Introduction: Simple Methods for Molding Fiberglass and Carbon Fiber

Picture of Simple Methods for Molding Fiberglass and Carbon Fiber

Here's a couple of low cost and quick procedures I use to manufacture simple parts using fiberglass and carbon fiber. While it's not the best method for producing parts that see structural loads (some form of consolidation like vacuum bagging/pressure molding to reduce internal voids should be used for that) or parts that need to have an extremely high finish leveI, I have molded everything from simple car parts to subwoofer enclosures to costuming/theatrical props using these methods.

Step 1: Materials

Clear packing tape
Blue foam insulation- available at many home improvement stores
Laminating resin- polyester resin and epoxy resin ( I use Bondo polyester resin for fiberglass and West Systems epoxy resin for carbon)
Paint brushes- I use a short bristled brush, sometimes I just cut the bristles down to anywhere between 1/2' to 1" llength
Mold release paste wax-available from Aircraft Spruce-
Woven fiberglass cloth and/or chopped strand mat- often called CSM
Woven carbon fiber cloth- available from Aircraft Spruce-
3M Super 77 spray glue (optional)
Rubber gloves
Filtering mask

Please use proper safety equipment when working with resins and fibers. Carbon fibers are extremely sharp when cut and the use of resins and melting of foam can produce nasty vapors.

Step 2: Method #1- Creating a Mold and Pulling a Positve

Picture of Method #1- Creating a Mold and Pulling a Positve

I used this method to modify existing bodywork on a friend's Ralt R5 CSR Mazda race car. His new tire/wheel set up was causing the front tires to hit the existing bodywork so he modified it by cutting holes in the fenders and then bolting on some plates with spacers so the front tires would clear when the suspension was fully compressed. The modifications were pretty ugly and un aerodynamic (not to mention the tires would still rub on occasion) so he asked me to make new fender parts that could be grafted onto the existing bodywork so he wouldn't have to buy all new bodywork for the front of his racecar. I made these modifications several years ago so I don't have photos of the entire sequence so I did drawings to fill in the gaps.

The first thing I did was to figure out how much I wanted to raise the fender line and then cut a template to be used to cut the blue foam. The blue foam is then cut using a hot wire cutter and is placed on top of the existing fender to check the fit.

Next the foam is taped down to the fender using clear packing tape- the tape prevents the polyester resin from melting the foam when applying the fiberglass. I then made some vents for the new fender using some wood molding- this is hot glued in place on the taped foam.

The taped foam then gets a coating of mold release wax.

Now the fiberglass cloth is wetted with polyester resin and is applied over the foam. You can use woven fiberglass cloth or chopped strand mat for this. If you use CSM you have to use polyester resin- the polyester resin melts the styrene binder in the CSM. All epoxy resin will do is make a huge mess.

One trick I do when using woven cloth is I pre cut my cloth and stick it down on my pattern with some 3M Super77 spray cement and then wet the woven cloth out with resin using a short bristled paintbrush. If the cloth (or CSM) has to be applied in sections make sure the sections overlap by at least one inch.

Once the resin has cured you can pull the fiberglass mold from the foam pattern.

To mold a carbon fiber positive from the fiberglass mold you first apply a mold release wax to the fiberglass mold.

Then start wetting out the carbon cloth with resin and laying it inside the fiberglass mold. When wetting out the resin use a short bristled paintbrush and a "stippling" action to make sure the resin has fully penetrated the carbon cloth as is placed in the mold. I tend to use epoxy resin when laminating carbon cloth as it produces a much stiffer final product.

I usually apply layers of carbon cloth at alternating 45/90 degree positioning. This is because the woven carbon cloth only has stiffness in the direction of its fiber orientation.

Before the resin has fully cured you can trim the excess material around the edges of the mold with scissors- this is much harder to do later.

Once the resin has fully cured the part can be removed from the mold.

After the molding process was finished all I had to do was bond the new carbon sections onto the existing bodywork and blend it in with some filler.

Step 3: Method#2- Lost Foam Molding

Picture of Method#2- Lost Foam Molding

I used this method to create an air duct for the oil cooler on a friend's Porsche GT5R racecar. This is a very quick and easy way to mold a hollow component or a part that has severe undercuts that would not alllow it to be molded in a single piece female mold.

The first step is to cut a pattern from blue insulation foam and wrap it with clear packing tape.

Next, cover the taped pattern with a mold release wax.

Now cover the pattern with woven fiberglass cloth and wet out the resin. Make sure to leave a section uncovered- this is where the foam will be melted.

After the resin has cured, melt out the foam by pouring acetone on the uncovered section (note that in drawing #4 it says kerosene- this is incorrect). Make sure you have a decent sized tray or bucket under your part- this will get messy! As the foam melts you can reach into your hollow part and pull out the packing tape and you are left with a hollow molding.

Step 4: Method#3- Using an Existing Item for a Mold

Picture of Method#3- Using an Existing Item for a Mold

In this case I wanted to make a hidden subwoofer enclosure for my car by molding a fiberglass tub that sat inside the spare tire so I wouldn't lose any trunk space.

I first started by covering the inside of the spare tire wheel with clear packing tape and giving it a good coat of mold release wax.

Next I cut a large ring from MDF and placed it on top of the spare tire.

Then I started laying up my fiberglass ( I used CSM for this) into the taped spare tire. The fiberglass was applied so that it overlapped the upper surface of the MDF ring. This made the finished molded tub much stronger and made it much easier to remove from the spare tire after molding.

After the resin cured I cut another MDF ring to mount my subwoofer into and joined it to the MDF ring that was molded into the fiberglass tub. I used some sealant between the two rings and also screwed them together.

Then I drilled a hole in the hole in the MDF for my subwoofer wiring and applied sealant to it after I ran my wiring to the woofer. The last step was to bolt the subwoofer into place.

The volume of the molded tub turned out to be perfect for a 10" subwoofer and it sounds great without taking up additional trunk space- and I get to keep my spare tire!


L_Morais29 (author)2017-04-14

Hey, do you know if the foam you used stands temperatures around 150ºC and high temperatures? If not, do you know any type of foam for molds that can cure in an autoclave?

Tofia made it! (author)2017-03-29

Hey! I want to build an outrigger canoe with carbon fibre and need help. What material should i get and where and how to start...

I have a rough sketch shown here... 16' long and 12" deep in the mid section.

AnikaM7 (author)2017-01-07

Any ideas on how to make a carbon fiber monoplane? A fellow peer of mine and I are constructing one for Science Olympiad and we were thinking carbon fiber would be a wise decision of material.

Honus (author)AnikaM72017-01-31

Sorry for the late reply! I honestly wouldn't even know where to begin on a project of that scope. I would look for a forum about homebuilt aircraft or maybe contact the people at Aircraft Spruce.

AmeliaEarheart (author)2016-12-07

I am trying to make a glass fibre epoxy resin composite with thickness of 5mm by hand layup method. i tried but couldnt achieve the desired thickness . I used four layers each at orientation difference of 45 degrees. if you could please suggest how to attain the desired thickness, it would be a great help. How should i go about it? Please help. Thanks :)

Honus (author)AmeliaEarheart2016-12-09

The only methods I know of to obtain a desired thickness are to either add more layers or sandwich something between layers to form a core. Cores are often foam sheet or even balsa wood.

AzJavi (author)2016-11-04

Awesome! I came here because I was looking for some fiberglassing techniques since I'm molding some skirts onto my show car and doing a custom sub box in the hatch area. I'm also doing a custom front bumper lip since they don't have any ready made ones for my car and I was going to use a friends lip that has a nice design but I am going to add to the top of his lip to make it fit my bumper better. This was very helpful for me. Thanks man!

BryanS145 (author)2016-10-06

ok so I'm trying to make like a "real carbon fiber overlay" type thing for the wood dash accent type stuff in my car. Im new to cars and all of this but I want the experience of doing this. I'm thinking of sanding down to the wood on all of the wood interior of the car. Possible process: make mold, lay carbon fiber to the original thickness of the resin on the parts beforehand, and use a strong adhesive like clear JB Weld. My question is, do you think this will work? My car is 1995 Mercedes Benz C280 if you need a reference.

Honus (author)BryanS1452016-10-06

Yes, I think that would work. So are you molding an entirely new piece or are you adding a carbon layer to the wood trim piece?

BryanS145 (author)Honus2016-10-10

I think just adding a carbon layer to the original piece

nmrobotics (author)2016-09-05

So I'm trying to make a mold of a laser rifle from fallout, how much would it cost to make? (Using method 3)

Honus (author)nmrobotics2016-09-05

The best way to do that is by making a matrix mold (sometimes called a mother mold.) This involves building up a layer of silicone on each half of the rifle and then this is supported by a fiberglass jacket. Molds of this type can cost hundreds of dollars (or more, depending on size.) You would have a really difficult time making a straight fiberglass mold for that.

Have a look at the Punished Props video of a matrix mold here-

kalecarson (author)2016-05-03

Question about the wax release. Does it matter on the specific wax you use? I've heard different things.

Honus (author)kalecarson2016-05-03

I've only used specific mold release wax. I can't really say if any other type of wax will work.

kalecarson (author)Honus2016-05-05

Thanks for responding so quickly
Another question, could i substitute polyester resin for fiberglass resin?

Honus (author)kalecarson2016-05-05

Absolutely- polyester resin is fiberglass resin. The other type of resin is epoxy resin. The difference is epoxy resin is tougher and it produces a nicer finished piece but it's a lot more expensive.

Hfeucht (author)2016-01-10

Hi! So I am working on a 49 ford pickup and making a custom dashboard insert since the previous owner cut out a huge chuck behind the steering wheel. Since I have to do small cut out circles for the gauges do you think the taping will still work fine? Or do you have any tips on working on small areas? The whole gap is about 12"x10". Also do you have to get your wax online? Or is that something a store would carry?

Honus (author)Hfeucht2016-01-10

It's hard to say but you might be better off making the insert and then cutting out the holes for the gauges afterward. Some stores carry mold release wax, some don't -just depends on stores in your area. The local independent hardware store that I frequent carries fiberglass supplies and wax.

ShannonK18 (author)2015-10-27

Will this work to make a skid plate for a fuel tank for a 1979 F250? The old skid plate rusted out and it will cost hundreds of dollars to make something we don't even like. The old skid also held up the fuel tank and the fuel tank is the main not the reserve.

Honus (author)ShannonK182015-10-27

Boy I don't know about that. For a skid plate I'd want something much tougher and more abrasion resistant. At a bare minimum you would probably want to use Kevlar material if you want to mold it.

ドラゴンマ (author)2015-10-19

how do you trim those intserts

Honus (author)ドラゴンマ2015-10-19

Use something like a Dremel tool with a thin cutoff wheel.

EG4 (author)2015-10-08


manuel.roales (author)2015-03-16

thanks sir for being being generous in sharing your knowledge. such a big help to someone starting a project. THANKS again sir!

brubble (author)2015-03-12

I'm thinking about building an inner fairing for my Harley that will house a small stereo, amp and some speakers. Do you think that would be too much weight for either of these methods, or can you suggest another method that will work?

Honus (author)brubble2015-03-13

There's absolutely no reason why it wouldn't work for that.

supinder (author)2014-11-18

Do you add the resin to over the fiber glass?

Honus (author)supinder2014-11-18

Yep- you brush it on using a stippling motion in order to make sure the resin has fully impregnated the fiberglass cloth.

Nemesis201077 (author)2014-04-04

This has been quite helpful.

I recently bought an older 4x4 to play with and the wings are prone to rust. Although mine are in reasonable condition they could be better. I was thinking of using the existing wings to create a mold. then use the mold to make some rust free wings for myself and a few friends on the Nissan Terrano forums.

dianahopper (author)2013-10-24

This is really a good tutorial. I was unknown about all these facts. Thanks for sharing the info about fibreglass moulding.

GraphixS6 (author)2013-05-31

This is a great method. However, You're using a lot of fiberglass, about twice as much which I imagine could get pretty expensive quickly (resin).

taylorknight6 (author)2013-05-08

I like this tutorial, but I have a few questions. Do the edges of the packing tape leave impressions in the fiberglass (like if they are overlapped)? And will the foam hold up to vacuum bagging?

Honus (author)taylorknight62013-05-08

The edges of the packing tape will leave an impression in the mold- any surface imperfection on your pattern will be reproduced in the mold. The foam I used will hold up to vacuum bagging.

USFRobotDesignerME (author)2012-11-27

Do you answer your private messages and not mind helping people who are just getting started with this?

Honus (author)USFRobotDesignerME2012-11-27

I'm happy to help- just ask! :)

Edgar (author)2012-08-19

You gotta love an Instructable that even goes as far as the Author drawing Instructions, to make sure people understand it!
Nice! :D

Honus (author)Edgar2012-08-19


Edgar (author)Honus2012-08-19


Edgar (author)2012-08-19

Since people keep asking me questions about carbon Fiber, on my Portuguese DIY Blog, I've made a Post, pointing to this Instructable:

ydeardorff (author)2012-07-07

Any instructable about how to make reversed identical body parts like fenders etc?

That would help me immensely. i have half a car done, and need to replicate the work on the other side.

Honus (author)ydeardorff2012-07-07

The best way to do that would be to design your body parts using a computer and then create templates to make your molds.

ydeardorff (author)Honus2012-07-07

The parts are already made on one side. So my only option i guess would be building a home made 3D laser scanner, and then make a 3 axis CNC big enough to carve the parts in layered format.
Again, very expensive, and time consuming. i was hoping for a more ghetto, but works idea.


808Dave (author)2012-03-18

Honus - still checking in after all these years?

Re: lost foam method:

A year or so ago, I suspected this might be a great method to make an irregularly-shaped diesel tank to add to my truck's crowded underbed spaces, so I poked around on the web for info to confirm that the sludge/goo that would result from melting the styrofoam with acetone wouldn't be a problem (thinking it would stick to insides of tank, leave a residue, etc.) I didn't stumble across your instructable then - but a major web-retailer of resin/materials said it /would/ be a problem, and advised me not to do it. Your experience makes me think that might have been bad advice.

Would you say I should plan to do hand-cleaning of the interior surfaces after the initial melting - so I'd need to have good access to all interior spaces in the tank for this? If there's residue, does it seem like it's going to be forever sticky (and maybe get mixed into fuel or cause other problems), or is this a non-issue?

Also - you mentioned something about epoxy/CSM turning into a big mess? My occasional experience with almost nothing but epoxy resin in the last ten or so years is that there isn't really a problem wetting out CSM fibers, and I'd definitely prefer to work with CSM in a project like this tank, since it conforms better to irregular surfaces. Maybe this has changed since you wrote this a few years back - styrenes in polyester resin being recognized as rather nasty toxins, etc. and I hear all but replaced with epoxy in many industrial applications. I do recall how magically the fibers used to melt invisibly into the polyester resin when I used it before (doesn't do quite that with epoxy) but I've not observed any alarming lack of strength in my dabblings. Do you still avoid epoxy/CSM combination?

Honus (author)808Dave2012-03-18

I wouldn't use a fiberglass container as a fuel tank. Any type of fuel will act as a solvent so that would be bad. Commercial composite fuel cells have an internal bladder.You would be better off creating a steel or aluminum tank.

I still avoid using epoxy with CSM since the CSM uses styrene as a binder and epoxy resin will not dissolve the binder. The only time I really use CSM these days is for making some molds using polyester resin, and even then not that often. I much prefer using epoxy resin.

808Dave (author)Honus2012-03-18

I might not use epoxy for a more volatile fuel, but with diesel, I have no qualms. Also, I plan to slosh the finished tank with a sealer, so it's not a problem, really.

Anyway, repeating my question about the 'lost styrofoam' process: Would you say I should plan to do hand-cleaning of the interior surfaces after the initial melting - so I'd need to have good access to all interior spaces in the tank for this? If there's residue, does it seem like it's going to be forever sticky (and maybe get mixed into fuel or cause other problems), or is this a non-issue?

general-Insano (author)808Dave2012-04-23

as someone who has melted styrofoam before as long as you melt and clear within the same day it won't be a problem.
day 1- slippery goo similar do a runny egg(pulverize styrofoam for best results)
day 2- still slippery has a bit more body
day 3- starting to harden but still shapeable
day 4- becomes even more hard but small holes are a problem
day 5- if you get rid of the smell of gas from styrofoam makes a interesting ball once formed

Honus (author)808Dave2012-03-18

Boy, I don't know. A lot of it is going to depend on how well you tape the foam pattern. If you do a good job taping it and then waxing you shouldn't have much clean up. I'd say you would definitely want to have really good access to get rid of any residue.

Have you run a test do determine the impact resistance of the fiberglass tank relative to a steel or aluminum tank?

crowd9 (author)2011-03-30

I imagine the final product has to resist compression/suction from air forces, and not vibrate/flap during a race. How many layers of CF did you use for the parts on this project to ensure strength?

Honus (author)crowd92011-03-30

It was so many years ago I honestly don't remember but it was several layers.

nixnope (author)2011-01-19

Nice info! Will need to try this!

About This Instructable




Bio: I'm a former bicycle industry designer turned professional jeweler. I like working with my hands and am happiest when I'm in the shop ... More »
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