Rev. 11-20011 of 69 Design for Cast and Molded Parts Team: Terese Bertcher Larry Brod Pam Lee Mike Wehr

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Design for Cast and Molded PartsTeam: Terese Bertcher

Larry Brod

Pam Lee

Mike Wehr

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Design for Cast and Molded PartsRevision 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


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Basic Casting Design Guidelines1. Visualize the Casting

2. Design for Soundness

3. Avoid Sharp Angles & Corners

4. Minimize the Number of Sections

5. Employ Uniform Sections

6. Correctly Proportion Inner Walls

7. Fillet All Sharp Angles

8. Avoid Abrupt Section Changes

9. Maximize Design of Ribs & Brackets

10. 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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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

Good Corner Design Incorrect Corner Design

Incorrect Corner Design Incorrect Corner Design

• Generous radius

• Uniform wall thickness

• Smooth flow transition

• Very sharp radii

• High stress concentration

• Sharp flow transition

• Inside / outside radius mismatch

• Non-uniform wall thickness

• Non-uniform flow transition

• Outside corner and inside radius


• Non-uniform flow transition

• Shrinkage stress / voids / sinksSink

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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

• Very thick wall sections


• Sharp inside and outside radii

Improved Part Design

• Thinner wall sections

• More uniform wall thickness

• 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.

Gating from “thick to thin” when possible to improve flow and allow thicker sections to be packed.

Internal runner to assist / improve the ability to pack the thick section when gating from “thin to thick” is necessary.

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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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• 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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• 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

Wall Thickness Transitions

Tapered Transition

Gradual TransitionStepped Transition

Core out thicker areas where possible

Poor Design

Better

Best

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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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• 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 2.53.0h)

•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

Correct Proportions

Radius (fillet)

Sink Mark

Shrinkage VoidsExcessive

Radius

Part Wall

Rib

Excessive Rib Wall Thickness

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• 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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• 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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• 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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• 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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• 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

May complicate die construction

External ribs may cause poor distribution of stresses

May complicate die construction

Sharp corners, small radii

H T H > T core out underside

Good distribution of stresses

Sharp corners, small radii, little draft

Generous draft and fillets, angular transitions

Ribs inside, good distribution of metals for all purposes.

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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

a c

B

b

d

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A B C

a

dcb

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A B

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• 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’ Min. Boss Height: .25”

1.5’< X < 6’ .75”

> 6’ 1.00”

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• 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 Gussets reinforce free standing bossesThick sections

cored outBosses attached to the walls using ribs

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Design Rules: Boss Design

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Lecture Topics


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• 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.

Injection Molding Process

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Conventional Injection Molding

Gas Assisted Injection MoldingGas Assisted Injection Molding

Sink

Gas Channels

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Video Clip of Injection Molding Process

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Lecture Topics


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Gating Location and Constraint Considerations

Spoke Gating (2 spokes) Diaphragm or disk gateSpoke Gating (4 spokes)

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Gating Considerations

Cavity

Spruce Puller (and cold slug well)

GateCore

Runner

PartSpruce

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Two plate single cavity mold

Three plate mold configuration (multi cavity)

Single parting

line

Primary spruce

Pin Gate

Parting Line 1

Parting Line 2

Secondary Spruce

Spruce Gate

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Reverse Injection

Cavity (stationary half)

Core (moving half)

Standard Configuration

Alternatives to Reverse Injection

Tunnel gating through knockout pin

Cavity (stationary half)

Core (moving half)

Logo..placed

At gate location

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Single top center gate

Hot manifold for a stack mold

Cold edge gate fed by hot manifold

Direct lateral gating of several cavities

Center gating of several cavities Cold edge gating of several cavities fed by hit manifold

Multiple top gating of single cavity

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Two Gates

•Improved filling pattern and pressure distribution

•Formation of one weld line

Three Gates

•Filling pattern and pressure distribution are better

•Formation of two weld lines

Fill is complete

Sections remain unfilled

Weld

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Three gates and flow leaders

• Most uniform filling pattern and pressure distribution

• Requires wall thickness variation or diagonal ribs

Spruce gated box shaped molding Uniform wall thickness

Corners: last to fill

Flow leaders / internal runners Local increases in wall thickness promote flow, uniform pressure drop extend from gate to corners of part

Filling pattern without flow leaders (uniform wall thickness)

Max Flow length (highest P)

Overpacking and changes flow direction

Improved filling pattern with flow leaders (non-uniform wall thickness)

Sides fill early

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Lecture Topics


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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


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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”

http://www.castsolutions.com/

Documents

Rev. 11-20011 of 69 Design for Cast and Molded Parts Team: Terese Bertcher Larry Brod Pam Lee Mike Wehr