Extrusion Machine for Puffing Food

A puffing extruder is composed of a tube and a screw rotating in the puffing tube. There are two categories: single screw extrusion puffer and double screw extrusion puffer. In the process of extrusion, the material enters the extuder cavity from the feeding port and is compressed, mixed and cut when driven forward by the screw. Due to the driving force, friction and shear force and heating, the material is compressed and heated, reaching a high temperature and high pressure state, and is turned into a paste condition. When the material is squeezed out of the mold holes, the temperature and pressure suddenly drop to normal ones, causing the water in the material to evaporate sharply and the volume to expand rapidly, which is called "Puffing".

A single screw extrusion puffer uses a singe screw which strings a series of various structures. The entire screw consists of three sections: feeding section, blending section and melting segment. After entering the tube from the feeding port, the material experiences solid conveying, melting and homogenizing in the screw so that the material is transformed from a loose state to a continuous plasticized-dough shape.

In the single screw extrusion process, the material closely surrounds the screw. When the screw rotates, the material moves forward in the spiral way. When the friction between the material against the screw is bigger than that against the barrel, the raw material can not achieve the forward extruding and conveying effect. The higher the moisture and oil in the material, the more obvious this trend is. In order to avoid these problems, most single screw extruder puffers now utilize a segmented screw with single and dual spirals, interleaved pressure rings and kneading elements jointly together with an inner-grooved barrel to fit the material processed.

Double screw extruder puffer is one of the multi -screw squeezers. It is developed on the basis of a single screw extrusion puffer. In the barrel of the double screw extrusion puffer, two screws are placed in parallel. Therefore, the double screw extrudes the puff. According to the relative position of the screw, it can be divided into meshing and non -meshing type, and the meshing type can be divided into partial meshing and full meshing; according to the spinning direction of the screw can be divided into two types: co-rotation and reverse rotation/counter-rotation. The reverse rotation can be divided into two types: inward and outward.

The nature of the co-rotating screw pressure zones is different. The raw material is rotated by the screw in the barrel, which produces a high -pressure area and a low -pressure area. Obviously, the material will flow along the two directions from the high -pressure zone to the low -pressure area: one is to form the two C -shaped material flow along the inner wall of the barrel which is the mainstream of the material; the other is through the screw meshing gap which forms a reverse flow. The reason for reverse flow is that the left screw pushes material into the gap while the right screw pulls the material out. As a result, the material moves forward in the shape of ∞ to change the direction of the flow. This not only helps the mixing and homogenization of the material, but also causes grinding (that is, shear) between the screw elements and receives a rolling effect.  The smaller rolling effect results in a less wearing of screw. This is how the material is processed by conveying, cutting, mixing plus heating outside the barrel. It becomes mature under high temperature and high pressure.

The reverse rotation dual screw extrusion puffer generally uses two screws that are exactly the same in size, but the screw direction is opposite. The difference between inward rotating and outward rotating is that the position of pressure area is different. The pressure generated by the inward rotation is higher at top and lower at bottom. This makes feeding quite difficult. At present, this reverse inward rotation is rarely used; the pressure generated by outward rotation is lower at top and higher at bottom, which is conducive to feeding. However, compared with the co-rotation, the C -shaped material flow formed in the screw cannot move from one screw to another. The mixing effect of materials is significantly reduced, and its self -cleaning ability is not as effective and stable as co-rotation.

Due to the pressure difference between the reverse rotation , the separation force F will push the screws to both sides. The screw is pressed to the barrel under the F force and the wearing of both barrel and screw is accelerated. The higher the speed, the bigger F and the more wearing. Thereby the RPM of screws is restricted. While co-rotating screws do not have the force to separate the two screws, so the wearing is less. It can run at high speed and achieve a high output. Therefore its application range is much wider.

Most of the heat required by the material comes from the intermeshing gap; The force of shearing, squeezing and mixing generates heat and makes the heating uniform. The size of gap has a great impact on the quality of the puffing.  With a small gap and a large shear force, the amount of material passed through is reduced; with a large gap, the amount of material passed increases, but the shear force is reduced. The features of forced-conveying and self -cleaning by the twin screws make the duration short and uniform in the tube; the good homogenization performance of the dual screw makes the heating uniform, accelerates the maturity of the material and reduces the fluctuation of the temperature, which improves the yield and quality of puffed products.