Design%20for%20Manufacturing%20and%20Assembly

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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.

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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
  concept on those goal.

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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

• Modularize multiple parts into single
  sub-assemblies.

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DFM and DFA Design Guidelines

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

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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 easier.

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DFM and DFA Design Guidelines

• Standardize parts to reduce variety.

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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
  variation in part dimensions.

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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
  adhesives.

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DFM and DFA Design Guidelines
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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
• 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
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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 GuidelinesInjection Molding
Fabrication of Plastics
Injection Molding
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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.
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DFM Design GuidelinesInjection Molding
Keep rib thickness less than 60 of the part
thickness in order to prevent voids and sinks.
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DFM Design GuidelinesInjection Molding
Provide smooth transition, avoid changes in
thickness when possible.
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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.

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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.

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Rotational Molding Machines
Vertical wheel 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.

• Very little scrap is produced

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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.)

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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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Rotational Molding Examples
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Rotational Molding Examples
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DFM Design GuidelinesSheet-metal Forming
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DFM Design GuidelinesSheet-metal Forming
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DFM Design GuidelinesSheet-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) to 10 m (propellers of ocean liners).

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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.
6. Finishing, cleaning and inspection operations.

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Sand Casting Terminology
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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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DFM Design Guidelines - Casting
Recommended minimum section thickness
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DFM Design Guidelines - Casting
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DFM Design Guidelines Machining