Development of a Micro Sheet Hydroforming Setup to Produce Micro Cup Products

Hydroforming is a very suitable technology due to some advantages like high strength to weight ratio, better surface quality and good thickness distribution of products [1-3]. Miniature hydroforming and especially micro-Hydroforming are an area to form products with less than 0.05mm thickness and it needs controlled blank holding and smooth container to tolerate high fluid pressure [3,4]. Size effect is also a problem in micro forming processes [5-7]. There is some research regarding micro forming processes. Hirota K et al. [8] carried out micro-extrusion process with less than 50μm size. Kim DJ et al. [9] carried out Finite element simulation of micro-rolling. Yiping L et al. [10] carried out micro forging simulation. Emil Egerer et al. [11] carried out hot micro forming simulation along with experimental validation. Zhuang W et al. [12] carried out an experimental investigation about micro-tube hydroforming. The thickness of the tube was 30.4μm. Fu MW et al. [13] carried out experimental and simulation studies of micro blanking and deep drawing compound process using copper sheet material [13]. Hung J et al. [14] carried out an experimental study to produce micro-flow channels with 0.051mm thickness. In this paper Micro Sheet Hydroforming (MSHF) work carried by author has been introduced.

Simulation of micro sheet hydroforming process

In this part simulation of Micro sheet hydroforming Using package DEFORM-3D is explained [15].

Preprocessing for micro sheet hydroforming

The difficulty in producing micro-components is the design for its tools and dies. Therefore, a simulation study is very important to gain the know-how of the entire process well in advance before starting the actual manufacturing. Here the blank of 9mm is formed into the cup as shown in Figure 1. Stainless steel (AISI-304) is used as a work piece material and its properties are given in Table 1 [16-18]. Different simulations are performed and some of the results are discussed in this section. Material properties of this kind of stainless steel are very similar to a kind of CuBe2% [19]. Figure 1 shows the micro-forming blank, which is meshed with 1, 45,200 tetrahedral mesh elements. The blank diameter is calculated by the formula, as shown in Figure 2 [20]. CAD model of the required shape has been given in Figure 3. Initial design for micro sheet hydroforming has been shown in Figure 4. In this situation punch is cited above of the liquid and will push the fluid into the die cavity therefore friction value is very high and deformation load will be increased. Simulations were carried out for micro forming to study the different kinds of defects and specifically to calculate the starting load required to start the deformation. Also, the thickness distribution is plotted. Some kind of defects like thinning, tearing, wrinkling and high load requirement are observed and efforts were carried out to get the optimum deformation which would be free from defects.

Figure 1:Micro-forming meshed blank.

Figure 2:Formula to calculate Blank Diameter [20]. Where, d1= 5.8mm, d2= 6.3mm and s=1.52mm, therefore blank diameter = 8.54mm which can be taken as 9mm.

Figure 3:Geometry of microcap, all dimensions in mm.

Figure 4:Front view of punch, sheet and die cavity for simple micro sheet hydroforming (fluid pressure from upside) (first condition).

Table 1:Material data for micro-forming simulation.

Design modifications

Attempts have been done to minimize the thinning and high load requirements. Design modifications are done as shown in Figure 5. Instead of a die cavity the container is used which is provided with pressurized fluid. This pressurized fluid also acts as a cushion minimizing the defects like tearing and wrinkling. Second modification is done by changing the die profile radius which plays significant role for the proper deformation. Also, the cup forming operation is carried out by designing actual punch and here the punch movement is considered as an important factor [21].

Figure 5:Front view of punch, sheet and die cavity for micro hydro mechanical deep drawing (fluid pressure from downside) (second condition).

Simulation result

Figure 6 shows effective stress distribution in appropriate deformation under second condition with container and fluid pressure from downside. Area of flange is more than enough and secondary operation (cutting) is required. Figure 7 shows appropriate deformation with some permitted wrinkles under second condition. Figure 8 shows strain distribution in appropriate micro cup. Figure 9 shows damage distribution.

Figure 6:Effective stress distribution in micro cup (with flange without wrinkling).

Figure 7:Effective stress distribution in micro cup (with permitted wrinkling and small flange).

Figure 8:Strain distribution in appropriate micro cup.

Figure 9:Damage distribution (damage value: 0.00039).

Forces in sheet hydroforming

In cup drawing many different processes take place simultaneously. The punch pushes the sheet into the die (Figure 10). The sheet under the punch pulls the remaining sheet in flange under the blank holder plate. As the punch advances, the sheet in flange is pulled over the die profile radius into cup wall. In doing so the sheet in flange suffers circumferential compression, bends under tension into die profile radius, slides over die profile radius and then it unbends into cup wall. Also, the sheet on the punch profile radius bends under tension. All these processes add to tension in the cup wall. The maximum stress thus occurs at the punch profile radius. The total stress in the cup wall is due to the cumulative effect of the following processes taking place during deep drawing.

Figure 10:Stresses in sheet in cup drawing [4,5].

A. Drawing of flange under friction between the die and blank holding plate.
B. Bending of sheet at the die profile radius.
C. Slipping of sheet into the cup wall.
D. Unbending the sheet into the cup wall.
E. Bending over punch profile radius (punch corner radius).

Total stress and forming load to start the deformation (punch force) for SHF can be calculated using equation no. 1 and 2 respectively [22,23]:

Where, σ_w: Stress in cup wall, P: Punch force, α: Angle between the punch axis and the cup wall, t: Thickness of cup wall, R_c: The mean radius of the cup at the section where σ_w is determined, R_b: The radius on flange where bending takes place, R: Current outer radius of flange, P_h: Hydrostatic or fluid pressure, σ₀: Yielding stress, λ: Thickening factor which has value between 1 and 2/√3, r_d: Die radius, μ: friction value.

Percent error should be calculated using [(measured-actual)/ actual] ×100 where the analytical result formula is taken as measured value and simulation result as actual one) [24]. Table 2 shows comparison between first condition (without container) and second condition (with container) for calculating total forming load by simulation. Also, Table 3 shows comparison between results of simulation and explained analysis about forming load of micro cup under micro sheet hydroforming. For analytical formula, the value of any data has been selected from table 3.5 and R_c(2.975≅3mm), R_b(3.17mm) and R(4mm), P_h(2MPa), λ(1.1), α(6.65°=0.037π radian) regarding geometry details obtained from figure 3.40 and t(0.05mm) have been selected to substitute in equations (1) and (2). It appears that in simple shape percent error is not very high. Figure 11 shows comparison of thickness distribution for micro sheet hydroforming in two methods. This study can also illustrate the advantage of design modification in the second condition (without die cavity).

Table 2:Comparison between first condition (without container) and second condition (with container).

Figure 11:Thickness distribution for micro sheet hydroforming.

Table 3:Comparison between results out of simulation and analysis.

Design and fabrication of a simple micro sheet hydroforming setup and experimental study

Based on concept of simulation results, a simple micro sheet hydroforming setup has been designed and developed to study the functionality of deformation of micro size sheet under hydro forming and evaluating of the defects. The thickness of the sheet is 0.05mm (less than 0.1mm) therefore it will be in the micro size category. Figures 12-14 show the detailed design of the setup. The setup has been fabricated for restriction of two plastic parts of the die and blank holder. A simple screw jack (1 ton capacity) provides force for punch moving from downside also punch load can be measured using one seated load cell above the jack as shown in Figure 12 & 15. Jack pushes the piston connected to the punch and punch will push fluid through the work piece and therefore work piece/sheet will be pushed inside of die. Figure 13 shows a closeup view of the die cavity. In this simple design two acrylic parts are in contact with each other. Fluid will come from downside and blank is already restricted between two in contact plates as shown in Figure 12 & 14. Acrylic material has been used for the die set as shown in Figure 14. Figure 15 shows detailed design of load cell to calculate total forming load of setup based on previous research [25-28]. Figure 16 shows developed micro sheet hydroforming setup.

Figure 12:Front view of the designed micro sheet hydro forming setup.

Figure 13:Close up view of die set of microcup.

Figure 14:Front view of the designed micro sheet hydro forming setup..

Figure 15:Seated load cell along with strain gauge for measuring the punch load.

Figure 16:Developed simple micro sheet hydroforming setup.

Explanations of the parts in Figure 16:
a) Upper plate for restriction of up movement of the die set
b) Middle plate for restriction of side movement of the die set
c) Lower plate for restriction of down movement of the die set
d) Legs (Diameter: 25mm)
e) Screw jack for providing the deformation force along with load cell.
f) Die set of micro sheet hydroforming

The maximum forming load by the simulation has been 1.66KN as shown in Table 2. This value has been obtained around 1.50KN from experimental study using load cell along with relevant strain gage after calibration [29,30]. Therefore, the result of FEM simulation has been good agreement with experimental study.

In this paper, research on micro sheet hydroforming work has been introduced and explained. A final element simulation has been carried out for deformation of a micro cup with 0.05mm thickness. The process has been simulated for two different conditions (with and without die cavity). Result of simulation and analytical study has been compared. A micro hydroforming setup has been designed and fabricated based on results of simulation. Result of FEM simulation has been good agreement in comparison with experiments by the setup.

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