lets hope this will work...

General Composites Discussions Composites Talk
#1

One of the most popular cars in South Africa is a VW Golf Mark 1(rabbit). It was in production from 1979 up to 2009. That means one in every 3 cars is a Mark1. Parts are widely available, and it’s the most popular cars for street tuning. But there are no carbon parts available. Carbon hoods for these cars do not exist. After a lot of people bugging me for a carbon hood, I decided to take on the challenge. Luckily it’s a plain and simple hood, practically flat and square. But there are ventilation grills on the top part of the hood.
I’m planning on closing these grills on my plug hood with body filler, because it’s going to complicate the mould.
These vehicles were not fitted with air conditioners, so the windows are always open. Vent grill won’t make a difference.
I’m planning to make the mould and the hood as cheap as possible, because these vehicles are cheap, and expensive carbon hoods won’t sell.
What I’m planning to do:

  1. make an outside mould with tooling gel coat (polyester gel coat)
  2. Then build it up with chopped strand matting (polyester resin)
  3. Same procedure for the inside of the hood.

That means I’m going to have 2 moulds.

Actual carbon part outer skin. Infuse with epoxy resins

  1. Use only 1 layer of 200g 2by2 twill weave carbon
  2. Use a 3mm core material, soric
  3. Then a 195g woven glass sheet.

Inside ribs structure (infuse with epoxy resin)

build purely out of fiberglass and paint it black.

Then bond the two together using an epoxy bonding material.

Do you guys think it’s the right procedure to keep the cost down? Or do you see a whole in my plans…?

#2

I would be concerned that the external carbon will expand less than the glass back as the temperature changes. The CTE differential between the two could cause the hood to warp when the temperature changes. The questions is whether the ribs/stiffeners on the inside are stiff enough to resist the CTE differential. I don’t know. :slight_smile:

Also you want to make sure that each side has reached a significant cure before bonding the two sides together. Since the tensile modulus of the carbon is much higher than glass the resin shrinkage will cause the panel to bow towards the carbon side of the sandwich if they are bonded early in their cure… If the shrinkage is complete between the two sides and then bonded together this won’t be an issue. Remember though that most of the shrinkage happens in the latter part of the cure. You need to let each panel reach full cure, or close to it, before joining.

You’ll also want to create some significant stiffening to the hood mold to prevent warping as the mold cures. Polyester has a higher rate of shrinkage which promotes mold warping. I would use at the bear minimum an isophthalic polyester which shrinks less.

Adam

#3

or use a rapid tooling system for the mould.

The other concerns I have is that 200 gr/m2 carbon is still relatively open weave. I would either beef it up with another layer of carbon, or a black coloured glass fabric.

The support structure on the inside can be glued to the hood with MS polymer putty. This dampens things somewhat, and is cheap.

#4

My biggest concern at this stage is to build a sturdy mould, i received a lot of “rumour” that a polyester mould would shrink and distort, and it’s a scare, because i need this tool to produce at least 20 parts. Forgive me if I’m wrong, but i can’t get any photos on a mass production sturdy mould on this forum. My initial idea is to build up the layers of the mould slowly, to ensure the curing and heat can settle. I’m planning to use wood and an “x” rib on the back side of the mould.
Is there anyone that can help me with images?
furthermore, the way i do mould lay ups is to apply my first layer of polyester and fiberglass when the gel coat is touch dry but still tacky, but the company i bought my materials yesterday told me it’s the wrong way, they say, the gel coat need to be almost complete dry and your finger must make a squeaky sound when you pull your finger over it???

Is that right?

#5

Upload images to photobucket or whatever, then place between [img ] tags.
a tooling system will minimize schrink and distortion. Otherwise a slow build-up of your laminate. Never place framing on to your mould surface, it will print. You could attach it to your flanges though.

I use gelcoat when it’s still tacky. It should remain a bit tacky anyways;)

#6

^Susho is right, why polyester and not epoxy if you’re concerned with shrink…

#7

well epoxy is WAY more expensive than polyester. i know its the right way becasue i want to make a lot of bonnets, but on the other side, im not sure if carbon bonnets will catch on amongst these guys. so im using this mold purely as a test to see if there is a market. you see im not planning to charge more that $450 per bonnet…so if it works, then yes ill make a more sturdy epoxy mold. but at this rate im planning to pay for the mold after the 4th bonnet…just an example how damn expensive these things are over here.

#8

…I would go broke at that price.

#9

Do you have contact with African Cats? They might be able to supply you with cheaper materials. Otherwise AMT.

#10

Yes im using AMT as my supplier. they give the best advice, wont say they are the cheapest, but ill pay a little bit more to recieve expert advice.

#11

Bruh…check your costing on this, materials are nothing in this business for an item like a hood…your REAL cost is labour, keep track of your hours and divide how much time it took you to make one, trim it, deal with packing, shipping and final end user. I think you will find you are working for nearly free on this.

^ I heard that…I would be bust also, OP, I know the market is larger there for that particular vehicle, but volume is not always good in composites, we’re still pretty labour intensive. Maybe look to the higher end models, as the old saying goes, I would rather sell 1 hood for a $1000 margin than 10 hoods at $100.

I am not trying to tell you how to run your business, but that does seem cheap from a place where things generally cost much more as you have noted.

#12

A polyester mold is fine for something the size of a Rabbit Hood. The shrinkage won’t be noticeable. Go ahead and make a sturdy mold the first time. The extra money and time to do so is nothing compared to making a whole new mold. Seriously. A few extra yards of fiberglass mat, a pint of resin, and 20 minutes work.

#13

Ok here are my next questions. The vent grills on the top of the hood are going to give me headaches, especially with the molding process, so my plan was to close them permanently with body filler and sand it smooth. Or do you think it’s going to be easier to mask it shut with tape and then sand it smooth inside the mold…?
Then, the inside support ribs are bonded to hood. But with a type of silicone putty? This means the ribs are about 6-7mm away from the bonnet. I know it’s easy to overcome this problem, but do you think i need to split the two parts to be able to make a better mold?