Flame polishing acrylic isn’t “melting the plastic smooth.” It’s controlled surface reflow driven by polymer chain mobility. Acrylic (PMMA — polymethyl methacrylate) is an amorphous thermoplastic. When you cut or machine it, the surface becomes microscopically rough — full of tool marks, stress fractures, and scattering interfaces that kill transparency. When a flame briefly passes over the edge: 🔥 The surface temperature rapidly rises above the glass transition temperature (Tg ≈ 105°C) 🔥 Polymer chains at the surface gain mobility 🔥 Surface tension pulls the material into a lower-energy, smoother configuration 🔥 Micro-scratches collapse and optical clarity returns But here’s the catch: If you dwell too long or overheat the surface: • You induce internal stresses • You risk bubbling (monomer vaporization) • You create crazing (microcrack networks) • You permanently weaken the material Professional flame polishing is a timing + heat flux control problem. The goal is surface ...