Last updated: 2026-09-04 · 5 min read

PBT vs. PET — How Do You Choose Between These Two Polyester Resins for Your Injection Molded Part, and What Are the Key Differences in Processing, Properties and Application?

PBT and PET are both semi-crystalline polyester resins — but PBT crystallizes 40–50% faster (cycle 15–25s vs. 25–45s), processes at lower temperature (240–270°C vs. 270–300°C), and flows better in thin walls. PET has 10–15% higher tensile strength and better hydrolysis resistance. Choose PBT for high-volume connectors and thin-wall electronics; choose PET for structural brackets and fluid-handling parts exposed to hot water or automotive fluids.

Key Facts

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  • Property comparison (unfilled): PBT tensile 55–60 MPa, PET 60–70 MPa. HDT at 1.8 MPa: PBT 60°C, PET 70°C. Melt temp: PBT 240–270°C, PET 270–300°C. Mold shrinkage: both 1.5–2.0% unfilled, 0.3–0.7% (flow) for GF30 grades.
  • GF30 comparison: PBT tensile 140–160 MPa, PET 160–180 MPa. Flexural modulus: PBT 9,000 MPa, PET 10,500 MPa. HDT: PBT 210°C, PET 230°C. PET is 10–15% stronger and stiffer in glass-filled grades.
  • Processing: Both require drying below 0.02% moisture. PBT: 80°C / 2–4h. PET: 120–140°C / 4–6h. PET mold temp 120–140°C (below 100°C = amorphous, brittle part). PBT mold temp 60–80°C.

    • Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.

    What Are the Key Property Differences Between PBT and PET?

    Six properties compared across unfilled and GF30 grades:

    Property PBT (unfilled) PET (unfilled) PBT GF30 PET GF30
    Tensile strength 55–60 MPa 60–70 MPa 140–160 MPa 160–180 MPa
    Flexural modulus 2,400 MPa 2,800 MPa 9,000 MPa 10,500 MPa
    HDT (1.8 MPa) 60°C 70°C 210°C 230°C
    Melt temperature 240–270°C 270–300°C 240–270°C 270–300°C
    Mold shrinkage 1.5–2.0% 1.5–2.0% 0.3–0.7% (flow) 0.3–0.7% (flow)
    Crystallization rate Fast (15–25s cycle) Slow (25–45s cycle) Fast (15–25s) Slow (25–45s)

    PET has 10–15% higher tensile strength and flexural modulus than PBT in both unfilled and glass-filled grades. PET processes at 20–30°C higher melt temperature. PBT crystallizes 40–50% faster — shorter cycle time. Both have similar mold shrinkage in unfilled grades but reduced and anisotropic shrinkage in glass-filled grades (0.3–0.7% flow, 0.7–1.2% transverse). The values are quoted from the resin supplier’s data sheet for the specific grade you select.

    When Do You Choose PBT Over PET — and When Do You Choose PET Over PBT?

    Four decision criteria:

    1. Choose PBT when cycle time is the priority. PBT crystallizes faster — 15–25s cycle vs. 25–45s for PET. At the same wall thickness, the 10–20s difference translates to 15–30% more parts per year from the same mold. For high-volume production (over 1 million pieces per year), cycle time drives the cost per part. PBT is standard for electrical connectors, automotive relays and consumer electronics housings.
    2. Choose PBT when thin-wall flow is the priority. PBT has lower melt viscosity than PET at the same temperature — it flows more easily into thin sections (below 1.0mm wall). For connectors with 0.5mm walls, PBT fills more reliably (lower injection pressure, less risk of short shots). PBT is standard for thin-wall electrical components and micro-connectors.
    3. Choose PET when mechanical strength is the priority. PET GF30 has 10–15% higher tensile strength and flexural modulus than PBT GF30. For structural parts under high loads, PET provides the higher stiffness-to-weight ratio. PET is standard for automotive structural brackets, load-bearing housings and industrial components.
    4. Choose PET when chemical resistance is the priority. PET has better resistance to hydrolysis (hot water and steam), automotive fluids (brake fluid, coolant) and solvents. At 80°C water immersion, PBT loses 30% tensile strength after 1,000 hours; PET loses less than 10%. For parts exposed to hot water or automotive fluids, PET retains properties longer.

    The decision tree is simple: if cycle time and thin-wall flow dominate → PBT. If mechanical strength and chemical/hydrolysis resistance dominate → PET. For dry-environment electrical parts → PBT. For wet or hot environments → PET.

    What Are the Processing Differences — and How Do They Affect the Mold Design?

    Four processing differences that affect mold design:

    Factor PBT PET Mold Design Impact
    Drying 80°C / 2–4h 120–140°C / 4–6h PET absorbs more moisture; insufficient drying → hydrolysis → brittle part
    Melt temperature 240–270°C 270–300°C PET requires higher-temp steel (H13/S136, 48–52 HRC); P20/718H acceptable for PBT only
    Mold temperature 60–80°C 120–140°C PET mold below 100°C → amorphous, brittle part; PET needs dedicated mold temp controller
    Shrinkage (GF30) 0.3–0.7% flow / 0.7–1.2% transverse 0.3–0.7% flow / 0.7–1.2% transverse Anisotropic shrinkage → warpage; balanced cooling and proper gate location required

    You cannot run PBT and PET in the same mold without modification — the processing temperatures differ by 30°C, the mold temperatures differ by 50–60°C, and the cycle times differ by 10–20 seconds. Running PBT at PET temperatures degrades the PBT (thermal decomposition). Running PET at PBT mold temperatures produces an amorphous, brittle part. If you need both resins, you need two separate molds.

    Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. The related guide covers adjacent topics.

    The PBT vs. PET decision is driven by crystallization rate and application environment. PBT crystallizes faster (shorter cycle, better for high volume). PET resists hydrolysis better (better for hot water and fluid exposure). Select the resin based on the application environment first, then optimize the process.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    How does hydrolysis resistance differ between PBT and PET?

    At 80°C water immersion for 1,000 hours: PBT loses 30% of its tensile strength; PET loses less than 10%. The difference is in the chemical structure — PET’s shorter glycol chain (ethylene vs. butylene) makes it less susceptible to water molecule attack.

    For parts exposed to hot water or steam (dishwasher components, hot water valves, automotive coolant parts), PET is the correct choice. For dry environments (electrical connectors, electronics housings), PBT is sufficient and more cost-effective.

    What is the optimal glass fiber content for structural parts?

    GF30 is the standard for structural parts — it provides 2.5–3× increase in tensile strength and flexural modulus over unfilled resin, with a good balance of strength, processability and surface finish.

    GF15 is better for cosmetic parts (better surface finish, lower warpage). GF50 provides maximum stiffness (3–4× increase) but with higher warpage risk and rougher surface. The values are quoted from the resin supplier’s data sheet for the specific grade you select.

    Can you use the same mold for both PBT and PET?

    No — the processing temperatures differ by 30°C (PBT: 240–270°C, PET: 270–300°C), the mold temperatures differ by 50–60°C (PBT: 60–80°C, PET: 120–140°C), and the cycle times differ by 10–20 seconds. Running one resin at the other’s conditions produces defective parts. Two separate molds with different temperature control systems are required.

    Key Takeaways

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  • PBT: faster cycle (15–25s), lower processing temp (240–270°C), better thin-wall flow.
  • PET: higher strength (+10–15%), better hydrolysis resistance, higher processing temp (270–300°C).
  • Choose PBT for connectors and thin-wall electronics; PET for structural and fluid-handling parts.
  • Both require drying below 0.02% moisture; PET needs higher drying temp (120–140°C vs.
    80°C).

    • If you are selecting between PBT and PET, send the part drawing, the application environment and the target volume. The review compares the two resins against your requirements (mechanical, thermal, chemical, cycle time) and recommends the resin with the best balance of performance and production cost.

    Sources: BASF Ultradur (PBT) Product Data · DuPont Rynite (PET) Technical Data · Omnexus — PBT vs PET Selection Guide · Celanese Celstran — PBT/PET Comparison · SABIC Valox (PBT) Data Sheets