Study on the influence of operating parameters of

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Research on the influence of operating parameters of twin screw extruder on processing quality

key words: operating parameters; Optimization; Statistical model; Twin screw food extruder; The specific volume

twin screw extruder is an efficient mechanical equipment, which can complete a number of unit operations in food processing, such as material conveying, mixing, shearing, sterilization, cooking, molding, etc. at the same time. It has the advantages of high production efficiency, low energy consumption, good product quality and so on. It is widely used in food extrusion processing

twin screw extruder is a multi input and multi output system, and extrusion processing is affected by many factors. At present, the extrusion theory of food raw materials processed by twin-screw extruder is not perfect, and there is no accurate mathematical model applied to twin-screw food extruder. It is difficult to predict the working condition and processing result of the extruder from material characteristics, structural parameters and processing conditions of the extruder. In recent years, researchers have carried out a lot of simulation research on the extrusion process, which is of great significance for in-depth understanding of the extrusion process of food and reasonably determining the structural parameters and operating parameters of the extruder. However, in view of the complexity of the extrusion process and the oversimplification of the simulation model, the simulation results often have great errors with the actual processing results. At present, experimental research still plays an important role in the research of food extrusion processing of twin-screw extruder

this paper takes corn flour as the processing object, studies the influence law of screw speed, barrel temperature and feeding speed on product quality (measured by specific volume) through experiments, and establishes the corresponding statistical model, in order to explore the extrusion processing mechanism of food and promote the development of extrusion processing technology. The research of this paper has a guiding role for the determination of operating parameters in the extrusion process of twin-screw extruder

1 experimental part

1.1 materials and experimental equipment

the experimental processing material is corn flour, and its moisture content is 17%

dse25 twin-screw food extruder with full meshing and co rotating produced by Brabender company in Germany is adopted. The screw of this machine adopts the most popular sectional combination type, with reverse screw and shear ring. Screw speed, temperature, die pressure, torque, feeding speed and other parameters can be displayed on the computer screen in real time in the form of curves and values

1.2 experimental design

1.3 determination of experimental parameters

2 results and analysis

2.1 experimental results

in the experiment, 8 samples were taken for each formula, the specific volume values were measured respectively, and then the average value was taken. The experimental results are shown in Table 2

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2.2 result analysis

2.2.1 analysis of the impact of various factors on specific volume

1) the impact of barrel temperature on product specific volume

figures 1 and 2 show that the specific volume increases with the increase of barrel temperature

the specific volume value has a great relationship with the gelatinization degree in the barrel during the extrusion process. When the gelatinization degree increases, the specific volume of the product is the condition and basic condition for sustainable development. With the increase of barrel temperature, the material absorbs more heat during extrusion, which is conducive to promoting the gelatinization of starch. Therefore, with the increase of barrel temperature, the specific volume of our company's testing machine has the advantages of increasing. In addition, the increase of temperature can accelerate the melting process and improve the melting degree of materials. Under the stirring and shearing action of the screw, the starch particles in the materials are easy to be destroyed, the mechanical strength is reduced, and it is easy to be expanded. However, the temperature cannot be increased arbitrarily, because too high a temperature will make the material "coking", affecting the product quality, and when the temperature is too high, the protein in the material will have a "Maillard" reaction, resulting in the loss of nutrients. According to the author's experience, for the extrusion of corn flour, the temperature should not exceed 185 ℃

2) effect of feeding speed on specific volume of products

under the condition of constant screw speed, the increase of feeding speed will improve the filling degree of materials in the barrel, strengthen the shear and friction effect of materials, and is conducive to the destruction of starch particles and improve the expansion degree. In addition, when the feeding speed increases, the pressure formed at the die mouth increases, the pressure difference between the pressure at the die mouth and the atmospheric pressure outside the die mouth increases, and the expansion degree of the material when extruded from the die mouth increases. However, further increase in the feeding speed will cause uneven mixing of materials in the barrel, resulting in uneven absorption of heat from the barrel, so that the gelatinization and melting degree of starch are different, and there are "hard lumps" in the extrudate, affecting the expansion degree and taste. Figures 1 and 3 show the effect of feeding speed on specific volume

3) effect of screw speed on specific volume of products

the influence of screw speed on specific volume is complex. When the screw speed is small, the residence time of the material is prolonged, which is conducive to absorbing heat from the barrel and promoting the gelatinization of starch. However, at this time, the shear effect of the screw on the material is small, the damage degree of starch particles in the material is not enough, and the extruded product is rigid and shriveled, with small specific volume. With the increase of screw speed, the shear and friction effects of materials increase. However, due to the reduction of the residence time of materials in the barrel, the absorption of heat is affected, which will reduce the gelatinization degree of starch, resulting in small specific volume of extruded products and poor expansion effect. Therefore, the screw speed should not be too high or too low in the processing process. Figure 2 and 3 show the influence of screw speed on specific volume

3 conclusion

this paper studies the effects of screw speed, barrel temperature and feeding speed of twin-screw extruder on the specific volume of products, and draws the following conclusions:

(1) the influence of various factors on the objective function is complex, and their interaction also affects the specific volume in addition to their individual effects

(2) with the increase of barrel temperature, the specific volume of products shows an upward trend, but too high temperature will cause "coking" of materials; The influence of screw speed and feeding speed on specific volume is complex, and there is a nonlinear relationship between them

source: Packaging Engineering Author: Xu Kefei, Li Depu Harbin Business University

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