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Integrated CAD/CAE/CAM system for injection moulding
by Yuan Zhongshuang, Li Dequn, Chen Xing, Ye Xiangao,Gao Xianke and Xiao Jingrong
Huazhong University of Science & Technology, China
An integrated CAD/CAE/CAM system, HSC-1.1, is described in this article. At the CAD/CAE stage the drawings of injection moulded parts can be transformed into the drawings of the mould parts interactively and, according to the user's needs, the mechanical check, runner balance analysis, flow simulation and cooling simulation can be carried out. NC tapes for wire cutting or milling machine tools can be generated at the CAM stage. The practice shows that the system is a useful tool for mould designers and manufacturers.
Introduction
Injection moulding is one of the most important polymer processing operations in industry today. It is superior for mass production of complex parts to high precision at low cost. For a long time, experience, intuition and trial and error have been key factors in mould designing, mould manufacturing and moulding operation. These approaches have become increasingly inefficient and costly, especially when applied to the moulding of large parts and parts of high precision or to the processing of new kinds of polymers. Now some of these problems are being solved successfully by combining recent advances in CAD, CAE and CAM technology.
In recent years more and more CAD/CAE/CAM systems for injection moulding have been developed and delivered in Western industrialized countries, such as C-MOULD 3.1 of AC Technology Inc. in the USA, CAD-MOULD of IKV in Germany, McKAM-ll of McCill University in Canada, and MoldFlow in Australia. With the help of these software packages the productivity and quality of injection moulded parts can be improved and the start-up time can be shortened.
CAD/CAE/CAM technology for injection moulding has been developed quickly in China since
1980. As a pioneer in this field in our country, we have studied and developed CAD/CAE/CAM technology of injection moulding for many years. Through five years' development and practical verification, an integrated CAD/CAE/CAM system for injection moulding, HSC-1.1, has been developed and put into use successfully in many factories.
System description
HSC-1.1 is developed on personal computers such as the PC386 and PC486. The desirable internal storage is 4 MB or more, and the external storage is more than 100 MB.
Fig. 1 shows the software requirements of HSC-1.1. The system is developed and run under the
environment of Operating System II (OS/2) or MS-DOS. In the system AutoCAD-10.0 is used only as a graphic editor and drawing software. Standard Fortran 77 and AutoLisp are used for programming. Except for a few of programs for graphic driver, all the software in the system is independent of computers, which ensures good portability of the system..
As shown in Fig. 2, HSC-1.1 integrates nine modules with their function design based on the requirements of users. All modules in the system are supervised by a main control program named control panel' or 'control menu'. Users can invoke any module by ordering the menu displayed on the screen by the control panel. The data exchange from one module to another is implemented automatically in the form of data files. Fig. 3 shows the data flowchart of HSC-1.1.
Function of CAD modules
The task of CAD modules is to transform the drawings of injection moulded parts into the drawings of mould parts efficiently and provide necessary data for simulation and NC modules. Due to complex cavities, a surface modelling program has been eveloped for graphic input. Planes, regular surfaces and bi-cubic surfaces can be created easily. The co-ordinates of points on surfaces
can be calculated by the programusing dimensions of the part drawing and the co-ordinates of the points input before. While the part drawing is being input one surface after another, the dimensions of the part will be transformed into the dimensions of the cavity and core through interaction. The data for the cavity and core are recorded for both mould design and simulation.
A database for standard mould sets has been set up which contains ten kinds of standard mould sets issued by the Electrical Ministry of the People's Republic of China. Each kind of mould set contains 13 series. Hence there are 31150 combinations of mould sets altogether in the database. Once the cavity layout is determined, all the standard mould parts can be selected automatically and dimensioned through interaction.
The system provides a group of functions for users to design the runner system, edit construction of cavity and core and arrange ejection pins and cooling lines. Finally all the mould part drawings including moving mould assembly, stationary mould assembly and general mould assembly drawing can be produced. Fig. 4 shows a mould assembly drawing of a switch socket made in the Shanghai No. 9 Radio Factory for colour TV sets.
Function of CAE modules
CAE modules include the interface between CAD and CAE modules, mechanical check for mould plates, runner balance analysis, flow simulation and cooling simulation. With the help of these CAE modules the mould construction design can be improved and possible defects in injection moulded parts such as degradation, short shots, and improper location of weld lines can be addressed before mould making.
The interface between CAD and CAE modules reads the geometric model of cavity and delivery system which is produced at the previous CAD stage and generates FEM mesh automatically. Using the interface, users can also select polymer, coolant, mould material and set process conditions such as injection temperature, injection time, coolant temperature etc. The interface reads the property data of the materials from the data bank and writes the mesh configuration, material properties and process conditions into data files which are the inputs for the CAE modules described below.
Currently the system uses a 2D finite-element method (FEM) to analyse the mould plate's strength and rigidity for a typical mould cross-section. An analysis program based on a 3D FEM is under development.
In order to guarantee the same qualities of the injection moulded parts produced in multi-cavities, each cavity must be filled simultaneously at the same pressure and temperature. This requires the runner system to be balanced. In HSC-1.1 the balance can be reached by correcting the dimensions of runners and gates designed by users at the preliminary design stage which are most probably not balanced.
The flow simulation program is one of the most basic and useful analyses in the system. The
governing equations for flow in cavity can be obtained by extending the classical Hele-Shaw flow to an inelastic, non-Newtonian fluid under non-isothermal conditions:
i-(rti) + J- (bv) =0
dX df/
+u+ v
dt dx By
Bz2
where P, Fare the pressure and temperature of melt, respectively; r|, y represent viscosity and shear rate; '—' denotes an average over z, the gap wise co-ordinate; p, Cp and K are density, specific heat and heat conductivity of the melt, respectively; and b is the half gap thickness. Because the thickness dimension of an injection moulded part is often much smaller than the other two dimensions, a powerful numerical scheme, which is the hybrid of the finite element and finite difference methods (FEM/FDM), is adopted in the solution. In the implementation of the scheme, the planar co-ordinates are described in terms of finite elements and the gapwise and time derivatives are expressed in terms of finite differences. A control volume approach is adopted to derive the finite-element formulation and track the melt front movement. By use of flow simulation, users can acquire information such as pressure, velocity and temperature distributions, total pressure drop, clamp force etc., which is very helpful in designing the deliver helpful in designing the delivery system and optimising the process conditions. Users can also animate the filling of the cavity and obtain an optimum flow pattern by changing the number and locations of gates.
Cooling simulation includes 3D steady and transient cooling analyses. The 3D steady cooling analysis uses the boundary element method (BEM). Formulas for modelling the cavity surfaces, cooling lines and exterior surfaces have been established and proven to be reliable and effective. Based on the steady cooling simulation, 3D transient cooling simulation has been developed. A novel BEM has been adopted in this module to eliminate numerical body integration. With the help of this module users can calculate heat transfer between cavity and cooling channels, reduce cooling time and predict temperature along mould and injection moulded part surfaces.
All results of CAE modules can be displayed dynamically with contour plots, shaded colour images and various curve plots to aid users in improving their design.
Function of CAM module
The cutter location files can be created based on the geometry of cavity and core which is modelled at the previous CAD stage. NC tapes can then be generated by use of postprocessors for both NC wire cutting machine tools and NC milling machine tools. Currently only the function of NC wire cutting is used in practice. A lot of injection moulds have been designed and manufactured by using the system HSC-1.1 in factories in our country.
Conclusions
HSC-1.1 is an integrated CAD/CAE/CAM system for injection moulding. All programs in the system are independent of computers except a few programs for the graphic driver. It ensures the system's good portability. The modular structure of the system guarantees that each module of the system has good expandability and maintainability. The practice shows that HSC-1.1 is a powerful tool for mould designing and manufacturing. It can assist engineers in cutting mould cost and improving mould quality. HSC-1.1 will find more and more applications in the mould industry.
References
1 WANG, K. K. : Polymer Plastics Technology Engineering, 1980, 1, p.75
2 MENGES, G.: Plastics Engineering, 1983, 8, p.37
3 KAMAL, M. R. etal.: Application of computer aided engineering in injection moulding' (Hanser Publisher, 1987, p.247)
4 AUSTIN, C: 'Application of computer aided engineering in injection moulding (Hanser Publisher, 1987, p. 137)
5 WANG, V. W., Ph.D. thesis, Cornell University, 1985 ?IEE: 1993 The authors are with Huazhong University of Science & Technology, Wuhan 430074, People's Republic of China