PMMA, also known as acrylic, is a highly transparent and glossy engineering plastic widely used in optical lenses, lamp covers, light guide plates, transparent housings, automotive light components, and household appliance panels. Its final quality, including light transmittance, surface finish, and dimensional stability, heavily depends on the mold material used. The mold material affects not only appearance and precision but also service life, processing cost, production stability, and long-term production reliability. For this reason, PMMA mold material selection must focus on four core requirements: mirror polishability, corrosion resistance, dimensional stability, and appropriate hardness. A well-chosen mold material can reduce black spots, scratches, flow marks, and rust-related defects, ensuring consistent high-quality production for long runs.
Basic Selection PrinciplesDuring injection molding, PMMA releases small amounts of acidic gas and is extremely sensitive to surface defects. The mold material must therefore support ultra-high gloss finishing, ideally reaching Ra ≤ 0.01μm for optical clarity. It must also resist corrosion from acidic gases and moisture to prevent rust, pits, or contamination on the cavity surface, which directly cause defects on transparent parts. Dimensional stability is critical to maintain precision under prolonged heating and pressure, reducing issues such as flash, sink marks, warpage, and size deviation. Hardness is typically controlled between HRC 45 and 50 to ensure wear resistance without causing excessive difficulty in polishing or increasing the risk of internal stress. Only materials that meet all these conditions can support stable, high-yield production of PMMA products.
Common Mold Materials for PMMAS136 stainless steel is the top choice for high‑end PMMA molds. It offers exceptional purity, corrosion resistance, and polishability, easily achieving a #10000 mirror finish. After quenching, it reaches HRC 48–52 and supports over one million cycles, making it ideal for optical parts, automotive lamps, and high‑precision light guide plates. NAK80 is a pre‑hardened mirror steel that requires no additional heat treatment. It provides good polishability up to #8000, stable machining, and short delivery times, making it suitable for medium‑volume production. However, its corrosion resistance is limited. 420 series stainless steel provides a balance between performance and cost. It offers decent polish and corrosion resistance after heat treatment and is widely used for medium‑precision transparent parts with budget constraints. It performs well in general transparent components where high optical grade is not required.
Process Matching and Application AdviceHardness, polishing, and heat treatment must be matched to material properties. For glass‑filled PMMA, hardness can be increased to HRC 50–54 for improved wear resistance. Polishing processes differ: S136 and 420 require multi‑stage fine polishing to avoid orange peel and pinholes, while NAK80 can be directly finished. Vacuum quenching is recommended for S136 and 420 to reduce deformation and improve internal structure stability. For high‑demand molds, surface treatments such as nitriding or PVD coating may be applied to extend service life and improve wear resistance. The cooling system design must also be considered, as uniform cooling helps reduce internal stress and deformation in PMMA parts.
Final Selection GuidelinesHigh‑grade optical products, medical components, and long‑run molds should use S136 for maximum stability and finish. Medium‑volume projects with cost sensitivity and moderate surface requirements are best suited for NAK80 due to its short manufacturing cycle. Small‑to‑medium runs with surface requirements and limited budget can use 420 stainless steel. Common steels like P20 or 718 are not recommended due to rusting and poor polishability, which lead to consistent quality issues. Proper material selection ensures stable production, high transparency, low defect rates, and long mold life in PMMA injection molding.
ConclusionPMMA mold material selection is a key step in quality control for transparent plastic parts. The goal is to balance polishability, corrosion resistance, stability, and cost according to product positioning and production volume. With appropriate material choice and standardized processing, including precise machining, correct heat treatment, and fine polishing, manufacturers can achieve consistent high quality, high productivity, and reduced overall cost in PMMA injection molding. A reliable mold material system directly supports improved product competitiveness and lower long-term production risks.
