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Injection Molding Fundamentals
Lesson 9 of 10 12 min

Materials Selection

Thermoplastic Categories

Thermoplastics for injection molding are categorized by performance and cost:

Commodity Plastics

High volume, low cost, general-purpose applications.

MaterialPropertiesApplications
PP (Polypropylene)Light, chemical resistant, living hingesPackaging, automotive interior, appliances
PE (Polyethylene)Flexible, chemical resistant, low costContainers, caps, toys
PS (Polystyrene)Rigid, clear (GPPS), easy processingPackaging, disposables, housings
ABSTough, good surface, electroplatableAutomotive trim, electronics, LEGO

Engineering Plastics

Higher performance, balanced properties.

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MaterialPropertiesApplications
PA (Nylon)Strong, wear resistant, absorbs moistureGears, bearings, connectors
POM (Acetal)Low friction, dimensional stabilityGears, clips, fuel system
PC (Polycarbonate)Impact resistant, transparent, heat resistantLenses, housings, safety equipment
PBTDimensional stability, chemical resistantElectrical connectors, housings
PETClear, barrier properties, recyclableBottles, packaging, fibers

High-Performance Plastics

Extreme environments, premium applications.

MaterialPropertiesApplications
PEEKHigh temperature, chemical resistantAerospace, medical, oil & gas
PPSChemical resistant, flame retardantUnder-hood automotive, electrical
PEI (Ultem)High temperature, flame retardantAerospace, medical, electronics
LCPThin-wall flow, dimensional stabilityConnectors, micro-parts

Key Properties for Selection

Mechanical Properties

PropertyWhat It MeasuresUnits
Tensile strengthResistance to pulling apartMPa
Flexural modulusStiffness in bendingGPa
Impact strengthEnergy absorption (Izod/Charpy)J/m or kJ/m²
Elongation at breakHow much it stretches before breaking%

Thermal Properties

PropertyWhat It MeasuresUnits
HDTTemperature at which part deforms under load°C
Continuous use tempMax long-term service temperature°C
Melt temperatureProcessing temperature°C
Thermal conductivityHeat transfer rateW/m·K

Other Properties

PropertyRelevance
Chemical resistanceExposure to fuels, solvents, cleaners
UV stabilityOutdoor applications
FlammabilityUL94 rating for electronics
TransparencyOptical applications
Food contactFDA/EU compliance

Material Comparison

Typical Property Ranges

MaterialTensile (MPa)Flex Mod (GPa)HDT (°C)Cost Relative
PP25-401.0-1.560-1001×
ABS40-502.0-2.590-1001.5×
PA6670-852.5-3.0180-2502×
PC55-702.2-2.5130-1402.5×
POM60-702.5-3.0100-1102×
PEEK90-1003.5-4.0250-31030×
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Reinforcements and Additives

Glass Fiber Reinforcement

Effects:
  • Increases strength and stiffness by 2-3×
  • Increases HDT significantly
  • Reduces shrinkage
  • Causes warpage if not balanced
  • Increases wear on tooling
Common Loading: 10-50% by weight

Other Additives

AdditivePurpose
Carbon fiberHigh stiffness, conductivity
Mineral fillersStiffness, reduced cost
UV stabilizersOutdoor durability
Flame retardantsUL94 ratings
ColorantsAesthetics
Impact modifiersToughness at low temperatures

Processing Considerations

Moisture Sensitivity

Hygroscopic materials must be dried before molding:

MaterialDrying RequiredTemp (°C)Time (hr)Max Moisture (%)
PPNo---
ABSRecommended802-40.1
PAYes804-160.2
PCYes1202-40.02
PETYes1404-60.02
PEEKYes1503-40.02

Shrinkage

MaterialShrinkage Range (%)
PP1.0-2.5
ABS0.4-0.7
PA661.0-2.0 (higher if crystalline)
PC0.5-0.7
POM1.8-2.5
PEEK1.0-1.5
Note: Glass-filled grades have lower shrinkage (0.2-0.5%).

Flow Characteristics

MaterialFlow (L/t ratio)Processing
PPExcellent (300+)Easy
ABSGood (200)Easy
PA66Good (200)Moderate
PCModerate (150)Moderate
PEEKLow (100)Difficult
LCPExcellent (350+)Easy but fast freeze

Material Selection Process

Step 1: Define Requirements

  • Operating temperature range
  • Mechanical loads
  • Chemical exposure
  • Regulatory requirements (food, medical, flame)
  • Aesthetic requirements
  • Cost target

Step 2: Screen Candidates

Eliminate materials that don't meet critical requirements:

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  • Temperature: HDT > max service temp
  • Chemistry: Resistant to exposure chemicals
  • Regulations: Meets required certifications

Step 3: Compare Trade-offs

Among remaining candidates, balance:

  • Performance vs. cost
  • Processability vs. properties
  • Availability vs. lead time

Step 4: Prototype and Test

  • Mold prototype parts
  • Test in actual application
  • Validate assumptions

Industry-Specific Selection

Automotive Interior

  • Standard: PP, ABS, PC/ABS
  • Needs: Aesthetics, scratch resistance, low odor
  • Requirements: OEM material specs, VOC limits

Automotive Under-Hood

  • Standard: PA66-GF, PBT-GF, PPS
  • Needs: Heat resistance, chemical resistance
  • Requirements: Temperature cycles, fluid exposure

Consumer Electronics

  • Standard: ABS, PC, PC/ABS
  • Needs: Flame rating, aesthetics, EMI shielding
  • Requirements: UL94 V-0 or V-1

Medical Devices

  • Standard: PC, ABS, PP (medical grades)
  • Needs: Biocompatibility, sterilization resistance
  • Requirements: USP Class VI, ISO 10993

Key Takeaways

  • Commodity plastics (PP, ABS): Low cost, high volume, general use
  • Engineering plastics (PA, PC, POM): Better performance, higher cost
  • High-performance (PEEK, PPS): Extreme conditions, premium cost
  • Glass fiber increases strength and stiffness but causes warpage
  • Hygroscopic materials (PA, PC, PET) must be dried before molding
  • Selection process: Define requirements → Screen → Compare → Test

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