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DOI 10.1007/s00170-004-2388-9 ORIGINAL ARTICLE Int J Adv Manuf Technol (2006) 28: 495503 Y.-M. Deng G.A. Britton Y.C. Lam Towards automatic shapemodifi cation in injection-moulded-plastic-part design Received: 6 May 2004 / Accepted: 23 August 2004 / Published online: 18 May 2005 Springer-Verlag London Limited 2005 AbstractInjection-moulded-plastic-part design must ensure that the part can be manufactured to the desired quality level by the injection moulding process. Simulation software has been widely used in industry to assess mouldability and measures of quality. However, it cannot be used to improve a design directly. Design modifi cations must be performed by the designer after evaluation of the simulation results. Based on the authors pre- vious work on injection moulding CAD-CAE integration, this paper explores the strategies and methods for automatic-part- shape-modifi cation to attain a desired part quality. An enhanced CAD-CAE integration model is developed. This model is used to specify the shape-modifi cation variables, as well as the mould- ability and other quality measuring criteria. The shape modifi ca- tion variables include positional and sizing parameters of each individual feature, as well as those associated with the part, such as part thickness. With this information, an iterative process of part-shape modifi cation and execution of simulation subroutines is carried out automatically, and the results are verifi ed and eval- uated. Optimal shape, according to the specifi ed criteria can thus be derived from the evaluation results. A software prototype has been developed. A design case study is presented to illustrate and demonstrate the usefulness of the proposed strategies and methods. KeywordsInjection moulding Plastic part design Shape modifi cation Y.-M. Deng (u) Y.C. Lam Singapore-MIT Alliance (SMA), N2-B2C-15, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore E-mail: .sg Tel: +65-67904273 Fax: +65-68627215 G.A. Britton CAD/CAM Lab, School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore 1 Introduction Injection moulding is a major manufacturing method for the fab- rication of plastic parts. Part design is one of the most important design tasks in injection moulding. During the design process, designers must take into account functional requirements of the part as well as its mouldability. The quality of a part is infl u- enced by many factors, such as selection of material, moulding machine, and injection mould, as well as processing conditions. Concurrent engineering techniques are recognised as facilitating strategies in ensuring successful part design 14. One way to apply concurrent engineering strategies in part design is to get part designers to work with both the customers and marketing personnel on the front end, as well as the mould designers, material suppliers, and process engineers on the back end 1. Another important factor in the success of concurrent engineering is the use of design support tools that can provide relevant information and feedback with various degrees of de- tail 5. Various computer-based tools have been developed to assist designers in assessing the mouldability, quality, and cost of a design. Among these tools, simulation packages are the most popular. For example, Moldfl ow is widely used in industry to simulate the moulding process before part design and moulding- process design are fi nalised and actual tooling is started. However, existing computer-based tools only provide infor- mation relating to the mouldability and performance of a design; that is, they can only act as an evaluation tool. They are not capable of directly improving design. To address this problem, a number of optimisation algorithms have been proposed based on the utilisation of the simulation software, such as gate loca- tion optimisation 6,7, feed system optimisation 3, moulding condition optimisation 8, cavity balancing 9, and part thick- ness optimisation 10. These optimisation algorithms require iterative executions of simulation routines. However, for each it- eration, modifi cation is only made to the CAE analysis model (mesh model and other input parameters). For example, in gate location optimisation, the gate location is changed, which can be implemented by setting a different mesh node of the analy- 496 sis model as the new gate location. Part-thickness optimisation is achieved by modifying the thickness attribute of each mesh element. For moulding- or processing-condition optimisation, changes are made to parameters such as melt temperature, mould temperature, and injection time, which are not relevant to the part- geometry model. As such, these optimisation algorithms are not oriented to part design, especially part-shape design and modifi cation. This area of research has largely been ignored. In fact, it is not uncommon that the shape and some features of the initial de- sign may have to be modifi ed to cater to the manufacturability requirements. This paper aims at developing the strategies and methods for automatic part- shape modifi cation. Modifi cation of a part- geometry model may be broadly classifi ed into geometric mod- ifi cation and topological modifi cation 11, or modifi cation and optimisation in sizing, shape, and topology 12. This paper will focus on geometric modifi cation, including sizing of the part and its features, as well as positional change of the features. To support the automatic modifi cation of part geometry, it is essential to have a model that allows the designers to specify the parameters of the part geometry to be modifi ed and how they are to be modifi ed. These parameters are referred to as shape modifi cation variables in this paper. Since the modifi cation vari- ables are related tothe part geometry, the model should be able to acquire information from the part-geometry model, or to incor- porate the part geometric model as its integral part. In addition, the model needs to incorporate information relevant to the CAE analysis, including the desired criteria for measuring quality. In our previous work, we have developed a CAD-CAE integration model 13 and based on this model, a CAD-CAE integrated sys- tem 14. This integration model provides a starting point for part-shape modifi cation. However, it does not support specifi - cation of shape modifi cation variables of part geometry, thus does not support automatic part-shape modifi cation. This paper presents an extension to the model to support automatic shape modifi cation. We will begin by briefl y introducing the existing CAD-CAE integration model in Sect. 2. The enhanced integration model will be presented in Sect. 3. Section 4 describes how this new model is used to modify the part shape. A software prototype implementing the strategies and methods is also presented. Sec- tion 5 studies a few design cases to demonstrate the useful- ness of the presented work. Finally, the paper is summarised in Sect. 6. 2 Brief description of CAD-CAE integration model The CAD-CAE integration model is an object-oriented feature- based model that incorporates both design and analysis informa- tion of an injection-moulded part. The model consists of a num- ber of hierarchically organised features such as part feature, wall feature, hole feature, rib feature, boss feature, and treatment fea- ture. The part feature holds the overall information of the part, while all other features are constituent components of the part, collectively referred to as the component features. The treatment features are features acting as the treatment of other compon- ent features, such as chamfer, fi llet, and round features. These features are defi ned by both their geometric and topological in- formation from the part CAD model, as well as the relevant CAE analysis data. They are referred to as the CAD-CAE features. Figure 1 illustrates the integration model. The thick grey lines show the relationships between the CAD-CAE features. The dashed line links the part feature to its corresponding CAD geometry, while the thin dark lines link the features to their re- spective CAE data. The model uses existing CAD and CAE systems as its under- lying platforms. The part geometry data is stored inthe part CAD database, which is establishedby the CAD platform. ActiveX au- tomation from the CAD system is employed for the model to access the part geometry data as well as the operations on these data. That is to say, the integration model only holds pointers to the part geometry. The model can directly use the exposed func- tionalities of the CAD system through its automation server. The analysis data include those that are related to the overall part, as well as those that are related to the component features of the part. For example, part material, boundary conditions, processing conditions, etc. are the overall CAEanalysis informa- tion. Thus, they are stored in the part feature. Suppressibility is used to suppress a feature of the CAD model to prevent it from being incorporated in the CAE analysis model. It is used to sim- plify the CAE model and applies to features such as rib, boss, hole and treatment. Wall, rib, boss (not hole) features have an attribute of thickness. Fig.1. Illustration of CAD-CAE integration model 497 Fig.2. Defi nition of wall thickness Note that depending on the underlying CAD system, the thickness attribute may or may not be provided by the CAD sys- tem. For example, for Solid Edge, a wall feature is defi ned by an extruded protrusion feature from the system. As such, the thick- ness may be obtained by the extrusion distance, or a dimension of the profi le that was used to defi ne the CAD feature, depending on how the CAD feature was created in the CAD system. Fig- ure 2 illustrates how the wall thickness is defi ned depending on the ways that the CAD geometry is created, with the thickness in Fig. 2a defi ned by the extrusion distance, and the thickness in Fig. 2b defi ned by one dimension of the profi le. These features also hold constraints on their respective rele- vant attributes. For example, the desired part-quality criteria may be defi ned as a constraint of the part feature, while the constraint on the gate location on a wall feature may be defi ned as the con- straint of the corresponding wall feature. Given that such an integration model is created and fully specifi ed, the relevant subroutines of the underlying CAE sys- tem can be activated to generate an analysis model (mesh) and perform CAE analysis. The analysis results are then examined to check whether any of the pre-defi ned criteria are violated. And if more than one design satisfi es all constraints, then evaluations may be performed so that the optimal one can be selected. 3 The enhanced integration model As has been elaborated, it is an important part of injection- moulded-part design to modify the part shape to take account of mouldability and other quality requirements. This section de- scribes extensions to the CAD-CAE integration model to address this issue. The extensions have been made to both the part feature and the component features. 3.1 Shape modifi cation variables A new attribute, namely, a list of shape modifi cation variables, including both positional modifi cation variables and sizing mod- ifi cation variables, is introduced into the CAD-CAE features. The shape modifi cation variable of the part feature is the overall thickness (a sizing modifi cation variable). Bychanging this mod- ifi cation variable, the part shape is modifi ed such that the thick- ness of all the component features is changed to the same thick- ness. The thickness may be varied until the part shape achieves the optimal mouldability and other quality measures. Obviously, part feature does not have any positional modifi cation variable. Component features have both positional and sizing modi- fi cation variables. Before proceeding further, it is necessary to emphasize that most of the component features have a corres- ponding profi le defi ned in the underlying CAD system. A profi le is a number of 2D curves either connected or not connected with each other. By extruding or rotating a profi le, a 3D CAD feature can be created, which is then used to construct more complex CAD geometry by means of Boolean operations. Figure 2 shows the profi le of a wall feature. Examples of some commonly used profi les of a hole, boss, and rib feature include a circle, two con- centric circles, and a 2D triangle respectively. With the help of the feature profi le, modifi cation variables can easily be defi ned. For example, the positional modifi cation variables of a hole feature can be defi ned by the coordinates of the centre of the circle that is used as the profi le to create the hole feature. The sizing modifi cation variable is the hole radius. Figure 3 illustrates these modifi cation variables. Similarly, the wall feature, rib feature, and boss feature also have their respective positional and sizing modifi cation vari- ables. Table 1 lists the modifi cation variables of all the CAD- CAE features. It is important to note that by specifying shape modifi cation variables, different types of shape modifi cation problems, either positional or sizing, or modifi cation to different features, can be handled in a unifi ed manner. It is not necessary to develop indi- vidual methods for the different problems on different features. Hence it benefi ts both model consistency and software develop- ment efforts. Fig.3. Positional and sizing modifi cation variables 498 Table1. Modifi cation variables of the CAD-CAE features Feature type Positional modifi cation variables Sizing modifi cation variables PartNoneOverall thickness Wall Coordinates of a key point on the profi le Wall thickness; dimensions of the profi le Hole Coordinates of the centre of the circle on the profi le Radius of the circle on the profi le Rib Coordinates of a key point on the profi le Rib thickness; dimensions of the profi le Boss Coordinates of the centre of the circles on the profi le Boss thickness; boss height; radius of the inner circle on the profi le Associated with each modifi cation variable are four parame- ters:currentvalue,startingvalue,endingvalue,andstepvalue.For sizingmodifi cationvariablessuchasthickness,radius,andheight, these parameters are suffi cient in defi ning how the correspond- ing variable should be varied during the part-shape-modifi cation process. For the positional modifi cation variables, which are the coordinates of the relevant key point on the profi le, e.g. the cen- tre of a circle, it may be necessary to specify how they are varied. Forexample, the hole position maybe variedwithina rectangular range or a circular range on the surface of the hole profi le. This is problem specifi c,hence itisuptothedesigner tospecify. 3.2 Criteria construction variables and the criteria for shape modifi cation The designer should also specify the mouldability and other quality measuring criteria as constraints or targets ofthe intended shape modifi cation operation. Some of these criteria are 16: Shear stress should not exceed the maximum recommended for the material type. Shear rate should not exceed the maximum recommended for the material type. Flow front temperature should not be more than 20C below the melt temperature. Cooling time should be uniform and minimised. All extremities should fi ll at the same time. All extremities should fi ll at the same pressure. Designers may also have some specifi c quality requirements, such as minimising the maximum shear stress, uniform end- of-fi ll temperature, uniform volumetric shrinkage, and uniform warpage. Some requirements may be imposed on a particular location or area of the plastic part, such as the shear stress re- quirement at the vicinity of snap fi ts and screw holes. To enable the designers to specify these criteria rather than hard-wire them in a computer program, we modify the part fea- ture by replacing the existing constraint attribute with two cri- teria attributes: a verifi cation criteria attribute and an evaluation criteria attribute. Each of these attributes contains a list of crite- ria. The verifi cation criteria refer to those that must be satisfi ed, such as the fi rst three criteria listed above; while the evaluation criteria refer to those that should be minimised, such as the last three criteria listed above. Case-specifi c requirements may also be classifi ed as either verifi cation criteria or evaluation criteria. Part feature is further extended to include a new attribute for storing a list of variables from which the verifi cation and evalua- tion criteria can be constructed. These variables are thus referred to as criteria construction variables. The following is a list of some of these variables (the units are put inside the brackets, where “SD” stands for standard deviation): v1 = maximum shear stress (MPa); v2 = maximum shear rate (1/s); v3 = maximum pressure (MPa); v4 = maximum fl ow front temperature (C); v5 = minimum fl ow front temperature (C); v6 = maximum end-of-fi ll temperature (C); v7 = minimum end-of-fi ll temperature (C); v8 = maximum cooling time (s); v9 = minimum cooling time (s); v10 = actual injection time (s); v11 = maximum volumetric shrinkage (%); v12 = minimum volumetric shrinkage (%); v13 = maximum clamp tonnage (tonnes); v14 = uniform end-of-fi ll temperature (SD); v15 = uniform cooling time (SD); v16 = uniform volumetric shrinkage (SD); v17 = uniform warpage (SD); v18 = uniform fi lling time at extremities (SD); v19 = uniform pressure at extremities (SD); The new attribute stores a list of criteria-construction vari- ables selected by the designers from these commonly used vari- ables. Each selected variabl