Working Principle of Mixers in The Plastics Industry​

The core function of plastic mixing equipment is to achieve physical mixing and preliminary plasticization of materials through mechanical forces (shearing, extrusion, stirring) and thermal effects (frictional heat or external heating). The process can be specifically divided into three stages:

Dispersive Mixing

High-speed rotating mixing components (such as blades and rotors) generate strong shearing forces on materials, breaking up agglomerated particles of additives or fillers (e.g., agglomerated color masterbatches and glass fiber bundles). This allows small particles to disperse evenly in the resin matrix, reducing local concentration variations.

Example: Dispersing 1% black masterbatch into polyethylene resin prevents color spots in the finished product.

Convective Mixing

The rotation of mixing components drives the overall flow of materials (e.g., rolling up and down and radial diffusion), enabling materials in different regions to displace and blend with one another, thereby achieving macroscopic uniformity of components.

Example: When mixing plasticizers with polyvinyl chloride (PVC) resin, convection ensures the plasticizer penetrates uniformly into the resin particles.

Thermal-Assisted Mixing

Frictional heat: Heat generated by friction between materials, mixing components, and the container wall softens resin particles, enhances intermolecular bonding, and facilitates mixing.

External heating: Some equipment uses jackets or heating rods to control material temperature (for instance, PVC mixing requires avoiding overheating-induced decomposition and demands precise temperature control), accelerating the dissolution of additives or the melting of resins.

II. Classification of Mixers in the Plastics Industry

Based on structure, operating mode, and mixing intensity, commonly used mixing equipment in the plastics industry falls into the following categories:

High-Speed Mixer

Structural features: A vertical cylindrical container with a high-speed rotating stirring paddle (predominantly Z-type or butterfly-type) at the bottom, a cover plate on top (capable of being vacuumed or filled with nitrogen), and some models feature jacket heating/cooling functions.

Working principle: The stirring paddle (typically rotating at 500-2000rpm) spins at high speed, driving materials to rise along the container wall before falling from the center, creating strong convection and shearing. Simultaneously, frictional heat is generated to warm the materials (e.g., PVC can be heated from room temperature to 100-120℃ during mixing).

Application scenarios: Suitable for preliminary mixing of powders and granules (such as color masterbatch preparation and premixing prior to plastic modification), with high mixing efficiency (batch size 50-500kg, mixing time 5-15 minutes).

Low-Speed Mixer

Structural features: A horizontal or vertical container with a low-speed rotating stirring paddle (usually 30-100rpm). The blades are mostly anchor-type, ribbon-type, or paddle-type, emphasizing the overall agitation of materials rather than intense shearing.

Working principle: The blades push materials slowly, minimizing particle breakage and frictional heat. This makes it suitable for temperature-sensitive materials or those requiring protection from excessive shearing (e.g., reinforced plastics with added glass fibers to prevent fiber breakage).

Application scenarios: Primarily used for cooling mixing after high-speed mixing (for example, PVC needs low-speed stirring to cool below 60℃ after high-speed mixing to prevent additive volatilization) or for mixing bulk materials with low uniformity requirements.

Kneader

Structural features: A horizontal container housing a pair of oppositely rotating Z-type or Sigma-type rotors (with differing speeds to create a shear differential). The container can be tilted for discharging and is equipped with a heating/cooling jacket.

Working principle: The rotors process high-viscosity materials (such as rubber, PVC paste, and high-filler composites) through extrusion, shearing, and kneading, ensuring full contact of materials in a closed space to achieve the integration of "melting - mixing - plasticizing".

Application scenarios: Suitable for mixing high-viscosity and high-filler materials (e.g., rubber mixing for automotive weather strips and mixing of insulation materials for cable products).

Continuous Mixer

Structural features: Mostly adopting a double-screw or single-screw structure. Materials are continuously fed from one end, mixed by screw elements (kneading blocks, thread segments), and continuously discharged from the other end. It can be integrated with heating and exhaust devices.

Working principle: Through the conveying and shearing action of the screw and the dispersing effect of mixing elements, continuous production is achieved, with mixing and plasticization occurring simultaneously (similar to the mixing function of an extruder but with a greater focus on mixing efficiency).

Application scenarios: Large-scale industrial production (such as continuous mixing and modification of raw materials for plastic pipes and films), suitable for materials with large batches and stable formulations.

Planetary Mixer

Structural features: A vertical container. The stirring paddle rotates around its own axis (rotation) and around the container's center (revolution). Some models are equipped with wall-scraping devices (to prevent material adhesion to the wall).

Working principle: "Revolution + rotation" generates strong centrifugal force and shearing force, ensuring materials mix without dead zones in the container. It is particularly suitable for high-viscosity and paste-like materials (e.g., plastic sol and adhesives).

Application scenarios: Precision mixing (such as mixing special plastics for the electronics industry and medical plastics) to ensure uniform dispersion of trace additives (e.g., flame retardants).