Proceedings of the National Conference on Trends and Advances in Mechanical Engineering,

YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012

536

INVESTIGATION AND ANALYSIS FOR THE WRINKLING BEHAVIOUR OF DEEP DRAWN DIE SHEET METAL COMPONENT

BY USING FAST FORM

Surya Prakash1, Dinesh Kumar 2 1, 2 Department of Mechanical Engineering, ITM University, Gurgaon, Haryana, India. email: suryaprakash@itmindia.edu, +91-9718238577

Abstract The manual design of any type of drawing die is complicated and tedious procedure, In spite of all precautionary measures there are several chances of denting, cracking and wrinkling which needs to be rectified. As the design and development of sheet metal deep drawn die is a comprehensive technique which needs accuracy in the apprehension of working for high order and its rectification in forming process. The appearance of dimensional deviations of shape and position, of the defects in the metal sheets that have been subjected to a cold plastic deformation process, represents a critical problem for the specific industry, especially for the mass production, like the machine manufacturing industry. Thus, there arises the need for development of a system for manufacturing wrinkle free surface of deep drawn components. The complex forces act on the sheet metal blank during drawing are so unpredictable that they are difficult to determine manually and mathematically. These forces cause wrinkles and other defects on the surface of the wall of component. The aim of this publication is to present the principal aspects and investigation that effect wrinkling. Firstly component 3D-modeled in CATIA for analyzing for detecting wrinkle prone area by using fast form software. The input to software comprises of initial graphics exchange system .Cold rolled extra deep drawing quality material of sheet metal component has been utilize. The out put in form of results received regarding wrinkle prone area are found in closed agreement matching with the practical results. One can therefore easily predict and detect the tendency of expected wrinkle formation and stress distribution in any drawn component. Some methods for preventing wrinkling in deep drawn part are also suggested. Keywords: Wrinkles, Forming, Sheet Metal, Deep Drawing

1. Introduction Deep drawing is a process for shaping flat sheet into cup shaped articles, without excessive localized thinning, fracture or wrinkling. This is done by placing a blank of appropriate size over shaped die and pressing the metal into the die punch. In production of passenger car body, motor bike components, deep-drawing is one of the most important manufacturing processes. A usual deep-drawing die is shown in figure 1 (a). It consists of a die cavity, a blank holder and a punch During the deep-drawing process, the blank is clamped between die cavity and blank holder. The blank holder is to avoid the occurrence of wrinkling and inducing required retracking force. While the punch is forming the blank into the die cavity, blank material flows into die cavity. This effect is called material flow. In deep-drawing processes, it is important to control material flow in order to get defect-free components [1].

Figure 1(a) Sheet metal deep drawing process

Depending upon several factors such as geometry, volume material type, deep drawing or stretch forming is used to form sheet metals. In sheet-forming process, however, several types of failures could occur, such as rupturing, necking, wrinkling and spring back which is undesirable [2]. Main defects in deep-drawing processes are cracks and sidewall wrinkling, depicted in figure 1(b) & figure 1(c).

Proceedings of the National Conference on Trends and Advances in Mechanical Engineering,

YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012

537

Figure 1 (b) cracks and sidewall wrinkling in a deep drawn part

Figure 1(c) Wrinkles formation on the side wall of the component

1.1. Deep drawing parts There are the components which are manufactured with the help of deep drawn dies. As shown in the figures-

Figure 2 Car engine part Figure 3 Deep drawn cup Figure 4 Deep drawn sheet

1.2. Wrinkles on deep drawing parts Material thinning and cracks may arise when local load in the blank increased the level of uniform elongation. In contrast, sidewall wrinkling occurred when tangential compressive stresses led to buckling in the sidewall area. In deep-drawing, wrinkling and cracks have to be avoided by control of material flow. Blank holder force, draw or lock beads, type and amount of lubricant as well as shape and size of initial blank represent possibilities to influence material flow. Wrinkling is usually undesired in final sheet metal parts for functional and aesthetic reasons. It is unacceptable in the outer skin panels where the final part appearance is crucial. Wrinkling on the mating surfaces can adversely affect the part assembly and part functions, such as, sealing and welding. In addition, severe wrinkles may damage or even destroy dies. Therefore, the prediction and prevention of wrinkling are extremely important in sheet metal forming parts. One of the variable blank holder forces known from literature has been published by Sheng and is illustrated in figure 5. The depicted blank holder force profile has been utilized for optimization of deep- drawing a conical cup. Optimization of variable blank holder force distributions was a deciding factor too.

Proceedings of the National Conference on Trends and Advances in Mechanical Engineering,

YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012

538

1.3. Summary of causes Of Wrinkling in Deep Drawn Parts: Several factors can cause wrinkles in deep drawn parts, including: • Blank holder pressure • Die cavity depth and radius • Friction between the blank, blank holder, punch and die cavity • Clearances between the blank, blank holder punch and die cavity • Blank shape and thickness • Final part geometry • Punch speed

Figure 5 Variable blank holder forces for a conical cup Figure 6 silencer protector of Yamaha bike

Wrinkling is a phenomenon of compressive instability of as a result, compressive hoop stresses are generated and thus wrinkling may be developed in the sheet metal under the holder (flange wrinkling) as well as those in the side-wall [3]. The prediction on the initiation of flange wrinkling has been addressed analytically and numerically in a number of previous works [4-8]. In depth study of deep drawing, with a view to provide explanation of certain less understood aspects of the process, especially in the case of deep drawing of non circular parts and components like protector silencer is taken as shown in figure 6.

2. Literature review Nonmu [8] is the first person who studied on the wrinkling defect in the deep drawing operation. He examined actual phenomenon of wrinkling in conventional deep drawing without blank holder by considering equilibrium of moment acting on half waves & critical blank thickness. M. M. Alkky & D.M.Woo [9] examined effect of die profile one near to tractrix form & other two with large radius of curvature on the drawing performance. He showed that punch load can be reduced by using tractrix type die with large radius of curvature. Yossifon and Tirosh [10] published a series of articles dealing with simple analysis of the deep drawing process as applied to the formation of cups from metallic materials such as copper, aluminum, steel and stainless steel. Shawki [11] has systematically investigated the influence of different test condition on LDR for two different types of profiles namely conical , tractrix and showed that tractrix die is more effective. Lo, Hsu and Wilson [12] expanded upon the earlier work of Yossifon and Tirosh by applying the deep drawing hydro forming theory to the analysis of the hemispherical punch hydro forming process. The purpose of this work was to determine a theoretical method of predicting failure due to wrinkling (buckling) or rupture (tensile instability) during the punch hydro forming of hemispherical cups. This work was basically an extension of the work done by Yossifon and Tirosh by incorporating a general friction-force expression into the analysis and expanding to more complicated geometries. In 1994 Naryansamy & Sowerby [13] showed that stainless steel 304 which has low value of anisotropy and high value of hardening rate has better resistance to formation of wrinkles when deep drawn through conical die.Wrinkling in sheet metal forming, with tearing, is one of the most important instabilities that occur in parts formed using stamp forming and deep drawing processes. This phenomenon limits the type of parts and geometries that can be formed using these techniques. Simulation of wrinkling behavior using the finite element method (FEM) in sheet metal stamping is an important predictive tool. An accurate finite element model that could accurately predict the formation of wrinkling could also be used at the tooling design stage of parts of various shapes.

Proceedings of the National Conference on Trends and Advances in Mechanical Engineering,

YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012

539

Klaus M. Wurster etc describes a procedure for automated optimization of blank holder force distribution for a deep-drawing process with segment-elastic blank holder. Thereby, it was possible to identify optimized blank holder force distribution in space and time without manual investigation of optimization results during finite element analysis prior to manufacturing of deepdrawing die. These could be traced back to Yoshida [14], Wang X. and Cao J. [15], Zhang LC, Yu TX, Wang R.[16], and Fatnassi A, Tomita Y, Shindo [17]. Colgan, M., Monaghan, J [18] – worked on the initial stages of a combined experimental and finite element analysis (FEA) of a deep drawing process. The objective of theses work was to determine the most important factors influencing a drawing process, utilizing the help of a design of experiments and statistical analysis. M. Firat[19] worked on the finite element simulations of a sheet metal forming process. His method helped in designing the forming interface for a stamping part by shifting the costly press shop try-outs to the computer aided design environment. The finite element models used in the sheet metal formability and stamping feasibility assessment studies are commonly based on the ideally rigid die-face design. The results have indicated the relative merits of the die-face distortions on the formability and springback deformations.M. Abbasi, M. Ketabchi, at al [20] worked tailor welded blanks (TWBs) that are steel sheets of different characteristics welded into a single flat blank prior to pressing in order to achieve the optimal material arrangement and weight reduction for cars, and to increase process efficiency and machine flexibility. The results also showed that wrinkle waves just formed in thin segment of TWB, and wrinkling initiated by development of three wrinkle waves. Agrawal, A., Reddy, N. V., etc study the determination of optimum process parameters for wrinkle free products in deep drawing process [21].

3. Methodology In figure 9 flow diagram is given that shows the steps involved in the present work. 3D data of component is transferred to FASTFORM for developing the blank. Different conditions on 3D deep drawn component for analysis with FASTFORM were applied.

Figure 8 Section view of Silencer Protector

Figure 7 Top view of the silencer protector

Proceedings of the National Conference on Trends and Advances in Mechanical Engineering,

YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012

540

Figure 9 (a) M e s h e d V i e w o f t h e c o m p o n e n t

Figure 10 Front view o f t h e c o m p o n e n t

3.1. Blank developments and analysis FAST FORM is the software which is utilized in the development of the blank and analysis. The accuracy of development blank is depends on the meshing size.

4. Result and Discussion

Figure 13 Silencer protector component analysis result

Figure 11 Blank developed on the Fast blank

Figure 12 Thickness strain distribution

Figure 9(b) 3D Left view of CATIA model

Proceedings of the National Conference on Trends and Advances in Mechanical Engineering,

YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012

541

From figure 11-17

• Low Strain - Minimal stretch or compression in either the major or minor directions.

• Strong Wrinkle Tendency - Slight stretch in one direction and compression in the other with material thickening. Wrinkles are very likely to occur.

• Wrinkle Tendency - Stretch in one direction and compression in the other with slight material thickening. Wrinkles may occur.

• Loose Material - Stretch in one direction and compression in the other with slight material thinning. Surface issues like "oil canning" may result.

• Semi-Tight Panel - Stretch in one direction and slight compression in the other with material thinning.

• Plain Strain - Stretch in only one direction with material thinning.

• Tight Panel - Stretch in two directions with highest material thinning. Stiff dent resistant panel with possible thinning problems may result.

Figure 14 Equivalent strain

Figure 15 Equivalent stress

Figure 16 X- Direction forming displacements

Figure 17 Forming Zone

Proceedings of the National Conference on Trends and Advances in Mechanical Engineering,

YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012

542

5. Methods for preventing wrinkling in deep drawn parts 5.1. Using a Blank Holder In most deep drawing processes, a constant blank holder pressure is applied throughout the entire drawing action. Variable blank holder pressure, however, has been employed with some success. A pneumatic or hydraulic blank holder cushion can vary the blank holder pressure linearly over the stroke of the machine. A numerically controlled (NC) die cushion can be used to provide a variable blank holder pressure over the course of drawing action. An NC die cushion can dramatically increase the allowable die cavity depth while preventing both wrinkling and cracking.

5.2. Die Cavity Design Choosing a flange radius that is just large enough to prevent cracking or can minimize the potential for wrinkles. Additionally, considering minimizing the part complexity and any asymmetry can also help. Incorporating a multi-step drawing process offers a variety of advantages in preventing wrinkling in deep-drawn parts. Designing the blank geometry to minimize excess material can reduce the potential for wrinkling. Adjusting the sheet metal grain in an asymmetrical design to minimize the compound of grain stresses and the general stresses of the deep draw process is something to take into consideration [24].

5.3. Other Factors

Lubricants reduce the friction between the blank and the punch and die cavity and can be liquid (wet) or films (dry). Generally, they are applied to the blank before drawing. While lubricants can facilitate the metal flow into the die cavity, consider increasing the blank holding force to account for the reduced friction. Today, computer aided design and finite element modelling are used to create part and die designs and to simulate the deep drawing process, significantly reducing the costs of tooling and labour in the design process.

6. Conclusion The present work investigate and analyses facts like in spite of all precautionary measures there are macro and micro level chances of denting, cracking, and wrinkling which needs to be diminished using probabilistic approach. Component & deep drawn die have been modeled. The IGES data exported easily to the Fast Form software. The component namely Silencer Protector is modeled in CATIA & the Fast Blank software has been utilized for the blank development. Accordingly die and punch system has been modeled and developed too. It was also observed that the wrinkles generated on the deep drawn parts are found in the thin sheet component and that wrinkles are generated when die and punch parts not matched and aligned suitably. For enhancing the quality of wrinkle free oriented drawn component hard chrome plating on die and punch was preferred. It was observed that wrinkle is strong when analysis is performed without blank holder. Based on the observations some methods for preventing wrinkling in deep drawn parts like using a blank holder, die cavity design, lubricants, finite element modelling suggested.

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