Masks and Stencils
- When we paint a window we often put tape on the edges of the glass, so we won't end up painting the glass
- this is often referred to as a mask
- it protects selected areas
- There are a number of different forms of masking
- you can buy or cut a stencil in the desired pattern
- you can either hold it over the surface or tape it down
- you can apply paint with a brush or a spray can
- you can usually reuse a stencil
- you can cover the surface with a protective tape or film (in art circle, this is a frisket)
- use a fine knife and cut out and remove sections of the film
- again you might paint the exposed area
- on glass you could etch the exposed area with a fluoride paste
- you could also sandblast the exposed areas
- you could electroplate a metal surface, selectively placing metal where you want
- metal etchings can be done in a similar manner
- a metal plate is coated with a varnish
- small tools are used to remove the varnish from selected areas
- the plate is then etched in acid
- the exposed areas become small grooves that can retain ink
- the plate is used to print
- a silk-screen is another form of a mask
- this is a cloth of finely woven silk (more likely nylon or rayon now)
- you can paint on a layer of wax or other coating in some areas of the cloth
- then you sandwich the screen (stencil )with paper and apply ink
- ink oozes through the exposed areas
- ink is blocked by the waxy areas
- the paper picks up a copy of your image
- often several different screens are made, each for a different color ink
- one stencil can make hundreds or thousands of prints
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- It is often desirable to be able to create masks in a more controlled manner (or with less artistic skill)
- it's obvious we are about to explore ways to create the masks photographically
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- The material of choice is a
photoresist
a typical photoresist is a solid polymer (large molecule)
- the resist is used to coat the entire surface we want to treat
- the material is light sensitive (usually only in the ultraviolet)
- it undergoes changes on exposure to light
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a negative photoresist generally becomes tougher on exposure to light
- the unexposed area is easily removed with warm water or with a solvent
- the result is a surface that has a mask
- some areas are exposed
- some areas are protected
- the pattern is a copy of the negative we used when we exposed the surface
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now we can modify the underlying surface
- we might etch it with acid
- we could simply etch the surface to a moderate depth
- this is the way photo-offset printing plates are made
- we could completely dissolve away the underlying surface
- we might electroplate and build up a new metal layer
- we might sand blast
- we would simply paint it if we wished
- we could expose it to chemicals that alter the surface
- (you can also coat silk-screen with a resist and make a photographic stencil for silk screening)
usually we finish by using a stronger solvent and stripping away the remaining photoresist
- There are positive photoresists
- materials that are easily damaged by light
- the areas that receive light end up unprotected
- the areas that were dark retain their protective plastic properties
Remark-
- Photoresists are capable of producing patterns of extremely fine detail
- this it the way all modern electronic chips are made
- details as small as 1 micron are routine
- We will work with photoresists, but we will not attempt that fine a resolution
- a. We will make a printed circuit board
- http://www.bgsu.edu/departments/chem/faculty/endres/projects/photoresists.htm
- b. we'll try to etch a pattern into a bottle or a glass plate
- c. we will try to electroform a thin metal object
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