Building molds for the tail cone

The first group of molds that I will make is for the tail cone. There are 5 separate molds involved.

The tail cone mold will consist of three separate pieces: left lower section, right lower section, and the third part is the curved upper surface of the tail cone.

I also added two access panels to my design, for airing up the rear tire and for access to the rear air shock. I will make molds for each of these panels. So I’ll be making 5 molds total.

I began with the lower right side of the tail cone, since there are no access panels located there. I began by creating 4 inch high walls of polyethylene sheet. These will be used to create the mold parting areas. I used hot melt glue gun to adhere chunks of the polystyrene foam onto the gel coat, and then hot melt glue to hold the polyethylene. Lower right I have a small clamp holding the 2 pieces that meet there, having just used a heat gun to soften and curve the polyethylene.

Getting the lower right side of the tail ready for mold making.

Truth be told, I was sort of dreading this part of the whole process. Cutting those poly sheets into a reasonably close contour to the body I hadn’t had luck with in the past.

This time went much better. I had purchased a bench-top band saw last year, and I mounted it and a bench-top belt/disc sander onto a rolling cart. So I rolled the cart right over next to the tail of the Zephyr and plugged it all in. Made my markings and cuts much faster to have it right there, and then hit the belt sander to smooth out the contours.

I then used molding clay to fill in the final gap between plug and poly. And I used quite a bit of the clay pressed into the backside of the poly sheet. Without the clay as backing, it is very easy to flex the polyethylene sheeting when sealing that gap, and then it flexes back on you, ruining the nice edge you just made….

To press the molding clay into the corner and create a nice clean parting area I simply use a popsicle stick and cut it to give me a wooden scraper blade. Occasionally I have to cut a new edge. And softening the clay in the shop microwave for a minute or so makes it a lot easier to work with.

Forgot to mention my waxing process!

I applied 4 coats of release wax to the entire tail cone area first. Then I glued the foam chunks on, put on the molding clay. Also, flash/release tape on seams and corners of the poly sheeting. Mask off nearby areas of the plug.

And then 2 more coats of release wax, followed by a light misting of PVA; I use FibRelease. I’m also making the access panel molds at the same time. Once that was dry, two coats of orange tooling gel coat.

Once the PVA was dry, I sprayed two coats of orange tooling gel-coat. I’m not concerned about overspray to the rest of the plug; it’s going to get re-sprayed gray gel coat next.

Two layers of orange tooling gel coat.

As I’m making these molds I’m also trying a few different things, or perhaps I should say new to me things. For this first large mold I used carbon fiber veil as a primary layer, applied next onto this orange gel coat. It went on very easily but I really doubt it adds any structural benefit.

Carbon fiber veil on the left, 4 ounce fiberglass cloth on the right

I want and need something that is going to bond well with that gel coat, to give it strength and structure; sorta like rebar in concrete. I recall Stephane at Katanga pointing out that when they build a WAW body, the first layer after the gel-coat is a thin layer of fiberglass cloth.

I had some 4 ounce fiberglass in the shop, so I tried using that when building the lower left access panel. No matter how much I tried, I could not get air bubbles to stop forming right at the sharp edge. Not surprising to see them, but they just wouldn’t cooperate.

After 10-15 minutes of fussing with it, I finally pulled that resin soaked piece off the plug, and quickly stuck carbon fiber veil on there. Far easier to work with, but again, not much structure.

But in the end the access panel molds came out well. That vertical black line on the right mold transferred from the plug, it marks where the rear wall will be. It won’t transfer to any pieces made.

Molds for the access panels

I made these with two layers of 3K CF cloth after that starter layer, and finished with CF veil on the exterior. They’re super lightweight, will be strong enough for making a couple pieces; I’m not going into production!


I’ve decided to add a layer of CF veil as the final layer on all my molds. It’s not very expensive, goes on easily, and gives a nice finished look to the mold, as well as making the outer surface of the mold easier on my hands, gloves, etc.

On the left is an older mold I made with CF fabric, no finishing layer. On the right is finished with a layer of CF veil.

The mold on the right is finished with one layer of carbon fiber veil; the left has no finish layer

I made a couple quick test pieces from the molds, to check fit. I found this blemish on the part I made, but the blemish wasn’t noticeable to me on the mold version. Those red marks are indicating where there is a noticeable dip. I will wet sand that area of the mold to try and reduce that dip before making my final piece. (a “positive” or high area that I can sand down on the mold, will help to remove a “negative” or low area on the finished piece.)

White test piece I made to check for defects; found one! Some judicious sanding is in order.

Construction of the large mold went well; I made sure to spend a lot of time getting the CF right down into those sharp corners. Two layers of 3K CF, tried to apply them at a 45 degree bias, and finished with CF veil. I also added a cross of UniDirectional or UD carbon fiber, to really stiffen the mold.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s