| Paper-based packaging materials are not harmful to the ecological environment and human health,and are favored by the packaging industry.Compared with the traditional paperboard forming processes such as artificial glue,push pin and so on,the stamping process can be directly formed,which has the advantages of high efficiency,various shapes of formed products,high forming accuracy and good sealing,and has a wide application prospect.In this paper,the single-layer and double-layer paperboard stamping of polypropylene(PP)coated kraft paper is analyzed by experimental study and numerical simulation.The influence of paperboard size on forming depth and forming law is described.The effects of blank holder clearance on wrinkle height,wall thickness distribution of longitudinal(mechanical direction,MD)and transverse(cross direction,CD),equivalent plastic strain and springback amount are explored.The stamping results of single-layer and double-layer paperboard are compared.It is revealed that the stamping process of double-layer paperboard can better inhibit the forming defects of PP coated kraft paper,and achieve high-quality and efficient tray stamping.The main work and conclusions are as follows:(1)Study on the mechanical properties of PP coated kraft paper.The tensile strength,elongation at break,elastic modulus and yield strength of six kinds of basis weight PP coated kraft paper were analyzed by single tensile test.The results show that the tensile strength decreases with the increase of the basis weight of the paperboard,and MD tensile strength is about 1.4 times of CD tensile strength;the elongation at break is not affected by the change of basis weight,but CD elongation at break is about 2.2 times of MD;the elastic modulus decreases with the increase of the basis weight,and the MD elastic modulus is about 2 times of CD elastic modulus;the yield strength decreases with the increase of the basis weight of the paperboard,and MD elastic modulus is about 2 times of CD elastic modulus.MD yield strength is about 1.8 times higher than that of CD.The analysis of stress-strain curve shows that PP coated kraft paper has orthotropic elastoplasticity.(2)Experimental and Simulation Study on forming size of single-layer and double-layer paperboard.For different sizes of PP coated kraft paper composed of 6 kinds of thicknesses and 8 kinds of diameters in any combination,press forming experiments and simulation studies were conducted to reveal the effect of paperboard thickness and diameter on forming depth and forming law.The appropriate paperboard size was selected in the forming process of single-layer and double-layer paperboard,and the forming effect of the two forming processes was compared.The results show that the diameter of paperboard is the key to determine whether the paperboard can be formed.When the size of paperboard is small,it is easy to crack and damage the surface of formed paperboard,and when the size of paperboard is large,it is easy to produce large wrinkle.Double paperboard stamping is more conducive to paperboard forming,which can significantly improve the ratio of good and best formed paperboard.The wrinkle and damage mechanism and manifestation of PP coated kraft paper were summarized.(3)Simulation study on blank holder clearance of single-layer and double-layer paperboard.The stamping process of 180 g and 225 g single-layer and double-layer PP coated kraft paper is simulated respectively.By analyzing the buckling and wrinkling of paperboard,wall thickness distribution and maximum plastic strain,the appropriate blank holder clearance is selected for the two stamping processes,and the auxiliary paperboard with appropriate thickness is explored for the stamping process of double-layer paperboard,finally the simulation results of single-layer paperboard and double-layer paperboard are compared.The conclusions are as follows:the appropriate blank holder clearance can significantly inhibit the excessive thinning and thickening of the wall thickness of the formed paperboard,and the wall thickness distribution of the double-layer paperboard is more uniform than that of the single-layer paperboard.With the increase of the blank holder clearance,the wrinkle thickness will gradually increase,but the wrinkle formed by the double-layer paperboard will be significantly larger than that of the single-layer paperboard.350 g PP coated kraft paper is most suitable to be used as the auxiliary paperboard of 180 g and 225 g PP coated kraft paper.(4)Experimental analysis of blank holder clearance between single-layer and double-layer paperboard.In the process of single-layer and double-layer forming,180 g and 225 g PP coated kraft paper were pressed under the conditions of 1.1 times of paperboard thickness(1.1t),2t,3t and 4t of blank holder clearance,respectively.The effects of blank holder clearance on the wrinkle height,MD and CD wall thickness distribution and springback amount of the formed paperboard were revealed.The proper blank holder clearance for tray forming was selected.The following conclusions were drawn:in the process of single-layer and double-layer stamping,the wrinkle height and the springback increase gradually with the increase of blank holder clearance.The double-layer stamping process can inhibit the damage of the forming part,make the wall thickness distribution more uniform,and reduce the springback of the forming paperboard,but it can not reduce the wrinkle height. |
