PMMA, also known as acrylic or organic glass, is widely used in optical lenses, decorative panels, home appliance shells, medical devices, and automotive lamps due to its high transparency, excellent weather resistance, high hardness, and good formability. However, PMMA has high brittleness, strong moisture absorption, temperature sensitivity, and moderate fluidity, making it a difficult transparent material to process. Even small deviations in parameters or improper operation can cause silver streaks, bubbles, flow marks, cracks, whitish marks, and reduced transparency, which directly affect product quality and yield. Based on full-process injection experience and industrial practice, this article summarizes detailed operational guidelines to ensure stable production of high-quality PMMA products.
Raw Material Pretreatment: Drying Is the Foundation to Avoid Moisture DefectsPMMA contains hydrophilic groups and absorbs moisture easily. Excessive moisture causes melt decomposition during injection, leading to silver streaks, bubbles, and cloudy patterns that severely damage transparency. Therefore, sufficient drying is the first critical step in PMMA molding and must be strictly implemented. Dehumidifying dryers are strongly recommended; ordinary hot-air dryers cannot fully remove moisture, and undried materials must never be used directly. The recommended drying temperature is 80–90°C for no less than 3–4 hours, and for seriously damp materials, 4–6 hours is required to reduce moisture content below 0.02%. After drying, the hopper must maintain 70–80°C to prevent re-absorption. Sealed hoppers are essential, and materials left unused for over 4 hours must be re-dried. Raw materials should be screened to remove impurities, agglomerates, and dust. Mixing different grades or batches is not advised, as inconsistent flowability causes molding instability and appearance defects.
Temperature Control: Precise Control to Prevent Overheating or Insufficient HeatingPMMA is extremely sensitive to temperature. Excessively high temperature causes decomposition, yellowing, carbonization, bubbles, and black spots, while low temperature results in insufficient filling, flow marks, and cold lines. Temperature settings must follow the principle of “stable and uniform, rather too low than too high”. Barrel temperature is controlled sectionally: feeding section 200–220°C, compression section 230–240°C, metering section 225–235°C. Nozzle temperature is 220–235°C, slightly lower than the front barrel to prevent drooling and decomposition. Transparent high-gloss parts can be molded at 10–15°C lower to reduce decomposition and improve gloss. For low-flowability PMMA, a slight increase of 5–10°C is allowed but must not exceed 260°C, the decomposition threshold. Real-time monitoring is necessary, with temperature fluctuation controlled within ±5°C. Regular inspection of heating coils and thermocouples prevents failures that cause mass defects.
Mold Design: Reasonable Structure Ensures Appearance and StabilityMold quality directly determines PMMA product appearance, dimensional accuracy, and stability. Considering PMMA’s high brittleness and moderate flowability, special attention must be paid to venting, runners, gates, cavity surfaces, and ejection systems. Poor venting easily causes short shots, gas lines, burning, and fogging. Venting grooves at parting lines, corners, and weld lines should be 0.015–0.03mm deep and 5–8mm wide. Runners should be short, thick, and smooth to reduce pressure loss. Gates preferably use round or fan shapes and should be properly enlarged to avoid excessive shear stress. Pin-point gates are not recommended due to stress concentration. The cavity surface must be polished to mirror level above #8000, free of pits and scratches. The ejection system must be balanced with multi-point support to avoid whitish marks and cracks. Ejection speed should be slow and uniform. Before production, the mold must be cleaned of oil, water, and rust residue. Regular maintenance, polishing, and lubrication maintain long-term stability.
Injection Parameters: Medium Speed and Medium Pressure to Reduce StressPMMA is sensitive to internal stress, so parameters should focus on reducing shearing and stabilizing filling. Injection speed adopts multi-stage control: low speed at the initial stage to avoid impact, medium speed during filling, and low speed at the end to stabilize pressure. Injection pressure is generally 80–120MPa, adjusted to just fill the cavity. Holding pressure is 50%–60% of injection pressure, with a time of 10–30 seconds. Excessively high pressure or long holding time increases internal stress and causes cracking. Cooling time must be sufficient, usually 20–60 seconds depending on wall thickness. Insufficient cooling leads to deformation and cracking, while excessive cooling reduces efficiency.
Internal Stress Control and Defect PreventionInternal stress is the main cause of cracking in PMMA products during storage, assembly, and use. Stress can be reduced by avoiding excessive pressure, speeding, and unbalanced ejection. Annealing at 70–80°C for 2–4 hours followed by slow cooling eliminates over 80% of internal stress. Common defects such as silver streaks and bubbles are solved by improving drying, lowering temperature, enhancing venting, and adjusting speed. Flow marks require higher mold and barrel temperatures and proper flow path design. Cracks and whitish marks need lower pressure, slower ejection, higher mold temperature, and annealing. Poor transparency comes from uncleaned molds, insufficient drying, overheating, or insufficient polishing.
Equipment Operation and Post-treatmentPMMA requires a gradually compressed screw with L/D ratio 20–25:1. Special screws are recommended for transparent products to avoid contamination. Before shutdown, the barrel must be cleaned with PP or PE. The production environment should be clean, dust-free, dry, and temperature-controlled at 20–25°C with humidity below 60%. Products must be handled gently to avoid scratches and fingerprints. Proper storage avoids direct sunlight, high temperature, humidity, and heavy stacking. Products used for screen printing, bonding, or plating must pass stress testing to prevent cracking during post-processing. Strictly following these guidelines ensures high transparency, stable dimensions, and consistent quality in mass PMMA production.
