PC (polycarbonate) materials possess characteristics such as high strength, good transparency, low molding shrinkage, and good processing performance, making them widely used in the electronics and electrical industries and in industrial parts. Today, Daliang will discuss with you the injection molding process and screw selection for PC materials.
PC has excellent performance, high transparency, good impact toughness, creep resistance, and a wide operating temperature range.
The processing characteristics of PC are: the melt viscosity is less sensitive to shear rate but highly sensitive to temperature, it has no distinct melting point, the melt viscosity is relatively high, the resin is prone to hydrolysis at high temperatures, and the molded products are prone to cracking.
Considering these characteristics, we need to pay special attention to the following: To increase the fluidity of the melt, instead of increasing the injection pressure, the injection temperature should be increased. The mold runners and gates should be short and thick to reduce pressure loss, and a higher injection pressure is required. The resin needs to be thoroughly dried before molding to control the water content below 0.02%. In addition, insulation measures should be taken during processing to prevent re-absorption of moisture. Not only is a reasonable product design necessary, but also a correct understanding of the molding process is crucial. For example, increasing the mold temperature and post-processing of the product can reduce or eliminate internal stress. Process parameters should be adjusted promptly depending on the specific product.
Injection Molding Process
1. The injection temperature must be determined after considering the shape and size of the product, the mold structure, and the performance and requirements of the product. Generally, the molding temperature is between 270 and 320°C. If the material temperature is too high, such as exceeding 340°C, the PC will decompose, the product color will darken, and defects such as silver streaks, dark lines, black spots, and bubbles will appear on the surface. At the same time, the physical and mechanical properties will also decrease significantly. 2. Injection pressure has a certain influence on the physical and mechanical properties, internal stress, and molding shrinkage rate of PC products, and a significant impact on the appearance and demolding properties of the products. Too low or too high injection pressure can cause certain defects in the product. Generally, the injection pressure is controlled between 80-120 MPa. For thin-walled, long-flow, complex-shaped products with small gates, a higher injection pressure (120-145 MPa) is used to overcome the resistance of melt flow and ensure timely filling of the mold cavity, thus obtaining a complete and smooth-surfaced product.
3. Holding pressure and holding time: The magnitude of the holding pressure and the length of the holding time have a significant impact on the internal stress of PC products. Too low a holding pressure results in insufficient shrinkage compensation, easily leading to vacuum bubbles or surface shrinkage. Too high a holding pressure can easily generate large internal stresses around the gate. In actual processing, this is often solved by using high melt 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, while large and thick products require a longer holding time. The length of the holding time can be determined through testing the gate sealing time.
4. Injection speed: Injection speed does not have a very significant impact on the performance of PC products, except for thin-walled, small-gated, deep-hole, and long-flow products. Generally, medium or slow speed processing is used, preferably multi-stage injection, typically using a slow-fast-slow multi-stage injection method.
5. Mold temperature: Generally, it is controlled at 80-100℃. For complex shapes, thinner products, and products requiring higher quality, it can be increased to 100-120℃, but it should not exceed the mold's thermal deformation temperature.
6. Screw speed and back pressure: Due to the high viscosity of the PC melt, to facilitate plasticization, degassing, and maintenance of the injection molding machine, and to prevent excessive screw load, the screw speed should not be too high, generally controlled at 30-60 r/min. The back pressure should be controlled between 10-15% of the injection pressure. 7. During the injection molding process of PC, the use of mold release agents must be strictly controlled, and the use of recycled material should not exceed three times, with the usage amount being around 20%.
The requirements for injection molding machines used to produce PC products are as follows: the maximum injection volume of the product (including runners and gates) should not exceed 70-80% of the nominal injection volume. The screw should be a single-threaded, constant-pitch screw with a check ring and a gradual compression type. The length-to-diameter ratio (L/D) of the screw should be 15-20, and the geometric compression ratio (C/R) should be 2-3.
2. Fillers, also called fillers, can improve the strength and heat resistance of plastics and reduce costs. For example, adding wood flour to phenolic resin can greatly reduce costs, making phenolic plastic one of the inexpensive plastics, while also significantly improving mechanical strength. Fillers can be divided into two categories: organic fillers and inorganic fillers. The former includes wood flour, shredded cloth, paper, and various textile fibers, while
