Injection molding of transparent plastics
Due to their excellent light transmittance and aesthetics, transparent plastics are widely used in electronics, medical devices, daily necessities, and other fields. However, their injection molding process differs significantly from that of ordinary plastics, placing higher demands on the production environment, equipment precision, and process parameter control. The core characteristic of transparent plastics is light transmittance, which is usually required to reach over 90%. This means that any defects that affect light transmittance, such as bubbles, silver streaks, black spots, and weld marks, must be avoided during the injection molding process. Common transparent plastics include polymethyl methacrylate (PMMA), polycarbonate (PC), and polystyrene (PS). Different types of transparent plastics require different injection molding process parameters due to differences in molecular structure and performance, and require targeted adjustments.
Pretreatment of raw materials before transparent plastic injection molding is a critical step in ensuring product quality. Because transparent plastics are highly hygroscopic, if the raw materials contain moisture, hydrolysis will occur during the high-temperature injection molding process, causing the plastic molecular chains to break, resulting in defects such as bubbles and silver streaks, seriously affecting the product’s transparency and mechanical properties. Therefore, transparent plastics must be thoroughly dried before injection molding. Taking PC plastic as an example, it is typically dried at a temperature of 120-130°C for 4-6 hours, and the moisture content of the dried raw materials should be controlled below 0.02%. PMMA plastic requires drying at 80-90°C for 3-4 hours. The drying equipment should use a hot air circulation dryer to ensure uniform heating of the raw materials and prevent the incorporation of dust and impurities during the drying process. A filter can be installed at the feed inlet of the drying hopper, and the interior of the drying equipment should be cleaned regularly.
The selection and commissioning of injection molding equipment significantly impacts the quality of transparent plastic molding. Due to the high viscosity and poor fluidity of transparent plastic melts, an injection molding machine with sufficient injection volume and strong plasticizing capacity is required. The screw and barrel surfaces should be highly polished, with a surface roughness Ra below 0.2μm to reduce melt flow resistance and prevent black spots or scorching caused by material retention. Use straight-through nozzles, avoiding right-angle nozzles with dead angles to prevent prolonged melt retention at the nozzle and decomposition. During equipment commissioning, check the parallelism of the mold clamping mechanism and the stability of the clamping force to ensure a tight mold closure. This prevents flashing due to insufficient clamping force, or mold deformation due to excessive clamping force, which can affect the product’s dimensional accuracy and light transmittance.
The injection molding process for transparent plastics requires precise control of process parameters, encompassing temperature, pressure, speed, and time. Temperature control is crucial. The barrel temperature should be appropriately set based on the plastic type. For example, the barrel temperature for PC plastic is typically 260-300°C, while for PMMA it’s 180-230°C. Excessively high temperatures can cause plastic decomposition, resulting in discoloration or black specks; excessively low temperatures can lead to uneven melt plasticization, affecting product transparency. Mold temperature is also crucial. Properly increasing the mold temperature can reduce melt cooling, facilitate gas discharge and melt flow, and reduce internal stress. For example, mold temperatures for PC plastic are typically 80-120°C, while for PMMA it’s 40-60°C. Injection pressure and speed must be matched. Insufficient pressure can result in underfilling, while excessive speed can easily generate turbulence and bubbles. A moderate injection speed is typically used to ensure smooth melt filling of the cavity. Holding pressure and time should be adjusted based on product thickness to minimize shrinkage and internal stress, preventing sinking or cracking.
Post-injection processing and testing of transparent plastics are also crucial. After molding, the product should be annealed to eliminate internal stresses and prevent cracking or deformation during use. The annealing temperature is typically 10-20°C below the plastic’s glass transition temperature. For example, the annealing temperature for PC plastic is 120-130°C. After 2-4 hours of holding, the product is slowly cooled. Furthermore, the product undergoes rigorous quality testing, including light transmittance testing, appearance inspection, and dimensional measurement. Light transmittance can be tested using a transilluminator to ensure compliance with design requirements. Appearance inspection requires visual inspection or a magnifying glass, ensuring there are no defects such as bubbles, scratches, or black spots. Dimensional measurement requires the use of precision measuring tools, such as a coordinate measuring machine, to ensure dimensional accuracy. High-quality transparent plastic products can only be produced through rigorous control of every step of the injection molding process.
