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.2.1 Difference Between a Single-Cavity and aMulti-Cavity MouldVery often, the impression in which molten plastic is beingfilled is also called the cavity. The arrangement of the cavitiesis called the cavity layout. When a mould contains more thanone cavity, it is referred to as a multi-cavity mould. Figures 3(a)and 3(b) shows a single-cavity mould and a multi-cavity mould.A single-cavity mould is normally designed for fairly largeparts such as plotter covers and television housings. For smallerparts such as hand phone covers and gears, it is always moreeconomical to design a multi-cavity mould so that more partscan be produced per moulding cycle. Customers usually deter-mine the number of cavities, as they have to balance theinvestment in the tooling against the part cost.2.2 Multi-Cavity LayoutA multi-cavity mould that produces different products at thesame time is known as a family mould. However, it is notusual to design a mould with different cavities, as the cavitiesmay not all be filled at the same time with molten plastic ofthe same temperature.On the other hand, a multi-cavity mould that produces thesame product throughout the moulding cycle can have a bal-anced layout or an unbalanced layout. A balanced layout isone in which the cavities are all uniformly filled at the sametime under the same melt conditions [15,16]. Short mouldingcan occur if an unbalanced layout is being used, but this canbe overcome by modifying the length and cross-section of therunners (passageways for the molten plastic flow from thesprue to the cavity). Since this is not an efficient method, itis avoided where possible. Figure 4 shows a short mouldingsituation due to an unbalanced layout.A balanced layout can be further classified into two categor-ies: linear and circular. A balanced linear layout can accommo-date 2, 4, 8, 16, 32 etc. cavities, i.e. it follows a 2nseries. Abalanced circular layout can have 3, 4, 5, 6 or more cavities,but there is a limit to the number of cavities that can beaccommodated in a balanced circular layout because of spaceconstraints. Figure 5 shows the multi-cavity layouts that havebeen discussed.3. The Design ApproachThis section presents an overview of the design approach forthe development of a parametric-controlled cavity layout designsystem for plastic injection moulds. An effective workingmethod of mould design involves organising the various subas-semblies and components into the most appropriate hierarchydesign tree. Figure 6 shows the mould assembly hierarchydesign tree for the first level subassembly and components.Other subassemblies and components are assembled from thesecond level onwards to the nth level of the mould assemblyhierarchy design tree. For this system, the focus will be madeonly on the “cavity layout design”. 3.1 Standardisation ProcedureIn order to save time in the mould design process, it isnecessary to identify the features of the design that are com-monly used. The design processes that are repeatable for everymould design can then be standardised. It can be seen fromFig. 7 that there are two sections that interplay in the stan-dardisation procedure for the “cavity layout design”: componentassembly standardisation and cavity layout configuration stan-dardisation. Fig. 6. Mould assembly hierarchical design tree.Fig. 7. Interplay in the standardization procedure.
3.1.1 Component Assembly StandardisationBefore the cavity layout configuration can be standardised,there is a need to recognise the components and subassembliesthat are repeated throughout the various cavities in the cavitylayout. Figure 8 shows a detailed “cavity layout design” hier-archy design tree. The main insert subassembly (cavity) in theFig. 8. Detailed “cavity layout design” hierarchical design tree.second level of the hierarchy design tree has a number ofsubassemblies and components that are assembled directly toit from the third level onwards of the hierarchy design tree.They can be viewed as primary components and secondarycomponents. Primary components are present in every moulddesign. The secondary components are dependent on the plasticpart that is to be produced, so they may or may not be presentin the mould designs.As a result, putting these components and subassembliesdirectly under the main insert subassembly, ensures that everyrepeatable main insert (cavity) will inherit the same subas-semblies and components from the third level onwards of thehierarchy design tree. Thus, there is no need to redesignsimilar subassemblies and components for every cavity in thecavity layout.3.1.2 Cavity Layout Configuration StandardisationIt is necessary to study and classify the cavity layout configur-ations into those that are standard and those that are non-standard. Figure 9 shows the standardisation procedure of thecavity layout configuration.A cavity layout design, can be undertaken either as a multi-cavity layout or a single-cavity layout, but the customersalways determine this decision. A single-cavity layout is alwaysconsidered as having a standard configuration. A multi-cavitymould can produce different products at the same time or the Fig. 9. Standardisation procedure of the cavity layout configuration.same products at the same time. A mould that producesdifferent products at the same time is known as a familymould, which is a non-conventional design. Thus, a multi-cavity family mould has a non-standard configuration.A multi-cavity mould that produces the same product cancontain either a balanced layout design or an unbalanced layoutdesign. An unbalanced layout design is seldom used and, as aresult, it is considered to possess a non-standard configuration.However, a balanced layout design can also encompass eithera linear layout design or a circular layout design. This dependson the number of cavities that are required by the customers.It must be noted, however, that a layout design that has anyother non-standard number of cavities is also classified ashaving a non-standard configuration.After classifying those layout designs that are standard, theirdetailed information can then be listed into a standardisationtemplate. This standardisation template is pre-defined in thecavity layout design level of the mould assembly design andsupports all the standard configurations. This ensures that therequired configuration can be loaded very quickly into themould assembly design without the need to redesign the layout.3.2 Standardisation TemplateIt can be seen from Fig. 10 that there are two parts in thestandardisation template: a configuration database and a layoutFig. 10. The standardization template.design table. The configuration database consists of all thestandard layout configurations, and each layout configurationhas its own layout design table that carries the geometricalparameters. As mould-making industries have their own stan-dards, the configuration database can be customised to takeinto account those designs that are previously considered asnon-standard.3.2.1 Configuration DatabaseA database can be used to contain the list of all the differentstandard configurations. The total number of configurations inthis database corresponds to the number of layout configur-ations available in the cavity layout design level of the moulddesign assembly. The information listed in the database is theconfiguration number, type, and the number of cavities. Table 1shows an example of a configuration database. The configur-ation number is the name of each of the available layoutconfigurations with the corresponding type and number ofcavities. When a particular type of layout and number ofcavities is called for, the appropriate layout configuration willbe loaded into the cavity layout design.3.2.2 Layout Design TableEach standard configuration listed in the configuration databasehas its own layout design table. The layout design tablecontains the geometrical parameters of the layout configurationand is independent for every configuration. A more complexlayout configuration will have more geometrical parameters tocontrol the cavity layout.Figures 11(a) and 11(b) show the back mould plate (coreplate) with a big pocket and four small pockets for assemblingthe same four-cavity layout. It is always more economical andeasier to machine a large pocket than to machine inpidualsmaller pockets in a block of steel. The advantages of machin-ing a large pocket are: Fig. 11. The back mould plate with pocketing.Table 1. Sample of the configuration database.Configuration number Type Number of cavitiesS01 Single 1L02 Linear 2L04 Linear 4L08 Linear 8L16 Linear 16L32 Linear 32L64 Linear 64C03 Circular 3C04 Circular 4C05 Circular 5C06 Circular 61. More space between the cavities can be saved, thus asmaller block of steel can be used.2. Machining time is faster for creating one large pocketcompared to machining multiple small pockets.3. Higher accuracy can be achieved for a large pocket thanfor multiple smaller pockets.As a result, the default values of the geometrical parametersin the layout design table results in there being no gap betweenthe cavities. However, to make the system more flexible, thedefault values of the geometrical parameters can be modifiedto suit each mould design where necessary.3.3 Geometrical ParametersThere are three variables that establish the geometrical para-meters:1. Distances between the cavities (flexible). The distancesbetween the cavities are listed in the layout design tableand they can be controlled or modified by the user. Thedefault values of the distances are such that there are nogaps between the cavities.2. Angle of orientation of the inpidual cavity (flexible). Theangle of orientation of the inpidual cavity is also listedin the layout design table which the user can change. Fora multi-cavity layout, all the cavities have to be at the sameangle of orientation as indicated in the layout design table.If the angle of orientation is modified, all the cavities willbe rotated by the same angle of orientation without affectingthe layout configuration.3. Assembly mating relationship between each cavities (fixed).The orientation of the cavities with respect to each other ispre-defined for each inpidual layout configuration and iscontrolled by the assembly mating relationship betweencavities. This is fixed for every layout configuration unlessit is customised.Figure 12 shows an example of a single-cavity layout con-figuration and its geometrical parameters. The origin of themain insert/cavity is at the centre. The default values of X1and Y1 are zero so that the cavity is at the centre of thelayout (both origins overlap each other). The user can changethe values of X1 and Y1, so that the cavity can be offset appro-priately.Figure 13 shows an example of an eight-cavity layout con-figuration and its geometrical parameters. The values of X andY are the dimensions of the main insert/cavity. By default, thevalues of X1 and X2 are equal to X, the value of Y1 is equalto Y, and thus there is no gap between the cavities. The valuesof X1, X2, and Y1 can be increased to take into account thegaps between the cavities in the design. These values are listedin the layout design table.If one of the cavities has to be oriented by 90°, the rest ofthe cavities will be rotated by the same angle, but the layoutdesign remains the same. The user is able to rotate the cavitiesby changing the parameter in the layout design table. Theresultant layout is shown in Fig. 14.Fig. 12. Single-cavity layout configuration and geometrical parameters. Fig. 13. Eight-cavity layout configuration and geometrical parameterswithout cavity rotation.Fig. 14. Eight-cavity layout configuration and geometrical parameterswith cavity rotation.A complex cavity layout configuration, which has moregeometrical parameters, must make use of equation to relatethe parameters.4. System ImplementationA prototype of the parametric-controlled cavity layout designsystem for a plastic injection mould has been implementedusing a Pentium III PC-compatible as the hardware. Thisprototype system uses a commercial CAD system (SolidWorks2001) and a commercial database system (Microsoft Excel)as the software. The prototype system is developed using theMicrosoft Visual C++ V6.0 programming language and theSolidWorks API (Application Programming Interface) in aWindows NT environment.SolidWorks is chosen primarily for two reasons:1. The increasing trend in the CAD/CAM industry is to movetowards the use of Windows-based PCs instead of UNIXworkstations mainly because of the cost involved in purchas-ing the hardware.2. The 3D CAD software is fully Windows-compatible, thusit is capable of integrating information from Microsoft Excelfiles into the CAD files (part, assembly, and drawing)smoothly [17].This prototype system has a configuration database of eightstandard layout configurations that are listed in an Excel file.This is shown in Fig. 15(a). Corresponding to this configurationdatabase, the layout design level, which is an assembly filein SolidWorks (layout.sldasm), has the same set of layoutconfigurations. The configuration name in the Excel file corre-sponds to the name of the configurations in the layout assemblyfile, which is shown in Fig. 15(b).Every cavity layout assembly file (layout.sldasm) for eachproject will be pre-loaded with these layout configurations.When a required layout configuration is requested via the userinterface, the layout configuration will be loaded. The userinterface shown in Fig. 16 is prior to the loading of therequested layout configuration. Upon loading the requestedlayout configuration, the current layout configuration infor-mation will be listed in the list box.The user is then able to change the current layout configur-ation to any other available layout configurations that are foundin the configuration database. This is illustrated in Fig. 17.The layout design table for the current layout configurationthat contains the geometrical parameters can be activated whenthe user triggers the push button at the bottom of the userinterface. When the values of the geometrical parameters arechanged, the cavity layout design will be updated accordingly.Figure 18 shows the activation of the layout design table ofthe current layout configuration.5. A Case StudyA CAD model of a hand phone cover, shown in Fig. 19, isused in the following case study.Prior to the cavity layout design stage, the original CADmodel has to be scaled according to the shrinkage value ofthe moulding resin to be used. The main insert is then createdto encapsulate the shrunk part. This entire subassembly isknown as the main insert subassembly (xxx cavity.sldasm), where “xxx” is the project name. Figure 20 shows the maininsert subassembly. After the main insert subassembly is cre-ated, the cavity layout design system can be used to preparethe cavity layout of the mould assembly.5.1 Scenario 1: Initial Cavity Layout DesignIn a mould design, the number of cavities to be built in amould is always suggested by the customers, as they have tobalance the investment in the tooling against the part cost.Initially, the customers had requested a two-cavity mould tobe designed for this hand phone cover. After the creation ofthe main insert subassembly, the mould designer loads a layoutconfiguration that is of a linear type which has two cavitiesusing this cavity layout design system. The correspondingconfiguration name is L02 and is listed in the user interfaceas shown in Fig. 21.5.2 Scenario 2: Modification in the Cavity LayoutDesignTechnical discussion sessions between the customers and moulddesigners are common. This enables changes to be made tothe 3D CAD files of both the product and mould as soon aspossible, prior to mould manufacture. Changes are almostalways inevitable and mould designers are never given anyextension in the lead time.In this case, during a technical discussion session, the cus-tomers changed their minds and needed a linear four-cavitymould instead of a two-cavity mould so that the production