laminating by vacuum-forming

[drcrash]

I've been thinking about vacuum-forming a sandwich construction in three vacuum forming steps, applying a spray adhesive in between.

The idea is to construct something stiff and light, similar to foamcore board, in the desired shape, using a thin layer of HIPs or ABS followed by a thin layer of extruded polystyrene or EVA, and another thin layer of HIPs or ABS.

The first problem I see with this is what to use as an adhesive---something that will cure pretty quickly, but not instantly, and won't suffer too much from a brief period of being mashed in hot plastic. (Or which might benefit, if it's the right thermoset; if it accelerates the cure, but not to the point of instantaneousness, that's okay.

The second problem I see is warpage. HIPs contracts significantly as it cools, and that sounds like bad news. If I'm forming around a male plug, I'd think that would be self limiting while it's on the plug, but I'm not sure what would happen when I took it off. (Even if it didn't warp, the residual stresses mike weaken the part.) But maybe I could fix this with proper annealing or something...?

Has anybody else done this? Does it work? Any advice?

Thanks,

Paul

 

[Pud]

Hi,

Cannot see why you would want to do this - what's the application?

Why not use ABS aheet? Be much cheaper. e.g:

http://www.spartech.com/plastics/royalite.html

There must be many others too...

Rgds

Harry

 

[drcrash]

Harry,

I have several applications in mind, all of which require a certain rigidity and light weight. (And ideally the ability to take whatever paint you want and not have it crack or flake off because the lightweight plastic flexes too much.)

One example would be for RC model planes. Hollow foam is great for wings and fuselages, but needs a tougher and preferably rigidifying skin. You can hand-lay thin fiberglass over it, but that's a pain. And it would be more rigid if you had a thin coat of hard plastic on the inside, as well, so that a skin is in tension whichever way the foam flexes, limiting the flex. Solid ABS thick enough to be rigid enough would just be too heavy, too expensive, and too difficult to form on a low-end vacuum forming setup.

I'd like to be able to do similar things, mostly for temporary sculptures of various kinds, which are lightweight and portable but large and apparently massive, even monumental. They can have a few internal spars, but the skin panels should be self-supporting, and shouldn't deform so much in a breeze that they crumple; something in the ballpark of foamcore board seems about right.

For other things, I need something a bit stronger, but still fairly rigid, cheap, and light; something like 93 or 125 mil styrene over 1/4" or 3/8" foam seems about right for those. (And about the most an inexpensive vacuum-forming machine can handle.) One example I'd like to use that for is making a small production run of a 3D router pantograph I designed, for scaling sculptures up or down. It's essentially a hinged system of tapered box beams, which should be very rigid and fairly strong but still quite light. (The "router" in question isn't full-size; it's a little rotary tool or maybe a spiral saw.) The box beams could be made out of something roughly like thick, rippled foamcore with fiberglass tubes glued inside the edges.

In general, I want to try to expand the range of things I can make with a simple vacuum-forming system, by laminating various materials to control strength and stiffness, for one-offs, prototypes or very small production runs.

Paul

 

[dwightdixon]

Seems like you would be better off injecting a polyurethane foam in between vac formed rigid skins. It is already done alot. No need for adhesive and you get the density you want. Expanded styrofoam bead is also commonly used for boat hulls with rigid vac formed shell and deck. Prelaminated plastic sheet is also commonly used for canoes. Lots of proven options already out there. Why reinvent the wheel. You need to look around.

 

[drcrash]

I have consider injecting polyurethane, but I don't currently see how to do it with simple, cheap machinery and very simple tooling. I can currently vac form thin sheets, including foam sheets, and if I could simply laminate them a la vacuum bagging, that seems acceptable for many purposes.

The problems I see with injecting polurethane are constructing two forms with the proper spacing between them, holding them rigid at the right spacing, and ensuring that the foam flows all through that thin, compound-curved gap. Perhaps those things are easier than I think, though.

My current technique for easily enlarging a form slightly is just to vacuum-form EVA foam over it, and then vacuum-form styrene over that. This is less than ideal, due to thinning, and of course it loses a lot of detail, but for my purposes it's usally okay, or even a plus. (The loss of detail often amounts to desirable smoothing of a rough plug. Sometimes I want more detail, but it should be added later anyway, because several objects have the same basic form but each one has custom details and finish.)

So if I could just laminate things like I already make, with equipment I already have, I'd be done. I could vacuum-bag laminate them together, but since I'm already vacuum-forming them, it seems much easier to just combine those steps, using a bit of glue. (And use a different kind of foam sheet, usually.)

I'm sure that for larger-volume manufacturing, there are vastly better ways to do it, but I usually don't want to make more than a half dozen of a particular form, or maybe a dozen. Sometimes just one, either because I'm creating a unique object in an easily-sculpted medium and using vacuum-forming to make it thin and light, or because I'm making an intermediate object that will be modified and used to make another plug.

That's why I still have 2 questions: (1) what kind of adhesive is likely to work, and (2) how to control warping, if possible.

If injecting polyurethane would be better, I need to know how to make it very easy and very cheap for very low volume production.