Design for Manufacturing and Assembly

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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.

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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.

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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

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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.

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DFM and DFA Design Guidelines

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DFM and DFA Design Guidelines

• Design to allow assembly in open spaces, not

confined spaces. Do not bury important

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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

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DFM and DFA Design Guidelines

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DFM and DFA Design Guidelines

• Design parts so they do not tangle or stick to each

other.

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DFM and DFA Design Guidelines

• Distinguish different parts that are shaped

similarly by non-geometric means, such as color

coding.

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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

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DFM and DFA Design Guidelines

• Design parts with orienting features to make

alignment easier.

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DFM and DFA Design Guidelines

• Provide alignment features on the assembly

so parts are easily oriented.

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DFM and DFA Design Guidelines

• Design the mating parts for easy insertion. Provide

allowance on each part to compensate for

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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.

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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.

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DFM and DFA Design Guidelines

• Joining parts can be done with fasteners (screws,

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

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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.

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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;

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.

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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.

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DFM Design Guidelines

Injection Molding

Injection Molding

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DFM Design Guidelines

Injection Molding

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.

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DFM Design Guidelines

Injection Molding

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.

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DFM Design Guidelines

Injection Molding

Provide smooth transition, avoid changes in thickness when possible.

Keep section thickness uniform around bosses.

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DFM Design Guidelines

Injection 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 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.

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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.

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Rotational Molding Machines

Rock and roll machine Vertical wheel machine

Turret machine

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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.

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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).

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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.

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DFM Design Guidelines

Sheet-metal Forming

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DFM Design Guidelines

Sheet-metal Forming

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DFM Design Guidelines

Sheet-metal Forming

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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)

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Casting Processes

1. 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.

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Casting Defects

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

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Casting Defects and Design

Consideration

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