Design for Cast and Molded Parts
Team:
Terese Bertcher
Larry Brod
Pam Lee
Mike Wehr
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Lecture updates
Revisions & Edits provided by: Seamus Clark, Scott
Leonardi, Gary Meyers, Joe Torres, Beatriz Dhruna,
John Fraser, Craig Jozsa, Dwayne Mattison, Darcy
McClure, Kevin O’Callaghan, Norm Opolsky, Henry
Gasahl, Rolf Glaser
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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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Die Casting vs. Sand Casting
Pros and Cons of using Die-Casting
Pros
●
●
●
●
●
●
Thinner walls
Closer Dimensional Limits
Reduced labor in setup
Smoother surfaces with a
potential for less stress risers
Lower finishing costs
Good solution for high
volume applications that
have stable designs and
design life.
Cons
● High initial die investment
● Limited # of casts (<100,000 typically)
● Limit to size of cast part (approx. 70 lbs for
aluminum)
● High temperature casts can reduce life of
dies
● Cannot be used on hollow parts such as
exhaust manifolds
● Minor casting updates difficult and costly.
● Die casting prevents the usage of stronger
alloys tolerated by sand-casting
The designer must decide what casting best suits their specific application as one type is NOT
necessarily better than the other!
Simplification of Die Configuration
Die Opening Direction
Cavity
Part
Core
Die Opening Direction
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Simplification of Die Configuration
Add a chamfer
to the part
Die Opening Direction
Cavity
Core
Die Opening Direction
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Simplification of Die Configuration
Support plate is
stationary. It
pushes lifter to
the left as ejector
plate moves
upward. Lifter
moves away from
undercut portion
of snap
Ejector pin
pushes the part
out of the core as
the lifter slides
away from the
snap undercut.
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Simplification of Die Configuration
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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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Design for Soundness
• Most metals, alloys, and plastics 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
• The mold and pattern should be made larger than the casting by the
amount of shrinkage
• Shrinkage of casting varies not only with material but also with shape,
thickness, casting temperature, mold temperature and mold strength
• Thicker areas will cool slower than thinner areas. Areas of transition
between thick and thin (ribs, walls, embosses, etc) will be prone to sink
marks. Different tool shops and different materials will require a
certain rib-to-wall thickness ratio.
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Design for Soundness
• The table below shows an average amount of
shrinkage for important cast metals
*
*
Design Rules: Disguising Sink Marks
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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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Avoid Sharp Angles & Corners
• When two or more sections conjoin, mechanical
weakness is induced at the junction interrupting
free cooling (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 a more uniform distribution
of strength
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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
• Outside corner and inside radius
• Non-uniform wall thickness
• Non-uniform flow transition
• Non-uniform flow transition
Sink
• Shrinkage stress / voids / sinks
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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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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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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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Employ Uniform Sections
• Thicker walls will solidify more slowly.
– This means they will feed solidifying thinner 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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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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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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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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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
– Improves flow of material
• The number of different size fillet radii used in a pattern
should be the minimized
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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Design Rules: Fillets & Corners
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Design Rules: Fillets & Corners
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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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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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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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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.
• Preferably the ribs connect the attachment to the
loading point.
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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 & 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
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
c
d
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Reducing Heavy Masses & Die
Simplification
A
B
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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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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’
Min. Boss Height:
.25”
1.5’< X < 6’
.75”
> 6’
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
– 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 the most commonly used plastic molding
process. It is a high speed, automated process for producing parts from both
thermoplastic and thermosetting plastic materials, and is used to create a large
variety of products with complex shapes and sizes.
Advantages:
o high production rates (multi-cavity tooling, low cycle times)
o repeatable high tolerances
o low labor cost
o minimal scrap losses
o little need to finish parts after molding
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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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Rev. 11-2001
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Rev. 11-2001
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Rev. 11-2001
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Rev. 11-2001
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Gas-Assisted Injection Molding Process
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Common Defects with Injection Molding
•
•
•
•
Knit lines - Caused by the melt-front flowing around an object
standing proud in a plastic part as well as at the end of fill where the
melt-front comes together again. Can be minimized or eliminated
with a mold-flow study when the mold is in design phase.
Sink marks – Excessive material thickness (in local area) or rib
thickness too large for wall thickness.
Flash - Parting line on the tool is damaged, too much injection
speed/material injected, clamping force too low. Parting lines should
be placed in non-visible locations, especially in class-A surfaces.
Dealing with these: Die draw, parting lines, boss locations, can all be
assessed while the interior surfaces are still in the clay design phase.
Suggestion is to add a case study around an IP or Console clay in the
studio development phase, and how it can be used to assess injection
molding capabilities.
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Impact of Design Guidelines on Molding Process
Steel / Product Conditions
Risk if violated
•Minimum Wall Thickness 2.5mm
•Increase Cavity Pressures
•Short Shots
•Wall stock designed with variation of more than +/- 15% from
normal wall
•Product stress caused by uneven pack and cooling
•Inconsistent gloss levels due to “hot spots”
•40% of normal wall to be maximum rib thickness at wall stock
intersection
•Sink marks
•Gloss variation
•1 degree per side minimum draft on ribs
•Sinking in mold
•Drag marks
•Distortion of “A” surface
•Ribs deeper than 17mm without ability to insert and / or vent
mold at bottom of ribs
•Gas traps and /or short shot
•Gas Burns
•Steel erosion at bottom of rib due to “Off Gassing”
•Minimum Rib thickness of 1mm at base of rib
•Short shots due to inability to fill thin sections
•Increase cavities pressures
•Over packing of part causing inaccurate shrink
•Injection Pressures in excess of 12,000 psi, as shown on
mold flow (Mold flow pressures are typically 15% / 25% lower
than actual machine pressures in production)
•Excess stress to cavity or core steel possible causing metal
fatigue leading to failure
•Inconsistent cavity fill and pack on older equipment
Rev. 11-2001
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Impact of Design Guidelines on Molding Process
Steel / Product Conditions
Risk if violated
•Hot drop quantity and / or positioning that limit mold cooling
•Internal stress in plastic
•Gloss variations do to lack of cooling
•Increase cycle time
•Standing “Fingers of Steel” that do not maintain a 1:1 height
to base ratio (i.e. 10mm high x 10mm square or Dia. Base)
•Possible failure of steel do to fatigue caused by molding
pressure or shutoff loads
•Minimum draft of 1.5 degrees for an un-textured surfaces and
1 degree / .001” (.025mm) of Texture Depth for grained
surfaces
•Drag marks
•Damage surface
•Grain scuffing
•Class “A” surface parting lines should be designed with
deliberate offset to facilitate matching and graining
•As a little as .001” mismatch of steel is visible on class “A”
surface
•Texture Mask (Shut-Off protection) is visible
•Split parting lines on Class “A” surface should be designed
into a flat surface, if the parting line is designed to a tangent of
a radius the surface will deteriorate rapidly
•Parting line deterioration
•“Frog Hair” Flash
•Heavy Surface Flash
•Deep “V” product designs that create “notch” conditions in
injection mold (a deep “V” is defined as anything less than 60
degrees included angle and with a depth of more than twice
the width
•Cracked or broken molds
•Catastrophic mold failure with loss of production
•Tolerances that are tighter than are repeatable in normal
mold cycles. General rule of thumb is +/- 0.5mm first 200mm
than +/- 0.0025mm per mm of length or width after
•Molded product that may not meet capability requirements
Rev. 11-2001
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Injection Mold Design Considerations
• Min draft on all faces → 0.5° (without grain)
• Draft as low as 0.3° for deep features (without
grain)
• Gate into thick regions of the part
• Maintain acceptable rib-to-wall ratio (to avoid
sink)
• Use action in tooling to avoid undercuts
• Consider draft angle requirements for intended
grain (if parts are to be grained)
Rib Design Guidelines - Injection
Molding
• Rib thickness at base should be no more than 4050% of wall thickness to avoid sink marks
• Rib height should be set to achieve 1mm MIN
thickness at peak (avoid “razor edge”)
Tooling Action - Injection Molding
Red hatched lines represent undercut area when using
the die draw direction specified. Adding a slide to form
this feature will alleviate the undercut issue.
Example:
”Witness lines” can be seen on the part in the
area where a slide was used to form the clip tower
• Action (i.e. slides/lifters) should be used to avoid
Part Graining - Injection Molding
Molded parts can be
grained to improve
appearance and reduce
gloss level.
• Each grain type has draft requirements for grain depth
(i.e. 60%, 80%, 100%, etc.). 100% depth requires
maximum draft angle for all grained surfaces.
• General requirement = 1° draft / 1 micrometer of grain
depth
Advanced Injection Molding Techniques
Pulsed Cooling
–Traditional techniques of cooling have circulated fluid through the mold at a
constant rate, and the local surface temp of the mold was therefore somewhat
uncontrolled.
–Advanced pulse cooling controls the temperature of the mold on a localized
basis by opening and closing valves which controls the mass flow of the fluid
and therefore the rate of heat transfer out of the mold.
Traditional Cooling
Pulsed Cooling
Advanced Injection Molding Techniques
Induction Heating
– Traditionally, the temperature of the mold during the filling cycle was a function of
the temperature of the resin, the facility, the temperature and flow of the coolant, and
length of time since the last shot.
–With the use of induction heating, the mold temperature can be raised above the
temperature of the resin before and during the filling phase.
–This can have many benefits including:
• lower injection pressure and clamping requirements
• improved flow lengths
• reduction of internal part stresses
• reduction or elimination of weld lines,
• Reduction of jetting, silver streaks, or sink marks.
Simple Induction heating of rod
Advanced Injection Molding Techniques
Live-Feed Injection Molding
–The live-feed injection molding process applies oscillating pressure at
multiple polymer entrances to cause the melt to oscillate
–The action of the pistons keeps the material in the gates molten while
different layers of molecular or fiber orientation are being built up in the mold
due to solidification
–This process provides a means of making simple or complex parts that are
free from voids, cracks, sink marks, and weld-line defects
Advanced Injection Molding Techniques
Multi-Material Molding – There are several types
– Once Step Process
• Co-Injection
• Sandwich molding
– Multi-Step Process
• Overmolding
• Two-shot molding
Advanced Injection Molding Techniques
Multi-Material Molding – Sandwich
– Sandwich molding is typically used to mold multi-layered plastic
packaging articles having a laminated wall structure. Each layer is
typically passed through a different annular or circular passageway in a
single nozzle structure and each layer is partially, sequentially, injected
through the same gate. Ref US Pat #7,559,756
Advanced Injection Molding Techniques
Multi-Material Molding – Co-Injection
– Co-Injection molding is similar to Sandwich molding, however in coinjection the materials may not come out in layers as in Sandwich.
– The same basic manufacturing principle applies – Muli-Materials are
simultaneously input into a single mold, and combined together to form
one part
Multiple Sprues & Materials
Advanced Injection Molding Techniques
Multi-Material Molding – Over-Molding
– Overmolding (or two-shot molding) is for parts where more than one
material is needed (i.e. plastic housing molded around metal plate)
– In these processes, only part of a product is molded in one material,
and that molded piece is manipulated so the second material can be
molded around, over, under, or through it to complete the final part.
– Overmolding is differentiated from two-shot in the fact that a part that
is overmolded may be done in a completely separate operation, or may be
made of a material other than plastic, so it is not a two shot process, but
still an overmold.
Overmolded Knife – Polymer Handle
Advanced Injection Molding Techniques
Multi-Material Molding – Two Shot
– Two shot molding is very similar to overmolding, only the molding is
done within a single manufacturing process.
– There are several processes of two shot molding, shown below is an
indexing process. Shot one is completed in cavity A, then the parts are
rotated, part A is transferred into cavity B. After Shot B is complete, the
final part is ejected.
Advanced Injection Molding Techniques
Multi-Material Molding – Combined Processes
– In this example, Overmolding and Two Shot molding are combined
• Polymer C is overmolded to the Metal Ring
• Polymer B is overmolded to the Metal Ring / Polymer C
• Polymer A is then overmolded to complete the assembly
Polymer A
Polymer B
Polymer C
Metal Ring
Advanced Injection Molding Techniques
Multi-Material Molding – Combining processes
– In the diagram below, this shows how an overmolded (2
shot) part and a sandwich molded part are brought together
via a hot plate weld.
Material C
Material B
Hot Plate Weld Joint
Material B
Material A
Material B
Advanced Injection Molding Techniques
Extrusion Blow Molding
Image Source: http://www.the-warren.org
Typical applications of blow molding:
-Bottles
-Automotive rear seat back panels (sedans)
-Fuel tanks
-HVAC ducts
DISAMATIC Casting Process
• Mold parting is vertical.
• All metal in gates, risers and casting cavities are contained
within the flaskless sand molds.
• There are casting size and weight limitations due to the
hydrostatic pressure built up within the mold.
• There is reduced flexibility for gating and risers.
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DISAMATIC Casting Process
Sand Bin
Cavity
Swing Pattern
RAM Pattern
Core
Pouring cup
Mold Pattern
Squeeze Piston
Sand Mold
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Mold Chamber
DISAMATIC Casting Process
Sand Blowing
Sand Shot
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DISAMATIC Casting Process
Sand Blowing
Sand Shot
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DISAMATIC Casting Process
Sand Blowing
Sand Shot
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DISAMATIC Casting Process
Sand Blowing
Sand Shot
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DISAMATIC Casting Process
Sand Blowing
Sand Shot
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DISAMATIC Casting Process
Mold Squeeze
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DISAMATIC Casting Process
Mold Squeeze
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DISAMATIC Casting Process
Mold Squeeze
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DISAMATIC Casting Process
Mold Squeeze
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DISAMATIC Casting Process
Supply sand
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Supply sand
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Supply sand
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Supply sand
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Supply sand
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Supply sand
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Supply sand
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
Swing Pattern drawing and swinging
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DISAMATIC Casting Process
RAM pattern drawing
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DISAMATIC Casting Process
RAM pattern drawing
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DISAMATIC Casting Process
RAM pattern drawing
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DISAMATIC Casting Process
RAM pattern drawing
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DISAMATIC Casting Process
RAM pattern drawing
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DISAMATIC Casting Process
RAM pattern drawing
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DISAMATIC Casting Process
RAM pattern drawing
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DISAMATIC Casting Process
RAM pattern closing
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DISAMATIC Casting Process
RAM pattern closing
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DISAMATIC Casting Process
RAM pattern closing
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DISAMATIC Casting Process
RAM pattern closing
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DISAMATIC Casting Process
RAM pattern closing
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DISAMATIC Casting Process
RAM pattern closing
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DISAMATIC Casting Process
Core Mask
Core
Core Setting
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DISAMATIC Casting Process
Core Setting
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DISAMATIC Casting Process
Core Setting
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DISAMATIC Casting Process
Core Setting
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DISAMATIC Casting Process
Core Setting
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DISAMATIC Casting Process
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Cycle time : 10 - 15 seconds/mold
Lecture Topics
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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)
Spoke Gating (4 spokes)
Diaphragm or disk gate
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Gating Considerations
Spruce
Cavity
Part
Core
Gate
Runner
Spruce Puller
(and cold slug well)
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Gating Considerations
Primary spruce
Spruce
Gate
Two plate single
cavity mold
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 ΔP)
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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Gating Considerations
• Gating system design is crucial in controlling the
rate and turbulence in the molten metal being
poured, the flow of liquid metal through the
casting's system, and the temperature gradient
within the metal casting.
– good gating system will create directional solidification
throughout the casting, since the flow of molten
material and temperature gradient will determine how
the casting solidifies.
Source: http://thelibraryofmanufacturing.com/metalcasting_troubleshooting.html
Gating Considerations
• Superheat (The difference between the solidification temperature
and the pouring temperature of the metal is called the superheat.)
– increases fluidity of the material for the casting
– increases gas porosity, increased oxide formation, and
mold penetration.
Gating Considerations
• Insulate Risers
– Since the riser is the reservoir of molten material for the
casting it should be last to solidify.
– Insulating the top will greatly reduce cooling in the
risers from the steep temperature gradient between the
liquid metal of the casting, and the room temperature
air.
Gating Considerations
• Sections of the Casting
– The flow of material is very important to the
manufacturing process. Do not feed a heavy section
through a lighter one.
Gating Considerations
• Connection Between Riser and Casting Must Stay Open
– If the passage linking the riser to the casting solidifies before
the casting, the flow of molten metal to the casting will be
blocked and the riser will cease to serve its function.
Die Casting vs Other Processes
• Die casting vs. plastic molding - Die casting produces stronger parts
with closer tolerances that have greater stability and durability. Die
cast parts have greater resistance to temperature extremes and
superior electrical properties.
• Die casting vs. sand casting - Die casting produces parts with
thinner walls, closer dimensional limits and smoother surfaces.
Production is faster and labor costs per casting as well as finishing
costs are lower.
• Die casting vs forging - Die casting produces more complex shapes
with closer tolerances, thinner walls and lower finishing costs. Cast
coring holes are not available with forging.
• Die casting vs. stamping - Die casting produces complex shapes
with variations possible in section thickness. One casting may
replace several stampings, resulting in reduced assembly time.
Source: http://www.diecasting.org
Lecture Topics
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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
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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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