Standard injection molding equipment handles polyesters with moderate to high injection pressures (70-140 MPa) depending on glass loading. General-purpose screws with compression ratio of 2.5:1 to 3.0:1 work well. Hot runner systems reduce waste and improve cycle time. Desiccant or dehumidifying hopper dryers required to achieve <0.02% moisture content before processing. Glass-reinforced grades wear screws and require hardened barrels, screws, and non-return valves for production runs.
Thermoplastic Polyester (PET/PBT) Overview
Polyester resins, specifically polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), are high-performance engineering thermoplastics known for excellent mechanical properties, electrical insulation, thermal stability, and chemical resistance. PET dominates packaging applications with its exceptional clarity and barrier properties, appearing in beverage bottles, food containers, and textile fibers. PBT serves automotive and electrical markets where rapid crystallization, dimensional stability, and low moisture absorption matter more than optical clarity. Engineers choose between PET and PBT based on crystallization speed, processing requirements, moisture sensitivity, and end-use performance demands.
A quick distinction: thermoset polyester, also known as unsaturated polyester, is commonly used in casting, sheet molding compounds, fiberglass layups, and composite prepregs. However, this article will focus only on thermoplastic polyesters.
Both polyesters process through injection molding, extrusion, blow molding, and thermoforming at temperatures ranging from 240-290°C. The semi-crystalline molecular structure provides stiffness, strength, and chemical resistance while maintaining processability. PBT crystallizes rapidly during cooling, enabling short cycle times (20-45 seconds) and high productivity in injection molding. PET crystallizes more slowly, requiring longer cooling periods or specialized nucleating agents for bottle production. Glass reinforcement (10-50%) transforms these materials into structural components replacing metals in automotive and industrial applications, raising heat deflection temperatures from 65-85°C to 200-230°C.
Material selection between PET and PBT hinges on application-specific trade-offs. Choose PET when optical clarity, high strength, and FDA food contact approval drive requirements, accepting slower crystallization and higher processing temperatures (260-290°C). Select PBT when rapid processing, dimensional stability in humid environments, and continuous use temperatures to 140°C justify its premium cost over PET. PBT absorbs less moisture (0.08-0.15% vs 0.1-0.3% for PET), maintaining tighter tolerances in electrical connectors and precision gearboxes. Glass-reinforced grades of both materials offer exceptional stiffness (8,000-14,000 MPa modulus) for thin-wall structural designs and metal replacement.
Regulatory compliance positions PET as the polymer of choice for food contact applications. FDA 21 CFR 177.1630 approves PET for direct food contact, supporting its dominance in beverage bottles and food packaging. Recycling code #1 identifies PET in consumer recycling streams, with established mechanical and chemical recycling infrastructure converting post-consumer bottles into fiber, sheet, and molding compounds. PBT achieves UL94 V-0 flame ratings in electrical applications, meeting safety requirements for switches, circuit breakers, and automotive electrical components. Both materials comply with RoHS and REACH regulations for electronics and automotive markets.
Packaging leverages PET transparency, strength, and barrier properties in beverage bottles, food containers, and thermoformed clamshells. Automotive applications specify PBT for electrical connectors, sensor housings, and fuel system components where chemical resistance and dimensional stability prevent warranty claims. Textile manufacturers extrude PET into fibers for clothing, carpets, and industrial fabrics, capitalizing on strength and abrasion resistance. Electronics benefit from PBT electrical insulation in switches, relays, and transformer housings. Industrial sectors use bearing-grade PBT for gears, bushings, and conveyor components where low friction and wear resistance extend service life. As a polyester resin supplier, Formerra provides access to PET and PBT grades for packaging, automotive, electronics, and industrial applications.
Performance Characteristics
Mechanical Properties
Tensile strength
50-70 MPa (unreinforced),
100-200 MPa (glass-reinforced)
Tensile modulus
2,800-3,100 MPa (unreinforced),
8,000-14,000 MPa (glass-reinforced)
Flexural strength
80-110 MPa (unreinforced),
140-280 MPa (glass-reinforced)
Flexural modulus
2,400-2,900 MPa (unreinforced),
7,000-12,000 MPa (glass-reinforced)
Elongation at break
50-300% (unreinforced),
2-4% (glass-reinforced)
Notched Izod impact
25-60 J/m (unreinforced),
50-100 J/m (glass-reinforced PBT)
Hardness (Rockwell)
R108-118
Thermal Properties
Continuous use temperature
115-140°C (PBT higher than PET)
Heat deflection temperature
55-65°C (unreinforced PET),
65-85°C (unreinforced PBT), 200-230°C (glass-reinforced PBT)
Melting temperature
245-265°C (PET),
220-235°C (PBT)
Glass transition temperature (Tg)
70-80°C (PET),
40-60°C (PBT)
Processing temperature range
260-290°C (PET),
240-270°C (PBT)
Coefficient of linear thermal expansion
70-80 x 10⁻⁶/°C (unreinforced),
20-35 x 10⁻⁶/°C (glass-reinforced)
Thermal conductivity
0.24 W/(m·K)
Flammability rating
UL94 HB (standard),
V-2 to V-0 (flame-retardant grades)
Operating Environment
Water absorption
PET: 0.1-0.3% (24h at 23°C), better dimensional stability in humid environments. PBT: 0.08-0.15% (24h), lowest moisture uptake among engineering thermoplastics. Both maintain properties in wet conditions better than nylon.
UV/weatherability rating
Fair to Good with UV stabilizers. Unprotected grades yellow and lose mechanical properties. PET more susceptible to UV degradation than PBT. Stabilized grades suitable for outdoor applications.
Hydrolysis resistance
Moderate. Prolonged exposure to hot water/steam (>80°C) causes hydrolytic degradation. PBT slightly better hydrolysis resistance than PET. Not recommended for continuous hot water contact without hydrolysis-resistant grades.
Stress cracking sensitivity
Good resistance to environmental stress cracking. Better than polycarbonate for chemical environments. Some solvents (chlorinated hydrocarbons, aromatic solvents) cause stress cracking under load.
Electrical Properties
Dielectric strength
18-25 kV/mm
Dielectric constant (1 MHz)
3.0-3.3
Volume resistivity
10¹⁴-10¹⁵ ohm·cm
Dissipation factor (1 MHz)
0.01-0.02
Comparative Tracking Index (CTI)
175-600 (varies by grade and reinforcement)
Arc resistance
120-180 seconds
Physical Properties
Specific gravity
1.31-1.38 (unreinforced),
1.45-1.65 (glass-reinforced)
Water absorption
PET 0.1-0.3%,
PBT 0.08-0.15% (24 hours at 23°C)
Shrinkage
1.5-2.5% (unreinforced),
0.3-1.0% (glass-reinforced)
Mold shrinkage anisotropy
Higher in flow direction with glass reinforcement
Chemical Resistance
Excellent resistance
Dilute acids, dilute bases, alcohols, mineral oils, greases, aliphatic hydrocarbons, salt solutions
Good resistance
Gasoline, diesel fuel, hydraulic fluids, detergents, many solvents at room temperature
Limited resistance
Strong acids (concentrated sulfuric, nitric), strong bases (sodium hydroxide >10%), hot water/steam (>80°C causes hydrolysis)
Poor resistance
Aromatic hydrocarbons (toluene, xylene), chlorinated solvents (methylene chloride), ketones (acetone), esters
Superior chemical resistance
to nylon for hydrocarbon and acidic environments
Strengths, Weaknesses, & Operating Limits
Key Strengths
- Excellent Dimensional Stability: Low moisture absorption (PBT 0.08-0.15%, PET 0.1-0.3%) maintains tight tolerances in humid environments. Superior to nylon, which absorbs 1.5-2.5% moisture and swells significantly.
- High Stiffness and Strength: Unreinforced grades provide 2,800-3,100 MPa modulus. Glass-reinforced grades reach 8,000-14,000 MPa, enabling thin-wall designs and metal replacement in structural applications.
- Excellent Electrical Properties: High volume resistivity (10¹⁴-10¹⁵ ohm·cm), good dielectric strength (18-25 kV/mm), and high CTI ratings make polyesters ideal for electrical connectors, switches, and circuit breaker components.
- Fast Crystallization (PBT): Rapid crystallization enables short cycle times (20-45 seconds) in injection molding, increasing productivity and reducing cost per part compared to slower-crystallizing polymers.
- Good Chemical Resistance: Resists dilute acids, bases, oils, greases, and most automotive fluids. Better hydrocarbon resistance than nylon, making PBT preferred for fuel system components and under-hood automotive applications.
- Recyclability (PET): Recycling code #1, widely recycled infrastructure. Post-consumer recycled PET (rPET) available for sustainable applications. Mechanical and chemical recycling processes established.
- FDA Food Contact Approved (PET): Multiple grades meet FDA 21 CFR 177.1630 for food contact, enabling beverage bottles, food containers, and packaging applications with proven safety record.
Known Weaknesses
- Hydrolysis Susceptibility: Prolonged exposure to hot water, steam, or humid heat (>80°C) causes chain scission and property loss. Limits use in autoclavable medical devices and continuous steam contact without special hydrolysis-resistant grades.
- Notch Sensitivity: Stress concentrations from sharp corners, threads, or molded-in inserts cause brittle failure. Requires generous radii and careful part design to avoid crack initiation points.
- Limited Heat Resistance: Maximum continuous use temperature 115-140°C restricts applications in high-heat automotive under-hood and electronics where PPA, PPS, or LCP offer better performance.
- UV Degradation: Unprotected grades yellow and lose properties under prolonged UV exposure. Requires UV stabilizers for outdoor applications, adding cost compared to inherently UV-stable materials like ASA.
- Processing Sensitivity: Moisture causes hydrolytic degradation during processing. Requires pre-drying to <0.02% moisture content (4 hours at 120°C for PBT, 4-6 hours at 160°C for PET). Inadequate drying produces splay marks, bubbles, and property loss.
Operating Limits
- Temperature extremes: Continuous use 115-140°C depending on grade (PBT higher than PET). Glass-reinforced grades extend service temperature. Short-term excursions to 180-200°C acceptable. Brittleness below -40°C limits cold-weather outdoor applications.
- Hydrolysis limits: Avoid prolonged contact with water/steam above 80°C. Standard grades unsuitable for autoclave sterilization (121°C) without hydrolysis-resistant formulations. Service life reduced in hot, humid environments above 85% RH at elevated temperatures.
- Chemical attack: Strong acids (pH <2), strong bases (pH >12), chlorinated solvents, aromatic hydrocarbons, and ketones degrade polyester. Stress cracking possible under load in marginal chemical environments.
- Processing constraints: Moisture content must be <0.02% before processing. Drying time 4-6 hours at elevated temperature. Melt temperatures 240-290°C. Regrind acceptable at 15-25% with proper drying and contamination control.
Typical Applications
- Beverage bottles (PET: water, soda, juice) with excellent clarity and barrier properties
- Automotive electrical connectors, sensor housings, ignition components (PBT)
- Food packaging, clamshells, thermoformed trays (PET)
- Textile fibers for clothing, carpets, industrial fabrics (PET)
- Electrical switches, circuit breakers, relay housings (PBT)
- Automotive exterior lighting, headlamp reflectors (PBT)
- Precision gears, bearings, bushings for low-friction applications
- Consumer electronics housings, appliance components
- Industrial conveyor components, wear strips (bearing-grade PBT)
Niche Applications
- Medical device housings requiring dimensional stability and chemical resistance (non-autoclave)
- Automotive fuel system components (filler necks, vapor canisters) for PBT chemical resistance
- High-performance textile fibers for outdoor apparel and industrial fabrics
- 3D printer filament (PET-G) for functional prototypes
- Electrical transformer bobbins, coil formers for high-temperature insulation
- Sheet and film for thermoforming, graphic overlays, membrane switches
- Bearing-grade components with PTFE/silicone lubrication for food processing equipment
Design, Assembly & Aesthetics
Surface finish capability
Excellent high-gloss achievable. Takes mold polish well. Glass-reinforced grades show fiber texture on surface. Gate blush possible with improper processing. Flow lines visible on large parts without proper gate design.
Sink/warpage/visible defects tendency
Moderate sink marks at heavy sections. Warpage risk with uneven wall thickness, especially in glass-reinforced grades due to shrinkage anisotropy. Post-mold shrinkage stabilizes within 24-48 hours.
Colorability
Good color range through masterbatch. Transparent, translucent, and opaque options. PET offers better clarity than PBT for transparent applications. Glass reinforcement limits transparency.
Color stability
Excellent with UV stabilization. Unprotected grades yellow with UV exposure. Heat-stable pigments required for processing temperatures (240-290°C).
Optical properties
PET offers excellent clarity for bottle applications (85-90% light transmission). PBT less transparent due to crystalline structure. Glass reinforcement eliminates transparency.
Scratch/chemical mar resistance
Moderate scratch resistance. Good resistance to oils, greases, cleaning chemicals. Surface hardness R108-118. Hard-coat treatments improve scratch resistance for optical applications.
Marking methods
Laser marking produces high-contrast marks. Pad printing, hot stamping, and inkjet printing work well. In-mold decoration possible. Screen printing suitable for graphics.
Coating/painting/plating suitability
Paintable with surface pretreatment (flame, plasma, corona). Metallization possible for decorative effects. Chrome plating used for automotive trim. Adhesion promotion required.
Joining methods
Ultrasonic welding produces strong joints. Vibration welding for larger parts. Hot plate welding for hermetic seals. Adhesive bonding with surface preparation. Mechanical fastening with proper boss design. Snap fits work well with proper geometry.
Frequently Asked Questions
Polyester resins include polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), semi-crystalline thermoplastics offering excellent mechanical strength, electrical insulation, and chemical resistance. PET dominates packaging with its optical clarity (85-90% light transmission) and FDA food contact approval (21 CFR 177.1630), appearing in beverage bottles, food containers, textile fibers for clothing and carpets, and thermoformed clamshells. PBT serves automotive electrical connectors, sensor housings, circuit breakers, and precision gears where rapid crystallization (20-45 second cycle times), dimensional stability (0.08-0.15% moisture absorption), and continuous use temperatures to 140°C justify premium cost over PET. Both materials process through injection molding, extrusion, and blow molding at 240-290°C, with glass reinforcement (10-50%) enabling metal replacement in structural applications.
PET and PBT differ primarily in crystallization speed, moisture absorption, and heat resistance. PET crystallizes slowly (45-90 second cycles), processes at 260-290°C, absorbs 0.1-0.3% moisture, and offers continuous use to 115°C with excellent optical clarity (85-90% light transmission) and FDA food contact approval for packaging applications. PBT crystallizes rapidly (20-45 second cycles for productivity), processes at 240-270°C, absorbs only 0.08-0.15% moisture (maintaining tighter tolerances than nylon), and provides continuous use to 140°C, making it preferred for automotive electrical connectors, circuit breakers, and precision components requiring dimensional stability. Glass-reinforced grades of both reach 8,000-14,000 MPa modulus for structural applications, but PBT's 10-20% cost premium over PET is justified when rapid processing, lower moisture sensitivity, or higher heat resistance prove critical.
PBT absorbs far less moisture than nylon (0.08-0.15% vs 1.5-2.5% at 24 hours), delivering superior dimensional stability in humid environments where nylon swells significantly. This fundamental difference makes PBT preferred for electrical connectors requiring tight tolerances (±0.1-0.2%) independent of humidity, precision gearboxes where dimensional changes cause noise or wear, and automotive under-hood applications where moisture fluctuates. PBT maintains consistent performance across 20-85% relative humidity while nylon dimensions change 0.3-0.5%, affecting assembly fits and electrical contact reliability. PBT also offers better hydrocarbon resistance than nylon for fuel system components. However, nylon provides higher impact strength, better wear resistance for unlubricated bearings, and lower cost, making material selection depend on whether dimensional stability or mechanical toughness proves more critical.
Injection molding processes polyesters at 240-290°C barrel temperature (PET higher, PBT lower) with 70-140 MPa injection pressure, requiring 4-6 hour drying at 120-160°C to reach <0.02% moisture for optical quality. PBT crystallizes rapidly enabling 20-45 second cycles for thin-wall parts while PET requires 45-90 seconds. Blow molding creates PET bottles using stretch blow molding at 260-280°C with precise preform heating and blowing pressure control for wall thickness uniformity. Extrusion produces PET textile fibers, sheet for thermoforming, and film at 260-280°C with cooling control maintaining optical properties. Thermoforming heats extruded PET sheet to 120-160°C for packaging clamshells and trays. All processes demand hardened screws for glass-reinforced grades, hot runner systems reducing scrap, and mold temperatures 60-120°C balancing crystallization with cycle time.
PET carries recycling code #1 with established mechanical and chemical recycling infrastructure converting post-consumer bottles into fiber, sheet, and molding compounds. Mechanical recycling sorts, cleans, and reprocesses PET bottles at 15-25% regrind ratios maintaining acceptable properties for packaging, textiles, and strapping applications. Chemical recycling depolymerizes PET into monomers (terephthalic acid and ethylene glycol) for repolymerization into virgin-quality resin, eliminating contamination concerns from mechanical recycling. Post-consumer recycled PET (rPET) reduces carbon footprint 60-70% versus virgin resin while meeting FDA food contact requirements for new bottles containing rPET content. PBT recycling is less established due to automotive and electronics applications making collection difficult, though post-industrial scrap recycles effectively at 15-25% ratios with proper drying. Both materials comply with RoHS and REACH regulations.
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