The principle of hot mixing technology

Mixing can be divided into hot mixing and cold mixing parts. Hot mixing is by no means as most people understand it, it is just a simple mixing and stirring process of materials. Hot mixing is that different components and dosage materials in the formula, driven by high-speed rotating blades, spread sharply along the inner wall of the mixer, and fall from the center of the machine to form a movement. Under the shearing action of the blade of the mixer and the material, as well as the material and the inner wall of the mixer, the transformation process from solid, unidirectional, inhomogeneous state to multiphase, homogeneous and partial gel state. During the compounding process, the density can be measured by the increase in apparent density. The apparent density of PVC resin is only 0.45-0.5sml. In high-speed mixing, due to the high shear force generated by heat, mutual impact and friction, the temperature of the material continues to rise, and the PVC particles and other components continue to penetrate each other. The surface gradually absorbs or adsorbs stabilizers, melted lubricants, processing aids, etc., and the apparent density of the mixture can be increased to 0.63-0.70g/ml.

The mixing process is a process of diffusion, convection and shearing. Diffusion is convenient to reduce the concentration difference between the components of each material, so that the micro-particles of the components migrate from the high concentration area to the low concentration area, so as to achieve uniform composition; convection and shear both use mechanical force for irregular flow, so that the material The composition achieves a fully uniform dispersion process. As the temperature continues to rise, the PVC particles continue to absorb stabilizers, lubricants, processing aids, etc. Newly coated with each component. When the temperature of the material rises to about 100°C, it is easy to remove moisture and other volatile substances in the resin and additives. Avoid bubble defects during extrusion. When rising to 110-120 ℃. It can form uniform and stable dry mixed powder.

The mixing process is also the PVC mixing process. The unmixed PVC resin at room temperature has different particle sizes and many small particles, which can easily cause uneven plasticization during the extrusion molding process. The PVC material is gradually melted, plasticized, and recrystallized to form a network structure during the mixing process. From normal temperature to below 50 ℃, the agglomerated PVC particles and larger particles are rubbed. After impact, although the apparent density does not change much, the large accumulated particles are cut and broken into small particles one by one, and absorb thermal energy and mechanical energy. Then the activity increases, and it gradually combines with other particles to increase the size of the particles. When the material temperature is below 90 °C, the heating rate is faster, and then the heating rate is slower. Before the temperature of 90°C, the PVC mixed with the three forces is relatively violent. When the temperature reaches 115°C, the small resin particles become large and uniform, and the small particles gradually disappear. And the particle edge becomes transparent and translucent due to partial gelation. Its partial gelation degree is determined by the mixing discharge temperature under certain conditions.

When the temperature reaches 120℃, the PVC particles tend to be stable, and the degree of gelation is deeper. In the whole process, the particles are repeatedly combined and homogenized, and the particles increase, which plays a densifying effect, so that the apparent density of the particles reaches the maximum value. Promote the uniform plasticization of extrusion molding, improve the output and welding, impact resistance and other properties of plastic products. The uniformity and dry flow are good. For dry-mixed powders, it can be observed that the PVC resin has both particle refinement, uniform particle size and morphological changes, and shows the characteristics of high density and partial gelation.

01 Hot mixing temperature setting

The temperature of the hot mixing material determines the uniform plasticization process of the extruded material. If the hot mixing temperature is too low, the material is unloaded for cold mixing before reaching the predetermined temperature requirement, and the material is not uniformly kneaded in the mixing tank, that is, the recombination and homogenization of the PVC particles are incomplete. It will affect the apparent density and gelation of the particles, the components of the raw materials cannot be coordinated and fully exerted, and the water analysis of the materials is incomplete. Affect extrusion plasticization degree. When the material is difficult to plasticize during extrusion, it is necessary to appropriately increase the set temperature of the extruder. However, sometimes the temperature of the barrel is adjusted too high, but the plasticization cannot be performed well, resulting in the decline of various physical and chemical properties of the product; when the mixing temperature is too high, the material is prone to degradation in the mixing tank. Even "paste", production is difficult to continue. In general, SG-5 type resin is used to produce PVC profiles, and 115℃-120℃ should be used as the rated temperature index of hot mixing. When the calcium carbonate filling amount is large. The temperature of the hot-mixed material can be appropriately increased to facilitate the balanced adsorption of calcium carbonate by PVC materials and low-melting components, and to improve the uniformity of the apparent density and flow velocity of the mixture.

02 Hot mixing time control

In addition to the need to control the hot mixing temperature, it is also necessary to pay close attention to the hot mixing time in order to monitor the temperature control. When the continuous mixing, the hot mixing reaches the discharge time is too short, less than seven minutes, each pot should be extended. Material interval; when the mixer is used for a long time, it is found that the temperature rise of the hot mixing is too slow, and the discharge temperature cannot be reached after more than 15 minutes. It can be unloaded in time, and carefully check whether the thermocouple and thermometer indicate failure. , and handled to prevent paste. When it is found that the blade of the mixer is seriously worn, it should be replaced in time to prevent the quality of mixing. Generally, when the mixing time exceeds twice the normal mixing time or the end of the blade is worn more than 15 mm, it should be replaced. blade.

03 Hot mixing program

The general feeding procedure for small and medium-sized enterprises is: add PVC main material, heat stabilizer and internal slip agent at low speed; add processing aid and CPE at high speed at 60°C; add external slip agent and titanium at 90°C White powder, calcium carbonate, pigments, etc. Adding heat stabilizer first can give full play to the thermal stabilization effect of PVC resin; adding internal slip agent early is beneficial to the initial gelation and dispersion effect of the material; processing aids will preferentially absorb internal lubricants and heat stabilizers if they are too early; carbonic acid The premature addition of calcium and titanium dioxide will not only preferentially absorb the internal slip agent and heat stabilizer, but also reduce the acceptance of PVC resin, reduce the stability of the mixture, and cause greater wear to the mixing tank; The friction and shear force of the mixture are reduced, which affects the mixing effect, delayed plasticization and the dispersion performance of the stabilizer; the premature addition of the color masterbatch will affect the color of the color profile, etc.; Great impact! should be corrected. Image

04 Total feed amount control of hot mixing single pot

The total volume of hot powder in each pot can not be ignored. Generally, the volume of each pot should be controlled at about 55%-75% of the volume of the hot mixing tank. If the mixing amount in a single pot is too small, the shear heat is low and the temperature rise Too slow, the mixing time is too long, the production efficiency is low, the single pot mixing amount is too much, the shear heat is too high, the local temperature rise is too fast, the mixing time is too short, the material components are not uniformly dispersed, if the mixing load is too high If it is too heavy, the speed of the frequency conversion motor will slow down or the pulley will slip, and the mixing time will be prolonged. The single-pot feeding range should be set according to the mixer, and the single-pot feeding amount should be adjusted accordingly.