Thermoforming time variability of PET-G and TPU aligner sheets across consecutive thermoforming cycles: an in vitro study

BACKGROUND: Thermoforming time is a critical parameter in the fabrication of thermoformed orthodontic aligners, as it influences polymer softening, material adaptation, and dimensional consistency. Manufacturer-recommended heating durations are typically based on standardized conditions and may not reflect the dynamic thermal behavior of thermoforming units during consecutive fabrication cycles. Empirical data quantifying heating-time variation and stabilization patterns remain limited. METHODS: This in vitro study evaluated heating-time variation across 20 consecutive thermoforming cycles in two commercially available thermoplastic aligner materials: a multilayer polyethylene terephthalate glycol/thermoplastic polyurethane composite (Zendura FLX) and a single-layer thermoplastic polyurethane sheet (Maxflex), both with a nominal thickness of 0.76 mm. Each material underwent ten independent experimental sessions (total n = 200 cycles per material) using a standardized pressure-forming unit under controlled environmental conditions (23 ± 1 °C; 50 ± 5% humidity). Heating was terminated at a predefined sag depth of 16 mm, determined using a laser-based measurement system. Linear mixed-effects models were used to assess cycle-dependent variation, and simple linear regression was applied to the pre-stabilization phase. RESULTS: Heating time progressively decreased across consecutive thermoforming cycles for both materials. Zendura FLX stabilized from cycle 6 onward, with a steady-state mean of 64.59 ± 3.58 s. Maxflex stabilized from cycle 12 onward, with a steady-state mean of 69.37 ± 1.12 s. During the transient phase, heating time decreased linearly by 8.66 s per cycle for Zendura FLX (R² = 0.78) and 4.17 s per cycle for Maxflex (R² = 0.97). Both transient and steady-state heating durations were significantly longer than Supplier-Recommended Thermoforming Times (p < 0.0001). CONCLUSIONS: Heating time during thermoforming decreased across consecutive cycles before reaching material-specific stabilization thresholds. For both materials, experimentally determined heating durations exceeded manufacturer-recommended times under controlled laboratory conditions, suggesting that thermoforming behavior may vary dynamically during repeated fabrication cycles.