The short answer
Unfilled general-purpose PBT typically runs 1.30–1.31 g/cm³ density, 55–60 MPa tensile yield strength and a melting point of about 223–225°C / 433–437°F. Heat deflection is only about 50–65°C / 122–149°F at 1.8 MPa but 115–165°C at 0.45 MPa, which is why load-bearing hot parts usually use glass-filled PBT. Melt flow is usually reported as MVR at 250°C / 2.16 kg, from about 9 cm³/10 min for high-viscosity grades to about 20 for standard injection grades. Dry it well before molding.
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Paste it as written. A polymer chemist will tell you which grade it calls for, suggest equivalents and quote it by email.
This page covers unreinforced PBT for injection molding. For glass-filled grades see glass-filled PBT, and for how PBT compares with PET see PBT vs. PET.
Typical spec window
| Property | Typical range | Test method |
|---|---|---|
| Melt flow (MVR) | About 9–21 cm³/10 min, grade-specific | ISO 1133, 250°C / 2.16 kg (some U.S. sheets give MFR under ASTM D1238 at other conditions, such as 265°C / 5 kg, and some high-viscosity grades are rated at 250°C / 1.2 kg) |
| Density | 1.30–1.31 g/cm³ | ASTM D792 or ISO 1183 |
| Tensile strength at yield | 55–60 MPa / 8,000–8,700 psi | ASTM D638 or ISO 527 |
| Flexural modulus | 2.3–2.5 GPa / 330,000–360,000 psi | ISO 178 or ASTM D790 |
| Notched impact, 23°C | Charpy 5–6 kJ/m²; notched Izod about 50 J/m / 0.9 ft·lb/in | ISO 179/1eA; ASTM D256 |
| Heat deflection, 1.8 MPa | 50–65°C / 122–149°F | ISO 75 or ASTM D648, unannealed |
| Heat deflection, 0.45 MPa | 115–165°C / 239–329°F | ISO 75 or ASTM D648, unannealed |
| Melting point | 223–225°C / 433–437°F | ISO 11357 or ASTM D3418 |
| Mold shrinkage | About 1.1–2.0% | ISO 294-4 or ASTM D955 |
Typical ranges from published data sheets for grades in this window. Your grade’s own data sheet and the lot COA are what count.
What it’s used for
- Low-load electrical parts such as connector bodies, bobbins and switch parts
- Pump housings, sprinkler and nozzle parts
- Door handles, knobs and keycaps
- Pens and other consumer parts
- Appliance parts that need good electrical and dimensional properties
- Base resin for parts that later move to a filled or FR PBT grade
How to tell grades apart
- Viscosity class. High-viscosity grades (around 9–10 at 250°C / 2.16 kg, such as Crastin S600F10 at MFR 10; Valox 315 is also high viscosity but SABIC rates it at 250°C / 1.2 kg) suit extrusion, thick parts and more toughness. Medium-viscosity grades (MVR around 14–21, such as Valox 325, Celanex 2002 and Ultradur B4520) are the usual injection choice for thin walls and fast cycles.
- Injection vs. extrusion grades. Some unfilled PBT is positioned for extrusion. BASF describes Ultradur B4500 for cast film, profiles and tubing, and B4520 as its standard injection grade, though their MVR is similar. Check the intended process on the data sheet.
- Nucleated and release grades. Fast-freezing (nucleated) grades and grades with internal mold release shorten cycles. Crastin S600F10, for example, lists mold release as an additive. Keep the same package once a part is approved.
- Hot, wet service. Standard PBT loses strength in hot water and steam over time. Hydrolysis-resistant grades exist for that service. Don’t assume a general-purpose grade will hold up there.
- Not GP. Glass-filled, flame-retardant and PBT/PC blend grades are separate families with their own specs and process windows.
What to put on your RFQ or PO
- Melt flow with the condition. MVR at 250°C / 482°F and 2.16 kg, or the producer’s own condition, with nominal value and range.
- Viscosity class and process. Injection grade, plus a nucleated or mold-release package if your cycle depends on it.
- Heat and impact. The heat deflection your part depends on, with the load (0.45 or 1.8 MPa), and the impact values you test.
- Flame rating. Standard unfilled PBT is HB. If you need V-0, that is an FR grade, not GP.
- Color. Natural, black or a color code, and whether you add masterbatch.
- Approvals. If any, the producer’s FDA or UL documents for the exact grade and color.
- COA values to check. Melt flow or viscosity number on each lot.
- Packaging. Gaylords or 25 kg (55 lb) bags. PBT must be dried before molding, so ask for lined gaylords or sealed bags.
- Quantity and release size. Total quantity, how much per release and how often.
A polymer chemist reviews every request and checks the grade against your process before we quote. Every prime lot from our suppliers ships with a COA, and orders start at one gaylord.
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- Same viscosity class from another producer. Valox 325, Celanex 2002, Ultradur B4520 and unreinforced Crastin grades in the same MVR range are the usual comparison set. Check shrinkage and warp on first articles.
- PET. Stiffer in some grades but slower to crystallize in the mold. See PBT vs. PET.
- Glass-filled PBT. For load at temperature, since glass raises the 1.8 MPa heat deflection sharply. See glass-filled PBT and PBT GF30 properties.
- PBT/PC blends. Better impact, and some grades carry flame ratings. See Valox 357.
- Nylon. Tougher, but it absorbs far more moisture. See glass-filled nylon vs. PBT.
Common processing problems
- Silver streaks from wet resin: splay silver streaks
- Brittle parts from moisture or overheating: brittle cracking parts
- Warp in flat or uneven-wall parts: warped parts
- Parts soften or creep under heat: parts soften under heat
- Flash on thin parting lines: flash
Common questions
Why is unfilled PBT heat deflection so low at 1.8 MPa?
PBT’s glass transition is only about 45–60°C, depending on the test method, so the unfilled resin softens under high load above that, even though it does not melt until about 225°C. At 0.45 MPa the same grades test about 115–165°C. Glass fiber is what raises the 1.8 MPa value.
Does PBT need to be dried?
Yes. Molding wet PBT breaks down the polymer and gives brittle parts. Producers list about 2–4 hours at 80–130°C / 176–266°F, depending on dryer and grade, to a maximum of about 0.02–0.04% moisture.
What melt flow PBT do I need for injection molding?
Most general-purpose injection parts run on medium-viscosity grades around MVR 14–21 at 250°C / 2.16 kg. Use a higher-viscosity grade (around 9–10) for thick parts, extrusion or more toughness. Match the grade that runs your mold now.
What flame rating does unfilled PBT have?
Standard unfilled PBT is typically UL 94 HB. Parts that need V-0 use flame-retardant PBT or PBT/PC grades, which are separate families.
Tell us the grade you run now
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province