1. Materials
Plastics

2. Introduction

3. What are Plastics Polymer
“Poly” – many
“mer” – unit
Many Units
Carbon based, high molecular weight, versatile synthetic materials that are built up from monomeric units

4. How plastics are made Addition or Condensation Reaction Addition
A simple combining of molecules without generating byproducts
Vinyls PE PP PS

5. Addition Reaction - Polyethylene

6. How plastics are made Condensation
Involves removing certain atoms from each molecule, allowing the molecules to combine
Byproducts are generated that must be removed
Nylons PC

7. Condensation Reaction - Polycarbonate

8. Types of Plastics Thermoplastic Thermoset
Soften with heated, then solidify when cooled
Only physical changes
Thermoset
Polymers that chemically react when heated to form a cross-linked polymer chain network
Not reformable with heating

9. Thermoplastics Amorphous Semi-Crystalline Random Structure Tg
Polystyrene, Polycarbonate
Semi-Crystalline
Organized Molecular Arrangement
Tg, Tm
Polyethylene, Polypropylene

10. Crystallinity

11. Thermoplastics
The ability of plastics to form crystals is largely dependent on the structure of the plastic molecule
Linear plastics with small side groups can form crystalline regions
HDPE, LDPE, Acetals, Nylon and PET

12. Structure Property Relationship
The Property of a Plastic Material formulation can be tailored to meet most end use applications
The properties are dependent on
The chemical composition of the polymer
Additives

13. Structure Property Relationship
Chemical Composition varies by
Structure of the repeat unit
Average molecular weight
Molecular weight distribution
Linear, branched or cross-linked

14. Structure Property Relationship
PMMA and PS are very different in behavior and properties because their repeat units are different

15. Molecular Distribution

16. Structure Property Relationship
Number-Average Molecular Weight (Mn)
Mn = NiMi / Ni
where Ni is the number of molecules of the ith species of molecular weight Mi.
Measured from colligative properties such as:
freezing point depression for low molecular weight
osmotic pressure for higher molecular weight
gel permeation or size exclusion chromatography

17. Structure Property Relationship
Weight-Average Molecular Weight (MW)
MW= NiMi2/ NiMi
where Ni is the number of molecules of the ith species of molecular weight Mi.
Measured using techniques such as:
light scattering
gel permeation or size exclusion chromatography.

18. Structure Property Relationship
Polydispersity(MWD) = MW / Mn
A measure of the distribution of molecular weights of polymer chains.

20. Effect of Mw on Viscosity
Low shear – lots of entanglements, Mw has direct effect on viscosity
Medium shear – reduced entanglements Mw has less effect on viscosity
High shear – few entanglements, Mw has no effect on viscosity
Low Shear
Medium Shear
High Shear
Log 
Log 
Log shear rate

21. Effects of MWD on Viscosity
Narrow MWD
Viscosity
Broad MWD
Shear Rate

22. Structure Property Relationship
Additives
Used to enhance specific properties
Combustion modifiers
Release agents
Blowing Agents
UV stabilizers
Fillers
Reinforcements
Colors
Additives are like medications, they have side effects

23. Plastics Behavior and Properties

24. Plastics Behavior and Properties
Mechanical Behavior
Flow Behavior
Short Term Mechanical Properties
Long Term Mechanical Properties
Thermal Properties
Electrical Properties
Environmental Properties
Other Properties

25. Mechanical Behavior Viscoelasticity Creep Stress Relaxation Recovery
Loading Rate

26. Viscoelasticity Elastic Viscous Plastics show both responses
The material returns to original shape after the load has been removed
Linear stress strain response
Viscous
The material will deform or flow under load
Nonlinear stress-strain response
Plastics show both responses
Short term load
elastic
Long term load
viscous

27. Creep
One of the most important results of plastics’ viscoelastic behavior
Deformation over time when a material is subjected to a constant stress
The polymer chains slip past one another
Some of the slippage is permanent

28. Creep

29. Stress Relaxation Gradual decrease in stress at constant strain
Same polymer chain slippage as in creep

30. Recovery
The degree to which a plastic returns to its original shape after a load is removed

31. Temperature and Loading Rate Effects
The rate at which the part is stressed or strained
Thermoplastics become stiffer and fail at smaller strain levels as the strain rate increases
At higher temperatures plastics lose their stiffness and become more ductile

32. Temperature and Loading Rate Effects

33. Flow

34. Types of Flow
Drag Flow
Caused by the relative motion of one boundary with respect to the other boundary that contains the fluid
Two major boundaries in injection unit are the barrel and screw surfaces
Since the screw is rotating in a stationary barrel, one boundary is moving relative to the other boundary
This causes drag flow to occur

35. Types of Flow Pressure Flow
Caused by the presence of pressure gradients
Pressure flow is what occurs downstream of the injection unit
Sprue, runner, gate and cavity
Flow occurs because the pressure is higher at the injection unit discharge than in the mold

36. Types of Flow For the overall system
The injection unit uses drag flow to move the material and build pressure
This pressure buildup at the discharge of the injection unit results in pressure flow through the mold

37. Shear Flow Induced by Drag Flow
Different layers of plastics move at different velocities with the maximum velocity being at the moving boundary and zero velocity at the wall

38. Shear Flow Induced by Pressure Flow
Different layers of plastics move at different velocities with the maximum velocity being at the centerline of flow and zero velocity at the walls

39. Shear Rate Difference in velocity per normal distance
The change in shear strain with time
Units of seconds-1
Drag Flow
Pressure Flow

40. Shear Stress The stress required to achieve a shearing type flow
Force divided by the area over which it acts
Units of Pascal or psi
Drag Flow
Pressure Flow

41. Shear Viscosity Internal resistance to shear flow
Ratio of shear stress to shear rate
Units of poise or Pa-sec

42. Shear Heat Viscous heat generation Heat generated due to shear flow
Conversion of mechanical energy to heat through friction
Amount is equal to the product of the viscosity and the shear rate squared

43. Effect of Temperature on Viscosity

44. Types of Fluids Newtonian Shear thinning(pseudo-plastic)
A fluid whose viscosity is independent of shear rate
Shear thinning(pseudo-plastic)
A fluid whose viscosity decreases with increasing shear rate
Shear thickening(dilatants)
A fluid whose viscosity increases with increasing shear rate

45. Flow Behavior

46. Power law Fluids Polymer melts are shear thinning fluids
The fact that the viscosity reduces with shear rate is of great importance in the injection molding process
Important to know the extent of the change of viscosity with shear rate
m is the consistency index
n is the power law index

47. Mechanical Properties

48. Mechanical Properties
Important in all applications
Stiffness
Hardness
Toughness
Impact Strength
The ability to support loads

49. Mechanical Properties
Mechanical property data is used to
Select materials
Estimate part performance
Predict deformation and stresses from applied loads

50. Mechanical Properties
Most data have been derived from laboratory tests and may not directly apply to your application
Data should be used for comparison purposes only because
Difference between testing and end use conditions
Material and processing variability
Unforeseen environmental or loading conditions

51. Types of Forces
There are four fundamental forces we deal with in the testing of mechanical properties of plastics
Tensile
Compressive
Shear
Torsion
These forces are tested alone and in combinations

52. Tension and Compression Forces
Pulling force
Compression
Pushing force

53. Shear and Torsion Force
Opposing forces at the same point
Torsional Force
Turning force

54. Stress and Strain
Stress is the force per area that is applied to the specimen
Units of psi or Pa
Strain is the change is dimension divided by the original dimension
No units

55. Stress-Strain

56. Terms and Definitions Proportional Limit Elastic Limit
The end of the region where the plastic shows linear stress-strain behavior
Elastic Limit
The point after which the plastic will permanently deform
Applications that cannot tolerate permanent deformations must stay under the elastic limit

57. Terms and Definitions Yield Point
Marks the beginning of the region in which the ductile plastic continues to deform without a corresponding increase in stress
Elongation at yield gives the upper limit for application that can tolerate a small deformation

58. Terms and Definitions Break Point Ultimate Strength
Shows the strain value at which the test bar breaks
Ultimate Strength
Measures the highest stress value
Used for general strength comparisons

59. Terms and Definitions Elastic Modulus
The slope of the linear region of the stress-strain curve
Ratio of stress-strain response
Used to compare materials and make structural calculations
Units of psi or Pa

60. Short Term Mechanical Test
Tensile
Flexural
Compressive
Impact
Hardness
Coefficient of Friction

61. Tensile Tester Measures a plastics stiffness
After the test bar is clamped in the jaw, the jaws then move at a constant rate of separation
The force required for movement is recorded

62. Tensile Test Data Tensile Modulus measure a plastics stiffness
Used for comparisons and structural calculations
The higher the modulus the greater the stiffness
Tensile stress at yield establishes an upper limit for applications that can tolerate a small permanent deformation

63. Tensile Test Data
Elongation at yield is the strain value at the yield point
Determines the upper limit for application that can tolerate small permanent deformations
Tensile Stress at Break is the stress applied at the time of fracture
Establishes an upper limit for
One time use applications that fail due to fracture
Parts that can still function with large deformations

64. Tensile Test Data
Elongation at Break measures the strain at fracture as a percentage of elongation
Useful for applications that fail by fracture
Ultimate Strength measures the highest stress value during the tensile test
Useful for comparing general strengths between plastics
Ultimate Elongation is the elongation at the breaking point

65. Stress Strain Curves

66. Stress Strain Curves

67. Poisson’s Ratio
Parts subjected to tensile or compressive stress deform in two directions
Poisson’s Ratio measures the lateral to longitudinal strains

68. Poisson’s Ratio Usually between 0.35 to 0.42 for plastics
Required for many structural analysis calculations

69. Flexural Tester

70. Flexural Test Data
Flexural Modulus is the ratio of stress to strain in the elastic region of the stress strain curve
Measures the plastics stiffness in bending
Compressive and tensile forces are both measured as a result of bending
Used in bending structural calculations
Test values for tensile modulus correspond well with flexural modulus for solid plastics

71. Flexural Test Data
Ultimate Flexural Stress is the highest value of stress on the stress-strain curve
Measures the level after which severe deformation or failure will occur

72. Flexural Properties

73. Compressive Tester Measures a materials hardness
The test specimen is compressed at a constant strain rate between two parallel platens until it ruptures or deforms by a certain percentage

74. Compressive Test Data Shows a materials hardness and load capabilities
Compressive Strength measures the maximum compressive stress recorded during the test
Useful in structural calculations for load bearing applications

75. Compressive Properties

76. Shear Strength
Measures the shearing force required to make holes or tears in the plastic
Useful in structural calculations for parts that may fail in shear
Data does not account for stress concentrations or mold-in stresses

77. Tear Strength
The force required to rip the plastic divided by the thickness
Provides relative data for comparing materials

78. Impact Tester

79. Impact Test
Impact Strength measures a plastics ability to absorb and dissipate energy
Hard to relate at actual part performance
Part geometry
Temperature
Stress concentrations
Molding stresses
Impact speed

80. Impact Tests Izod is most widely used
Uses horizontally notched sample to concentrate impact
Charpy uses a vertically notched sample
Use for comparing materials relative impact strength

81. Tensile and Impact
Impact Strength and Tensile Modulus provide insight into a plastics mechanical nature
High impact strength and large tensile modulus suggest a tough material
High impact strength and small tensile modulus indicates a ductile, flexible material
Low Impact strength and a large tensile modulus typify a brittle material

82. Hardness Tester
A load is applied to an indentor, which presses against the plastic

83. Hardness Data

84. Abrasion Resistance
Abrasion Resistance is measured by applying a Taber Abrader with 250gr weight and a CS 10-F textured abrader to a test specimen for a set number of cycles
Then measuring the changes in volume and transparency

85. Abrasion Resistance Data

86. Coefficients of Friction
Ratio of the friction force, the force needed to initiate sliding, to the normal force, the force perpendicular to the contact surface

87. Coefficients of Friction (Static) Ranges for Various Materials

88. Long Term Mechanical Properties
Creep
Stress Relaxation
Fatigue

89. Creep Short Term testing gives us data for periodic loading
It is not unusual for plastic parts to be subjected to continuous loading or loads that last a long time
The viscous nature of plastics make these long term loading to be of interest even if small
Creep is the deformation or strain due to viscous or cold flow
To design parts that are subjected to long term loading, the designer must utilize creep data

90. Examples of Creep

91. Creep The time and temperature dependent creep modulus of a polymer is
Manufacturers generate creep data by subjecting molded test specimen to varying stress level and measuring the change in dimension over time

92. Creep Data

93. Creep Sample Problem
How much would the material be strained after 1000 hours at a constant stress of 2800 psi?

94. Stress Relaxation
Stress relaxation data is used for applications where strain levels remain constant over a long period of time
When plastics are stretched, compressed, bent or sheared to a fixed value of strain, the stress value decrease with time due to the viscous effects(molecular relaxation)

95. Stress Relaxation Examples

96. Stress Relaxation
The time and temperature dependent relaxation modulus of a polymer is
Stress relaxation data is generated by applying a fixed strain to molded samples and measuring the gradual decrease in stress with time

97. Stress Relaxation Data

98. Stress Relaxation Sample Problem
What is the stress of the polycarbonate after 104 hours at a 2% constant strain?

99. Fatigue
Fatigue properties are used when designing parts that are subjected to repeated or cyclic loadings
Tests are ran in bending, torsion and tension

100. Fatigue Curves

101. Fatigue Example Problem
What is the amount of stress that will lead to failure after 1 million cycles for
Tensile = 34N/m2
Bending = 38N/m2

102. Thermal Properties

103. Thermal Properties Glass Transition Temperature Melting Temperature
Coefficient of Thermal Expansion
Deflection Under Load
Thermal Conductivity
Specific Heat
Vicat Softening Temperature

104. Glass Transition and Melting Temperature
Specific volume vs temperature provides
Tm = melting temperature
Tg = glass transition temperature

105. Melting Temperature
While cooling the melt, the specific volume of the melt sharply drops at a temperature which is termed as Tm.
This is due to the crystalline regions forming
Only for semi-crystalline plastics

106. Glass Transition Temperature
While cooling non-crystalline polymer melt there is no sharp drop in specific volume and the melt becomes highly viscous and it appears like solid.
Since the glass behaves in this manner the temperature at which the specific volume curve changes its slope is called Tg- glass transition temperature.

107. Glass Transition Temperature
Polymer becomes :
hard, stiff and brittle below Tg
highly viscous but solid at Tg
rubbery, flexible and softer above Tg
Both amorphous and semi-crystalline plastics have Tg

108. Coefficient Of Linear Thermal Expansion
Measures the change in length per unit length of a material per unit change in temperature
Expressed in in/in/°F or cm/cm/°C
Used to calculate the dimensional change resulting from thermal expansion
Very important when components of an assembly are made of different materials

111. Heat Deflection Under Load
Used to compare elevated temperature performance of plastics under load
Temperature requirements often limit plastics choice more than any other factor
Does not represent the upper temperature limit
Molding factors, sample preparation and thickness significantly affects the values

112. Heat Deflection Under Load
The test bar is loaded on a support, the temperature raises until the applied load causes the bar to deflect

113. Vicat Softening Temperature
Ranks the thermal performance of plastics according to the temperature that causes a specified penetration by a lightly loaded probe
Used as a general indicator of short term, high temperature performance
Less sensitive to sample thickness and molding effects
Often used as the ejection temperature

114. Vicat Softening Temperature
A flat ended probe contacts a plastic specimen submerged in a heated oil bath
A specified load is applied and the temperature is increased
Temperature of the oil bath when penetration takes place

115. Thermal Conductivity
Indicates a materials ability to conduct heat energy
Measured in Btu*in/(hr*ft2*°F) or W/(°K*m)
Used to calculate heating and cooling requirements in mold filling, thermal insulation or heat dissipation analysis

116. Thermal Conductivity Data

117. Specific Heat
Reflects the heat required to cause a one degree temperature change in a unit mass of material
Measured in Btu/lb/°F or KJ/kg/°C
Used in heat transfer calculations from mold filling and cooling analysis

118. Electrical Properties

119. Electrical Properties
Resistivity
Dielectric
Dissipation
Arc Resistance

120. Resistivity Measure of a plastics electrical insulating properties
Used to compare plastics as electrical insulators
Indicates current leakage through an insulating body
Should be at least 108 ohm*cm to be considered an insulating material

121. Volume Resistivity Data

122. Dielectric Strength and Constant
Dielectric Strength measures the voltage an insulating material can withstand before electrical breakdown occurs
Best indicator of a material’s insulating capabilities
Measured in volts per mil of thickness
Higher values indicate better insulating characteristics

123. Dielectric Strength Data

124. Dielectric Strength and Constant
The Dielectric Constant is the ratio of the capacitance of a plate electrode system to a test specimen
Lower values indicated better insulating characteristics

125. Dissipation Factor
Measures a plastics tendency to convert current into heat
Important in applications such as radar and microwave equipment that run at high frequencies
Lower values indicate less power loss and heat generation

126. Arc Resistance
Measures the number of seconds a plastics surface will resist forming a continuous conductive path while being exposed to high voltage electric arc
Plastics with higher values are used in closely spaced conductors, circuit breaker and distributor cap applications

127. Environmental Properties

128. Environmental Properties
Pay close attention to the environment to which the part will be exposed during
Processing
Secondary Operations
Assembly
End Use
Chemical exposure and weather conditions may determine which plastic you choose

129. Environmental Properties
Water Absorption
Hydrolytic Degradation
Chemical Resistance
Weatherability
Gas Permeability

130. Water Absorption
Plastics absorb water to varying degrees, depending on their molecular structure, fillers and additives
Adversely affects both mechanical and electrical properties and causes swelling
Measures the amount of water absorbed as a percent of total weight

131. Hydrolytic Degradation
Exposing plastics to moisture at elevated temperature can lead to hydrolysis
A chemical process that severs polymer chains by reacting with water
Reduces the molecular weight and degrades the plastic
Degree of degradation depends on
Exposure time
Temperature
Stress levels

132. Chemical Resistance Chemical Resistance of a plastic depends on
The chemical
Exposure time and temperature
Stress level
Type of chemical attack varies with the plastic and the chemical
Degradation
Stress cracking
Swelling
Consider all substances a part will encounter
Manufacturing
Assemble
Storage
End Use

133. Weatherability
Plastics in outdoor use are exposed to weather that can affect the performance of the part
Ultraviolet radiation can cause embrittlement, fading and surface cracking
Actual and accelerated testing
Additives and higher molecular weight can improve stability

134. Gas Permeability
Measures the amount of gas that can pass through a plastic in a given time
Used in packaging and medical applications, where the plastic forms a barrier

135. Other Properties

136. Other Properties Density Specific Gravity Specific Volume
Transmittance
Refractive Index
Flammability

137. Density Mass per unit volume
Useful in converting volume into part weight and cost calculations
Expressed in lb/ft3 or Kg/m3

138. Specific Gravity
The ratio of a material's density to the density of water
Used in a variety of calculations and comparisons when relative weight matters

139. Specific Volume
The reciprocal of density
Measured in ft3/lb or m3/Kg

140. Density and Specific Volume Data

141. Transmittance Measures a material’s transparency
Haze is the percentage of transmitted light passing through a plastic that is scattered
Luminous transmittance is the ratio of transmitted light to incident light

142. Transmittance Data

143. Refractive Index
Ratio of light’s velocity in a vacuum to its velocity as it passes through a plastic
Important in optical lens and light-pipe calculations

144. Refractive Index Data

145. Flammability
Most Plastics need an additive to meet flame resistance ratings
Oxygen Index measures the percentage of oxygen need to support flame in a plastic sample
UL 94 Classes
Established by Underwriter Laboratories to classify the burning behavior of plastics