Outside mold - Cast Plastic

mjgreenwood

mjgreenwood

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On my first couple of builds I noticed a hump in the upper bout on my uke bodies. (this would crack on some sides and really showed up on binding). A product of a bad home made template.
I also had a problem with the fit of my bending mold profile to the outside mold, not very exact.

So I went from hand drawn templates to acrylic from Kevin Waldron. Huge difference, I would recommend their products to anyone.

The problem I have how is making a good negative for my outside mold, I tried a variation of Mr. Howlett's method and the sand to the line deal. Neither was getting me to an exact fit. After scrapping the last set I want exact :)
So I'm trying a cast plastic mold (hard aluminum filled stuff). Has anyone tried this?

At first I was just going to cast a half to create a router template but then figured I'd cast the entire mold. Ideas? Waiting for the products delievery.
The plastic will hold a screw and is millable and hard, any pro/cons?
uke.jpg
 
Are you using the Devcon F type aluminum?? Man, that's gotta be expensive. I think that a well made outer form and good fitting inner/spreader made from good birch ply is as accurate as you need. For the way I do things, I have to relieve certain parts of the spreader to get things to fit tight. Curious to know how it turns out for sure.
 
smooth-on makes the product, Task-18. I'm waiting on a possible freebie from a brother-in-law otherwise It's gonna be about 40-50 bucks in material. If it works I'll be able to pour multiple molds consistanly without the fuss. My spreaders will be mdf or birch.
I can also pour a 1/2" thick template and then use that as a router guide. That would only be a few bucks but then I'd have to route all the peices to the mold. It may cost more in material but the time spent trying to get the mold just right has some value as well.

Like I said after the first mold and forms not coming out right I'm excited to have a dead on set of tools. My first two bodies came out really well with the exception of the buldge from the bad form. If I get this right I think I'll be on my way to move my learning focus to neck work (and finishing something).

The other big difference is the bening mold. I switched from bulbs to the heat blankets. Got away from the aluminum bar type mold to a solid ply mold. So I souldn't have any buldge.
 
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I think you're over-thinking this whole thing. We don't have any problems with hand-made molds and bending forms. Frankly, it's all in the bending...get that right, and you won't have to over-engineer molds.

This isn't rocket surgery...
 
If you want to go that way.....only use 1/2 of the body to make the mold. Cast two pieces and you'll have a symmetrical mold. Any slight difference between left and right will show up if you cast it all at once. No one notices if your waist is too long or the bout is too pointy if both sides are the same.

Best way to make a mold is to make one template in 1/4" MDF or plywood. Then cut all the parts to that template with a router and guide bearing. No one notices if your waist is too long or the bout is too pointy if both sides are the same.
 
Considering how many millions of ukes, guitars, violins, etc. have been successfully made using hand made wooden molds (and I have some out of the old Chicago Harmony factory that probably accounted for thousands apiece...), I just don't see where the problem is.
 
Hmmm, it occurs to me that you may think that the mold is there to achieve the shape of the instrument. If so, not so... The bending forms or hand bending on a pipe is what establishes the real shape of the sides. If you have lumps, they're there from the bending, and that's where you need to improve your methods. The mold is really just an assembly fixture, not a mold in the same sense as plastic injection molding or equivalent. The traditional Spanish method of guitar making doesn't even require a mold for building the instruments; they're built quite free form with the sides hand bent. Of course it takes practice...
 
I've made dozens of molds and forms in my time, all precision made because in my regime they act as reference points. Free form building as in the Spanish heel method is highly skilled and still results in some asymmetry (and there is nothing wrong with that). Some of the greatest American guitars made by the Larson brothers are nearly all asymmetric. The increasing mechanisation of what was once hand processes is forcing us to demand a degree of machine-like precision in hand processes.
 
I was just reading through the original post and thought that you are looking a the mold (assembly fixture) as the way to achieve something that it has very little effect if any on.

As Rick has just said, it's your bending that is the problem. You most certainly are not going to smooth out any curves with your outside fixture. It's only going to hold it in a approximate and stable shape while you install linings, top and back. That shape is directly related to how well you bent those sides and cut them to correct length.

I build in the Spanish Method with no mold at all. Bends need to go smooth, correct and little to no tension, then glued to the soundboard. If my students can do it on their first instrument, pretty much anyone should be able to as well.
 
Pete Howlett said:
I've made dozens of molds and forms in my time, all precision made because in my regime they act as reference points. Free form building as in the Spanish heel method is highly skilled and still results in some asymmetry (and there is nothing wrong with that). Some of the greatest American guitars made by the Larson brothers are nearly all asymmetric. The increasing mechanisation of what was once hand processes is forcing us to demand a degree of machine-like precision in hand processes.
Click to expand...
Don't call e'm Molds Pete it just encourages that lot over there.:D
I'm just finishing off a new Soprano mould(correct English spellink):)..it's my sixth so far...and this one is spot on and should last my lifetime out...I'm just waiting for some more bits to finish it and it will be ready to go into action next week.
*edit*
And what Allen just said I agree with as well "you gotta bend e'm correct before they go in the MOULD". :)
 
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You could always get your template CNC cut from a single piece of 1/4inch hardboard. That would give you both male and female pieces. You would then use them to make your large moulds.
 
I had mine cut on a CNC by a friend. I gave him the drawing then asked for a profile piece, an outside mold and blanks for the side bending mold that accounted for the side thickness. Total bill was $60 plus cost of 3/4" baltic birch ply. He also drilled registration holes so everything could be lined up. Much easier and saved me a bunch of time.
 

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I understand that the shape comes from the bending, my original bending form (bar type) had a imperfection in it. This same imperfection came up in the original mold.
So I just want to take all the little imperfections out of this that I can. I also wanted to be able to make a nice master set for routing additional molds without any fuss in the future. My new bending mold is spot on and I'm excited for a warm day to get out there and give that a go.

Making the casting mold took all of a few minutes at the router table with some scrap MDF, so I think it's work a try.
 
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