Cutting plastic for first time, perhaps I am going wrong a tip or two be great thanks

[suesi34e]

Hi guys,
I have to cut some 10mm to 10.8mm grooves at a depth of 39mmin HDPE. I am thinking of the following:
10mm 4 tip carbide end mill 50mm long
4000 rpm which is my max spindle speed x and y feedrate 1800mm per minute 2500mm max for my mill
300mm in the z cutting at 3.9mm per pass 10 passes.
I believe weld back could be an issue. I was thinking of using an airline to try and blow the loose away but I thought I may just do more damage than good and stop the waste from exiting the groove. I got to thinking perhaps a vacuum may be better to try and suck the waste away.
I also have to surface finish the HDPE. I was wondering if Iwould get away with a 38mm TCT surfacing bit I use in my router I think I used to run it at about 12000rpm so I am not sure what it would be like at 4000rpm
I am pretty clueless when it comes to plastic and metal so Iwill welcome any thoughts or criticism.
I downloaded G wizard but I found it a little confusing and I have to crack on with machining the part I will have to have a look over it when I get more time!
All the best and many thanks for any help in advance
Suesi