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This paper presents a practical prototype knowledge-basedsystem, called IKMOULD, for mould design in the injectionmoulding process. It attempts to tackle the problem in apractical and integrative way, unlike the stand-alone andmathematical programs which have been developed in thepast to solve only a part of the problem. A total quantitativeand structured approach is not feasible in dealing with thecomplex and multirelated design problems generally involvedin mould design. In this system, the computational module, theknowledge-based module and the graphic module for generat-ing mould features are integrated within an interactive CAD-based framework. The knowledge base of the system can beaccessed by mould designers through interactive programs sothat their own intelligence and experience can also be incor-porated with the total mould design. The approach adoptedboth speeds up the design process and facilitates design stan-dardisation which in turn increases the speed of mould manu-facture.Keywords: CAD/CAM; Injection mould design; Interactivedesign procedure; Knowledge-based expert systems1. IntroductionPlastic, which is one of the most versatile materials in themodern age, is widely used in many products throughout theworld. 61730
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The injection moulding process is the most commonmoulding process for making plastic parts. It is a process bywhich plastic pellets or powders are melted and pressurisedinto a cavity to form a complex 3D part in a single cyclicoperation. The engineering tasks involved in injection mouldingare the design of the geometry of parts and moulds, machiningand polishing of cavity/core surfaces and cooling lines,assembly of plates, pins and mould bases, and prototype testswith a proper choice of material and processing parameters.Correspondence and offprint requests to: C. K. Mok, Departmentof Manufacturing Engineering and Engineering Management, CityUniversity of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong.E-mail: meckmokKcityu.edu.hkIn injection moulding, the design of the moulds is of criticalimportance for product quality and efficient processing. In mostcases, the quality of the mould is responsible for the economicsof the entire process. For example, cycle times are determinedprimarily by mould temperature control (cooling time). Thequality of the moulded parts is determined primarily by themould, even though constant machine conditions, achievablethrough improved machine fabrication technology and the useof process control systems combined with uniform materialproperties, are also an essential prerequisite. Frequently, mould-ing problems are traceable to moulds which either show signsof premature wear or cause processing difficulties because ofpoor thermal and rheological design, or else they are character-ised by operating tolerances which are too close for themachine to guarantee its constant machine parameters.
Production start-up is usually delayed owing to the long timerequired for mould design and manufacture and, especially, tothe very frequently required mould corrections which, in thefinal analysis, are attributable to poor mould design. Moreover,the requirements increase as high-quality industrial parts areincreasingly demanded. These facts clearly show that currently,the moulds could become the weakest link in the productdevelopment cycle. Hence, in any improvement programs,moulds must receive the major attention, which should beconcentrated mainly in the area of mould design. Such effortsshould be directed primarily to improving the entire designprocess in terms of the results achieved, to accelerate pro-duction start-up, and to enable all designers to make use ofcurrently available mould design knowledge in a practicablemanner.Nowadays, mould design faces increasing deadline pressureand the design itself is predominantly based upon the experi-ence of the mould designer. Mould designers are required topossess thorough and broad experience, because detaileddecisions require a knowledge of the interaction between differ-ent parameters. A change of an inpidual factor in a morefavourable direction could have a negative effect on othercritical factors. Unfortunately, at present, it is impossible tocover the growing demand for experienced designers.At present, most CAD systems provide only the geometricmodelling functions which facilitate the drafting operationsof mould design, but do not provide mould designers with the necessary knowledge to develop good mould designs.Conventional computer-aided engineering packages are usu-ally good at data processing for information-intensive prob-lems or good at number manipulation for formulation-inten-sive problems. The former involves the computer-aideddrafting and graphics, and data reduction and transformation[1], whereas the latter involves numerical (or mathematical)modelling and analysis [2]. However, in design problems,especially in mould design which involves a substantialpractical knowledge component about the functions andstructure of a mould, human heuristic knowledge and empiri-cal knowledge are needed in addition to information-inten-sive and formulation-intensive knowledge. Therefore, con-ventional computer-aided design technology is unsuitable forprocessing the heuristic and empirical type of knowledgewhich is critical in the mould design problems.Injection mould design involves extensive empirical knowl-edge (heuristic knowledge) about the structure and functionsof the components of the mould. It thus has very good potentialfor success as a knowledge-based system application. Thispaper describes a prototype CAD-based mould design pro-cedure developed by using a knowledge-based approach. Theprocedure provides a mould designer with an interactivecomputer-aided design system for a knowledge base, incorpor-ating methods, and elements of mould features; and graphicmodules for constructing mould assemblies and parts. Theprocedure begins with the plastic part which is to be mouldedand finishes by generating a general configuration and detaileddrawings of a completed mould.The system can now handle the design of small-sized injec-tion moulds of four common types, namely standard two-platemoulds, stripper moulds, standard sliding-splits moulds andthree-plate moulds.2. A Summary of Injection Mould DesignThis summary of injection mould design establishes the termin-ology and introduces aspects of the design procedure involved.Figure 1 illustrates the basic components of a typical injec-tion mould.(a) Feed system. The feed system accommodates the moltenplastic material coming from the injection nozzle of themoulding machine and distributes it into each cavity.Normally, the feed system consists of a sprue (23), runner(4) and gate (19).(b) Cooling system. To remove the heat from the moulding,it is necessary to supply the mould with a system ofcooling channels (3) through which a coolant is pumped.(c) Ejection system. After the moulding has solidified andcooled down, it has to be removed from the mould. Afterthe mould is opened, the ejector of the machine willactuate the ejection system to move forward to knockout the mouldings. In Fig. 1, ejector pins (16) and sprueejector (10) are used to eject the parts and runner,respectively. The return pins (15) are used to push theejector plate assembly (13, 14) back when the mouldis closed.(d) Mould construction.