Corner Fillets / Inside Radius Creation

I am trying to find the best / easiest / cheapest way of filleting the corners of a melamine mold (female mold with unwaxed polyester gelcoat & resin layup). I went out and bought the ingredients to make a few gallons of home-made modeling clay. After the first test batch, I'm not convinced that it will work as well as the non-drying store-bought clay that is traditionally used in mold making. My plan, as of now, is to use drywall mud and a popsicle stick. Anyone have experience with this technique? Anyone need a dozen boxes of corn starch and a 20 pound bag of table salt? Thanks in advance for any suggestions & advise.

 

I just use bondo for quick radii. You can bye wax fillets and a heated round tool that works quite well.

 

Polyester filler (Bondo) for hard mould or plasticine for instant one off result, warm it up & fillet it with ball ended tool.

 

When making rounded fillets, try a piece of alloy tube to the desired radius.
Sand gently the cut edge then drag it along the track that you wish to fill. The alloy will leave like a pencil mark in two places (where it touches the sides). Use these lines to mask using fine line plastic tape, and you will have perfect one off rounds when you use the same tube to scribe in the bog. The tape lines are perfectly placed to leave the minimum residue naturally. Peel off the tape over itself to cut a nice fine and even edge.

Hope this helps, if there is anything not understood, please ask.

 

Neat trick! I'm gonna' try that next time.

-jim lee

 

Yes , that does work well.

I have used PVC pipe in the same way.

Slightly damp rag wiped over the pipe prevents dragging the filler.

 

Sure, use any medium you like to make the fillet, but it is the line drawn for the masking that makes the job perfect.

You can of course mask out from the sides by the thickness of the radius, but the alloy tube simply draws that line for you.

 

Yes, that is good. Thanks Lubber. Most know this but long runs of tape are much better than short, never lay your fine line down on edge, particularly in a dusty shop. Reminds me, I can't buy fine line in my town because they always put a price tag on the edge of the roll and handle it roughly!
One more thing, When using Bondo, even when waxed, my part sticks in places. I don't want to spray PVA every time I need a small part - any secrets? ( I am going to try this Plasticine stuff. I Never use modeling clay because it dries out mid-job and creates a mess.)

 

Thank you for your ideas and suggestions. I have tried several methods and materials and had very little luck with most. Threw a few trials at the wall. Finally, I got a great result using Plasticine. My home-made clay was water based and it seems rather critical that it is instead the non-drying oil-based "Plasticine" style of clay. I custom built a radius tool out of an aluminum rod and a perfectly round cast iron ball. Heating the cast iron ball by dunking it into hot water allowed me to shape and smooth (nearly melting) the clay to a near perfect finish. With the tape lines in place (nicely pre-marked by dragging the ball along the corner as suggested) the result is gorgeous compared to earlier attempts.

 

And then you wax the flat surfaces, or they were waxed first as this stuff is soft?

 

Typically, I think the mold is waxed first and then one final (careful) wax coat is applied after the fillets. But in my case I am using a heated tool to apply the fillets and I suspect that would melt the wax instantly so I plan to do all of my waxing after applying the fillets. The clay that I'm using stays nice and stiff in colder temperatures so I'm not worried about waxing after filleting - although the clay is still malleable so I will need to wax very carefully in the corners. The clay almost has the consistency (and mine is blue so it also looks) like a racquet ball. With a soft touch it could be called "hard" but if you poke it with a finger it will dent and deform easier than a racquet ball - it's neat stuff.

 

Thanks, I'll get some.

 

The trick with PVA, at least what we do, is to wipe it on with a rag. We just use pieces of T-Shirt. You should not be able to see the green tint from it. Doesn't take much.

-jim lee

 

And you're sure that it's doing something beyond what the wax does?

 

I don't think I'll be using PVA. I figure the wax should be fine.