Researchers at the University of Nottingham and the University of California, Berkeley, have developed a resin chemistry that curbs the overheating responsible for warped and fused parts in computed axial lithography (CAL), a volumetric additive manufacturing process that cures an entire 3D object at once by projecting light into a rotating vat of resin.

The study, published in Nature Communications, found that introducing reversible addition-fragmentation chain transfer (RAFT) polymerization into common CAL resins sharply reduced the exothermic runaway that has limited the technique’s accuracy and scale.

CAL builds parts through free radical polymerization (FRP), a reaction that generates heat rapidly once triggered. That heat can set off a self-reinforcing cycle known as the Trommsdorff, or gel, effect, in which warmer regions of resin cure faster and release still more heat, distorting or fusing features that were meant to stay separate. The researchers added a RAFT agent, which regulates polymer chain growth by shuttling radicals between growing chains, to slow this runaway process without slowing the print itself.

Cutting the temperature rise

Testing a common CAL resin, pentaerythritol tetra acrylate, the team found that prints made without a RAFT agent underwent a temperature rise of 59 degrees Celsius during polymerization. Adding a dithiobenzoate RAFT agent called CPBD cut that rise to 27ºC at a 0.1% loading and to 3.5ºC at 0.3%, according to the study. 

Thermal and shadowgraph imaging showed resin without RAFT agent overcured within minutes, while resin containing 0.2% RAFT agent showed no overcuring even two minutes after the object had formed.

Denser, multi-material parts

The chemistry also addressed thermal buoyancy, a defect in which heat-driven convection displaces parts mid-print. A test object made of three different-sized spheres fused into a single mass when printed with standard FRP resin, but formed as separate, correctly spaced parts using the RAFT formulation, achieving a resolution of 150 micrometers between features. 

The authors also printed nested and interlocking geometries and used the RAFT agent’s retained reactive end groups to graft additional polymer coatings onto finished parts after printing, a step they said could support future multi-material fabrication.