How to Control Moisture in Plastic Materials for Injection Molding
Moisture control is a critical factor in plastic injection molding because excess moisture in raw materials can cause defects such as cracks, whitening, bubbles, and reduced mechanical performance. In some cases, moisture can also lead to material degradation during processing, affecting both appearance and part quality.
To achieve stable molding results, manufacturers must control the moisture content of plastic materials through proper drying methods, suitable equipment selection, and effective handling after drying.
Why Moisture Control Matters in Injection Molding
When plastic materials absorb too much moisture, the water can react during processing and create surface and internal defects in molded parts. Common moisture-related problems include:
- Surface cracking
- Whitening or silver streaks
- Air bubbles or voids
- Reduced mechanical properties
- Material degradation during melting
For this reason, drying plastic materials before molding is an essential step in many injection molding applications.
Main Factors That Affect Plastic Drying
1. Drying Temperature
Drying temperature varies by material and plays an important role in achieving proper moisture removal. If the temperature is too low, the material may not dry completely, or the drying time will become too long. If the temperature is too high, the plastic may degrade, which can reduce its physical and mechanical properties.
2. Airflow
Drying airflow also affects drying performance. In general, higher airflow can improve the drying effect, but it also increases energy consumption. Common drying airflow is typically in the range of 1.5 to 3.5 m³/(kg·hr), although the exact requirement depends on the plastic material.
3. Drying Time
Different plastics require different drying times depending on their hygroscopicity, pellet size, and initial moisture content. Insufficient drying time may leave residual moisture in the material, while excessive drying can waste energy or overheat the resin.
4. Dew Point Temperature
The dew point of the drying air is especially important for hygroscopic plastics. For ordinary plastics, a dew point of ≤ -20°C can usually provide effective drying. In general, the lower the dew point, the better the drying effect.
How to Choose the Right Drying Equipment
Different plastics require different levels of drying performance, so the choice of drying equipment should match both the material type and the product requirements.
Common Hot-Air Dryers
For plastics with low hygroscopicity, such as PP, PE, PS, PVC, and PPS, a standard hot-air dryer is usually sufficient. These materials do not require extremely low moisture levels, so common hot-air drying can help reduce equipment and operating costs.
Partial Hot-Air Dehumidifying Dryers
For medium-hygroscopic plastics such as ABS, PMMA, PC, POM, and PBT, where moisture content often needs to be controlled to ≤ 0.05%, partial hot-air dehumidifying dryers are often a suitable choice.
Hot-Air Dehumidifying Dryers
For strongly hygroscopic plastics such as PA, PET, and PES, where moisture content may need to be reduced to ≤ 0.02%, hot-air dehumidifying dryers are required to achieve complete drying and stable processing quality.
Prevent Moisture Regain After Drying
Drying alone is not enough. After the material has been dried, it must be protected from moisture regain before it enters the injection molding machine.
When floor-mounted drying systems are used, dried materials should be conveyed with low-dew-point hot air, and shut-off suction boxes should be used to prevent the material from absorbing moisture again. Insulated pipelines can also help maintain material temperature and reduce reabsorption of moisture.
When using a hopper mounted on the injection molding machine, the hopper size should match the production output. Materials should not remain in the hopper for too long, as this increases the risk of moisture regain when the material temperature drops.
For applications with stricter moisture requirements, even hopper dryers may need hot-air recirculation or supplemental dehumidified air to maintain proper material condition during processing.
Conclusion
Effective moisture control in injection molding depends on selecting the right drying temperature, airflow, drying time, and dew point, along with choosing the proper drying equipment for each plastic material. Just as importantly, manufacturers must prevent moisture regain after drying to maintain consistent molding quality.
By matching the drying method to the resin type and product requirements, injection molders can improve part quality, reduce defects, and achieve more stable production performance.