PMMA, commonly known as acrylic or plexiglass, is a non-crystalline thermoplastic with excellent optical properties. Its most prominent characteristic is its extremely high light transmittance (up to 92%), superior to ordinary inorganic glass, and it also possesses good weather resistance, surface hardness, and electrical insulation. However, PMMA is relatively brittle, has moderate impact resistance, and extremely high melt viscosity, which places specific requirements on processing technology.

 

Extremely strong hygroscopicity: Polar ester groups in PMMA molecular chain have a strong affinity for water, with a water absorption rate of 0.3-0.4%. Trace amounts of moisture can vaporize during processing, leading to bubbles, streaks (silver streaks), cloud-like spots on finished product, and causing a decrease in molecular weight, severely degrading optical and mechanical properties. Therefore, deep and thorough drying is an uncompromising prerequisite for PMMA processing.
High melt viscosity and temperature sensitivity: PMMA has poor melt flowability and is a thick-walled, slow-flowing material. Its viscosity is extremely sensitive to temperature changes; increasing temperature significantly improves flowability, but there is a clear upper limit to degradation.
Narrow thermal processing window: Continuous use temperature of PMMA is approximately 80-90℃, and its processing temperature needs to be much higher to ensure flowability. However, once thermal decomposition initiation temperature (approximately 260-270℃) is exceeded, degradation occurs rapidly, causing material to yellow, produce gases, and develop black spots.
Susceptibility to residual internal stress: Due to relatively high rigidity of molecular chains, molding stress is easily frozen under rapid or uneven cooling, leading to decreased solvent resistance (e.g., cracking upon contact with alcohol or ketones) and dimensional stability issues.

1. Pre-treatment: Drying
This is the first and most crucial step in determining success of PMMA injection molding.
Equipment: A dehumidifying dryer must be used; ordinary hot air drying is ineffective.
Temperature: 80-90℃. Excessive temperature may cause particle surface to soften and stick together.
Time: At least 4-6 hours, depending on material layer thickness and initial humidity.
Moisture Content Requirement: Must be reduced to below 0.02%, ideally below 0.01%.
Management: Dried material should be temporarily stored in an insulated hopper (70-80℃) and used as soon as possible. If exposed to humid air for more than 1 hour, it must be re-dried.
2. Molding Temperature
A precise balance must be achieved between improving flowability and preventing thermal degradation.
Ball Temperature Setting:
Rear Zone (Feed Section): 200-220℃. Primarily serves a preheating function; excessive temperature can easily cause bridging at discharge port.
Middle Zone (Compression Section): 230-250℃. Main plasticizing zone, providing most of heat required for melting.
Front Zone (Metering Section): 240-260℃. Ensures melt homogenization, preparing for injection.
Melt Temperature: Actual measured values should be strictly controlled within 230-250℃ range. Do not exceed 260℃.
Nozzle Temperature: Can be slightly lower than front zone by 5-10℃, approximately 230-245℃. Open nozzles can be used, but spring-loaded needle valve nozzles are recommended to prevent drooling.
3. Mold Temperature
Mold temperature has a decisive influence on internal stress, surface gloss, and transparency of PMMA products.
Range: 60-80℃. For high optical quality requirements or complex products, 70-80℃ is recommended.
Function Analysis: Higher mold temperatures can:
Significantly reduce melt cooling rate, allowing molecular chains to relax, greatly reducing molding internal stress, and improving resistance to solvent streaks.
Improves melt's ability to replicate within mold cavity, achieving extremely high surface gloss.
Maintains a good temperature at melt front, which is beneficial for thin-walled filling and reduces weld line strength.
Promotes uniform cooling, preventing turbidity or deformation caused by temperature differences.
4. Injection Pressure and Speed
Injection Speed: Medium to low speed injection should be used. Due to high viscosity of melt, a certain injection pressure is required, but high-speed injection should be avoided. High-speed injection generates excessive shear heat, which may lead to localized overheating and degradation, is also prone to generating jetting marks at gate, compromising optical uniformity of product. A multi-stage control of "slow-fast-slow" is usually used for smooth mold filling.
Injection Pressure: Due to high melt resistance, a relatively high injection pressure is required, typically ranging from 80-120 MPa.
Holding Pressure and Time: Holding pressure should be set to 50-70% of injection pressure. Holding time should be sufficient to compensate for shrinkage during melt solidification stage and prevent shrinkage marks. However, excessively high holding pressure and excessively long holding time will increase internal stress, should be optimized to prevent shrinkage marks in product.

 

5. Back Pressure and Screw Speed
Screw Speed: Low speed is required, recommended 20-50 rpm. High shear heat generated by high speeds is one of main causes of PMMA degradation.
Back Pressure: Use relatively low back pressure (3-10 Bar), only enough to remove gas from melt and ensure uniform plasticization.
6. Mold and Auxiliary Design
Runners and Gates: Should be designed to be short and wide to reduce flow resistance. Gate size must be large enough; fan gates, round gates, or direct gates are preferred. Avoid point gates to prevent high shear and stress concentration.
Ventilation: Good venting is required; vent depth should be 0.02-0.03 mm. Poor venting can lead to trapped gas, scorching, and incomplete filling.
Surface Treatment: Cavity must be highly polished to a mirror finish; any minor scratches will be magnified on transparent parts.
Cooling System: Requires uniform and efficient cooling to maintain a constant high mold temperature and avoid localized overcooling.

Bubbles, Silver Streaks: Primary and most likely cause is incomplete drying. Drying system needs immediate inspection. Secondly, excessively high material temperature can lead to decomposition.
Yellowing, Black Spots: Caused by excessively high barrel temperature or prolonged material residence time in barrel, leading to thermal degradation. Thoroughly clean barrel, lower temperatures in each zone, and optimize cycle to reduce residence time.
Surface Flow Marks, Ripples: Caused by excessively low melt or mold temperature and slow injection speed. Appropriately increase material and mold temperatures, optimize injection speed profile.
Cracking (especially after solvent contact): Caused by excessive internal stress. The most effective solution is to significantly increase mold temperature. Also, check for sharp corners in product and optimize holding pressure parameters.
Obvious Weld Lines: Increase mold and material temperatures; increase injection speed (within permissible limits); optimize gate location; add venting.

PMMA is often compared to PC, which is also a transparent material. Core differences in their processing are:
Drying: Both require strict drying, but PMMA requires a lower drying temperature (80-90℃ vs. PC's 120℃) and a longer drying time.
Processing Temperature: PMMA has a narrower processing temperature window and a lower upper limit (approximately 260℃ vs. PC's 320℃), making it more sensitive to overheating.
Mold Temperature: PC requires higher mold temperatures (80-120℃) to relieve stress, while PMMA achieves good results at 60-80℃, but requires extremely high uniformity.
Toughness: PC has good toughness and can withstand higher injection speeds; PMMA is brittle and requires a gentler molding process to prevent internal stress cracking.