[drermer]

I need to make some prototype heat spreaders out of a thermally conductive polymer. I want 2.5" x 3.5" x 1.25" (or there about) blocks that I can then machine

The company that makes the material will sell a compression molded block at an outrageous price with a significant lead time. I get the impression that they don't want to do it. I'm also concerned that the compression molded material will be porous, which is unacceptable for my application.

I can get pellets shipped the next day at a resonable price.

Are there any companies that do this sort of thing, i.e. make standard sized injection molded blocks without the cost of having a mold made?

Any info would be appreciated.

[mc_n_g]

Why will it be porous? I would worry more about the part warping and have potential heat sinks. I would also worry about the part warping because of the compressed stress in the material when you machine it.
Most likely their cost is manufacturing a mold and mounting it to their machine for a small run. You are paying for all the labor and material to get your blocks made.
For as many 'standard' sizes as you can come up with that is a lot of molds to have on hand. The other thing is you state this is a thermally conductive polymer. Is there a market for thermally conductive polymer blocks?
Can you do injection or compression molding yourself or can you only machine? Meaning your only have milling equipment and not plastics equipment.

[jetski]

You can have large blocks molded in Scottsburg Indiana at Engineered Block sales. I can tell you like the previous M C ng lots of stress is molded in this way. I machined some glass filled Valox and about 1/2 the way through the part made a bang that sounded like a shot gun blast. We did not know where the sound came from untill the block was done and we had set up another one to cut and bang again. A crack in both parts due to stress. The Valox was 4" x 12" x 12" hope this helps. Good luck.