汽車活塞托架三維模型及注塑模具設(shè)計【一模一腔】【說明書+CAD+UG】,一模一腔,說明書+CAD+UG,汽車活塞托架三維模型及注塑模具設(shè)計【一模一腔】【說明書+CAD+UG】,汽車,活塞,托架,三維,模型,注塑,模具設(shè)計,說明書,仿單,cad,ug International Journal of Machine Tools Stereolithography; Rapid tooling; Injection moulding techniques are improving and are becoming increasingly process 10. It has shown that SL injection mould tooling (Fig. 1). The back-filled mixture added strength to the inserts and allowed heat to be conducted away from the mould. The modular steel mould bases were two standard ARTICLE IN PRESS C3 Corresponding author. base plates machined with a cylindrical pocket to fit the steel frames and the inserts 12. The SL tools were then tested in a 50ton Battenfeld production moulding machine 0890-6955/$-see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2006.09.022 E-mail address: (S. Rahmati). 1 Professor (b) Flexural failure; (c) Shear failure. Tools & Manufacture 47 (2007) 740747 application of computational fluid dynamics (CFD) and finite element method (FEM), which will combine the fluid and stress analysis to model the SL tool. 5.2. Crack propagation and fatigue Flexural stresses can also induce a fatigue type process, spanning a number of moulding cycles. In this situation, the cube pivots as in Fig. 8(b) without being fractured but a crack is initiated at the intersection between the face of the cube in tension due to flexural stresses, and the core face perpendicular to it. During subsequent cycles, the crack propagates through the base of the cube eventually resulting in failure. Failure analysis of the SEM images has revealed that the crack propagates through the cubes prior to the ultimate failure. Micro- scopic pictures of mouldings numbered sequentially indicate that the crack has started well before the ultimate flexural failure. Fig. 10 is a picture taken of the cross section of a moulding before the actual failure happened, where subsequent injection mouldings have exhibited a positive flaw corresponding to the inverse of the crack generated. Fig. 11 shows the flexural failure of a similar cube to that seen in Fig. 10, after a number of shots. Crack initiation in SL tools occurs predominately at stress concentrations, such as sharp corners or at stair steppings (an inherent property of SL parts). Crack Fig. 10. Moulding showing the attached plastic of crack before failure. ARTICLE IN PRESS MachineS. Rahmati, P. Dickens / International Journal of formation may also result from flaws or microscopic defects created during photo-polymerisation process due to material discontinuities 15. Sharp corners, stair stepping, voids or flaws are a cause or source of crack initiation. Fatigue failure can be minimised by introducing fillets at the sharp corners in order to reduce the stress concentration and crack propagation. Evidence of the crack failure as shown in Fig. 12, can be seen on the fracture surface in the form of striations, where each one of these marks represents crack growth. At the tip of the crack and in a small region near the tip, the yield strength of the material is exceeded. In this region, plastic deformation occurs and the stresses are limited by yielding 17. After each cycle, the crack grows in the same manner until a critical crack length is reached. At this point, the crack tip can increase in velocity and spread all the way across the cube resulting in failure. Fig. 11. Flexural failure as a result of crack propagation. Fig. 12. SEM observation revealing striation marking on the fractured surface. 5.3. Shear failure During shear failure, the feature is sheared off in the direction of the melt flow. Fig. 13, shows the cross section of a sheared SL cube. Notice that the SL cube has been pushed across by the flow of plastic. The shear stress at a point in a section is given by 18: t VQ Ia , (2) where V is the shear force at the given section, Q is the first moment of the area about the neutral axis, I is the moment of inertia of the cube section with respect to the neutral axis, and a is the width of the cross-section. As the shear stress calculation results show in Table 2, the maximum shear stresses produced in the SL tool during operation are below the shear strength of the SL tool. Moreover, the SL Fig. 13. SL cube being sheared off during injection moulding process. Tools & Manufacture 47 (2007) 740747 745 tool can survive at injection temperatures beyond 401Cas shown in the last column of the Table 2. Fig. 14, shows the maximum shear stresses at various points of the cube base versus the average shear stress. The plot of the maximum shear stresses at various points results in a parabolic curve. 6. Conclusions SL tools have been successfully tested where failures were observed after 500 shots. SL tool failure mechanisms have been investigated and different scenarios have been demonstrated. Using a thermoplastic with a melting temperature of 2003001C in epoxy SL tooling which has a Glass transition temperature (T g ) of about 60901C, seems unrealistic or impossible. However, the key point to the success of this technique is the very low thermal conductivity of the SL tool and the short injection time (Fig. 15). These two factors are the key to the success of the SL injection mould tooling, which are overlooked by many. ARTICLE IN PRESS stress Machine Table 2 Shear stresses acting on the SL cubes Shear area A S (mm 2 ) Shear force V (N) Shear Cube 1 36 421.64 11.71 Cube 2 30 421.64 14.05 Cube 3 24 421.64 17.57 Cube 4 18 421.64 23.42 0 S. Rahmati, P. Dickens / International Journal of746 Although epoxy has a very low tensile or shear strength at high temperatures, during the first few seconds of injection in which the maximum pressure is exerted on the tool, the heat has not been able to penetrate. Therefore, the tool strength is still maintained and low conductivity of the epoxy works in favour of the process initially. It can be concluded that the tool must be cooled down in each cycle to as low as 40501C before the next injection is made. Tool cooling can be achieved either through free convection, which takes 45min or through forced convection by means of an air jet which reduces the cycle time to 1, 2min. The results of the work can be summarised as follows: 1/4 N.A. 1/2 0 Fig. 14. Distribution of the shear stresses 0 200 400 600 800 1000 1200 1400 1600 1800 0 1020304050 Pressure (psi) Time (sec) Fig. 15. Plot of temperature and pressure C15 C15 C15 C15 versus average shear stress 24.3 55.9 24.3 46.4 t ave (Mpa) Shear strength at 401C (Mpa) T MAX (1C) 24.3 65.3 24.3 61.5 Tools & Manufacture 47 (2007) 740747 More than 500 parts were produced using the epoxy SL core and cavity using external air jet to cool the tool to 451C. Tool failure during injection is independent of the plastic temperature. Failure during injection may occur either at low tool temperature when tool toughness is not sufficient, or at high tool temperature (above epoxy T g ). As experience and theoretical calculations confirm, flexural stresses during the injection process are the most probable cause of failure. Reducing the features aspect ratio of tool decreases the chances of flexural failure. shear stress at 1/4 fron N.A. shear stress at N.A. 11.71 MPa 13.18 MPa 17.57 MPa 13.18 MPa across the largest cube base. 6070 809010 0 10 20 30 40 50 60 70 80 90 100 110 120 Temperature (Deg C) Pressure Temperature time during injection cycle. C15 Shear stress failure during injection is less likely than flexural failure in particular when the SL tool is warmed to over 401C prior to injection. References 1 D. Chen, F. Cheng, Integration of product and process development using rapid prototyping and work cell simulation technology, Journal of Industrial Technology 16 (1) (2000). 2 J.A. McDonald, C.J. Ryall, D.H. Wimpenny, Rapid Prototyping Casebook, Professional Engineering Publishing, UK, 2001. 3 M.A. Evans, R.I. Campbell, A comparative evaluation of industrial design models produced using rapid prototyping and workshop- based fabrication techniques, Rapid Prototyping Journal 9 (5) (2003). 4 A. 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Dickens, Stereolithography injection mould tool failure analysis, Eighth Annual Solid Freeform Fabrication, Texas, 1997, pp. 295305. 11 S. Rahmati, P.M. Dickens, C. Wykes, Pressure effects in stereo- lithography injection moulding tools, Seventh European Conference on Rapid Prototyping and Manufacturing, Aachen, Germany, 1998, pp. 471480. 12 G. Menges, P. Mohren, How to make injection molds, Hanser, Munich, ISBN:0-02-947570-8, 1986. 13 G.C. Ives, J.A. Mead, M.M. Riley, in: R.P. Brown (Ed.),Handbook of Plastics Test Methods, second ed, London, ISBN:0-7114-5618-6, 1981. 14 R.A. Douglas, Introduction to Solid Mechanics, Sir Isaac Pitman & Sons Ltd., London, 1989. 15 R.W. Hertzberg, J.A. Manson, Fatigue of Engineering Plastics, Academic, New York, 1980. 17 J.W. Dally, F.R. William, Experimental Stress Analysis, 3rd ed, McGraw-Hill, ISBN 0-07-015218-7, 1991. 18 F. Cheng, Statistics and Strength of Materials, 2nd ed, McGraw-Hill, ISBN 0-07-115666-6, 1997. ARTICLE IN PRESS S. Rahmati, P. Dickens / International Journal of Machine Tools & Manufacture 47 (2007) 740747 747