Long glass fiber polypropylene: LGF PP and LFT pellets from 20% to 60% glass

Long glass vs. short glass PP

Both are glass-reinforced polypropylene. The difference is how the glass gets into the pellet and how long it stays in the part. Short-glass PP is made on a twin-screw extruder that chops the fibers as it mixes. Long-glass PP is made by pulling continuous glass roving through a melt bath or die so that each fiber is coated, then cutting the strand into pellets. The fibers in the pellet are as long as the pellet.

Typical comparison for coupled grades. Individual grades vary.

Glass levels and where each one fits

Glass level is by weight. Confirm the level against the grade sheet and an ash value on the COA.

What long glass gains you: typical values

The table puts one short-glass grade beside long-glass grades from producer data sheets, all tested to ISO methods. Use it to see the pattern, not to design a part. Stiffness tracks the glass level. Impact and creep track the fiber length left in the part.

Values compiled from individual producer and compounder data sheets for specific grades. Notched Charpy (ISO 179, kJ/m²) and notched Izod (ASTM D256, J/m) do not convert into each other, so compare the same test. Confirm against the data sheet for the exact grade.

Molding long-glass PP without breaking the fibers

The pellet arrives with long fibers. Whether the part keeps them depends on the press. Every sharp edge, tight gate and high-shear screw section cuts fibers, and a part that loses most of its fiber length molds like short-glass PP at a higher price. These are common guidelines from producer and compounder molding guides. Follow the sheet for your grade first. Melt and mold temperatures for PP are on our processing chart.

• Screw. A general-purpose three-zone screw with a low compression ratio, 2:1 to 2.5:1, and an L/D of about 16:1 to 22:1 in one compounder’s guide. Mixing sections add shear and cut fibers, so ask the producer before using one. Keep the feed throat open enough that long pellets do not bridge.
• Check valve. A fully free-flow, fluted screw tip valve (a three-piece ring check). Avoid restrictive tips.
• Nozzle and sprue. A generous, constant-diameter path with no sharp edges and no internally tapered tip. One guide gives about 5.6 mm / 0.25 in for the nozzle and sprue orifice, and a sprue as short as possible that tapers from about 5.6 mm / 0.25 in to about 8.7 mm / 11/32 in.
• Runners and gates. Full-round runners of about 5.6 mm / 0.25 in with no sharp corners, and large gates. One guide sets a minimum gate thickness of about 2 mm / 0.080 in, and larger is better for structural parts. Open-channel hot runners are acceptable.
• Back pressure and screw speed. Use the minimum back pressure that gives a consistent shot. One guide suggests 25–50 psi (0.17–0.34 MPa) without saying whether that is hydraulic or melt pressure, so check what your press gauge reads. Avoid high screw speeds, which add shear.
• Barrel profile. A reverse profile, hotter at the feed, softens the pellets before they reach the compression zone and cuts fiber breakage there.
• Regrind. Keep it low. One compounder sets 5% as the maximum that does not change properties significantly.
• Drying. Unfilled PP does not need it. Filled and reinforced PP can pick up surface moisture, so follow the grade sheet. Our drying chart gives about 75°C / 167°F for 2 hours when PP needs it, and some long-glass sheets ask for more.
• Check your result. Burn off a molded part and look at the fibers, or send it to a lab for fiber length analysis. It is the only way to know the process kept the fibers long.

Coupling, color, UV and interior odor

• Chemical coupling. PP does not bond to glass on its own. Structural long-glass grades use a glass sizing made for PP plus a maleic anhydride grafted PP (PP-g-MA) coupling agent, so load passes from the polymer into the fibers. Uncoupled grades are much weaker. Ask whether the grade is chemically coupled. See maleic anhydride grafted polyolefins.
• Color. Most structural parts are black. Natural grades are sold for coloring at the press with a color concentrate, and concentrate pellets let down with PP are another route. Glass can show at the surface, especially with a cold mold, so long-glass PP is often used for hidden or covered parts.
• UV. Exterior and sunlit parts need a UV-stabilized grade. On weathered surfaces the resin erodes and the glass starts to show, so ask for weathering data if the part is visible outdoors.
• Heat aging. Under-hood and appliance parts need a heat-stabilized grade. Ask for oven-aging data at the part's temperature, since heat aging depends on the stabilizer package, not the glass level.
• Odor and emissions for vehicle interiors. Instrument panel carriers and door modules sit inside the cabin, so OEM specs often add odor (VDA 270, a panel rating), emissions (VDA 277 headspace total carbon, or VDA 278 thermal desorption VOC and FOG values) and fogging (SAE J1756 or DIN 75201). Producers sell low-emission versions of long-glass grades for this. Ask which spec the grade has been tested to, and see automotive resin sourcing for FMVSS 302 and PPAP.

Where long-glass PP is used

Typical uses only. The part's load, temperature and spec decide the grade.

D-LFT: when the molder compounds its own

In direct long fiber thermoplastic (D-LFT) molding, the molder feeds PP and continuous glass roving into its own compounding extruder and sends the melt straight to an injection or compression press. There is no pellet. It suits very high volume parts, such as front-end and underbody parts, where a molder can justify the extruder line. For D-LFT, we can quote the base PP, the PP-g-MA coupling agent and additive masterbatch through our supply partners, not the long-glass pellet. Tell us the line and the formula and a polymer chemist will match the components.

Specs and test methods to check

The COA often reports a short list, often glass content and a flow or density value. Agree which values are on every lot COA before the first order.

What to put on the RFQ, and how it is priced

Long-glass PP is a specialty compound, so its price does not track the PP index one for one. Glass content, the pultrusion step, pellet length, coupling and stabilizer packages, color and interior emissions testing all sit on top of the base resin, so long-glass grades often cost more than short-glass PP at the same glass level. Many structural grades are proprietary to one producer, so a true second source can mean a requalification. We do not post prices. We quote a specific grade, quantity, package and ship-to city, through our supply partners and case by case, from one gaylord (about 1,000 to 1,600 lb) to truckloads, and every prime lot from our suppliers ships with a COA. If nylon is the material you are trying to replace, see glass-filled PP vs. nylon and glass-filled nylon.

• The grade you run now, or the OEM or Tier 1 material spec number. If the part is approved to one grade, a substitute needs customer approval first.
• Glass level and type. Long glass, the percentage, and ready-to-mold or concentrate to blend down.
• Pellet length. For example 10–12 mm for injection molding, and the feed equipment you run.
• Base resin and coupling. Homopolymer or copolymer base, chemically coupled, heat-stabilized and UV-stabilized as needed.
• Color. Black, natural for coloring at the press, or a color match, and the concentrate you plan to use.
• Interior and odor requirements. VDA, SAE or OEM emissions and odor specs, if the part is in the cabin.
• COA values. Glass content by ash on every lot, plus density or flow as agreed. See what a COA shows.
• Quantity and ship-to. This order, yearly use and the city. Our RFQ package page lists what helps a chemist match a structural grade.

Common questions

If your grade is not listed, tell us what you run. We source commodity, engineering and specialty resins.