Ppt Plastics 19oct12
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Transcript of Ppt Plastics 19oct12
© 2012 Zuti Engineering Solutions Pvt. Ltd.
Engineering plastics And basic design guidelines
© 2012 Zuti Engineering Solutions Pvt. Ltd.
What are Plastics? ●A plastic material is any of a wide range of synthetic or semi-synthetic
organic solids that are moldable.
●Plastics are typically organic polymers of high molecular mass, but they
often contain other substances.
●They are usually synthetic, most commonly derived from
petrochemicals, but many are partially natural
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classification There are two types of polymers
Thermoplastics &Thermosetting polymers.
Thermoplastics
Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be molded again and again. Examples include
• Polypropylene (PP)
• High Impact Polystyrene (HIPS)
• Expandable Polystyrene (EPS)
• Crystal Polystyrene (GPPS)
• Acrylonitrile Butadiene Styrene (ABS)
• Low Density Polyethylene (LDPE)
• Poly vinyl chloride (PVC)
• Polycarbonate (PC)
• Nylon 6 (PA6, PA66)
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Thermosetting polymers
While thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as repeat units, derived from monomers; each polymer chain will have several thousand repeating units.
Thermosets can melt and take shape once; after they have solidified, they stay solid. In the thermosetting process, a chemical reaction occurs that is irreversible.
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Applications
● Due to their relatively low cost, ease of manufacture, versatility.
● plastics are used in an enormous and expanding range of products, from paper clips to spaceships.
● They have already displaced many traditional materials, such as wood, stone, horn and bone, leather, paper, metal, glass, and ceramic, in most of their former uses.
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Wide range of applications
●Automotive
●Home appliances
●Electronics
●Electrical
●Packaging
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Automotive applications
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Household applications
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Electronic
applications
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Packaging applications
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Plastics Product Design
Considerations
•Wall Thickness
•Drafts
•Radii
•Ribs/Bosses/Gussets
•Undercut
•Holes
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Uniform Wall Thickness
Minimizes
•Warpage
•Molded-in Stress
•Sink Marks
•Voids Improved Design
Poor Design
Uniform wall thickness promotes better flow
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Efficient Wall Design
Minimum Wall Thickness Increases Productivity:
•Reduce Cycle Time •Reduce Material Consumption
Limits to Minimum Wall Thickness:
•Flow Length
•Structural Stiffness & Strength
•Flammability Rating •Uniform Thickness
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3h
h
h
h
3h
Poor
Good
Best
Wall Transition When wall thickness transitions cannot be
avoided, the transitions should be made
gradually, on the order of 3 to 1. This
avoids stress concentration
and abrupt cooling
differences
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Why Warpage? Factors Affecting Warpage
•Uneven Wall Thickness
•Unequal Cooling Rate
•Unequal Pressure
(Uneven Shrinkage Causes Warp)
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Molded-in Stress
Mold-in
Mold-in
Assembly
Assembly
Application
Application
Low Molded-in
Stress
High Molded-in
Stress
High Performance
Low Performance
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Molded-in Stress
•Over packing •Uneven Shrinkage
Causes
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Overpackng
•More Material than Required in the Mold Cavity
•Results in Molded in Stresses
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Shrinkage •Decreased spacing between polymer chains
at cooler temperatures.
•Must be accommodated
in tooling design.
•Differs from material to
material.
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Factors Affecting Shrinkage:
•Material Properties
•Part Geometry
•Manufacturing
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Shrinkage and Part Geometry (Stress Due to Uneven Shrinkage)
Non-Uniform Part Thickness Cools Unevenly
Causing High Molded-in Stress
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Voids Already Cooled Surface
Will Not Yield to Shrinking Interior
Sink Marks Surface Yields to the
Still Shrinking Interior Mass
Uneven Shrinkage Defects
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Material Considerations
Some materials shrink differently than others
and require different part & tool
design considerations
Crystalline Amorphous Filled Amorphous
- - - Part
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Void
Rapidly cooling skin pulls apart the slower cooling
core, resulting in a void.
Void Example
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Coring should be employed where possible to eliminate material masses in the part. Coring results in more efficient designs and faster more productive cycle times. It also provides more uniform shrink and avoid sink marks.
Coring
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Differential Shrinkage Leads to: •Warpage
•Voids
Gating from thick to thin sections reduces differential shrinkage. It avoids a restricted flow and reduces molded in stress. It also allows for
more uniform packing.
Gating Wall
Transitions
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Parting lines are the lines on the part where the tool
halves come together. Often the parting line will be noticeable as a bump on the surface of the part which is
caused by the material that seeped into the joint.
Ejection is the process of removing the solidified part from
the mold. Common ejection methods include ejector
pins,ejector sleeves, ejector blades and stripper plate. All this leave “witness mark” on the finished part.
Parting Line and
Ejection
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On the surfaces in the draw of the die, a minimum of 1/2 degrees should be specified. Typical draft is 1 degrees. More draft aids
ejection but may generate a material mass on sections contained in one side of the mold.
Draft
No Draft Angle
Difficult Ejection Easier Ejection
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Draft Draft is the tapering of surfaces parallel to the line of draw (opening and
closing of the mold) for easier part removal.
The larger the draft angle,
the easier the ejection.
Typically draft angle is 10 with 1/20 on
ribs
The amount of draft angle depends on the depth
of the part in the mold, and its required end use
function
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Mismatch
Specify mismatch on the parting line. Note establishes what is
acceptable to the engineer for the molder.
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• For blind holes , the length
over diameter ratio should remain below 2. As the diameter of holes increases above 3/16” , the length over
diameter ratio can increase to 3. •For through holes, support of the core pin can be obtained on
both ends or the pins can meet
in the center which allows the diameter to length ratio to
increase to 4. For diameters greater than 3/16”, the length
to diameter ratio should remain below 6.
Holes and Depressions
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Holes
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Mold - Filling Plot
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Reinforcement
Structures
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T
R => .020”
The Larger the Better
Limited by the possibility of sinks and voids
Thermoplastics are notch sensitive!
t t = 0.6 T for T < .125” t = 0.4 T for T > .125”
1/2 to 1 1/2 degrees draft
Rib Design
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H > 3T
T
H < 3T
2T
Improved Design
Rib Height is Limited By the Thickness of the Base.
Rib Design Strategies Initial Design
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RIB DESIGN
• Should not be placed at the corners of the part. • Instead of one heavy rib use series of Ribs. • Should be rounded to add Strength.
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Careful of wall thickness at intersection
Radius all corners
Rib Intersections
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Bosses are commonly found on injection molded plastic parts. They are the standing features on a part which often serve as
mounting or fastening points. They also facilitate alignment during assembly.
Bosses
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Boss Design
•Should be avoided at the corners of the part. •Should be of round shape.
•If boss is of square or rectangular shape, corners should have a radii of at least 1/64”.
D 2D
W
T
R = .25 T H = 2 to 5T
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Gussets are often placed at points of attachment, support, or contact with other components. Gussets are effective in stiffening
structural features and in distributing stress
Gussets may be considered a subset of ribs and the guidelines
for ribs apply to gussets.
Gussets
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•The thickness of the gusset at the intersection with the nominal wall should be 50% of the nominal wall.
•The height of the gusset can be 95% of the height of
the boss it attaches to. Generally the height will be less than 4 times the nominal wall thickness and the preferred height is 2 times the nominal wall.
•The length of the gusset may vary from 30 to 100% of the height of the gusset.
•The intersection of the gusset with the feature or the nominal wall should have fillet with a radius of 25% of
the nominal wall.
•The spacing between gussets should be at least twice the nominal wall thickness
Guidelines for Gussets
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A fillet radius should be between 25% to 60% the nominal wall
thickness. The larger fillet radius is suggested for load carrying features. A minimum radius of 0.020 in (0.508 mm) is
suggested. Break any sharp corner with at least a 0.005 in (0.127 mm) radius.
The outside corner radius should be equal to the
inside radii plus the wall thickness (R = r + t)
Radius, Fillets and Corners
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Stress Concentrator
T
r = 0.5 T
R = 1.5 T
Corners
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Points of Consideration…for Product Designer
• Product should be molded i.e. Part can be withdrawn after Molding.
• Parting line is going to be extended up to Work piece, check for its
smoothness.
• Check for draft / undercut, specially at ribs, bosses etc.
• Avoid round at parting line.
• Prefer larger radius at vertical round.
• Check draft in direction of slide core movement.
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Thank you