Thanks again to Andy at handprint.co.uk who emailed with some more advice about our miniature silkscreen idea. Below is an abridged copy of his email:
To get good results you need to have sufficient room on screen to load it with enough ink for several prints, enough space to have a good run up, and enough for a good follow through without bashing into the frame which will almost certainly mess the things up.
Also, there is “snap off”. To get a nice clean print, the screen should only contact the substrate under the edge of the squeegee – as the squeegee passes over the screen it will pull the fabric in the direction of travel. When the squeegee has passed, the mesh will spring back and if it is still in contact with the substrate, it will smudge the ink which was laid down when the mesh was elongated. Getting optimum snap-off is problematic when using a small screen without sufficient “spare” between the image and the frame. Balancing the pressure required to ensure contact without overdoing it and squelching too much ink through is not easy.
Also, if you think about it you are deforming the fabric by printing with snap-off. The frame will prevent the fabric from deforming evenly at the edges, leading to distortion of the image.
Exposing the stencil is also harder with small screens. I expose my screens in a vacuum frame. This ensures that the film +ve is squashed tight against the emulsion layer and minimises the risk of the light undercutting the image. It’s a pretty good vacuum, but even so, there is an area against the frame where the vacuum can’t ensure contact, even on a good day! So it pays to have the edge of the image well away from the frame.
It may be that the accuracy you required doesn’t merit taking account of all the stuff that I’ve been telling you, but I feel that it’s good to come from a position of knowing what the ideal setup should be like and making modifications that you think you can get away with, rather than constructing something and trying to get it to work even though it has basic inbuilt flaws that only become apparent when you’ve built it and tried to use it!
Well, since our PCB traces are often 0.5mm (and sometimes below that) accuracy is one thing we’re keen to preserve. Allowing the silk to stretch to the printed surface and “snap-back” is also pretty important too – we don’t want any chance of smudging these relatively small PCB traces at all. So we’re going to need a bit more room around the outside of our copper board to get a really good print.
Following Andy’s suggestions, we’ve redrawn our MDF laser-cut pattern to make the frame much larger, but still try to keep within a maximum of an A4 footprint.