Classification of extruder

Classification of extruder

1. By functional characteristics

It can be divided into single-function extruder and multi-function extruder. Single-function extruder is simple in structure, parameters can`t be changed at will, it is specially designed for a single product, large limitations, poor adaptability (mostly small extruder). And multi-function extrusion function through the change of some parameters and parts such as screw, barrel, the replacement of die head, implement different requirements of length to diameter ratio, compression ratio, shape of die hole, hole area, screw rotation, the change of the process factors such as temperature, pressure, which can meet various demands, is suitable for the production of various products.

2. By processing method

It can be divided into dry expansion and wet expansion. Dry puffing is a kind of extruded puffing without water or external heating, which simply depends on the heat generated by the friction between the material and the Extruder Screw and the cylinder wall. Because soybeans contain a lot of oil, they lubricate well. Dry puffing machine has simple structure and low price, but it is difficult to operate, the extrusion temperature is not easy to control, the nutrition damage is large, and the energy consumption is large and the yield is low. With the development of extrusion technology, there are more and more types of processed materials, and the performance difference is also very big. Many materials have low oil content, low water content and poor fluidity, so they cannot be expanded by dry method. In order to ensure the smooth process of extrusion, some additional steam and water are often added before and during the extrusion to help increase the moisture of the material, reduce friction and improve the fluidity. This is wet expansion.

At present, there are dry expansion with no steam and wet expansion with some steam. During the process of dry expansion of full-fat soybean, about 3~4% water is lost, the granules are large and the color is yellow (brown). In wet expansion process, 3~5% steam is generally added (steam pressure is 3kg/cm²), after expansion, the product basically retains the original water, the particles are fine, the color is golden, the production efficiency is higher, the power consumption is less, most existing manufacturers use this method for processing. Due to the addition of steam and water in the material, the moisture content in the material increases, so it is easier to flow and less friction during extrusion, so the extrusion temperature is lower than that of dry expansion, which is easier to control. In addition, the damage to the nutrient composition of the material is less than that of dry puffing, but the equipment is more complex (must be equipped with a modulator).

Some materials cannot be extruded without water at all. Nowadays, it is called dry puffing that a small amount of water is added before extrusion and the safe water can be stored by direct cooling treatment without drying after extrusion. But before the extrusion adds the moisture to be many, after the extrusion product must pass through the dry and the cooling treatment to achieve the storage safe moisture content method is called the wet method expanded.

Many studies have been done abroad on the role of water in extrusion puffing process and its effect on puffing machine and puffing performance. The addition of water in the extrusion process can promote starch paste and protein denaturation, increase the yield and reduce the driving power required by the spindle. But does not add the moisture to be able to increase in the extrusion puffing chamber the shear force, in addition to can cause the vitamin loss to increase and the amino acid composition to be destroyed, but also can increase the screw and the cylinder wall abrasion, and reduces the output, increases the electricity consumption. According to research company Wenger USA, the water content of materials in the extrusion chamber is 27%, when the water content is reduced to 15%, the production cost will increase by 2.7 times, the electricity consumption will increase by 4.7 times, and the abrasion will increase by 4.8 times.

3. Sort by machine structure

It can be divided into Single Screw Extruder and double screw extruder.

The structure of single screw extruder is relatively simple and the price is relatively low. In the single screw extrusion chamber, the materials are basically tightly around the screw rod. As the screw rotates, the material moves forward along the thread like a nut. However, when the friction between the material and the screw is greater than the friction between the material and the barrel, the material and the screw will produce a common rotation (rotation), which can`t realize the forward extrusion and conveying of the material. The higher the moisture and oil content of the material is, the more significant the trend will be. In order to avoid this problem, it is necessary to control the moisture and oil, in order to reduce the lubrication of the material and the inner wall of the cylinder. Therefore, the single-screw extruder has some difficulties in the production of high moisture and high oil content materials. In addition, at the end of the single screw extruder, when there is no material to continue to enter, the material in the extruder chamber will not be discharged automatically, it has no automatic cleaning function, so after the machine is stopped, the extruder chamber must be opened manually to clear the accumulated material in the machine.

Double screw extruder has many advantages, such as complicated structure, difficult manufacture (especially high power twin screw machine) and high price. This type of material and the screw will not occur co-rotation, so that the inner wall of the barrel can be made of smooth surface, can avoid unnecessary friction, reduce the energy consumption of operation. At the same time, the clearance between the screw and the barrel can be minimized to reduce the possibility of backflow of materials along the clearance. Even high moisture, high oil content of the material is not easy to produce countercurrent, can be more high pressure and higher temperature extrusion processing.

Twin-screw extruder has three meshing modes: non-meshing, partial meshing and full meshing.

The rotation direction can be divided into three forms: rotation in the same direction, inward reverse rotation and outward reverse rotation. They all have self-cleaning ability, and have their own characteristics. But at present it seems that the twin screw extruder which rotates in the same direction and rotates in the same direction with tight meshing trapezoid thread has more obvious advantages and is the mainstream of current development.