Engineering Plastic Materials

This course introduces the material behavior that makes plastic product design different from designing with metals. It begins with the factors that influence material selection and part performance, then examines polymer structure, property modification, and representative plastic material families and their application characteristics.

Core Topics

Plastic material selection
Temperature, moisture, time, load, and environmental effects
Creep, stress relaxation, impact, and chemical resistance
Thermoplastics, thermosets, amorphous, and crystalline polymers
Fillers, modifiers, blends, and molecular-weight effects
Properties and applications of common plastic material families

Detailed Course Outline

How Plastic Material Behavior Differs from Metals
- Changes in material properties with temperature
- Effects of humidity and moisture absorption
- Effects of sustained loading and time
- Chemical exposure
- Sunlight and ultraviolet exposure
- Creep under sustained load
- Stress relaxation under clamp loads and spring applications
- Designing with properties appropriate to actual service conditions rather than room-temperature values alone
Material Effects on Part Performance
- Changes in yield strength and modulus with temperature
- Moisture effects on part dimensions
- Moisture effects on strength and impact properties
- Differences in chemical resistance among plastics
- Environmental stress cracking
- Permanent deformation and incomplete recovery of plastic components
- Designing around allowable deflection
- Relating loads, stresses, environment, and duration to material selection
Application Conditions to Consider
- Static loads
- Cyclic loads
- Impact loading
- Thermal stresses
- Operating temperature range
- Duration of exposure at temperature
- Chemical and environmental stresses
- Processing-induced stresses
- Assembly stresses
- Overmolding
- Machining
- Spin welding
- Press fits
- Snap fits
- Screw assembly
- Shipping stresses
- Cold-temperature handling and impact
- Installation conditions
Material Selection Criteria
- Physical properties at normal use temperatures
- Physical properties at temperature extremes
- Creep modulus at the expected temperature, stress level, and load duration
- Chemical resistance under stress
- UL flame-rating requirements
- Molding characteristics
- Mold shrinkage
- Relationship between shrinkage and dimensional variation
- Sink and warpage considerations
- Longitudinal and transverse shrinkage
- Effects of part thickness and crystallinity on shrinkage
- Effects of glass-fiber orientation
- Effects of glass bead and mineral fillers
Molecular Weight and Material Consistency
- Relationship between molecular weight and material strength
- Relationship between molecular weight and viscosity
- Higher molecular weight and increased flow resistance
- Batch-to-batch molecular-weight variation
- Molecular-weight distribution
- Relationship between narrow molecular-weight distribution and repeatability
- Effects of viscosity variation on molding pressure requirements
- Non-Newtonian behavior of plastic melts
- Effect of shear rate on viscosity
- Interaction between viscosity and part geometry
- Wall-thickness effects on required filling pressure
- Flow behavior in rectangular and round sections
Using Material and Design Data
- Material supplier design guides
- Material-specific engineering data
- Using creep, impact, moisture, temperature, and chemical data during material selection
- Recognizing limitations of published material-property values
- Using application-specific conditions when interpreting published data
Strain Rate and Loading Rate
- Effect of loading rate on plastic mechanical behavior
- Static versus high-speed loading
- Increasing stiffness with increasing strain rate
- Reduction in strain-to-failure at higher loading rates
- Relationship between temperature and ductility
- Balancing low-temperature impact strength with high-temperature stiffness
- Balancing impact performance with creep resistance
- Allowable strain considerations
Temperature, Moisture, and Mechanical Properties
- Stress-strain behavior under different environmental conditions
- Dry-as-molded versus moisture-conditioned properties
- Effect of relative humidity on nylon properties
- Temperature effects on tensile behavior
- Moisture effects on flexural modulus
- Moisture-related dimensional change
- Interaction of temperature and moisture during material selection
Creep and Stress Relaxation
- Time-dependent deformation in plastic materials
- Creep under sustained load
- Stress relaxation under fixed deformation
- Initial strain
- Increasing deformation with time
- Recovery after removal of load
- Permanent deformation after creep
- Time-dependent stress-strain behavior
- Isochronous stress-strain curves
- Effect of stress level on creep rate
- Apparent modulus
- Using creep modulus rather than short-term modulus for sustained-load applications
Impact Strength
- Notched and unnotched impact behavior
- Izod impact testing
- Gardner falling-dart impact testing
- Effect of temperature on impact strength
- Effect of moisture condition on impact performance
- Effect of fillers on impact behavior
- Effect of part thickness on impact strength
- Notch sensitivity
Moisture During Processing and Service
- Moisture content during molding
- Material drying considerations
- Hydrolytic stability
- Long-term heat and humidity exposure
- Retention of mechanical properties after moisture exposure
- Moisture effects on impact strength
- Material selection for hot-water and steam environments
UV Exposure and Property Retention
- Effects of ultraviolet exposure on plastic materials
- Use of stabilizers and modifiers
- Deterioration of stabilizer effectiveness over time
- Preference for base material properties that inherently meet application requirements when possible
Chemical Resistance and Environmental Stress Cracking
- Chemical type and concentration
- Exposure time
- Exposure temperature
- Stress level in the component
- Polymer-chain degradation
- Stress cracking and crazing
- Swelling caused by weak solvents
- Changes in mechanical properties after chemical exposure
- Intentional and accidental chemical exposure
- Manufacturing and assembly chemicals
- Cutting oils
- Degreasers and cleaning solvents
- Printing dyes and paints
- Adhesives and lubricants
- Relationship between temperature, concentration, stress, and chemical attack
- Using compatibility data as a screening tool rather than final proof of suitability
- End-use testing for stressed components
Variation Within a Polymer Family
- Property differences among grades of the same polymer type
- Molecular-weight effects within a material family
- Lot-to-lot variation
- Variation within individual production lots
- Supplier process-control capability
- Effects of polymer-chain length on melting point
- Effects on tensile and compressive strength
- Effects on toughness and impact strength
- Effects on glass-transition temperature
- Importance of material consistency when selecting suppliers and grades
Thermoplastics and Thermosets
- Basic differences between thermoplastic and thermoset materials
- Ability of thermoplastics to be remelted and reformed
- Cross-linked structure of thermosets
- Resistance of thermosets to creep
- Heat and solvent resistance
- Impact and mechanical-property differences
- Representative thermoplastic families
- Representative thermoset families
- Polyester
- Vinyl ester
- Epoxy
- Phenolic
- Urethane
- Polyimide
Amorphous and Crystalline Polymers
- Basic structural differences
- Representative amorphous polymers
- Representative crystalline polymers
- Relationship between polymer structure and material behavior
- Relationship between structure and shrinkage
- Relationship between structure and dimensional behavior
- Comparison of material stiffness and modulus
Fillers, Modifiers, Alloys, and Copolymers
- Property modification through fillers
- Glass-fiber reinforcement
- Mineral fillers
- Glass bead fillers
- Impact modifiers
- Blending and copolymerization
- Combining polymer systems to alter material properties
- Effects on stiffness, toughness, dimensional stability, and processing
- Coefficient-of-friction considerations
Polyethylene Family
- Low Density Polyethylene — LDPE
- High Density Polyethylene — HDPE
- Ultra High Molecular Weight Polyethylene — UHMWPE
- Differences in flexibility, stiffness, strength, wear, and moldability
- Moisture and corrosion resistance
- Impact performance
- Wear and abrasion applications
- Representative consumer, piping, packaging, and mechanical applications
Polypropylene and Polyolefin Copolymers
- Polypropylene properties and applications
- Comparison with polyethylene
- Mechanical strength and stiffness
- Fatigue resistance
- Integral-hinge applications
- Temperature capability
- Chemical resistance
- UV limitations and stabilization
- Low-density applications
- Polyallomers and polyethylene/polypropylene copolymers
- Balancing moldability with mechanical properties
Thermoplastic Elastomers
- TPE material behavior
- Santoprene as a representative thermoplastic elastomer
- Combining rubber-like properties with thermoplastic processing
- Flexibility and hardness ranges
- Temperature capability
- Fatigue and tear resistance
- Oil, grease, acid, base, and aqueous chemical resistance
- Solvent-related swelling considerations
- Flame-retardant and application-specific grades
Vinyl Materials
- PVC
- Rigid and plasticized PVC
- CPVC
- PVDC
- Corrosion and weather resistance
- Temperature limitations
- Flammability behavior
- Electrical-insulation applications
- Pipe and structural applications
- Toughness and notch-sensitivity considerations
- Chemical-resistance limitations
Styrenic Materials
- Polystyrene
- High-impact styrene
- Styrene copolymers
- ABS
- Differences in hardness, brittleness, toughness, and impact strength
- Moldability and processing characteristics
- UV limitations
- Chemical and solvent resistance
- Stress-cracking and crazing considerations
- Representative consumer-product applications
Polycarbonate and Polycarbonate Blends
- Polycarbonate — PC
- Impact strength
- Temperature resistance
- Transparency
- Structural applications
- PC/ABS blends
- PC/PET blends
- Using blends to balance properties
Polyesters and High-Performance Thermoplastics
- PET
- PBT
- Wear resistance
- Coefficient of friction
- Flexural modulus
- Dimensional stability
- Moisture-resistance advantages compared with nylon
- Glass-reinforced polyester grades
- Notch sensitivity
- Hot-water, steam, and oxidizing-chemical limitations
- PEEK
- PPS
- High-temperature applications
- Chemical resistance
- Glass-filled high-performance grades
Acetal
- Polyoxymethylene — POM
- Homopolymer and copolymer grades
- Mechanical properties
- Moisture absorption compared with nylon
- Dimensional stability
- Wear and sliding applications
- Filled and lubricated grades
- Molding shrinkage
- Use in gears, cams, bushings, and mechanical components
Polyamides — Nylon
- Nylon 6
- Nylon 6/6
- Nylon 6/10
- Nylon 6/12
- Nylon 11 and Nylon 12
- Mechanical-property differences among nylon families
- Moisture absorption
- Effect of moisture on strength, stiffness, toughness, and dimensions
- Operating-clearance considerations
- Processing-temperature differences among grades
- Reinforced and specialty nylon systems
- Representative structural and mechanical applications
Applying Material Selection to Product Design
- Matching the material class to actual product stresses
- Matching material capability to the complete temperature range
- Accounting for duration of loading
- Accounting for moisture and chemical exposure
- Accounting for processing and assembly stresses
- Considering molding behavior and dimensional requirements
- Evaluating filled, modified, blended, or copolymer grades when appropriate
- Recognizing that published property values are only a starting point
- Verifying material suitability under actual application conditions

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