Injection Molding Glossary

Key terms and definitions used in plastic injection molding machines and spare parts.

A

Accumulator

A hydraulic energy storage device that stores pressurized fluid to supplement pump flow during peak demands, such as injection or clamping phases.

B

Back Pressure

The pressure applied to the screw during plasticization (recovery phase) to improve melt homogeneity and color mixing.

Barrel

The cylindrical steel housing surrounding the screw in which plastic pellets are melted. Contains heater bands and thermocouples for temperature control.

Bushing

A cylindrical sleeve used as a bearing surface or guide, commonly found in the mold sprue, ejector pins, and tie bar assemblies.

C

Clamping Force

The force applied by the machine to keep the mold halves closed during injection. Measured in tons or kilonewtons (kN). Insufficient clamping force causes flash.

Clamping Unit

The section of the machine responsible for opening, closing, and holding the mold. Can be toggle-type or hydraulic direct-pressure.

Check Ring

A ring at the tip of the screw that acts as a non-return valve, preventing molten plastic from flowing backward during injection.

Controller Board

An electronic circuit board (PCB) that manages various machine functions such as temperature control, motion control, or I/O communication.

Cooling Channel

Internal passages in the mold or machine through which cooling water circulates to regulate temperature and reduce cycle time.

Cycle Time

The total time to complete one full injection molding cycle, including mold close, injection, cooling, mold open, and ejection.

D

Directional Control Valve

A hydraulic valve that controls the direction of oil flow in the hydraulic circuit, enabling forward and reverse motion of cylinders and motors.

E

Ejector Pin

A metal rod that pushes the finished part out of the mold cavity after the mold opens.

Encoder

A sensor that converts mechanical position or motion into electrical signals, used to provide feedback on screw position, platen position, or motor speed.

F

Flash

A thin layer of excess plastic that seeps out at the parting line of the mold, typically caused by insufficient clamping force or worn mold surfaces.

G

Gear Pump

A positive-displacement hydraulic pump that uses meshing gears to pressurize fluid. Common in hydraulic injection molding machines.

H

Heater Band

An electrical heating element clamped around the barrel to maintain proper melt temperature. Available in ceramic and mica types.

Hydraulic Cylinder

A linear actuator powered by pressurized hydraulic fluid, used for clamping, injection, ejection, and core pulling functions.

Hydraulic Oil

The working fluid in a hydraulic injection molding machine. Proper viscosity grade and cleanliness are critical for machine performance and longevity.

I

Injection Pressure

The pressure applied to the molten plastic by the screw or plunger during the injection phase to fill the mold cavity.

Injection Unit

The section of the machine responsible for melting, metering, and injecting plastic into the mold. Includes the hopper, barrel, screw, and nozzle.

L

L/D Ratio

Length-to-diameter ratio of the screw. Affects melt quality and throughput. Standard ratios range from 20:1 to 25:1.

M

Mold

A precision-machined tool consisting of two or more parts that form the cavity into which molten plastic is injected to create the desired part shape.

N

Nozzle

The component at the end of the barrel that connects to the mold sprue. Transfers molten plastic from the barrel into the mold.

P

Piston Pump

A variable-displacement hydraulic pump that uses reciprocating pistons. Offers better energy efficiency and pressure control than gear pumps.

Platen

The large flat steel plates that hold the mold halves. Includes the stationary (fixed) platen and the moving platen.

PLC (Programmable Logic Controller)

The main industrial computer that controls the sequencing, timing, and monitoring of all machine operations.

Proportional Valve

A hydraulic valve that provides variable flow or pressure control proportional to an electrical input signal, enabling precise speed and force control.

Purging

The process of cleaning the barrel and screw by running cleaning compound or a different resin through the system to remove contamination or previous material.

S

Screw

A helical-flighted shaft inside the barrel that rotates to convey, compress, and melt plastic pellets, then acts as a plunger to inject the melt into the mold.

Screw Tip

The pointed end of the screw assembly that includes the check ring (non-return valve). Critical for consistent shot size and preventing backflow.

Servo Motor

An electric motor with precise position and speed control, used in servo-hydraulic and all-electric injection molding machines for energy efficiency.

Shot Size

The volume or weight of plastic material injected into the mold in a single cycle. Determined by screw diameter and stroke length.

T

Thermocouple

A temperature sensor inserted into the barrel or nozzle that provides feedback to the controller for accurate temperature regulation.

Tie Bar

Heavy steel rods (typically four) that connect the stationary platen to the rear platen, guiding the moving platen and withstanding clamping forces.

Toggle Mechanism

A mechanical linkage system used in toggle-type clamping units to multiply the clamping force generated by the hydraulic cylinder.

V

Vane Pump

A hydraulic pump using sliding vanes in a rotor to create flow. Offers moderate pressure capability and is commonly used in injection molding machines.