--- title: "Barrel and screw of an injection molding machine" url: "https://www.imspares.com/injection-molding-machine-parts/barrel-screw/" --- # Barrel and screw of an injection molding machine How 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. The barrel and screw melt the plastic and push it into the mold. The screw turns inside the heated barrel to melt each shot, then moves forward like a plunger to inject it through the nozzle. Together with the screw tip, the check ring, the barrel front flange and the nozzle, they form the **plasticizing unit**, the part of the injection unit that wears the most. The numbered drawing shows each part; this guide explains what each one does, how they wear and how to specify a replacement, and each part has its own guide, linked below. ## How the barrel and screw work Every cycle has two movements of the same screw: 1. **Plasticizing (screw recovery).** The screw turns inside the heated barrel. Its flights carry pellets forward from the feed throat, and the shear heat of the turning screw, together with the heat from the heater bands, melts them. The melt collects in front of the screw tip and pushes the screw backward until the set [shot size](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/shot-size/) is reached. A small [back pressure](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/back-pressure/) on the screw keeps the melt dense and well mixed. 2. **Injection and hold.** The injection cylinder drives the screw forward like a plunger. The check ring closes, so the melt in front of the screw is pushed through the nozzle into the mold. The screw then holds pressure while the part cools and shrinks, and a little melt is left in front of the tip as the **cushion**. Because the same screw both melts and injects, any wear between the screw and the barrel shows up directly in the shot: melt leaks backward instead of going into the mold. ## The parts of the barrel and screw ### Barrel A thick steel tube with a precisely finished bore, heated in zones by heater bands, each zone measured by its own thermocouple. The bore is either nitrided or bimetallic (compared below). See the [barrel guide](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/barrel/). We supply complete [bimetallic barrel and screw assemblies](https://www.imspares.com/parts/mc-bc-01/) and [screw barrel assemblies](https://www.imspares.com/parts/mc-sb-01/). ### Screw A general-purpose screw has three zones: deep flights in the **feed zone** take in the pellets, the flights get shallower through the **compression zone** to compress and melt the plastic, and the shallow **metering zone** delivers an even, fully melted shot to the front. Screws are described by their diameter and their [L/D ratio and compression ratio](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/l-d-ratio/); general-purpose injection screws typically have an L/D of about 20:1 to 25:1. Wear shows first on the outside diameter of the flights. The [screw guide](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/screw/) covers its parts and types; see also our [injection screws](https://www.imspares.com/parts/mc-bc-02/). ### Screw tip and check ring The [screw tip](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/screw-tip/) screws into the front of the screw and carries the non-return valve. The [check ring](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/check-ring/) slides forward to let melt pass while the screw turns, and is pushed back onto its seat ring to seal when the screw injects. A worn check ring is the most common reason for an unstable cushion, and it costs much less to replace than a screw. Removing the tip needs a [screw head wrench](https://www.imspares.com/parts/mc-gmp-01/) that matches it. ### Barrel front flange Also called the [barrel head](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/barrel-head/), front head, end cap or cannon head. On most machines it is bolted to the front of the barrel; it closes the barrel, leads the melt into the nozzle and carries the nozzle thread. Its sealing face must be clean and flat: a damaged face leaks melt between the flange and the barrel. See the [barrel front flange](https://www.imspares.com/parts/mc-bc-08/) we supply. ### Nozzle The [nozzle](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/nozzle/) screws into the front flange and touches the mold's sprue bushing on every shot. Its tip radius must be the same as or slightly smaller than the sprue bushing radius, and its orifice slightly smaller than the sprue opening, or the shot leaks and the sprue sticks. Open nozzles are the most common; [shut-off nozzles](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/shut-off-nozzle/) are used for materials that drool and [filter nozzles](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/nozzle-filter/) for material that may carry contamination. See [injection nozzles](https://www.imspares.com/parts/mc-bc-07/) and our article on [nozzle drooling, stringing and blockages](https://www.imspares.com/blog/injection-nozzle-problems-troubleshooting/). ### Heater bands and thermocouples Heater bands, usually ceramic or mica, are clamped around the barrel and the nozzle in zones (rear, middle, front and nozzle), each zone with a thermocouple that feeds the temperature controller. A failed band or thermocouple leaves a zone too cold or too hot, which can give unmelted pellets or degraded plastic and puts extra load on the screw. See the [heater bands and sensors guide](https://www.imspares.com/injection-molding-machine-parts/heaters/). ### Hopper and feed throat Pellets fall from the [hopper through the feed throat](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/hopper-and-feed-throat/) onto the screw. The throat is water-cooled so the pellets stay solid until the screw takes them in; if it runs too hot, the pellets soften and bridge, and the screw starves. A hopper magnet in the material path catches loose iron and steel before it can reach the screw. ### Injection cylinder and screw drive The [injection cylinder](https://www.imspares.com/injection-molding-machine-parts/injection-unit/injection-cylinder/) moves the screw forward and holds pressure; see the [injection cylinders](https://www.imspares.com/parts/mc-bc-06/) we supply. On hydraulic machines a [hydraulic plasticizing motor](https://www.imspares.com/parts/hy-pw-08/) turns the screw, while electric machines use a servo motor (see the [plasticizing motor guide](https://www.imspares.com/injection-molding-machine-parts/injection-unit/plasticizing-motor/)). Worn cylinder seals cause slow or uneven injection: see our [hydraulic cylinder seal kits](https://www.imspares.com/parts/mc-sb-08/). ## Settings that load the screw and barrel How the machine is set affects how fast the plasticizing unit wears: - **Back pressure and screw speed.** Both add shear heat to the melt. They improve mixing, but very high settings lengthen recovery, raise the melt temperature and wear the screw, check ring and barrel faster. - **Shot size compared with barrel capacity.** A shot that uses only a small part of the barrel capacity leaves the plastic in the barrel for many cycles, where it can degrade; a shot close to the full stroke leaves little cushion and less melting length. - **Screw design for the material.** The screw's L/D ratio, compression ratio and surface should suit the plastic you run, especially heat-sensitive or filled materials. ## Signs of a worn screw and barrel - The cushion varies from shot to shot, or the screw creeps forward during hold. - Screw recovery takes longer than it used to at the same settings. - Short shots or weight variation that process changes do not cure. - More black specks, burnt material or streaks, and slower color changes. - Scratches or scoring on the screw flights when it is pulled. Several of these also come from a worn check ring alone, a failing heater band or a pressure problem in the hydraulics, so measure before replacing. Our guide to [measuring barrel and screw wear](https://www.imspares.com/blog/measuring-barrel-screw-wear-injection-molding/) explains how, and [when to replace the barrel](https://www.imspares.com/blog/when-to-replace-injection-molding-machine-barrel/) covers the decision. ## Nitrided or bimetallic? A **nitrided** barrel has a thin hardened surface layer of the barrel steel itself and suits most unfilled plastics. A **bimetallic** barrel has a much thicker lining of wear-resistant alloy cast and bonded inside the bore, and lasts much longer with glass-filled, mineral-filled and flame-retardant plastics. Screws are matched in the same way, with nitrided flights for general use and hard-faced or coated flights for abrasive materials. The [bimetallic barrel guide](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/bimetallic-barrel/) compares the two in detail. ## What to send for a replacement quote To quote a screw, barrel, check ring or nozzle, send: 1. The machine brand and model, and the injection unit size if you know it. 2. The screw diameter and the plastics you run (especially fillers such as glass fiber). 3. Photos of the old part with the main dimensions: overall length, flight diameter, the drive end (spline or key) and the tip thread. 4. The OEM part number or a drawing, if you have one. [Request a quote](https://www.imspares.com/request-quote/) or send the details on WhatsApp. ## Maintenance that makes them last - Never turn the screw before the barrel has reached temperature and soaked: turning it on cold plastic can break the tip or twist the screw. - Purge thoroughly before shutdown and when changing to a material that degrades easily. - Keep the feed throat cooling water running, and clean the hopper magnet. - Check the heater bands and thermocouples regularly; one cold zone puts extra load on the screw. - Record recovery time and cushion: a slow change over weeks is the earliest sign of wear. - When the screw has to come out, follow the steps in [how to remove the screw](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/removing-the-screw/), and inspect the tip set, front flange and nozzle while everything is apart. ## Part by part - [Screw](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/screw/): What 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. - [Barrel](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/barrel/): What 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 ring](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/check-ring/): What 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. - [Screw tip](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/screw-tip/): What the screw tip of an injection molding machine does, ring, ball and free-flow tip types, signs of wear, and how to remove and refit a screw tip safely. - [Nozzle](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/nozzle/): How 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 nozzle](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/shut-off-nozzle/): 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. - [Nozzle filter](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/nozzle-filter/): What 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 head](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/barrel-head/): What 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. - [Hopper and feed throat](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/hopper-and-feed-throat/): How the hopper and feed throat of an injection molding machine feed the screw, why the throat is water-cooled, what a hopper magnet does, and feeding problems. - [Bimetallic barrel](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/bimetallic-barrel/): What 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 ratio](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/l-d-ratio/): What 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 pressure](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/back-pressure/): What 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 calculation](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/shot-size/): How 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 screw](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/removing-the-screw/): How 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. ## Questions ### What does the check ring do on an injection molding screw? The check ring is a non-return valve on the front of the screw. While the screw turns, the ring is pushed forward and melt flows past it to the front of the barrel. When the screw moves forward to inject, the pressure in front pushes the ring back against its seat, so the melt goes into the mold instead of flowing back along the flights. ### How do I know if the screw and barrel are worn? Typical signs are a cushion that varies from shot to shot, the screw creeping forward during hold pressure, longer screw recovery times, short shots, more black specks and slower color changes. A worn check ring causes several of the same signs, so measure the barrel bore and the screw flight diameter to confirm. ### Should the screw and barrel be replaced together? Measure both first. If the barrel bore is still close to its original size, a new screw (or a new check ring) restores the clearance. If the barrel is worn, a new screw in it still leaves too much clearance, so both are usually replaced as a matched assembly. ### What is the difference between a nitrided and a bimetallic barrel? A nitrided barrel is alloy steel whose bore is hardened in a thin surface layer, suitable for most unfilled plastics. A bimetallic barrel has a much thicker lining of wear-resistant alloy cast and bonded inside the bore, which lasts much longer with glass-filled, mineral-filled and flame-retardant materials. ### What information is needed to quote a screw and barrel? The machine brand and model, the screw diameter, the plastics you process, and photos of the old screw and barrel with their main dimensions: overall length, flight diameter, the drive end and the tip thread. A drawing or the OEM part number makes it faster. ## Replacement parts - [MC-BC-01 Bimetallic Barrel & Screw Assembly](https://www.imspares.com/parts/mc-bc-01/): High-quality bimetallic barrel and screw assembly for plastic injection molding machines. - [MC-BC-02 Injection Screw](https://www.imspares.com/parts/mc-bc-02/): Replacement injection screw for injection molding machine plasticizing unit. - [MC-BC-07 Injection Nozzle](https://www.imspares.com/parts/mc-bc-07/): Precision-machined injection nozzle for plastic injection molding machines. - [MC-BC-08 Barrel Front Flange (Front Head)](https://www.imspares.com/parts/mc-bc-08/): Barrel front flange (front head) for Haitian MA series injection molding machines. - [MC-GMP-01 Screw Head Wrench (3-Prong / 4-Prong / 6-Prong)](https://www.imspares.com/parts/mc-gmp-01/): Specialist screw head wrench for removing injection molding machine screw heads. - [MC-SB-01 Screw Barrel Assembly](https://www.imspares.com/parts/mc-sb-01/): Complete screw and barrel assembly for injection molding machine plasticizing unit.