Introduction
Mixing is the first and one of the most important processes in the production of PVC plastic products. Before extrusion, all raw materials and additives must be fully mixed and evenly dispersed to ensure stable processing and consistent product quality.
In PVC processing lines, conical twin-screw extruders are commonly used. However, their material conveying capability is much stronger than their mixing capability. Therefore, the uniform dispersion of additives is mainly achieved during the mixing stage before extrusion.
The primary objective of mixing is to produce a dry blend that has:
- Uniform distribution of all additives
- High apparent density
- Good flowability
- Loose and dry structure suitable for extrusion
The mixing process generally consists of two stages:
- Hot mixing
- Cold mixing
Hot Mixing Process
Principle of Hot Mixing
In hot mixing, raw materials are placed inside a high-speed mixer equipped with rotating impeller blades.
Under the action of the rotating blades:
- The raw materials move rapidly along the inner wall of the mixer
- Then fall toward the center of the mixer, forming a vortex motion
During this process, continuous collision, friction, and shear occur between the material, the blades, and the mixer wall. As a result, the material gradually transforms from a solid, single-phase, non-uniform state into a multi-phase, homogeneous mixture with partial gelation.
At the same time, the temperature of the mixture gradually increases until it reaches the preset mixing temperature.
Mixing Mechanisms
The hot mixing process mainly involves three mechanisms:
Diffusion
Diffusion allows different material components to distribute more uniformly. Particles move from areas with higher concentration to areas with lower concentration, achieving a balanced composition.
Convection
Mechanical forces generate irregular flow of materials, helping to distribute the additives throughout the mixture.
Shear
Shear forces created by the rotating blades help break agglomerates and disperse additives, ensuring uniform mixing.
Functions of Hot Mixing
Hot mixing plays several important roles in PVC processing:
- Partial Gelation (Pre-plasticization)
PVC particles begin to soften slightly, allowing low-melting substances such as lubricants to melt and adhere to the surface of PVC resin. - Improved Dispersion
Additives such as stabilizers, fillers, pigments, and lubricants are evenly distributed within the mixture. - Higher Apparent Density
The dry blend obtains higher bulk density, improving flowability, transportation, and extrusion efficiency. - Removal of Moisture and Volatile Components
Heating helps remove moisture and low-volatile substances, which could otherwise affect product quality.
Hot Mixing Temperature
Typical hot mixing temperatures are:
- Rigid PVC: 110–130°C
- Soft PVC: slightly lower than rigid PVC
When the calcium carbonate filler content increases, the hot mixing temperature may be raised slightly to improve the adsorption of additives and increase the apparent density of the mixture.
However, the temperature must be carefully controlled.
If the temperature is too low:
- PVC plasticization becomes uneven
- Volatile substances cannot be fully removed
If the temperature is too high:
- PVC may begin to decompose
- Materials may form lumps or paste
Cold Mixing Process
Principle of Cold Mixing
After hot mixing, the heated dry blend is transferred into a cold mixer for rapid cooling.
The cold mixer usually contains a water-cooled jacket around the mixing chamber. Cooling water circulates through this jacket to remove heat from the mixture.
The capacity of the cold mixer is typically about three times larger than the hot mixer, allowing enough space for cooling and material movement.
As the blades continue to rotate, the material is constantly stirred and rolled, gradually releasing heat until the mixture cools down.
Functions of Cold Mixing
Cold mixing has two primary functions:
Prevent Thermal Decomposition
Rapid cooling prevents the material from remaining at high temperatures for too long, which could cause PVC degradation.
Prevent Moisture Reabsorption
Cooling also prevents the hot mixture from reabsorbing moisture from the environment, which could negatively affect extrusion performance.
Cold Mixing Temperature
The typical final temperature after cold mixing is:
40–45°C
If the temperature is too high:
- The dry blend may retain excess heat during storage
- Stabilizers may be consumed prematurely
- PVC may decompose or burn during extrusion
If the temperature is too low:
- Cooling efficiency decreases
- Overall mixing efficiency may be reduced
Conclusion
The hot and cold mixing stages are essential for producing high-quality PVC compounds. Proper mixing ensures:
- Uniform dispersion of additives
- Stable processing performance
- Improved extrusion efficiency
- Consistent product quality
Careful control of temperature, mixing time, and formulation is necessary to achieve optimal results in PVC production.
