20 replies to this topic

[Keith Young]

After much searching, I have yet to find a good description of fabricating a composite hood. My freind will soon buy a WRX STI and I have designed a hood for it. We were discussing fabricating it, but I realised how little I know on the subject. What is the best fibre? Fibre glass, kevlar, Carbon fibre? Cost will be an issue, and this is something that we would want to just fabricate in his garage. I know near nothing of the proper materials or resins, or how to make the mold, anything about the fabrication.

Does anyone have any good tips on this, or any good sources of information on the subject?

Cheers

Keith

[saudoso]

About fabrication:

This is the process by the fiberglass boats are made, should work:

1- Build the hood with wood. If you have a 3d model of it, just plot the cross-sections, and build it.

2- Finish it with gel-coat (resin used in fiber glass finishing), sand it and polish it like it was the final piece. Now you have a "plug", from where you will build the mold.

3- Spray the plug with silicon and then start covering it with fiber-glass. Reinforce the shell of the mold with wood or steel rods.

4- Now you will have a mold, just turn it upside down and then repeat the process, to have a hood.

This is a rough view of it, many sites around sell trainning videos with the process and also sell material. Some stores even give free workshops on how using their material. Good look!

[Keith Young]

I know the very basics, it is where material type and weight come in first off that I dont know enough about. Also I have heard wood plugs arent entirely accurate or Class A surfaced. I have heard autoclave and other terms thrown around, but dont really know what they are. I am sure this would need to be heated or have some sort of vacuum so pull out air bubbles. This is a complicated process and I am hoping to find complicated answers, either through a response or a link provided, which I could print off and basically have everything I needed but the design and the tools/materials.

I need all those hard learned tips, I dont have the time or money to go into this blindly and learn from my own mistakes, and even with the ultimate information at my disposal, I am sure after fabrication I could look back and say "I wont do this little thing in this manner again" even if the part is a success.

Cheers

Keith

[Greg Locock]

Vac bagging is fairly painless. You do it to the unfinished side, as it makes wrinkles. You use a bleed layer, which basically absorbs the exess resin that is sucked out by the vacuum, and provides a path for the air, and then a polythene sheet taped over the lot, and a source of vacuum.

Once you have your layup made, then you assemble this lot on top, tape it up, apply the vacuum and leave to set. I don't know what stops the bleed layer from sticking to the main layup - it has been a long time!

I'd recommend S glass and Polyester resin. I strongly suggest de-aerating the mixed resin with an ultrasonic shaker and vacuum before you do the layup.

An autoclave is an oven. You need to cure your resin to at least 20 degrees higher than it will get in practice, and ideally more than that.

Wht are you going to use as a structure? A hood that is going to be stiff enough is going to need a fair amount of reinforcement underneath unless it is small, and flanged, or ridiculously thick.

[hydra]

What about drape-forming lexan or some other suitable plastic?

[Greg Locock]

What is the point of this? weight saving or do you want to make it a different shape?

If you just want to replicate the original then you can take a brave pill and use the original part as the male mould (aka plug), just polish it up (very well) apply release agent and then make your female mould. You will need to attach some structure to the female mould to prevent it warping, and edges from curling.

I can't remember if we sealed our plug. It was made of craftwood. It had ten layers of wax on it before we were happy with the finish.

Oh, I've never used polyester resin for large parts, I'm not sure if it needs baking. Epoxy definitely does for parts like this.

[dosco]

Originally posted by rough_wood
I know the very basics, it is where material type and weight come in first off that I dont know enough about. Also I have heard wood plugs arent entirely accurate or Class A surfaced. I have heard autoclave and other terms thrown around, but dont really know what they are. I am sure this would need to be heated or have some sort of vacuum so pull out air bubbles. This is a complicated process and I am hoping to find complicated answers, either through a response or a link provided, which I could print off and basically have everything I needed but the design and the tools/materials.

I need all those hard learned tips, I dont have the time or money to go into this blindly and learn from my own mistakes, and even with the ultimate information at my disposal, I am sure after fabrication I could look back and say "I wont do this little thing in this manner again" even if the part is a success.

Cheers

Keith

It depends on what you're looking for.

Methods:
Wood plugs with hand-layup is the most basic method. This method can be used with any fiber, and any resin that will cure at room temp.

The next step is vacuum bagging. This can be done with pre-impregnated cloth ("pre-preg") or wet-layup cloth, or "resin transfer molding." The mold could be of any type (wood plug, metal plug, etc), but you must be able to seal a vacuum around the mold. A plastic "bag" is put around the whole mold (while the resin is still uncured), and then a vacuum pump is attached to the bag....the air is drawn out, and the atmospheric pressure "pushes" on the bag, squeezing the whole mess toghether while the resin cures.

The next step is autoclave+vacuum bagging. The autoclave is a huge oven that can be pressurized such that the pressure squeezing the vacuum bag is greater than atmospheric. The reason for the oven part is high-temperature curing of the resin.

There are also computer-controlled fiber layup systems.....but I don't think you'll be setting one of those up in your garage.

Fiber: Dry cloth is exactly that. Dry. You either wet it during a hand-layup process, or use "resin transfer molding in which resin is sucked through the layup during vacuum bagging.

Pre-impregnated cloth has the "just right" amount of resin already impregnated in the cloth. This type of fiber requires vacuum bagging and usually an autoclave. Keep it in the freezer to prevent the resin from curing.

Resin: There are many resin systems.....
Two-part epoxy (both polyester and non-polyester resins)

Prepreg resin (all kinds of resin "flavors" here)

The thing to note with the resins is the temperature the final part will be exposed to. There are certain resins that are much more suited to high-temperature service than other resin types.

Tooling: Wood plugs...build a wooden plug with the shape you want. Finish it, apply release agent, layup fibers, etc etc etc

Foam-core layup....cut a foam core of what you want, layup the fibers over the foam, etc etc etc

Metallic tooling....can be tricky. Such tooling is used for high-precision parts molded in an autoclave. Carbon composites are built on Invar tooling (invar is a nickel alloy that has the same thermal expansion properties as carbon fiber...it is expensive, difficult to machine, and really only for autoclave type molding). Metallic tooling is so expensive that is should be left for the rich boys or companies making high-volume parts.

I'll have to dig around for links/URLs......

[dosco]

Oh, almost forgot.....

Wood molding is good for prototype parts.

Vacuum bagging helps reduce voids and improves fiber packing, and therefore improves structural performance of the final product.

Autoclaving further increases fiber density, and is superior for high-strength structural requirements. Of course, creating a pressure vessel is a BFD and probably should not be done in your garage.

Surface finish of the final part is a function of surface finish of the mold......wooden tooling can be highly finished (many layers of finish and lots of sanding with fine-grit abrasives) to result in very nice looking parts. On the flip side, metallic tooling with crappy surface finish will result in composite parts with poor finish qualities. Some of the finish imperfections can be painted over, so depending on if you want a "natural" finish or a painted finish, you can decide how much effort you want to put into the tooling.

[dosco]

for some basic info: http://www.acp-composites.com/

If you really start digging, you can use different weaves and layup weave "directions" to maximize structural properties......which can be done with any type (FG, CF, etc) of cloth.

Like I said, it depends what you want.

[Keith Young]

The reason for this is to get him a different looking custom hood, and hopefully we do it right and save some weight. Also I want to do this so I can get experience in this composite fabrication art, and for that matter when people ask "where did you get that hood" its always nice to be able to say "i made it". From this experience I could make more things like this for myself in the future. I kind of thought that autoclave was an oven, and pre preg is another word i dont know but i would assume it is already wet composite or something.

When using a vacuum bag, do you have to use any specific precautions to keep the fabric from wanting to suck up inside the vacuum tube?

Probably wood would be a good enough surface for this application. After removing the part, can you sand it without damaging fabric at all? Then add a clear coat or something?

[dosco]

Originally posted by rough_wood
The reason for this is to get him a different looking custom hood, and hopefully we do it right and save some weight. Also I want to do this so I can get experience in this composite fabrication art, and for that matter when people ask "where did you get that hood" its always nice to be able to say "i made it". From this experience I could make more things like this for myself in the future.

As a first project, esp without vacuum bagging, I would be suprised if you saved any weight at all. Even with a bag setup, precise fiber orientation is what yields the maximum strength to weight ratio. On many race cars, the composite body panel is just stiff enough to allow it to be handled and placed on the car. When on the car, there are enough "pickup points" to reinforce the panel. You could conceivable make a "floppy" hood that "stiffens" when it is attached to the car.

With that said, a basic vacuum bagging setup is relatively inexpensive. OTOH if you decide to use RTM, you're in for added expense and possible manufacturing problems.

Originally posted by rough_wood
I kind of thought that autoclave was an oven, and pre preg is another word i dont know but i would assume it is already wet composite or something.

See my previous post. The autoclave is a high-pressure oven (not really "high pressure," but higher than atmospheric). Pre-preg isn't "wet" per se, more like "mildly tacky/sticky." It "wets" when heated so that the resin flows through the layup.

Originally posted by rough_wood
When using a vacuum bag, do you have to use any specific precautions to keep the fabric from wanting to suck up inside the vacuum tube?

Yes. The tube where the vacuum is drawn is not usually located "above" the fiber layup. The setups I've seen there is a section of the mold that has a little "trough" in it, that sits below the plastic bag....the vacuum tube is attached there.

Originally posted by rough_wood
Probably wood would be a good enough surface for this application. After removing the part, can you sand it without damaging fabric at all? Then add a clear coat or something?

Be careful when sanding. If you just skim it with like 2000 grit, you'll be fine....but it is possible to over-sand and hit the fibers (especially with lower grit abrasives). OTOH you can add additional layers of resin (or other finishing agents like primer/paint) that add enough thickness where you can lightly sand between coats.

Of course, if you really polish the mold surface, the finished part's surface will be very smooth as well and shouldn't require any finish sanding.

Another thing - are you going to paint the hood when it's done? Or leave it "natural?" (ie as produced from the mold without paint).

[cmoore]

Try this. There is some very good information throught the forum and on the companies website.

http://www.fibreglas...hpBB2/index.php

CMoore

[Keith Young]

There we go, thats exactly what I was looking for. Thanks alot for the help guys. I have all I need now, but lets keep this topic going for others who have similar questions about composites.

Does anyone know how to properly choose, prepare, shape, and apply honeycomb to a composite part?

[GBarclay]

If this was me learning this art from the beginning again - I would KISS (keep it simple s*****)

Carbon, epoxy, bagging, autoclaves, pre-pregs, honeycombs. All of these are a big expense, all need experience to use, messing any ONE of these up will mean your completed part is useless and not to spec.

Start small, start with glass, and poly resin. Start with a small part - definitely not a hood. An STi hood is an ambitious project, even more so if you have never worked with any of this stuff before. And a small simple mistake would mean a large $$$ waste of materials and labor. Your buddy is going to save a lot of time and money BUYING a carbon hood from a ricer magazine. And be warned, half of the "carbon" hoods sold are not carbon fiber at all.

Sorry to rain on your parade, but in all honesty, modern composite fabrication is not something you want to learn from a book (or a website), on your time, with your $$$.

[Keith Young]

Trust me I am not so optimistic to think I could do an A job 1st try. I plan on making very simple shapes, ones that I wont even have a use for, just to learn from them. I would like to follow instructions on my first tries, then develope my own method from there. Obviously one cant just tackle a huge project in such an artform.

My first projects would possibly be a flat peice, then sharp corners to see how sharp one can go with certain weight fabrics, and just start experimenting with small 1X1 foot shapes, to get a feel for different things. I think you understand what I mean.

[GBarclay]

Starting small and simple will be a good way to learn. If you make mistakes then there is no great harm done and you have not wasted a lot of money.

There are two very good books out there dealing with composite fab.
The names escape me at the moment but the author is Forbes Aird (I think spelt correctly)

OK, I found one of them - Fiberglass & Composite Materials: An Enthusiast's Guide to High Performance Non-Metallic Materials for Automotive Racing & Marine Use

as found on - http://www.fetchbook...hBy_Author.html

Also try getting literature on the WEST epoxy system, a very effective epoxy brand that is geared towards the DIY'er as well as the pro. http://www.westsystem.com/

[BRIAN GLOVER]

You have the original hood, so use that for the buck. Can you find a used hood somewhere, this will save you plenty time. Using the original hood may result in a repaint, but it will still be less time consuming. Forget anything other than fiberglass unless this is your business.
I would check with your car club to see if others would like your design so that you can proceed with production in mind. A one off is a bitch of amount of work to do and it would behoove you to purchase a after market hood and modify that. If it is for pure sex, I would chop up the original hood and make the part from that.

3m has a product for each of the following processes:

Tape off the hood at the bottom under the edge radius.

Buff the hood real well and spray a release agent over the surface or apply many layers (at least 5) of floor wax taken to a gloss each time. Use No. 8 cloth and lay 5 layers on top of 3 coats of a fill prima. Forget gel coats even the final product. Allow to cure in the sun outside. Heat source is always from the bottom to the top. In this case the bottom is the last layer of glass. Don't do this in cold weather. It is a large area and will distort. when cured over 2 days, reinforce this mould with bucking tubes of steel.

Take a working part off your new mould of 3 layers. Again, no gel layer. Trim the edge.
Remove taping from the underside of the original hood. You will be using the steel pressing reinforcement under the hood for a cast of the location points. Your hood will be a two piece unit. If you don't do this, the new hood will flex like a bitch in the wind. The modifications to the hood are made after the hood is located and attached as a duplicate functioning hood.

You will seal off all manufacturing location holes and take a mould of the outer perimeter reinforcement bracing and make a working part from this mould. Trim this to fit in the hood recess on the actual car. Cut recess for a steel hinge plate and bond it to your working part. If you are going for production, you will have to develop a jig for each process.

Remove hood springs and bolt this rectangular part to the hinges and check for correct operation and fit.

Bond this part to the underside of your new hood and clamp it to your original mould to cure.

If you don't have a used hood, the under edge of the hood can be made using wax strips that come in a wide variety of sections that can be easily shaped to fit along the body where the hood fits. Once the wax strip is in place, press the 'new' hood onto it to the correct height. The wax will stick to the fiber glass. remove from car and place inverted on your work table. Beauty of prima filler is that it can easily be repaired if damaged or scratched.
Insulate the top edge of the hood with tape. Take a mould as described above.

If this interests you, I will give you product numbers for all the materials used.
Are you sure you want to continue?

Now you have duplicated the hood in fiber glass. Time for the mods.
Never use wood. If you are going to make a scoop, use an air-conditioning insulation polyurethane foam block. They come in 5 different grades and densities. Cut a block to fit the hood and affix it the hood with an appropriate adhesive.

Carve out the shape of your scoop. If you screw it up,replace the block and start again. Once you are satisfied with the design and have made it symmetrical with templates, there are other methods to gain symmetry but this will suffice for this size job.

Apply a thin layer of 3m modeling clay about 1/8 '' thick over the foam. This clay can be worked to a near perfect gloss surface with fine detail. It is expensive stuff, but how much is your time worth. Also you will need the carving tools. Have you ever sculptured models or carvings before? The foam can be discarded and the clay is recoverable with a year shelf life. Now again, if you want this for production, remember, to release a mould, it cant go around corners larger than 275 degrees. If your design is too complex it will have to be finished after you take the final hood out the mould. For production, you will need split moulds. If you have to cut holes into the hood, the edges can be finished with this clay also.

When you are satisfied with your creation, you can take a localized mould of the modified parts of the hood and then work these pieces into your original buck mould.
Another benefit of the primer filler inplace of a gel coat apart from ease of effort and better finish is a better paint job and color match. It also more chip resistant for surfaces that are exposed to the wind.

The car on the left was done with this method. carved from a big block of foam and covered in clay. Working body panels came off the clay. A 1/4 scale model using the same method was used and scaled to full size. I have not found a faster more accurate method.

Originally posted by rough_wood
After much searching, I have yet to find a good description of fabricating a composite hood. My freind will soon buy a WRX STI and I have designed a hood for it. We were discussing fabricating it, but I realised how little I know on the subject. What is the best fibre? Fibre glass, kevlar, Carbon fibre? Cost will be an issue, and this is something that we would want to just fabricate in his garage. I know near nothing of the proper materials or resins, or how to make the mold, anything about the fabrication.

Does anyone have any good tips on this, or any good sources of information on the subject?

Cheers

Keith

[Keith Young]

Making a duplicate hood but with composites would be much easier, but the whole reason for us to do this would be to design and fabricate our own hood. What is the cheapest and easiest way to make a mold from CAD data?

[Greg Locock]

Print your drawing out on paper, stick it to craftwood, jigsaw round the lines, stick the sandwich together, hand finish.

That's the cheap way.

The easiest is to pay someone else to do that!

[McGuire]

Originally posted by BRIAN GLOVER
The car on the left was done with this method. carved from a big block of foam and covered in clay. Working body panels came off the clay. A 1/4 scale model using the same method was used and scaled to full size. I have not found a faster more accurate method.

Great how-to piece, Brian. Very clear and evocative description...made my arms and neck itch just reading it.

[BRIAN GLOVER]

You've been there and done that too? Long time since Ive done that. It's hunting and fishing now.

The foam is not glass and is non toxic. It is easy to work with virtually instant results. I run a line from my compressor with a regulator and filter to a mask so I breath outside air. The working panels are far from the final part as you know.

I'm looking for a Mark X Jag now so I can put a Rat motor into it. Yeah.

Originally posted by McGuire


Great how-to piece, Brian. Very clear and evocative description...made my arms and neck itch just reading it.