Short answer: add the fibre downstream of the melting zone, use moderate 30° or 45° kneading rather than 90° blocks, keep a wide-pitch conveying section immediately after the fibre inlet, and specify a wear-resistant steel because glass is abrasive.
Glass-fibre compounding asks the screw to do two things that fight each other. The fibre bundles must be opened and wetted out by the polymer, which needs stress. The individual filaments must survive at a useful length, which forbids stress. Every configuration decision is a trade between dispersion quality and retained fibre length.
Retained length drives mechanical properties. A compound with well-dispersed but crushed fibre can measure correctly on fibre content and still fail on impact strength, because the length distribution has collapsed.
| Zone | Element | Reason |
|---|---|---|
| Feed to melt | Wide-pitch conveying, then 45° kneading | Melt the polymer fully before any fibre arrives |
| Fibre inlet | Wide-pitch conveying, deep channel | Volumetric room for the bundle stream, low shear at entry |
| Wetting | 30° or 45° narrow-disc kneading, short stack | Enough stress to wet out, not enough to mill the fibre |
| Distribution | SME or slotted conveying | Even out fibre without further breakage |
| Vacuum | Conveying plus reverse element for the seal | Remove volatiles without a high-shear zone |
| Discharge | Narrow-pitch conveying | Pressure build with minimum work |
A 90° kneading block is the most effective dispersive element available, and that is exactly the problem. Its neutral-to-backward pumping action holds material in the high-shear gap for a long time, which mills glass filaments down to dust. The visible symptoms are a drop in impact strength and a rise in fines, often with no change in fibre content by ash test.
If dispersion is genuinely inadequate with 30° and 45° blocks, the usual fix is not a more aggressive block but a longer wetting section, a higher melt temperature at the fibre inlet, or a coupling agent review — all of which address wet-out without destroying length.
Downstream addition through a side feeder is standard for loadings above roughly 20 percent, for two reasons. First, the fibre never passes through the melting zone, so it is not exposed to the highest stress in the process. Second, the melting zone can be optimised for polymer alone, which is easier and more stable.
The element immediately after the side feeder matters more than most operators expect. A narrow channel there starves the fibre inlet and produces surging. A wide, deep channel accepts the fibre stream and lets it combine with the melt before any significant shear is applied.
Glass fibre erodes flight tips and kneading disk edges steadily. At 30 percent loading, nitrided elements are a poor choice; powder-metallurgy or HIP-clad grades at 58–62 HRC, with surface hardness in the 900–1100 HV range, are the realistic baseline. The dispersion and wetting zones wear fastest, so budget for replacing those elements before the conveying sections.
| Symptom | Likely cause | Action |
|---|---|---|
| Impact strength below specification | Fibre length destroyed | Remove 90° blocks, shorten the kneading stack |
| Visible unwetted bundles | Insufficient wetting stress | Add a 45° block, raise melt temperature at inlet |
| Surging at the side feeder | Channel too narrow after inlet | Open the pitch, deepen the channel |
| Rapid tip wear | Wrong steel grade | Move to PM or HIP-clad, check clearance |
Both benefit from a polished flight, which reduces particle trapping.
Downstream through a side feeder for loadings above roughly 20 percent. It keeps the fibre out of the melting zone, where stress is highest, and lets the melting section be tuned for polymer alone.
30° or 45° with narrow discs. Both wet out the fibre with far less length reduction than 60° or 90° blocks. Use the shortest stack that achieves wet-out and stop there.
The most common cause is a more aggressive kneading configuration than the previous build, which shortened the fibre. Compare retained fibre length, not just fibre content, before changing anything else.
A powder-metallurgy or HIP-clad grade at 58–62 HRC, with surface hardness around 900–1100 HV. Nitrided elements wear quickly at this loading, particularly in the wetting and dispersion zones.
Yes. Higher loading needs more volumetric capacity at the inlet and a wider channel immediately downstream, and it accelerates wear. Above about 40 percent, the feed and wetting sections usually need to be redesigned rather than adjusted.