---
title: "Injection Nozzle Problems: Drooling, Stringing, and Blockages Explained"
url: "https://www.imspares.com/blog/injection-nozzle-problems-troubleshooting/"
author: "IMSPARES Technical Team"
date: "2026-10-31"
last_modified: "2026-10-31"
category: "Troubleshooting"
tags: ["injection nozzle","nozzle drool","stringing","nozzle replacement","cold slug"]
---

# Injection Nozzle Problems: Drooling, Stringing, and Blockages Explained

Common injection nozzle issues and how to fix them. Covers drool, stringing, cold slugs, nozzle blockages, and when to replace the nozzle tip or body.

The [injection nozzle](https://www.imspares.com/parts/mc-bc-07/) is a small component that causes a disproportionate number of headaches. It's the last point of contact between the machine and the mold, and any malfunction here shows up immediately in your parts.

Let's go through the most common nozzle problems, their causes, and what to do about them.

## Nozzle Drool

[Drool](https://www.imspares.com/troubleshooting/#nozzle-drool) is when molten plastic oozes out of the [nozzle tip](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/nozzle/) between shots. You'll see it as a blob on the sprue or as material dangling from the nozzle when the carriage retracts.

**Why it happens:**
- [Nozzle temperature](https://www.imspares.com/injection-molding-machine-parts/heaters/nozzle-heater/) too high
- [Back pressure](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/back-pressure/) too high during screw recovery
- Suck-back (decompression) not enough or not set at all
- Worn nozzle tip — the orifice has enlarged from wear, and it can't hold material back
- Wrong nozzle type for the material — some resins flow more easily and need a smaller orifice

**What to do:**
1. Set suck-back / decompression after screw recovery. Start with a few millimeters and increase until the drool stops — too much suck-back draws air into the melt.
2. Reduce nozzle temperature in small steps (about 5°C at a time). The nozzle zone is often set too hot.
3. Check the nozzle tip bore size. If it's worn, replace the tip.
4. For materials like nylon that drool badly, consider a [shut-off nozzle](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/shut-off-nozzle/).

## Stringing

Strings are thin filaments of plastic that stretch between the nozzle and the molded part when the mold opens. They're related to drool but specifically happen at the moment of mold separation.

**Common causes:**
- Melt temperature too high (general, not just nozzle)
- No sprue break (carriage doesn't retract before mold opens)
- Sprue bushing interface worn (poor seal between nozzle and sprue)
- Material with poor melt strength (like PET or some grades of PP)

**What to do:**
1. Enable sprue break in the machine sequence — retract the carriage a few millimeters before the mold opens.
2. Reduce melt temperature across all zones.
3. Check the nozzle radius and sprue bushing radius match. The nozzle radius should be slightly smaller than the sprue bushing radius. If you see a witness mark (ring impression) that doesn't line up, the fit is wrong.
4. Increase cooling time slightly — gives material at the sprue tip time to solidify.

## Nozzle Blockage

[A blocked nozzle](https://www.imspares.com/troubleshooting/#nozzle-blocked) is exactly what it sounds like — material can't flow through. The machine will alarm on high injection pressure or the parts will be short shots.

**Why nozzles block:**
- Cold start without proper soak time. The material in the nozzle from the last run has solidified. If you inject before it's fully melted, you push a cold slug into the nozzle orifice and block it.
- Material degradation. If material sits in the nozzle too long at high temperature, it crosslinks or carbonizes. Carbon buildup inside the nozzle eventually restricts flow.
- Foreign object. Occasionally a piece of metal (broken screw tip, [check ring](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/check-ring/) fragment) migrates to the nozzle and blocks it.

**What to do:**
1. Always purge thoroughly before starting a new run. And when restarting after a long stop, give the barrel and nozzle enough soak time to fully melt the residual material.
2. For a blocked nozzle: retract the carriage, remove the nozzle while it is still hot, and clean it out. Use brass tools so you don't scratch the bore or the sealing seat.
3. Inspect the nozzle bore for buildup. If the internal surface is pitted or rough from carbon, replace the nozzle — cleaning won't restore it.

## Cold Slug Marks

Cold slugs are small solidified pieces of material from the nozzle tip that get injected into the mold at the start of the shot. They show up as marks on the part surface — usually near the gate.

**Causes:**
- Nozzle temperature too low at the tip
- [Thermocouple](https://www.imspares.com/parts/ec-ht-02/) not positioned correctly on the nozzle band
- Excessive dwell time between shots — material cools in the nozzle tip
- Poor thermal contact between heater band and nozzle body

**What to do:**
1. Check the nozzle [heater band](https://www.imspares.com/parts/ec-ht-01/). Is it tight against the nozzle body? Air gaps dramatically reduce heat transfer.
2. Check the thermocouple position. It should be on the nozzle body, not on the heater band.
3. Raise the nozzle temperature in small steps (about 5°C at a time).
4. Make sure the mold has a proper cold slug well opposite the sprue to catch cold slugs before they enter the runner.

## When to Replace the Nozzle

Replace the [injection nozzle](https://www.imspares.com/parts/mc-bc-07/) when:
- The bore is visibly worn (enlarged orifice)
- The sealing surface (where it contacts the sprue bushing) is damaged or pitted
- Internal surface is heavily carboned and can't be cleaned
- The thread is stripped or damaged
- You're switching to a different material family that requires a different nozzle geometry

We supply nozzles for Haitian MA II, MA IIS and MA III machines, from the MA600 up to the MA24000, with M30×2, M39×2, M45×4 and M60×4 threads in several lengths. They are machined from SKD61 / H13 tool steel hardened to HRC 48-52, in open, shut-off and filter types. [Request a quote](https://www.imspares.com/request-quote/mc-bc-07/) with your machine model and the nozzle length, or [contact us](https://www.imspares.com/contact/) if you're not sure which one you need.