DFMA II Design Guidelines

Ken Youssefi SJSU: PDM II Slide 1

Design for Manufacturing and Assembly II:

Design Guidelines

by Kenneth Youssefi

Product Design and Manufacturing Minicurriculum

San José State University


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 are axiomatic: they are based on accurate and deep knowledge of each process. They should be obtained from a production engineer knowledgeable about the process. The manufacturing processes (tools and equipment) are constantly refined and improved.


DFM Design GuidelinesInjection Molding

Injection MoldingFabrication of Plastics

Process Overview(see also Injection Molding lecture notes)



Minimize section thickness; cooling time is proportional to the square of the thickness. Reduce cost by reducing the cooling time.

Do

Provide adequate draft angle for easier part removal from mold.

Don’t



Keep rib thicknesses 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.



Keep section thickness uniform around bosses.

Provide smooth transitions, avoid changes in thickness when possible.



• 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

Standard thickness variation.

• 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.


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 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


Rotational MoldingLimitations

• Cannot 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


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 moltenmetal 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 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.


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 – Machining


End

DFMA II ~ Design Guidelines

DFMA II Design Guidelines

Documents

Transcript of DFMA II Design Guidelines