[sdiy] Silkscreen process

[Quixotic Nixotic]

On 7 Mar 2007, at 23:06, Hallvard Tangeraas wrote:

> There are a few things however I don't understand:

I have extensive personal experience of silkscreen. I don't do it any  
more but I used to have my own setup at home for many years. I have  
printed on metal, glass, fabric, slate, papers and plastics.

I have also printed etch resist on circuit boards commercially as a  
full time job, together with the solder mask and annotation, all by  
hand.

The silk screen itself is very fine. Normally sawtoothing due to the  
mesh will not be seen.

Stencils can be made in two ways. You either coat the screen directly  
in a photo-sensitive emulsion, leave it in the dark to dry, and  
expose it to UV via a film positive image, or you use what is called  
an indirect stencil.

Direct stencils are way messier to use, you need to get an even coat,  
and take way more cleaning off too, but are much cheaper. You can  
even make your own photo-emulsion using ammonium bichromate and  
soluble PVA glue (often sold as 'school glue' or similar). I've done  
it. A bit slow on the exposure time, but it works.

As the photo-stencil is held by the screen, your Os cannot drop out.

Direct stencils are prone to showing mesh sawtoothing, but on fabrics  
this would not be seen. It may not be seen on finer flat work either,  
it depends.

Indirect stencils are rolls of photosensitive gelatine-like but dry  
material on a clear polyester backing. You cut a piece off the big  
roll and expose the stencil through a film, usually using a mercury  
vapour lamp as the UV source. I expect PCB exposure units will work  
also.

Your film should be what is called 'right-reading emulsion-side up  
positive' for silkscreen. This means the photo side of the film is on  
top and black parts of the film will print. When you expose, this is  
in intimate contact with the polyester side of the stencil.

You then wash out the stencil, still on its backing, so again the Os  
cannot drop out.

While it is wet you place it under the screen and gently mop up the  
water from the top. Usually with a few layers of newsprint and a  
small roller. The idea is to squash the gelatine-like stuff into the  
mesh, without distorting the image.

If you degreased your screen correctly beforehand, when the stencil  
is dry the backing falls off and you then have a printable screen. If  
not, bits of your image stay on the film and you swear a lot.

I always had a good stencil hit rate, because I would use household  
chlorine bleach with 600 grit wet and dry sandpaper on the screen to  
degrease it first. The screen would still last many tens of times  
over. It all depends how careful you are.

Its expensive, especially now digital printing has taken over, to set  
up for silkscreen. But you can do amazing and accurate things with it.

Of course there are many other aspect to it too. You should be able  
to print using the pressure of your thumbs and forefinger only. This  
goes even for a largish piece of work. If you need to ride a seat on  
the top of the squeegee to get any ink through, then there is a  
problem with the ink consistency.

You should also not find it a messy process. If it gets out of  
control with ink up your arms, then whoaah! Stop and clean up. I got  
so I could silkscreen in my Sunday best suit, not that I had one.

Another BIG secret to silkscreen is called liftoff or snapoff. Your  
screen does not touch the piece of work you are printing on. You set  
up so it sits just above the work. When you pull the ink down the  
screen onto your work, (and you ALWAYS pull the ink towards you from  
the top, not flop it up and down like you are filleting a haddock or  
something) the screen should lift back off about 3-4 inches behind  
the squeegee. This action is crucial in getting a nice clean print.

You pull down from top and push the ink back up again after your  
print. This floods the ink back over the stencil, so it keeps the  
holes wet and open. This gives you time to stay calm, get the next  
piece and position it. The screen hinges from the back, lift and rest  
it on your head while you mess about underneath. Props and legs just  
get in the way. Use clamps and small blocks (double sided tape is  
good too) either side so the screen sits in the same place each time.

You also have to make a 'lay' so your work drops into the same place  
every time. Easiest is to get the film you made the stencil from and  
tape it over your panel or whatever you are printing. You then tape a  
metal ruler as a temporary handle to the workpiece-with-film and  
slide it around until from the top the stencil overlays the film. You  
get used to using your fingers as pivot points on top and can usually  
get there quite quickly. Lift up the screen and tape pieces of  
material the same height as your panel (its important it is the same  
height, or hard corners will cut your screen when you print) around  
your panel, each end overlapping the next as you go around, so you  
get a sort of swastika shape. This way you don't need to cut  
anything. Now you have a well to pop your piece in each time.  
Remember to take the film off the first registration piece, I have  
sometimes printed onto this by mistake.

You also don't need to mess about with the whole screen area. If you  
want a small image on a big screen, just do a stencil. On the top use  
brown parcel tape to mask off the image and tape either glossy paper  
or I use brown 'kraft' wrapping paper around to cover all the rest.  
When you are done, simply peel the tape and paper off and bin it. You  
then only have a tiny area to wash up afterwards, all the ink and  
crap is on the binned paper.

It's a great process you can do on the kitchen table, but I doubt it  
is cost-effective these days,

John