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INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 13, No. 7, pp. 1239-1242 JULY 2012 / 1239 DOI: 10.1007/s12541-012-0164-6 1. Introduction A spring guide pin for a press die set is usually manufactured via machining process such as cutting and turning in order to achieve dimensional accuracy and sufficient strength.1 However machining process is highly energy consuming because it uses up much material and time. Also it requires extra steps such as heat treatment in order to attain necessary strength. Fig. 1(b) shows the spring guide pin produced via machining process, and there is about 60% material loss with machining process (Fig. 1(a). The paradigm of manufacturing technology is shifting towards sustainable production (Fig. 2) in this age of energy crisis. Sustainable production is achievable by minimizing environmental effects during the whole manufacturing cycle of a product. Such production can be designed by means of minimal manufacturing that uses minimal material and energy while producing least amount of waste. There are number of research on going concerning environmental effects due to manufacturing methods.2-4 Fig. 1 Spring guide pin made by machining process: (a) before and (b) after machining process Fig. 2 Shift of paradigm in manufacturing technology Green Alternative Processing Technology for a Spring Guide Pin of Stamping Die Set Myeong-Sik Jeong1, Sang-Kon Lee1, Ji Hyun Sung1, Kang-Eun Kim1, Shinok Lee2, Kang-Won Lee2, and Tae-Hoon Choi1,# 1 Green Transformation Technology Center, Korea Institute of Industrial Technology, Daegu, Republic of Korea, 704-230 2 Daegu-Gyeongbuk Regional Division, Korea Institute of Industrial Technology, Daegu, Republic of Korea, 704-230 # Corresponding Author / E-mail: thchoikitech.re.kr, TEL: +82-53-580-0131, FAX: +82-53-580-0130 KEYWORDS: Green manufacturing, Sustainable production, Multi-stage forging, Spring guide pin, FE-analysis In this research, a multi-stage cold forging process for a spring guide pin of stamping die set is designed through alternative process for sustainable production. Highly energy and material consuming process needs to be converted to an energy saving process. Up to now, a spring guide pin for stamping die set is usually manufactured by machining process in order to achieve dimensional accuracy. However, machining process has some disadvantages such as excessive material loss, low productivity, and poor mechanical properties. In order to overcome these problems, a multi-stage cold forging process is applied to manufacturing spring guide pin for achieving less material loss and higher productivity. Multi-stage forging process requires careful design steps to prevent the defects of the final product. So, the design must consider the material flow during the whole process and the loading limit of the press in order to avoid product defects such as folding and underfilling. To develop multi-stage forging process for spring guide pin the forming load and material flow at each step is analyzed using a commercial finite element code, DEFORM. The guide pin is then manufactured by the developed multi- stage forging process, and is compared to the simulation results. Also the efficiency of the developed forging process is compared to the machining process. The developed process is a green manufacturing process minimizing material loss, which achieves cost reduction through improved material usage and productivity compared to the more widely used machining process. Manuscript received: December 30, 2011 / Accepted: January 25, 2012 KSPE and Springer 2012 1240 / JULY 2012 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 13, No. 7 Therefore such energy consuming process needs to be converted to an energy saving process. For this purpose forging process is applied to manufacturing the spring guide pin for achieving less material loss and higher productivity. However a wide spread of such process is limited by two barriers: the geometrical inaccuracy and the insufficient strength caused by defects such as underfills and folds. These barriers can be overcome if the material flow is fully understood for the given geometry. In this study, multi-stage forging process for spring guide pin of a press die set is designed considering the material flow using the FE-analysis. To design multi-stage forging process for spring guide pin the forming load and the material flow at each step is analysed using commercial finite element code DEFORM. Then, the result is compared to the original process for assessing the efficiency of the developed process. 2. Design of cold forging process 2.1 Development of the forging stage During cold upsetting of an axisymmetric object, the maximum strain is dependent on the number of forming stages. To avoid defects such as buckling during upsetting process with large deformation, a number of forming stage needs to be determined based on the geometry of plastic deformation. The initial diameter (D0) and length (Lt) of the billet for spring guide pin is 24.5 mm and 219 mm respectively. Also the upsetting ratio s=L0/D0 is 2.95 for plastic deformation length (L0) of 63.5 (Fig. 3). This case requires a double-stroke process.5 But to increase formability the multi-stage cold forging process is designed as a three stage process with the initial stage of head diameter reduction (Fig. 4). Fig. 3 Initial and target dimension for spring guide pin Fig. 4 Three stage forging process for spring guide pin 2.2 Condition for FE analysis The material of the spring guide pin is AISI1020, and the relationship between effective stress and strain for FE analysis is fitted from Capan and Brans results.6 The flow stress can be expressed with a power law formula as shown in equation (1): 0.297 382.434242.747=+ (1) FE simulation of the multi forging process is conducted using an implicit finite element code DEFORM. This process is modelled in 2D because it is an axisymmetric deformation mode. The workpiece is assumed to be perfect plastic material, and the punch and die are assumed to be rigid. A shear friction factor m=0.12 between workpiece and punch is used for the simulation. The punch speed is 100 mm/s and number of elements of the workpiece is 3000. Table 1 summarizes the simulation conditions for FE analysis. Fig. 5 shows the shape of punch at each stage of the three stage process. The chamber angle and radius R are shown in Fig. 5(d). The preform shape of the intermediate stage is determined by the radius R2 of the second punch, and the radius R2 is determined from the simulation results considering the loading limit and the defects during the third stage. 2.3 Results of FE simulation Fig. 6 is FE simulation results of the first and second stage forging process using the punch designed in Fig. 5. The head diameter of the billet is reduced to 22.47 mm from 24.5 mm after the first stage, and it is confirmed that no defects such as barrelling and bursting occurred. The diameter of the broker head reduced to 20.16 mm after the second stage. The predicted load from the FE analysis stayed below the 250 ton loading limit of the press as shown in Fig. 6(d). However this preform with R2=5 causes folding defect during the third forging stage as shown in Fig. 8. This is due to the slower material flow of the flange in the radial direction than in the axial direction. Table 1 Simulation condition of FE analysis Simulation mode Isothermal Geometry Axisymmetric Velocity of punch 100 mm/s Number of mesh 3000 Friction value 0.12 (a) First punch (b) Second punch (c) Third punch R1 1.5 1 72 R2 5 2 71 R3 1 (d) Dimension of punch Fig. 5 Shape of each punch for the multi-stage forging process INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 13, No. 7 JULY 2012 / 1241 (a) Billet (b) First stage broker (c) Second stage broker (d) Predicted load of first and second stage Fig. 6 FE result of the multi-stage forging process (a) Initial (b) Stroke: 72.5 (c) Stroke: 81.8 (d) Folding defect Fig. 7 FE analysis results: Second punch R2=5 Fig. 8 Preform for 3rd stage forging To avoid folding defect the preform geometry for the final stage needs to be modified, and this is achieved by re-designing the punch in the second stage. For optimal design of the preform, the preform shape with radius from 5R to 15R is designed and observed for deformation behaviour as shown in Fig. 8. It can be seen that folding defect occurs when radius R of the second punch is below 12R. When radius is greater than 14R it hits the loading limit of the press. Fig. 9 summarizes this result, and the suitable radius for the second punch of the multi-stage cold forging is determined to be 12R to 13R. 3. Multi-Stage cold forging experiment 3.1 Process condition for Experiment To evaluate the validity of the punch and die shape, a multi- stage forging experiment is performed using the punches in Fig. 10. The radius of the second punch is set to 12R and the material shape at each stage is shown in Fig. 11. No defects such as barrelling and bursting occurred during the multi-stage forging process, and no cracks and fractures are found in the final product. But there is small amount of flash forming and FE analysis shows similar results (Fig. 12). Such flash may affect lifetime of the spring guide Fig. 9 The load at 3rd punch as a function of R2 Fig. 10 Punch of multi-stage forging process for spring guide pin Fig. 11 The spring guide pin at each stage 1242 / JULY 2012 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 13, No. 7 pin, and more work needs to be done to minimize the flash. This can be realized through modification of the die geometry. 3.2 Comparison of the developed process to the original process Table 2 shows the improvement of the developed process compared to the original process. When the developed multi-stage cold forging process is applied to the spring guide pin for a press die set the initial billet diameter is reduced by 50% and the material usage increases to 100%. Also the processing capacity per minute is two hundred times the original process. These improvements bring down the unit cost by 50%. In other words, the developed process is an energy saving process because it minimizes material loss and reduces energy used for producing the original material. Figure 14 shows the amount of energy consumed between machining and cold forging for producing a single spring guide pin. The calculation was done using SolidWorks Sustainability tool with computation method that complies with the ISO 1444. When the cold forging process was applied, the consumption of energy can be reduced about 10%. This means that the developed process is more green process than the machining process to manufacture the spring guide pin. 4. Summary In this study, a multi-stage cold forging process for the spring guide pin of a press die set is developed considering energy saving during the manufacturing cycle. The process is designed using FE analysis, and preform shape is optimized considering the material flow and the loading limit of the press. The suitable radius of the second punch is 12R to 13R, and the validity of the developed process is verified through experimentation. The developed process is a green manufacturing process not only minimizing material loss but also saving energy. Also this process achieves cost reduction through high material usage and high productivity compared to the more widely used machining process. ACKNOWLEDGMENTS This research is supported by “The Technical Support for Manufacturing Technology Innovation of Small and Medium Sized Manufacturing Companies in Daegu & Gyung-Buk Province”. REFERENCES 1. Boothroyd, G. and Knight, W. A., “Fundamentals of machining and machine tools,” CRC, 2006. 2. Park, C. W., Kwon, K. S., Kim, W. B., Min, B. K., Park, S. J., Sung, I. H., Yoon, Y. S., Lee, K. S., Lee, J. H., an