Today, we're discussing injection molding of PCR materials. One of key issues is moisture content and predrying requirements. Following describes commonly used drying methods in injection molding to help process engineers better select drying equipment that meets these requirements and ensure process stability.
Plastic pretreatment and drying are crucial steps in processes like injection molding and extrusion, directly impacting product quality and production efficiency. Following are common plastic pretreatment and drying techniques:

Plastics may contain moisture, volatiles, or other impurities before processing, which can affect processing performance and final product quality. Main objectives of pretreatment are:
- Moisture Removal: Prevents moisture from vaporizing during high-temperature processing, leading to defects such as bubbles and silver streaks.
- Volatile Removal: Prevents release of volatiles during processing, which can affect product surface quality.
- Homogenization: Ensures uniform size, shape, and composition of plastic particles, improving processing stability.

Drying is the most common step in plastic pretreatment, especially for hygroscopic plastics (such as PA, PC, and PET).
2.1 Hot Air Drying
- Principle: Utilizes hot air convection to heat plastic pellets, evaporating moisture.

 

- Equipment: Hot air dryer, hopper dryer.
- Applicable Materials: General-purpose plastics (such as PE and PP) and some engineering plastics.
- Advantages: Simple equipment and low cost.
- Disadvantages: Low drying efficiency, not suitable for highly hygroscopic plastics.
2.2 Dehumidification Drying
- Principle: A dehumidifier is used to remove moisture from air, and dry air is then used to heat plastic pellets.

 

- Equipment: Dehumidification dryer.
- Applicable Materials: Highly hygroscopic plastics (such as PA, PC, and PET).
- Advantages: Excellent drying efficiency, suitable for high humidity environments.
- Disadvantages: High equipment cost.
2.3 Vacuum Drying
- Principle: Lowering boiling point of water in a vacuum environment allows water to evaporate at low temperatures.
- Equipment: Vacuum dryer.
- Applicable Materials: Materials sensitive to high temperatures (such as PEEK and PPS).
- Advantages: Low drying temperature, preventing thermal degradation of the material.
- Disadvantages: Complex equipment and high cost.
2.4 Infrared Drying
- Principle: Using infrared radiation to directly heat plastic pellets, rapidly evaporating water.
- Equipment: Infrared dryer.
- Applicable Materials: Suitable for small batches or special materials.
- Advantages: Fast drying speed.
- Disadvantages: High energy consumption, not suitable for large-scale production.
2.5 Microwave Drying
- Principle: Using microwave energy to directly heat water inside plastic pellets.
- Equipment: Microwave dryer.
- Applicable Materials: Materials requiring high uniformity.
- Advantages: Fast drying speed and good uniformity.
- Disadvantages: High equipment cost and complex control.

Drying performance depends on following key parameters:
- Drying temperature: Select according to heat resistance of the plastic, typically between 60℃ and 120℃.
- Example: Drying temperature for PA is 80℃ to 90℃, and drying temperature for PC is 120℃.
- Drying time: Determined by hygroscopicity and moisture content of plastic, typically 2 to 6 hours.
- Example: PA requires 4 to 6 hours, and PET requires 4 to 5 hours.
- Air dew point: During dehumidification drying, the lower dew point, the better drying effect, typically requiring a dew point below -40℃.
- Material thickness: Thickness of plastic granules in dryer should be moderate; excessive thickness will result in uneven drying.

4.1 Screening and Filtration
- Purpose: To remove impurities and dust from plastic granules.
- Equipment: Vibrating screen, magnetic separator.
- Application: Ensures material purity and prevents clogging of processing equipment.
4.2 Preheating
- Purpose: Raises temperature of plastic pellets to reduce energy consumption during processing.
- Equipment: Preheating hopper.
- Application: Suitable for high-viscosity plastics (such as POM and PPS).
4.3 Mixing and Color Matching
- Purpose: Uniformly blends plastic pellets with masterbatch, additives, etc.
- Equipment: Mixer, blender.
- Application: Suitable for products requiring specific colors or properties.
4.4 Recycled Material Processing
- Purpose: Washes, crushes, and dries recycled plastics to make them reusable.
- Equipment: Washers, crushers, dryers.
- Application: Reduces costs and environmental pollution.

Dew point is a key indicator of air moisture content, plays a crucial role in plastics manufacturing and other manufacturing fields. Understanding concept and role of dew point can help us select right dehumidifier equipment, ensuring product quality and production efficiency.
During transportation and storage, plastic pellets can absorb moisture, depending on plastic. Many plastics are inherently hygroscopic, and water vapor can even penetrate deep into their molecular structure. Environmental factors affect moisture content, so moisture content of plastic pellets fluctuates seasonally.

Dew point is temperature at which moisture in air begins to condense into liquid water. When air is saturated with moisture, dew forms when temperature drops. This temperature is known as dew point. Dew point is commonly used to describe moisture content in air and is crucial in many industries, especially in production and manufacturing.
Relative Humidity: Relative humidity is percentage of actual water vapor density in air to saturated water vapor density at same temperature.

The key indicator for effective drying in a dehumidification dryer is dew point, which is equivalent to ambient humidity. In other words, the lower dew point, the less moisture there is in air inside dryer.

 

Drying process consumes a large amount of heat energy. During drying process, both heat and mass (moisture) must be transferred simultaneously to ensure that moisture vapor pressure (concentration) on the surface of material is higher than moisture vapor pressure in external space, and that heat source temperature is higher than material temperature.
Heat is transferred from high-temperature heat source to wet material in various ways, vaporizing surface moisture and dissipating it to external space, resulting in a difference in moisture content between surface and interior of material. Internal moisture diffuses toward surface and vaporizes, gradually reducing moisture content of material, thereby completing drying of the entire material.
Drying rate of material depends on the surface vaporization rate and internal moisture diffusion rate. Typically, drying rate in the early stages of drying is controlled by surface vaporization rate. Thereafter, as long as external drying conditions remain unchanged, drying rate and surface temperature of material remain stable. This stage is called constant-rate drying stage.
When moisture content of material decreases to a certain level, diffusion rate of internal moisture to surface decreases and becomes less than surface vaporization rate. Drying rate is then primarily determined by internal diffusion rate and continues to decrease as moisture content decreases. This stage is called decreasing-rate drying stage.

Using a dew point meter, different plastic materials have different dew point requirements. Dew point is moisture content.