Notice the cleanliness of the entry point.
"NO" sharp bits have to be left there.
smooth out all sharp edges.

the reason is that if you leave any sharp particles;
1) It will gouge the tube you are trying to form letting loose little particles of PVC.
2) These particles roll between the mandrel and the tube modifying the thickness.
By creating little pimples on the finished surface.
Ruining the "LOOK" :-) 

Note:
If you have a bench grinder with a metal brush. 
Or a brush on your angle grinder.
It makes the polishing process so much easy.

tip: You know the polishing is done when you can confidently 
pass your finger along, and around. Without fear of cutting yourself :-)