This content systematically sorts out the troubleshooting and solutions for the haze and low transparency defects of plastic products from four dimensions: raw material, process, mold and environment. In terms of raw materials, the first priority is to verify the moisture content of raw materials. For highly hygroscopic materials such as PET and PA, sufficient high-temperature dehumidification and drying must be carried out before processing to control the moisture content below 0.05%, preventing water vapor from vaporizing into micro-bubbles during melting and damaging the light transmittance. It is necessary to select raw materials with stable quality and minimize the use of recycled materials with excessive impurities. Meanwhile, check the additives in the formula, including plasticizers, lubricants and stabilizers, to confirm that their addition amount does not exceed the standard and their compatibility with the base material is well matched, so as to avoid the precipitation haze formed by excessive additives migrating to the product surface.
In terms of process, adjust the barrel temperature to ensure full plasticization of the material. Neither too low temperature that leads to incomplete plasticization, nor too high temperature that causes thermal degradation of the material and generates impurities is allowed. Appropriately increase the melt back pressure and control it within the range of 0.5 to 1.5 MPa, to promote uniform mixing of the melt and discharge the entrained air. Reduce the screw rotation speed to decrease shear heat generation and air entrainment. Adopt the multi-stage injection mode of slow-fast-slow and lower the injection speed, to reserve sufficient time for the gas in the cavity to be discharged. Appropriately increase the mold temperature to ensure that the melt can fully fit the cavity surface and reproduce the high-gloss finish effect. At the same time, reasonably reduce the clamping force to avoid over-tight clamping that blocks the exhaust channel of the mold cavity.
In terms of mold, finely polish the surface of the mold cavity to reduce the surface roughness, and reduce the condensation attachment points for water vapor and volatiles. Regularly dredge and clean the cooling water channels to ensure uniform temperature on the mold surface, and prevent water vapor from condensing due to excessively low temperature in local areas. At the positions with concentrated trapped gas such as weld lines and the end of material flow, reasonably open exhaust grooves with a depth of 0.02 to 0.04 mm to discharge the gas in the mold cavity in time.
In terms of environment, install dehumidification equipment in the production workshop to control the ambient relative humidity between 40% and 60%. Strengthen the ventilation of the workshop to timely discharge the water vapor and volatiles generated during the production process, and reduce the accumulation of haze in the workshop.
You can directly follow the steps below to quickly locate the root cause of the problem. No additional equipment is required throughout the process, and troubleshooting can be completed only through on-site observation and simple operations:
First, check the state of the newly produced products. If there are obvious haze marks on the products right after demolding, prioritize troubleshooting the raw material and mold links. Take a small amount of raw materials to be processed, dry them with a dehumidifying dryer at the standard temperature corresponding to the material for 4 hours, and then use this batch of dried raw materials for a separate trial production. If the haze on the produced products is significantly reduced or even disappears, it indicates that the problem lies in the excessive moisture content of the raw materials, and the previous raw material drying process is not in place. If the haze situation of the products does not improve at all after using the dried raw materials, proceed to troubleshoot the mold. First, raise the mold temperature by 5 to 10 degrees Celsius, and then carry out trial production for several cycles. If the haze layer becomes lighter, it means that the previous mold temperature was too low, the melt condensed rapidly after contacting the cold mold, and a fine uneven structure was formed on the surface.
Then observe the distribution position of the haze layer. If the haze is concentrated around the gate, it can basically be determined that the shear at the gate is overheated, the local material degrades or a large number of tiny bubbles are generated. This can be verified by adjusting and reducing the injection speed near the gate. If the haze is distributed at the end of the material flow and the corner positions of the product, it is most likely that the exhaust in these areas is not smooth, and the haze is caused by trapped gas. This can be confirmed by appropriately reducing the clamping force for trial production and seeing the haze disappear. If the product is completely transparent right after demolding and gradually shows haze after being placed for a period of time, it can basically be determined that the additives in the formula precipitate, which is commonly known as "blooming". This can be verified by replacing additives with better compatibility and reducing the addition amount of additives. If the haze evenly covers the entire surface of the product without obvious local concentration features, prioritize checking the ambient humidity. In the high-humidity weather in Qingdao, the water vapor in the workshop condenses on the surface of the cold mold and adheres to the product surface to form a uniform water haze. This cause can be confirmed after turning on the workshop dehumidification equipment to reduce the ambient humidity and the haze disappears.
