---
title: "Shut-off nozzle for injection molding: types and uses"
url: "https://www.imspares.com/injection-molding-machine-parts/barrel-screw/shut-off-nozzle/"
---

# Shut-off nozzle for injection molding: types and uses

What a shut-off nozzle does in injection molding, needle, sliding and bolt types, spring or cylinder actuation, when to use one, common faults and maintenance.

A shut-off nozzle is an injection molding nozzle with a built-in valve that closes the melt channel between shots. It stops material drooling or stringing from the nozzle when the injection unit moves away from the mold, and lets the screw recover with back pressure while the nozzle is closed. Shut-off nozzles are used mainly with low-viscosity materials such as nylon and in processes that need the front of the barrel sealed, and they add moving parts that an open nozzle does not have.

A shut-off nozzle takes the place of the standard nozzle, item 1 in the drawing. For nozzle sizing and sealing against the sprue bushing, which apply to every nozzle, see the [nozzle guide](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/nozzle/).

## How a shut-off nozzle works

1. **Injection and hold.** The valve is open, and the melt flows through the nozzle into the mold as it would through an open nozzle.
2. **After hold.** The valve closes. The screw can now recover with back pressure, and the injection unit can retract from the sprue bushing without melt running out of the tip.
3. **Next shot.** The valve opens again before or as injection starts.

On machines with an actuated valve, the controller has a shut-off nozzle function that sets when it opens and closes. The valve must be open before the screw injects; the machine manual describes the interlocks.

## Types of shut-off nozzle

| Type | How it is opened and closed | Notes |
|---|---|---|
| Spring-loaded needle | A spring holds a needle in the orifice; injection pressure pushes it open | No outside control; the spring can weaken with heat |
| Actuated needle | A hydraulic or pneumatic cylinder moves the needle through a lever, on a signal from the controller | Positive opening and closing, with timing set on the machine |
| Sliding (contact) type | Pressing the nozzle against the sprue bushing slides it open; retracting closes it | Opens only when the nozzle is in contact with the mold |
| Bolt or cross-slide type | A pin or bolt across the melt channel, moved by a cylinder, opens or blocks it | Straight flow path when open |

The actuated types need a cylinder, a valve and wiring or hoses, and the nozzle body is longer than a standard nozzle. Check the space between the barrel head and the mold before choosing one.

## When to use a shut-off nozzle

- Low-viscosity materials that drool from an open nozzle, such as nylon.
- Processes where the injection unit retracts from the mold every cycle and the screw should recover while it is away.
- Foamed materials, where the melt must stay under pressure until it is injected.

Before fitting one for drool, check the simpler fixes: lower the nozzle temperature, set decompression (suck-back) after recovery, reduce back pressure, or use a reverse-taper tip. Our article on [nozzle drooling, stringing and blockages](https://www.imspares.com/blog/injection-nozzle-problems-troubleshooting/) goes through them.

Avoid shut-off nozzles for heat-sensitive materials such as rigid PVC, where any spot in which melt can hang up leads to degradation, unless the nozzle maker and the material supplier approve it. Abrasive filled materials wear the needle and seat faster.

## Common faults and their causes

- **Drool or leaking when closed:** worn needle or seat, degraded plastic on the seat, a weak spring, or an actuator that does not close fully.
- **Valve not opening fully:** injection pressure rises, fill slows and short shots appear. Causes are a sticking needle, a fault in the actuating cylinder, its valve or its signal, or a worn linkage.
- **Black specks or burnt streaks:** melt hanging up in dead spots inside the nozzle, often because the nozzle runs too hot or the material sits too long.
- **Seized needle:** carbonized plastic around it after overheating or a long stop.
- **Oil or air leaks** at the actuating cylinder, or play in the lever and pins.

## Maintenance and cleaning

1. Purge the barrel and keep the nozzle at processing temperature.
2. Before removing the nozzle, make sure the valve is open or the pressure in front of the screw has been relieved. A closed shut-off nozzle can trap molten plastic and gas under pressure.
3. Retract the injection unit and remove the nozzle hot, as described on the nozzle page, wearing heat-resistant gloves and a face shield.
4. Take the nozzle apart following the maker's instructions and clean the parts while warm, with brass tools.
5. Inspect the needle tip and its seat for wear and scoring, the spring for loss of length, and the actuator seals and linkage for leaks and play.
6. Replace worn parts, reassemble with high-temperature anti-seize on the threads, and check the open and close timing on the machine before production.

## What to send for a quote

1. The machine brand and model, and the nozzle thread.
2. The type and the actuation (spring, hydraulic, pneumatic or contact).
3. The overall length, and the tip radius and orifice to suit the mold's sprue bushing.
4. The materials you run and the problem you want to solve.
5. Photos of the old nozzle and its actuator, and the OEM part number if you have it.

The [injection nozzles](https://www.imspares.com/parts/mc-bc-07/) we supply include open, shut-off and filter types. [Request a quote](https://www.imspares.com/request-quote/) with the details above.

## Questions

### What is a shut-off nozzle used for in injection molding?

A shut-off nozzle closes the melt channel between shots, so material does not drool or string from the nozzle when the injection unit moves away from the mold. It also lets the screw recover with back pressure while the nozzle is off the sprue bushing.

### What is the difference between a spring-loaded and a hydraulic shut-off nozzle?

In a spring-loaded nozzle a spring holds the needle closed and the injection pressure pushes it open, with no outside control. In a hydraulic or pneumatic nozzle a cylinder opens and closes the valve on a signal from the machine, so the timing is set in the controller.

### Can a shut-off nozzle be used with PVC?

It is generally avoided for rigid PVC and other heat-sensitive materials, because any place where melt can hang up inside the valve lets it degrade. Ask the material supplier before using one with a heat-sensitive grade.

### Why does a shut-off nozzle still drool?

Usually because the valve is not closing fully: a worn needle or seat, degraded plastic on the seat, a weak spring, or an actuator that does not complete its stroke. Too high a nozzle temperature or back pressure makes it worse.

## Replacement parts

- [MC-BC-07 Injection Nozzle](https://www.imspares.com/parts/mc-bc-07/): Precision-machined injection nozzle for plastic injection molding machines.