Introduction

Laser engraving text on acrylic / perspex does not create a lot of contrast. This leads to the subject of painting text on the acrylic. I have developed some useful methods of achieving this that I thought I would share.

My techniques achieve a good hard wearing result. One of them allows for painting everything except the text, to allow for back lighting. Note: As I often have to bend and mould Perspex, I needed a procedure that could be implemented on the final moulded pieces. This meant that using a laser cutter to remove paint was not an option.

Technique 1, Painting the text.

This effect is a little more difficult than would appear at first glance. You want to paint the whole area and remove only the paint that’s not in the text. Problem is doing this by washing with solvents will thin the paint and dilute the finish, reducing the contrast. There is a neat trick you an use however, it involves pouring water over an oil based paint to form a gum in the lettering, which is resistant to solvents.

• Laser engrave the text with a deep cut (1mm -ish).
• Prepare the piece (bend, polish etc.)
• Spray paint with a cheap oil based paint (the kind that gums with water). The “Fiddly Bits” brand works well.
• Wait 30sec to a minute.
• Place piece under running water for a couple of seconds.
• You should now be looking at a complete disaster, the finish is ruined and everything is a kinda horrid sticky mess.
• Get a thin micro-fiber disposable cloth, dip in turpentine.
• Scrub the surface (will require elbow grease and persistence).
• Presto, the only stuff you cant scrub out is this awesome looking text.

Technique 2, Painting everything that is not text.

This seems like an odd way of doing things, but offers some interesting advantages:

• Done on translucent acrylic, you can now do back-lighting
• You can paint gradients / flames etc. and still have blacked out text.
• You can always find the right coloured paint on short notice, while acrylic colours are at the mercy of your supplier.

This technique is actually a lot simpler than the first technique, but a lot less forgiving to errors.

• Engrave text with the quick shallow cut. You basicly just want to rough the surface.
• Prepare the piece (bend, polish etc.)
• Wipe turpentine over the piece with a cloth.
• Gently (half) dry the surface with a dry cloth. Leave the thinnest smear don’t dry out the letters.
• Wait 20 seconds to allow for some evaporation to occur.
• Spray paint a very thin layer.
• The text area will still have some turps, and also possesses a higher surface tension due to its roughness.
  • The paint will move away from the text forming on the flat, smooth areas.
• Repeat coats using this procedure as needed.

   
(Left, final product. Right. first coat)

I spent a lot of time calculating track widths to make sure my circuit board could handle the current I was using.
Then I had to do more calculations for font ratios. so that that all lines were wide enough to work with the silk screening process.

Sick of this; I quickly used matlab to create a few charts so I could look up the answers quickly.
I thought I would share these charts and the matlab scripts, hope they are useful.

Font Ratio Chart

To my experience, In Eagle Cad (and others) font ratios for silk screens work best using vector fonts. The cam processor, other tools, and factory will often use vector fonts regardless; – so using vector fonts tends to keep things compatible. In the text properties dialogue you must set the ratio correctly so that (Size * Ratio) > “Factory Minimum Line Width”.  To make it worse, fonts are best wrangled using Mils (1/1000th of an inch [what’s an inch]).

Typically in Eagle Cad, I would set up text for silk-screening as follows.

Anyway, here is the chart

To use it:

• Find the colour that corresponds to your manufactures minimum silk screen resolution.
• Find your font height in the x-axis
• The corresponding ratio is given in the y-axis.

PCB Track Width Chart

Track width is related to a lot of factors (acceptable temperatures. how insulated the tracks are from the air, current, acceptable power loss, etc). For anything complicated (internal PCB layers, high currents/voltages, installation in a vehicle/ heater / oven) go do proper calculations!

Looking for a good track width calculator, I found this and like it a lot:  http://www.4pcb.com/trace-width-calculator.html

99% of the time, as a hobbyist, you just want an external PCB track that does not raise in temperature by more than 10ºC. You would be using either 0.5, 1 or 2 oz copper tracks (eek, more imperial units). So I implemented the formula presented on the calculator mentioned above in matlab.  Then I plotted 3 lines, one for each common copper thickness, creating charts that seem correct for normal hobbyist type situations.

Four charts follow metric and imperial versions of high and low current situations. Find the graph that suits you and keep it handy.

Disclaimer: Your mileage may vary. A) I may be wrong, and accept no liability for that. B) Silkscreens, lacquer, protective coatings, hot electrical components may invalidate these figures.  If your doing anything medical / military /  safety critical / mass produced / potentially dangerous; this page is not an appropriate source of information, go find an engineering book, or something peer reviewed.

Metric – High Amps

Metric – Low Amps

Mils (Imperial)- High Amps

Mils (Imperial)- Low Amps

Source Code

Fonts

%----------------------------------------------------------------------------------%
%                                    BUSYDUCKS.COM                                 %
%                            Making you pro-duck-tive                              %
%                                                                                  %
%  Author: Duckman   Date: 10/3/15   Ver: 1.0   Licence: Creative Commons (by-sa)  %
%                                                                                  %
%  Calculates silckscreen font ratios.                                             %
%  Compile with Matlab / Possibly Octave                                           %
%                                                                                  %
%  Permision given to freely copy/paste "code snippets" into your own code. For    %
%  other uses (e.g. derivative works) the Creative Commons Attribution Share-      %
%  alike license applies (cite busyducks.com). This means commercial use is ok.    %
%----------------------------------------------------------------------------------%
s = [10 : 5 : 100];
r = zeros(7, max(size(s)))

for x = 2:8
    r(x-1,:) = ((x*100) ./ s)';
    plot(s, r);
end

plot(s, r);
title('PCB Silk Screen Font Ratios')
xlabel('Font Height (Mils)')
ylabel('Font Ratio (%)')
legend('2 Mil', '3 Mil', '4 Mil', '5 Mil', '6 Mil', '7 Mil', '8 Mil');
grid on

Track Width

%----------------------------------------------------------------------------------%
%                                    BUSYDUCKS.COM                                 %
%                            Making you pro-duck-tive                              %
%                                                                                  %
%  Author: Duckman   Date: 10/3/15   Ver: 1.0   Licence: Creative Commons (by-sa)  %
%                                                                                  %
%  Calculates Track widths.                                                        %
%  Compile with Matlab / Possibly Octave                                           %
%                                                                                  %
%  Permision given to freely copy/paste "code snippets" into your own code. For    %
%  other uses (e.g. derivative works) the Creative Commons Attribution Share-      %
%  alike license applies (cite busyducks.com). This means commercial use is ok.    %
%----------------------------------------------------------------------------------%
function plotTraceWidth (metric, small)
%Formula from: http://www.4pcb.com/trace-width-calculator.html
%sample usage: plotTraceWidth(true, false);  
%will save a .png to the current working directory;  

amps = [0.1 : 0.1 : 10];
thickness = [0.5, 1, 2];

if(small)
    amps = amps * 0.2;
end

%external layers, IPC-2221
 k = 0.048;
 b = 0.44;
 c = 0.725;
 tempRise = 10; %deg C

factor = (k*tempRise^b);

for it = 1:3
    t = thickness(it);
    area = amps./factor;
    area = area.^(1/c);
    width(it,:) = (area./(t*1.378))';
end

if (metric)
    %convert to metric
    mmPerMil = 0.0254;
    width = width .* mmPerMil;
end

plot(amps, width);
title('PCB Track Width')
xlabel('Amps')
legend('0.5 oz', '1 oz', '2 oz', 2);

name = 'track_width';
if (metric)
    ylabel('Track Width (mm)')
    ticks = [1:15];
    name = [name '_metric'];
    if(small)
        ticks  = [0.2:0.2:2];
        name = [name '_small'];
    end
else
    ylabel('Track Width (Mils)')
    name = [name '_mils'];
    ticks = [50:50:2000];
    if(small)
         ticks = [10:10:200];
        name = [name '_small'];
    end
end
set(gca,'Ytick', ticks)
grid on
saveas(gcf, [name '.png']);