Today I tried again mill the corner blocks.
I lowered the spindle speed to 1000 rpm and it seems to bee about rigth for now I have no more melting plastic.
And even multiple ;-)
Milling time is under 4 minutes for one piece and 22 minutes for batch of six.
I still have some problems on keeping the slippery plastic on the CNC mills vacuum table.
I had to use a piece of MDF board that is sanded both sides so that only the core that is not so dense and passees vacuum through is left (16 mm board sanded to 10 mm) this has been a good way before with thin or other ways problematic materials but was not good enough so I added two sided tape between sanded MDF and plastic and it seems to help but is not feasible for mass production ;-) I probably have to make a jig so I can fasten the milled piece with screws.
Because I am using a vacuum table to hold the milled piece on it's place I have to mill the pieces so that about 1 mm is left to hold the vacuum. After milling I took the pieces apart with band saw and cleaned them with hand router that is installed upside down on a table.
Finally I deburred the sides with a knife.
I still have to drill the holes open to test the pieces.
After initial measurements it seems that the milled plastic is still moving a bit and is a bit warped on the milling table and so the milling depth varies +- 0.3 mm because of the double sided tape I used is not even thickness .
For next batch I will try fastening the plastic with screws on a MDF board and if that works I have to optimize the program for now it lifts the spindle over 100 mm when moving to make it run faster.