DFMA Notes on Selection of Materials and Processes

Lesson 247 of IEKC Industrial Engineering ONLINE Course Notes.

Engineering in Industrial Engineering -  Machine work study or machine effort improvement, value engineering and design for manufacturing and assembly are major engineering based IE methods. All are available as existing methods.

Product Design for Manufacture and Assembly, Third Edition

Geoffrey Boothroyd, Peter Dewhurst, Winston A. Knight

CRC Press, 08-Dec-2010 - Technology & Engineering - 712 pages

https://books.google.co.in/books/about/Product_Design_for_Manufacture_and_Assem.html?id=W2FDCcVPBcAC

Note: It is important to read the books by Boothroyd to understand the full method of DFMA. The DFMA method is to be combined with Value Analysis and Engineering to do product industrial engineering. In the note only attempt is made to make readers aware of issues raised and solutions proposed by DFMA method.

2. Selection of Materials and Processes 43

2.1 Introduction 

2.2 General Requirements for Early Materials and Process Selection 

2.3 Selection of Manufacturing Processes 

2.4 Process Capabilities 

2.5 Selection of Materials 

2.5.1 Grouping of Materials into Process Compatible Classes

2.5.2 Material Selection by Membership Function

2.5.3 Material Selection by Dimensionless Ranking 

2.6 Primary Process/Material Selection 

2.7 Systematic Selection of Processes and Materials 

2.7.1 Computer-Based Primary Process/Material Selection 

2.7.2 Expert Processing Sequence Selector 

2.7.3 Economic Ranking of Processes

References 

This topic is covered in process planning subject also. Of course it is covered in the design of machine elements subject.

Selection of Materials and Processes

Designers tend to conceive parts in terms of the processes and materials with which they are most familiar. As a consequence,  more economic processes and process/material combinations may not be considered. According to a survey quoted by Boothroyd,   more than half of those surveyed professed little or no knowledge of metal extrusion, two-thirds knew little about glass-reinforced molding, and over three-quarters were uninformed about plastic extrusion, technical blow molding, and sintering. For less common processes, such as hot isostatic pressing, outsert molding, and superplastic forming, the percentage of designers claiming some process knowledge was only 6, 7, and 8, respectively. Similar results were found for materials showing a surprising lack of familiarity with even many more common materials. The overall implication of these findings is that because material and process combinations are likely to be chosen from those with which designers are most comfortable, the possibilities of using other processes that may be much more cost effective may be missed.

GENERAL REQUIREMENTS FOR EARLY MATERIALS AND PROCESS SELECTION 

In order to be of real design value, the information on which the initial selection of material/process combinations and their ranking is to be based should be available at the early concept design stage of a new product. Such information might include, for example:

Product life volume

Permissible tooling expenditure levels

Possible part shape categories and complexity levels

Service or environment requirements

Appearance factors

Accuracy factors 

It is important to realize that for many processes the product and process are so intimately related that the product design must use an anticipated process as a starting point. In other words, many design details of a part cannot be defined without a consideration of processing. For this reason, it is crucial that an economic evaluation of competing processes be performed while the product is still at the conceptual stage. Such an early evaluation will ensure that every economically feasible process is investigated further before the product design evolves to a level where it becomes process specific. As a design progresses from the conceptual stage to production, different methods can be used to perform cost modeling of the product. At the conceptual stage, rough comparisons of the costs of products of similar size and complexity may be sufficient.  As the design progresses and specific materials and processes are selected, more advanced cost modeling methods may be employed.

These may be particularly useful in establishing the relationship between design features and manufacturing costs for the chosen process. 

Relationship to Process and Operations Planning

There is an obvious relationship between the initial selection of process/material combinations and process planning. During process planning the detailed elements of the sequence of manufacturing operations and machines are determined. It is at this stage that the final detailed cost estimates for the manufacture of the part are determined.  The initial decision on the material and process combination to be used for the part is most important, as this will determine the majority of subsequent manufacturing costs. The goal of systematic early material and process selection is to influence this initial decision on which combination to use, before detailed design of the part has been carried out and before detailed process planning is attempted.

SELECTION OF MANUFACTURING PROCESSES

The selection of appropriate processes for the manufacture of a particular part is based upon a matching of the required attributes of the part and the various process capabilities. Once the overall function of a part is determined, a list can be formulated giving the essential geometrical features, material properties, and other attributes that are required. This represents a "requirement list" that must be filled by the material properties and process capabilities. 

Most component parts are not produced by a single process, but require a sequence of different processes to achieve all the required attributes of the final part. The forming or shaping processes are used as the initial process, and then material removal and finishing processes are required to produce some or all of the final part features.  Experience shows that it is generally most economical to make the best use of the capabilities of the initial manufacturing process in order to provide as many of the required attributes of a part as possible.

There are hundreds of processes and thousands of individual materials. Moreover, new processes and materials are being developed continually. 

Processes can be categorized as:

Primary processes

Primary/secondary processes

Tertiary processes

In the context of producing component parts in this text, the term primary process will refer to the main shape-generating process. Such processes should be selected to produce as many of the required attributes of the part as possible and usually appear first in a sequence of operations. Casting, forging, and injection molding are examples of primary shape-generating processes.

Primary/secondary processes, on the other hand, can generate the main shape of the part, form features on the part, or refine features on the part. These processes appear at the start or later in a sequence of processes. This category includes material removal and other processes such as machining, grinding, and broaching.

Tertiary processes do not affect the geometry of the part and always appear after primary and primary/secondary processes. This category consists of finishing processes such as surface treatments and heat treatments. Selection of tertiary processes is simplified, because many tertiary processes only affect a single attribute of the part. For instance, lapping is employed to achieve a very good surface finish, and plating is often used to improve appearance or corrosion resistance.

PROCESS CAPABILITIES 

Included in the process capabilities of various manufacturing processes are shape features that can be produced, natural tolerance ranges, surface roughness capabilities, and so on. These capabilities determine whether a process can be used to produce the corresponding part attributes. 

Manufacturing processes have varying levels of compatibility with the basic goals of DFA of simplified product structure and ease of assembly. This also needs to be compiled.

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https://www.youtube.com/watch?v=ZFLO-xn26gA

Part 2 - https://www.youtube.com/watch?v=JHWgnuwrD7c

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SELECTION OF MATERIALS

Some textbooks and handbooks are available,  for example, the detailed handbook system of the Fulmer Research Institute in the United Kingdom. Software systems based upon comprehensive databases of material properties are available, such as the Mat.DB system from the American Society of Metals  and the Cambridge Material Selector.  

These procedures are aimed at the selection of specific materials based on detailed material property specifications  Several approaches can be adopted to rationalize the search for suitable materials for application during early product design.

2.5.1 Grouping of Materials into Process Compatible Classes

Rather than presenting a single comprehensive materials database, it is preferable to divide the material databases into classes related to the principal shape-generating processes used in discrete parts manufacture.  The separate material databases should include, for example, standard metal stockforms (wire, rod, etc.), sand and permanent mold-casting alloys, die-casting alloys, metal powders, thermoplastics granules, thermoplastic sheet and extruded stockforms, and so on.

A more efficient procedure at the stage of conceptual design is to have, for each process, an associated supermaterial specification that comprises the best attainable properties of all the materials in the corresponding category. If a new alloy is added, say, to the die-casting material database,  the diecasting supermaterial specification is  updated as necessary. 

The approach to preliminary process selection through supermaterial specification is compatible with the tradeoff and compromise decisions that are part of early design work. A typical scenario might involve the specification of possible shape attributes, size, and one or more production and performance parameters. The next step would be to change the input specifications or add to the specification list, or to investigate a process further for acceptable associated materials.

2.5.2 Material Selection by Membership Function

Material selection can be done using fuzzy logic models with qualifiers for suitable materials as "about" and "in the neighborhood of" the given requirements  Fuzzy logic relies on the concept of a membership function to determine how well an object fits into a defined set. A simple example may help to illustrate the advantages and flexibility of this approach. For instance, if pressing and sintering has been selected as a candidate primary process and the user has restricted the material to one with an ultimate tensile strength between 35 and 40 kpsi, then a conventional search may  yield 15 candidate materials. A fuzzy search with the qualifier "close to" would yield 29 candidate materials.  . The qualifier "approximately" produces 38 materials.  These additional materials may make the designer think in terms of choosing better economic materials that could satisfy the design requirements.  

2.5.3 Material Selection by Dimensionless Ranking 

An aspect of material selection, which is a great source of difficulty, is the distinction between the fundamental material properties, which are given in material databases, and the actual design requirements, which are usually based on a combination of different property values. For the present purposes, material cost per unit weight will be included as a property of the material, so that economic constraints on design can be considered in just the same manner as weight constraints, strength constraints, and so on. Thus, for a structural member in an aerospace product the designers may be interested in maximum stiffness per unit weight, while for a high-volume consumer product, maximum stiffness per unit cost may be more important. In the first case, the materials would be compared on the basis of a function of Young's modulus divided by density, and in the second case a combination of Young's modulus, density, and cost per unit weight would be used for comparison purposes. Some derived parameters, which are commonly used in mechanical design, have been established in the literature. A  procedure for comparing materials based on either single fundamental properties, or the general form of derived parameters is required. Such material comparisons may typically be required on the basis of total performance, best performance per unit weight, or best performance per unit cost. 

PRIMARY PROCESS/MATERIAL SELECTION

Systematic procedures can be developed for the selection of primary process/material combinations. Such procedures operate by eliminating processes and materials as more detailed specification of the required part's attributes occurs. 

SYSTEMATIC SELECTION OF PROCESSES AND MATERIALS

The development of computer-based procedures for process/material selection from general part attributes can have a significant impact on early product design, and several approaches to this problem have been made.

1 Computer-Based Primary Process/Material Selection 

Wilson and co-workers  developed a Fortran-based computer program given the acronym MAPS. A more recently developed primary material/process selector uses a commercially available relational database system. This selector has the acronym CAMPS, for computer-aided material and process selection. In the selector, inputs made under the headings of part shape, size, and production parameters are used to search a comprehensive process database to identify processing possibilities. However, process selection completely independent of material performance requirements would not be satisfactory, and for this reason, required performance parameters can also be specified by making selections under the general categories of mechanical properties, thermal properties, electrical properties, and physical properties. As many selections as required can be made, and at each stage the candidate processes are presented to the system user. Processes may be eliminated directly because of shape or size, or when performance selections eliminate all of the materials associated with a particular process. For the materials in the CAMPS system for each process, the type of supermaterial specification described above is utilized. The supermaterial specifications are maintained automatically by the program.

The CAMPS system also classifies all possible selections into ranges labeled A through F. This is intended to ensure ease of use in the early stages of design when precise numerical values for many of the parameters would not be known.

2 Expert Processing Sequence Selector 

An approach to the preliminary selection of materials and processes has been described above. While this approach may generally result in selection of appropriate combinations of materials and primary processes, in some cases matching of the material and primary process alone to the finished part attributes, without considering viable sequences of operations, may lead to the omission of some appropriate combinations of primary processes and materials. An expert processing sequence generator has been investigated to enhance this aspect of material and process selection.

With this procedure the user classifies the geometry and specifies the material constraints for the part. The result is a list of viable sequences of processes and compatible materials. The procedure is divided into four steps: geometry input, process selection, material selection, and system update. The geometry of the part is first classified according to its size, shape, cross section, and features. Using pattern-matching rules, processes are then selected that would form the geometry of the part. Material selection uses fuzzy set theory materials. 

The geometrical classification of a part is concerned with the following characteristics: 1. The overall size 2. The basic shape 3. The accuracy and surface finish 4. The cross section 5. Functional features—projections, depressions, etc.

As described earlier, processes are classified as either primary, primary/secondary, or tertiary to take advantage of the natural order of processes in a sequence. Rules, formulated from knowledge about processes and materials, are used to select sequences of processes and materials for part manufacture.

Next, the material database is searched for the primary process selected and uses the fuzzy logic approach described earlier to choose candidate materials. Since the properties of a material are related to how the material is processed, each process has its own material database. Materials are selected by mapping the user's input onto the material properties. Material properties that can be affected by tertiary manufacturing processes are not used to exclude materials from consideration, at this stage. For example, corrosion resistance could be achieved by plating an otherwise unacceptable material.

Primary/secondary processes are selected in a similar manner to form any features of the part that cannot be formed by the primary process. Similarly, tertiary processes are selected to fulfill material requirements that the candidate material could not fulfill. A viable sequence of processes is found when all of the geometrical and material goals specified by the user are satisfied. 

Economic Ranking of Processes

The viable material/process combinations determined by the selection procedures described above require evaluation as to which is the most suitable, usually by estimating which will be the most economic. This requires the availability of procedures for realistically evaluating manufacturing costs early in the design process. Several of the later chapters deal with simplified cost-estimating procedures for various processes. However, at the very early design stages even simpler methods for cost assessment can be used for the ranking of alternative material/process combinations. As an example of how early cost estimates can be made for a particular process, machining will be considered. From a detailed analysis of cost estimating for machined parts  it can be concluded that, in general, the time to remove a given volume of material in rough machining is determined mainly by the specific cutting energy (or unit power) of the material and the power available for machining. For finish-machining a given surface area, the recommended speeds and feeds for minimum machining cost could be used. Also, it is possible to make appropriate allowances for tool replacement costs.

A large amount of statistical data is available on the shapes and sizes of machined components and the amount of machining carried out on them. Statistical data is also available on the sizes of the machine tools relative to the sizes of the workpieces machined. Combining this data with information gathered on machine costs and power availability, it can be shown that estimates of machined component costs can be made based on the minimum of design information, such as might be readily available early in the design process.

The information required can be divided into three areas: 1. Workpiece and production data 2. Factors affecting nonproductive costs 3. Factors affecting machining times and costs

The first item under the heading of workpiece and production data describes the shape category of the workpiece. It was found in previous studies that common workpieces can be classified into seven basic categories. Other items under this first heading include: the material, the form of the material (standard stock or near-net shape), dimensions of the workpiece, cost per unit weight, average machine and operator rate, and batch size per setup. A knowledge of the workpiece and production data not only allows the cost of the workpiece to be estimated, but also allows predictions to be made of the probable magnitudes of the remaining items necessary for estimates of nonproductive costs and machining costs.

A comparison of different estimates  for several other workpieces reveals that the initial crude estimate based on typical workpieces is quite accurate and probably sufficient for the purposes of early cost estimating when various material and process combinations are being considered and before the component has been designed. However, these considerations should preferably occur after the proposed product has been simplified as much as possible, through design for assembly analysis.

This example for machining illustrates that it is possible to obtain reliable cost estimates based on some initial general design information and that such cost estimates can be refined as more detailed design information becomes available.

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https://www.youtube.com/watch?v=9WR2l3L-thU

Part 2: https://www.youtube.com/watch?v=emWDFW1Cj-I

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Farag, M.M., Materials and Process Selection in Engineering, Applied Science Publishers, Barting, United Kingdom, 1979.   4th Edition - 2021 - Details

Crane, F.A.A., and Charles, J.A., Selection and Use of Engineering Materials, Butterworths, London, 1984. 

Hamley, D.P., Introduction to the Selection of Engineering Materials, Van Nostrand Reinhold, New York, 1980. 

Fulmer Institute, Fulmer Materials Optimiser, Fulmer Institute, Stoke Poges, United Kingdom, 1975. 

American Society of Metals, Mat.DB User's Manual, ASM International, Cleveland, Ohio, 1990.

Cambridge Materials Selector Software, Granta Design Limited, Cambridge, United Kingdom, 1998.

Ud. 11.12.2021

 Pub. 29.11.2021