Mixing is the first process in the production of PVC plastic products. In the process of extrusion, usually use
conical twin-screw extruder, and its positive conveying function is far greater than the mixing effect on
materials.Therefore, the uniform mixing and mixing of various additives are mainly accomplished by mixing
machine before extrusion.
The main purpose of mixing is to make the raw materials fully mixed and evenly distributed.and form a dry
mixture with high apparent density, good fluidity and loose drying.
Mixture consists of hot mixing and cold mixing.
Hot mixing principle
Under the impetus of a high-speed rotating impeller blade, the raw material disperses sharply along the inner
wall of the mixer and falls down from the center to form a whirlpool motion. Under the shear action caused by
the collision and friction between the material and the blade and the inner wall of the hot pot, the material is
transformed from solid, single-phase and non-uniform state to multi-phase, homogeneous and partial gel state,
and the temperature of the raw material keeps rising and reaches the set temperature.
Hot mixing process includes three processes: diffusion, convection and shear. Diffusion makes the distribution
of material components more uniform, and makes the component particles migrate from more areas to less
areas, so as to achieve uniformity of composition. Convection and shear are irregular flow by mechanical force,
so that the material components can be uniformly dispersed.
Function of hot mixing:
1.Raw get a certain degree of gelation (pre plasticization), and low melting point substances, such as
lubricants can melt into or adhere to the PVC surface.
2.Get good initial gel effect and dispersing effect
3.The material obtains a larger apparent density, the flow ability is enhanced, the transportation is convenient,
and the extrusion efficiency is improved.
4. Eliminate the moisture and low volatile components in raw materials, and eliminate the impact of these
components on product quality.
Hot mixing temperature:
The set temperature of hot mixing is 110 - 130 degrees for rigid PVC products, and the soft PVC temperature
is slightly lower than that of rigid PVC. When calcium carbonate Increase, the temperature of hot mixing
material can be increased appropriately, which is beneficial to the equilibrium adsorption of calcium carbonate
by PVC materials and low melting point components, and to the increase of apparent density. When the
temperature of heat mixing is too low, the plasticization of PVC material is not uniform, and volatile matter
can’t fully removal. When the temperature of heat mixing is too high, PVC materials are easy to decompose,
Cold mixing principle
The barrel of the cold mixer is sandwiched with cooling water flowing in the sandwich, and the volume of the
cold mixer is about three times that of the hot mixer. The heat raw materials enter the cold mixer is absorbed
by the cooling water in the interlayer of the inner wall of the barrel and gradually cooled by itself under the
elatively spacious environment and the continuous agitation and rolling of the rotating blade.
Function of cold mixing:
1. Prevent materials decompose stored at high temperature for a long time.
2. Prevent raw materials reabsorpt the moisture by cooling at high temperature, which will cause adverse
effects on extrusion.
Cold mixing temperature:
The terminal temperature of cold mixing is 40- 45degree,. If the temperature of cold mixing is too high, the
dry mixtures will be stored if they are not fully cooled, which will cause excessive waste heat, excessive
consumption of stabilizers, lead to raw materias decompose and burning. If the temperature of cold mixing
is too low,its not conducive to improving the mixing efficiency.