Female Mold for Carbon Fiber Motorcycle Bodywork

Introduction: Female Mold for Carbon Fiber Motorcycle Bodywork

About: Professional front-end web developer and hacker since I was a kid. I'm constantly working on many personal projects and/or tinkering with something random.

I've taken up teaching myself how to build my own carbon-fiber/kevlar composite motorcycle fairings using some old fairings as my template. Originally I was just going to use the old fairings as my mold and layup directly on them but changed my mind and decided to do it right.. So I started building female molds out of plaster. I tried making a small mold out of fiber glass but after researching further decided to give the plaster a shot and it came out surprisingly well I think. The surface is not totally smooth but I was planning for that and will smooth it out with sandpaper and glaze. I'm just glad I separated the part without cracking the mold and the impression successfully got all of the edges.

Step 1: Pour the Plaster

I laid the plaster (100% plaster of paris - 1 large box from Home Depot) last night at about 6:30 finishing up at close to 8. I used cheese cloth soaked in plaster directly against the part and after fully covering it with cheesecloth, I mixed up the last bit of plaster (about half a bucket) and poured that over the top just to build up a base for the skin I'd layed down.

The next day I started around 3:30pm (having let it sit outside all night and all day up until then). In the middle of summer Los Angeles weather I was satisfied it was dry.

This is how it looked after wiggling it loose a little bit.

Step 2: Flip and Trim

First thing to do is flip it over. You'll see I let the plaster wrap around the edges and covered the backside with papertape..

Flipping it over and removed the paper tape I had used to seal it off as you can see.. I had draped some of the plaster and cheesecloth up and around the part to ensure I got the edges.

Step 3: Strengthen Your Mold

I read somewhere that mixing wood glue or coating the plaster in wood glue would strengthen the plaster, so I flipped it over, coated it with wood glue and used a heat gun on it for about 10 minutes to cook in the glue and to comfort my nerves that the plaster was totally dry.

Step 4:

After that using a combination of scissors and a cordless Dremel I carefully trimmed away the excess plaster so I'd be able to lift out the part.

Step 5: Prepare the Removal of Your Part

And positioned some scrap wood around the part to prevent sensitive edges from crumbling due to their own weight once the part was freed. I also positioned an assortment of clips, screwdrivers, a bondo squeegee and a plaster piece of junk I found in my tool bag around the part to act as my wedges.

I was concerned it might not come loose because I really didn't use much release prior to pouring the plaster and definitely didn't give it much time to dry but I was able to separate the part VERY easily in about 5-10 minutes being probably more careful than I needed to be.

Step 6: Strengthen Your Mold AGAIN

After I got the part free, I did the same thing to the inside that I'd done to the outside.. Wood glue and heat gun for about 10 minutes.

Step 7: Pretty Much Done

Here you can see the comparison between the part and the mold. I think I will come back with a marker to draw out the edges just so it's totally clear since there are some cut away areas.

Step 8: CLEAN UP

And last but not least.. the longest part of the process. I cleaned up and organized my work area because I'm OCD like that.
You see I used a pallet on top of a trash can as my work table and built a wooden bed lined with trash bag plaster to hold my work.. With a cardboard top and some string for peace of mind.

My buckets for mixing plaster. Off to the far right I have some more part in a large Rubbermaid container waiting to have plaster poured over them.

I just wrapped up and am going to take a break a bit longer and probably come back and work on smoothing it out so I can start the resin infusion / carbonfiber build up tomorrow

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    6 years ago

    I cover the part with painters tape before putting it into whatever I am using to make a mold. This usually results in a fairly smooth mold surface without spending ages sanding the part. it is also easier to get the part out of the mold (as the tape just peels off). You could also cover the mold with tooling gel before laying the carbon fiber to get a more professional finish.

    For a cf mold, you should really use a 2 part casting resin instead of plaster. Plaster really isn't good for cf. It would even be better if you used bondo to make the mold. Personally, I like to use liquid latex and then cover the outside with polyester resin before removing it.

    If you intend to paint the cf part, you don't need to worry about the surface quality as much. It is easy to use bondo to fill any holes, sand the bonds and then paint.

    There is very little chance of getting a shiny presentable bare carbon fiber finish with a plaster mold anyway.


    Reply 6 years ago

    Thanks for the tips Robert. You are correct. I tried making molds of 3 separate parts using plaster and was only successful in completing one carbon fiber piece. The other two molds, including the one in this instruct able ended up being trashed. I have now started using liquid latex but have not been able to do any large pieces because the liquid latex is so expensive. Almost $100 per gallon. :/

    I've also been using a resin infusion method of creating the carbon layup but getting an airtight seal seems to be a struggle no matter how well I thought I prepared.


    7 years ago on Introduction

    I first thought that perhaps doing the body work on the female pattern would be easier than smoothing out the original part prior to making the pattern and I was feeling too lazy to take the time to properly prep the part beforehand.. I discovered that sanding the plaster pattern smooth is much more labor intensive than it would be to do the prep work on the actual part first so I've decided to redo this because I was not satisfied with the quality of the surface area of the pattern.

    I will rewrite this instructable later after I finish my second attempt.


    Reply 6 years ago

    you'll never be able to smooth out a female mold once it's made. You'll end up ruining it trying, or changing the size of the part as you sand the surface away.

    An easy way of getting a smooth surface on your original part is to use shrink wrap on it when making the mold. It works suprisingly well.

    It is worth remembering that carbon fiber never picks up small details from the mold surface. A smooth mold will give you a cf part that needs less sanding but sanding the final part and adding a glossy clear-coat works as well.

    Use pva mold release on the mold when making the cf part.