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, excellent dimensional stability, good electrical insulation, and a wide operating temperature range. Its transparency makes it ideal for applications where visibility is important. PC is widely used in the manufacturing of electrical appliances, instrument casings, and structural components in electronics.
There are many modified versions of PC, such as those reinforced with glass fiber, mineral fillers, or flame retardants. These modifications help enhance certain properties like heat resistance or fire safety. However, because PC has low melt flow and requires high processing temperatures, some modified grades may need specialized injection molding equipment to ensure optimal results.
1. **Plastic Processing** PC has a relatively high moisture absorption rate, so pre-drying is essential before processing. Pure PC should be dried at 120°C for over 4 hours, while modified PC is usually dried at 110°C for the same duration. Drying time should not exceed 10 hours. The drying process can be checked using the air extrusion method to ensure the material is sufficiently dry.
Recycled materials can make up to 20% of the blend, and in some cases, 100% recycled content may be used depending on product quality requirements. However, it's crucial to avoid mixing different masterbatches when using recycled materials, as this can lead to serious property degradation in the final product.
2. **Injection Molding Machine Selection** Due to cost considerations and the need for flame resistance in many PC applications—especially in electrical products—it’s important to choose an injection molding machine that can handle modified PC materials. Flame-retardant PCs and plastic alloys require a plasticizing system with good mixing performance and corrosion resistance. Standard screws may not perform well, so it's recommended to use specialized PC screws, such as those offered by Ramada, for better results.
3. **Mold and Gate Design** The typical mold temperature ranges from 80–100°C, and when glass fiber is added, it increases to 100–130°C. For small parts, needle gates are commonly used, with the gate depth set at 70% of the thickest section. Other gate types include ring or rectangular gates.
Larger gates are preferred to reduce shear-related defects. The vent hole depth should be less than 0.03–0.06 mm, and the flow path should be as short and smooth as possible. A draft angle of around 30'–1° is typically recommended for easier ejection.
4. **Melt Temperature** The processing temperature can be determined using the air injection method. Typical PC processing temperatures range from 270–320°C, while modified or low-molecular-weight PC grades may require lower temperatures, around 230–270°C.
5. **Injection Speed** A faster injection speed is often preferred for better filling, especially in parts like electrical switches. Some applications may use a slow-to-fast injection profile for improved surface finish and reduced warping.
6. **Back Pressure** A back pressure of about 10 bar is generally sufficient. It should be adjusted carefully to avoid issues like air bubbles or color mixing.
7. **Residence Time** Prolonged residence time at high temperatures can cause material degradation, leading to discoloration or gas emission (such as COâ‚‚). It's important to avoid cleaning the barrel with materials like LDPE, POM, ABS, or PA; instead, PS is recommended for cleaning purposes.
8. **Important Notes** Some modified PC materials may develop dark brown liquid bubbles if they are recycled too many times or if the components are not mixed evenly. This issue can affect both the appearance and performance of the final product, so careful monitoring during processing is necessary.
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