Negri Bossi Alarm NB20: Screw Speed Deviation — Diagnostics & Troubleshooting
Published: 2026-06-04
Negri Bossi Alarm NB20 on Vector and EOS series machines triggers when the Amico controller detects that the actual screw speed during plastication deviates from the setpoint by more than 15% for more than 3 seconds. This is not an immediate stop...
Negri Bossi Alarm NB20 on Vector and EOS series machines triggers when the Amico controller detects that the actual screw speed during plastication deviates from the setpoint by more than 15% for more than 3 seconds. This is not an immediate stop — it is a cycle deviation warning that indicates the plasticizing unit cannot deliver molten polymer at the programmed rate.
Root cause: (1) Worn hydraulic motor — internal leakage reduces the motor's output RPM at a given flow rate. (2) Worn screw/barrel — increased clearance reduces plasticating efficiency, requiring more screw revolutions to melt the same shot volume. (3) Bridged feed throat — polymer granules not flowing into the screw flights, causing the screw to run empty. (4) Incorrect back pressure setting — excessive back pressure reduces screw recovery speed. (5) Material change without adjusting screw RPM — high-viscosity materials (PEEK, PPS) require lower screw speeds to avoid shear heating.
Alarm Details
| Alarm Code | NB20 |
|---|---|
| Brand / Machine | Negri Bossi |
| Severity | Medium (Cycle Delay / Warning) |
| Component | Screw Drive Motor / Hydraulic Motor |
| Affected Systems | Plasticizing Unit / Amico Controller |
Troubleshooting Protocol
Identify which zone/component triggered the alarm. Record the error code, timestamp, and any measured deviation values shown on the diagnostic screen.
Power down the machine and follow Lock-Out Tag-Out procedures. Visually inspect the affected component for physical damage, loose connections, polymer leakage, or carbonized material.
Using a multimeter, check resistance/continuity on the affected circuit. Verify SSR functionality and fuse integrity. Compare readings to OEM specifications.
Replace the failed component with an OEM-approved part. Do not substitute with generic equivalents unless validated for the specific machine model and operating conditions.
Restart the machine, verify the alarm is cleared, run a test cycle, and document the root cause, repair performed, and parts replaced in the machine maintenance log.
Equivalents & Cross-References
| Equivalent / Alternate | Action |
|---|---|
| Engel Alarm 405 | |
| KraussMaffei MC45 |
Frequently Asked Questions
How can I trend screw recovery time to predict screw/barrel wear?
On the Amico controller, navigate to Process Monitor > Plastication > Recovery Time Trend. Plot recovery time against cycle count over the last 500-1000 cycles. A gradual increase (e.g., 3.2s → 3.8s → 4.5s over one month) indicates progressive screw/barrel wear. Schedule screw/barrel replacement when recovery time increases 30% above original baseline. A sudden step change (3.2s → 6.1s in one shift) indicates a catastrophic failure (broken screw, seized check ring).
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References & Industry Standards
- ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
- UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
- MatWeb. Material Property Data for Engineering Thermoplastics. matweb.com
- ISO 1043. Plastics — Symbols and Abbreviated Terms. iso.org