Injection molding process of three commonly used transparent plastics
Commonly used transparent plastics in injection molding include polymethyl methacrylate (PMMA), polycarbonate (PC), and polystyrene (PS), each with unique properties and injection molding process requirements. PMMA, commonly known as organic glass, offers excellent light transmittance and weather resistance, but its heat resistance and impact strength are relatively low. When injection molding PMMA, process parameters must be strictly controlled to ensure the transparency and surface quality of the finished product. For example, PMMA’s poor melt flowability requires a high injection temperature, typically 230-260°C, and a moderate injection pressure, typically 80-120 MPa, to avoid excessive pressure that increases internal stress in the product and can cause cracking.
Polycarbonate (PC) is a transparent plastic with excellent overall properties, boasting high impact strength, heat resistance, and dimensional stability. It is widely used in electronics, automotive, and medical devices. The injection molding process for PC is complex, and its high melt viscosity requires high injection temperatures and pressures. The injection temperature for PC is typically 270-310°C. If the temperature is too low, the melt fluidity is poor, making defects such as material shortages and weld marks more likely to occur. If the temperature is too high, the material may degrade, affecting the transparency and mechanical properties of the finished product. Injection pressure is generally 100-150 MPa, with a holding pressure of 60%-80% of the injection pressure. The holding time should be sufficiently long to minimize shrinkage and internal stress in the product. An electronics factory, producing PC mobile phone cases, achieved a 5% increase in the case’s light transmittance without internal stress cracking by controlling the injection temperature at 290°C, adjusting the injection pressure to 120 MPa, and extending the holding time to 5 seconds.
Polystyrene (PS) is a low-cost, transparent plastic with good fluidity and processing properties, but it has low impact strength and poor heat resistance, making it suitable for the production of daily necessities and packaging materials. The PS injection molding process is relatively simple, with low injection temperatures, typically 170-200°C, and relatively low injection pressures, typically 50-80 MPa. Due to PS’s rapid cooling rate, the mold temperature can be controlled at 40-60°C, shortening the molding cycle. However, it is important to note that PS is prone to internal stress, and excessive holding pressure and rapid cooling should be avoided during the injection molding process. A packaging factory, when producing PS transparent boxes, used a lower holding pressure (50% of the injection pressure) and a slower cooling rate, effectively reducing internal stress and lowering the box breakage rate from 10% to 2%.
During the injection molding process, all three types of transparent plastics require careful attention to prevent the formation of impurities and bubbles to ensure the transparency of the finished product. The raw materials must be thoroughly dried before use to remove moisture, as moisture converts to steam at high temperatures, forming bubbles within the finished product. The drying temperature for PMMA is 80-90°C, and the drying time is 3-4 hours; the drying temperature for PC is 120-130°C, and the drying time is 4-6 hours; and the drying temperature for PS is 60-70°C, and the drying time is 2-3 hours. During the production of PMMA lenses, a certain eyewear factory encountered numerous bubbles due to insufficient drying of the raw materials. By extending the drying time to 4 hours, the bubble problem was completely resolved, and the lens qualification rate reached over 98%.
Mold design also significantly impacts the injection molding process for the three transparent plastics. The mold cavity surface should be highly smooth to ensure the surface quality and transparency of the finished product. Polishing is typically required, with a roughness Ra of less than 0.02μm. Gate and runner design should avoid turbulence and shear overheating in the melt to reduce internal stress and weld marks in the product. For PMMA and PC, due to their higher melt viscosities, larger gates and runners are recommended. For PS, due to its better fluidity, smaller gates and runners can be used. A lighting manufacturer, producing PC lampshades, optimized gate position and adopted fan-shaped gates to ensure uniform melt filling of the cavity, reducing weld marks and significantly improving the uniformity of the lampshades’ light transmission. Furthermore, the mold should have ample venting slots to expel trapped air, preventing surface burns and bubbles from forming due to trapped air.
