340 Plastics Engineered Product Design Vehicle Oil Pun The oil pan is a critical component in auto and truck applications because any failure would be catastrophic. It must withstand high temperature and impact, keep the bolt torque intact for good sealing, resist vibration and abuse, especially during assembly, and in many instances, functions as a structural member. The pan is divided into basic elements and analyzed as follows: (1) sides are considered to be flat plates uniformly loaded with rigid supports. (2) flanges are analyzed as beams on elastic foundations because of the gasket/metal inter- facing, and (3) bottom section is considered as a plate but needs to withstand high impact loads (dynamic loading). After the bolt torque is established, the washer size must be determined by computation based upon compressive and shear stresses and flange deflection. The flange thickness will also be determined fkom calculations. After it has been established that these values are safe, the remaining sections such as the sides and bottom can now be designed for thickness and shape based on the values of the static and/or dynamic loads (impact) supplied as part of the input data. If some elements show either high stress or excessive deflection (1% elongation in the elastic range) ribs or gussets can be added and/or the wall thickness increased in selective areas only. History has shown that theoretical analysis yields a design very close to the final manufactured product and meets intended performance requirements dictated by laboratory as well as field testing. Attuchwent There will be cases where attachments become necessary. One of the most frequent types is threaded parts. In most instances, these fasteners are used repeatedly to attach or remove components. To ensure that the reliability and the life of the frequently used joint are maximized, a threaded insert is used rather than threading the plastic material. In this manner, the wall thickness of the insert and plastic part is reduced, or, if a boss is used, it is smaller in diameter and shorter in height compared to its plastic counterpart. Since ketal is much larger than a higher torque can be applied (shear stress is also much larger). Extremely close toleranccs such as might be necessary for a seal or a bearing may not be within the capabilities of the STX molding process. Machining is not recommended because it would break the nylon surface and expose the glass fibers that then act as wicks for fluid. An aluminum insert that has been finish machined is used as a substitute. Since the ratios of the moduli are quite large (about 20:l for steel and 1O:l for aluminum), there will be no deformation of the metal insert and only the plastic will be highly stressed. It is most important that 4 - Product design 341 these stresses are calculated so that the boss does not split during assembly or that the metallic ring does not become loose because of long term relaxation effects. Design ~ limitation and constraint - As reviewed throughout this book, designing acceptable products requires knowledge of the different plastics and their processing limitations such as individual advantages and disadvantages. Although there is no limit theoretically to the shapes that can be created, practical considerations must be met such as available and size of processing equipment and cost. These relate not only to the product design, but also the mold or die design, since they must be considered as one entity in the total creation of a usable, economically feasible product. One of the earliest steps in product design is to establish the configuration that will form the basis on which strength calculations will be made and a suitable material selected to meet the anticipated requirements. During the sketching and drawing phase of working with shapes and cross-scctions there are certain design features with plastics that have to be kept in mind to obtain the best cost-performances and avoid degradation of the properties. Such features may be called property detractors or constraints. Most of them are responsible for the unwanted internal stresses that can reduce the available stress level for load- bearing products. Other features may be classified as precautionary measures that may influence the favorable performance of a product if they are properly incorporated. As an example a weld line(s) can exist in a product that could have met design requirements if the weld line(s) did not develop. The designer did not contemplate the potential for weld line (s). However the person designing the mold took a logical approach to simplify its construction and reduce cycle time to mold the product based on thc requirements specified for the product. Result was in reducing the cost of the mold and fabrication time. To meet this objective the design of the mold causcd one or more weld lines to develop in the product. With conventional injection molding, molded products can be designed that create unwanted weld line(s). The so-called line forms when two melt flow fronts meet during the filling of an injection mold cavity. This action can also occur during extrusion through a die, etc. Depending on how the weld line forms it could have very little strength and under the most ideal molding conditions it may obtain up to possibly 85% strength retention. To eliminate any problem the product requirements 342 Plastics Engineered Product Design have to account for loss in properties ifweld line(s) occur or specie that no weld lines are to occur. Weld lines are also called knit lines. During processing, such as by injection molding and extrusion, weld lines can occur. They can form during molding when hot mclts meet in a cavity because of flow patterns caused by the cavity configuration or when there are two or more gates. With extrusion dies, such as those with “spiders” that hold a center metal core, as in certain pipe dies, the hot melt that is separated momentarily produces a weld line in the direction of the extrudate and machine direction. The results of these weld lines could be a poor bond at the weld lines, dimensional changes, aesthetic damages, a reduction of mechanical properties, and other such conditions. The top set has a single gate for each specimen, the center set has double gates that are opposite each other for each specimen, and the bottom set has fan gates on the side of each specimen. The highest mechanical properties come with the top set of specimens, because of its melt orientation being in the most beneficial direction. The bottom set of specimens, with its flow direction being limited insofar as the test method is concerned, results in lower test data performance. With the double-gated specimens (the center sct) weld lines develop in the critical testing area that usually results in this set’s having the potential lowest performance of any of the specimens in this diagram. Fabricating techniques can be used to reduce this problem in a product. However, the approach used in designing the product, particularly its mold (relocate gates), is most important to eliminate unwanted orientation or weld lines. This approach is no different from that of designing with other materials like steel, aluminum, or glass. With moldings that include openings (holes), problems can develop. In the process of filling a cavity the flowing melt is obstructed by the core, splits its stream, and surrounds the core. The split stream then reunites and continues flowing until the cavity is filed. The rejoining of the split streams forms a weld line. It lacks the strength properties that exist in an area without a weld line because the flowing material tends to wipe air, moisture, and/or lubricant into the area where the joining of the stream takes place and introduces foreign substances into the welding surface. Furthermore, since the plastic material has lost some of its heat, the temperature for self-welding is not conducive to the most favorable results. A surface that is to be subjected to load bearing should be targeted not to contain weld lines. If this is not possible, the allowable working stress should be reduced by at least 15%. Under the ideal molding conditions up to about 85% of available strength in the solidified plastic can be developed. At the other extreme where poor process controls exist the weld line could approach zero strength. In fact the two melt fronts could just meet and not blend so that there is relatively a microscopic space. Other problems occur such as influencing aesthetics. Prior to designing a product, the designer should understand such basic factors as those reviewed in this book. Recognize that success with plastics, or any other material for that matter, is directly related to observing design details. The important factors to consider in designing can be categorized as follows: shape, part thickness, tolerances, ribs, bosses and studs, radii and fillets, drafts or tapers, holes, threads, colors, surface finishes and gloss levels, decorating operations, parting lines, shrinkages, assembly techniques, production volumes, mold or die designs, tooling and other equipment amortization periods, as well as the plastic and process selections. The order that these factors follow can vary, depending on the product to be designed and the designer's familiarity with particular materials and processes. F COMPUTER-AIDED DESIGN b Technoloqy overview Computer technology requires a completely different methodology of engineering design. It has revolutionized the speed and efficiency of the plastic design functions. The more the entire design function is studied, the more repetitive tasks are uncovered in that function. The computer’s ability to perform these tasks untiringly and with blazing speed is the basis for these productivity gains. The computer continues to provide the engineer with the means to simplifjr and more accurately develop a design timewise and costwise. It provides a better understanding of the operating requirements for a product design, resulting in maximizing the design efficiency in meeting product requirements. The computer is able to convert a design into a fabricated product providing a faster manufacturing startup. Other benefits resulting from the computer technology include (1) ease of developing and applying new innovative design ideas, (2) fewer errors in drawings; (3) good communications with the fabricator, (4) improved manufacturing accuracy; and (5) a faster response to market demand. Many of the individual tasks within the overall design process can be performed using a computer. As each of these tasks is made more efficient, the efficiency of the overall process increases as well. The computer is suited to aid the designer by incorporating customer inputs, problem definitions, evaluations, and final product designs. Computer-aided design (CAD) uses the mathematical and graphic- processing power of the computer to assist the mechanical engineer in the creation, modification, analysis, and display of designs. Many factors have contributed to CAD technology becoming a necessary tool in the 5 - Computer-aided design 345 .” * XI engineering world, such as the computer’s speed at processing complex equations and managing technical databases. CAD combines the characteristics of designer and computer that are best applicable to the design process. There is also the combination of human creativity with computer technology that provides the design efficiency that has made CAD such a popular design tool. CAD is often thought of simply as computer- aided drafting, and its use as an electronic drawing board is a powerhl tool in itself. The functions of a CAD system extend far beyond its ability to represent and manipulate graphics. Geometric modeling, enginccring analysis, simulation, and communication of the design information can also be performed using CAD. In every branch of engineering, prior to the implementation of CAD, design has traditionally been accomplished manually on the drawing board. The resulting drawing, complete with significant details, was then subjected to analysis using complex mathematical formulae and then sent back to the drawing board with suggestions for improving the design. The same procedure was followed and, because of the manual nature of the drawing and the subsequent analysis, the whole procedure was time-consuming and labor-intensive. For many decades CAD has allowed the designer to bypass much of the manual drafting and analysis that was previously required, making the design process flow more smoothly and much more efficiently. It is helpful to understand the general product development process as a step-wise process. However, in today’s engineering environment, the steps outlined have become consolidated into a more streamlined approach called concurrent engineering. This approach enables teams to work concurrently by providing common ground for interrelated product development tasks. Product information can be easily communicated among all develop- ment processes: design, manufacturing, marketing, management, and supplier networks. Concurrent engineering recognizes that fewer alterations result in less time and money spent in moving from design concept to manufacture and from manufacturing to market. The related processes of computer-aided engineering (CAE), computer-aided manufacturing (CAM), computer-aided assembly (CAA), computer- aided testing (CAT), and other computer-aided systems have become integral parts of the concurrent engineering design approach. Design for manufacturing and assembly methods use cross- disciplinary input from a variety of sources (design engineers, manufacturing engineers, materials & equipment suppliers, and shop floor personnel) to facilitate 346 Plastics Engineered Product Design - the efficient design of a product that can be manufactured, assembled, and marketed in the shortest possible period of time. CAD, CAE, CAM, CAA, and CAT are the directions all types of plastics product design, mold or die making, and the fabricating line. The number and complexity of plastic products being produced are greater every year, but the number of experienced product designers, mold/die designers, and fabricators generally have not kept pace. The answer to this “pcople power” shortage has been to increase “design to productivity” through the use of CAD/CA.E/CAM/CAA/CAT. Computers and people - -~ - v YI Computers have their place but most important is the person involved with proper knowledge in using and understanding its hardware and software in order to operate them efficiently. The computer basically supports rather routine tasks of embodiment and detailed operation rather than the human creative activities of conceptual human operation. Recognize that if the computer can do the job of a designer, fabricator, and others there is no need for these people. The computer is another tool for the designer, fabricator, and others to use. It makes it easier if one is knowledgeable on the computer’s software capability in specific areas of interest such as designing simple to complex shapes, product design of combining parts, material data evaluation, mold design, die design, finite element analysis, etc. By using the computer tools properly, the results are a much higher level of product designing and processing that will result in no myths. Successhl products require the combination of various factors that includes sound judgment and knowledge of processing. Until the designer becomes familiar with processing, a fabricator must be taken into the designer’s confidence early in development and consulted fi-equently. It is particularly important during the early design phase when working with conditions such as shapes and sizes. There are certain features that have to be kept in mind to avoid degradation of plastic properties. Most of these detractors or constrains are responsible for the unwanted internal stresses that can reduce the available stress for load bearing purposes. The industrial production process as practiced in today’s business is based on a smooth interaction between regulation technology, industrial handling applications, and computer science. Particularly important is computer science because of the integrating functions it performs that includes the tool manufacture, primary processing 5 * Computer-aided design 347 equipment, auxiliary equipment, material handling, and so forth up to business management itseg. This means that CIM (computer-integrated manufacturing) is very realistic to maximize reproducibility that results in producing successful products. The use of computers in design and related fields is widespread and will continue to expand. It is increasingly important for designers to keep up to date continually with the nature and prospects of new computer hardware and software technologies. For example, plastic databases, accessible through computers, provide product designers with up-dated property data and information on materials and processes. To keep material selection accessible via computer terminal and a modem, there are design database that maintain graphic data on thermal expansion, specific heat, tensile stress and strain, creep, fatigue, programs for doing fast approximations of the stiffening effects of rib geometry, educational information and design assistance, and more. Today’s sofnvare developers are laced with a serious challenge con- cerning how to produce a safe and reliable product in the shortest possible time frame. This is not a new problem; it has simply been exaggerated in recent years by pressures from the marketplace, and the manufacturing industry certainly is not immune to those pressures. Manufacturers including throwing large budgets into software develop- ment tools and manpower have sought many solutions. Geometric modeling Geometric modeling is one of the major uses of the CAD systems. It uses mathematical descriptions of geometric elements to facilitate the representation and manipulation of graphical images on the computer’s screen. While the central processing unit (CPU) provides the ability to quickly make the calculations specific to the element, the sofnvare provides the instructions necessary for efficient transfer of information between user and the CPU. There are three types of commands used by the designer in CAD geometric modeling. Its first allows the user to input the variables needed by the computer to represent basic geometric elements such as points, lines, arcs, circles, splines, and ellipses. The second is used to transform these elements that include scaling, rotation, and translation. The third allows the various elements previously created by the first two commands to be joined into a desired shape. During the whole geometric modeling process, mathematical operations are at work that can be 348 Plastics Engineered Product Design easily stored as computerized data and retrieved as needed for review, analysis, and modification. There are different ways of displaying the same data on the CRT (cathode ray tube) screen, depending on the needs or preferences of the designer. One method is to display the dcsign as a 2-D representation of a flat object formed by interconnecting lines. Another method displays the design as a 3-D view of the product. In 3-D representations, there are the four types of modeling of wireframe modeling, surface modeling, solid modeling, and hybrid solid modeling. The wireframe model is a skeletal description of a 3-D part. It consists only of points, lines, and curves that describe the geometric boundaries of the object. There are no surfaces in a wireframe model. The 3-D wireframe representations can be conhsing because all of the lines defining the object appear on the 2-D display screen. This makes it difficult for the viewer to tell whether the model is being viewed from above or below, inside or outside. It is the simplest of the CAD/CAM modeling methods. The siniylicity of this modeling method also implies simplicity in the database. With the surface modeling one defines not only the edge of the 3-D part, but also its surface. One of its major benefits is that it allows mass- related properties to be computed for the product model (volume, surface area, moment of inertia, etc.) and allows section views to be automatically generated. The surface modeling is more sophisticated than wireframe modeling. In surface modeling, there are the two different types of surfaces that can be generated: faceted surfaces using a polygon mesh and true curve surfaces. NLTRBS (Non-Uniform Rational B-Spline) is a B-spline curve or surface defined by a series of weighted control points and one or more knot vectors. It can exactly represent a wide range of curves such as arcs and cones. The greater flexibility for controlling continuity is one advantage of NURBS. It can precisely model nearly all kinds of surfaces more robustly than the polynomial- bascd curves that were used in earlier surface models. The computer still defines the object in terms of a wireframe but can generate a surface to cover the frame, thus giving the illusion of a real product. However, because the computer has the image stored in its data as a wireframe representation having no mass, physical properties cannot be calculated directly from the image data. Surface models are very advantageous due to point-to-point data collections usually required for numerical control (NC) programs in CAM applications. Most surface modeling systems also produce the stereolithographic data required for rapid prototyping systems. An important technique is the solid modeling that defines the surfaces of a product with the added advantages of volume and mass. It takes the surface model one step hrther in that it assures that the product being modeled is valid and realizable. This allows image data to be used in calculating the physical properties of the final product. Solid modeling sohare uses one of two methods: constructive solid geometry (CSG) or boundary representation (B-rep). CSG method uses engineering Boolean operations (union, subtraction, and intersection) on two sets of objects to define composite models. B-rep is a representation of a solid model that defines a product in terms of its surface boundaries that are faces, edges, and vertices. Hybrid solid modeling allows the user to represent a product with a mixture of wireframe, surface modeling, and solid geometry. By using CAD software, its hidden-line command can remove the background lines of the part in a model. Certain features have been developed to minimize the ambiguity of wireframe representations. These features include using dashed lines to represent the background of a view, or removing those background lines altogether. This hidden- line removal feature makes it easier to visualize the model because the back faces are not displayed. Shading removes hidden lines and assigns flat colors to visible surfaces. Rendering adds and adjusts lights and materials to surfaces to produce realistic effects. Shading and rendering can greatly enhance the realism of the 3-D image. I_-___II_ Design accuracy and efficiency - - - I_ I___ l_l *il”r”i.T CAD permits reviewing a design quickly and permits ease in accomplishing the design evaluation. Design accuracy can be checked using automated tolerancing and dimensioning routines to reduce the possibility of error. Layering is a technique that allows the designer to superimpose images upon one another. This can be quite useful during the evaluative stage of the design process by allowing the designer to check the dimensions of a final design visually against the dimensions of stages of the design’s proposed fabricator, ensuring that sufficient material is present in preliminary stages for the correct fabrication. CAD permits checking on interference potential problems. This pro- cedure involves making sure that no two parts of a design occupy the same space at the same time. Automated drafiing capabilities in CAD systems facilitate the design presentation, which is the final stage of the design process. CAD data, stored in computer memory, can be sent to a [...]... Shawbury, Shrewsbury, Shropshire SY4 4NR, U.K., tel: +44 -19 39-250-383, 358 Plastics Engineered Product Design Fax: +44 -19 39-2 51- 1 http://www.rapra.net 18 , PIASPEC It is a Materials Selection Database tel: 212 -592-6570, http://www.plaspec com Plastics Desi@ Lilwary The PDL Electronic Databooks (also available in hardcopy) provide properties of thermoplastics, elastomers, and rubbers The world’s largest... require approximately 15 0 MB, and the operating system itself usually requires about 10 0 MB A 16 inch highresolution (10 24 x 768), 256-color monitor should also be considered 374 Plastics Engineered Product Design a minimum requirement CD-ROM drives and fast modems with transfer rates of 28,800 band are essential for non-networked tasks For 3-D modeling and FEA applications, a Pentium 10 0 MHz processor... Hoechst, and Hulls; followed with Dow, GE, Ciba, etc The CAMPUS Plastics Database is a registered trademark of CWFG GmbH, Fr&rt/Main, Germany, tel: +49 2 41 963 14 50, Fax: +49 2 41 963 14 69, htip://www.CAMPUSplastics.com 356 Plastics Engineered Product Design CENBASE The database is available on CD-ROM, and contains the equivalent of over 15 0,000 pages of data http://www.centor.com/ cbmat/ DART A diagnostic... 704-5 71- 4005) Polymer Software PC-based polymer research tools DTW Associates, Inc P.O Box 916 , Ardmore, PA 19 003 USA, tel: 1 610 642 0380, Fax: +16 10 642 2599, http://www.dtwassociates.com ProHeZp XPM Features a powerfbl shop-floor algorithms scheduler that monitors machines in real time with Windows-based software, - 5 Cornputer-aided desiqn 359 updating its drag-and-drop bar charts during production... force concentrations in the section and determines if the material and design shape selected will meet product performance requirements In reviewing mechanical enginccring analysis, one can perform using one or two approaches, namely analytical or experimental Using the 362 Plastics Engineered Product Design analytical method, the design is subjected to simulated conditions, using any number of analytical... graphically Designing There are the practical and engineering approaches used to design products Both have their important place in the world of design With experience most products usually use the practical approach since they - 5 Computer-aided design 363 are not subjected to extreme loading conditions and require no computer analysis Experience is also used in producing new and complex shaped products... stages of design, CAD systems enable the designer to make more effective use of experimental data, especially where analytical methods are thought to be unreliable for the given model CAD also provides a useful platform for incorporating experimental results into the design process when experimental analysis is performed in earlier approaches of the process 368 Plastics Engineered Product Design P... to be reviewed A2ibP.e Design 3 - 0 Algor reports that it has added support for the Alibre Design 3-D parametric modeling package from Alibre, Inc The new software provides capabilities for opening Alibre design assembly a n d part geometry in Algor, a midplane mesh engine for converting thin solid features in a model to plate or shell elements, 354 Plastics Engineered Product Design and a joint creation... impacting surfaces Films change colors in proportion to the amount of pressure applied A 360 Plastics Engineered Product Design Window-based system scans and interprets the exposed film, rendering high definition, digital enhanced images, and statistical reports Sensor Products, Inc., East Hanover, NJ 07936 (tel 97388 417 55) Troubleshooting IM ProbLems Molders training programs fiom SME Supply Chain Software... PDLCOM Published by the Plastics Design Library, PDLCOM is an exhaustive reference source of how exposure environmeiits influence the physical characteristics of plastics http://www.nace.org/ naceframes/Store/pdlindex.htm PDM It is for product development management and training as opposed to product data or document management It extends CAD data to a manufacturing organization's non -design department such . U.K., tel: +44 -19 39-250-383, 358 Plastics Engineered Product Design Fax: +44- 19 39-25 1- 1 18 , http://www.rapra.net PIASPEC It is a Materials Selection Database tel: 212 -592-6570, http://www.plaspec. NC (tel. 704-5 71- 4005). Polymer Software PC- based polymer research tools. DTW Associates, Inc. P.O. Box 916 , Ardmore, PA 19 003 USA, tel: 1 610 642 0380, Fax: +1 610 642 2599, http://www.dtwassociates.com. CAMPUS Plastics Database is a registered trademark of CWFG GmbH, Fr&rt/Main, Germany, tel: +49 2 41 963 14 50, Fax: +49 2 41 963 14 69, htip://www.CAMPUSplastics.com 356 Plastics Engineered