Aop,
I have made several photo etch designs myself, and that taught me a few things.
I use a program called DesignCAD 3D Max (not the same as the program "3D Max"). It costs $100 US, although it is often on sale for less.
https://www.turbocad.com/designcad/desi ... fgodfCwEswThere is a 2D version that costs $50 US:
https://www.turbocad.com/designcad/desi ... fgodhPUFbgIt has a great user forum where you can find help. It is a complete CAD program and it does have a learning curve. But it is easier to use than other CAD programs I have worked with.
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One thing to keep in mind is that the etching process doesn't cut straight through the metal like a saw blade does. As the etchant cuts into the metal between the photo resist it also cuts under the resist pattern, so a cross section of etch cut sides are not parallel - it is wider in the middle of the sheet. This is more apparent with thicker materials and not much of a problem with thinner materials. But, as a rule of thumb, the narrowest cut you can make reliably is about 1.2 times the thickness of the metal. Anything narrower than this may etch away completely. For really fine details you need to use thin materials.
For large pieces this will be unnoticeable, but if you try to etch fine gratings or screens they may not work. However, this is dependent upon the photo etch equipment and process, and some companies are very good. I have seen some very fine photoetched screens. Each company should have instructions for what you can and cannot do.
It isn't enough just to have a bunch of lines to show where the edges of the etch are to be. You have to fill in a solid color between the lines to indicate where the metal should be removed and where it should remain. The photo etch company can't read your mind so you have to show them. BUT, there are two methods of creating the photo resist on the metal - positive and negative. For the positive method you color the areas where the metal should remain, and for the negative method you fill in where the metal should be removed. You need to know which method the photo etch company wants.
The comment about colors and layers probably has to do with partial etching. Normally the same pattern is used on both sides of the material to protect the metal that is not to be etched. This also controls the undercutting. However, you may want to etch details into one side of the sheet only - such as fold grooves. For this you must supply two images - one for the top side with the fold grooves and one for the back side without the fold grooves. It is common to give each layer a different color to make it easy to give instructions how the job is to be done. You can also have fold grooves in different places on both sides.
Also, you are not limited to just two layers. I have seen jobs where thicker sheets have been etched several times to produce complex multi-level etches on one side, with things like rivets and such. I haven't tried it myself. For this you will need different colors for each layer and instruction how the layers are to be "stacked" for the job.
Phil