Design for Manufacturing and Assembly

1. Design for Manufacturing and Assembly • Design for manufacturing (DFM) is design based on minimizing the cost of production and/or time to market for a product, while maintaining an appropriate level of quality. The strategy in DFM involves minimizing the number of parts in a product and selecting the appropriate manufacturing process. • Design For Assembly (DFA) involves making attachment directions and methods simpler. UC Berkeley

2. DFM and DFA Benefits It reduces part count thereby reducing cost. If a design is easier to produce and assemble, it can be done in less time, so it is less expensive. Design for manufacturing and assembly should be used for that reason if no other. It increases reliability, because if the production process is simplified, then there is less opportunity for errors. It generally increases the quality of the product for the same reason as why it increases the reliability. UC Berkeley

3. DFM and DFA • DFM and DFA starts with the formation of the design team which tends to be multi-disciplinary, including engineers, manufacturing managers, cost accountants, and marketing and sales professionals. • The most basic approach to design for manufacturing and assembly is to apply design guidelines. • You should use design guidelines with an understanding of design goals. Make sure that the application of a guideline improves the design concept on those goal. UC Berkeley

4. DFM and DFA Design Guidelines • Minimize part count by incorporating multiple functions into single parts. Several parts could be fabricated by using different manufacturing processes (sheet metal forming, injection molding). Ask yourself if a part function can be performed by a neighboring part. UC Berkeley

5. DFM and DFA Design Guidelines • Modularize multiple parts into single sub-assemblies. UC Berkeley

6. DFM and DFA Design Guidelines • Design to allow assembly in open spaces, not confined spaces. Do not bury important components. UC Berkeley

7. DFM and DFA Design Guidelines • Parts should easily indicate orientation for insertion. Parts should have self-locking features so that the precise alignment during assembly is not required. Or, provide marks (indentation) to make orientation easier. UC Berkeley

8. DFM and DFA Design Guidelines • Standardize parts to reduce variety. UC Berkeley

9. DFM and DFA Design Guidelines • Design parts so they do not tangle or stick to each other. UC Berkeley

10. DFM and DFA Design Guidelines • Distinguish different parts that are shaped similarly by non-geometric means, such as color coding. UC Berkeley

11. DFM and DFA Design Guidelines • Design parts to prevent nesting. Nesting is when parts are stacked on top of one another clamp to one another, for example, cups and coffee lids UC Berkeley

12. DFM and DFA Design Guidelines • Design parts with orienting features to make alignment easier. UC Berkeley

13. DFM and DFA Design Guidelines • Provide alignment features on the assembly so parts are easily oriented. UC Berkeley

14. DFM and DFA Design Guidelines • Design the mating parts for easy insertion. Provide allowance on each part to compensate for variation in part dimensions. UC Berkeley

15. Insertion from the top is preferred. DFM and DFA Design Guidelines • Design the first part large and wide to be stable and then assemble the smaller parts on top of it sequentially. UC Berkeley

16. DFM and DFA Design Guidelines • If you cannot assemble parts from the top down exclusively, then minimize the number of insertion direction. Never require the assembly to be turned over. UC Berkeley

17. DFM and DFA Design Guidelines • Joining parts can be done with fasteners (screws, nuts and bolts, rivets), snap fits, welds or adhesives. UC Berkeley

18. DFM and DFA Design Guidelines UC Berkeley

19. To determine whether it is possible to combine neighboring parts, ask yourself the following questions: Minimizing the Number of Parts • Must the parts move relative to each other? • Must the parts be electrically or thermally insulated? • Must the parts be made of different material? • Does combing the parts interfere with assembly of other parts? • Will servicing be adversely affected? If the answer to all questions is “NO”, you should find a way to combine the parts. UC Berkeley

20. Minimizing the Number of Parts • The concept of the theoretical minimum number of parts was originally proposed by Boothroyd (1982). During the assembly of the product, generally a part is required only when; • A kinematic motion of the part is required. • A different material is required. • Assembly of other parts would otherwise be prevented. • If non of these statements are true, then the part is not needed to be a separate entity. KISS – Keep It Simple Stupid UC Berkeley

21. DFM Design Guidelines Another aspect of design for manufacturing is to make each part easy to produce. The up to date DFM guidelines for different processes should be obtained from production engineer knowledgeable about the process. The manufacturing processes are constantly refined. UC Berkeley

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42. DFM Design GuidelinesInjection Molding Fabrication of Plastics Injection Molding UC Berkeley

43. DFM Design GuidelinesInjection Molding Provide adequate draft angle for easier mold removal. Minimize section thickness, cooling time is proportional to the square of the thickness, reduce cost by reducing the cooling time. UC Berkeley

44. Avoid sharp corners, they produce high stress and obstruct material flow. DFM Design GuidelinesInjection Molding Keep rib thickness less than 60% of the part thickness in order to prevent voids and sinks. UC Berkeley

45. Keep section thickness uniform around bosses. DFM Design GuidelinesInjection Molding Provide smooth transition, avoid changes in thickness when possible. UC Berkeley

46. Standard thickness variation. DFM Design GuidelinesInjection Molding • Use standard general tolerances, do not tolerance; • Dimension Tolerance Dimension Tolerance • 0 ≤ d ≤ 25 ± 0.5 mm 0 ≤ d ≤ 1.0 ± 0.02 inch • 25 ≤ d ≤ 125 ± 0.8 mm 1 ≤ d ≤ 5.0 ± 0.03 inch • 125 ≤ d ≤ 300 ± 1.0 mm 5 ≤ d ≤ 12.0 ± 0.04 inch • 300 ± 1.5 mm 12.0 ± 0.05 inch • Minimum thickness recommended; • .025 inch or .65 mm, up to .125 for large parts. • Round interior and exterior corners to .01-.015 in radius (min.), prevents an edge from chipping. UC Berkeley

47. DFM Design GuidelinesRotational Molding Rotational molding process consists of six steps • A predetermined amount of plastic, powder or liquid form, is deposited in one half of a mold. • The mold is closed. • The mold is rotated biaxially inside an oven. • The plastics melts and forms a coating over the inside surface of the mold. • The mold is removed from the oven and cooled. • The part is removed from the mold. UC Berkeley

48. Turret machine Shuttle machine Rock and roll machine Rotational Molding Machines Vertical wheel machine UC Berkeley

49. Rotational Molding Advantages • Molds are relatively inexpensive. • Rotational molding machines are much less expensive than other type of plastic processing equipment. • Different parts can be molded at the same time. • Very large hollow parts can be made. • Parts are stress free. • Very little scrap is produced UC Berkeley

50. Materials Polyethylene (most common), Polycarbonate (high heat resistance and good impact strength), Nylon (good wear and abrasion resistance, good chemical resistance, good toughness and stiffness). Rotational Molding Limitations • Can not make parts with tight tolerance. • Large flat surfaces are difficult to achieve. • Molding cycles are long (10-20 min.) UC Berkeley