Polycarbonate (PC) is a well-known thermoplastic material, often referred to as "bulletproof rubber" due to its outstanding mechanical properties. It offers high impact resistance, a wide operating temperature range, excellent electrical insulation, and good dimensional stability. Additionally, it is transparent and maintains its structural integrity under various conditions.
PC is widely used in electrical appliances, instrument housings, and electronic components. To enhance its performance, PC is often modified by adding glass fiber, mineral fillers, flame retardants, or blended with other plastics. However, due to its low melt flow and high processing temperatures, many modified grades require specialized injection molding equipment.
Before processing, PC must be dried thoroughly because of its high moisture absorption. Pure PC should be dried at 120°C for over 4 hours, while modified PC is typically dried at 110°C for the same duration. Drying time should not exceed 10 hours. The air extrusion test is commonly used to check if the drying process is sufficient.
Recycled materials can be added up to 20% in some cases, and in others, 100% recycled material may be used depending on product quality requirements. However, different masterbatches should not be mixed with recycled materials, as this could severely affect the final product’s properties.
When selecting an injection molding machine, it's important to consider that modified PC, especially flame-retardant versions, requires a plasticizing system with good mixing and corrosion resistance. Standard screws may not perform well, so it's advisable to choose a dedicated PC screw, such as those offered by Ramada.
Mold design plays a crucial role in achieving quality parts. The mold temperature is usually between 80–100°C, while with glass fiber, it can go up to 100–130°C. Small parts often use needle gates, with gate depth around 70% of the thickest section. Other gate types include ring and rectangular gates. Larger gates are preferred to reduce shear-related defects. Vent hole depth should be less than 0.03–0.06 mm, and the flow path should be short and smooth. A draft angle of about 30'–1 is typically recommended.
The melt temperature is generally determined using an air injection method. Typical PC processing temperatures range from 270–320°C, while modified or low molecular weight PC may require 230–270°C.
Injection speed is usually set to a faster rate for better filling, especially in applications like electrical switch parts. Some processes may use a slow-to-fast injection approach.
Back pressure should be kept around 10 bar, adjusted to avoid air bubbles or color mixing.
Residence time at high temperatures can lead to material degradation, resulting in COâ‚‚ emissions and yellowing. Therefore, it's important to avoid cleaning the barrel with LDPE, POM, ABS, or PA. Instead, PS is recommended for cleaning.
It's also worth noting that some modified PCs may develop dark brown bubbles due to repeated recycling or poor mixing of components. Proper handling and processing techniques are essential to maintain product quality.
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