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Design for Cast and Molded Parts

• Team Terese Bertcher
• Larry Brod
• Pam Lee
• Mike Wehr

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Design for Cast and Molded Parts

• Revision Team 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

• 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 RulesCorners Radii
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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
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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
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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 21.
• With a ratio less than 21 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 21 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
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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 RulesRib Dimensions
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Design RulesRib Wall Thickness
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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
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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
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Reducing Heavy Masses Die Simplification
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Reducing Heavy Masses Die Simplification
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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

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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
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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 Assisted Injection Molding
Gas Assisted Injection 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
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Gating Considerations
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Gating Considerations
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Gating Considerations
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Gating Considerations
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Gating Considerations
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Gating Considerations
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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