[sdiy] rack enclosures

[Quixotic Nixotic]

On 8 Mar 2007, at 07:44, Hallvard Tangeraas wrote:

> So there relationship between what kind of fabric is used (i.e. how  
> big the holes are) and the paint (how think or thick it is) must be  
> quite important if you want I good result I presume.

Within reason. A finer mesh prints a bit less ink but holds finer  
detail. The consistency of the ink is usually dictated by the  
process. Too thin an ink and it will spread when printed. Too thick  
and it will block your corner detail. There is a range in between  
where you are good to go.

How hard you pull the squeegee has as much effect as anything. Too  
much pressure gives you a thinner print, not thicker. It also drags  
the image, leading to premature mesh/stencil failure through stress.  
You also get smudged prints.

The thickest print will happen if your ink is like runny cream on  
mom's apple pie and you have flooded the ink over the image  
beforehand. You then pull down towards you - use the thumb and  
forefingers only to avoid undue pressure. You are dragging the ink  
over the item, all you need to do is keep it in intimate contact with  
your work as you pull. The squeegee will be leaning towards you about  
30 degrees, its sharp edge dragging the ink.

You are not trying to force ten litres/pints of ink through the mesh  
with pressure, it doesn't work like that. If the ink doesn't print  
right, then there is a problem elsewhere, with the ink or the screen.

Ink gets thicker as you do a run, the lighter solvents evaporate off  
or get absorbed in the masking paper around your image. So you need  
to scoop it up - a round ended cheap kitchen palette knife is good -  
and mix it back into your ink pot, adding more thinners. But be  
careful with epoxy ink, too much thinners and you will destroy its  
strength and it will not cure well.

A standard mesh will be a monifilament polyester, sometimes it's  
orange (direct stencils work better on this, less glare when exposed)  
and sometimes milky/clear/white. A mesh will typically be between  
175-305 threads per inch, or 68-120 per cm. A 100 to 120 metric mesh  
is good for most things.

Get a screen stretched professionally if you can. It's worth the  
money. A slack screen is hopeless. A screen is usually stretched  
using a device. This is often a strong rectangular frame with bars on  
the outside of the four sides. You wrap the four sides around the  
bars and then screw them to the stretching frame. As the screws go  
in, the bars go out, stretching the silk (which is polyester these  
days). You then use an epoxy glue to fix the mesh to your screen.

Good tutorial video here.

There are special frames that have tensioning bars around the edge of  
the screen itself. You can do the whole thing yourself, with no glue.  
The stretching frame is the printing frame itself.

There are ways of stretching a screen yourself, but I doubt you will  
ever get it stretched enough, certainly not if you are a novice. When  
wetted with water, a stretched screen will boom like a drum.  
Seriously tight.

>> The people who do my boxes, have an ink that they mix some kind of  
>> hardening catalyst with, and then bake it as well.

The ink I have used many many times is Sericol's 'Polyscreen' two  
part epoxy ink. This is what is used for the white annotation on PCBs  
as well as front panels. The translucent green solder mask is a very  
similar two part ink. A bake is not essential but it helps. Not too  
hot though.

Data sheet is here:
http://www.sericol.com/USApi/Polyscreen.pdf

This uses its own catalyst and thinners. It must be mixed and used  
fresh on a grease-free surface. And you must wash up your screen  
immediately afterwards. Any ink that has set has ruined your screen  
for life. So wash up and take the stencil off too. get those last  
bits of ink out.

A good indirect stencil material is called Sericol 5 star. This gets  
exposed (a 1000 watt halogen will work) soaked in a solution of  
hydrogen peroxide, and washed out with water.

There is a good rundown of everything you would need including UK  
prices here:
http://www.megauk.com/screen_printing.php

You don't need that expensive bench. I have done fine registration  
work with nothing more than two bits of hinged 2" x 1" and 4 G  
clamps. Quick. Easy. Simple.

John