Bumpiness can be due to few reasons I can think of right now, such as :

-Non-uniform pressure application

-Surface finish of the mold

-try cooling the slab even slower.

-geometry of the mold (corners can be tricky)

- is polymer powder spread uniform?

-temperature to which you heat

-Most important- Does your mold has vent holes for the air or any gasses produced to escape during the cure?

These might be sink marks, or vent marks.

To reduce sink marks keep the pressure up during cooling to improve "packing".

To reduce vent marks, try adding a little excess solid material at the center of plate so you get some flow towards the edge as the material melts.

Thanks for the suggestions . The mold has one port for vacuum. I heated the polymer powder for 20 mins and the heated at the same temperature under vacuum for 20 mins , then applied pressure ~1000 psi under vcacuum for 15 mins. Finally cooled it down to room temp by forced air. During cooling the pressure dropped to less than 500 psi.

I will try next time to keep the pressure up during cooling. The polymer doesn’t flow much , it is partially crosslinked material.

Do you think crushing the polymer further into a ball mill to 10-20 micron will help eliminating these bumpiness. Thanks.

I don't think milling would help. I am concerned about the time that the material is at a high temperature. Do you really need to preheat for 40 minutes before molding ? I am also concerned about the pressure, do you think you could achieve a result with 100 psi ?

The time is needed for the polymer to melt completely before molding. And dont you think I need higher pressure to get a bump free slab? Isnt 100 psi little low. I am applying 1000 psi.