Introduction to Injection Molding Process

　　Introduction to the characteristics and injection molding process of commonly used transparent raw materials for injection molding

　　1. Characteristics of commonly used transparent raw materials

　　Transparent plastic, due to its high transmittance, inevitably requires strict surface quality requirements for plastic products, without any defects such as spots, pores, whitening, haze, black spots, discoloration, and poor gloss. Therefore, strict or even special requirements must be paid attention to and put forward for the design of raw materials, equipment, molds, and even products throughout the entire injection molding process.

　　Secondly, due to the high melting point and poor flowability of transparent plastics, in order to ensure the surface quality of the product, it is often necessary to make slight adjustments to process parameters such as high temperature, injection pressure, and injection speed, so that the injection of plastic can not only fill the mold, but also avoid internal stress that may cause product deformation and cracking.

　　Due to the high melting point and poor fluidity of transparent plastics, in order to ensure the surface quality of the product, it is often necessary to make slight adjustments to process parameters such as high temperature, injection pressure, and injection speed, so that the mold can be filled with plastic during injection without generating internal stress and causing product deformation and cracking.

　　2. Issues to be noted in terms of technology

　　In order to reduce internal stress and surface quality defects, attention should be paid to the following aspects in the injection molding process.

　　1) The injection temperature should be higher than the injection humidity, provided that the plastic resin does not decompose;

　　2) Injection pressure: Generally high to overcome the defect of high melt viscosity, but too high pressure will generate internal stress and cause difficulty and deformation during demolding;

　　3) Injection speed: Generally, it should be low when meeting the requirements for mold filling, and slow fast slow multi-stage injection can be used;

　　4) Holding time and forming cycle: under the condition of meeting the requirements of product mold filling without generating dents or bubbles; It is advisable to keep it as short as possible to minimize the residence time of the molten material in the machine barrel;

　　5) Screw speed and back pressure: On the premise of meeting the plasticization quality, it should be as low as possible to prevent the possibility of detachment;

　　6) Mold temperature: The cooling quality of the product greatly affects its quality, so the mold temperature must be able to accurately control its process. If possible, the mold temperature should be higher.

　　Due to the need to prevent deterioration of surface quality, release agents are generally minimized during injection molding; When using recycled materials, it should not exceed 20%

　　Injection molding process for commonly used transparent raw materials

　　In addition to the common issues mentioned above, transparent plastics also have some process characteristics, which are described as follows:

　　1. PMMA has high viscosity and slightly poor flowability, so high material temperature and high injection pressure are necessary for injection molding. The influence of injection temperature is greater than that of injection pressure, but an increase in injection pressure is beneficial for improving the shrinkage rate of the product.

　　The injection temperature range is wide, with a melting temperature of 160 ℃ and a decomposition temperature of 270 ℃. Therefore, the material temperature adjustment range is wide and the processability is good. Therefore, improving fluidity can be started from the injection temperature. Poor impact resistance, poor wear resistance, easy scratching, and brittle cracking, therefore, it is necessary to increase the mold temperature and improve the condensation process to overcome these defects.

　　2. PC has high viscosity, high melting temperature, and poor flowability. Therefore, injection molding at a higher temperature (270

　　-Between 320T, the range of material temperature adjustment is relatively narrow, and the processability is not as good as PMMA. The injection pressure has a small impact on flowability, but due to its high viscosity, a higher injection pressure is still required. Therefore, in order to prevent the generation of internal stress, the holding time should be as short as possible.

　　The shrinkage rate is large and the size is stable, but the product has high internal stress and is prone to cracking. Therefore, it is advisable to improve the fluidity by increasing temperature instead of pressure, and reduce the possibility of cracking by increasing mold temperature, improving mold structure, and post treatment. When the injection speed is low, defects such as ripples are prone to occur at the gate. The temperature of the radiation nozzle needs to be controlled separately, the mold temperature needs to be high, and the resistance of the runner and gate needs to be small.

　　3. PET has a high molding temperature and a narrow range of material temperature adjustment (260-300 ℃), but after melting, the flowability is good, so the processability is poor, and often anti flow devices are added in the nozzle. The mechanical strength and performance are not high after injection, and it is necessary to improve the performance through stretching and modification. Accurate control of mold temperature is to prevent warping. Therefore, it is recommended to use Hot runner die. The mold temperature is high, otherwise it will cause poor surface gloss and difficulty in demolding.

　　4. Defects and Solutions of Transparent Plastic Parts

　　1. Silver streaks: Due to the anisotropy of internal stress during mold filling and condensation, the stress generated in the vertical direction causes the resin to flow upward, while the refractive index is different from the non flow direction, resulting in shiny silk streaks. When it expands, it may cause cracks in the product. In addition to paying attention to the injection molding process and molds, the products undergo annealing treatment. If the PC material can be heated to above 160 ℃ for 3-5 minutes, then cooled naturally.

　　2. Bubbles: Due to the inability to expel water vapor and other gases from the resin (during the mold condensation process) or insufficient mold filling, the condensation surface condenses too quickly, forming a "vacuum bubble".

　　3. Poor surface gloss: Mainly due to the large roughness of the mold, on the other hand, premature condensation prevents the resin from replicating the surface state of the mold, all of which cause small unevenness on the surface and cause the product to lose its luster.

　　4. Vibration pattern: refers to the dense ripple formed from the center of the straight gate, which is caused by the high viscosity of the melt. The front material has already condensed in the mold cavity, and later the material breaks through this condensation surface, resulting in vibration patterns on the surface.

　　5. Whitening and fog halo: mainly caused by dust falling into the raw material or excessive moisture content in the air.

　　6. White smoke and black spots: mainly formed by the decomposition or deterioration of the resin in the barrel caused by local overheating of the plastic inside the barrel.