Experimental Study On Compression Molding Mechanism Of Machine-harvested Seed Cotton

Xinjiang is an important cotton producing area in China.With the continuous improvement of the level of agricultural mechanization and the continuous advancement of cotton mechanization in Xinjiang,the cotton mechanical harvest rates steadily rise.But there are still many problems:machine-harvested seed cotton is loose,low in density,inconvenient for transportation and storage and high in cost,which reduces the working efficiency of cotton picker.Seed cotton compression molding can solve such problems.Therefore,studying the compression performance and molding performance of machine-harvested seed cotton could provide necessary data support and theoretical basis for the development and design of related equipment.Based on its physical and mechanical properties and combined with the safe compression density,the machine-harvested seed cotton was taken as research object in this paper,and the mechanical properties during the compression process are studied.The stress change law in the shape retention stage of machine-harvested seed cotton compression was revealed by using the material rheology theory.The compression test bench for machine-harvested seed cotton was built,and compression molding test was carried out.And the key factors affecting compression molding are obtained.We carried out a multi-factor combination test to obtain the compression parameter range with lower cotton seed crushing rate and better molding properties.And the main research contents and conclusions of this thesis are as follows:(1)Research on physical and mechanical properties of seed cotton.The main planting machine-harvested cotton in Shihezi area during the harvest period was taken as the research object.Through statistical analysis and experimental tests,it was found the moisture content of the seed cotton was between 7～11%.The natural bulk density was 37.5～46.28 kg/m~3,which was significantly positively correlated with the moisture content and the trash content(P<0.05).The mechanical property parameters of cottonseed were tested,and it was concluded the primary and secondary order of factors affecting were:moisture content,compression orientation and cottonseed variety.(2)Compression characteristics and compression process analysis of machine-harvested seed cotton.Through the compression test of machine-harvested seed cotton,the relationship between compression density,compression force and bulk modulus can be described by power function and exponential function respectively(P<0.01).At the same time,it is obtained that the effects of the moisture content,the trash content,compression speed,initial density and cross-sectional dimension on the compression characteristics are different.That is,the influence of various factors on pressure and bulk modulus is linear at low density,and the linear relationship is enhanced at high density,and gradually becomes nonlinear.Through theoretical analysis,it is concluded that the relationship between pressure and distance is an exponential function.And it is obtained that the single cottonseed crushing condition is that the compressive force is greater than the maximum bearing capacity of the cottonseed.Compression test results of machine-harvested seed cotton show that when the compressive density is greater than 250 kg/m~3,the cottonseed crushing rate increases rapidly.(3)Studying on stress relaxation characteristics of machine-harvested seed cotton.The five-element Maxwell stress relaxation rheological model was established by the successive remainder method.The stress relaxation characteristic parameters were obtained,and it was concluded the equilibrium elastic modulus was negatively correlated with the moisture content,holding time,and cross-sectional dimension(P<0.05),and positively correlated with the trash content and compressive density(P<0.01).The rapid decay time was negatively correlated with the moisture content and holding time(P<0.01),and positively correlated with compressive density(P<0.01).The residual stress ratio was negatively correlated with the moisture content,compressive density and holding time(P<0.01).Finally,a stress relaxation dynamic model was established to simulate the stress relaxation process of machine-harvested seed cotton.The maximum deviation from the experimental value was 9.46%.(4)Molding characteristics and compression molding test of machine-harvested seed cotton.The compression test bench was built.Firstly,the single factor test of compression molding was carried out and the main influencing factors were identified.Then,taking the dimensional stability coefficient and the cottonseed crushing rate as the evaluation indicators,a multi-factor combination test was carried out,and the primary and secondary order of the factors affecting the dimensional stability coefficient were:initial density,compressive force,moisture content and holding time.The primary and secondary order of factors affecting the crushing rate of cottonseed are:initial density,compressive force,moisture content and holding time.Finally,under the condition that the crushing rate of cottonseed is 2～3%and the dimensional stability coefficient is 3～3.5,the optimal compression parameters range is:moisture content is 6～11.7%,initial density is 47.87～74.84 kg/m~3,compression force is 3～5.32 k N,the holding time is 50～239.75 s.