Further to what Dave has said - if you are working commercially, don't forget to advise your customers about cleaning appropriately!
Just recently we got back some returns - well over 6months after the customer purchased the product. All of them had become extremely brittle at places where we had bent the plastic, and around a some of the laser cut holes - yet, when straight off the production line, they easily pass a 2m drop-test onto a concrete floor.
The customer had been keeping their kit nice and shiny using a generic "bathroom cleaner", which we found was acting in the way Dave mentions. Bear in mind though, that even water vapour can lead to this kind of cracking if there is enough stress within the plastic. Doubling the stress way more than doubles the chance of cracking, so its well worth paying attention to.,
Some tips for reducing the stresses are...
- Bending over a larger radius reduces the stress at bends.
- Using a radius/fillet at internal corners -the stresses get concentrated at 'any sharp internal angle.
- Cutting thick plastic progressively over two or more passes - the faster the laser head moves, the less heat is pumped into the plastic, and it can cool a little between passes.
- Avoiding parts that are very narrow - as the cut nears its end, small parts become tiny 'islands' of plastic, so heat from the laser can't be conducted away by the rest of the plastic sheet.
- Prefer cast plastic to extruded - extruded plastic has stresses 'locked in' by the machines that form it into sheets.
- For very critical applications, the finished parts can be annealed - a special heating and cooling process that 'relaxes' the internal stress.
If you're interested to see the stresses in your (transparent) parts, it's easily done with a pair of polarising filters. You can get polarisers from most camera shops, and many sunglasses have polarised lenses (the ones where you see 'rainbows' in car windscreens). Put one behind the plastic and one in front, and slowly rotate one of them - you will see coloured bands appear in the plastic. They are rather like the contours on a map - closer together = trouble!. If you flex the plastic, you can even see the stresses changing right before your eyes.
PS) Why not treat yourself for Xmas - get some polarisers anyway, the effect looks real pretty!!