--- title: "Injection unit of an injection molding machine" url: "https://www.imspares.com/injection-molding-machine-parts/injection-unit/" --- # Injection unit of an injection molding machine How the injection unit of an injection molding machine works: injection cylinder, piston rod, screw motor, guide columns, linear guides and common faults. The injection unit of an injection molding machine melts the plastic and injects it into the mold. It is made of the **plasticizing unit** (nozzle, barrel head, barrel, screw, screw tip and check ring, and the heater bands around them), the **injection cylinder** and piston rod that drive the screw forward, the **plasticizing motor** that turns the screw, and the **carriage** that moves the whole unit to and from the mold on guide columns or linear guides. The numbered drawing shows the plasticizing unit and injection cylinder in section; this guide explains how the injection unit works as a whole, how its mechanical and hydraulic parts wear, and what to send for a replacement. ## What the injection unit does in the cycle 1. **Carriage forward.** Carriage cylinders move the injection unit forward until the nozzle touches the sprue bushing of the mold, then hold it there with the **nozzle contact force** so the melt cannot leak at the joint. 2. **Injection.** Oil flows into the injection cylinder, which drives the screw forward like a plunger. The check ring closes and the melt is pushed into the mold at the set speeds. 3. **Hold pressure.** At a set screw position, pressure or time, the machine switches from speed control to pressure control and holds pressure while the part cools and shrinks. 4. **Recovery (plasticizing).** The plasticizing motor turns the screw. Melt collects in front of it and pushes the screw back, against the **back pressure** held in the injection cylinder, until the shot size is reached. 5. **Decompression.** The screw is pulled back a short distance to relieve the melt pressure, so the nozzle does not drool. 6. **Carriage back** (when the process needs it), to break the sprue or to purge. How the barrel and screw melt the plastic is explained in the [barrel and screw guide](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/); this guide covers the parts that move them. ## How injection speed and pressure are controlled On a hydraulic machine, the **oil flow** into the injection cylinder sets the injection speed, and the **oil pressure** sets the injection and hold pressure. Proportional valves, or a servo-driven pump, adjust both through the cycle; see [proportional valves](https://www.imspares.com/injection-molding-machine-parts/hydraulics/proportional-valve/) and the [hydraulics guide](https://www.imspares.com/injection-molding-machine-parts/hydraulics/). The pressure on the melt is higher than the oil pressure. The injection cylinder's piston area is much larger than the cross-section of the screw, so the force from the oil is concentrated on a smaller area: melt pressure equals oil pressure multiplied by the **intensification ratio**, the ratio of the two areas. The ratio depends on the cylinder and the screw diameter: on the same injection unit, a smaller screw gives a higher injection pressure and a larger screw a bigger shot. A [linear position transducer](https://www.imspares.com/injection-molding-machine-parts/electrical/linear-transducer/) measures the screw position for the shot size, the switchover to hold and the cushion. Electric machines replace the injection cylinder with a servo motor and a ball screw, and the plasticizing motor with a second servo motor. The sequence is the same. ## The parts of the injection unit ### Nozzle, barrel, screw and check ring The plasticizing unit: the nozzle, the barrel front flange, the barrel, the screw, and the screw tip and check ring, items 1 to 5 in the drawing. They are covered in the barrel and screw guide linked above and its pages on the [screw](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/screw/), the [check ring](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/check-ring/) and the [nozzle](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/nozzle/). ### Heater bands The bands around the barrel and nozzle, each zone with its own thermocouple, bring the plastic to processing temperature. See the [heaters guide](https://www.imspares.com/injection-molding-machine-parts/heaters/). ### Hopper and feed throat Pellets fall from the hopper through the water-cooled feed throat onto the screw. See [hopper and feed throat](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/hopper-and-feed-throat/). ### Injection cylinder Item 8 in the drawing: a double-acting hydraulic cylinder that drives the screw forward to inject, holds pressure, and provides back pressure during recovery. Some machines have one cylinder in line with the screw; many have two cylinders side by side. Worn seals cause slow, weak or uneven injection. See the [injection cylinder](https://www.imspares.com/injection-molding-machine-parts/injection-unit/injection-cylinder/) page. ### Injection piston rod The hardened, chrome-plated rod that joins the injection piston to the moving plate and carries the injection force. Its surface runs through the rod seals, so scoring or pitting on it causes oil leaks. See the [injection piston rod](https://www.imspares.com/injection-molding-machine-parts/injection-unit/injection-piston-rod/) page. ### Plasticizing motor The low-speed, high-torque hydraulic motor that turns the screw during recovery. Its speed sets the screw speed, and its torque must overcome the resistance of the plastic. See the [plasticizing motor](https://www.imspares.com/injection-molding-machine-parts/injection-unit/plasticizing-motor/) page. ### Guide columns and bushings On many twin-cylinder injection units, the moving plate carrying the screw drive slides on guide columns through bushings, which keep it square to the barrel. Worn bushings let the plate rock and load the screw and the seals unevenly. See [guide columns and bushings](https://www.imspares.com/injection-molding-machine-parts/injection-unit/guide-columns/). ### Linear guide and carriage The injection unit carriage runs on guide rails on the machine base, on many machines linear guide rails with ball carriage blocks. They keep the nozzle in line with the mold as the carriage moves. See the [linear guide](https://www.imspares.com/injection-molding-machine-parts/injection-unit/linear-guide/) page. ### Carriage cylinders One or two hydraulic cylinders between the machine base and the injection unit. They move the carriage and set the nozzle contact force. Their seals are the same kind of wear part as those of the injection cylinder; see [cylinder seal replacement](https://www.imspares.com/injection-molding-machine-parts/clamp-cylinder/cylinder-seal-replacement/). ## Common problems and their symptoms - **Slow or weak injection, or set pressure not reached.** Hydraulic supply, valves, or worn injection cylinder seals. See [hydraulic pressure loss](https://www.imspares.com/blog/troubleshooting-hydraulic-pressure-loss/). - **Cushion varies, or the screw creeps forward during hold.** Most often the check ring; then the injection cylinder. - **Screw does not turn, turns slowly or unevenly.** The plasticizing motor, its supply valve, a cold barrel zone, or a worn drive coupling. - **Recovery time increases.** A worn screw and barrel, a cold zone, or internal leakage in the plasticizing motor. - **Melt leaks between the nozzle and the sprue bushing.** Nozzle and sprue radius or orifice mismatch, too little nozzle contact force, or the nozzle off center because of worn guides. - **Carriage moves jerkily or noisily.** Dry or damaged linear guides, or worn guide bushings. - **Oil leaks at the cylinders.** Worn rod seals or a damaged piston rod. Measure before replacing: injection time, recovery time, peak injection pressure and cushion, compared with records from when the machine ran well, show which part has changed. ## What to send for a replacement quote 1. The machine brand and model, and the injection unit size or screw diameter. 2. The part name and, if you have it, the OEM part number or the nameplate data (for a motor, its model and shaft type). 3. Photos of the old part and its main dimensions: bore and rod diameter and stroke for cylinders, length and thread sizes for rods, rail width and length for linear guides. 4. For seal kits, the cylinder bore and rod diameter, or the markings on the old seals. [Request a quote](https://www.imspares.com/request-quote/) with these details. ## Maintenance that keeps the injection unit accurate - Grease the linear guides and lubricate the guide column bushings as the machine manual specifies; on some machines the centralized lubrication system serves them (see [toggle lubrication](https://www.imspares.com/injection-molding-machine-parts/toggle/toggle-lubrication/)). - Wipe the piston rods and check them for scores and rust, especially after a long stop. - Look for oil leaks at the cylinders, the plasticizing motor and the hoses at every planned stop. - Check that the lock nuts on the piston rods and the guide column nuts are tight. - Keep the hydraulic oil clean and at the right temperature; see [hydraulic oil problems](https://www.imspares.com/blog/hydraulic-oil-problems-injection-molding/). - Check that the nozzle is centered on the sprue bushing after mold changes. - Record injection time, recovery time and cushion: slow changes over weeks are the earliest sign of wear. ## Part by part - [Injection cylinder](https://www.imspares.com/injection-molding-machine-parts/injection-unit/injection-cylinder/): What 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. - [Injection piston rod](https://www.imspares.com/injection-molding-machine-parts/injection-unit/injection-piston-rod/): What the injection piston rod of an injection molding machine does, how its chrome surface and threads wear, how to inspect it, and what to send for a quote. - [Plasticizing motor](https://www.imspares.com/injection-molding-machine-parts/injection-unit/plasticizing-motor/): How the hydraulic plasticizing motor turns the screw of an injection molding machine, the motor types, signs of a worn motor, and what to send for a quote. - [Guide columns and bushings](https://www.imspares.com/injection-molding-machine-parts/injection-unit/guide-columns/): What the guide columns, guide bushings and nuts of an injection molding machine do, signs of worn bushings, how to check the play and what to send for a quote. - [Linear guide](https://www.imspares.com/injection-molding-machine-parts/injection-unit/linear-guide/): How the linear guide rail and carriage blocks under the injection unit of an injection molding machine work, signs of wear, greasing, and replacement. ## Questions ### What are the main parts of the injection unit? The plasticizing unit (nozzle, barrel head, barrel, screw, screw tip and check ring, and the heater bands around them), the injection cylinder and its piston rod that push the screw forward, the plasticizing motor that turns the screw, and the carriage that moves the whole unit to and from the mold on guide columns or linear guides. ### What does the injection cylinder do? It drives the screw forward to inject the melt into the mold, holds pressure while the part cools, and controls back pressure while the screw recovers. On hydraulic machines it is a double-acting hydraulic cylinder; some designs use two cylinders side by side. ### What turns the screw on a hydraulic injection molding machine? A low-speed, high-torque hydraulic motor at the back of the injection unit, called the plasticizing motor or screw drive motor. Electric machines turn the screw with a servo motor instead. ### Why is injection slow or weak? Common causes are a hydraulic supply problem (pump, valves or oil), worn injection cylinder seals, and a worn check ring that lets melt flow back over the screw. Check the injection pressure reached and the cushion before replacing parts. ### What is the injection unit carriage? The frame that carries the whole injection unit. Carriage cylinders move it forward on its guides until the nozzle touches the mold's sprue bushing, hold it there with the nozzle contact force, and pull it back for purging or sprue break. ## Replacement parts - [HY-PW-08 Hydraulic Plasticizing Motor (NHM Series)](https://www.imspares.com/parts/hy-pw-08/): NHM series radial piston hydraulic motor for injection molding machine screw pre-plasticizing drive. - [MC-BC-06 Injection Cylinder](https://www.imspares.com/parts/mc-bc-06/): Injection hydraulic cylinder for injection molding machine injection unit. - [MC-GZ-01 Injection Piston Rod](https://www.imspares.com/parts/mc-gz-01/): Injection unit piston rod for injection molding machine injection cylinder. - [MC-SB-07 Piston Rod Seal Set](https://www.imspares.com/parts/mc-sb-07/): Piston rod seal set for injection molding machine hydraulic cylinders. - [MC-SB-08 Hydraulic Cylinder Seals & Y-Ring](https://www.imspares.com/parts/mc-sb-08/): Hydraulic cylinder seal kits and Y-ring piston rod seals for injection molding machine cylinders. - [MC-SB-10 Piston Rod Nut](https://www.imspares.com/parts/mc-sb-10/): Injection cylinder piston rod nut for Haitian and equivalent injection molding machines. - [MC-SG-08 Linear Guide Rail & Slider](https://www.imspares.com/parts/mc-sg-08/): Linear guide rail and slider block for injection molding machine injection unit or carriage.