Modeling the Thermal Behavior of Photopolymers for In-Space Fabrication

Future long-duration space missions will require in-situ, on-demand manufacturing of tools and components. Photopolymer-based processes are attractive for this purpose due to their low energy requirements, volume efficiency, and precise control of curing. However, photopolymerization generates significant heat, which is difficult to regulate in microgravity where natural convection is absent, leading to defects such as surface blistering and deformation. In this work, we combine experimental studies and modeling to address these thermal challenges. We report results from International Space Station (ISS) experiments and a dedicated parabolic flight campaign, which confirm that suppressed convective heat transfer in microgravity exacerbates thermal buildup and defect formation. Building on these observations, we present a predictive thermal model that couples heat transfer, light absorption, and evolving material properties to simulate polymerization and temperature evolution under terrestrial and microgravity conditions. Laboratory validation demonstrates strong agreement between model predictions and measured temperature profiles. Applying the model to the ISS experiments, we show that the model accurately reproduces experimentally observed blistering in TJ-3704A, a commercial acrylate-based polymer resin, while also predicting defect-free outcomes for Norland optical adhesives. The model functions as a design tool for defect-free in-space manufacturing, enabling selection of polymer properties, exposure strategies, and environmental conditions that together inhibit excess thermal buildup, paving the way for scalable, reliable in-situ manufacturing during future missions.

💡 Research Summary

This paper addresses a critical challenge for long‑duration space missions: the need for reliable, on‑demand manufacturing of tools and components in microgravity. Photopolymer‑based additive manufacturing is attractive for space because it requires minimal energy, compact hardware, and offers precise spatial control of curing. However, the exothermic nature of photopolymerization generates substantial heat, and in the absence of natural convection in microgravity this heat cannot be dissipated efficiently, leading to defects such as surface blistering, bubbling, and deformation.

The authors first report observations from the International Space Station (ISS) during the RAKIA Ax‑1 mission, where three resins—commercial acrylate TJ‑3704A and Norland optical adhesives NOA61/63—were used to fabricate spherical lenses via Fluidic Shaping. While all three resins produced defect‑free optics on Earth, only the lenses made from TJ‑3704A exhibited severe surface blistering and frame deformation on the ISS. The authors hypothesized that the lack of convective heat transfer in microgravity caused excessive temperature rise during curing.

To test this hypothesis, a series of parabolic‑flight experiments were conducted on a Zero‑G aircraft. Sixteen custom desiccators each held 2–4 polymer samples (30 mm diameter, 1–5 mm thickness). UV LEDs (365 nm) were used to initiate polymerization, and embedded thermocouples recorded temperature histories at 25 Hz. Fans mounted inside the desiccators provided forced convection when activated; otherwise, natural convection occurred under 1 g and none under 0 g. The experimental matrix included: (i) 1 g with natural convection, (ii) 0 g with natural convection (i.e., no convection), and (iii) 0 g with forced convection. Video recordings captured visual evidence of boiling, bubbling, and blister formation.

Results showed that samples cured in microgravity without convection reached peak temperatures roughly 20 °C higher than comparable 1 g samples, and blister density was approximately double. Forced convection reduced peak temperatures by ~20 °C and virtually eliminated defects, confirming that convection is the dominant heat‑removal mechanism. However, forced airflow is impractical for space manufacturing of larger or precision parts because it distorts free‑surface shapes and cannot be applied in vacuum.

Building on these observations, the authors developed a predictive thermal model that couples heat conduction, light absorption, and evolving material properties. The model solves the unsteady heat equation:

ρ(T,φ) cₚ(T,φ) ∂T/∂t = ∇·