Structural Optimization And Performance Analysis Of Single-screw Extruder For Wheat Flour

As single-screw food extruder has the features of simple design, easy to be manufactured, low cost and so on, thus it is favored by the manufactures. In order to obtain the puffed wheat flour products with high quality, we designed and manufactured the single-screw food extruder on basis of extrusion theory. The influence of structure parameters and process parameters on expansion was studied by numerical simulation and experimental method. The specific work is as follows:1. At first, the screw extrusion process and the particles’ flow in the hopper were analyzed in theory, then designed and manufactured the single-screw food extruder according to the theory. Utilize the finite element method to verify the full flow of the single screw and the results show that the screw can transport the food material stably and meet the expansion pressure condition. Carry on experimental study on the extrusion prototype and find that the prototype can meet the working conditions and performance requirements.2. Established three-dimensional heat-flow coupled steady state finite element model in the POLYFLOW software. Simulated the influence of different factors on the melt flow. The results show that: material in the single thread screw can be extruded better and generate more heat, double threads screw can shear the material more strongly; the decrease of the flight clearance is good for extruding, shearing and generating heat, but too small flight clearance may lead to some bad phenomenon; the shear rate and heat of the material is in proportion to the screw speed, the pressure field is influenced by the screw speed weakly; the temperature of the melt material has a positive correlation with the die pressure, but the shear rate has little to do with the die pressure.3. Using Overlapping mesh method and particle tracking technique to simulate and analyze the effects of various factors on mixing characteristics of melt flow. And results show that: single thread screw can mix the material better comparing with the double threads screw; the decrease of flight clearance is helpful for material mixing; the increase of screw speed is helpful for shearing material, but the mixing effect on the distribution of material will decrease gradually to the same; in a certain range, the mix and distribution of materials is in proportion to the die pressure, but if the die pressure is too large, the mixing effect of the material will decrease.4. The single-factor experiment method and response surface experiment method were used to study the effects of various factors on the expansion. Through single-factor experiments, the effects of various factors on expanding products were studied, and the results show that: if the moisture content of the material is too low, that may lead to the occurrence of scorch and excessive degradation to products, too high moisture content will affect the expansion, gelatinization and water reabsorbing capacity; with the increase of the width of screw channel, the gelatinization, expansion and water reabsorbing capacity of the products will increased firstly and then decreased;expansion rate, gelatinization and water reabsorbing capacity of the products were negatively correlated with the diameter of the hole in the die mold; gelatinization and softness of the products will improve along with the increase of glutinous rice flour content, but the expansion of the products will decrease at the same time, and there are some burnt points on the products. From the response surface experiment we can learn that: impact of the interaction of screw configuration and diameter of the hole in the die mold is significant; due to the single thread screw will lead to uneven expansion, so we’d better choose the screw with double threads, under the premise of ensuring the expansion degree and the water absorption index of the product, the optimum process condition is as follows: the moisture content of material is 24.2%, the diameter of the hole in the die mold is 3 mm, the best configuration of the screw is between 2# and 3#, that means the width of the screw channel is about 20 mm.The study of this paper provides a reference for the design and optimization of the single screw extruder, which is conducive to shorten the development time and save the cost of research and development.