Drying of plastics
Drying plastics is a crucial preparatory step before injection molding. Its purpose is to remove moisture and other volatiles from plastic particles to prevent these substances from adversely affecting the quality of the molded part during the injection molding process. Many plastic materials, such as polyamide (PA), polycarbonate (PC), and polyethylene terephthalate (PET), are highly hygroscopic and easily absorb moisture from the air during storage and transportation. If used directly in injection molding, this moisture vaporizes under the high temperature inside the barrel, forming bubbles. This can lead to defects such as silver streaks, pores, and surface depressions in the molded part. It can also cause degradation of the plastic’s molecular chains, reducing the part’s mechanical properties. Therefore, thorough and effective drying of plastics is a crucial prerequisite for ensuring smooth injection molding production and improving part quality.
The effectiveness of plastic drying depends primarily on parameters such as drying temperature, drying time, wind speed, and ambient humidity. These parameters require precise control based on the plastic type, hygroscopic properties, and moisture content requirements. Drying temperature is a key factor influencing drying efficiency. Excessively high temperatures may cause surface oxidation or degradation of plastic particles, affecting processing performance and part quality. Excessively low temperatures prevent effective moisture removal, preventing drying requirements. For example, the drying temperature for polyamide (PA6) is typically set at 80-90°C, while that for polycarbonate (PC) is higher, typically 120-130°C. Drying time is dependent on the plastic’s moisture content, particle size, and the performance of the drying equipment. Plastics with high moisture content or larger particles require longer drying times to ensure adequate diffusion and removal of moisture. Furthermore, an appropriate wind speed ensures that evaporated moisture is promptly removed from the drying equipment, maintaining a low humidity environment and improving drying efficiency. Generally, a wind speed of 2-3 m/s is ideal.
Commonly used plastic drying equipment includes hot air circulation dryers, dehumidifying dryers, and vacuum dryers. Different types of equipment are suitable for different plastic materials and drying requirements. Hot air circulation dryers are the most commonly used drying equipment. They operate by heating and circulating air within a drying drum to remove moisture from plastic particles. They are suitable for plastic materials with low drying requirements, such as polyethylene (PE) and polypropylene (PP). Dehumidifying dryers, based on hot air circulation, incorporate a dehumidifier to remove moisture from the air before heating it into the drying drum. They are suitable for highly hygroscopic plastics with high drying requirements, such as polyamide ( PA) and polycarbonate (PC), and can control the moisture content of plastics to below 0.02%. Vacuum dryers heat and dry plastics in a vacuum environment. Because a vacuum lowers the boiling point of water, they can achieve efficient drying at lower temperatures. They are suitable for heat-sensitive plastics, such as polyvinyl chloride (PVC), to avoid degradation caused by high temperatures.
The storage and transportation process of dried plastics also need to be protected from moisture to prevent the dried plastics from reabsorbing moisture. Dried plastics should be stored in sealed hoppers or containers and the internal environment should be kept dry. For plastics that need to be stored for a long time, desiccant can be placed in the storage container to further absorb moisture. In the process of transporting the dried plastic to the barrel of the injection molding machine, the transportation path should be shortened as much as possible to reduce the contact time with the air. At the same time, the transportation pipeline should be kept sealed to prevent humid air from entering. In addition, for some highly hygroscopic plastics, such as polyamide (PA), they should be used for injection molding as soon as possible after drying. If they are left for too long (usually more than 4 hours), they need to be dried again to ensure that their moisture content meets the injection molding requirements.
Testing the drying quality of plastics is crucial for ensuring the quality of injection molding production. Common testing methods include gravimetric testing, dew point testing, and instrumental analysis. The gravimetric method calculates moisture content by measuring the mass difference between the plastic before and after drying. This method is simple to use but offers limited accuracy. The dew point method indirectly determines the degree of dryness of plastics by measuring the dew point temperature in the drying air, making it suitable for online monitoring. Instrumental analysis, such as the Karl Fischer titrator, can accurately measure moisture content in plastics with an accuracy of up to 0.001% and is a common method for laboratory testing. In actual production, the appropriate testing method should be selected based on the type of plastic and the molding requirements. Regular quality testing of dried plastics should be performed to ensure that moisture content remains within acceptable limits. As the injection molding industry continues to demand higher quality products, future plastic drying technologies will evolve towards intelligent and efficient technologies. These include developing drying equipment with automatic monitoring and adjustment capabilities to achieve precise control of the drying process, and adopting new energy-saving heating technologies to reduce energy consumption during the drying process, providing strong support for green injection molding production.
