congratulations, you joined the crowd of those who suffer from the results of so-called castable resin and complain about bad casting surface.
Surface defects may be caused by multiple effects. In order to verify your initial guess of resin residue just take a random, reasonably solid pattern with a clear structure. Put it into a new crucible and burn to the burn-out temperature recommended by the resin manufacturer. You may check every 50°C what happens - this will give you some extra insights. If there’s any visible residue at the recommended burn-out temperature, you might want to change to another resin or/and another 3D printer.
If there are no residues, then your problem originates from other causes along the process. These may be (among others):
- chemical reaction of the resin with the liquid/fluid of the investment during the setting phase,
- swelling of the pattern caused by hydrophilic properties of the resin,
- expansion of the pattern without melting during ramp-up temperature,
- and many more.
I assume that you are not interested to contribute an academic essay on possible causes and effects. Therefore it is not reasonable to investigate on the causes of chemical reaction, swelling and the like.
A more pragmatic approach would be to use an investment with increased thermal expansion and preferably one with setting expansion as well. By this you can eliminate the effect of intrinsic force generated by pattern expansion. We have developed Invest RP for this purpose and you may contact the smart guys at Stuller for this.
To get back to your inital question to get a better burn-out result, the approach to add air or oxygen, to be precise, will lead to a faster carbonization but the result will be the same as long you have residues.
J. Tyler Teague is savvy with this, and if I’m not mistaken he addressed the procedure of how to increase air ventilation in burn-out kilns either in the MJSA Journal or in this forum. You might contact him for advice if he’s willing to give a helping hand.
All other causes, addressed or not, you don’t want to know and don’t want to solve either but turn to a resin/printer without these troubles.