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
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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.
Ken Youssefi
UC Berkeley
2
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
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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.
Ken Youssefi
UC Berkeley
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DFM and DFA Design Guidelines
• Modularize multiple parts into single sub-assemblies.
Ken Youssefi
UC Berkeley
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DFM and DFA Design Guidelines
• Design to allow assembly in open spaces, not
confined spaces. Do not bury important
components.
Ken Youssefi
UC Berkeley
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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.
Ken Youssefi
UC Berkeley
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DFM and DFA Design Guidelines
• Standardize parts to reduce variety.
Ken Youssefi
UC Berkeley
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DFM and DFA Design Guidelines
• Design parts so they do not tangle or stick to each
other.
Ken Youssefi
UC Berkeley
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DFM and DFA Design Guidelines
• Distinguish different parts that are shaped
similarly by non-geometric means, such as color
coding.
Ken Youssefi
UC Berkeley
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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
Ken Youssefi
UC Berkeley
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DFM and DFA Design Guidelines
• Design parts with orienting features to make
alignment easier.
Ken Youssefi
UC Berkeley
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DFM and DFA Design Guidelines
• Provide alignment features on the assembly
so parts are easily oriented.
Ken Youssefi
UC Berkeley
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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.
Ken Youssefi
UC Berkeley
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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.
Insertion from the top
is preferred.
Ken Youssefi
UC Berkeley
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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.
Ken Youssefi
UC Berkeley
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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.
Ken Youssefi
UC Berkeley
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DFM and DFA Design Guidelines
Ken Youssefi
UC Berkeley
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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.
Ken Youssefi
UC Berkeley
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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.
KISS – Keep It Simple Stupid
Ken Youssefi
UC Berkeley
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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.
Ken Youssefi
UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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UC Berkeley
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DFM Design Guidelines
Injection Molding
Fabrication of Plastics
Injection Molding
Ken Youssefi
UC Berkeley
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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
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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.
Ken Youssefi
Avoid sharp corners, they
produce high stress and
obstruct material flow.
UC Berkeley
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DFM Design Guidelines
Injection Molding
Keep section thickness uniform
around bosses.
Provide smooth transition,
avoid changes in thickness
when possible.
Ken Youssefi
UC Berkeley
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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
5 ≤ d ≤ 12.0
± 0.04 inch
12.0
± 0.05 inch
125 ≤ d ≤ 300 ± 1.0 mm
300
± 1.5 mm
•
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.
Ken Youssefi
UC Berkeley
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.
Ken Youssefi
UC Berkeley
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Rotational Molding Machines
Vertical wheel machine
Turret machine
Shuttle machine
Rock and roll machine
Ken Youssefi
UC Berkeley
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Rotational Molding
Advantages
Ken Youssefi
•
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
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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).
Ken Youssefi
UC Berkeley
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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.
Ken Youssefi
UC Berkeley
51
Rotational Molding Examples
Ken Youssefi
UC Berkeley
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Rotational Molding Examples
Ken Youssefi
UC Berkeley
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DFM Design Guidelines
Sheet-metal Forming
Ken Youssefi
UC Berkeley
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DFM Design Guidelines
Sheet-metal Forming
Ken Youssefi
UC Berkeley
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DFM Design Guidelines
Sheet-metal Forming
Ken Youssefi
UC Berkeley
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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).
Ken Youssefi
UC Berkeley
57
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.
Ken Youssefi
UC Berkeley
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Sand Casting Terminology
Ken Youssefi
UC Berkeley
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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.
Ken Youssefi
UC Berkeley
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Casting Defects and Design Consideration
Ken Youssefi
UC Berkeley
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DFM Design Guidelines - Casting
Recommended minimum section thickness
Ken Youssefi
UC Berkeley
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DFM Design Guidelines - Casting
Ken Youssefi
UC Berkeley
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DFM Design Guidelines – Machining
Ken Youssefi
UC Berkeley
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