Manufacturing process of bezel frame for strength ... - Springer Link

Journal of Mechanical Science and Technology 30 (3) (2016) 1103~1109 www.springerlink.com/content/1738-494x(Print)/1976-3824(Online)

DOI 10.1007/s12206-016-0214-6

Manufacturing process of bezel frame for strength-reinforced TFT LCD module by progressive hemming of SUS304 stainless steel sheet† Kyung-Hun Lee1, Dae-Cheol Ko2, Jung-Min Lee3, Chan-Joo Lee3, Seon-Bong Lee4 and Byung-Min Kim5,* 1

Division of Marine Engineering, Korea Maritime and Ocean University, Busan 49112, Korea 2 ERC/ITAF, Pusan National University, Busan 46241, Korea 3 Dongnam Regional Division, Korea Institute of Industrial Technology, Jinju 52791, Korea 4 Faculty of Mechanical and Automotive Engineering, Keimyung University, Daegu 42601, Korea 5 School of Mechanical Engineering, Pusan National University, Busan 46241, Korea (Manuscript Received March 26, 2015; Revised September 15, 2015; Accepted October 29, 2015) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract In this study, the design procedure for a progressive multi-hemming forming process has been proposed for manufacturing bezel frames for strength-reinforced TFT LCD (Thin-film-transistor liquid-crystal display) modules. First, a strength analysis was performed using Finite element (FE) simulations to determine the minimum number of required folding edges. Subsequently, anoother FE analysis was carried out in order to investigate the effects of the process parameters on the dimensional accuracy of the bezel frame and to design the progressive hemming process. The analytical results were validated by hemming experiments performed using SUS304 stainless steel with a thickness of 0.3 mm. Finally, the quality of the bezel frame was estimated through measurements of its dimensional accuracy and bending stiffness. From the experimental results, it was confirmed that a bezel frame with a height distribution of 1.5±0.05 mm and a respectable bending stiffness of 70 N/mm could be manufactured using the proposed process. Keywords: Bezel frame; Progressive hemming; Multi-stage bending; Bending stiffness ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction With the increasing popularity of the slim product trend, electronic devices, such as phones, PDAs (Personal digital assistants), and tablets, have been progressively miniaturised and, as a result, the thickness of TFT LCD (Thin-filmtransistor liquid-crystal display) modules has been reduced. Consequently, the thinner TFT LCD modules have lower structural strength and less resistance to crash. In order to overcome these problems, the bezel frame, which is assembled with the backlight units of electronic devices, is a key component for maintaining the form of TFT LCD modules and preventing impact defects, as illustrated in Fig. 1. The manufacturing process for a strength-reinforced bezel frame consists mostly of the bending of a stainless steel sheet and the joining of the sheet with injection moulded parts used as plastic-reinforced corners. However, this process leads to low bending stiffness and impact resistance. Because of the *

Corresponding author. Tel.: +82 51 5103074, Fax.: +82 51 5813075 E-mail address: [email protected] † Recommended by Associate Editor Jin Weon Kim © KSME & Springer 2016

demand for high structural strength and crashworthiness quality, the need to manufacture bezel frames by a metal forming process is apparent. Bending and hemming processes are often used as the final stages of sheet metal forming operations. In particular, the hemming process is used not only to improve the deformation resistance (to work-harden an edge of the sheet metal) but also to attach one sheet metal component to another. The process design of hemming has been established through practice and relies heavily on designer experience and simple analytical tools. However, to advance the process, a new paradigm in designing dies and preforms is required. The new approach is based on a shift toward scientific techniques such as finite element analysis and quick-execution computational methods. These techniques have the potential to increase process efficiency and reduce material springback in the hemming process. The earlier studies on the hemming process were based on theoretical and experimental methods. Muderrisoglu et al. experimentally studied the effects of flange radius and flange height on the hemming operations for an aluminium sheet [1]. Zhang et al. investigated the curved surface-straight edge

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Fig. 1. Research background and necessity of hemming process.

hemming of automotive panels and developed a regression model as a hemming design guideline to predict radial springback, hemming load, and creepage/growing [2]. In addition, flanging and convex edge hemming of aluminium-killed drawing quality steel were investigated by Livatyali et al. and Livatyali and Larris [3, 4]. They proposed a design map of the failure zones for wrinkling and hem-out and revealed the effect of important factors in convex edge hemming. Over the last decade, many researches have sought to apply FE (Finite element) analysis to the hemming process. Livatyali and Altan presented the prediction and elimination of springback in the flanging and hemming processes by FE method [5, 6]. In their study, flanging with coining was investigated as a method to eliminate springback and to improve part quality. Lin et al. proposed a computational DOE (Design of experiments) study of the hemming process for automotive aluminium alloys [7]. This approach became the basis for process parameter selection in order to avoid hem surface cracking and provided important insights for achieving acceptable formability. Furthermore, a computational response surface study of three-dimensional aluminium hemming using solid-to-shell mapping was suggested by Lin et al. [8]. They presented some significant variables in the hemming process such as pre-hemming angle on roll-in/roll-out, nominal surface curvature on sheet springback, initial sheet strain, and flange die radius. Maoût et al. investigated the multi-step classical hemming process on simple samples with complex geometries and a pre-strain state linked to the forming of curved surfaces [9]. The use of shell elements led to slight differences between pre-hemming and hemming. Moreover, a new hemming process with counteraction force was proposed by Wanintradul et al. [10]. In this process, hemming with an angled stopper could prevent creepage and retain the sharp radius from a two-step flanging process. However, the proposed hemming process was limited to a double-folded flange part. Most of these studies focused on the plastic deformation mechanism, numerical simulation techniques, roller hemming, or hemming with simple geometries like L- and U-sections. However, their results are difficult to apply to actual production because of the lack of research on the die design for multi-step hemming, especially in the case of TFT LCD bezel

Fig. 2. FE model of three-point bending test.

frames, which have a multi-folded edge shape. The aim of this study was to develop the procedure for manufacturing bezel frames for strength-reinforced TFT LCD modules by a progressive hemming process. First, a strength analysis using the software Abaqus was performed to determine the minimum number of required folding edges. Then, another FE simulation was carried out using the software DEFORM-2D in order to investigate the effects of the process parameters on the dimensional accuracy of the bezel frame and to design the progressive hemming process. The analytical results were validated by hemming experiments performed using SUS304 stainless steel with a thickness of 0.3 mm. Finally, the quality of the bezel frame was estimated through measurements of its dimensional accuracy and bending stiffness.

2. Strength analysis for determining the minimum number of required folding edges 2.1 FE analysis model FE simulations of the three-point bending test were performed under the Abaqus/Standard software environment to determine the minimum number of required folding edges. The 3D FE model is shown in Fig. 2. The bending analysis was categorized into six types according to the positions of the rollers, bending direction, and number of folding edges. The stress roller and support rollers were assumed to be rigid bodies, and the friction between the material and the rollers was neglected. The descent rate of the stress roller was 12 mm/min, and the limited displacement was 1 mm. The relevant parameters of the model are shown in Table 1. The material used in the model was SUS304 stainless steel, and its thickness, density, elastic modulus, and Poisson’s ratio are 0.3 mm, 8000 kgm-3, 193 GPa and 0.29, respectively.


2.2 Results of FE analysis The deformations resulting from the different number of folding edges were compared, as shown in Fig. 3. The blue areas indicate the elastic zone, and the areas with other colours represent the plastic zone with different plastic strains. The maximum equivalent strain was produced at the contact surface between the sheet and the stress roller. With the increase in the number of folding edges, the values slightly increased in both cases A and B. The required bending stiffness for bezel frames for strengthreinforced TFT LCD modules is about 50 N/mm. Fig. 4 shows a graph of bending stiffness at different numbers of folding edges. In this study, the bending stiffness was defined as the maximum bending load divided by the stroke. The specimen with five folding edges was found to have the highest bending stiffness values of 50 N/mm and 68 N/mm in cases A and B, respectively. However, it should be noted that the three-point bending test compared the load of the stress roller per stroke for the bending of a non-work-hardened bezel frame. The maximum bending stiffness of real products may be higher because of strain hardening. Table 1. Material properties of SUS304 stainless steel sheet. Direction (from the RD)

0º

45º

90º

Y. S. (MPa)

202.4

206.7

207.7

T. S. (MPa)

83935

877.6

854.7

K (MPa)

1207.7

1245.2

1224.3

n

0.424

0.488

0.449

r-value

0.503

0.701

0.724

3. Process design of progressive hemming for manufacturing bezel frame This study identified the necessity to develop an integrated forming technology, i.e. multi-hemming using a progressive process by which the strength and production rate can be increased. Fig. 5 shows the FE analysis procedure for a sequence of progressive hemming processes, namely first bending, second bending, third bending, and hemming. Sectional dies, which have a different parting line between the upper and lower die sets, were used to prevent crack initiation in the bending part in the second bending process [11], as illustrated in Fig. 5(c). 3.1 Design of corner radius of die and bending angle in multi-stage bending process The commercial FE code DEFORM was used to predict the deformation behaviour of SUS304 stainless steel sheet. The FE analysis was performed to evaluate the effects of the process parameters on the dimensional accuracy of the hemming parts. As illustrated in Fig. 5(a), the bending angle (θ) and the corner radii of the die (R) were considered as the process parameters in this study. The material used in the analysis was SUS304 stainless steel, and its stress–strain relationship is described in Table 1. The frictional factor at the interface between the tool and the material was assumed to be 0.1, in accordance with other literatures [12, 13]. The thickness of the initial blank and the final height of the hemming part were 0.3 mm and 1.5 mm, respectively. The Blank holding force (BHF) and stroke of the cam punch were 300 N/mm and 3.2 mm, respectively [11]. The hemming part had five folding edges in order to satisfy the required bending stiffness of 50 N/mm.

No. of ①②③ folding

(a) Number of folding edges: 3EA

No. of ① ② ③④ folding

(b) Number of folding edges: 4EA

No. of ① ② ③④ ⑤ folding

(c) Number of folding edges: 5EA Fig. 3. Effective strain distribution on deformed bezel frame [11].

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K.-H. Lee et al. / Journal of Mechanical Science and Technology 30 (3) (2016) 1103~1109 70

1st bending

Case A Case B

Bending stiffness (N/mm)

65 60 55

Mises stress (MPa) 964 846 728 610 492 375 257 139 21.0

2nd bending

Mises stress (MPa) 1340 1170 1010 845 681 517 353 189 25.6

hemming

3rd bending

Mises stress (MPa) 1520 1330 1140 954 764 575 386 196 6.75

50 45

Mises stress (MPa) 1540 1350 1160 970 h 1.5 781 591 402 213 23.3 *after sprinback

(a) θ = 30º, R = 0.3 mm

40 1st bending

35 3.0

3.5

4.0

4.5

5.0

Number of forlding-edges

Fig. 4. Bending stiffness at different numbers of folding edges.

Mises stress (MPa) 925 813 702 590 478 366 254 142 30.7

2nd bending

Mises stress (MPa) 1290 1140 976 817 657 498 339 179 19.8

hemming

3rd bending

Mises stress (MPa) 1670 1460 1260 1050 842 635 428 221 14.4


(b) θ = 45º, R = 0.3 mm st

1 bending

(a) Process variables

Mises stress (MPa) 934 820 707 593 479 366 252 138 24.7

2nd bending

Mises stress (MPa) 1130 985 845 705 565 425 285 145 5.41

hemming

3rd bending

Mises stress (MPa) 1380 1210 1050 883 717 551 385 219 53.2


(c) θ = 60º, R = 0.3 mm (b) First bending process

(c) Second bending process

(d) Third bending process

(e) Hemming process Fig. 5. Procedure of FE analysis.

Fig. 6. Deformed shapes and effective stress distribution of TFT LCD module bezel frame.

Moreover, the required tolerance at each bending point after the hemming process was ±0.05 mm. The deformed shape and distribution of effective stress at various θ in the case of R = 0.3 mm are shown in Fig. 6. The maximum bending stress was predicted on the inside corners of the folding edges. The maximum bending stresses resulted from the restriction of the material inflow between the folding edges and the locally severe deformation at the inside corners of the hemmed part. There are some dimensional errors at each bending point not only during the bending process but also after the hemming process. The deformation in the outer layer is remarkably greater, which is the neutral plane move to the inner side of the bent sheet. As the bending angle decreases, the tensile strain at the outer layer increases, and the width of thickness of the bent material is eventually smaller. As shown in Table 2 and Fig. 7, the deformed shape was undesirable in all the case studies. The negative and positive dimensional errors were observed at the bending points B and D, respectively, and the minimum dimensional errors (-0.07, +0.08 mm) were predicted in the case of R = 0.3 mm and θ = 45º.

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Table 2. Dimensional error at each bending point after hemming process. No.

θ (°)

Table 3. Dimensional error at each bending point after hemming process (θ = 45º, R = 0.3).

Dimensional error (mm)

R (mm) A

B

C

D

θ (°)

No.

R (mm)

Dimensional error (mm)

δ (mm)

A

B

C

D

1

30

0.2

-0.03

-0.09

-0.04

0.06

2

30

0.3

-0.01

-0.10

0.03

0.12

1

0.36

0.07

0.04

-0.08

0.02

0.18

2

0.38

0.07

0.03

-0.08

0.02

0.40

0.05

0.04

-0.03

0.04

3

30

0.4

0.02

-0.10

0.08

45

0.3

4

45

0.2

-0.02

-0.03

0.03

0.13

3

5

45

0.3

-0.05

-0.07

-0.02

0.08

4

0.42

0.06

0.01

-0.07

0.03

-0.01

0.16

5

0.44

0.06

0.02

-0.05

0.05

45

0.4

-0.10

-0.10

7

60

0.2

0.01

-0.04

-0.10

-0.03

8

60

0.3

-0.06

-0.05

-0.13

0.01

9

60

0.4

-0.16

-0.05

-0.17

0.06

0.17

Absolute dimensional error (mm)

6

C

0.15 0.13

C.L

A

After hemming

0.11

d = 0.36 d = 0.38 d = 0.40 d = 0.42 d = 0.44

1.5 mm

D

B

0.09 0.07 0.05 0.03 0.01 A

B

C

D

Measuring position

Fig. 9. Comparison of absolute dimensional errors at different δ (θ = 45º, R = 0.3).

Fig. 7. Comparison of absolute max. dimensional error at different R and θ. (a) After third bending process

Punch stroke: 1.8 mm

Fig. 8. Redesigned dies in second and third bending processes.

Mises stress (MPa) 1560 1370 1170 983 791 600 408 217 25.3

3.2 Design of stepped height in multi-stage bending process (b) During hemming process

The die design in the bending process needed to be adjusted to compensate for the negative dimensional error. Fig. 8 shows a schematic illustration of the redesigned dies for the second and third bending processes. The important parameter is the stepped height (δ) between the bending points O and B. An FE analysis was performed to investigate the effect of δ on the dimensional accuracy of the hemming parts. Table 3 and Fig. 9 show the dimensional errors at each bending point after the hemming process for various δ in the case of θ = 45º and R = 0.3 mm. The height difference of 0.4 mm led to the successful hemming shape and the required dimensional precision of ±0.5 mm. The deformed shape and distribution of effective stress in the case of R = 0.3 mm, θ =

(c) After hemming process Fig. 10. Deformed shape and effective stress distribution of TFT LCD module bezel frame (θ = 45º, R = 0.3 mm, δ = 0.4 mm).

45º, and δ = 0.4 mm is shown in Fig. 10. The stepped height prevented an excessively downward deflection at the bending point B during the hemming process, thereby decreasing the

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Table 4. Comparison of dimensional errors in simulated and experimental results (θ = 45º, R = 0.3, δ = 0.4 mm). *unit: mm Measuring point

A

B

C

D

Experimental result

0.01

0.01

-0.03

0.03

Simulated result

0.05

0.04

-0.03

0.04 Fig. 13. Experimental setup for three-point bending test.

Table 5. Results of three-point bending tests. No. of specimens

1

2

3

Bending stiffness (N/mm)

70.12

71.01

71.50

three-point bending test was conducted on the bezel frame specimens, using the test equipment shown in Fig. 13. From these tests, it was found that the bezel frame with five folding edges manufactured by the multi-step hemming process had a sufficient bending stiffness of about 70 N/mm.

5. Conclusions

(a) Progressive bending dies

(b) Hemming die

Fig. 11. Photographs of multi-step hemming dies. A

C Slice-secton

0.3 mm

D

B

Fig. 12. Comparison of cross-sectional shapes in simulated and experimental results (θ = 45º, R = 0.3 mm, δ = 0.4 mm).

dimensional errors.

4. Experimental results The design procedure for the manufacturing process of the bezel frame was verified by experiments at Young E&T Co., Ltd. in Korea. The progressive dies used to manufacture the bezel frame are shown in Fig. 11. The multi-hemming sequence consisted of three bending operations and one hemming operation. The punch and blank holder were made from one piece to prevent wrinkling around the flange. The comparison between the cross-sectional shapes of the products obtained from the FE analysis and experiment in the case of θ = 45º, R = 0.3 mm, and δ = 0.4 mm is shown in Fig. 12. It can be seen that the experimental result is in good agreement with the FE analysis result. Table 4 represents the comparison of dimensional errors in the simulated and experimental results. All the values at each bending point were found to satisfy the dimensional accuracy. Therefore, on the basis of the above results, it was shown that the procedure of die design proposed in this study could be effectively applied to multi-hemming forming by a progressive process. In order to validate the strength of the final product, the

The design procedure for a progressive multi-hemming forming process has been suggested in this study for manufacturing bezel frames for strength-reinforced TFT LCD modules. Based on the results of the FE analysis and experiment using the design procedure, the following conclusions were drawn: (1) In the multi-stage bending process, it is important to determine the corner radius of the die and the bending angle to improve dimensional accuracy. It is also necessary to prevent an excessively downward deflection at bend point B by applying a stepped height. (2) The experiment for manufacturing a bezel frame was performed using the progressive process. The height of the hemmed part measured from the experiment was found to be in good agreement with that in the FE analysis. (3) From the experimental results, it was confirmed that a bezel frame with a height distribution of 1.5±0.05 mm and a respectable bending stiffness of 70 N/mm could be manufactured from an SUS304 stainless steel sheet of initial thickness 0.3 mm by using the proposed multi-hemming forming process.

Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2012R1A5A1048294) and PNU-IFAM Joint Research Center.

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[11] G. H. Kim, S. H. Lee and B. M. Kim, The die design of STS304 Bezel Frame for the strength reinforcement in hemming process, Transactions of Materials Processing, 17 (2008) 436-442. [12] Z. Wei, Z. L. Zhang and X. H. Dong, Deep drawing of rectangle parts using variable blank holder force, International Journal of Advanced Manufacturing Technology, 29 (2006) 885-889. [13] D. C. Ko, H. S. Choi, W. S. Jang and B. M. Kim, Progressive process design of door lock striker with double bosses by the wall-thickening process, International Journal of Advanced Manufacturing Technology, 66 (2013) 1191-1199.

Kyung-Hun Lee received his bachelor’s and doctor’s degree at Pusan National University, Korea, in 2007 and 2013, respectively. Dr. Lee is concurrently an assistant professor of Division of Marine Engineering at Korea Maritime and Ocean University in Busan, Korea. Byung-Min Kim received his bachelor’s, master’s and doctor’s degree at Pusan National University, Korea, in 1979, 1984 and 1987, respectively. Dr. Kim is currently a professor of School of Mechanical Engineering at Pusan National University in Busan, Korea.