Injection Molding Guide · Updated October 3, 2026
PA6 GF30 Floating Fiber: 9 Injection Molding Adjustments That Actually Fix It
White, rough streaks on the surface of your PA6 GF30 parts — that is floating fiber, one of the most common defects with glass-reinforced nylon. The good news: it is almost always a process problem. You can usually reduce or eliminate it by adjusting the machine, without changing the material or cutting new tooling. Below are the fixes in the order we try them on the shop floor, with parameter ranges taken from real production trials.
Table of Contents
1. What Is Floating Fiber, and How Do You Recognize It?
Floating fiber (also called fiber show, or fu xian 浮纤 in Chinese) shows up as white, matt, slightly rough streaks or patches on the part surface. Under a loupe you can see individual glass fibers standing out of the surface. It is worst near the gate, at the end of fill, around weld lines and on thin ribs — the places where the melt has to turn, slow down or knit together.
It is easily confused with moisture silver streaks. The quick way to tell them apart: silver streaks look shiny and silvery, follow the flow lines, and usually disappear once the material is dried properly. Floating fiber stays even on perfectly dried material, and the surface feels rough, like fine sandpaper.
Why it matters: cosmetic rejection is the obvious cost, but exposed fibers also hurt paint adhesion, and on sliding or sealing surfaces they accelerate wear. So this is not just an appearance problem.
2. Why Does It Happen? (Read This Before You Touch the Machine)
Glass fibers are solid particles — typically 10–13 µm in diameter with a retained length of roughly 0.2–0.4 mm in the melt. The PA6 matrix is a liquid. During filling, the melt advances with a "fountain flow": the material at the front rolls from the core toward the mold wall and freezes there. The resin, being less viscous, moves ahead of the fibers; the fibers get carried to the surface and trapped in the frozen skin before the resin can cover them.
Three things make it worse: a cold mold, which creates a thick frozen skin; a low melt temperature, which makes the resin viscous and poorly wetting; and a high glass loading — at 30% there are simply more fibers, so more of them end up at the surface.
So every adjustment below is chasing one of two goals: cover the fibers with a thicker resin skin, or keep the fibers well dispersed in the core. Keep that in mind and the parameter changes stop looking random.
3. Nine Adjustments, in the Order We Try Them
3.1 Dry the pellets properly (always step one)
PA6 absorbs moisture fast. An opened bag can pick up harmful moisture within hours, and that moisture turns into steam in the barrel — producing silver streaks that are constantly misdiagnosed as floating fiber.
Dry at 80–90 °C for 4–6 hours in a dehumidifying dryer, and target a moisture content below 0.1%. Check with a moisture meter if you have one. Keep dried pellets covered and feed them through a hopper dryer; do not leave them standing in the hopper overnight. And do not over-dry: very long times above 100 °C start to oxidize the material and can make parts yellow and brittle. See our complete nylon drying guide for details.
3.2 Raise the melt temperature (step two)
For PA6 GF30, a typical barrel profile is rear 240–250 °C, middle 250–260 °C, front 260–270 °C, nozzle 265–275 °C. Keep the actual melt below about 300 °C.
If floating fiber is your problem and your current settings sit at the low end of that window, raise the whole profile by 10–20 °C. Lower viscosity means the resin wets the fibers better and the skin closes over them more easily. Watch for yellowing or a burnt smell — that is your ceiling.
3.3 Raise the mold temperature (step three — usually the biggest single lever)
Mold temperature for PA6 GF30 should be at least 80 °C. For cosmetic parts, 90–100 °C, and 110–120 °C if you run a hot/cold (variotherm) mold.
A hot mold slows the freezing of the skin, so the resin has time to cover the fibers. If float fiber is concentrated at the gate and end of fill, a 20–30 °C jump in mold temperature is often enough to make it disappear. The trade-off is a longer cycle — that is simply the price of a good surface.
3.4 Set a multi-stage injection speed (step four)
The general rule: fill fast. High injection speed reduces the velocity difference between resin and fibers, so they travel together and stay inside. Slow filling gives the fibers time to separate and float.
But do not just set one high speed. Use a profile: slow start for the first 10–15 mm to avoid jetting at the gate, fast middle, then slow down for the last 5–10 mm before switch-over to avoid flash and improve the end-of-fill area. If the part has thin ribs, slow down slightly through them.
3.5 Adjust back pressure and screw speed (step five)
A slightly higher back pressure than you would use for unfilled PA6 helps disperse the glass evenly in the melt. A practical range is 0.5–1.5 MPa (about 5–15 bar on most machines; read your own display unit). Increase it in small steps of roughly 0.2–0.3 MPa and watch the surface after each step.
Too much back pressure breaks the fibers and overheats the melt — you will see yellowing and a bad smell before you see better parts. Keep screw speed moderate, around 30–50 rpm, so the glass is not beaten up during plastication.
3.6 Check holding pressure and the switch-over point (step six)
Excessive holding pressure can push fibers through the still-soft skin. If the float fiber is worst around the gate, try reducing holding pressure by 10–20% and shortening hold time slightly, as long as you do not introduce sink marks.
Also confirm the switch-over point is not too late. Over-packing the last few millimetres of screw stroke does nothing for the surface, adds stress, and can bring the fiber problem back at the gate.
3.7 Review the gate, runner and wall thickness (step seven)
Small gates shear the fibers and concentrate them; a gate that is too thin for 30% GF is a common root cause. Enlarge the gate or switch to a fan or side gate. Keep the nozzle bore reasonably large (at least 4 mm) — a narrow nozzle breaks fibers and creates fiber-rich melt.
Gate into the thickest wall, keep the flow length short (at 1 mm wall, try to stay under roughly 80 mm flow length for 30% GF), and for cosmetic parts consider a second gate to shorten the flow.
3.8 Improve venting (step eight)
Trapped air compresses, heats up and pushes fibers to the surface — it also causes burn marks at the end of fill. Vent depth for PA6 is 0.02–0.04 mm, typically 0.03 mm. Add vents at the end of fill and at weld lines. If you cannot vent enough, vacuum venting is the next step up.
3.9 Change the material or add a processing aid (step nine, only if the process is maxed out)
Drop to a lower glass content for the visible surface (25% instead of 30%) if the mechanical spec allows.
Add a nylon-compatible lubricant or flow-aid masterbatch at 0.3–1%. Small doses reduce fiber/wall friction; big doses weaken weld lines.
Use a compatibilizer (maleic-anhydride-grafted) to improve fiber/matrix bonding — it helps the surface and the mechanicals together.
Ask your supplier for a "low float fiber" or "cosmetic" grade. Several PA6 GF30 grades are formulated with special sizing and coupling agents specifically for surface quality.
4. Quick Parameter Reference for PA6 GF30
These are typical starting windows. Your grade and machine will shift them, so always verify against the material datasheet and your own trials.
| Parameter | Typical range | Note |
|---|---|---|
| Drying | 80–90 °C × 4–6 h, dehumidifying dryer | Moisture ≤ 0.1%; check with a moisture meter |
| Barrel rear / mid / front | 240–250 / 250–260 / 260–270 °C | Nozzle 265–275 °C; do not exceed ~300 °C |
| Mold temperature | 80–100 °C | 100–120 °C for cosmetic parts or variotherm |
| Injection speed | Fast fill, 3-stage profile | Slow at gate to avoid jetting, slow at end |
| Back pressure | 0.5–1.5 MPa | Raise in small steps; watch for yellowing |
| Screw speed | 30–50 rpm | Protect fiber length |
| Holding pressure | Moderate | Lower 10–20% if the gate area shows fibers |
| Venting | 0.02–0.04 mm depth | End of fill + weld lines |
5. A Real Case From the Shop Floor
A customer of ours molds electric hedge-trimmer housings in black PA6 GF30. The float-fiber reject rate was around 40%. The parts were running at a barrel of about 245 °C, a mold of 60 °C, and a single-speed fill.
We made four changes in sequence: (1) dried the material at 85 °C for 5 hours, with moisture confirmed below 0.1%; (2) raised the barrel profile by about 15 °C; (3) raised the mold temperature from 60 °C to 90 °C; (4) changed to a three-stage speed profile and raised back pressure from roughly 0.3 MPa to 0.8 MPa.
Float fiber dropped to near zero on the first full run. Cycle time went up by about 12 seconds, but the customer shipped the parts without rework. The lesson: do not chase a single parameter. The combination of a hot mold, a hot melt and a fast staged fill is what closes the skin.
6. Common Mistakes That Make It Worse
Spraying mold release on the cavity to "hide" the fibers — it makes the surface worse and contaminates weld lines.
Cranking injection speed without raising the mold temperature — you get jetting, silver streaks and fiber show at the gate.
Skipping drying because "the bag was sealed" — a sealed bag still breathes through pinholes, and moisture streaks get misdiagnosed as float fiber.
Dropping the mold temperature to save cycle time — the fastest way to bring float fiber back.
Adding lubricant in big doses — a few percent of flow aid can halve weld-line strength.
7. Frequently Asked Questions
No. Silver streaks are shiny, follow the flow lines and usually disappear after proper drying. Floating fiber stays even on dry material and feels rough like fine sandpaper, with individual glass fibers visible under magnification.
Raising the mold temperature to 80–100 °C usually gives the fastest visible improvement, but always dry the material properly first, so you are not chasing moisture streaks instead of float fiber.
Use a dehumidifying dryer at 80–90 °C for 4–6 hours, targeting moisture below 0.1%. Check with a moisture meter. An opened bag of PA6 can pick up harmful moisture within hours.
Generally high, using a staged profile: slow start to avoid jetting, fast middle fill, slow down before switch-over. Slow filling gives the fibers time to separate from the resin and float.
A nylon-compatible lubricant or flow aid at 0.3–1% reduces fiber show in many cases, but overdosing weakens weld lines and mechanical properties. Treat it as a step before changing the grade, not as a cure-all.
It is mostly a cosmetic defect. But where fibers are missing from a highly loaded thin section, or exposed on a sealing or wear surface, it can reduce performance. Fixing the surface usually fixes the fiber distribution too.
8. Summary
In one sentence: dry the material properly, push melt and mold temperature up, fill fast with a staged profile, keep back pressure moderate — and if fibers still float, look at the gate and then at the grade. Process first, material second, mold third. That order solves most PA6 GF30 float-fiber problems without a single euro of new tooling.
