stretch forming visor/ vac forming alternative

Hi everyone, I've been mulling over how I'm going to pull off the visor for a guy manuel helmet I'm planning. I'm reluctant to take the plunge into vac forming so I've been doing some research & seen that model/rc aircraft builders use stretch forming to create the cockpit domes, so I'm wondering if there's any reason this wouldn't work on a larger scale? My theory is using a form of the visor shape, then laying the heated sheet of plastic over it & pulling a frame (not sure of the lingo but basically a piece of wood with the outline of the form shape cut out) down over that to make sure it conforms to the shape of the form. Does this make any sense, and if so would it work? My fear is it will either destroy the visibility or tear the heated plastic, any ideas/past experience?

So, if I understand your frame/outline cutout idea, you're looking to use this template to conform the sheet to the sides of your buck. Assuming you don't create scratches in the plastic as you push this frame through your sheet, your buck would have to be a very simple shape - you can't have any undercuts or else the frame won't make it conform to these features.

But your idea makes me wonder if there's a better 'poor-mans' way of getting the plastic to move inwards without using a vacuum setup...

In the little bit of vac-forming I've done, the 1st part of getting the sheet to conform to the buck is physically pushing the sheet down onto the buck until the sheet can make a seal. This is done with a rectangular frame (since the stock sheet is typically rect). At this point, the vacuum takes over and draws the sheet the rest of the way down and, more importantly, 'inwards' to conform to the sides of the buck.

If you still utilize step 1 of this process - stretching the sheet down onto the buck - then you just need to find a way to draw the sheet inwards without overstretching the top. Imagine your frame that holds the sheet is hinged so that when it's in the oven getting the plastic warm it's a rigid rectangle, but when you flip it upside down the hinges allow it to collapse inwards as you stretch the sheet down and under your buck. Might be a bit messy, might get folds in the sheet, and it might encapsulate your buck, etc. But, it might be possible.

So what you are describing sounds like Vac-forming without the Vac. I think the problem you will run into is that on a larger scale, you won't be able to keep the material at a universal temperature. That will make forming very difficult. Vac-forming is no where near as difficult as it looks. Anything 24"x24" or smaller can be done well with a shopvac providing the suction. You can build a simple base with pegboard and use your oven to heat the plastic. I built a device of terror out of scrap and electric griddles to use in the shop, but you don't have to go that far.

This is a write up for the first vac-former I built. I have since rebuilt it into a 24"x24" with two electric griddles, but this covers my first attempts in vac-forming: http://knowonesdesigns.com/2013/01/11/new-year-new-post-new-tool/

* I AM NOT RESPONSIBLE IF YOUR HOUSE BURNS DOWN AND YOU DIE. Just had to put that out there, liability issues and all.