Materials Chapter 5. Plastics Material Selection

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Material Selection
Material Selection
• Getting the optimum balance of
performance, quality and cost requires a
careful combination of material and plastics
part design
• Once end use requirements have been
specified, the designer can begin searching
for plastic materials that are suitable for the
application
Material Selection
• Material selection/screening process is
accomplished by comparing material
properties with a property profile derived
from the end use requirements
• Best to select 3-6 materials during the initial
process based on the property profile and
costs
Material Selection
• Material Selection should be based
primarily on the functional requirements of
the parts
– Those items without which the part will not
work
• Then go to design considerations
– Those items involving manufacturing factors
without which the part cannot be made
profitably
Material Selection
• Functional requirements are physical properties
– The ability to operate over a wide range of temperatures
– Resistance to chemicals and environments
– Any other condition that can be described by testable
parameters
• Some functional are more important than others
– These should be given a top priority
Material Selection
• Design considerations are the
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Raw material costs
Processing cost
Tooling costs
Recyclability
Familiarity of material
Acceptance of material in the market place
Material Selection
• Final choice of material almost always
involves compromises between functional
requirements and design considerations
Property Profile
Step 1
• Develop property profile
• The polymer required is classified and
ranked according to the following
representative grouping of functional
requirements
• The list can be expanded or filtered to fit
particular needs
Step 1
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Rigid
Flexible
Opaque
Translucent
Transparent
High strength to weight
Ordinary strength to weight
High Temperature resistance
Sterlizable
Ductile at low temperatures
Resistant to chemicals
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Dielectric constant
Arc resistance
Nonflammable
Slow burning
Very low density
Ordinary density
Wear resistance
Friction coefficient
Acceptable for food
Post-consumer recyclable
Weather resistant
Step 2
• It is now possible to refer material property
data sheets and databases to arrive at a short
list of candidates that match with the
property profile developed in Step 1
• If similar application exist, begin with the
current materials
Step 3
• Material properties are used to select likely
candidates with corresponding property value or
ranking listed
• Best to begin with properties that cannot be
enhance by design
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Coefficient of thermal expansion
Transparency
Chemical resistance
Softening temperature
Step 3
• Then go to mechanical properties, which
can generally be enhanced by design
– Wall thickness
– Ribs
– Reinforcements
Step 4
• The next step in the elimination process is to
consider items such as
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Creep modulus
Dielectric strength
Mold shrinkage
Costs
• The designer may want to consider statistically
designed experiments to narrow the list of
candidates
Material Selection Example
• There are a wide range of properties within
polymer families and between families
• The problem here is to select a injection
moldable polymer based on mechanical
properties as a first priority
• The property profile is developed first
Material Selection Example
• Property Profile
Material Selection Example
Material Selection Example
• In the preceding table all the resins checked
yes meet the preliminary specification
• The next step is to consider secondary
matters such as creep, dielectric strength,
shrinkage and cost.
Material Selection Example
Material Selection Example
• From the data in the preceding table, the higher
cost resins can be eliminated first
• The filled resins can be eliminated next because
they are more difficult to process
• This leaves 6/6 nylon and PPO
• The HDTUL of nylon is must lower than PPO
• The shrinkage values for PPO are much less than
nylon
• PPO is the best choice
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