November 21, 2023
PC plastic material is a common amorphous engineering material with high transparency up to 98%, good impact toughness, creep resistance, and large sensitivity in the operating temperature range. PC has good mechanical properties, but poor flow properties, so the injection molding process of this material is difficult. For these characteristics, we must pay special attention to different treatment:
To increase the fluidity of the melt, it is not necessary to increase the injection pressure but to increase the injection temperature. The runner and gate of the mold are required to be short and thick to reduce the pressure loss of the fluid. At the same time, a higher injection temperature is required. pressure.
In addition, PC materials have 10 precautions in mold design and injection molding process that require special attention.
1. Combined with the material characteristics, when designing the glue nozzle, it is necessary to use S-shaped side glue inlet or fan-shaped glue inlet. Because PC has poor fluidity, when designing the glue mouth, the thickness of the glue inlet can be designed to be thicker to avoid high pressure. During filling, the shear stress of the material is increased. In addition, bubbles are likely to occur on the surface during molding, or abnormal phenomena such as silver streaks may occur near the glue inlet. Therefore, the design needs to be optimized into an S-shaped glue feeding pattern to allow the material to flow during the filling process. , first perform buffering in the flow channel. Note that the PC material does not allow the design of point glue or diving glue, because PC is commonly known as bulletproof glass glue.
2. If you are designing a long product, the shrinkage rate can be scaled normally, but one side in the length direction can be reduced inward by 0.2~0.4MM to prevent the shrinkage rate in the length direction from being smaller than the width direction. If the size in the length direction is large, , the entire mold core will be scrapped and there is a risk, because PC transparent parts, strictly speaking, the mold core is not allowed to be welded, so it is more convenient to make the mold core small first, and if the size is not within the tolerance range, then enlarge it.
3. For ejection design, in the case of external appearance, the peripheral ejection block is usually used for ejection, or an overflow bag is added for ejection.
4. Exhaust design: because PC material has poor fluidity during molding, it is recommended to exhaust air around the front mold core. The exhaust depth can be 0.03~0.04MM.
5. PC is extremely sensitive to water, so it must be fully dried before injection molding to reduce the moisture content to less than 0.02%. General drying conditions for PC: 100~120°C, for at least 4 hours.
6. The barrel temperature is generally selected between 270 and 320°C. If the material temperature is too high and exceeds 340°C, PC will decompose, the color of the product will become darker, and silver threads, dark stripes, black spots, and bubbles will appear on the surface. and other defects, and the physical and mechanical properties are also significantly reduced. PC is also very sensitive to temperature. The melt viscosity decreases significantly as the temperature increases. The barrel temperature is 250~320℃. (It is best not to exceed 350℃). Appropriately increasing the barrel temperature is good for PC plasticization, but the material Do not leave the barrel in place for more than 15 minutes.
7. Injection pressure: if the fluidity is poor, high-pressure injection is required, but the large residual internal stress of the plastic part (which may lead to cracking) must be taken into account. Injection speed: medium speed for wall thickness, high speed for thin wall.
8. The size of the holding pressure and the length of the holding time have a great influence on the internal stress of PC products. If the holding pressure is too small, the feeding effect is small and vacuum bubbles or dents will appear on the surface. If the holding pressure is too high, it is easy to generate large internal stress around the gate. In actual processing, it is often solved by high material temperature and low holding pressure. The selection of holding time should depend on the thickness of the product, gate size, mold temperature, etc. Generally, small and thin products do not require a long holding time. On the contrary, large and thick products should have a longer holding time. The length of the pressure holding time can be determined by testing the gate sealing time.
9. Injection speed: it has no obvious impact on the performance of PC products. Except for thin-walled, small gate, deep hole, and long-process products, medium or slow speed processing is generally used. Multi-stage injection is best, and generally used Slow-fast-slow multi-stage injection method.
10. Mold temperature: mold temperature control: 85~120°C, generally 80-100°C is sufficient. For products with complex shapes, thinness, and high requirements, it can also be increased to 100-120°C, but it cannot exceed the mold temperature. Heat distortion temperature. High mold temperature can reduce the difference between mold temperature and PC material temperature, which can reduce the internal stress of the part.