Why are 3D-printed molds inhibiting PDMS curing?

Silicone curing inhibition using 3D-printed molds is a commonly encountered issue, that we addressed previously by proposing a universal post-treatment of molds. Still, curing inhibition mechanisms and how this was solved by UV and/or thermal treatments remained a mystery. Here, we report that curing inhibition is caused by leaching of phosphine oxide photo-initiator fragments from the molds, and that our post-treatment recipe allowed completion of the resin polymerization, elimination of these small fragments of photo-initiators by vaporization as well as their recombination into large molecular weight compounds that remained trapped in the molds.