A string is a thin thread of melt pulled from the tip of the sprue as the mold opens. Drool is melt that runs out of the nozzle between shots. Both mean the melt at the nozzle tip is too fluid, or still under pressure, when it should be sitting still. Start with resin moisture, nozzle temperature and decompression.
Also described as: strings from the sprue, angel hair, nozzle drool, drooling, stringy parts.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| A thin thread runs from the sprue to the nozzle when the mold opens. Strings land on the mold face and get molded into the next part. | The nozzle tip is too hot, so the sprue is still molten where it should break off. | Nozzle temperature, set and actual; where the nozzle thermocouple and heater band sit; cooling time |
| Melt runs out of the nozzle between shots or when the carriage is back. | The melt is too thin, or pressure is left in the barrel after the screw recovers. | Nozzle and front zone temperatures; decompression (suck back); back pressure |
| The drool is foamy, or the nozzle spits and pops. Parts show splay. | Wet resin. Steam pushes melt out of the nozzle, and in nylon, polyesters and polycarbonate the water also thins the melt. | Pellet moisture at the feed throat; dryer temperature, dew point and drying time |
| Stringing only on nylon, PET or PBT jobs. Other resins run clean on the same press. | These resins turn very fluid once melted, and an open general-purpose nozzle tip does not hold them back. | Nozzle tip type; nozzle temperature; moisture |
| A hair or string at each gate on a hot runner tool. | The hot tip is too hot or the gate area is not cooled enough, so the gate has not frozen when the mold opens. | Tip temperature; cooling at the gate insert; decompression before mold open |
| A cold lump or a dull streak at the gate, and a frozen blob on the nozzle face. | Drooled melt froze in the nozzle or sprue bushing and was pushed into the next shot. | Fix the drool first; the cold slug well |
| Stringing or drool started with a new lot or a new grade. | The new material has a higher melt flow, so it is thinner at the same temperature. Or the lot arrived wetter. | Melt flow on the COA of the good lot against the bad lot; pellet moisture |
Checks in order, cheapest first
- Check the moisture first. Test pellets taken at the feed throat. Wet resin is behind a large share of drool and stringing on nylon, PET, PBT, polycarbonate and TPU, and no nozzle setting will fix it.
- Lower the nozzle temperature in small steps. Give each step several shots to settle. Too low and the nozzle freezes off or sends a cold slug into the part. If the thermocouple sits far from the tip, the tip can run much hotter than the reading.
- Add decompression after screw recovery. Pulling the screw back a short distance takes the pressure off the melt at the nozzle. Use only as much as it takes. Too much draws air into the nozzle and shows up as splay or bubbles near the gate.
- Lower back pressure, screw speed and front zone temperatures. All three add heat and pressure to the melt in front of the screw. Check the actual melt temperature with a probe. A long residence time breaks down the resin and thins the melt.
- Lengthen cooling time, or delay the sprue break. The sprue has to be solid where it parts from the nozzle. A few more seconds, or a later carriage pull-back, lets it freeze.
- Compare lots. If the problem arrived with a new lot, compare melt flow on the COAs and test the moisture as received. Check whether the regrind ratio has gone up, since regrind is often thinner and wetter than virgin.
- Look at the nozzle and the sprue bushing. A reverse-taper tip made for nylon breaks the sprue inside the tip. A smaller orifice, a mechanical shut-off nozzle or better cooling at the sprue bushing are the next steps. Confirm that the nozzle orifice is slightly smaller than the sprue bushing opening.
- Change the resin last. Stringing and drool are usually not a fault in the resin. If the material is dry and the settings and nozzle are right, a stiffer-flowing grade is the next place to look.
Safety. Melt from a drooling or spitting nozzle causes serious burns. Wear a face shield and heat-resistant gloves at the nozzle, keep the purge guard closed, and never look into the nozzle or the feed throat.
The numbers
Typical drying conditions and moisture targets for the resins that string and drool most when they are wet:
| Resin | Drying temperature | Drying time | Target moisture |
|---|---|---|---|
| Nylon 6, unfilled | 176°F / 80°C | 2–6 h | 0.03–0.12% |
| Nylon 66, unfilled | 175–180°F / 80–82°C | 2–4 h from a fresh bag | 0.03–0.2% |
| PET, injection grade | 250–275°F / 120–135°C | 2–4 h virgin; 4–6 h regrind | <0.02% (200 ppm) |
| PBT | 250°F / 121°C typical | 2–4 h | 0.02% |
| Polycarbonate | 248–250°F / 120–121°C | 2–4 h | ≤0.02% |
| TPU | 194–230°F / 90–110°C (aliphatic grades 104–140°F / 40–60°C) | 1–4 h | <0.02–0.05% |
| ABS | 176–203°F / 80–95°C | 2–4 h | 0.1% general; 0.02–0.05% for plating |
| Acrylic (PMMA) | 158–208°F / 70–98°C | 2–4 h | <0.02% strict; 0.05–0.1% non-critical |
Typical values from published drying guides. Targets differ between producers and grades, so confirm against the data sheet for your grade. The full list is in the resin drying chart.
Why nozzles string and drool
- The nozzle is an open pipe. Between shots, nothing holds the melt in except its own thickness and a cool spot at the tip. If the melt is thin enough, or there is pressure behind it, it runs out.
- Some resins are thin as soon as they melt. Nylon, PET and PBT have a sharp melting point and a low melt viscosity. A few degrees too hot at the tip is enough to make them string. Amorphous resins such as ABS and polycarbonate soften gradually and are more forgiving.
- Water does two things. It turns to steam in the barrel and pushes melt out of the nozzle. In nylon, PET, PBT, polycarbonate and TPU it also breaks the polymer chains (hydrolysis), which thins the melt and weakens the part.
- Back pressure leaves the melt compressed. When the screw stops turning, that pressure is still there. Decompression releases it.
- A string is a sprue that has not frozen through. If the core of the sprue is still molten at the point where it should break, the mold opening draws it out into a thread.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A lower melt flow grade of the same resin | A thicker melt is less inclined to run out of the nozzle or draw into a thread. | Harder to fill thin walls and long flow paths, and higher injection pressure. A new grade for your customer to approve. |
| Resin supplied in moisture-barrier packaging | Nylon and other moisture-sensitive resins packed in sealed, lined bags or lined boxes start much closer to dry. | Usually a higher price and smaller package sizes. Once a package is opened the resin picks up moisture again, so it still needs a dryer. |
| A melt flow range on the specification | Stringing that comes and goes by lot often traces to melt flow moving between lots. | Nothing in the part. It takes an incoming check, and it may narrow your choice of sources. |
| Less regrind, dried with the virgin resin | Regrind has been through the barrel at least once, so it is often thinner in the melt, and it picks up moisture while it waits. | More virgin resin and more scrap to store or sell. |
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks less makes a bigger part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Splay. Silver streaks on the part surface from moisture or gas. Wet resin often gives you splay and drool together, but splay can occur with a nozzle that holds perfectly. See splay and silver streaks.
- Flash. Flash is a thin film attached along the parting line or around a pin. A string is a loose thread that can land anywhere on the mold face. See flash.
- Jetting. A snake-like line molded into the surface, starting at the gate on every shot. A molded-in string turns up in a different place each time. See jetting.
- Cold slug marks. A dull streak or lump at the gate from frozen material in the nozzle. Drool is one cause, and a nozzle set too cold is another. See flow lines and flow marks.
Quick answers
How do I stop stringing from the sprue?
Start with resin moisture, nozzle temperature and decompression. Test pellet moisture at the feed throat, then lower the nozzle temperature in small steps and add a little decompression after screw recovery. A longer cooling time or a later sprue break lets the sprue freeze where it parts from the nozzle.
Why is my nozzle drooling?
A nozzle drools when the melt at the tip is too thin, or pressure is left in the barrel after the screw recovers. Check nozzle and front zone temperatures, decompression and back pressure. Foamy drool or a nozzle that spits and pops points to wet resin. Wear a face shield and heat-resistant gloves at the nozzle.
What causes angel hair on molded parts?
Angel hair is a thin thread of melt pulled from the tip of the sprue as the mold opens. The nozzle tip is too hot, so the sprue is still molten where it should break off. Strings land on the mold face and get molded into the next part.
Is stringing caused by the resin?
Stringing and drool are usually not a fault in the resin. Wet resin, a hot nozzle tip and pressure left at the nozzle are the usual causes. If the problem started with a new lot, compare melt flow on the COAs and test the moisture as received.
Why does nylon string more than other resins?
Nylon, PET and PBT have a sharp melting point and a low melt viscosity, so they turn very fluid once melted. A few degrees too hot at the tip is enough to make them string, and an open general-purpose nozzle tip does not hold them back. Moisture thins the melt further.
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