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ORIGINAL ARTICLEY.-S. Ma T. TongAn object-oriented design tool for associative cooling channelsin plastic-injection mouldsReceived: 17 December 2002/ Accepted: 17 December 2002/Published online: 17 October 2003C211 Springer-Verlag London Limited 2003Abstract Due to the demand for short product devel-opment cycles, plastic injection mould designers are re-quired to compress their design time and toaccommodate more late changes. This paper describesan associative design approach embedded in a coolingchannel module of a mould design software package. Itgives a set of comprehensive object definitions forcooling circuits, and addresses balanced or unbalanceddesigns. CAD algorithms that have been developed arebriefly explained. With this new approach, moulddesigners can easily propagate changes between mouldplates or inserts and the cooling system without the needfor tedious rework. Hence, this approach can signifi-cantly reduce the total design time and the impact of latechanges.Keywords Cooling circuit Plastic mould design CAD/CAM Associative design Design automation1 IntroductionCurrently, most CAD systems are unable to capturedesign intent completely and unambiguously. Rich de-sign information cannot be fully represented in CADmodels and late changes in the product developmentcycle cause a lot of rework. It has been acknowledgedthat CAD interoperability should cover integration withknowledge-based engineering (KBE) systems 1. How-ever, there is no mechanism to enable design intentinformation flow. Such an information gap is veryobvious in plastic injection mould design as well. Moulddesigners are facing increasing pressure to reduce thedesign time, and yet are expected to assure mouldquality.Various CAD tools for designing plastic injectionmoulds have emerged since the early 1970s 2, most ofwhich focused on moulding flow analysis and optimi-sation algorithms 3, 4, 5. Recently, the design of mouldsub-systems, such as core/cavity inserts 6, 7, runners 8,9, gate locations 3, 4, 5 and cooling systems 10, havebeen the focus. For cooling system design, Wang et al.11 suggested a strategy with three stages, initial designwith one-dimensional approximation, two-dimensionaldesign with optimisation, and three-dimensional designwith cooling eect analysis. They have developed aprogram that uses 3D-boundary element methods toanalyse 3D heat transfer. All the above-mentioned toolsare only able to generate geometrical information. Therepresentation and reuse of rich design information atdierent levels are not addressed.Object-oriented (OO) software technology has beenapplied to meet the information representation gap inmould design 12. Object definitions can provide a greatdeal of help in sorting out complicated entities, espe-cially for part-independent parts and features. However,maintaining the relationships among geometrical entitiesand making them customisable is still not a trivial task.The CAD software development approach that canachieve persistent relationships among geometricalentities is referred to as the associative design approach.One way to build design intent and process knowledgeinto a CAD system is in the form of a process wizard,which is basically an application program coupled with aset of sequenced user interfaces (UIs) to guide users tocomplete certain interactions with the computer system.MouldWizard from EDS Inc. is one such process-basedwizard 13. This paper introduces the associative designapproach applied in its cooling channel module. Marketfeedback shows that this concept can significantly reducethe gap between human knowledge and consistentcomputer representations.The cooling system in a mould aects not only thequality of resultant moulding parts but also eciency inY.-S. Ma ( design variables,such as locations, types of cooling channels, and 3Dlayout of circuits, are usually modified frequently toaddress late part design changes as well as mould designoptimisation. The modification process is laborious anderror prone because designers have to edit and updateCAD models repetitively. Mok et al. 16 developed acooling system that can automatically retrieve certaincircuit patterns, such as straight or U types, but theassociation among geometrical entities is not discussed.An expert system for designing cooling systems wasintroduced by Kwon et al. 10. The system consists offour levels: layout design, analysis, evaluation anddecision-making. A decision-making module evaluatesthe redesign of cooling channels based on the rulesstored in a knowledge base. However, there is no inte-gration with a parametric CAD system.In summary, an ecient and user-friendly coolingsystem design tool is highly sought; such a system can beexpected to free mould designers from tedious geomet-rical updating and to keep design models consistent, sothat the total mould design cycle time can be shortened.This paper presents a cooling channel design tool thatprovides substantial automation for cooling circuitgeneration with associative links among cooling holesand their drilling faces.1.1 Generic issues associated with capturingdesign intentsIn the industry, cooling channels are usually designed inthe form of cooling circuit, but represented as HOLEfeatures using CAD tools. On the other hand, experi-enced mould designers found that solid cylinders arealso commonly used instead to represent cooling chan-nels. In the latter approach, when the design is finalised,all channels are united to form a cooling circuit. Withsuch united circuits defined with the help of CAE anal-ysis tools, the cooling eect can be evaluated 11. Thesecircuits are not converted into holes until the design hasbeen finalised and is ready for CAM tool path genera-tion. With this form of representation, a CAD systemcan display/draw cooling channels for visual inspection,without displaying detailed features of the core/cavityinserts and mould plates. Repositioning and modifyingcylinders also require fewer steps than using HOLEfeatures. It enables automatic checking for collisionsbetween cooling channels and other mould features,such as cavities and ejecting-pin holes.However, representing cooling channels in the formof solid cylinders has several problems. First, many stepsare still required for a simple channel, such as creating acylinder, chamfering the blind end in the case of a blindhole, and running through a series of dialogue boxes toposition and orient it. Commonly, there are manychannels in a cooling circuit, so creating them involves alot of repetitive commands. When modifications areneeded, cylinders have to be edited repetitively again.This situation is error-prone. Second, cooling channelsare not self-identified. For automatic heat transferanalysis or collision checking, identifying cooling chan-nels is particularly important. Third, they cannot pro-vide orientation information for plugs, nozzles, orbaes to be inserted into cooling channels in a user-friendly drag-and-drop manner. Hence, mould designershave been frustrated with tedious steps.1.2 Semantic definitions of a cooling systemAn object-oriented software design approach can beapplied to address the issues discussed in the abovesection. Defining a set of object types or classes thatprovides self-contained definitions of cooling systemsand enables dynamic updating to validate the coolingsystem, is essential. In Fig. 1, the simplified semanticstructure of a cooling system and its related componentmember types is shown. Each component type is definedas an object class.A cooling channel is defined as a continuous straighthole that contains cooling liquid (water in most cases). Itcan be contained in a single mould component (plates orinserts), or it cuts across several. In this paper, hole isused to describe the geometrical shape of a coolingchannel on a single mould component, however, itsrepresentation is not the same as traditional HOLEfeatures (see the next section). An example of a coolingcircuit is shown in Fig. 2. Holes 15 are cooling chan-nels. A cooling circuit represents all the inter-connectedcooling channels between an inlet and an outlet. Severalcooling circuits form a cooling system. In Fig. 2, holes15 jointly form a cooling circuit. A circuit can haveseveral cooling channels with dierent orientations.These channels consist of cooling holes which are drilledfrom dierent faces of the mould plates or inserts. Theface used to drill a cooling hole is called the penetratingface. Naturally, a cooling hole has one penetrating faceand the hole-drilling vector always leaves from the pe-netrating face and points to the other end. Usually,Fig. 1 Semantic structure of a cooling system80cooling holes are perpendicular to the penetrating face.However, in order to cater to some special cases, thisconstraint is not imposed for the purposes of this article.In practice, some cooling channels cut across multipleblocks; an example of this is shown in Fig. 3. It consistsof several connected collinear cooling holes (hole 1, hole2 and hole 3). Such channels are specially named col-linear cooling channels.In many cases, multi-impression design is used forthe mould layout. There are two approaches to creat-ing cooling circuits then: balanced and unbalanced.A cooling system is referred to as balanced if the samecooling circuit pattern is applied to every impression.Otherwise, the cooling system is unbalanced. Usually, ifthe mould is designed with a balanced multi-impressionpattern 14, and the designer wishes to have an identicalcooling circuit for each impression section, then thebalanced approach is used. In this case, because eachcircuit is designed mainly to cover one impression; thecooling eect can be better controlled to satisfy heat-transfer requirements. This is especially recommendedfor complex moulding parts where the cooling eect canbe optimised using simulation packages 11. With thisapproach, a CAD function that is commonly requiredby mould designers is to reflect the changes in thecooling circuit pattern on the individual impressions.On the other hand, the designer may want to treat themould as a whole and design cooling circuits withoutconsidering the impression pattern; if this is the case hecan apply the unbalanced approach.1.3 Detailed representationsA detailed component structure of a cooling system isgiven in Fig. 4. A hole is represented with a straight lineand an optional cylindrical solid. This straight line iscalled the cooling guideline for the hole. More precisely,a cooling guideline is a straight-line segment startingfrom the cooling-hole penetrating centre point to theholes end centre point. In Fig. 2, AB is the coolguideline for hole 1, and CD is for hole 2. Guidelinescontain hole-drilling vectors.At both the start and end points of each cooling hole,the following types of hole-ends can be selected, asshown in Fig. 5: (1) Drill-through, (2) Counter-bored,(3) Blind without extension and (4) Blind with extension.Such geometrical feature information is represented asFig. 3 A typical collinear cooling channel Fig. 4 Detailed component structures of a cooling systemFig. 2 An example of cooling circuit81attributes attached to guidelines. The cylindrical solidscan be generated anytime if it is needed based on theinformation stored with each guideline.Traditionally, cooling lines are also used to representa cooling circuit 11, but they are separate entities fromthe containing solids, such as mould plates or inserts.One of the design ideas in this paper is that everyguideline has start and end point that are associated withthe corresponding penetrating and exiting faces, exceptfor the end points of blind holes. Therefore, if these faceschange their positions, the associated points can be de-rived dynamically and updated accordingly. In otherwords, cooling guidelines are always associated withtheir penetrating and exiting faces.The cooling guidelines of all the holes within acooling circuit are grouped as a guide path. In Fig. 2,five guidelines, AB, CD, EF, GH, and IJ, form a guidepath. In this paper, as shown in Fig. 4, a guide pathrepresents each cooling circuit completely while coolingguidelines can have certain attributes to describe thecooling-hole types, diameters, etc.In fact, cooling cylindrical solids are generated onlywhen needed for viewing, checking physical collisionamong dierent features/components, or creating de-tailed features on plates or inserts. These cooling solidscan be deleted to simplify the display; as long as theguide paths are available, these cooling solids can beregenerated. At later stages, after confirming the coolingsystem design, geometrical holes may be still needed forCAM application or component structure detailing.They can be achieved by subtracting cooling solids fromtheir corresponding plate/insert bodies.A guide path is also used to maintain connectivityamong its guidelines. To validate and verify this condi-tion, a validator method is defined in the guide pathclass. The collinear cooling channel is the specialobject type that is created. From Fig. 4, it can be seenthat a cooling circuit may contain such collinear coolingchannels as well as simple channels. Each collinearchannel can be represented by a group of guidelinescalled the collinear path. Obviously, its element guide-lines must be connected from head to tail continuouslyalong a straight line. In Fig. 3, AB, CD and EF form acollinear path and represent through hole 1 (with bothcounter-bored ends), through hole 2, and blind hole 3respectively. It can be seen that within a cooling circuit,cooling elements are associated because they are vali-dated instantly upon any change.As shown in Fig. 4, the contents and representationof a circuit object change according to the context andusers choices, for example, a circuit can be displayedgraphically as a set of inter-connected guidelines, or as aset of cylindrical solids. A cooling circuit is self-con-tained with geometrical and non-geometrical informa-tion in the form of rich attributes.In summary, with this object structure design, coolingchannels and their related mould plates or inserts can beautomatically updated if some elements, such as pene-trating faces or drilling-hole types are modified at laterdesign stages. Since all the cooling channels are createdin an associative approach, then the process knowledge,such as penetrating faces, drilling directions and conti-nuity within a circuit can be embedded within the CADmodel and stored persistently.2 Implementation aspects2.1 Embedded links and parametersIn a cooling design session with this module, guidelinesare initially created through a graphical user interface.To associate the start and end points of every guidelinewith the penetrating and exiting faces, with the exceptionof the end points of blind holes, a smart point conceptis used 13. A smart point is a point on the surfaceassociated with the face at the kernel database level. Itkeeps the persistent link with the corresponding face.Here, word smart represents the associative nature ofan entity to another related entity. Since guidelines arecreated based on such smart end points, then the cor-responding guidelines are also called smart lines. Each ofthem is connected to one (for blind holes) or two smartpoints (for through holes).A cooling solid cylinder can be generated automati-cally along its smart guideline by sweeping a circularsection profile 13. For a blind hole, a cone end is added.For a cooling circuit, its cylindrical solids are then uni-ted as the solid representation. These geometrical fea-tures are represented with attributes attached toguidelines. Such related attributes include type of end(see Fig. 5), cooling hole diameter, and depth anddiameter of the counter-bored portion, if applicable.They are used for cooling hole editing and cooling solidregeneration.2.2 Functions and algorithmsThe main functions that have been developed in thismodule to meet the requirements for cooling systemdesign are listed here:Fig. 5 Types of cooling cylinder ends82a. Addition of smart guidelines to form guide pathsb. Modification/repositioning of guidelinesc. Deleting of circuit guide pathsd. Creation of cooling solidse. Modification of the cooling solidsf. Deletion of the cooling solidsg. Creation of balanced or unbalanced cooling designsfor a multi-impression mould.These functions are briefly described below.2.3 Creating and editing the smart guide pathof a cooling circuitTo create the first guideline of a guide path, the userneeds to select a face on an intended solid as the inletpenetrating (planar) face of the circuit (see Fig. 2). Aplane equation can be extracted from the selected planarface. The initial start point A for the guide path on thisface can be found based on the users indication point; asmart point is then created. The default direction togenerate the first cooling guideline is set to the reversedirection of the face to normal, and it is displayed on thegraphic window. The user can interactively modify theinitial point position and guideline direction with dif-ferent submenus activated from the UI shown in Fig. 6.Then, the user can dynamically drag a cooling line orinput a value of the length for the guideline of a blindhole, or choose another face to indicate the ending facefor a throughhole. In the latter case, another smart pointwill be created at the end point of the guideline. Aftercreating the first guideline, a sequence number, 1, isdisplayed near it.To create the next guideline (see Fig. 2), a drillingvector is required. The user can indicate the bottompenetrating face at point P. Then, the next guidelinedirection is set to be in the reverse normal direction ofthe selected face. The vectors start point C is determinedwith reference to the previous guideline AB and thenearest point to the users indicated point P. This is anembedded rule implemented in this work. To make thevect