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Views: 0 Author: Site Editor Publish Time: 2025-09-30 Origin: Site
In the PVC processing technology, mixing technology serves as the first procedure in the production of PVC products and plays a crucial role. However, many enterprises often do not attach sufficient importance to the mixing process, leading to misunderstandings among workers regarding mixing technology—they believe that simply mixing the materials evenly is sufficient. This article will elaborate on the functions, principles, and processes of PVC mixing to help readers gain a comprehensive understanding of PVC mixing technology.
PVC products are formed by mixing a polydisperse system composed of PVC resin, stabilizers, modifiers, fillers, colorants, and other additives, which is then used in molding processing. The main purpose of mixing is to uniformly blend these raw materials into a dry blend with high bulk density, good fluidity, and a dry, loose texture.
The mixing process involves two aspects: blending and dispersing.
Blending: It refers to changing the spatial distribution of two or more components. Through blending, materials of different components can be evenly distributed, ensuring the consistency and stability of subsequent processing.
Dispersing: It means that the physical properties of one or more components change during blending, such as a reduction in particle size or dissolution into other components. Dispersion helps to achieve full contact and reaction between components, thereby improving product performance.
Blending and dispersing are generally carried out and completed simultaneously. During the mixing process, through mechanical actions such as crushing and grinding, the particle size of the mixed materials continuously decreases to achieve uniform dispersion.
The PVC mixing process mainly consists of two stages: hot mixing and cold mixing.
Hot mixing plays a vital role in PVC processing. It is a process where materials are driven by high-speed rotating stirring blades to spread sharply along the inner wall of the mixer and fall from the center, forming a vortex-like movement. Under the shearing action generated by the collision and friction between the materials, the blades, and the inner wall of the mixer, the materials transform from a solid, single-phase, heterogeneous state to a multi-phase, homogeneous, and partially gelled state, with the temperature of the materials rising continuously.
Gelation (pre-plasticization) of materials to a certain extent: Many substances with low melting points (such as lubricants) melt and penetrate or adhere to the surface of PVC.
Achieving good initial gelation and dispersion effects for materials.
Increasing the bulk density of materials and enhancing their fluidity: This facilitates material transportation and improves extrusion efficiency.
Removing moisture and low-volatile components from raw materials: Eliminating the impact of these components on product quality.
The end temperature of hot mixing is a key factor affecting mixing quality. After long-term experimental exploration, the most suitable end temperature for hot mixing is currently 110-120℃. If the hot mixing temperature is too low, the PVC materials will plasticize unevenly, and residual volatile components will affect performance. If the temperature is too high, the PVC materials will consume more stabilizers in the mixing tank, easily degrade, or even form paste, which affects production.
The hot mixing process involves three actions: diffusion, convection, and shearing. Diffusion makes the distribution of material components more uniform; convection and shearing use mechanical force to achieve uniform dispersion of material components. As the temperature rises continuously, PVC particles continuously absorb stabilizers, lubricants, etc., and are gradually coated by various components. When the material temperature rises to approximately 100℃, it is easy to remove moisture and other volatile substances from the PVC resin and additives. When the temperature reaches 110-120℃, a uniform and stable dry blend can be formed.
Hot mixing is also a gelation process of PVC mixtures. Unmixed PVC resin has uneven particle sizes at room temperature, with many small particles, which easily cause uneven plasticization during extrusion molding. During hot mixing, the PVC resin gradually melts, plasticizes, and recrystallizes to form a network structure. When the material temperature reaches 115℃, the small PVC resin particles become larger and more uniform, the small particles gradually disappear, and the edges of the particles become transparent or translucent due to partial gelation.
The discharge temperature of hot mixing determines the progress of uniform plasticization of PVC materials. Generally, most PVC product manufacturers take 120℃ as the rated temperature index for hot mixing discharge. When a large amount of calcium carbonate is used, the hot mixing discharge temperature can be appropriately increased to facilitate the balanced adsorption of calcium carbonate by PVC materials and low-melting components, thereby improving bulk density.
Hot mixing time is also an important parameter that needs to be controlled. During continuous mixing, if the time to reach the discharge condition is short (e.g., less than 7 minutes), the interval between each batch of materials should be extended. If the temperature rises too slowly after long-term use of the mixer (e.g., the discharge temperature cannot be reached even after more than 12 minutes), the materials should be discharged in time, and the thermocouple and thermometer should be checked for indication failure. The wear of the mixer blades also needs to be checked regularly, and severely worn blades should be replaced promptly.
The feeding sequence for hot mixing has a certain impact on mixing quality and mixer tank wear. The usual feeding sequence is as follows: add PVC resin and heat stabilizers at low speed; add impact modifiers, processing aids, and internal lubricants at high speed and 60℃; add fillers, titanium dioxide, ultramarine, external lubricants, UV powders, color masterbatches, antioxidants, etc., at high speed and 80℃.
The mixing quantity is also a factor that cannot be ignored. Generally, each batch of materials should be controlled at approximately 75% of the volume of the hot mixing tank to avoid excessive plasticization (or uneven plasticization) of materials caused by excessively long (or short) mixing time.
The working principle of cold mixing differs from that of hot mixing. The barrel of the cold mixer has an interlayer through which cooling water flows, and the volume of the cold mixer is generally about 3 times that of the hot mixer. The hot-mixed materials entering the cold mixer are continuously stirred and tumbled by the rotating blades in a relatively spacious environment. Their heat is absorbed by the cooling water in the interlayer of the barrel wall, and the materials themselves gradually cool down.
PVC materials should be subjected to cold mixing immediately after hot mixing to prevent degradation caused by long-term storage of hot-mixed materials at high temperatures. At the same time, if the hot-mixed materials are cooled in a natural state without cold mixing, they are prone to reabsorb moisture, which has an adverse impact on extrusion. Cold mixing not only prevents the moisture absorption and moisture regain of PVC materials when cooling at high temperatures but also is a process in which the intermolecular moisture of PVC materials is further discharged during the exothermic process.
The rated discharge temperature of cold mixing is generally 40℃, and the cooling water temperature is usually controlled at 13-15℃.
PVC mixing technology is an important link in the PVC processing process. In-depth understanding of the functions, principles, and processes of PVC mixing can optimize the mixing process and improve product quality and production efficiency. It is hoped that this article can be helpful to readers and contribute to the development of the PVC processing industry.
