Newbie looking at 6040 (China CNC) for polypropylene 15mm sheet

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My idea is to then use 15mm polypropylene sheet instead to get the same sort of light wight properties and strength (small sized case, about 100x45mm).

In my opinion Acetal (Delrin, POM) is the best material. It is not cheap, but it is stable, keeping it's form even in varying temperature and humidity, machinable and you can handle it as you would handle soft metal, yet it is very strong, you can tap it and use ordinary screws for metal. It is not melting easily and the chips produced are nice. I am using it and in my opinion it is the best for milled plastic cases, though it is not light.

Are these China CNC milling machines okay for this?
I have seen them eat 10mm aluminium so this should be fine right?

Can't speak about "China CNC milling machines" in general since I never had one (built and designed my own) but I am pretty sure they can handle Acetal, as well as aluminium, assuming you are using the right feed rate and spindle speed. My first spindle was a cheap 400W DC motor with the maximum RPM of 12000. I used that one for two years on PCB, acrylic and Acetal and is still working, though now replaced with a 1.5kW 24kRPM brushless 3 phase spindle which is of course better. Never the less, it shows that even cheap motors can be used, but you need to adjust the feed rate accordingly. Anyway, with the DC motor I never really dared to work on aluminium, the new one on the other had cuts aluminium just fine.

My understanding of milling plastic is speed and sharp tools to reduce heat and thus the plastic melting.
There is less horse power needed as it is more speed than anything, though I still want a powerful spindle for future proofing..

It doesn't matter which material you work on, speeds and good tools are always critical. Even aluminium melts if you are not doing it right or use bad tools. Anyway, how fast you have to spin the spindle depends on the type of tool and the material, as well as the feed rate. Use a feed rate calculator, practice and learn. Different plastics behave differently, there are even different material qualities, just like there are different grades of aluminium and other material.

I cut everything dry, not using mist and the only air which is blown around the cutter is the one which is sucked in by the dust shoe, so there is not much cooling. Melting is only an issue if you are using the wrong speeds and feeds, or cut too lightly and the chips are far too small. If you must shave material you must be more careful, it is better to produce chips than dust.