Material for keyboard overlays?

[John Ioannidis]

What kind of plastic were the keyboard overlays for the 41, 71, and 48 made from?

[Garth Wilson]

Lexan.

[Randy]

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Lexan

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That would be a trademarked version of rigid polycarbonate sheeting used primarily in window glazing applications. Care to guess again?

All HP calculator overlays, permanent and removable, were made from a specific type of vinyl. Did you just want to know the material or how they were made?

[Ángel Martin]

I'd like to know how they were made. Was a laser involved or the keyholes were punched through using a molded device? And the text/graphic, was it silk-screened or something simpler?

[Randy]

No lasers, no molding. Just screen printing and die cutting.

HP overlays were made from a 3M product line called "Scotchcal" which is still available. The product is comprised of the base substrates made of vinyl with various surface textures, the inks required for printing on said substrates and the pressure sensitive adhesives that might be applied after printing.

The process for the HP41 keyboard surround was:

1. Start with a clear, matte finish substrate
   
2. On what becomes the back, print the orange key legends
   
3. Print gold surround band
   
4. Print the "four corner bleed" black
   
5. Die cut the key holes
   
6. Apply the adhesive from a die cut carrier sheet
   
7. Apply to the calculator

If the overlay was to be removable, there was no adhesive applied and the die cut shape was perhaps a bit different with locking tabs, depending upon the application.

I suspect the entire process up to the last step was done on a continuous roll as that is how the base substrate and adhesives are sold - which makes for a cost effective, continuous process.

You'll find that you can only buy the materials in bulk and it is quite costly. For small runs, you'll do much better to ask 3M for references to companies that use their products to produce finished product to specification.

[John Ioannidis]

I didn't realize that the keyboard overlays were made the same way as the actual keyboard surrounding material. It makes sense, of course.

I knew they were die-cut -- anything else would not have made sense.

Obviously, the reason I'm asking is that I want to make my own (I have a hobby laser-cutter and a variety of printers). The main concern here is that paper is completely out of the question, so I have to find a material that I can print on where the print won't smear if I touch it with my grubby fingers, and that I can also laser-cut safely.

I'm investigating, but suggestions are welcome.

[Richard Ottosen]

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The main concern here is that paper is completely out of the question, so I have to find a material that I can print on where the print won't smear if I touch it with my grubby fingers, and that I can also laser-cut safely.

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Remember, that you can NOT cut vinyl or PVC with a LASER. They create hydrochloric acid when cut which damages the optics of the LASER.

A clear polyester film such as Mylar can be LASER-cut. Overhead projector film is one possible source of material although it may be too thin for your purposes.

[John Ioannidis]

I know you can't cut PVC. I'm not an idiot.

[Randy]

An afterthought: It is possible that in the high volume world of HP, the overlays were printed via rotary pad transfer which more suits roll stock than direct on material screening. I'm more familiar with the Scotchcal inks for screen printing so I did assume that was the method, personal bias there.

For low volume work, a steel rule die would be out of the question, cost wise. I don't think laser would work on anything other than thin metal (which has been done for the 41 by a contributor here). The one method that would certainly work for vinyl and suit low volume would be water jet cutting as that has replaced die cutting for many materials.

[Garth Wilson]

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Lexan

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That would be a trademarked version of rigid polycarbonate sheeting used primarily in window glazing applications. Care to guess again?

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A product I designed in 1995 to go into the panels of private aircraft used a Lexan overlay with dead-front annunciators. ("Dead front" means the area looks black until a light goes on behind, then the words or icons become visible.) We had Bay Area Labels (http://www.bal.com/products-frontPanelOverlays.htm) make it, and I talked with them plenty in the process. "Lexan" is what the material got called a lot, so I suppose that was the brand of polycarbonate they used. We also got quotes from Nelson Nameplate, and it says "Lexan" right on the quotes they gave us, so I know I'm not making it up. It was flexible though. The adhesive was made by 3M, and we stuck it down to an aluminum panel. We required that the adhesive hold in the temperature extremes the product could meet in storage, ranging from Alaska's winter to being parked on the tarmac in Phoexix's summer, cooking. BAL also could make clear windows of different colors in it and make membrane switches and probably other things I'm forgetting. They made a die to cut the outline and the holes we needed.