Injection Molding Glossary - Key Terms & Definitions

A

Absolute Encoder: An encoder that sends the shaft position as a digital value over a serial link, so the position is known at power-on. Multi-turn versions also count revolutions.
Accumulator: A pressure vessel that stores hydraulic oil under nitrogen pressure and releases it when the machine needs more flow than the pump can deliver, mainly for high-speed injection. It also absorbs pressure peaks in the circuit.
Accumulator Pre-Charge: The nitrogen pressure in an accumulator's bladder with no oil pressure on it, filled and checked through the gas valve. A pre-charge that has dropped or was set wrong reduces the usable oil volume, so high-speed injection gets shorter or slower.
Air Shot: A shot injected with the nozzle retracted from the mold into a suitable container, for example to measure the real melt temperature with a preheated probe. Purged melt is hot and can spit, so wear a face shield and gloves.
All-Electric Machine: An injection molding machine with separate servo motors for the clamp, injection, screw rotation and ejector instead of hydraulic cylinders and motors. Injection is driven by a servo motor and a ball screw.
Anti-Vibration Pad: A rubber pad combined with a steel plate, placed under each leveling bolt of the machine. The rubber damps vibration and grips the floor, and the steel plate spreads the load and gives the bolt a seat.
Anti-Wear Hydraulic Oil (HM/HLP): The mineral hydraulic oil most injection molding machines use, classed HM under ISO 6743-4 or HLP under DIN 51524-2. It is commonly used in grade ISO VG 46 or ISO VG 68, as the machine manual specifies.
Auto-Tuning: A temperature controller function that heats a zone, watches how it responds and calculates its own PID settings. It is run when a zone overshoots or swings, for example after a heater band or thermocouple change.
Auxiliary Cylinder: A smaller hydraulic cylinder that works beside the main clamping cylinder. Depending on the machine, it adds clamping force as a booster, moves the platen quickly before the main cylinder builds force, or drives the ejector.

B

Back Pressure: The resistance the machine applies against the screw as it turns and moves back during screw recovery, which compacts and mixes the melt. Higher back pressure gives a more even melt but lengthens recovery, adds shear heat and wears the screw, check ring and barrel faster.
Baffle Plate: A plate in the hydraulic oil tank that separates the return side from the suction side. Returning oil has to flow around it, which gives air time to rise, dirt and water time to settle and the oil time to lose heat.
Barrel: The thick-walled steel tube that the screw turns in, fed through the feed opening at the rear and closed at the front by the barrel head. It carries the heater bands, withstands the full injection pressure on every shot, and its bore wears together with the screw flights.
Barrel Capacity: The volume of melt the screw sweeps over its full injection stroke, also called the theoretical shot volume. A shot that uses only a small share of it leaves plastic in the barrel for many cycles, where it can degrade; a shot close to the full stroke leaves little cushion.
Barrel Head: The steel piece bolted to the front of the barrel that closes it, leads the melt from the bore into the nozzle and carries the nozzle thread. Also called the barrel front flange, front head, end cap or cannon head.
Barrel Temperature Zone: One of the separately controlled heating sections along the barrel, usually the rear (feed), middle (compression) and front (metering) zones, plus the nozzle. Each zone has its own heater bands, thermocouple and set temperature.
Barrier Screw: A screw with a second flight in the melting section that separates the melted plastic from the unmelted pellets. It gives more even melting at higher outputs than a general-purpose screw.
Bell Housing: The housing that bolts the hydraulic pump to the motor flange and keeps the pump shaft centered on the motor shaft. The coupling inside then only has to carry the torque and absorb small misalignment.
Bimetallic Barrel: A steel barrel whose bore is lined with a layer of wear-resistant alloy, cast and bonded to the steel: the steel carries the pressure and the alloy liner takes the wear. It lasts much longer than a nitrided barrel with glass-filled, mineral-filled and flame-retardant plastics.
Bimetallic Screw: A steel screw whose flight lands, the surfaces that run against the barrel, carry a welded or sprayed layer of hard, wear-resistant alloy.
Bladder Accumulator: The most common accumulator type: a steel shell with a rubber bladder inside, the bladder filled with nitrogen and the oil around it. Oil pushed into the shell compresses the gas, and the gas pushes the oil back out when the system needs it.
Breather: The vent on the hydraulic oil tank that lets air in and out as the oil level changes.
Bushing: A sleeve that works as a plain bearing or guide, made of a softer material such as bronze so that it wears instead of the pin, bar or column that runs in it. On the machine, bushings are used at the toggle pins, in the platen bores for the tie bars and on the guide columns.

C

Carriage Cylinder: One of the one or two hydraulic cylinders between the machine base and the injection unit that move the carriage, bringing the nozzle to the mold, and set the nozzle contact force.
Cartridge Valve: A two-way slip-in valve, also called a logic valve: a poppet that opens or closes a passage depending on whether its control chamber is pressurized or vented. Several in one manifold switch the large flows of the clamp and injection circuits with a low pressure drop and very little leakage.
Case Drain: The separate line that returns to the tank the oil that leaks past the pistons of a piston pump and collects in its housing. A restricted case drain builds pressure in the housing and can damage the shaft seal and the pump.
Cavitation: The condition in which a hydraulic pump cannot draw enough oil, for example because of a blocked suction filter, a low oil level or cold, thick oil. The pump gets noisy, movements become slow or unsteady, and the pump wears quickly.
Centralized Lubrication: A system that pumps oil or grease through distributors and tubes to every toggle pin and bushing, and on many machines to the tie bar bushings and platen slides, at an interval set in the controller. When it fails, the joints run dry and wear quickly.
Ceramic Heater Band: A heater band with a resistance coil threaded through ceramic blocks inside an insulated steel shell. It handles higher temperatures and loses less heat outward than a mica band, but it is thicker and needs more space.
Check Ring: The ring on the screw tip that works as a non-return valve: melt pushes it forward and flows past while the screw turns, and the injection pressure pushes it back onto the seat ring so the melt goes into the mold instead of back along the screw. A worn check ring is the most common cause of an unstable cushion.
Clamping Cylinder: The hydraulic cylinder that opens and closes the mold. On a toggle machine it is mounted on the rear platen and drives the crosshead; because the toggle multiplies its force, it is much smaller than the cylinder of a direct hydraulic clamp.
Clamping Force: The force that holds the mold halves closed against the melt pressure during injection and hold, measured in kilonewtons (kN) or tons. Too little force lets the parting line open and the parts flash; on a toggle machine the force is set by the mold height adjustment.
Clamping Unit: The section of the machine that opens and closes the mold and holds it shut during injection: the platens, the tie bars and the toggle or clamping cylinders. It is built as a toggle clamp, a direct hydraulic clamp or a two-platen clamp.
Cold Junction Compensation: The correction a temperature controller applies to a thermocouple signal: it measures the temperature at its own terminals, the cold junction, and adds it back to work out the temperature at the tip.
Compression Ratio: The channel depth of a screw in the feed zone divided by the channel depth in the metering zone. It shows how much the screw compresses the plastic as it melts; general-purpose screws typically have a ratio between about 2:1 and 3:1.
Compression Zone: The middle section of a general-purpose screw, also called the transition zone, where the flights get shallower to compress and melt the plastic.
Control Panel: The operator panel on the front of the machine, a screen with keys or a touchscreen, where the operator enters the process settings, stores them for each mold, runs the machine and reads alarms. It sends the settings to the CPU board in the electrical cabinet.
Controller Board: Any of the circuit boards in the electrical cabinet that make up the machine controller, such as the main CPU board, the I/O modules, the drive interface board and the power supply board. Many apparent board faults come from the power supply or a loose connector, so those are checked first.
Cooling Channel: A passage in the mold, or in parts of the machine such as the feed throat, through which cooling water circulates to remove heat. A circuit restricted by scale or dirt gives uneven temperatures, longer cycles and warped parts.
Cooling Water Manifold: The distributor that feeds cooling water from the factory supply to the machine's circuits, usually the oil cooler, the feed throat and the mold, and collects the water coming back. Its valves and flow meters set and check each circuit in one place.
Cracking Pressure: The pressure difference needed to lift the poppet or ball of a check valve off its seat against its spring and start the flow. It is set by the spring and given in the valve data.
Crosshead: The block on the end of the clamping cylinder's piston rod that drives the toggle. Moving along the machine axis, it folds and straightens the upper and lower toggle links together through short connecting links.
Cushion: The small amount of melt left in front of the screw tip at the end of hold, so the screw does not bottom out. A cushion that varies from shot to shot or runs to zero most often points to a worn check ring.
Cycle Time: The time for one complete molding cycle: mold close, injection, hold, cooling, mold open and ejection. Screw recovery normally runs during the cooling time, so recovery that takes longer than the cooling time lengthens the cycle.

D

DC Bus Capacitor: An electrolytic capacitor in a servo drive that stores energy between the rectifier and the power module and smooths the DC voltage. It ages with heat and time, is a common cause of drive faults, and holds a dangerous charge after the supply is switched off.
Decompression: A short backward movement of the screw after recovery, also called suck-back, that relieves the melt pressure so the nozzle does not drool. It moves the screw back without adding melt, so it does not count toward the shot.
Direct Hydraulic Clamp: A clamping unit in which a large hydraulic cylinder presses on the platen directly, with the clamping force set by the hydraulic pressure rather than by the mold height adjustment as on a toggle machine. It has few moving joints; its main wear parts are the cylinder seals.
Directional Control Valve: A solenoid valve whose spool connects the pump to one side of a cylinder or hydraulic motor and the other side to the tank, so it starts, stops and reverses a movement. A spool that sticks or shifts only part way makes the movement slow, weak or jerky.
Double-Acting Cylinder: A hydraulic cylinder with an oil port at each end, so oil pressure moves the piston in both directions. Because the rod takes up part of the piston area, it pulls back with less force than it pushes and, with the same flow, moves back faster.
Drive Interface Board: The board that links the machine controller to the servo drive or frequency inverter, passing the speed and pressure commands to the drive and bringing its ready and fault signals back to the controller.

E

Ejector Cylinder: A small double-acting hydraulic cylinder on the back of the moving platen that pushes the connecting plate and ejector rods forward to eject the part, then pulls them back.
Ejector Pin: A hardened steel pin, fixed in the mold's ejector plate, that pushes the molded part off the core when the mold opens. Ejector pins belong to the mold, so worn or broken pins are replaced by the toolroom or the mold maker.
Ejector Plate: In the mold, the plate that carries the ejector pins and pushes them forward to eject the part. On the machine, the connecting plate behind the moving platen that drives the ejector rods is often called the ejector plate too.
Ejector Rod: A hardened steel rod, also called a knock-out rod, that passes through the moving platen and carries the force of the machine's ejector to the mold's ejector plate. All the rods used on one mold must be the same length so the plate moves square.
Electrical Cabinet: The cabinet, often called the electrical panel, that holds the controller boards, the servo drive, contactors, power supplies and terminal blocks. The three-phase supply enters it through the main isolator switch.
Encoder: The sensor at the rear of a servo motor that measures the position and speed of the shaft for the servo drive, which cannot run the motor without its signal. On hydraulic machines the mold, screw and ejector positions are usually measured by linear transducers instead.

F

Feed Throat: The water-cooled opening below the hopper through which the pellets fall onto the screw. Cooling keeps the pellets solid; a throat that runs too hot makes them soften and bridge, so the screw cannot pick up material.
Feed Zone: The rear section of the screw, with deep flights that take in the pellets from the hopper.
Fixed-Displacement Pump: A hydraulic pump that delivers the same volume of oil every revolution, such as a vane or gear pump. Driven at constant speed, it keeps delivering oil while the machine waits, and the surplus flows back to the tank and heats the oil.
Flash: A thin layer of excess plastic at the parting line or around the ejector pins, where melt has escaped from the cavity. It comes from clamping force that is too low or uneven, injection or hold pressure that is too high, or worn mold surfaces.

G

Gear Pump: A fixed-displacement hydraulic pump in which meshing gears carry oil from the suction port to the outlet. It is used as the main pump on some smaller machines, in auxiliary circuits and, as an internal gear pump driven by a servo motor, on many servo-hydraulic machines.
Guide Column: A hardened, ground steel shaft fixed between two plates, along which a moving plate slides on bronze or self-lubricating bushings so it stays square and in line. In the injection unit, that plate carries the screw drive.

H

Heater Band: An electric resistance heater clamped around the barrel or the nozzle, which passes its heat into the steel by contact. The main types are ceramic and mica bands, with mineral-insulated bands and cast heaters also used, and a replacement must match the size, power and voltage of the original.
Hold Pressure: The pressure the screw keeps on the melt after injection, also called packing pressure, while the part cools and shrinks. The machine switches from injection speed to hold pressure at a set screw position, pressure or time.
Hopper: The container on top of the injection unit that holds the plastic pellets and feeds them through the feed throat onto the screw. A slide gate under it shuts off the flow of pellets so the hopper can be emptied or the barrel purged.
Hopper Magnet: A magnet in the material path above the feed throat that catches loose iron and steel, such as bolts, washers and fragments from grinders, before it can score the screw and barrel, break the check ring or block the nozzle.
Hydraulic Check Valve: A valve in the oil circuit that lets oil flow one way only. It is a different part from the check ring on the screw, which is a non-return valve for the plastic melt.
Hydraulic Circuit Diagram: The drawing in the machine manual that shows every pump, valve, cylinder and hydraulic motor as standard symbols joined by lines for the oil passages. It shows which valve controls which movement and how pressure and flow are set.
Hydraulic Cylinder: A linear actuator in which oil pressure on a piston moves a rod, used for the clamp, injection, carriage and ejector movements and for core pulls. Most cylinders on an injection molding machine are double-acting, with piston seals inside and a rod seal set in the front head.
Hydraulic Oil: The working fluid of a hydraulic machine, which carries the pump's power to the cylinders and motors, lubricates the pump and valves and takes heat to the cooler. Most machines use a mineral anti-wear oil (HM/HLP) in the ISO VG grade the machine manual specifies.
Hydraulic Power Unit: The motor, bell housing, coupling and pump assembly mounted on the oil tank, with the filters and cooler around it. It turns electrical power into the oil flow that drives every hydraulic movement of the machine.

I

I/O Module: The board between the controller's CPU and the machine: its inputs bring in the signals from switches and sensors, and its outputs switch the valve solenoids, relays and contactors. A failed channel usually stops one function only.
Incremental Encoder: An encoder that gives two pulse trains, A and B, offset by a quarter of a pulse, so the drive can count pulses and tell the direction, plus a once-per-revolution index pulse (Z) as a reference. Its resolution is given in pulses per revolution (ppr).
Injection Cylinder: The double-acting hydraulic cylinder at the back of the injection unit that drives the screw forward to inject, holds pressure while the part cools and provides back pressure during recovery. Some machines have one cylinder in line with the screw; many have two side by side.
Injection Pressure: The pressure applied to the melt by the screw to fill the mold during injection. On a hydraulic machine it can be given as the oil pressure in the injection cylinder or as the pressure in the melt, which is higher by the intensification ratio.
Injection Unit: The section of the machine that melts the plastic and injects it into the mold: the hopper, the plasticizing unit, the injection cylinder and piston rod, the plasticizing motor, and the carriage that moves the unit to and from the mold.
Intensification Ratio: The area of the injection piston divided by the cross-section of the screw. Multiplying a hydraulic pressure by it gives the pressure in the melt, which is why the melt pressure is higher than the oil pressure.
Internal Gear Pump: A gear pump in which a small gear drives a larger ring gear with internal teeth, with a crescent-shaped piece between them. It runs more quietly and gives a smoother flow than an external gear pump, which is why many servo-hydraulic machines use it.
ISO VG: The viscosity grade of an oil defined by ISO 3448: the number is the oil's nominal kinematic viscosity at 40 °C in mm²/s. An ISO VG 46 oil is thinner than an ISO VG 68 oil at the same temperature.

K

K-Type and J-Type Thermocouples: The two thermocouple types common on injection molding machines. Each type gives a different voltage at the same temperature, so a replacement must be the type the controller input is set for, connected with extension cable of the same type.

L

L/D Ratio: The flighted length of a screw divided by its nominal diameter. A higher L/D gives a more uniform melt but a longer residence time; general-purpose injection screws typically have an L/D of around 20:1.
Leveling Pad: One of the adjustable feet under the machine base: a threaded leveling bolt through the base frame, standing on a pad on the floor and held by a lock nut. Turning the bolt raises or lowers that point of the machine.
Limit Switch: A mechanical switch with a roller lever or plunger that a cam presses at a set position, used on the safety gate and to detect the ends of travel. Switches for safety functions have positive-opening contacts, which the actuator forces apart so the circuit opens even if a contact has welded.
Linear Guide: A hardened steel guide rail bolted to the machine base, with carriage blocks that slide along it on recirculating steel balls, carrying the injection unit carriage. It keeps the nozzle in line with the sprue bushing with very little friction.
Linear Transducer: A rod or slider sensor that measures the position of a moving part continuously along its whole stroke: the moving platen, the screw, the ejector and, on some machines, the carriage. A potentiometric type uses a wiper on a resistive track that wears; a magnetostrictive type measures a magnet along a waveguide without contact.
Liner Bushing: A plain bronze sleeve that lines a platen bore, such as those of the rear platen, so the platen can move along the tie bar without wearing the bar or the cast iron.
Locating Ring: A round steel ring on the fixed half of the mold that fits into the center bore of the stationary platen. It centers the mold so that the sprue bushing lines up with the machine nozzle.

M

Metering Zone: The front section of the screw, with shallow flights that deliver an even, fully melted shot to the front of the barrel.
Mica Heater Band: A heater band with a flat resistance element between mica sheets in a thin steel sheath. It is thinner and cheaper than a ceramic band, but it has a lower temperature limit and needs a very tight fit on the barrel.
Mineral-Insulated Heater Band: A heater band whose element is embedded in compacted magnesium oxide powder inside a thin metal sheath. It is thin, transfers heat well and suits zones that are short of space or need a lot of power.
Mold: The tool, made of two or more parts, whose cavity gives the molded part its shape. Its fixed half is clamped to the stationary platen and its moving half to the moving platen; the ejector pins, sprue bushing and locating ring belong to the mold, not the machine.
Mold Clamp: A clamp that holds a mold half on its platen, pulled down by a bolt in a T-slot or tapped hole, with its front on the mold flange and its back on a support called the heel. The nearer the bolt is to the mold, the more of its force presses on the mold rather than on the heel.
Mold Height: The thickness of the closed mold, measured between the faces that sit against the platens. The machine's data sheet gives the minimum and maximum mold height it can accept.
Mold Height Adjustment: The mechanism that moves the rear platen and the toggle along the tie bars to suit the mold thickness, by turning the adjusting nuts on all four bars together. On a toggle machine it also sets the clamping force, because it sets how far the tie bars stretch when the toggle locks.
Mold Protection: A low-pressure stage at the end of mold closing, before the mold halves touch, so that a part left in the mold stops the clamp instead of being crushed.
Moving Platen: The platen that travels along the tie bars to open and close the mold, sliding on its tie bar bushings. It carries the moving half of the mold and, on its back, the machine's ejector.

N

Nitrided Barrel: A barrel of alloy steel whose bore is hardened in a thin surface layer by nitriding. It suits most unfilled plastics, while a bimetallic barrel lasts much longer with glass-filled or mineral-filled ones.
Nozzle: The heated, threaded steel part screwed into the barrel head that carries the melt into the mold. On every shot its rounded tip presses against the mold's sprue bushing to form a seal, so its radius and orifice must suit the bushing.
Nozzle Contact Force: The force with which the carriage cylinders hold the nozzle against the mold's sprue bushing, so the melt cannot leak at the joint.
Nozzle Filter: A filter insert in the nozzle that catches contamination in the melt, such as metal particles and unmelted pellets, before it reaches the mold. It protects small gates and hot runner nozzles, at the cost of a higher injection pressure than an open nozzle.
Nozzle Heater: A small heater band, coil heater or cast heater around the nozzle, with its own thermocouple. It keeps the melt at temperature between shots, because the nozzle touches the cooled sprue bushing on every shot and would otherwise freeze off.

O

Oil Cooler: A water-cooled heat exchanger on the hydraulic return line that removes the heat the pump and valves put into the oil, so the oil reaches the tank at a steady working temperature. It is built as a shell-and-tube or a plate type.
Open Nozzle: The most common type of injection nozzle, with no valve or other moving parts in its melt channel. Low-viscosity materials such as nylon can drool from it, which is where a shut-off nozzle is used instead.

P

P, T, A and B Ports: The standard port names on a hydraulic valve: P is the pressure port from the pump, T the return port to the tank, and A and B the work ports to the two sides of a cylinder or motor.
PID Control: The control method most machine temperature controllers use to hold each barrel zone at its set point. The proportional part reacts to how far the zone is from the set point, the integral part removes a steady offset and the derivative part reduces overshoot.
Pilot-Operated Check Valve: A check valve that also opens in the reverse direction when pressure is applied to its pilot port. It is used on cylinders that must hold their position without drifting.
Piston Pump: A hydraulic pump in which pistons in a rotating cylinder block stroke against a tilted swashplate. On injection molding machines it is usually a variable-displacement axial piston pump, which wastes less energy than a fixed pump but needs cleaner oil.
Piston Rod: The ground, hard chrome-plated steel rod that carries a cylinder's force out through the front head. Any scratch, dent or pitting on the chrome cuts the rod seals on every stroke.
Piston Rod Nut: The nut that holds the piston on the threaded end of the piston rod and is locked so it cannot work loose. A loose nut lets the piston knock on the rod, which damages the seals and the threads.
Piston Seal: A seal in the piston grooves, usually a Y-ring, that stops oil passing from one side of the piston to the other. A worn piston seal leaks inside the cylinder, so it shows as slow, weak or creeping movement rather than as an oil drip.
Plasticizing Motor: The motor that turns the screw during screw recovery. On hydraulic machines it is a low-speed, high-torque hydraulic motor at the back of the injection unit, often a radial piston motor; electric machines use a servo motor.
Plasticizing Unit: The barrel with its heater bands, the screw, screw tip, check ring, barrel head and nozzle together: the part of the injection unit that melts the plastic and the part that wears the most.
Platen: One of the heavy cast iron plates that hold the mold halves and pass the clamping force through the mold. A toggle machine has three: the stationary platen, the moving platen and the rear platen.
Platen Parallelism: How parallel the mold faces of the moving and stationary platens are to each other, measured as the difference in the distance between the two faces at their corners. An error shows as flash on one side and uneven wear on the mold.
PLC (Programmable Logic Controller): A general-purpose industrial controller that reads inputs, runs a program and sets outputs in a fast repeating scan. Most injection molding machines use a dedicated injection molding controller that works the same way; general-purpose PLCs with a touch panel are used on some machines and in retrofits.
Power Supply Board: The board that converts the cabinet supply into regulated low DC voltages, typically 24 V DC for the I/O, sensors and valves and 5 V DC for the logic circuits. A weak or noisy supply causes faults that look like failures of other boards.
Pressure Compensator: The control on a variable piston pump that keeps full flow until the outlet pressure reaches its setting, then reduces the displacement so the pump delivers only what is needed to hold that pressure.
Pressure Gauge: A mechanical dial that shows the hydraulic pressure without needing power: a Bourdon tube inside tries to straighten as the pressure rises and turns the needle. A glycerin filling damps the needle against pump pulsation and machine vibration.
Pressure Transducer: An electronic sensor that turns the hydraulic pressure into an electrical signal, for example 4 to 20 mA or 0 to 10 V, for the controller or the servo drive. The machine uses it to display the pressure, raise alarms and, on many machines, control injection, hold, back pressure and clamping pressure in a closed loop.
Projected Area: The area of the molded parts and runners seen in the direction the mold opens, like their shadow on the parting plane. Multiplied by a cavity pressure factor and a safety margin, it gives the clamping force the mold needs.
Proportional Valve: A hydraulic valve that sets pressure or flow in proportion to an electrical signal from the controller, through an amplifier. On fixed pump machines a combined pressure and flow (P/Q) valve turns the speeds and pressures set on the screen into oil pressure and flow.
Proportional Valve Amplifier: The electronic card, or module built onto the valve, that turns the controller's command signal into a regulated current for the proportional valve coil. It also sets the ramps, the minimum and maximum current and the dither, so the valve and amplifier are set up together.
Proximity Sensor: An inductive sensor that switches when a steel target comes close, without touching it, used for positions such as the mold stroke and ejector home and for mold protection. PNP and NPN versions are not interchangeable: the replacement must match what the controller input expects.
Pump Coupling: The flexible jaw coupling inside the bell housing that joins the motor shaft to the pump shaft, with an elastomer insert between the jaws of its two hubs. It carries the torque and absorbs vibration; a worn insert often shows as a knock at start-up.
Purging: Running melt out of the barrel with the nozzle away from the mold, to empty it or to clear out the previous material or color, sometimes with a purging compound. The barrel is purged before the screw is pulled, before shutdown and when changing to a material that degrades easily.

R

Rear Platen: The platen at the clamp end of a toggle machine that anchors the toggle and carries the clamping cylinder. The mold height adjustment moves it along the tie bars; two-platen machines have no rear platen.
Relief Valve: A valve that opens at a set pressure and returns oil to the tank, limiting the highest pressure in the system to protect the pump, hoses and cylinders. On injection molding machines the main relief valve is often called the safety valve.
Resolver: A rotary transformer used on some servo motors to measure the shaft position, with no electronics in the motor. It tolerates heat and vibration well.
Return Spring: A compression spring that pushes the ejector back after it has pushed the part out, so the ejector rods and pins are clear before the mold closes. Return springs are used on the machine's connecting plate and on the ejector plate of many molds.
Return-Line Filter: A filter where the return oil enters the tank, catching particles made in the system before they reach the tank. It has a finer element than the suction filter, usually with a bypass valve and an indicator.
Reverse Temperature Profile: A barrel temperature setting in which the rear zones are as hot as or hotter than the front zones. It is used when each shot takes a large share of the barrel's capacity, or when a material needs more heat early.
Rod Seal: The seal in a cylinder's front head that keeps the oil inside as the piston rod moves. It is the seal you can see leaking, usually because it is worn or has been cut by a scratch, dent or pitting on the rod.

S

Safety Gate: The sliding guard over the mold area. Switches pressed by the gate tell the controller that it is closed, and until they do, the interlock stops the mold from closing so the operator cannot be trapped between the mold halves.
Screw: The flighted steel shaft that turns inside the heated barrel to carry and melt the plastic, then moves forward without turning to inject the melt like a plunger. A general-purpose screw has three zones: feed, compression and metering.
Screw Recovery: The phase of the cycle, also called plasticizing, in which the screw turns to melt the next shot. The melt collects in front of the screw tip and pushes the screw back, against the back pressure, until the set shot size is reached.
Screw Tip: The hardened, pointed part screwed into the front of the screw, also called the screw head. On most machines it carries the non-return valve: the check ring slides on it and seals against the seat ring, and the three are replaced as a set.
Seal Kit: The full set of seals for one hydraulic cylinder: piston seals, wear rings, rod seals, wiper and the static O-rings, normally all replaced together when the cylinder is opened.
Seat Ring: The ring behind the check ring on the screw tip, also called the check seat. The check ring is pushed back onto it to seal when the screw injects.
Servo Drive: The power electronics unit that powers and controls a servo motor. It converts the mains supply to DC and then to a variable three-phase output, using the motor's encoder to control speed and torque precisely.
Servo Motor: A permanent-magnet AC motor whose servo drive reads its encoder to control speed and torque precisely. On a servo-hydraulic machine it drives the hydraulic pump; on an all-electric machine separate servo motors drive the clamp, injection, screw rotation and ejector.
Servo Pump: A fixed-displacement hydraulic pump, most often an internal gear pump, driven by a servo motor and a servo drive. The drive changes the motor speed to deliver only the flow each movement needs and uses a pressure transducer to hold the set pressure, which saves energy and heats the oil less.
Servo-Hydraulic Machine: A hydraulic injection molding machine whose pump is driven by a servo motor that changes speed so each step of the cycle gets the flow and pressure it needs. While the machine waits, the motor slows down or stops.
Shear Heat: Heat produced in the plastic by the turning screw and by back pressure, in addition to the heat from the heater bands. It can push a front barrel zone above its set point even with its heater switched off.
Shot Size: The volume, or weight, of melt that the screw pushes into the mold in one stroke. The shot volume is the screw's cross-section area multiplied by the injection stroke; multiplied by the material's melt density, it gives the shot weight.
Shot Weight: The shot volume the screw sweeps in one stroke multiplied by the melt density of the material. The same machine therefore has a different shot weight for each material.
Shut-Off Nozzle: A nozzle with a valve that closes the melt channel between shots, so material does not drool or string when the injection unit moves away from the mold. In a spring-loaded type the injection pressure pushes the needle open; in a hydraulic or pneumatic type a cylinder opens and closes it on a signal from the machine.
Solid-State Relay (SSR): An electronic switch that turns a zone's heater bands on and off on the temperature controller's command. It is the usual choice because it switches often without wearing; some machines use contactors instead.
Specific Back Pressure: The pressure in the melt in front of the screw during recovery: the hydraulic back pressure set in the injection cylinder multiplied by the machine's intensification ratio.
Sprue Bushing: The part of the mold that the nozzle tip presses against on every shot, where the melt enters the mold. The nozzle tip radius must be the same as or slightly smaller than its radius, and the nozzle orifice slightly smaller than its opening.
Stationary Platen: The fixed platen at the injection side of the machine, also called the fixed platen, that carries the fixed half of the mold. The nozzle reaches through its center bore to the sprue bushing, and the locating ring centers the mold on it.
Suction Filter: A strainer on the pump's suction line, in or on the oil tank, that keeps dirt and metal particles out of the pump; many also carry a magnet. A blocked suction filter starves the pump, which cavitates and gets noisy.
Swashplate: The tilted plate in an axial piston pump on which the piston shoes slide. Its angle sets the stroke of the pistons, and so how much oil the pump delivers.

T

T-Slot: A slot across a platen's mold face that takes the T-bolts or T-nuts of the mold clamps. Its lips carry the full force of each clamp bolt, so they crack or spread if bolts are overtightened or clamps are set badly.
Thermocouple: The temperature sensor of one heating zone: two wires of different metals joined at the tip, set into the barrel wall or the nozzle body, that produce a small voltage depending on the temperature. A thermocouple that is not pushed fully home reads low, so the zone overheats without an alarm.
Tie Bar: One of the four steel bars that run through the corners of the platens and join the stationary platen to the rear platen. The moving platen slides along them, and when the mold is clamped they stretch slightly and hold the full clamping force.
Tie Bar Bushing: A flanged bronze or self-lubricating sleeve in a bore of the moving platen that lets the platen slide along the tie bar and wears in place of the bar. Worn bushings let the platen drop and rock.
Tie Bar Distance: The clear space between the four tie bars, given horizontally and vertically on the machine's data sheet, also called tie bar spacing. A mold has to pass between the bars to be loaded, so it is a main limit on mold size.
Tie Bar Nut: The large threaded nut on the end of a tie bar that holds the bar in the platen and carries its share of the clamping force through its threads. A lock ring stops the nut from turning under the load that rises and falls on every cycle.
Toggle Bushing: The sleeve bearing pressed into a bore of a toggle link, in which a toggle pin turns. It is made of bronze or a self-lubricating composite, softer than the pin, so that it wears instead of the pin and the link.
Toggle Link: One of the steel levers, also called toggle arms, that join the rear platen to the moving platen. The clamping cylinder folds them to open the mold and straightens them to close it, and in the straight position they multiply the cylinder's force into the full clamping force.
Toggle Mechanism: The linkage between the rear platen and the moving platen of a toggle machine, in which the clamping cylinder drives a crosshead that folds or straightens the toggle links through short connecting links. As the links come into line they multiply the cylinder's force into the full clamping force and hold the mold shut.
Toggle Pin: The hardened steel pin, also called a hinge pin, at each pivot of the toggle. The links turn on it through bushings, and when the toggle locks, the full clamping force passes through the pins.
Two-Platen Machine: A clamp design with no rear platen: the moving platen locks onto the tie bars with split nuts, and clamp cylinders build the clamping force by pulling on the bars.

V

Valve Manifold: A steel block drilled with oil passages that carries the directional, proportional, cartridge, check and relief valves and connects them without pipes. Also called the valve block.
Vane Pump: A fixed-displacement hydraulic pump in which vanes sliding in the slots of a rotor carry oil around the inside of an oval cam ring. It is found on many conventional machines with a fixed pump, often as a double pump in a high-low circuit.
Variable-Displacement Pump: A hydraulic pump that can change its displacement while it runs, on injection molding machines usually an axial piston pump whose swashplate angle sets the flow. It reduces the flow when the set pressure is reached instead of sending surplus oil over a relief valve.

W

Wear Ring: A ring on a cylinder's piston, also called a guide ring, that keeps the piston centered in the bore so the steel never touches the cylinder barrel. A guide ring in the front head does the same for the piston rod.
Wiper Seal: The seal at the outer end of a cylinder's front head that scrapes dirt and dust off the piston rod as it retracts, so the dirt never reaches the rod seal.

Y

Y-Ring: A seal with two lips in a Y-shaped profile, also called a U-cup or lip seal, used as a piston seal. Oil pressure pushes into the space between the lips and presses them against the bore and the groove, so the seal grips harder as the pressure rises.

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