Analysis of Common Defects in PC Injection Molding and Its Solutions (Part One)

Polycarbonate or PC is an engineering plastic with excellent performance, which not only has high transparency, excellent impact toughness, but also has features of creep resistance, non-toxicity, wide temperature ranges, good dimensional stability, excellent electrical insulation and good weather resistance. Therefore, it is widely used in instruments and apparatus, lighting appliances, electronic and electrical equipment, household appliances, packaging and other industries. In addition, with the rapid advancement of polymer material technology, new varieties of its modification continue to appear, which greatly expands its application fields. At the same time, its processing technology has drawn more and more attention from all aspects. PC is a linear polymer containing benzene ring, isopropyl and ester bond in the molecular backbone structure. This structure makes PC rigid, flexible and high temperature resistant, but has high melt viscosity of the resin and is sensitive to moisture, which will bring certain difficulties to the injection molding process. Its processing characteristics are that there is no obvious melting point, and the melt viscosity is high at the normal processing temperature which is from 230 to 320℃. The viscosity is less sensitive to shear rates but highly sensitive to temperature, which is similar to Newtonian fluid behavior. Being sensitive to moisture, the resin is easy to hydrolyze under high temperature and the product is easy to generate internal stress. This shows that PC is a relatively difficult plastic to process. Therefore, in the actual production process, we encounter many problems. Now we will analyze and discuss several common product defects.
   
1. Color changing, blackening, yellow streaks and black spots appearing on the product
Relatively speaking, PC has better heat resistance. Usually, when ordinary PC materials are processed, the melting temperature can be set at 240 to 300°C. Even if it stays for a long time, it will generally not decompose. However, why do discoloration often occurs in the production of some electrical appliances? This is because the current market competition is fierce. In order to reduce production costs, most manufacturers use PC modified materials or recycled materials when producing mid-end and low-end electrical appliances. Some manufacturers even use their own blended materials mixed by flame retardants and fillers. Because these materials are mixed and the plasticization requirements are high, the process control is more difficult, which causes problems like this. In view of the above phenomenon, it is necessary to consider and find solutions from the following aspects: 
  
(1) In terms of process conditions, the melting temperature should be mainly considered. Generally, the temperature of the barrel should be lowered step by step, especially the temperature of the first two sections, and different temperatures are used for different materials. For example, using polyethylene (PE) modified PC to produce large electrical appliances, the temperature of the barrel should generally be controlled at about 230°C. When ABS or PS modified PC is used to produce small electrical parts such as switches and sockets, the temperature of the barrel should generally be controlled at about 250°C; the temperature of the barrel should generally be controlled at about 280°C when PBT modified PC is used to produce lighting products. Of course, requirements for product shapes, sizes, mold structures and product performance must be consider fully for the final selection of the molding temperature. Also, the raw materials should be fully dry to reduce the possibility of catalytic cracking of the hot melt due to trace moisture. In addition, if the screw speed is too fast; the back pressure is too high; the injection rate is too fast; and if the nozzle aperture, runner and gate size are too small, etc., high shear heat will be generated in the melt, which will cause melt fracture in PC. Moreover, the gas in the mold cavity cannot be discharged in time, causing partial burns and blackening of the product.
 

(2) In terms of equipment, because PC has features of high melt viscosity, poor fluidity, high injection pressure, strong bonding force with metallography and strong corrosiveness of decomposition products to metals, it is necessary to choose small or special processing equipment and chrome plated scrolls when selecting processing equipment. Moreover, dead corners, excess materials, gaps and cracks are not allowed in the plasticization system. Generally speaking, if there is no problem with the process conditions and the melt is found to be discolored during air injection, this indicates that there is a problem with the plasticizing system. It is necessary to check the plasticizing system one by one, starting from the nozzle to the nozzle flange, screw and barrel. Sometimes the product will periodically show the discoloration of two or three molds at regular intervals. This is mostly related to the excess material in the plasticizing system. When the PC decomposer exceeds a certain amount, it has its own catalytic effect and causes a large melt to decompose, especially plastics with flame retardants are added. In this case, it is necessary to find out the excess material locations such as scrolls sticking materials, material storage and barrels sticking materials, which need to be solved by cleaning, repairing and polishing.

   
(3) In terms of materials and operating methods, if black spots are found as soon as the machine is turned on, this is mostly related to the material storage of the barrel. Therefore, you must pay attention to the operation method. When the material stored in the barrel is PC before starting up, clean the barrel three to four times with new material at the molding temperature by air injection. If the material stored is the other material, especially the material with poor thermal stability such as PVC, POM, etc., which requires that the temperature cannot be increased when the machine is turned on, and PC materials cannot be used to clean the barrel. Only materials with good thermal stability such as PS and PE can be used at lower temperatures. After the material is cleaned, the temperature of the barrel should be increased to the normal processing temperature of PC, and then clean with PC material before processing.
 
During the processing, if the production needs to be temporarily suspended, the temperature of the barrel must be lowered to below 160°C for heat preservation to prevent the material from decomposing and discoloring for too long, because the glass transition temperature of PC is 160°C. When the production task is completed, the barrel can be cleaned by materials with good thermal stability such as PS, PE, etc., and the machine needs to be shut down after being emptied.
 
If there is always discoloration during the production process, first check whether or not there are any problems with the materials. For example, whether other materials and foreign materials are mixed; whether there are quality problems in the new materials; whether the pouring gate materials are qualified; whether the compounding method is correct. After excluding them one by one, check for other reasons. Another factor is that the environmental pollution is more serious, such as a lot of dust in the air, the mold being contaminated and the filter of the self-baking hopper not working and sucking more dust particles. This requires the processing workshop to be kept clean and tidy at all times. It is best to cover the air inlet and outlet of the hopper with fine gauze. This is very necessary when transparent products are processed.