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Int J Adv Manuf Technol (2001) 17:453–461
2001 Springer-Verlag London Limited
Three-Dimensional Kernel Development for Injection Mould Design
T. L. Neo and K. S. Lee
Department of Mechanical and Production Engineering, National University of Singapore, Singapore
Today, many software “plug-ins” have been developed on high-level 3D modelling platforms to facilitate processes such as FEM analysis, CAM, injection mould design, simulation and visualisation. Such an arrangement is advantageous in many ways. However, it is not without shortcomings. Ideally, these “plug-ins” could also be developed using low-level 3D kernels for higher flexibility and better portability. This paper examines the various issues and methodologies related to the development of such 3D-based applications. The emphasis is placed on the software aspect. First, a methodology for the development of 3D-based applications is proposed. The idea is then implemented by developing an injection mould design application using a low-level 3D kernel called Parasolid. Based on design concepts used in an established mould design appli cation, IMOLD, the development of a mould base design module is illustrated. An object-oriented programming langu age has been chosen for the development of the software on a Windows NT platform.
Keywords: 3D kernel; Computer-aided design (CAD); Injec tion mould design; Parasolid
1. Introduction
Three-dimensional CAD systems have increasingly been used to speed up the product realisation process. One of the first steps involved in the automation of the product design process is the creation of the component parts in a 3D modelling application. The 3D model, upon creation, is called the digital master copy. This 3D digital model forms the key to a wide spectrum of process automation.
Creating the 3D digital model of component parts is only the very first step. There are several other secondary tasks that must to be done before the part can be manufactured. Such tasks include finite-element analysis, jigs and fixtures design, injection mould design, computer-aided manufacturing, simul
Correspondence and ofprint requests to: K.-S. Lee, Department of
Mechanical and Production Engineering, National University of Singa pore, 119260 Singapore. E-mail: mpeleeksKnus.edu.sg
ation, and visualisation. Today, many application Plug-ins have been developed on high-level 3D modelling platforms to facili tate these secondary tasks. The 3D-modelling platform provides the plug-in software with a library of functions as well as an established user interface and style of programming. As a result, the development times for these plug-ins are signifi cantly reduced.
Such an arrangement is advantageous in many ways. How ever, it has its shortcomings, especially in the long run. In order to develop a plug-in for established software, the devel opers must adhere to the many constraints imposed. There is a need to be consistent with the style of the parent software. The developers must be able to achieve any functionality they need with only the set of library functions provided. Most end-users need both the parent software and the plug-in. In many cases, however, they may be more interested in using only the plug-in software. An example of such a situation is in injection mould design. These users, however, must purchase the entire software package which includes many features and functions that they do not need. Such a large program is often very demanding on the hardware, which also means higher cost. The plug-in software is also very dependent on developments in the parent software. Whenever a new version is updated for the parent software, the plug-in developers have to follow-up on the changes. These shortcomings may not exist if these applications were developed on a low-level platform. Ideally, these plug-ins could be developed using low-level 3D kernels for higher flexibility and better portability. In many instances, such a move is both feasible and advantageous.
Traditionally, injection mould design is carried out directly on a CAD system. The entire injection mould, consisting of perhaps hundreds of components, is modelled and assembled on CAD systems such as AutoCAD, Pro/Engineer, and Uni graphics. As the injection mould design process is recursive, it is very time-consuming to re-model and re-assemble the design. In this aspect, 3D CAD systems such as Pro/Engineer and Unigraphics, which are feature-based, have a significant advantage over 2D CAD systems such as AutoCAD. To further speed up the injection mould design process, plug-ins were developed on these 3D systems to automate certain stages of the design process. Examples of such add-on applications include IMOLD (Intelligent Mold Design and Assembly Sys-
454 T. L. Neo and K. S. Lee
tem, developed at the National University of Singapore, based on Unigraphics), Expert Mold Designer (based on CADKEY) and Moldmaker (based on EUCLID). As each is based on a specific CAD system, there is no plug compatibility.
In 1994, Mok and Cheung [1] presented work on the devel opment of an injection mould design application based on Unigraphics. In 1997, Shah [2] proposed a 3-tier architecture for standardising communications between geometric modelling kernels and applications that require geometric modelling ser vices. His objective is to achieve plug compatibility between 3D applications that are based on Parasolid [3] (a 3D kernel, developed at the University of Cambridge) and ACIS. This, however, involved an extensively developed 3-tier modelling husk. In this paper, the author attempts to develop a lightweight injection mould design application using a low-level 3D kernel directly. The focus is on the flexibility and speed of the software development. Design concepts and procedures were taken from IMOLD [4,5], a complete mould design and assembly 3D application. Although the discussion is limited to injection mould design only, the methodology applied can easily be applied in other 3D-based applications that are of a similar nature.
A combination of developer tools was chosen for this pur pose. Before the methodology is discussed, brief introductions to some of these tools are first presented. They are, IMOLD, Parasolid version 10.1, Visual Cversion 6.0, and the Microsoft Foundation Classes.
2. IMOLD as a Mould Design Application
IMOLD (Intelligent Mold Design and Assembly) is an estab lished 3D-based application that is dedicated to injection mould design. It is developed on top of an advanced CAD system called Unigraphics. The development is carried out using the applications programming interface (API) provided. The software enables mould designers to create a design rapidly by providing the tools that are commonly needed. Standard components parts, that are often required in the design, have been pre-created in the software and can be readily used by the designer. This reduces the design time significantly. The mould design process is divided into several stages, providing the designer with a consistent method of creating the mould design. They are, namely:
1. Data preparation.
2. Filling system design.
3. Mould base design.
4. Inserts and parting design.
5. Cooling system design.
6. Slider and lifter design.
7. Ejection system design.
8. Standard parts library.
Each stage can be considered as an independent module of the program. The 3D-based requirements for each module vary only slightly. The success in developing the mould base module implies feasibility in developing all the other modules.
3. Parasolid as a 3D Kernel
Parasolid is designed to be the centre or “kernel” of any system that is based on 3D model data. It is essentially a solid modeller, which can be used to:
1. Build and manipulate solid objects.
2. Calculate mass and moments of inertia, and perform clash detection.
3. Output the objects in various ways, including pictorially.
4. Store the objects in some sort of database or archive, and retrieve them later.
Parasolid is one of the most advanced 3D kernels among CAD applications. It is the 3D kernel of Unigraphics and Solid-Works. Its unique tolerant modelling functionality enables it to accept data stored in other modeller formats. Parasolid model files are thus very potable. It is, therefore, a superior platform for the development of stand-alone applications.
The 3D-based application interacts with Parasolid through one of its three interfaces (see Fig. 1). These are called the Parasolid kernel (PK) interface, the kernel interface (KI) and the downward interface. The PK interface and the kernel interface sit “on top” of the modeller (side-by-side), and are the means by which the application models and manipulates the objects, as well as controls the functioning of the modeller. The downward interface lies “beneath” the modeller, and is called by the modeller when it needs to perform data-intensive or system type operations. It consists of three parts: frustrum; graphical output (GO); and foreign geometry. These are briefly explained below.
3.1 The KI and PK Interface
The KI and the PK are interfaces for the programmer to access the modelling capabilities in the Parasolid kernel. They are standard libraries of modelling functions. The programmer calls these modelling functions in their programs. As the KI is to be phased out soon, we chose to use the PK interface.
Fig. 1. Parasolid components.
3D Kernel Development for Injection Mould Design 455
3.2 The Frustrum
The frustrum is a set of functions, which must be written by the applications programmer. The kernel calls them when data must be saved or retrieved. When using Parasolid, the appli cations programmer must first decide how to manage the storage of data, which Parasolid outputs through the frustrum. Transferring data through the frustrum usually involves writing to, or reading from, files. The format and location of the files is determined when writing the frustrum functions.
3.3 The Graphical Output (GO)
The graphical output is another set of functions, which is to be written by the applications programmer. When a call is made to the PK rendering functions, the graphical data generated are output through the GO interface. The graphical data are then passed to a 3D rendering package. OpenGL, a software inter face to graphic cards, is a rendering package that is used for our purpose.
3.4 The Foreign Geometry
The foreign geometry provides functionality for the develop ment of customised geometrical types such as in-house curves and surfaces. These are used together with the standard geo metrical types for modelling within Parasolid.
4. Object-Oriented Programming Using Visual Cand the Microsoft Foundation Classes
Object-oriented programming (OOP) has been the undisputed option for software developers. It is among the most advanced developmental tools available. The Microsoft Visual Studio is such a software package. It features several developmental tools that are meant for Internet-based and Windows-based programming. Among these tools are the Visual C(VC) and the Microsoft Foundation Classes (MFC). The VCis a powerful development tool for object-oriented programming, whereas the MFC is a framework of Cclasses that are dedicated to Windows-based programming. Together, these provided the applications programmer with powerful develop ment features and functionalities such as auto-code generation, and wizard-based operations. These greatly improved pro ductivity. The entire user-interface for our program is developed using the VCand the MFC.
5. System Design
The direct development of a 3D-based add-on application using a 3D kernel requires several issues to be addressed. They consist of 3 main stages at the highest level. First, the identifi cation of the crucial features and functions required for the plug-in application. Secondly, the development of the design
for the application framework. Lastly, the design and develop ment of the individual modules in the framework with appropri ate developmental tools.
5.1 Identification of Essential Modules
Parasolid, as a 3D kernel, provides only a number of libraries and a conceptual framework for 3D application development. It is thus necessary for the developers to identify and develop the other essential facilities that are provided in a 3D CAD system. In order to identify the required facilities, it is important to understand the discrepancies between the two. Table 1 summarises the main differences in the facilities provided by a 3D kernel and a 3D CAD system. Some of these facilities, such as features and parametric modelling, are both time-consuming and technically demanding to develop. As most plug-ins do not use all the facilities of the parent software, it is possible to develop only those required by the plug-ins using low-level 3D kernels, producing a stand alone version.
Items 7 to 9 in Table 1 are prerequisites for the development of 3D-based applications using Parasolid. By studying the requirements of the plug-in application, other essential facilities can be identified. A framework for the application is then proposed, based on the facilities provided by the Parasolid ker nel.
5.2 Framework for 3D-Based Applications
A framework is developed with reference to the facilities provided by the developmental tools and the requirements of the application. It is designed so that there are minimum dependencies between individual code modules. This may result in a small degree of code duplication. In exchange, there is better portability of the program codes, greater ease of mainte nance and a better prospect for future expansion. The overview of this framework is illustrated in Fig. 2. The details of the various modules are discussed in the following sections.
5.2.1 Windows-Based User-Interface (A)
Parasolid does not provide the programmer with a user interface. Thus, the development of the 3D-based application at every single stage will involve designing the user-interface from scratch. The necessary developments involve:
1. Environmental setting and display of the 3D-based appli cation.
2. Interactive graphical interface and execution procedure for all application functionality.
5.2.2 3D Developer Layer (B)
Since different 3D-based applications require 3D-facilities to different extent, the framework must provide for these vari ations. A 3D developer layer (See Fig. 2, Item B) is conceptual ised to handle such variations. It is a library of objects or classes that are developed, based on the Parasolid kernel. The extent of development depends on the requirements of the
456 T. L. Neo and K. S. Lee
Table 1. Summary of facilities provided by a 3D kernel and a CAD system.
Facilities
3D kernel
3D CAD system
1.
Basic 3D modelling
Low-level and general functions provided
High-level and specific functions provided
2.
Assemblies
Several library functions provided
Complete system provided
3.
Feature-based modelling
Not provided
Established feature set provided
4.
Parametric modelling
Not provided
Often provided
5.
Free-form modelling
Low-level functions provided
Often provided
6.
Drafting
Not provided
Complete system provided
7.
Interactive user-interface
Not provided
Always provided
8.
Visualisation of 3D objects
Conceptual framework and several library
Completely developed
functions provided
9.
File management system
Conceptual framework and several library
Completely developed
functions provided
Fig. 2. Overview of 3D-based application.
application identified in the previous section. Besides catering for variations in application requirements, the 3D developer layer also acts as a programming interface for non-Parasolid developers. Such an interface can also be re-used for sub sequent development of other 3D-based applications. The 3D developer layer essentially consists of three main sections. They are used for 3D modelling and assembly, 3D visualisation and 3D data management, respectively.
I. 3D Modeling and Assembly. The 3D modelling and assembly module is the most important and elaborate of all three sections. It is analogous to the application-programming interface (API) provided by most CAD systems. The module consists of a library of 3D-based objects or classes, which are used for the development of the core application modules. The basic 3D functionality that is required by most 3D applications must be developed first. Depending on the requirements of the individual 3D-based application, other more advance features are subsequently added.
II. 3D Visualisation. The display of 3D objects in a Windows client area requires a software graphics interface. The graphical output together with a selected graphical interface, are used for the rendering of 3D objects in the 3D-based appli cation, as well as the management of the viewing projections and transformations. Here, a library of classes is developed for such purposes.
III. 3D Data Management. The 3D data management module is developed on top of the frustrum. The frustrum is the module in the Parasolid kernel that facilitates archiving and access of 3D part files. A library of classes are developed using the frustrum for handling:
1. 3D object file format.
2. File management operations such as opening and saving a 3D object file.
5.2.3 Application Modules (C)
These are the actual 3D-based application modules that sit between the 3D developer layer and the application user interface. The design of these modules depends mainly on the nature of the applications and often differs greatly. The main bulk of the developmental work is carried out in this area. The ease of the development, however, depends on the capabili ties of the 3D developer layer.
5.2.4 Other Software Modules (D)
Very often, the 3D-based application may require functionality from other existing software modules or application modules. Therefore, such a link may exist. An example of such a requirement is illustrated in the implementation section of this paper.
5.3 Development of Individual Modules
Each module to be developed is studied and analysed before a suitable design is produced. The ease of development depends greatly on the design of the framework and the developer tools selected. The next section illustrates the implementation of the
Fig. 3. Overview of the injection mould base design application.
above methodology on a 3D-based injection mould base design and assembly application.
6. Implementations
Applying the system design, a 3D-based injection mould design application is developed. This is achieved using the develop mental tools mentioned in the earlier sections. The mould base module is chosen for illustration, as it requires the widest range of 3D functionality, including the generation of assemblies.
6.1 Framework of Application and the Requirements of Each Module
A framework for the application is designed with reference to the developmental work identified. Figure 3 illustrates the
framework for the Mold Base design application. The details of the requirements in each module are discussed as follows:
6.1.1 Windows NT User-Interface (A)
Mould base design is an iterative process. The Mould designer first selects a standard mould base from the catalogue, and then repeatedly makes modifications to the dimensions of the mould base until all the design requirements are met. It is, therefore, necessary to consider an interactive user-interface for such purpose. Using the Visual Cand the MFC, a Windows-based interface is developed. These include:
1. Creation, display and management of menu bar items, con text menu items and toolbar buttons for easy access to functionality of the application.
2. Creation, display and management of dialogue boxes to guide the user or to obtain user input.
458 T. L. Neo and K. S. Lee
Fig. 6. Cavity