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.
Ken Youssefi
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.
Ken Youssefi
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.
Ken Youssefi
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.
Ken Youssefi
UC Berkeley

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

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

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

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

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

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

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

15. 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.
Insertion from the top is preferred.
Ken Youssefi
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.
Ken Youssefi
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.
Ken Youssefi
UC Berkeley

18. DFM and DFA Design Guidelines
Ken Youssefi
UC Berkeley

19. Minimizing the Number of Parts
To determine whether it is possible to combine neighboring parts, ask yourself the following questions:
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.
Ken Youssefi
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
Ken Youssefi
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.
Ken Youssefi
UC Berkeley

22. DFM Design Guidelines Injection Molding
Fabrication of Plastics
Injection Molding
Ken Youssefi
UC Berkeley

23. DFM Design Guidelines Injection 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.
Ken Youssefi
UC Berkeley

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

25. DFM Design Guidelines Injection Molding
Keep section thickness uniform around bosses.
Provide smooth transition, avoid changes in thickness when possible.
Ken Youssefi
UC Berkeley

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

27. DFM Design Guidelines Rotational 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.
Ken Youssefi
UC Berkeley

28. Rotational Molding Machines
Turret machine
Vertical wheel machine
Shuttle machine
Rock and roll machine
Ken Youssefi
UC Berkeley

29. 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
Ken Youssefi
UC Berkeley

30. Rotational Molding Limitations
Can not make parts with tight tolerance.
Large flat surfaces are difficult to achieve.
Molding cycles are long (10-20 min.)
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).
Ken Youssefi
UC Berkeley

31. Rotational Molding
Nominal wall thickness
Polycarbonate wall thickness is typically between .06 to .375 inches, .125 inch being an ideal thickness.
Polyethylene wall thickness is in the range of .125 to .25 inch, up to 1 inch thick wall is possible.
Nylon wall thickness is in the range of .06 to .75 inch.
Ken Youssefi
UC Berkeley

32. Rotational Molding Examples
Ken Youssefi
UC Berkeley

33. Rotational Molding Examples
Ken Youssefi
UC Berkeley

34. DFM Design Guidelines Sheet-metal Forming
Ken Youssefi
UC Berkeley

35. DFM Design Guidelines Sheet-metal Forming
Ken Youssefi
UC Berkeley

36. DFM Design Guidelines Sheet-metal Forming
Ken Youssefi
UC Berkeley

37. DFM Design Guidelines - Casting
Casting, one of the oldest manufacturing processes, dates back to 4000 B.C. when copper arrowheads were made.
Casting processes basically involve the introduction of a molten metal into a mold cavity, where upon solidification, the metal takes on the shape of the mold cavity.
Simple and complicated shapes can be made from any metal that can be melted.
Example of cast parts: frames, structural parts, machine components, engine blocks, valves, pipes, statues, ornamental artifacts…..
Casting sizes range form few mm (teeth of a zipper) to 10 m (propellers of ocean liners).
Ken Youssefi
UC Berkeley

38. Casting Processes
Preparing a mold cavity of the desired shape with proper allowance for shrinkage.
Melting the metal with acceptable quality and temp.
Pouring the metal into the cavity and providing means for the escape of air or gases.
Solidification process, must be properly designed and controlled to avoid defects.
Mold removal.
Finishing, cleaning and inspection operations.
Ken Youssefi
UC Berkeley

39. Sand Casting Terminology
Ken Youssefi
UC Berkeley

40. Casting Defects
Hot spots – thick sections cool slower than other sections causing abnormal shrinkage. Defects such as voids, cracks and porosity are created.
Ken Youssefi
UC Berkeley

41. Casting Defects and Design Consideration
Ken Youssefi
UC Berkeley

42. DFM Design Guidelines - Casting
Recommended minimum section thickness
Ken Youssefi
UC Berkeley

43. DFM Design Guidelines - Casting
Ken Youssefi
UC Berkeley

44. DFM Design Guidelines – Machining
Ken Youssefi
UC Berkeley