Design for Manufacturing and Assembly

Ken Youssefi UC Berkeley 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. A primary strategy in DFM involves minimizing the number of parts in a product.

• Design For Assembly (DFA) involves making attachment directions and methods simpler.


DFM and DFA BenefitsIt 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.


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.


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.


DFM and DFA Design Guidelines

• Modularize multiple parts into single sub-assemblies.



• Design to allow assembly in open spaces, not confined spaces. Do not bury important components.


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.


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


DFM and DFA Design Guidelines• Design parts so they do not tangle or stick to each

other.



• Distinguish different parts that are shaped similarly by non-geometric means, such as color coding.


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


DFM and DFA Design Guidelines• Design parts with orienting features to make

alignment easier.



• Provide alignment features on the assembly so parts are easily oriented.


DFM and DFA Design Guidelines• Design the mating parts for easy insertion. Provide

allowance on each part to compensate for variation in part dimensions.


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.


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.


DFM and DFA Design Guidelines• Joining parts can be done with fasteners (screws,

nuts and bolts, rivets), snap fits, welds or adhesives.




Minimizing the Number of PartsTo 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.


Minimizing the Number of PartsThe 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;

1. A kinematic motion of the part is required.

2. A different material is required.

3. 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


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.


DFM Design GuidelinesInjection Molding

Injection MoldingFabrication of Plastics



Minimize section thickness, cooling time is proportional to the square of the thickness, reduce cost by reducing the cooling time.

Provide adequate draft angle for easier mold removal.



Keep rib thickness less than 60% of the part thickness in order to prevent voids and sinks.

Avoid sharp corners, they produce high stress and obstruct material flow.



Provide smooth transition, avoid changes in thickness when possible.

Keep section thickness uniform around bosses.



• 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 in 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.

Standard thickness variation.


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.


Rotational Molding Machines

Rock and roll machine

Vertical wheel machine

Turret machine

Shuttle machine


Rotational MoldingAdvantages

• 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


Rotational MoldingLimitations

• 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).


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.


Rotational Molding Examples


Rotational Molding Examples


DFM Design GuidelinesSheet-metal Forming






DFM Design Guidelines - CastingCasting, 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).


Casting Processes1. Preparing a mold cavity of the desired shape with

proper allowance for shrinkage.

2. Melting the metal with acceptable quality and temp.

3. Pouring the metal into the cavity and providing means for the escape of air or gases.

4. Solidification process, must be properly designed and controlled to avoid defects.

5. Mold removal.

6. Finishing, cleaning and inspection operations.


Sand Casting Terminology


Casting DefectsHot spots – thick sections cool slower than other sections causing abnormal shrinkage. Defects such as voids, cracks and porosity are created.


Casting Defects and Design Consideration


DFM Design Guidelines - Casting


DFM Design Guidelines - Casting


DFM Design Guidelines – Machining

Design for Manufacturing and Assembly

Documents

rotational

rotational

dfa design

dfm design

metal forming

ken youssefi

uc berkeley

manufacturing