Build question

It would be a daunting task to make a full female mould for a single craft. Wonder what it would cost for just the mould?

 

The big question is the desired surface finish of the boat. With a male form there is usually fairing and painting involved. A female form produces a smooth, fair finish because all of the fairing and sanding was on the plug. A direct machined female form will need fairing and sanding, either on the forms inside or on the finished product.

For a small one off a proper female mould is not that much more expensive as long as the labour is free. The plug is wood or styrofoam, then it's fairing compound and the mold is some polyester resin. Good quality sanding paper is going to cost more then the materials for the plug and mold.

 

What about making a mold on a 3-D printer? I know it would require a huge machine; also typical printed surfaces aren't smooth but 'pixelated'. There is, though a printing process that, IIRC, is done submerged in some kind of fluid which causes the deposited material to sort of melt together and produces a smooth surface. Lotta $$$$, of course.

 

It isn't so bad if you can accept that the hull may require a coat of primer before being sprayed.Given the size of a Moth you could probably do it from two 8X4 sheets of 50mm MDF for example or if you wanted to use tooling block it would cost a fair bit more.As I posted earlier,you begin with a slab large enough to contain the hull-plus a bit for stiffness.Subtract the hull from the block and divide the remaining shape into slabs to suit the material thickness nest them within the confines of the sheet(s).Use a CAM program to determine the roughing and finishing toolpaths and put the pieces on a CNC router.Set up extractor-press green button and wait.

Once machined,carefully assemble and apply sealer and/or paint to suit your requirements after the glue has cured.If you sit the fairly heavy female mould on a couple of sheets of melamine faced chipboard you will be able to seal a vacuum bag to the surface once the joint has been sealed to stop leakage.Coat with release agent and apply carbon,when cured extract your Moth.Obviously there are a lot more details than this,but it is an outline of the process.

As for cost,how long would it take to do the job the traditional way at commercial rates and what would that cost?The 3D printing alternative takes you through many of the same steps and you still wind up with a surface full of fine ridges that you have to finish and it takes an eternity to produce a 3D print of any size,always supposing you can get access to a large 3d printer.These days 3 axis CNC routers are relatively common,finding somebody who can use one to produce 3D shapes may be a hurdle as quite a few of the operators have only used them for panel processing and may not have the software for 3D work.It has become a lot less expensive than it used to be to get hold of the software and the computers in general use these days don't take too long to generate the toolpaths.

 

I'm personally not very excited about the prospect of CNC female.

Molds are generally started by applying a thick gel over the well waxed plug followed by sufficient lamination schedule incorporating structural stiffeners (steel pipe). This is what provides the flawless surface of the final product.

In my view this cannot be achieved using CNC to create a female. CNC is often used to make plugs though.

 

I have done it both ways and for something the size of a Moth that gets painted after moulding the minor surface blemishes won't matter.If you use a good tooling block you can fairly easily achieve a surface that needs light sanding with 320 grit paper before being sealed and waxed before moulding.I sincerely doubt that anybody has ever made a Moth mould with steel pipe reinforcement.That happens with much bigger and heavier boats that anticipate a gelcoat finish direct from the mould for production purposes.

 

Pipe reinforcement for a moth plug would indeed be unnecessary. In fact a more flexible mold in this case would greatly help with release.

If I was carrying out this project using carbon fiber I'd take the time to make a plug. At 3.4m LOA this isn't a huge task given that it has a fairly simple form.

 

Plenty of people make molds using CNC routers, typically only good for a couple of pulls (depends on plenty of variables though). Just split it down the centerline to mill in 2 pieces (or 3D print) sand (much easier if the mold is split), seal then join the pieces (using the indexing blocks you also machined in) and you’re ready to start.
Just makes sure you can actually pull the part (or break apart the mold to get the part out also works)

 

It is relatively easy to directly build the mold. And skip the plug stage.
Moth shapes are pretty simple, and most important, small.
You can strip plank or press plywood into shape, fair and glass the inside and paint with epoxy primer.
We have also built boats with the outer skin laminated to the core and then pushed into female form. Some foresight such as using masking tape on the tool surface to relieve the skin where you will be taking the panels together.
SHC