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What do you use to make your molds? Ply or MDF? and why?

Im sure there was something else I wanted to ask.... but my memory is failing in my old age

Thanks

Jonny

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"Anything that happens, happens. Anything that in happening causes something else to happen, causes something else to happen. Anything that in happening happens again, happens again. Though not necessarily in that order." Douglas Adams

Birch ply. Stability and longevity. When I build a prototype I first build a MDF temporary mold. Once the design becomes part of my product line I either make or have John Hall make me a production mold or molds from birch plywood. Ply has lamination in slightly apposing grain orientations. That makes plywood very stable. MDF is coated on two sides and the ends can and will adsorb moisture and swell if not sealed and will not hold screw for mounting latches well at all.

Oh and here is a trick that will sooner or later save you some grief. Line the inside of your mold with .005” thick Teflon or UHMW tape. This will prevent any un-noticed glue globules from adhering your rim semi permanently to the mold

And MDF is just heavier and a bit of a pain when lugging the mold around, sanding on a dish, etc...

Plywood for me from now on. I have two molds out of MDF and will be re-making them both out of plywood next time I build either of those body sizes.

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Does it have to be birch ply or will the regular lumber yard, one good side variety do?

birch is more stable and you want none or at least as few voids as possible and to get that grade of ply it is likely be birch.

I use 3/4" nominal birch plywood (which measures out to 11/16" thick), NOT baltic birch mind you. The difference being the birch is good both sides, but there are slight voids between the laminations. The baltic of course is effectively with out voids.

I think the birch plywood is just fine. No need to spend nearly double $$, especially if you put a couple of seal coats of zinsser's on it.

I buy birch plywood for $45 a sheet (4'x8'), and Baltic birch is $55 a sheet (5'x5')

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"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Thank you gentlemen! I saved a bunch of regular ply from my adventures with shop building thinking I could make some molds for some shapes that I came up with. Looks like I should get some birch.

thanks,
most of the ply i've seen/bought round here (not that i have been out of my way to look at it mind you) is very uneven in flatness, i sense problems here?, oo and how do you know if its birch ply or not?

also, for you little jigs and fixtures, do you prefer ply or mdf (im guessing the same as above)?

thanks

Jonny

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"Anything that happens, happens. Anything that in happening causes something else to happen, causes something else to happen. Anything that in happening happens again, happens again. Though not necessarily in that order." Douglas Adams

A tip for you Hesh my friend,

When you buy a sheet of ply wood, get them to cut it into 4 pieces at 2'x4'. It shouldn't cost you anything, it's easy to fit in your car, and easy to work with in your shop versus a full size sheet. No likely you'll be making anything bigger than 2'x4'. That's what I do

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"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Johnny, it really depends on the jig.

MDF doesn't hold screws in the end very well at all, so I don't like it for jigs that I have to make 90* corners on.

I like MDF for patterns though (cheaper than acrylic) and it works well for router patterns (seal with shellac though)

I would think Baltic birch would be less expensive for you over in the UK.

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"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

I really have to say that I like Todd's outside mold making suggestions. Makes for a great mold Thanks Todd.

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"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

construction grades of ply may be unlevel. Cabinet grades will be fine sanded on one side or both. Birch has a whitish tent as compared to pine and will have few knots. any fixture that has anything screwed to it needs to be ply. MDF is a heat formed/compressed matrix of almost dust like fibers and resin outer finish, cut MDF and you expose the dust like matrix. This is not suited for scews that will be under any laoding. It is really intended to be a glued togather product. because the fibers are comressed with no real grain structure left when you pre drill then drive a screw threads into it, the fibers have nowhere to go but to push outward and deform the compression. Saturating the pre drilled holes with CA helps with how long the scews will hold in the powder like matrix but sooner or later they will tear out if they see loading on a regular basis. MDF is well suited where it is the compressive load member of an assemby. It does not work very well at all wher it is bearing shear loading screws or other attachments and fasteners.

For just under $100, John Hall made me a custom shaped mold using voidless birch ply. It came in last Friday, and I couldn't be happier that I decided to have him do something seemingly simple for me.

Ditto: I just got a mold from Blue Mountain (John Hall) and couldn't be happier.

I make molds from whatever scraps I have laying around, usually particle board. I don't think I have seen any plywood over here that was so awful that it couldn't make a serviceable mold out of it. What's the big deal about voids in this application?

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I'm an idiot, Blues Creek, not Blue Mountain...I mixed two names up. Sorry guys...

No prob Joe my friend - I knew what you meant..... John Hall has great molds and all of mine came from him.

Thank you all for sharing your knowledge with me

I have just finished my exams, so I should have more time on my hands for atleast a couple of months, so i shall have a little experimental mould making session as soon as possible.

Thanks

Jonny

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"Anything that happens, happens. Anything that in happening causes something else to happen, causes something else to happen. Anything that in happening happens again, happens again. Though not necessarily in that order." Douglas Adams

Given birch plywood prices around the SF Bay Area, I find it's actually a bit cheaper to have John Hall (Blues Creek Guitars) make all my molds and bending patterns for me.....and it's a whole lot simpler too! John does a great job on them.

BTW, I hardly ever use MDF. It warps far too easily and oh, the dust!!

Dave F.

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Cambrian Guitars

"There goes Mister Tic-Tac out the back with some bric-brac from the knick-knack rack"

having looked at John's website, i think i might see what he can do. his molds look very good and seem reasonably priced (i think ill pick up a radisued dish at the same time).

it just appears to be a matter of find the correct plans to go with the molds. Can any one help me here pls?

Thanks for all your help and advice,

Jonny

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"Anything that happens, happens. Anything that in happening causes something else to happen, causes something else to happen. Anything that in happening happens again, happens again. Though not necessarily in that order." Douglas Adams

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Ask John, he'll steer you the right direction for what you are looking for.

Don't forget the adjustable type mold espoused by Charles Fox either. It has some strong points. I've used both and actually like the adjustable better. I'm probably in the minority though.
Terry

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