PLA foam is an excellent start. It could be better than polystyrene foam, because PLA contains more oxygen to form gaseous compound with carbon. I could only imagine POM-H (polyoxomethylene [-OCH2-]n) foam leaving less solid residues.
Within a ceramic mould deposited on the foam, I expect very little air mass is available, so the compounds present are together very reductive. I expect CO rather than CO2, which will pick some hydrogen to form things like H2CO. The rest of PLA [-C3H4O2-]n would be four H for one C if no hydrogen combined with CO.
If the foam has closed cells, the foaming gas contributes to the gross composition. With open cells, it will be air. The amount isn't huge, but it can increase radically the toxicity of the evolved gas. If polystyrene is foamed by a halogenated gas, this makes toxic fumes.
What do the up to four H for one C do? I won't risk a prediction. There will be some graphite and high-carbon compounds, but the amount doesn't result from a simple reaction equation. The rest can evolve as CH4, C2H4 and thousands more.
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Out of curiosity, I've let Propep compute the equilibrium products of hot polymers brought to 1400°C and 1atm. This is NOT what happens in your case, because the casting process is very far from an equilibrium. Also, iron will catch some oxygen. The evolved gases recombine as they cool down, for instance CH3, H, HCO disappear for sure. These are molar fractions. Propep hides the least abundant species. I've take polybutadiene for its small H/C ratio. Even smaller at polystyrene, but Propep failed.
----- POM
7.2750e-007 CH3
2.2516e-004 CH4
4.9990e-001 CO
5.9635e-005 CO2
1.4937e-005 C2H2,acetylene
1.4589e-008 CH2CO,ketene
4.6162e-007 C2H4
8.0184e-005 H
3.4620e-008 HCO
4.9952e-001 H2
1.0892e-007 HCHO,formaldehy
1.9706e-004 H2O
----- PLA
8.0500e-007 CH3
2.4911e-004 CH4
4.0002e-001 CO
3.4496e-005 CO2
2.2871e-005 C2H2,acetylene
1.6148e-008 CH2CO,ketene
7.0665e-007 C2H4
6.4143e-005 H
2.7699e-008 HCO
3.9954e-001 H2
8.7137e-008 HCHO,formaldehy
1.1394e-004 H2O
Condensed species
1.9995e-001 C(gr)
----- Polybutadiene
1.2192e-006 CH3
5.3350e-004 CH4
2.4496e-005 C2H2,acetylene
1.5134e-006 C2H4
4.8583e-005 H
4.2792e-001 H2
Condensed species
5.7147e-001 C(gr)
----- PS
Propep failed to find an equilibrium.