CAD知识的模具设计英文文献和中文翻译(3)
First, the plastic part has to be drawn or retrieved from thepart drawing database. Then an enquiry routine (block 5)selects the questions to put by reviewing answers given aboutthe part in terms of its geometry, dimensions, material, etc.,and also about the mould specifications in terms of number ofcavities, mould design features, etc. After the designer answersall the questions, the software creates a coded description ofthe part and the mould. The purpose of the code is two-fold.First, it is used to make reference to the existing moulddatabase and look for part codes identical to or close to theexisting part (block 6). If the search finds appropriate part(s)and their respective mould(s), then they can be retrieved andreviewed to see whether they are suitable for use. It is normallyrequired to modify existing moulds to make them suitable fora different part, but this can usually be done fairly quicklyusing the system CAD facilities. The second and main use ofthe code is to access the knowledge base of the system.Within the next stage of the design procedure, a functionaldesign analysis is performed so that the methods related to thevarious functional designs can be chosen (block 7). Themethods file forms part of the knowledge base and it containsa long list of methods for achieving the various functionaldesigns of an injection mould. Since only some of thesemethods are applicable to the particular part under consider-ation, it is necessary to extract the list of possible methods tobe posed to the designer for his selection. This task is nowcarried out by the method screening routine (block 8) whichuses the part code to select the recommended list of methods.Once a method has been selected, a code associated with eachchosen method is generated.Often, a method has alternative mould features (hardwarearrangements). The code associated with the chosen method isthen used to access and show to the designer the alternativemould features of the chosen method from the knowledge baseof the alternative mould features through the mould featurescreening routine (block 9). Separating the fundamentalmethods from their technical details in the present procedureguides the designer to focus on basic methods before dealingwith their implementation. This logical approach speeds up thedesign process by reducing the variety of choices.When a particular mould feature of a chosen method hasbeen selected, a code associated with each mould feature isproduced and used to access and prompt the designer to inputinformation concerning inpidual dimensions, and position andorientation from the knowledge base of mould feature geometrythrough the autographic routine. In the knowledge base ofmould features geometry, the detailed dimensions of thegeometry of all the mould features are stored in terms ofparameters. Once the graphic information has been input, thegeometry of the chosen mould feature is drawn.The designer then proceeds to another functional designthrough a similar procedure. When all the functional designshave been completed, a complete drawing is generated. Finally,all the designer has to do is to review and modify the moulddrawing if necessary.5.3 System DevelopmentThe program was written in UNIX C and implemented on theMcDonnell Douglas Unigraphics II CAD System. This programwas written in the “C” programming language primarily fortwo reasons. First, it is efficient, both in memory and speedwhich enables the inference program to run on small computers,such as IBM PC compatibles. Secondly, it is portable acrossa variety of computers. Thus, switching of the program toanother CAD package can be sorted out without muchdifficulty.The Unigraphics system is a very powerful CAD software.It possesses many internal user functions to facilitate program-ming and enhance program capability in graphic displays. Itis also fully compatible with the most widely used flow simulation software “MOULDFLOW”. This feature allows forthe future expansion of the program to link up with MOULD-FLOW to provide a more comprehensive mould designknowledge-based system.5.4 The Coding SystemThe code is a representation of all the necessary informationabout the plastic part such as shape, undercut features, material,and other specifications. In designing the coding system, con-sideration has been given to the characteristics of most injectionmoulded parts and moulds. The design of injection mouldsdepends critically on the shape and features of the product. Itis almost impossible to develop a unique definition for all theshapes of plastic parts as their shapes can vary infinitely.Nevertheless, in real-life the shapes of a large percentage ofcommonly encountered plastic parts can be approximated aseither rectangular or circular. For example, most of the plastichousings and casings are basically rectangular in shape; andmost of the knobs, buckets, and cups are basically circular inshape. The Opitz system [28], which is the popular classi-fication system for mechanical parts, has also been referred toin developing part of the coding system.The code is pided into three sections and has a total oftwelve digits. The first section which describes the plastic partcontains the first four digits.