Stencils are used everywhere where, but to produce them to perfection you need to pay attention to artwork you make. This guide will show how to make stencil artwork for laser cutting. Stencils can be cut with CO2 laser, fiber laser, CNC and blade vinyl plotters. regardless of what machine is used to cut them the all get cut and unlike a printer all cutters cut through material fully so all unsupported parts will fall off.
So lets say we want to spray paint this image below.
If you print out our image it will look perfect on paper, font is good size is good BUT if you send this art for laser cutting as stencil this art is not good and you will NOT get what you expected to be on paper. This font has letters B, oo, e,o that have inner elements. When laser cutting machine cuts out black area you get sign style letters that you can stick to something and you are OK but stencil needs negative space and as result all inner elements will get lost. Here is what happens when art is cut as stencil.
As inner elements of stencil were not supported by bridges ir tabs they got separated from main stencil body and when you spray that art you get big black holes in certain letters.
To avoid this, bridges or tabs need to be added to stencil artwork. This can be dome by cutting letters with eraser tool. So basically walls of the letters are separated and this allows main body your material to hold inner components. Example of properly setup stencil artwork will look like one blow
Same technique is used to make actual artwork with more complex thing then just plain text signs.
Another thing is to keep in mind about stencils is wall thickness of your bridges. If walls are too small then there us chance that bridge will burn out when lasered and inner part will be lost. Best to keep bridge walls to be at least same at material thickens though in many cases it can be smaller. For example if we would cut artwork above from 1/8″ (3mm) acrylic min wall thickness should be 3mm , bigger the wall more reliable it is. However if our stencil is very small day 6″ wide we can stil push bridges down to 1mm this will make stencil more fragile but its still usable. If we go less 1mm on acrylic parts will just brake.
When it comes to lasers two most important parts that affect your cutting results are laser beam aliment and beam quality itself. Laser sources such as Fiber lasers are pre-aligned at the factory with special tool and usually under microscope so output results are very good (with fibers we seen focused spot have perfect circle at 0.015mm) For CO2 lasers situation is not that easy and in most cases you need to have optical elements along the laser path to deliver laser beam. Any element you add to machine has some tolerance and end result gets degraded. No matter how good you are in tuning there will be some losses BUT proper tuning and setup to makes a big difference.
So step one is to align your optics. If we build our own machine we make parts the go into each other, no option to move too much so that guaranteed best results. Cylinder inverted into cylinder will have less chance of moving around and it wil maintain a straight vector which is the same as your laser beam travel path. Once mechanical aliment is done, alwyas check optical results.
When we work with LaserCutZ custom designed components we always make parts that can be mounted or host mounting hardware. As seen on image above test gel is installed onto of beam expanded. Aliment jig has reference rig so we know it matches mechanical part and it has cross chair lines so we can see what quadrant laser will hit. Laser test jig is nothing more then acrylic that can be cut by any CO2 laser machine, but outing hardware needs to be part of machine. If your machine is lucky (engineered well enough) then you can align it absolute perfection. Either way once test jig is installed fire laser 10% 20ms pulse and you will see exactly where it hits. As seen on image above it had 2 spot, one was too low, and tube was adjusted and 2nd one is perfectly in the center of cross chair. This part is good. We can move to next aliment point.
Next test point is where laser existing the mounting fixture. If main glass tube was not aligned properly then exit point will not be exactly in the center (even though entry point could be in perfect center) So we mount new test plate on exit point and check. To make live easier and safer this time we mounted red laser pointer so we can see what we will hit. Lucky due to experience doing laser maintenance our exit point hit exact center of cross chair so no further fixing is needed.
All laser sources are not created equally! CO2 tubes have less controlled spot, RF tubes are little better, both need some shaping at the end. Why do I care about spot quality? Initial laser spot will be carried through optical path get more distortion and finally gets focused to laser cutting material. If we have bad spot initially, we will project it and get not perfect circular cutting kerf so you may se some ribs on edge of material or some times have perfect cut through and some times not or the cutting or engraving line changes thickness all the time. Those are small not always critical problems but they do make difference when you need something perfect. Very often this whats determines good machine form bad cheap one. As we are crazy about quality of cuts lets look laser spots in details.
Perfect Laser Spot after Modifications
This CO2 laser spot was generated after we tuned laser beam with sharper and beam expanded. As we see from picture spot shape is nearly perfect circle, such spot is easier to deliver though optical path and focus onto cutting material.
Distorted Laser Spot form Glass Tube
Spot in this section is not absolutely bad, in fact it comes from very good China glass tube but its not yet perfect. As we can see shape is not a circle and there are some outside sparks, if we will be doing paper cutting or scoring we will have hard time.
As we see laser spots can be different and they can affect quality, lets find out how to make them better. One way is to use beam shapers, those are just metal screws thta block little of the laser beam outliers and kinda make more circular shape. Downturn of this screws taht catch laser will generate heat and those laser beans can reflect some where (even back reflect to laser source) Better option is beam expanders that, those will take some energy form our beam but usually that is form outlier rays.
Expanding lens is installed right after CO2 glass tube exit point, instead of expanding beam we turned it to combining direction, as result original rays coming from tubes get compressed a little and forming absolutely perfect beam spot. Note as we compressed ray density, our maximum laser travel distance will be reduced but usually its not a problem as on most machines total thavel path is 1-1.5m which is ok, laser attenuation will not be too big while quality improvement is dramatic