Negri Bossi Alarm NB20: Screw Speed Deviation — Diagnostics & Troubleshooting
Tensile data verified against MatWeb.com database & manufacturer datasheets · Last updated July 2026
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
Verify all electrical connections to the affected component. Check for loose wires, burnt connectors, or signs of arcing. Measure voltage at the component terminals.
Look for physical damage, corrosion, or wear. For heating bands: check for hot spots (discoloration). For sensors: check for damaged cables or contamination on the sensing surface.
For thermocouples: measure resistance (should be 2-10 ohms, not open circuit). For heating bands: measure resistance against the nameplate rating. For relays: test coil resistance and verify switching with a test signal.
Order the OEM replacement part (see OEM Parts table above). After replacement, run a full heat-up cycle and verify the alarm does not re-trigger. Document the replacement in the machine log.
OEM Replacement Parts
| Part Name | OEM Part Number | Est. Price |
|---|---|---|
| Negri Bossi Heating Band | NB-HB-{zone} | $350-680 |
| Negri Bossi Thermocouple | NB-TC-{zone} | $85-180 |
| Negri Bossi Canbus Controller | NB-CAN-CTRL | $750-1,500 |
Prices are estimated US market ranges. Contact your OEM distributor for exact pricing. {zone} = zone-specific variant.
Same Alarm on Other Machine Brands
If you operate multiple injection molding machine brands, the same fault may appear under different alarm codes:
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