Mechanism Of The Three-dimensional Surface Formation And Process Design In Continuous Roll Forming

Three-dimensional surface sheet metal parts have the advantage of light weight, highstrength and attractive appearance, etc., and are widely used in various fields ofmanufacturing. With the rapid development of manufacturing technology, the demand forthree-dimensional surface sheet metal parts shifts from mass production to small batches orsingle-piece production. Die forming has the characteristic of high precision, highconsistency, and is an important method for forming three-dimensional surface part.However, it is not suitable for forming large-size part, and each part needs a set of die. Themanufacturing cost of die forming is high and production cycle is long, this process is not fitfor small quantity and diverse products. Therefore, a new flexible forming to adapt to thereguirment of the manufacting of three dimensionnal surface part is urgently needed.Continuous roll forming is a new flexible forming technology, and can formthree-dimensional surface sheet metal parts in a continuous and flexible way. In continuousroll forming process, the large plastic in sheet metal is large, and therefore springback afterunloading is small. In addition, continuous roll forming process can form different shapeparts without changing forming tools, the technology has a good prospect. Curently, thestudy of continuous roll forming is still in the initial stage, and there are many problems tobe solved such like forming principle, process design method and precision controlling offormed parts. In this paper, based on theoretical analysis, numerical simulation andexperiment of sheet metal bending deformation of continuous roll forming process, thebending equation of longitudinal and transverse were set forth, and process design methodwere presented.The main contents and conclusions of this paper are as follows：(1)According to the characteristics of the bending deformation in continuous rollforming, deformation mechanism of sheet metal in longitudinal and transverse directionswas analyzed. In continuous forming process, the sheet metal is bent in transverse directionbefore entering contact deformation zone, and in the zone near to the contact line, thetransverse curvature of sheet metal is closed to the curvature of the flexible roll, and in thecontact deformation zone, sheet metal is uneven thinned in thickness direction, and unevenelongated in longitudinal direction, finally the sheet metal is changed into doubly curvedsurface part with the rotation of flexible rolls. Basic equations of longitudinal bending deformation were established, and relationship between longitudinal curvature and the profileof the flexible roll and the distribution of roll gap was established. Two typical distribution ofroll gap are, the gap narrow in middle area and wide on both sides, and the gap wide in middlearea and narrow on both sides. Plane assumption was proposed and used in bendingdeformation analysis, and three aspects of plane assumption are verified. Elongation inlongitudinal direction and thinning in thickness direction and deformation in transversedirections were studied. Based on simulation results, the law of elongation in longitudinaldirection and thinning in thickness direction is verified.(2)Design method of continuous roll forming process based on the target surface waspresented. In some cases, the equation of transverse section line and longitudinal curvatureare unclear from CAD model. NURBS description of transverse section line and the formulato calculate longitudinal curvature are provided. The profiles of upper and lower flexiblerolls were designed based on the transverse curvature of the target surface, and thedistribution of the roll gap was designed based on the longitudinal curvature of the targetsurface, a common formula for roll gap design was set up, and the thinning in thicknessdirection of sheet metal should be linear distribution. The shape of flexible roll is adjusted bycontrol unit, and affected by the number of control unit. Based on the given process designmethod, the continuous roll forming tests for negative Gauss curvature surface and positiveGauss curvature surface were carried out. The experiment results show that the the surface ofthe formed are smooth, and forming results are great. The above results demonstrated thatthe proposed process is a feasible and effective way of forming three-dimensional surfaceparts.(3)Based on numerical simulation, distribution of stress and strain fields in formingprocess were analyzed and curvature precision in longitudinal and transverse directions anderror distribution between simulation part and experimental part were given. Wrinkling mayoccurs when the thinning in thickness direction is nonlinear, and the reason of wrinkling wasdiscussed, and the region easy to wrinkling was analyzed. Stress and strain in wrinkling areaare much larger than non-wrinkling area, and the wrinkling is severe when the offset occurbetween distribution of roll gap in forming process and design roll gap. Interaction forcebetween flexible roll and sheet metal change the profile of the roll gap again, the thinning inthickness direction will be nonlinear, and the shape of the formed part is changed.Deformation of the work roll assembly with flexble wire shaft as a backup roll was analyzedthrough numerical simulation, and found elastic deformation of flexible work roll is small,and elastic deformation of flexble wire shaft is large due to its structure.(4)The continuous roll forming tests for negative Gauss curvature surface and positiveGauss curvature surface were carried out on the device adopt flexble wire shaft as backuproll and linear shaft as backup roll. When the device adopt the flexble wire shaft as backuproll, repeatedly compensation can improve the accuracy of the roll gap, and the accuracy forlinear shaft is high without compensation.The analyzed results show that formed part hassmooth surface, and the distribution of thickness and longitudinal curvature are consistent with the theoretical analysis. These results show that the proposed design method ofcontinuous roll forming process is a feasible and effective way to forming three-dimensionalsurface parts.