The injection molding machine nozzle is the heated, threaded steel part on the front of the barrel that carries the melt into the mold. It screws into the barrel head (front flange), and on every shot its rounded tip presses against the mold's sprue bushing to form a seal. The tip radius and the orifice diameter must suit the sprue bushing, and the nozzle design must suit the material, or the nozzle leaks, drools or leaves the sprue stuck in the mold.
The nozzle is item 1 in the drawing. For how it fits with the barrel and screw, see the barrel and screw guide.
Parts of an injection molding machine nozzle
- Nozzle body. Screws into the barrel head and carries the melt channel. It has its own heater, usually a small band, and its own thermocouple, controlled as a separate zone; see the nozzle heater guide.
- Nozzle tip. The front end, with a spherical radius that seats in the sprue bushing and the orifice the melt leaves through. On many nozzles the tip is a separate part screwed into the body, so it can be replaced on its own.
- Melt channel. The bore through the body and tip. It should be smooth, with no steps or dead spots where melt can hang up and degrade.
How the nozzle seals against the sprue bushing
When the injection unit moves forward, the carriage cylinders (or the carriage drive, on electric machines) press the nozzle tip into the spherical seat of the sprue bushing with the nozzle contact force. Two dimensions decide whether that contact seals:
- Radius. The nozzle radius must be the same as or slightly smaller than the sprue bushing radius. If it is larger, the nozzle touches only at the outer edge and melt leaks into the gap, where it freezes and builds up.
- Orifice. The nozzle orifice must be slightly smaller than the sprue bushing opening. If it is larger, the step between them holds the sprue in the bushing when the mold opens.
The nozzle must also be centered on the sprue bushing. To check the contact, put a thin coat of marking paste on the nozzle tip, bring it forward against the mold, and look at the print: it should be an even ring around the orifice.
Nozzle tip size: radius and orifice diameter
The radius and orifice are set by the mold: they come from the sprue bushing, which is shown on the mold drawing or can be measured with radius gauges and pin gauges. Nozzle tips are made in a range of standard radii and orifice sizes, so when a mold moves to another machine, change the tip rather than accept a mismatch.
A larger orifice lowers the pressure loss and the shear in the nozzle but makes drooling more likely. A smaller orifice drools less but raises the pressure loss and freezes off sooner. Use the size the mold maker specifies, adjusted only within what the sprue bushing allows.
Nozzle tip material
- Hardened tool steel for most materials.
- Wear-resistant steels or hard coatings for glass-filled and mineral-filled plastics, which erode the orifice.
- Corrosion-resistant steels for plastics that release corrosive gases.
- Copper-alloy tips, which conduct heat better and keep the melt at the tip from freezing, but are softer and wear faster.
Nozzle designs
| Design | What it is | Used for |
|---|---|---|
| Open (standard) nozzle | Open bore with no valve | Most materials and molds |
| Extended nozzle | A longer body that reaches a sprue bushing set deep in the mold, heated along its length | Molds with a recessed sprue bushing |
| Reverse-taper nozzle | The bore narrows a short way inside the tip and widens again toward the orifice, so the sprue breaks off at the narrow point | Low-viscosity materials that drool, such as nylon |
| Shut-off nozzle | A valve closes the bore between shots | Drool-prone materials; see the shut-off nozzle guide |
| Filter nozzle | A slotted insert or screen catches contamination | Regrind and recycled material; see the nozzle filter guide |
Signs of nozzle wear or trouble
- Drool or strings of plastic between shots.
- Plastic leaking around the tip at the sprue bushing.
- Cold slugs in the part, or a blocked nozzle.
- Dents, pitting or a flattened ring on the tip radius.
- An orifice that has worn larger than its original size.
- Plastic leaking at the thread, or a damaged thread.
Many of these come from settings rather than wear. Our article on nozzle drooling, stringing and blockages goes through the causes and fixes.
Removing and fitting a nozzle
Never stand in front of the nozzle while a barrel with plastic in it is heating: gas trapped behind a frozen nozzle can blow molten plastic out.
- Retract the injection unit and keep the nozzle and barrel at processing temperature.
- Remove the nozzle heater and thermocouple as the machine manual describes.
- Unscrew the nozzle hot, with a wrench on its flats, wearing heat-resistant gloves and a face shield.
- Clean the thread and the seat in the barrel head, and the nozzle bore, with brass tools.
- Coat the new nozzle's thread with high-temperature anti-seize and tighten it to the torque in the manual.
- Refit the heater and thermocouple, heat to temperature, and check the contact with the sprue bushing.
What to send for a replacement nozzle
- The machine brand and model.
- The nozzle thread and the overall length.
- The tip radius and orifice, or the sprue bushing radius and opening of the mold.
- The design you need (open, extended, reverse taper, shut-off or filter) and the materials you run.
- Photos of the old nozzle and, if you have it, the OEM part number.
The injection nozzles we supply for Haitian MA II, MA IIS and MA III machines come with M30×2, M39×2, M45×4 or M60×4 threads in several lengths. Request a quote with the details above.
Updated by the IMSPARES technical team.