We really recommend minimum 6mm between your objects, but you can go thinner if you want to.
For 1/2" foam, here are a few examples of what to expect if you want to cheat the guidelines.
These windowpane patterns show what it looks like to have 10mm, 6mm, and 3mm spacing between your objects.
With the 6mm spacing, everything stays together. With the 3mm spacing, it really depends on how long the cuts are and what is around it, but there can be major warping and it probably isn't doing what you want it to at that point.
Because we cut with a laser, the cut isn't straight down... the exit is going to be wider than the entrance. Usually this is actually an advantage for tool drawers... nice tight fit at the top but not too hard to push it in. It also guarantees that the sheet isn't moving around while we cut so the registration stays perfect even with a large number of thin sections (where a vacuum hold down might fail).
But this does limit our section width. For example with the 6mm spacing, we effectively add approximately 0.25mm spacing on each object so the actual "rib" between objects will be around 5.5mm. The lines on the front will be nice and sharp.
On the back side of your part, the cut is wider at the exit, the "rib" width will be around 4.5mm. Exact width depends on cutting parameters, foam color, foam thickness, and how flat the sheet is sitting. These lines are never as sharp as the front of the part, and will occasionally show some re-melting or other cosmetic flaws. Occasionally with fine details we can get much larger vaporization on the back side.
Here's an example of what we typically see on the reverse.
And our typically edge profile. Its a bit exaggerated on this small part and its a corner section you'll see nearly double the taper. Edge quality depends on design details and the exact piece of foam, but this is pretty typical for us. If you have particular edge requirements for the back side, please let us know and we'll try to figure out if our process can work for you.
With the laser, its a bit non-linear what happens when you start getting thinner and there isn't enough material to support the cut. if you go too thin, the lack of structure combined with rapid cooling will may twist and distort the part. So we suggest 6mm.
We have cut outlines for 100s of tools and every single one has fit without being too loose. There are a lot of reasons for this, but the good news is that it works for us in our workshops and we hope it will work for you too.
If you still have any questions, please feel free to contact us here.