Function and Principle of PVC mixture

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,

even paste.

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.