Thread: Freeform Vacuum Forming

lots of model train geograhy is just made out of papermache. stick sheets of newspaper together with pva glue.

Not exactly the look i'm after. I've done stacks of site models before but what i want is basically a sheet of plywood that conforms to a mold of my site. In essence, a single sheet of badly misshapen plywood that looks strangely like my site

i would have cnc routered out of a sheet of foam, if i had a cnc i would do it for you but i dont.

Forming compound curves as you want to do requires lots of heat as well as pressure. The pressure is usually applied mechanically rather than by vacuum, but if you could somehow apply the heat to your form it cold be possible.

Originally Posted by AlexS

Forming compound curves as you want to do requires lots of heat as well as pressure.

Thanks for that. One question... Is this heat and pressure needed even if you are gluing down one veneer layer at a time, or is it only necessary for situations where you are trying to press an already laminated sheet of plywood? hope that makes sense

Johnny

An interesting technical challenge, but your ultimate objective escapes my comprehension.

In practical terms, a single planar material cannot be deformed into non-planar without serious distortions and locked-in stresses, which may tend to return it to an un-stressed state.

Scale models of block forms typically exaggerate the vertical scale by a factor of two, as an aid to understanding; you seem to seek this AFAICT.

Are contour models now out of favour? They've historically been used to enhance understanding. And, as Bob says, paper mache would easily provide the shape itself. Leaving the contour edges exposed would serve both purposes.

Notwithstanding all that, you could achieve your objective by making your own plywood from veneers, either strips or single sheets. In either case, a stout negative mold will help to clamp the pieces together while the glue cures. And a stout positive, cast from the negative, will help even more. Concrete has usually been used for these molds. Boosting the vertical exaggeration by another 10% should allow for elastic rebound of the cured sandwich. Either strips or single sheets should be softened by steam, and cutting the strips will be purgatory. I'd suggest at least 3 layers of veneer, more or less perpendicular to one another.

This sounds a little like repairing a steam locomotive under way: Technically interesting, but not much fun.

Cheers,
Joe

Perhaps another way to think of it is to imagine that you are using paper instead of veneer (ie the paper represents the softened veneer). Could you achieve what you want with sheets of paper? Would the paper require cutting in order to get the distortions right. Would it require many many strips of paper to achieve your contours?

If it can't be done with paper, I can't imagine it being done with veneer. You have set yourself a huge challenge.

Originally Posted by WoodWad

Thanks for that. One question... Is this heat and pressure needed even if you are gluing down one veneer layer at a time, or is it only necessary for situations where you are trying to press an already laminated sheet of plywood? hope that makes sense

Johnny

I can't answer definitively, as I've never tried it, but I'd imagine not.

sounds like you're making it really hard for yourself WW,
(unless this project specifically requires formed ply)
I've done a handfull of site models for architects (none in wood)
but stack laminating on the contours and shaping would be far easier than all the rigamarole involved in vac forming and IMO the end result would be just as interesting

Fibreglass and resin??

If you are going to carve a base to mould over, I would be inclined to do the the moulding in fibre glass matt and resin. You could use a foam base for the former, epoxy resin for the base coat on the foam (as it doesnt harm foam), a fill coat of resin to hide the weave of the cloth, then melt the foam blank with thinners to eliminate it.

Easy to do but time consuming, sheet of ply for a flat base, hot glue gun/screw/nail on your high points(cardboard/strips of wood... what ever you can find that will hold staples).
Buy the cheapest 2 way stretch jumper material, some fine chicken wire and some spray adhesive(dont $kimp here), 1 or 2ltr's of fibreglass resin and a 2ltr tin of car bog.
Staple down the wire bending it to the desired shape as you go.
Spray the "rough side" of the material then the wire, wait a few minutes then lightly stretch the jumper material over the model stapling it around the back or side edges of your base padding/smoothing it down as you go.
Mix up a batch of fibreglass resin/hardener and paint the jumper material till its soaked, give it another coat once set(you can add some fibre matting for strength if needed).
Give it a clean up with some heavy abrasive(flap disc's on grinder are the quickest method)you may need to touch it up with more material soaked in resin if you cut through by accident, mix up the car bog and smooth it over using your hands(latex gloves might be handy here ) as a squeegee, let it set and rough sand it to shape then repeat the car bog again and sand it with a finer grit to finish it.

if you went to my hardware store and looked thru the bracing ply rack They are different shapes
les