# Design of a Combined Redrawing-Ironing Process to Manufacture a CNG Pressure Vessel Liner
## 1 Introduction  
A storage vessel that consists of a metallic liner wrapped with a carbon fiber-resin composite to work at high pressure has been widely applied to fuel containers for compressed natural gas (CNG) vehicles. The liner is manufactured by cold D.D.I. (deep drawing and ironing) to obtain the desired diameter and thickness of the cylinder and by hot spinning to form a dome shape. The D.D.I. process shown in Figure 1 is a continuous process that uses deep drawing to reduce the diameter of the billet and subsequent ironing to reduce the thickness of the workpiece. To manufacture a CNG pressure vessel with the liner dimensions shown in Figure 2, a first stage (deep drawing), second stage (redrawing + ironing 1 + ironing 2), and third  
Figure 1: Scheme of the existing D.D.I. process for manufacturing CNG pressure vessel. stage (redrawing + ironing) have been performed [1; 2]. In the first stage, an initial thick blank is manufactured into a cup shape using a tractrix die without a blank holder to prevent wrinkling. The tractrix die reduces radial stresses and punch loads occurring at the blank wall and increases the formability. In previous studies, wrinkling or buckling in deep drawing of both aluminum and copper sheets was investigated using a tractrix die [3]. [PERSON] et al. suggested a rapid determination method for the LDR (limiting drawing ratio) based on the characteristic limit load, which consumes less time, and performed an experimental investigation of an automotive aluminum alloy sheet [4]. A linear buckling analysis of the pressure vessel was conducted, and the maximum buckling load was estimated [5]. [PERSON] et al. designed a modified conical tractrix die to reduce the deformation force and to enhance the LDR, and the formability of a tailor friction stir welded sheet blank was improved by 27% [6]. [PERSON] et al. developed the standard interpretation model of non-linear finite element analysis of an E-glass/epoxy filament wound composite pressure vessel [7]. [PERSON] et al. proposed a simple process for the deep drawing of elliptical cups without a blank holder. The effects of die and punch geometry according to the half-cone angle and punch filled radius on the LDR and on the drawing load were investigated for the optimization of the process [8]. [PERSON] and [PERSON] presented the optimization of process parameters for reducing the risk of failure due to wrinkling and fracture in the deep drawing of a rectangular cup. The blank holder force, friction coefficient, punch radius, and initial blank shape were chosen as design parameters based on wrinkling and fracture criteria [9]. [PERSON] and [PERSON] proposed an analytical method for estimating the LDR of the redrawing stages in a deep drawing process to predict the limit ratio for redrawing with/without intermediate annealing processes. The presented method was in good agreement with the experimental and numerical result, and the effect of process parameters on the LDR was investigated [10]. [PERSON] suggested a new equation incorporating the value of the LDR derived as above and the critical drawing load based on the maximum principal load for localization of plastic flow. The maximum drawing load in cup-drawing with a flat-nosed punch was developed [11]. [PERSON] et al. conducted an ironing analysis of a deep-drawn aluminum cup. The outer wall thickness was reduced with varying the TRR (thickness reduction ratio) through a die ring with various die angles. Based on this stress, the most optimal condition was selected [12]. [PERSON] et al. conducted a simulation of an ironing process for a caliber bullet case to determine the minimum forming force on a variety of die angle and thickness reduction rates [13]. [PERSON] et al. proposed a new ironing method called constrained ironing for producing thin-walled cans and components with uniform thickness [14]. The second stage of the existing D.D.I.