Nozzle Stringing / Drooling
Causes, Solutions & Prevention in Injection Molding
Stringing occurs when residual melt in the nozzle tip is too fluid and elongates as the mold opens.
Below you will find general information about this defect. To get a pinpoint solution tailored to your specific machine, material, and process parameters using the world’s most comprehensive injection molding troubleshooting database — sign up and access AI-powered troubleshooting, 7 engineering calculators, and detailed project tracking.
🔍Symptoms & Visual Identification
⚠️Root Causes by Category
⚙️ Process
- Nozzle zone temperature too high
- Melt temperature too high overall
- Decompression suck-back timing incorrect should be BEFORE recovery not after
- Holding pressure too low excess melt at tip
🔲 Mold
- Sprue bush has no cold slug well degraded front causes strings
- Nozzle tip radius does not match sprue bush radius
- Barrel mold axis misalignment nozzle under lateral stress
🧪 Material
- High MFI material low viscosity more prone to stringing
- PA and PBT especially prone may require spring-loaded shut-off nozzle
- Moisture aggravates wet PA especially
📋 General
- Barrel not sealed properly between nozzle and machine head
✅Solutions & Corrective Actions
⚙️ Process
- Reduce nozzle zone temperature
- Perform decompression BEFORE screw recovery
- Increase holding pressure
- Reduce overall melt temperature
🔲 Mold
- Add cold slug well to sprue bush
- Verify nozzle tip radius equals sprue bush radius
- Check barrel mold alignment
🧪 Material
- For PA and PBT install spring-loaded shut-off nozzle
- Lower MFI grade of same family if stringing persists
- Ensure material is fully dried
🛠Step-by-Step Troubleshooting
Check Process Parameters
Reduce nozzle zone temperature
Inspect the Mold
Add cold slug well to sprue bush
Verify Material Condition
For PA and PBT install spring-loaded shut-off nozzle
❓Frequently Asked Questions
What causes nozzle stringing / drooling in injection molding?
Nozzle Stringing / Drooling is caused by several factors including: Nozzle zone temperature too high; Melt temperature too high overall; Sprue bush has no cold slug well degraded front causes strings; Nozzle tip radius does not match sprue bush radius; High MFI material low viscosity more prone to stringing; PA and PBT especially prone may require spring-loaded shut-off nozzle.
How do you fix nozzle stringing / drooling in injection molding?
Start by checking key parameters: Nozzle temperature, Suckback distance, Decompression, Cooling time at gate. Reduce nozzle zone temperature
Which plastics are most affected by nozzle stringing / drooling?
Common materials include: PP, PA6, POM, PET, TPE. Pay special attention to drying, shrinkage and mold temperature for each grade.
How do I prevent nozzle stringing / drooling in future production runs?
Prevention relies on three pillars: (1) process optimisation — verify Nozzle temperature, Suckback distance; (2) mold maintenance — check gates, vents and cooling channels; (3) material control — ensure correct drying and virgin/regrind ratio.
What is the difference between nozzle stringing / drooling and similar defects?
Thin plastic string pulled from nozzle tip as mold opens. Causes visual defect on sprue or part and machine contamination. This distinguishes it from related surface defects during visual inspection.
📐Related Calculators
Run these engineering calculators to verify your process parameters and prevent this defect.
🔗Related Defects
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