Key Points for Controlling Plastic Raw Material Drying Quality in Injection Molding Production

In injection molding production, the drying of plastic raw materials is a critical process that determines product quality. Excessive moisture content not only causes appearance defects such as sliver marks, bubbles, and fogging on molded parts but also leads to hydrolysis of certain polymers, reducing mechanical properties, increasing brittleness, and lowering production stability. To scientifically control raw material drying, a systematic management system must be established based on material characteristics, equipment parameters, on-site operation, and quality inspection.

I. Develop Differentiated Drying Plans Based on Material Hygroscopicity

Plastic materials vary greatly in hygroscopicity and must be processed with different drying methods and standards, eliminating one-size-fits-all operations.

Low-hygroscopic general-purpose plastics

Represented by PP, PE, and PS, these materials hardly absorb moisture and only need to remove surface-attached moisture. New materials can be used without drying under normal environments; damp or recycled materials can be dried with ordinary hot air dryers at a temperature of 70℃—80℃ for 1—2 hours to meet production requirements.

High-hygroscopic engineering plastics

Materials such as PA, PC, PET, PBT, and TPU are highly prone to absorbing moisture from the air, and moisture can penetrate deep into the pellets. Hot air drying alone cannot meet the standards, so dehumidifying dryers must be used. For example, PA66 needs to be dried at 80℃—90℃ for 4—6 hours, and PC needs to be dried at 100℃—120℃ for 3—4 hours; otherwise, hydrolysis and degradation are likely to occur.

Heat-sensitive plastics

Materials such as POM and PVC are sensitive to temperature and prone to decomposition and discoloration at high temperatures, so the drying temperature must be strictly controlled. The drying temperature of POM should not exceed 80℃, and the duration should be controlled within 3 hours to avoid yellowing of raw materials and generation of irritating gases.

II. Precisely Control Four Core Drying Parameters

The drying effect is jointly determined by four key parameters: temperature, time, air volume, and dew point. Any out-of-control parameter will lead to drying failure, which needs to be standardized and controlled item by item.

Drying temperature

The temperature should be set within the heat-resistant range of the material. Too low temperature results in slow moisture diffusion and incomplete drying; too high temperature causes thermal aging, caking, and adhesion of raw materials. In production, the actual temperature of the material layer should be monitored in real time to ensure that the deviation between the set temperature and the actual temperature does not exceed ±5℃.

Drying time

The duration must ensure that the hot air penetrates the entire layer of raw materials, allowing internal moisture to fully migrate to the surface. For damp raw materials, recycled runner materials, and large-particle raw materials, the drying time should be extended by 30%—50% on the basis of the standard duration to avoid the problem of dry surface but wet interior.

Air volume control

Air volume is responsible for carrying away the volatilized water vapor. Insufficient air volume will form drying dead corners, while excessive air volume will cause a lot of heat loss. Daily cleaning of filter screens and pipeline dust is required to ensure smooth air circulation and maintain stable hot air circulation efficiency.

Dew point control

For high-hygroscopic raw materials, the dew point of drying air should be controlled below -40℃. The higher the dew point, the higher the air humidity and the weaker the drying capacity. Ordinary hot air dryers are only suitable for low-hygroscopic materials and cannot meet the deep drying needs of engineering plastics.

III. Standardize On-Site Operations to Prevent Reabsorption of Dried Raw Materials

If dried qualified raw materials are exposed to a humid environment, they will quickly absorb moisture, leading to the failure of previous work, so on-site management is particularly important.Raw materials should be stored in a dry and ventilated warehouse, avoiding direct stacking on the ground. Unused raw materials after opening should be sealed and packaged in a timely manner to reduce the contact time with air. Dried raw materials should be put into production within 2 hours as much as possible; if the shutdown exceeds 1 hour, the raw materials in the hopper should be kept warm and sealed to prevent moisture absorption after cooling. Recycled runner materials must be dried separately and mixed with new materials in proportion after passing the inspection, and the mixing ratio is generally not more than 20%. At the same time, control the humidity of the production workshop; when the humidity is high, turn on the dehumidification equipment to reduce the impact of the environment on the raw materials.

IV. Establish a Quantitative Testing Mechanism to Scientifically Verify the Drying Effect

The traditional judgment method relying on visual observation has large errors and is likely to cause quality hidden dangers, so a quantitative testing standard needs to be established.In the production process, a rapid moisture analyzer can be used to detect the moisture content of raw materials. The moisture content of high-hygroscopic materials such as PA should be less than 0.05%, that of PC and PET should be less than 0.02%, and that of general-purpose plastics should be controlled within 0.03%. The hot-melt test method can also be used: put a small amount of raw materials into high-temperature equipment for melting; if no continuous bubbles are generated, it indicates that the drying is qualified. At the same time, establish a drying operation account, recording the raw material model, drying parameters, test results, and operators to achieve full process traceability.

V. Strengthen Equipment Maintenance to Ensure Stable Operation

The operating status of drying equipment directly affects the drying quality. Regular maintenance can avoid drying out of control caused by equipment failure. Clean the dust and agglomerated raw materials in the hopper and air duct daily to prevent blockage of the hot air channel; check the core components such as fans, heating pipes, and dew point sensors weekly to ensure normal operation; calibrate the temperature and time control system monthly to avoid parameter drift. Dehumidifying dryers should set the regeneration temperature and switching time according to the specifications to ensure sufficient molecular sieve regeneration and maintain stable output of low dew point air.

Raw material drying is a basic process of injection molding production. Only by accurately setting parameters based on material characteristics, standardizing on-site operations, implementing testing mechanisms, and doing a good job in equipment maintenance can we eliminate molding defects caused by moisture from the source, improve the appearance and internal quality of products, reduce the defective rate, ensure the continuous and stable operation of injection molding production, and provide strong support for enterprises to improve production efficiency and economic benefits.