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The basic principle of PVC material mixing
The preparation of dry mixed powder is to add the PVC resin and various additives and additives in the formula, after correct calculation and accurate weighing, into a high-speed hot mixer for hot mixing. After reaching the specified temperature and time, it is discharged into the mixer for cooling to obtain a powder mixture with a certain volume density, uniform looseness, and easy flow, which can be used for granulation in the next process using a single screw extruder or direct molding using a double screw extruder.

Mixing is one of the important processes in PVC processing. Be sure to operate according to the requirements.
The basic principle of mixing
The mixing process is generally completed through three actions: diffusion, convection, and shear. In the actual process, the three functions act simultaneously, but under certain conditions, one of them is dominant.
Diffusion: Diffusion is driven by the concentration difference between each component, causing the particles of each component to migrate from areas with high concentrations to areas with low concentrations, thereby achieving uniform composition.
Convection: Convection is the process of causing two or more materials to flow into mutually occupied spaces in order to achieve uniform composition.
Shear action: Shear action is a mixing process that utilizes mechanical shear force to promote the uniformity of various components of a material.
The influence of mixed process conditions
The influence of mixing temperature: When the mixing temperature is less than the glass transition temperature of PVC around 80 ℃, the material is mainly compacted by mechanical force, so that each component is surrounded by PVC particles. When the temperature is higher than 80 ℃, the PVC particles begin to soften, absorb other components and gradually compact, the temperature further rises, and small particles and some large particles gradually gel, so the apparent bulk density increases continuously.
Influence of mixing time: the density, homogenization and partial gel plasticization of materials in the mixing process should be completed within a certain time. Prolonged mixing time also helps stabilizers and other additives diffuse more evenly into the resin, improving material stability and uniformity.
The impact of the weight of each batch of materials: If the feeding amount of each batch is too small, the material turnover is poor, the friction heat generated is low, the temperature rise is slow, and the time consumption is long, which also affects the uniform mixing of materials. On the contrary, if the feeding amount is too large, the motor load is too high, and the spindle transmission decreases, it also affects the material's flipping condition and mixing quality.
Discharge temperature: during the preparation of dry mixed powder, the material has been compacted, homogenized and preliminarily gel to a certain extent, so its quality will directly affect the extrusion rate, process parameters and product quality of the extruder. If the discharge temperature is low, the pre plasticization of the material is poor, and the bulk density of the dry powder material is low, not only does the extruder yield decrease, but it also leads to poor plasticization during the extrusion process, loss of luster on the surface of the product, and a decrease in strength. On the contrary, if the temperature is too high, it is prone to agglomeration and even overheating decomposition, so the temperature of hot mixed discharge is an important process parameter. The discharge temperature for general high-speed mixing is 100-120 ℃.
Material status during material mixing process
Suspended PVC resin is generally composed of 1-2 μ Irregular circular primary particles of m are piled together, with varying particle sizes and an average particle size of generally 120-180 μ M. During the hot mixing process, when the temperature reaches around 50 ℃, the originally large particles become smaller and the smaller particles gradually disappear. This is obviously because during the thermal mixing process, large particles are sheared and broken into small particles, which absorb thermal and mechanical energy and increase their activity. They gradually combine with other particles, resulting in an increase in average particle size. When the temperature rises to about 120 ℃, the particles become large and uniform, the small particles almost disappear, and become transparent or translucent at some part or edge of the particles. This phenomenon indicates that the PVC particles have partially gel due to the absorption of heat energy.
When the temperature reaches about 130 ℃, the particle size of PVC particles tends to be stable, and the gel degree is deeper. From this, it can be seen that under high-speed mixing and stirring, PVC powder undergoes morphological changes such as particle refinement, recombination, and even increase in particle size; It also has the effect of increasing apparent density and partial gel. The increase of particles will play a role in densification, making the apparent density of particles reach the value, thus improving the extrusion output; Recombination and homogenization are beneficial for the uniform plasticization of the extrusion molding process; Partial gel can accelerate the plasticizing process of materials during extrusion.

Technical parameters and related performance of SRL W800/2000 hybrid unit:

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