Extrusion granulation(一） - spiral extrusion granulation
There are two types of screw extrusion granulator: front extrusion and side extrusion. There are some differences in the screw shaft and blade types of the two extruder granulators, the front extrusion screw Angle is smaller, and the side extrusion screw Angle is larger. Generally, the diameter of the pre-extruded particles is larger, and the hardness is also higher. Smaller diameter particles can be produced by side extrusion, but the hardness is worse than that of front extrusion. It is worth noting that the material is easy to heat up during pre-extrusion, and special attention should be paid to materials that are heat sensitive or heat sticky in the powder.
Usually the granulator is equipped with an extrusion blade to scrape the hole plate adhesion. The blade shapes installed in the two basic types of spiral extruder granulators are different, and the front extruder type adopts spiral column extension or propeller blade; The side extrusion type uses a conical cylinder or a toothed blade. The material is extruded through the surrounding screen holes perpendicular to the rotating shaft, so that the outflow direction of the material is at a right Angle to the rotating shaft. Due to its thin orifice plate, the obtained particle density is lower, but the production efficiency is higher. The shape and size of the small holes are determined according to the production requirements. If the particles with higher hardness need to be prepared, the thickness of the separator and the hole plate should be increased to withstand the higher extrusion force, and the power of the motor should be increased.
The screw extruder granulator uses the rotating thrust of the screw rod to compress the soft material and transport it to the front of the granulation hole plate with a certain aperture, forcing the extrusion through the small hole to granulate. The machine is divided into three functional areas, namely feeding area, pressing area and extrusion area.
The feeding area is composed of feeding hopper and other parts, the main function is to introduce the soft material into the spiral groove, after installing the mixed soft material part in the feeding area, directly add the solid material and binder, that is, the soft material can be mixed evenly. After the soft material enters the spiral groove, the soft material is sent to the compression zone by the spiral shaft. The rotating axis can be divided into single rotating axis and double rotating axis, and the particles produced by single rotating axis have higher density. The double rotary shaft mechanical ability can avoid the bridge phenomenon of the material at the feeding port, ensure the continuous granulation and large capacity.
In the compression zone, the air between the clumps is compressed and escapes. In order to make the air escape smoothly and avoid the uneven hardness of the particles caused by the air existing in the particles, an exhaust hole is often installed in the compression area.
In the extrusion area, the screw shaft is designed according to the amount of pressure required, and its main function is to transport material, usually in the screw
It is used to leave a certain space between the top of the spiral shaft and the formed hole plate, which is conducive to the compression of the material. According to the rheological characteristics of the material (such as viscosity, elasticity, plasticity, deformation and fluidity) and the size of the space, it can be compressed to a higher density, the better the viscosity, deformation and plasticity of the material, the greater the density of the material after compression, and the lower the vice versa. Under the same conditions, the material density increases with the increase of the space between the top of the spiral shaft and the formed hole plate, so the machinery with low pressure has a smaller compression space, and the machinery with higher pressure has a larger compression space. High pressure extrusion granulation equipment is less used in practical production because of its large wear, poor extrusion shape, power consumption, high operating cost, on the contrary, the application of low pressure extrusion granulation equipment is more common.
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