The short answer
Adding short glass fiber makes a plastic stronger, stiffer and more heat and creep resistant, and it cuts shrinkage. In exchange, parts are heavier, less ductile and more prone to warp, and the material wears screws and molds faster. 30–33% glass is the common balance; 50% and up is for stiff, metal-replacement parts.
Glass-filled grades of nylon, PBT, PET, polypropylene, PPS and other resins are among the most widely used engineering materials. Knowing what the glass changes helps you pick the right level and avoid molding problems.
How glass level changes properties (nylon 66 example)
| Property | Unfilled | 15% glass | 30% glass | 50% glass |
|---|---|---|---|---|
| Density (g/cm³) | ~1.14 | ~1.23 | ~1.36–1.37 | ~1.56–1.57 |
| Tensile strength (MPa, dry) | ~80 | ~120 | ~180–190 | ~230–250 |
| Tensile modulus (GPa, dry) | ~3 | ~5.5 | ~9.5–10 | ~16 |
| Heat deflection, 1.8 MPa | ~70–90°C | ~240°C | ~250–255°C | ~255–260°C |
| Mold shrinkage, flow direction | ~1.5–2.0% | ~0.6% | ~0.3–0.5% | ~0.2–0.3% |
Typical values for dry-as-molded PA66 grades; other resins follow a similar pattern. Heat deflection rises sharply in semi-crystalline resins like nylon and PBT, but only modestly in amorphous resins like polycarbonate.
What glass improves, and what it costs
Improves
- Tensile and flexural strength
- Stiffness, often two to five times unfilled
- Heat deflection temperature
- Creep and fatigue resistance
- Shrinkage and thermal expansion (lower)
Costs
- Higher density and part weight
- Lower elongation and ductility
- Warp from uneven shrinkage
- Weaker weld lines and rougher surface
- Faster screw, barrel and mold wear
Molding tips for glass-filled resins
- Dry to the base resin's spec; glass does not change it.
- Run a warmer mold for better surface finish and less fiber show.
- Place gates to control fiber orientation and keep weld lines out of loaded areas.
- Limit regrind; each pass shortens fibers and lowers strength.
- Use wear-resistant screws and hardened tool steel for long runs.
See also glass-filled vs. mineral-filled, carbon vs. glass-filled nylon, glass-filled nylon vs. PBT and warped parts.
Grades we source
We source glass-filled nylon, PBT, PET, polypropylene, PPS, PPA and other resins at 10% to 60% glass, from one gaylord up, with a COA on every lot, tested to ASTM or ISO methods. Surplus lots, including glass-filled PBT/PET, are listed on the surplus resin page.
Common questions
What does glass fill do to plastic?
Glass fibers make plastic much stronger and stiffer, raise heat deflection temperature (a lot in semi-crystalline resins), reduce creep and cut mold shrinkage. They also reduce elongation, raise density, wear screws and molds faster and can cause warp from uneven shrinkage.
What does 30% glass-filled mean?
Thirty percent of the compound's weight is short glass fiber; the rest is the base resin and additives. 30–33% is the most common level for engineering parts.
Is 50% glass-filled better than 30%?
It is stiffer and stronger, and closer to metal in rigidity, but heavier, harder to fill, more abrasive and less tough. It suits metal-replacement and structural parts; 30% is the better balance for most parts.
Why do glass-filled parts warp?
Fibers line up with the flow, so the part shrinks less along the flow than across it. That difference pulls the part out of shape. Gate location, wall thickness, mold temperature and a mineral or bead co-filler all help.
Does glass fill make plastic brittle?
It lowers elongation, so parts bend less before breaking. Notched impact can stay similar or even rise in some resins, but snap fits and thin ribs need care.
Does glass-filled resin need special equipment?
Not special, but wear-resistant screws, barrels and hardened tool steel last much longer with glass-filled resins. Drying requirements are the same as the base resin.
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