For previous reading, please refer to PART 01: What Exactly is Injection Molding? A Visual Guide to the Entire Injection Molding Process.
In previous article, we provided a panoramic view of the entire injection molding process. You saw how plastic granules are reshaped into products, but all of that depends on a core physical entity – injection molding machine.
However, many newcomers only know that it is a machine that continuously produces products in a cycle, without knowing its internal structure.
In this article, we will comprehensively introduce main parts and three core systems of an injection molding machine: mold clamping system, injection system, and control system.

 

Main structural names of injection molding machine parts (Image from Sumitomo data)

Core task of mold clamping system is to safely, smoothly, quickly open and close mold, and to apply a huge clamping force during injection to prevent mold from being forced open by high-pressure melt, thus preventing flash.
1. Core Components and Working Principle
Clamping Cylinder/Servo Motor: Power source. Hydraulic press uses a large-diameter cylinder for propulsion, while electric motor uses a servo motor and ball screw for drive.
Moving and Fixed Platen:
Fixed Platen: Stationary, used for mounting mold.
Moving Platen: Reciprocates on guide rails, used for mounting mold. Clamping force ultimately acts between these two platen sections.
Tie Rods: Four giant steel columns connecting front and rear platen sections. They not only serve as guides but also act as load-bearing beams to withstand clamping force. Their strength and diameter directly determine machine's clamping tonnage.
Toggle Linkage Mechanism (Mechanical Clamping): This is essence of most hydraulic injection molding machines. It utilizes linkage force amplification principle to amplify relatively small thrust of clamping cylinder by tens of times, transforming it into a huge clamping force.

 

1: Base 2: Fixed Mold Plate 3: Moving Mold Plate 4: Guiding Columns
Clamping Cylinder (Direct Pressure Clamping): Used in a few machines, it directly generates clamping force using a large-diameter cylinder. Structure is simple, but energy consumption is high.
2. Detailed Working Cycle

 

3. Key Points
Clamping Force Calculation and Selection: Clamping force must be greater than product of melt pressure inside mold cavity and projected area of product. Insufficient selection will lead to flash, while excessive selection will waste energy and damage the machine.
Mold Thickness Adjustment: Adjust mold thickness and high/low pressure positions through automatic or manual mold adjustment.
Parallelism and Balance of Four Guiding Columns: This is foundation for ensuring mold life and product quality, requires regular maintenance.

Core task of injection system: To uniformly heat and plasticize solid plastic granules into melt, inject it into mold cavity with precise pressure, speed, and metering.
1. Core Components and Working Principle
Hopper: Raw material inlet, usually equipped with a magnetic frame to adsorb metal impurities.
Barrel (Material Barrel): A long, pressure-resistant steel cylinder surrounded by a heating coil, serving as "furnace" for plastic melting.
Screw: A special spiral rod with threads on its surface, featuring three functional sections:
Functions of Each Component of Screw
Feed Section: Transports granular material to the front end of screw and preheats granular material;
Compression Section: Dissolves resin using heating and frictional heat;
Metering Section: Completely dissolves and homogenizes resin.

 

Nozzle: Discharge port at the front of barrel, in close contact with mold sprue sleeve. Various types are available (straight-through, spring-loaded, hydraulic valve type), used to prevent melt drooling.
Injection Cylinder/Servo Motor: Power source driving screw for injection forward and backward retraction.
Rotary Cylinder/Hydraulic Motor/Servo Motor: Power source driving screw rotation for plasticizing.
2. Detailed Working Cycle
Feeding and Plasticizing: Screw rotates, conveying granules forward from hopper. Under combined action of external heating and screw shear heat, plastic gradually melts and homogenizes within barrel. Melt accumulates at screw head, generating back pressure that pushes screw backward, completing "metering" stroke.
Injection Unit Advancement: After plasticizing, injection unit advances as a whole, bringing nozzle into close contact with mold sprue sleeve.
Injection: Injection cylinder pushes screw, like a syringe piston, injecting molten material into mold at high speed and pressure.
Holding Pressure: During holding pressure phase, screw maintains a certain pressure, moves forward slightly to compensate for cooling and shrinkage.
Cooling and Plasticizing (Simultaneous): In later stages of holding pressure, mold cools, and simultaneously, screw begins to plasticize new material for next mold rotation (this is the key to efficiency of injection molding machines—overlapping actions).
Injection Unit Retraction: To prevent nozzle from freezing in runner due to prolonged contact with cold mold, injection unit is sometimes retracted (not a mandatory action).
3. Key Points
Screw design is crucial: Different plastics (such as PVC, PC, PA) require specialized screws with different compression ratios, length-to-diameter ratios, and thread designs.
Temperature control is vital: Barrel is precisely controlled by PID control in multiple sections (usually 4-7 sections). Improper temperature profile settings can lead to decomposition or poor plasticizing.
Back Pressure: Resistance at the rear of screw during plasticizing. Increasing back pressure improves melt density and uniformity, but it also increases melt temperature and prolongs plasticizing time.

Core task of control system is to receive instructions and coordinate actions, speeds, pressures, temperatures, times of mold clamping and injection systems, as well as all auxiliary devices (such as robotic arms and mold temperature controllers), ensuring accurate reproduction of production process in each mold.
1. Core Level
Human Machine Interface (HMI): Typically a color touchscreen. Operators set all process parameters (temperature, pressure, speed, position, time) and monitor machine status here.
Programmable Logic Controller (PLC): An industrial computer, core of logic control. It executes preset programs, processes input signals (such as limit switches and pressure sensors), and issues output commands (such as energizing a solenoid valve).
Sensors and Actuators: Position sensors (electronic rulers), pressure sensors, temperature sensors (thermocouples), etc., provide real-time feedback of physical information.
Proportional pressure valves, proportional flow valves, servo motors, heating coils, switching valves, etc., execute specific actions.
Power and Drive System:
Hydraulic System: An electric motor drives a hydraulic pump to generate pressurized oil, which, through a complex valve block pipeline, drives various cylinders. Proportional valve is key to achieving stepless, precise control of pressure and speed.
Fully Electric System: Multiple servo motors drive mold-locking, injection, and screw rotation actions, respectively, with servo drivers precisely controlling position and torque.
2. Control Flow Overview
When you press "Semi-Automatic" start button:
PLC checks conditions such as "Safety door closed" and "No foreign objects in mold."
If these conditions are met, PLC issues a command: mold-locking solenoid valve is energized, and hydraulic fluid drives mold-closing cylinder.
An electronic ruler provides real-time feedback on moving platen position, which PLC uses to control speed switching at different stages.
After mold is locked in place, a signal triggers stage to advance and begin injection.
Pressure sensors monitor injection pressure, ensuring it matches set curve…
All actions cycle repeatedly according to a preset time and sequence.

Traditional Hydraulic Injection Molding Machine: Uses a hydraulic system (hydraulic pump, cylinder, valves) as its core power source.
All-Electric Injection Molding Machine: Uses a high-performance servo motor and precision ball screw as its core drive components.
1. Injection System
Function: Plasticizes, meteres, and injects molten plastic.
Common Components:
1. Hopper: Stores and conveys raw materials.
2. Barrel: Heates and houses the screw.
3. Heating Coil: Provides external heating.
4. Screw: Core of plasticizing and injection.
5. Nozzle: Channel connecting to mold.
Core Drive Differences:
1. Traditional Hydraulic Machine: Injection is completed by injection cylinder pushing screw forward; plasticizing (metering) is completed by hydraulic motor driving screw rotation via gears/belts.
2. All-Electric Machine:
2.1 Injection: Injection servo motor drives screw via a ball screw (converting rotational motion into precise linear motion) to achieve injection and pressure holding.
2.2 Plasticizing/Metering: An independent plasticizing servo motor directly drives screw rotation via a synchronous belt or gears.
Advantages Comparison: Electric drive offers more precise injection speed and pressure control, higher repeatability, faster response, eliminates hydraulic oil leakage and heat generation issues.
2. Mold Closing System
Function: Enables mold opening, closing, and locking.
Common Components:
1. Fixed Platen (Head Plate), Moving Platen (Second Plate), Tail Plate, Tie Rods: Form mold closing frame.
2. Ejection Device: Ejects product.
Core Drive Differences:
1. Traditional Hydraulic Press:
Direct Pressure Type: Relies on a large-diameter mold closing cylinder to directly generate the clamping force.
Toggle Type (Arched Type): Utilizes mold closing cylinder to push toggle mechanism, generating a mechanical amplification effect to lock mold.
2. All-Electric Motor:
Electric Mold Closing: Driven by a mold closing servo motor, a ball screw or high-efficiency nut directly moves moving platen. Clamping force is precisely controlled by output torque of servo motor. Electric toggle mechanisms are also commonly used, amplifying rotational motion of servo motor into clamping force via toggle lever.
Advantages Comparison: Electric mold clamping and opening/closing speeds are fast, position control is precise, energy consumption is extremely low (power is only consumed during operation), noise is low, large clamping cylinders and complex hydraulic circuits are unnecessary.
3. Power and Drive System (Core Difference)
This is the most fundamental difference between two types of injection molding machines.
1. Traditional Hydraulic Injection Molding Machine: Hydraulic System
Components: Hydraulic pump (fixed displacement/variable displacement), various cylinders, control valves, oil tank, cooler, etc.
Function: Provides unified fluid power for all actions.
Characteristics: Powerful, mature technology, relatively low cost, but disadvantages include high energy consumption (continuous pump operation), fluctuating oil temperature, risk of oil leakage, high noise, and need for cooling water.
2. All-Electric Injection Molding Machine: Multi-Servo Motor Drive System
Composition: Multiple independent servo motors (typically including: injection motor, plasticizing motor, mold clamping motor, ejection motor, and injection table movement motor) and their matching drivers, encoders, and ball screws/synchronous belts.
Function: Each motor independently handles a core action, coordinated by a central controller.
Features:
Energy Saving: Consumes energy only during operation; virtually no energy consumption during standby, saving 40%-70% more energy than hydraulic presses.
Precision: Closed-loop control of servo motors ensures extremely high accuracy and repeatability in position, speed, and pressure control.
Clean and Environmentally Friendly: No hydraulic oil required, eliminating oil pollution; clean working environment suitable for cleanrooms.
High Efficiency and Speed: Fast response speed; typically shorter cycle times.
Simple Maintenance: No need to handle hydraulic oil, coolers, or oil filters.
4. Control System (Both are similar, but electric systems have higher requirements)
Function: Machine's "brain," coordinating all actions and process parameters.
Common core components: Programmable Logic Controller (PLC), Human-Machine Interface (HMI) touchscreen, temperature controller, and various sensors.
Differences: All-electric injection molding machines have higher requirements for control system, requiring more powerful algorithms to coordinate multiple servo motors and achieve complex synchronous movements and smooth switching. Their parameter settings directly correspond to motor's torque, speed, and position, resulting in more direct and linear control.
5. Machine Body and Auxiliary Systems
Machine Body: Similar in structure. All-electric machines, by eliminating large hydraulic system, have a more compact structure and may occupy less space.
Safety Doors and Lubrication Systems: Same function.
Cooling System: Significantly simplified in all-electric machines. Typically, only barrel feed port and mold need cooling; forced cooling of hydraulic oil is unnecessary, reducing cooling water consumption and associated facilities.

When facing a machine, understanding its nameplate and parameter table is crucial: Clamping Force (tons): e.g., "850T". This is the most core specification, determining size of products you can make.
Injection volume (grams, g or ounces, oz): e.g., "3000g". Based on PS (polystyrene) standards, this refers to maximum weight of melt that screw can eject in one stroke.
Injection pressure (MPa or kgf/cm²): e.g., "180MPa". Maximum pressure that screw head can provide during injection.
Screw diameter (mm): e.g., "110mm". Diameter affects plasticizing capacity and injection rate.
Tie bar inner distance (horizontal × vertical, mm): e.g., "900 × 900mm". Determines maximum external dimensions of mold that can be installed.
Mold opening stroke (mm): e.g., "1300mm". Determines height (depth) of product.
Machine size and power: Related to workshop layout and power configuration.
For further reading, please refer to PART 03: How to Calculate Clamping Force? A Comprehensive Guide to Projected Area, Cavity Pressure,.