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MANUFACTURING PROCESSES

MANUFACTURING PROCESSES. The selection strategy for processes is similar to that for materials Design requirements should be put in terms of constraints and objectives to be used to screen, rank, and finally select an appropriate process.

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MANUFACTURING PROCESSES

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  1. MANUFACTURING PROCESSES

  2. The selection strategy for processes is similar to that for materials Design requirements should be put in terms of constraints and objectives to be used to screen, rank, and finally select an appropriate process

  3. Each of the three process families – shaping, joining, and surface treatment – has its own set of characteristic attributes One process attribute applies to all three families – compatibility with material

  4. There are limits to the size of a component that a process can make

  5. Along with mass, there is a limiting value for section thickness that a given shaping process can handle

  6. Casting and molding rely on material flow in the liquid or semi-liquid state Lower limits on section thickness are imposed by the physics of flow Flow of liquid metal or polymer into thin sections is opposed by surface tension and viscous forces Loss of heat into the mold increases viscosity and may result in too early solidification

  7. Upper limits to size are set by problems of shrinkage The outer layer of a casting or molding solidifies first, giving it a rigid skin When the interior solidifies, the change in volume can distort the product or crack the skin Keeping section thickness and cross-section uniform reduce the chances of differential shrinkage

  8. Metal shaping processes such as rolling, forging, or extrusion involve flow – solid metals flow by plastic deformation or creep The minimum thickness that can be achieved from these processes is limited by plastic flow Very thin sections cause substantial friction forces and stick to the tools, even at very large pressures

  9. A key attribute of a process is the families of shapes it can make There are three generic classes of shape, each subdivided in two

  10. Tolerance Dimension y specified as y = 100 ± 0.1 mm Roughness Measure of the irregularities of a surface specified as an upper limit such as R < 100 μm

  11. Processing costs for precision and surface finish increase exponentially as the requirements are made more severe

  12. Important joining-specific considerations are: • the geometry of the joint, • the thickness of the material they can handle, • the way the joint will be loaded

  13. The most important finishing-specific attribute is the function of the treatment Functions can range from protection, performance enhancement, or can be primarily aesthetic (for beauty)

  14. The manufacture of a component consumes resources, each of which has an associated cost The final cost is the sum of those of the resources it consumes

  15. Material: material cost and mass per unit; f is the scrap fraction for unused material Tooling: cost is dedicated and written off for production of n units; nt represents life of tool in case replacement is necessary Capital: non-dedicated cost; written off against time for a given hourly rate; t wo is the write-off time, L is the load factor, and ń is the production rate Overhead: gross overhead costs divided by the production rate

  16. The most economical process for a part depends on the production size At small batch sizes the unit cost is dominated by the ‘fixed’ costs of tooling As batch size increases, the variable costs of material, labor, and other overhead become dominant

  17. Each process has a range of batch sizes for which it is likely to be economically competitive

  18. Much of a cost model relies on user-defined parameters Approximate values are used for cost modeling in computer based modeling Default values can be replaced if more accurate data is available

  19. Figure 18.21 Three process groups for ceramics, each able to handle the required shape, mass, section thickness, tolerance, and roughness Constraint on economic batch size indicates powder methods as the preferred choice

  20. Screening the material, mass, and section thickness leads to three possible process: machining, injection molding, and compression molding All three processes can provide the proper surface quality, but a batch size of 50,000 is not economical for machining

  21. Figure 18.22 If a shaping process does not produce the required surface quality, a surface treatment process can be used in addition if the combined processes are more economical than a satisfactory single process For the con-rod, forging and powder methods do not provide adequate tolerance and roughness values – following with precision machining would satisfy the design requirements

  22. Figure 18.23 The final choice of process for the con-rod will be based on cost Based on a batch size of 10,000, forging is the cheapest process – as forging is likely to be followed by a machining process, the cost difference between forging and die casting is insignificant

  23. SUMMARY • Selecting a process involves 3 steps: • Translation of requirements • Screening of the possible options • Ranking of the processes • Further investigation of the ranked processes by detailed examination of the documentation is then performed.

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