I have not made a granulator, but I think it might be worth a shot to use the blades from a dado head, or several dado heads, depending on how wide the material that you need to shred would be. Wood working blades are so cheap compared to milling cutters.
You'd need to space the blades/chippers at regular intervals on a simple arbor, and provide a slotted stator blade to serve as the cutting anvil. The spacers on the arbor would be large diameter, allowing only about 1/8 inch of sawblade to protrude. This would prevent uncut material from sneaking through uncut, or wrapping round the mandrel.
The stator blade would look like a block of wood with saw kerfs for the chippers to pass through. Of course, you'd need a tiny bit of clearance so that the chippers could pass through unimpeded.
The mandrel would need a good method of retention to prevent the whole assembly from moving endwise and allowing the chippers to rub on or destroy the stator blade.
You might want to experiment with moderate blade speeds, as you don't want to melt the plastic and end up with a big stuck, gooey mess. Perhaps a positive infeed system would be worth a consideration, rather than hoping that gravity will do the job.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)