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Technical document

Knowledge of pressure regulation during plastic injection molding

2025-01-22 11:30:27 Pressure Regulation in Plastic Injection Molding
In injection molding, whether with hydraulic or electric machines, all motions generate pressure. Proper pressure control is crucial for quality products.

Pressure Regulation and Measurement System

Hydraulic Machines

On hydraulic injection presses, the oil circuit drives operations like screw rotation in plasticizing (controlling back pressure), slide carriage movement, injection screw's axial motion during injection and holding, mold closing, and part ejection.


The screw's role in melting and homogenizing material is vital. Back pressure adjustment prevents overheating, and excessive flow rates from mixing elements must be avoided to prevent polymer degradation.


Injection speed control has options such as open - loop (using proportional valves), closed - loop (with closed - loop proportional valves), and closed - loop with speed sensors. Closed - loop control ensures consistent pressure during injection, holding, and for back pressure. Pressure can be monitored in the hydraulic line or at the nozzle/cavity, and adding temperature detection aids process management. Understanding material's pressure tolerance helps predict part weight and dimensions, and adjusting holding pressure can control shrinkage.

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Electric Machines

Fully electric machines use brushless synchronous motors with permanent magnets and ball screws. Force is measured using load cells and extensometers as there's no hydraulic fluid. Back pressure is controlled by adding resistance to the injection motor's axial movement.

Hydraulic Equipment: Discharge and Pressure Adjustment

Centrifugal pumps in hydraulic systems typically produce 140 bar pressure, suitable for injection. Variable - displacement pumps and accumulators reduce energy consumption during peak discharge. Fixed - displacement pumps' selection depends on oil volume needs. Injection presses use multiple proportional servo valves to control various functions like mold opening/closing speed and pressure, injection, and feeding. The servo valves convert weak electrical signals to hydraulic outputs, and oil temperature in the hydraulic circuit (45 - 55°C) affects discharge. Precise control of servo valves reduces hysteresis.

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Injection Nozzle Pressure Measurement

Accurate and repeatable pressure detection is crucial in injection and holding. In open - loop systems (still used, especially in high - tonnage models), and closed - loop systems, proper pressure sensing is needed. Pressure - activated passages are used to limit filling pressure. Measuring pressure at the nozzle (closer to the cavity) is better than in the hydraulic line. Nozzle pressure detection can also control plasticization via back pressure adjustment.


Direct or probe - based (e.g., piezoelectric sensors) measurement methods exist. Nozzle pressure detection, though less effective than cavity pressure detection, has advantages like no mold modification and no marks on parts. Melt pressure control in the cavity can prevent over - filling, improve process control, and avoid issues like material charring.


Sensors for nozzle pressure detection should not interfere with molding, work under high pressure (2500 bar) and temperature (350 - 400°C), be small, robust, wear - resistant, provide long - term accuracy, and support high - speed sampling and standard communication protocols. Due to these complex requirements, hydraulic machines still use sensors in the hydraulic line, and electric machines use force detection, rather than melt sensors.

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