Well then you have access to material tables. IRC, there is not a huge range in density in pure solid plastics (not gas, or filled with mineral or metallic fillers). SG typically ranges between 0.8 and 1.6 for most plastics. IRC the densest plastic I've encountered is teflon which is roughly twice as dense as water (SG 2).
Still pretty far off of a SG of 3.8. There's some metallic filler involved I'm pretty sure.
Plastic in a molten state is quite compressible (well not like water anyways). Still it's hard to hit the density of the solid state in the liquid state (esp for crystalline plastics). It appears that a high injection pressure results in reduced shrinkage during solidification, but I don't think that you can get very high gains in solid density. If you mould at low pressure, you get alot of shrinkage during the phase change so the final solid denisity isn't too far from moulding at high pressure. HP moulding lets you cram in more plastic and acheive less shrinking during cooling.
Want nearly free GBB gas?