Cast & Molded Parts - Jonathan Weaver's Home Page

Report
Design for Cast and Molded Parts
Team:
Rev. 11-2001
Terese Bertcher
Larry Brod
Pam Lee
Mike Wehr
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Design for Cast and Molded Parts
Revision Team:
Rev. 11-2001
Seamus Clark
Scott Leonardi
Gary Meyers
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Lecture Topics
•
•
•
•
•
Basic Casting Design Guidelines
Injection Molding Process
Gating Considerations
Case Study – Corvette Brake Pedal
Case Study – M1 Abrams Tank
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Lecture Topics
•
•
•
•
•
Basic Casting Design Guidelines
Injection Molding Process
Gating Considerations
Case Study – Corvette Brake Pedal
Case Study – M1 Abrams Tank
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Basic Casting Design Guidelines
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Visualize the Casting
Design for Soundness
Avoid Sharp Angles & Corners
Minimize the Number of Sections
Employ Uniform Sections
Correctly Proportion Inner Walls
Fillet All Sharp Angles
Avoid Abrupt Section Changes
Maximize Design of Ribs & Brackets
Avoid Using Bosses, Lugs & Pads
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Visualize the Casting
• It is difficult to follow section changes and shapes
from blueprint.
• Create a model to scale or full size to help
designer to:
–
–
–
–
–
See how cores must be designed, placed or omitted
Determine how to mold the casting
Detect casting weaknesses (shrinks / cracks)
Determine where to place gates and risers
Answer questions affecting soundness, cost and
delivery
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Simplification of Die Configuration
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Simplification of Die Configuration
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Simplification of Die Configuration
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Simplification of Die Configuration
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Design for Soundness
• Most metals and alloys shrink when they solidify
• Design components so that all parts increase in
dimension progressively to areas where feeder
heads (risers) can be placed to offset shrinkage.
• Disguise areas of shrinkage when unavoidable
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Design Rules: Disguising Sink Marks
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Avoid Sharp Angles & Corners
• When two or more sections conjoin, mechanical
weakness is induced at the junction and free
cooling is interrupted – most common defect in
casting design.
– Replace sharp angles with radii and minimize heat and
stress concentration
– In cored parts avoid designs without cooling surfaces
– A rounded junction offers uniform strength properties
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Design Rules:Corners & Radii
Incorrect Corner Design
Good Corner Design
• Generous radius
• Very sharp radii
• Uniform wall thickness
• High stress concentration
• Smooth flow transition
• Sharp flow transition
Incorrect Corner Design
Incorrect Corner Design
• Inside / outside radius mismatch
• Non-uniform wall thickness
• Non-uniform wall thickness
• Non-uniform flow transition
• Non-uniform flow transition
Sink
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• Outside corner and inside radius
• Shrinkage stress / voids / sinks
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Minimize the Number of
Sections
• A well designed casting brings the minimum number of
sections together at one point.
• Staggering sections (where possible)
– Minimizes hot spot effects
– Eliminates weakness
– Reduces distortion
• Where staggering sections is not possible use a cored hole
through the center of the junction.
– Helps to speed solidification
– Helps to avoid hot spots
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Employ Uniform Sections
• Thicker walls will solidify more slowly.
– This means they will feed solidifying inner walls.
– Results in shrinkage voids in the thicker walls
• Goal is to design uniform sections that solidify
evenly.
– If this is not possible, all heavy sections should be
accessible to feeding from risers.
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Design Rules: Wall Uniformity
Original Part Design
Improved Part Design
• Very thick wall sections
• Thinner wall sections
• Non-uniform wall thickness
• More uniform wall thickness
• Sharp inside and outside radii
• Inside and outside radii
(when possible)
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Correctly Proportion Inner Walls
• Inner sections of castings cool much slower than
outer sections.
– Causes variations in strength properties
• A good rule of thumb is to reduce inner sections to
90% of outer wall thickness.
• Avoid rapid section changes
– Results in porosity problems similar to what is seen
with sharp angles.
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Design Rules: Wall Uniformity
Part gated from “thin to thick” hinders packing of thicker
sections and can create flow problems.
Internal runner to assist / improve the ability to pack
the thick section when gating from “thin to thick” is
necessary.
Gating from “thick to thin” when possible to improve
flow and allow thicker sections to be packed.
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Correctly Proportion Inner Walls
• Whenever complex cores must be used, design for
uniformity of section to avoid local heavy masses
of metal.
• The inside diameter of cylinders and bushings
should exceed the wall thickness of castings.
– When the I.D. is less than the wall it is better to cast the
section as a solid.
– Holes can be produced by cheaper and safer methods
than with extremely thin cores
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Fillet All Sharp Angles
• Fillets (rounded corners) have three functional
purposes:
– To reduce the stress concentration in a casting in
service
– To eliminate cracks, tears and draws at re-entry angles
– To make corners more moldable by eliminating hot
spots
• The number of fillet radii in one pattern should be
the minimum possible.
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Fillet All Sharp Angles
• Large fillets may be used with radii equaling or
exceeding the casting section.
– Commonly used to fulfill engineering stress
requirements
– Reduces stress concentration
• Note: Fillets that are too large are undesirable –
the radius of the fillet should not exceed half the
thickness of the section joined.
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Fillet All Sharp Angles
• Tips to avoid a section size that is too large at an
“L”, “V” or “Y” junction.
• For an “L” junction :
– Round an outside corner to match the fillet on the
inside wall. (If this is not possible the designer must
make a decision as to which is more important:
Engineering design or possible casting defect)
• For a “V” or “Y” junction:
– Always design so that a generous radius eliminates
localization of heat.
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Design Rules: Fillets & Corners
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Avoid Abrupt Section Changes
• The difference in relative thickness of adjoining
sections should not exceed a ratio of 2:1.
• With a ratio less than 2:1 the change in thickness
may take on the form of a fillet.
• Where this is not possible consider a design with
detachable parts.
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Avoid Abrupt Section Changes
• With a ratio greater than 2:1 the recommended
shift for the change in thickness should take on the
form of a wedge.
– Note: wedge-shaped changes in wall thickness should
not taper more than 1 in 4.
• Where a combination of light and heavy sections
is unavoidable, use fillets and tapered sections to
temper the shifts.
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Design Rules: Section Changes
Tapered Transition
Better
Wall Thickness Transitions
Stepped Transition
Poor
Design
Gradual Transition
Best
Core out thicker
areas where possible
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Maximize Design of Ribs &
Brackets
• Ribs are only preferable when the casting wall
cannot be made strong or stiff enough on its own.
• Ribs have two functions:
– They increase stiffness
– They help to reduce weight
• Common mistakes that make ribs ineffective:
– Too shallow
– Too widely spaced
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Maximize Design of Ribs &
Brackets
• The thickness of the ribs should be approximately
80% of the adjoining thickness and should be
rounded at the edge.
• The design preference is for ribs to be deeper than
they are thick.
• Ribs should solidify before the casting section
they adjoin.
• The space between ribs should be designed such
that localized accumulation of metal is prevented.
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Design Rules:Rib Dimensions
General Guidelines for Rib Dimensions*
•Component wall thickness: h
•Draft per side(0): 0.5º  1.5º
•Rib height (L):  5h (typically 2.53.0h)
•Rib spacing (on center):  2h  3h
•Base radius (R):  0.25h  0.40h
•Rib thickness (t): 0.4  0.8h
*Exact rib dimensions are material specific
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Design Rules:Rib Wall Thickness
Excessive
Radius
Rib
Part Wall
Shrinkage
Voids
Sink
Mark
Radius
(fillet)
Excessive
Rib Wall
Thickness
Correct Proportions
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Maximize Design of Ribs &
Brackets
• Generally, ribs in compression offer a greater
safety factor than ribs in tension.
• Exception: Castings with thin ribs in compression
may require design changes to provide necessary
stiffening and avoid buckling.
• Thin ribs should be avoided when joined to a
heavy section or they may lead to high stresses
and cracking
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Maximize Design of Ribs &
Brackets
• Avoid cross ribs or ribbing on both sides of a
casting.
– Cross ribbing creates hot spots and makes feeding
difficult
– Alternative is to design cross-coupled ribs in a
staggered “T” form.
• Avoid complex ribbing
– Complicates molding, hinders uniform solidification
and creates hot spots.
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Maximize Design of Ribs &
Brackets
• Ribs meeting at acute angles may cause molding
difficulties, increase costs and aggravate the risk
of casting defects.
• “Honeycombing” often will provide increased
strength and stiffness without creating hot spots.
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Design Rules: Rib Manufacturability
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Design Rules: Rib Design
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Maximize Design of Ribs &
Brackets
• Brackets carrying offset loads introduce bending
moments-localized and in the body of the casting.
• Tips to avoid this problem:
– Taper “L” shaped brackets and make the length of
contact with the main casting as ample as possible.
– Brackets may frequently be cast separately and then
attached, simplifying the molding.
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Maximize Design of Ribs &
Brackets
• A ribbed bracket will offer a stiffness advantage,
but avoid heat concentration by providing cored
openings in webs and ribs.
– The openings should be as large as possible
– The openings should be consistent with strength and
stiffness
• Avoid rectangular-shaped cored holes in ribs or
webs.
– Use oval-shaped holes with the longest dimension in
the direction of the stresses
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Recommended Configurations
HT
H > T core out
underside
Ribs inside, good distribution of
metals for all purposes.
May complicate die
construction
Good distribution of
stresses
Generous draft and fillets,
angular transitions
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Sharp corners,
small radii
May complicate die
construction
External ribs may cause poor
distribution of stresses
Sharp corners, small
radii, little draft
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Avoid Using Bosses, Lugs &
Pads
• Bosses and pads can have adverse effects on
castings:
– They increase metal thickness
– They create hot spots
– They can cause open grain or draws
• If they must be incorporated into a design you
should blend them into the casting by tapering or
flattening the fillets.
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Reducing Heavy Masses & Die
Simplification
a
c
A
B
d
b
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Reducing Heavy Masses & Die
Simplification
a
A
B
C
b
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c
d
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Reducing Heavy Masses & Die
Simplification
A
Rev. 11-2001
B
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Avoid Using Bosses, Lugs &
Pads
• The thickness of bosses and pads should be less
than the thickness of the casting section they
adjoin but thick enough to permit machining
without touching the casting wall.
• Exception: Where a casting section is light the
following should be used as a guide:
Casting Length:
< 1.5’
1.5’< X < 6’
> 6’
Rev. 11-2001
Min. Boss Height:
.25”
.75”
1.00”
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Avoid Using Bosses, Lugs &
Pads
• Bosses should not be used in casting design when
the surface to support bolts may be obtained by
milling or countersinking.
• A continuous rib instead of a series of bosses will
permit shifting hole location.
• Where there are several lugs and bosses on one
surface, they should be joined to facilitate
machining.
– A panel of uniform thickness will simplify machining
– Make the walls of a boss at uniform thickness to the
casting walls
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Design Rules: Boss Design
Poor Boss Designs: result in the potential for
sink marks and voids.
Sinks / Voids / Cooling stresses
Improved Boss Designs
Bosses attached to
the walls using ribs
Rev. 11-2001
Thick sections
cored out
Gussets reinforce free
standing bosses
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Design Rules: Boss Design
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Lecture Topics
•
•
•
•
•
Basic Casting Design Guidelines
Injection Molding Process
Gating Considerations
Case Study – Corvette Brake Pedal
Case Study – M1 Abrams Tank
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Injection Molding Process
• The injection molding process is a high speed,
automated process that can be used to produce
plastic parts with very complex geometries.
• A typical die casting machine is shown in the next
slide. Due to the combined effects of flow through
both the machine and the mold, large pressure
drops associated with mold filling can occur.
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Injection Molding Process
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Injection Molding Process
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Injection Molding Process
Conventional
Injection Molding
Sink
Gas
GasAssisted
Assisted
Injection
InjectionMolding
Molding
Gas Channels
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Video Clip of Injection Molding
Process
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Lecture Topics
•
•
•
•
•
Basic Casting Design Guidelines
Injection Molding Process
Gating Considerations
Case Study – Corvette Brake Pedal
Case Study – M1 Abrams Tank
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Gating Location and Constraint
Considerations
Spoke Gating (2 spokes)
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Spoke Gating (4 spokes)
Diaphragm or disk gate
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Gating Considerations
Spruce
Cavity
Part
Core
Gate
Runner
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Spruce Puller
(and cold slug well)
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Gating Considerations
Primary spruce
Spruce
Gate
Two plate single
cavity mold
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Secondary
Spruce
Single
parting
line
Parting
Line 2
Pin Gate
Three plate mold
configuration (multi cavity)
Parting
Line 1
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Gating Considerations
Standard
Configuration
Cavity (stationary half)
Alternatives to Reverse Injection
Core (moving half)
Logo..placed
At gate location
Reverse Injection
Cavity (stationary half)
Core (moving half)
Tunnel gating through
knockout pin
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Gating Considerations
Single top center gate
Center gating of several cavities
Multiple top gating of single cavity
Cold edge gating of several cavities
fed by hit manifold
Cold edge gate fed by hot manifold
Direct lateral gating of several
cavities
Hot manifold for a stack mold
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Gating Considerations
Fill is
complete
Weld
Two Gates
•Improved filling pattern and
pressure distribution
•Formation of one weld line
Sections
remain unfilled
Three Gates
•Filling pattern and pressure
distribution are better
•Formation of two weld lines
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Gating Considerations
Spruce gated box
shaped molding
Filling pattern without flow leaders
(uniform wall thickness)
Uniform wall thickness
Max Flow length
(highest P)
Corners: last to fill
Sides fill early

Flow leaders / internal runners
Local increases in wall thickness
promote flow, uniform pressure drop
extend from gate to corners of part
Overpacking and changes
flow direction
Improved filling pattern with
flow leaders (non-uniform wall
thickness)
Three gates and flow leaders
• Most uniform filling pattern
and pressure distribution
• Requires wall thickness
variation or diagonal ribs
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Lecture Topics
•
•
•
•
•
Basic Casting Design Guidelines
Injection Molding Process
Gating Considerations
Case Study – Corvette Brake Pedal
Case Study – M1 Abrams Tank
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A Design Study in Aluminum
Casting
The Brake Pedal for the Chevrolet Corvette
Casting\Corvette Case Study.pdf
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Lecture Topics
•
•
•
•
•
Basic Casting Design Guidelines
Injection Molding Process
Gating Considerations
Case Study – Corvette Brake Pedal
Case Study – M1 Abrams Tank
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A Design Study in Steel Casting
The Ice Cleat for the M1 Abrams Tank
Casting\ice_cleat M1 Abrams.pdf
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References
• The case studies were obtained from the
Engineered Casting Solutions website.
– URL: http://www.castsolutions.com/
• Modern Casting, May 2001 v91 i5 p50., “Basics
of Gray Iron Casting Design: 10 Rules for
Engineered Quality”
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