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.
Mechanical Components
Barrels & Cylinders
Bimetallic barrels, nitrided cylinders, and barrel assemblies
Parts of the injection unit
Point at a number or a part of the drawing.
Screws into the front of the barrel and seals against the mold’s sprue bushing. The melt passes through it on every shot.
Also called the front head or end cap. It closes the front of the barrel and holds the nozzle.
Thick steel tube the screw turns in. Bimetallic barrels have a hard alloy lining that resists wear from glass-filled and abrasive plastics.
Melts and meters the plastic. Its flights are deep in the feed zone, get shallower through the compression zone and stay shallow in the metering zone.
A non-return valve on the front of the screw. The ring lets melt pass while the screw turns and seals when it injects, so the shot does not flow back.
Bands around the barrel and nozzle that heat the plastic to its processing temperature, each zone controlled by its own thermocouple.
Pellets drop from the hopper through the feed throat into the screw. The throat is usually water-cooled so the pellets do not melt and stick before the screw.
Hydraulic cylinder that drives the screw forward to inject and holds pressure while the part sets. Its seals are wear parts.
Filters Category, zone, brand
- MC-BC-01Barrels & CylindersIn stock
Bimetallic Barrel & Screw Assembly
High-quality bimetallic barrel and screw assembly for plastic injection molding machines.
- Barrels & CylindersIn stock
- MC-BC-06Barrels & CylindersIn stock
Injection Cylinder
Injection hydraulic cylinder for injection molding machine injection unit.
- Barrels & CylindersIn stock
- MC-BC-08Barrels & CylindersIn stock
Barrel Front Flange (Front Head)
Barrel front flange (front head) for Haitian MA series injection molding machines.
How these parts work
How each part works, how it wears and what to send for a replacement quote, with drawings.
- Machine zone guideBarrel and screw of an injection molding machineHow the barrel and screw of an injection molding machine work: screw zones, check ring, front flange, nozzle, signs of wear and what to send for a quote.
- Machine zone guideInjection unit of an injection molding machineHow the injection unit of an injection molding machine works: injection cylinder, piston rod, screw motor, guide columns, linear guides and common faults.
- ScrewWhat the screw of an injection molding machine does, the names of its parts, its feed, compression and metering zones, common screw types and signs of wear.
- Injection cylinderWhat the injection cylinder of an injection molding machine does, single and twin designs, signs of worn seals or a scored bore, and how to specify a new one.
- BarrelWhat the barrel of an injection molding machine does, how it is built and heated, its temperature zones, nitrided and bimetallic bores, wear and replacement.
- Check ringWhat the check ring (non-return valve) on an injection molding screw does, how it opens and seals, the signs of a worn ring, and how to replace it.
- NozzleHow the injection molding machine nozzle seals against the mold, nozzle tip radius and orifice size, tip materials, nozzle designs, wear signs and replacement.
- Shut-off nozzleWhat 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.
- Nozzle filterWhat an injection molding nozzle filter does, slotted-insert and screen designs, when to fit one, the signs of a blocked filter, and how to clean or replace it.
- Barrel headWhat the barrel head (end cap, front flange or cannon head) of an injection molding machine does, how it seals, signs of leaks and wear, and how to replace it.
- Bimetallic barrelWhat a bimetallic barrel is, how its alloy liner is cast, how it compares with a nitrided barrel, what a bimetallic screw is, and when the upgrade is worth it.
- L/D ratioWhat the L/D ratio and the compression ratio of an injection molding screw mean, how to measure and calculate them, typical values, and how they affect the melt.
- Back pressureWhat back pressure is in injection molding, how the machine creates it, how it affects melt quality, recovery time and wear, and how to set and troubleshoot it.
- Shot size calculationHow to calculate shot size and barrel capacity on an injection molding machine: swept volume, shot weight for your material, stroke, cushion and capacity use.
- Removing the screwHow to remove the screw from an injection molding machine, step by step: preparation, nozzle, barrel head and screw tip removal, pulling the screw and refitting.
Related articles
When Should You Replace Your Injection Molding Machine Barrel?
Practical signs that tell you it's time to replace your barrel. Worn barrels waste material, cause defects, and cost you money every cycle. Here's how to know when enough is enough.
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