help!!!! I don't know anything about mold making!

Hello, I need advice on making a mold.

I build cedar strip canoes, and I'm trying to make a jig that will allow me to cut the rolling bevel on the stem for a cedar strip canoe. (I know this would be an utter waste of time and material for a handful of boats, but I think I know what I'm doing...

This would likely be straightforward to many of you, I assume, but I don't know anything about this. I started with the idea making a mold and casting the part, but after making one I decided that it would be much better to make the part out of real wood.

But I had mold release problems when I made the mold and when I made the actual part, and I don't understand why. I thought I did everything right, though I am essentially making everything up as I go.

I made my plug out of foam covered with bondo. I was very careful with it, sanding it to atleast 400 grit (maybe 1500? I don't remember how far I went) and applying several coats of Honey Wax mold release compound. The plug was the shape of the bow of the canoe, and the mold I made just wrapped around to tip and down 3/4". I made the mold with fiberglass cloth and epoxy resin.

When I tried to remove my mold from the plug, I inadvertently destroyed the plug; I had to pry the bondo out of my mold. It all came off eventually, albeit in 1/4" chunks.... I had used strips of ABS plastic in the plug, and the epoxy released from that no problem. But the bondo was another story completely.

When I made my final part in this mold, and I had the same problem. My part came out of the mold, but it was very difficult. I had carefully sanded the mold beforehand, and I waxed it liberally to prevent (I hoped) the problem I had encountered beforehand. But I got a very painful and time consuming release.

So I'd like suggestions on what could have gone wrong and what I can do to prevent it in my next project. I don't think it was heat--I made samples before I made my part, and the test mixture really didn't get that hot when it was curing; and the sample was thicker than the part I was making (I used MAS low viscosity epoxy with medium hardener).

I need to know what went wrong because I'm in the process of designing a different method of making theses stems. Now I'm intending to laminate the stems with wood veneer and epoxy, and after the part is cured I'll cut the rolling bevel with the help of a router and jig.

The jig will match the surface of the canoe, the idea being that if the router is sitting on the curved surface of the canoe, the blade will accurately cut the rolling bevel I need.

I'm making the plug out of foam and bondo again (it's ~3' x 3'). This time I want to make the mold out of pourable urethane foam, but I've never worked with the product and don't know what the best way is to get a smooth surface. It's a one time use mold (hopefully??), but I anticipate needing to do some fine tuning to it before I make my final part. Also important is a release agent for the foam...

I'm looking for any advice, suggestions, products, links, etc. to help me successfully complete this project (the first time???). I hope I'm not redundant in posting this. I've done some looking, but I haven't found much information directly about this type or project and the problems one will encounter with it.

Let me know how badly I've confused you, and I'll try to explain.

thanks in advance!!!

 

A simple solution for a male plug is to cover it with a smooth stretchable layer. The expensive solution is to get some very thin silicone rubber sheet. It stretches in all directions so can be clamped tight over a male plug. It has very smooth surface finish. Neoprene rubber also works but the surface is slightly porous and needs some wax sealing otherwise the epoxy will stick.

The low cost method is to stretch builders plastic sheet over the plug and heat shrink it down. It also is best if you stretch it when heated rather than just requiring it to shrink all the way. It will tend to get tiny creases if it is oversize to start with. You can use duct tape to seal on edges or tape it to the plug.

For a female plug there are mould release agents available. There is a thread on mould release. Someone else may be able to point you to it.

The place linked here has good gear for mould making so might provide some idea of what is available:
http://www.solidsolutions.com.au/index.php

Rick W.

 

Fredyg, you need some pva release agent, you can sponge it or spray multiple mist coats to prevent a "stick up" although the process your developing seems a bit extreme for a canoe, maybe a spokeshave & follow a "bearding line development" for the stems & if you don't wanna show the endgrain of the strips laminate on a capping when plankings compleat. All the best from Jeff.

 

One big problem is the Bondo, it makes a terrible mold surface. The surface needs to be sealed with something like a sandable primer (polyester or epoxy) or it will be attacked by the resin resulting in the sticking you experienced. More wax and PVA will also help.

 

Thanks. The mold release product I used didn't mention a pva--just said to apply several coats and buff... I'll give it a go; anything will be better than the last two attempts.

My process might be extreme for one canoe, but maybe not if I build enough of them? I plan to produce a wood/ epoxy canoe that is more competitive with high end fiberglass and kevlar canoes than hand made cedar strip canoes usually are (usually they're twice the price of a comparable manufactured canoe). The only way I know to accomplish this is to completely re-think the construction process, which means different methods, molds and techniques. I'm expecting to complete a boat in 15-20hrs, which I hope will prove to be a conservative estimate. This compares to 40-60hrs per canoe with traditional techniques.

Now, if the economy will just cooperate with my plan....

Darren

 

Fredyg, good luck with the economy, I've got a "job" after 20+ years as self employed, with your mold/plug surface pva isn't compatible with all waxes so check, years ago we used to mold off all kinds of stuff(like masonite etc) & would seal with shellac then wax with johnsons floor(now has silicone so no longer good) wax & simply wipe the pva solution on with a damp sponge & mold off it as soon as the pva was dry, the wax I use now is ceara wax which is like a carnuba type so far as I know. Its gunna be tough to compete on a labour basis especially in the surface prep & finish on the cedar strip, maybe make a mold too for a contact molded boat off your canoe to give some price options? Again all the best in your endeavours & make sure you still do it for the enjoyment & pride of workmanship too 'cos commercial realities & customers can suck the life outa ya, regards from Jeff.

 

We have used Awl Grip 545 primer on many molds and plugs and it works well. It seals bondo nicely and can be sanded to a very smooth surface or even polished. But as has been mentioned, you must seal the bondo some way as it has a very porous surface which is unsuitable as a release surface.

Jimbo