDF mould.
So I think, I used the terms correctly?

Yes, they're quite heavy, we used about 2 tons of MDF for all plugs and moulds for the car, I believe. And you have to carefully glue the layers.

BTW, you can't use foam plugs for autoclave curing at 80°C and 4 bar, can you?

For updates on our production, check our website frequently, especially the galleries. www.amz.ethz.ch (http://www.amz.ethz.ch)

Thomas

[RiNaZ]

Did you make your bodywork mold from MDF? if so, then yes, you used the term correctly.

If you're going in the autoclave, then foam plugs is probably not a good idea. Although, foam plugs covered with glass and Duratech products are strong enough for the autoclave should you decided to go with foam. And there is also the PBHT foam which is a high density foam that can withstand the high temperature and pressure.