For previous article, please refer to PART 04: How should barrel temperature be set? This article will guide you through front, middle, an.
In injection molding process, pressure and speed are two of the most crucial and interdependent control variables. Their settings not only determine instantaneous state of melt filling behavior but also profoundly affect final quality of product, energy consumption of production process, and operating status of equipment system. This article will delve into definitions, working principles, intrinsic relationships of injection pressure and injection speed, analyze energy consumption impact mechanism of excessively high pressure settings, compare fundamental differences between electric and hydraulic injection molding machines in terms of pressure detection location and method.

Injection Speed: Refers to linear velocity (unit: mm/s) of screw during injection phase. In modern injection molding processes, it is set as a target value or control variable. Its core function is to control flow rate of melt entering mold cavity, directly affecting melt front advancement method, shear rate, molecular orientation, and thermal history.
Injection Pressure: Refers to pressure (unit: bar, kgf/cm², MPa) exerted by hydraulic system (or servo motor) on screw to maintain set injection speed, ultimately transmitted to melt. During injection phase, it is more of a result value or process variable. Its core function is to provide driving force necessary to overcome all resistances during melt flow.

Injection pressure and speed are not independent forces, but rather tightly coupled through rheological properties (viscosity) of melt and geometry (flow resistance) of mold.
When injection speed v increases, injection pressure P required to maintain that speed must increase accordingly to overcome viscous resistance that increases dramatically with increased flow rate.
When processing high-viscosity (high η) materials (such as PC, PMMA), even at medium injection speeds, very high injection pressures are required.
When melt flows through narrow areas (such as small gates, thin walls), flow resistance increases dramatically, and pressure required to maintain a specific speed rises significantly.
This is reflected in injection molding machine control logic as follows:
During injection stage, technician sets "injection speed curve." Control system (whether a hydraulic proportional valve or a servo drive) is tasked with driving screw to move strictly according to preset speed curve. Force output by control system in real time (manifested as hydraulic pressure or motor torque) is what we see on screen as "injection pressure curve." Therefore: Injection speed is "cause," and injection pressure is "effect." Increasing speed setting will increase pressure until it reaches machine's pressure limit.
Pressure Limitation: When pressure required to reach set speed exceeds machine's maximum injection pressure limit, screw will be unable to maintain that speed, and actual speed will decrease. At this point, pressure reaches upper limit and remains there, and system enters a "pressure-limited" state. This usually leads to incomplete filling (speed is too slow, and pressure is limited, causing short shots).

 

Illustration: Relationship between Injection Speed and Injection Pressure Limitation Curve
As shown in figure, in the middle stage, to maintain high-speed filling, pressure requirement increases; if set pressure is too low, when pressure limit is reached, speed cannot be maintained and will continue to decrease. At this point, V/P switching position will not be reached, causing short shots.

It is precisely based on this coupling relationship that multi-segment injection technology is of great significance. For example: Using low speed at gate: Reducing speed (v↓) directly lowers required pressure (P↓), thus avoiding jetting marks and excessive shear heat that can result from high-speed shearing.
Using high speed during rapid filling of main body: Increasing speed (v↑) achieves better melt front temperature and fusion effect; at this time, system must provide correspondingly high pressure (P↑).
End-stage deceleration: Reducing speed (v↓) at the end of filling reduces requirement for venting speed, also reduces risk of excessively high pressure peaks due to increased end resistance.
In summary: Injection pressure setting must be greater than maximum pressure required to meet injection speed.

Setting injection pressure (or maximum pressure limit) too high, even if actual production does not require such high pressure, will lead to significant energy waste on multiple levels.

In hydraulic injection molding machines, injection pressure is provided by a hydraulic pump and regulated by a proportional pressure valve. Energy consumption impact is mainly reflected in:
a. Overflow loss and constant pressure difference characteristic of pressure valves. Working principle of a hydraulic system is "pump provides flow, and valve establishes pressure." When set injection pressure is high, proportional pressure valve needs to maintain a high pilot pressure to set main system pressure. To establish and maintain this high pressure, hydraulic pump must continuously output sufficient flow. However, at certain stages of injection (such as during low-speed passage through gate), actual required flow is small, but system pressure has already been set at a high level. This causes a large amount of high-pressure oil to leak back to tank through overflow port of pressure valve or through other means; this process is called "overflow loss." Overflow under high pressure means that work done by pump (converted from electrical energy) is not used to drive screw, but is directly converted into heat energy of oil. The higher set pressure, the greater proportion of overflow loss, and the more significant ineffective energy consumption.
b. Increased internal leakage in system. High pressure of hydraulic system exacerbates internal leakage between all moving parts, such as cylinder piston seals, valve core and valve body clearances, etc. Leaked oil also needs to be replenished by pump, and this energy is also converted into heat.
c. Increased Work to Overcome Mechanical Friction. Higher system pressure means that components such as piston rod and screw of hydraulic cylinder bear greater axial force, which may slightly increase frictional losses in guide rails, bearings, etc.

Energy consumption logic of all-electric machines is different, but its energy consumption is still directly related to pressure and speed settings.
a. Copper and Iron Losses of Servo Motors
Output torque of a servo motor (corresponding to injection pressure) is proportional to square of current. When a very high injection pressure is set or actually required, motor driver needs to output a large current. This leads to a significant increase in "copper loss" (I²R loss) of motor windings, with electrical energy being directly converted into heat energy. At the same time, high torque operation may also increase iron core loss of motor.
b. Regenerative Energy and Braking Resistance
During injection process of an all-electric machine, servo motor works as an electric motor. However, during certain deceleration or pressure holding phases, motor may be in a generator state. If system pressure is set too high, causing motor to frequently operate in high-torque range, regenerative electrical energy generated may exceed system capacitor's recovery capacity. Excess energy needs to be dissipated as heat through braking resistor, resulting in energy waste.

Although ultimate goal is the same—to measure and control "injection pressure"—electric and hydraulic injection molding machines differ fundamentally in their pressure detection locations, sensing elements, and feedback logic due to their fundamentally different drive principles.

Pressure detection in hydraulic machines directly targets "hydraulic system oil pressure."
a. Detection Location
Main Location: Oil inlet line of injection cylinder. This is typically measured by installing a pressure sensor (usually a piezoresistive or piezoelectric sensor) on oil line connecting to rodless chamber (side that drives screw) of injection cylinder.
Alternative/Auxiliary Locations: Sometimes sensors are also installed at hydraulic pump outlet or on main pressure valve block for monitoring.
b. Detection Principle and Signal Logic
Physical Quantity: Sensor directly measures hydraulic oil pressure (unit: bar or psi).
Conversion to Injection Pressure: Control system calculates axial force acting on screw (F = Poil × Acylinder) based on measured oil pressure (Poil) and effective area (Acylinder) of injection cylinder piston. This axial force is then divided by cross-sectional area of screw (Ascrew), finally converted and displayed as injection pressure acting on melt (Pinj = F / A_screw).
Characteristics:
Indirect Measurement: Displayed pressure is calculated, its accuracy depends on accurate input of cylinder and screw area parameters, does not consider frictional losses between screw and barrel. Actual pressure acting on melt is slightly lower than displayed value.
Responsive Hydraulic Circuit Pressure: Its reading directly reflects state of hydraulic system. For example, changes in oil temperature leading to viscosity changes, and wear of hydraulic valves causing internal leakage, will directly affect detected pressure value and stability.
Pressure Fluctuations: Due to compressibility of hydraulic oil and response characteristics of valves, its pressure curve may exhibit slight fluctuations and overshoot.

Electric Motor Pressure Detection: Pressure detection by electric motors is based on "output torque of servo motor."
Detection Location: Core location: Inside servo motor or servo driver. All-electric machines do not require hydraulic oil; their "pressure" comes from rotational torque output by servo motor.
Sensing Elements: A high-precision rotary encoder and/or torque sensor (such as a strain gauge) is typically mounted at motor's tail end. More commonly, indirect and precise torque measurement is achieved through "current feedback."
b. Detection Principle and Signal Logic
Physical Quantity: Servo driver monitors current (I) of drive motor in real time and with precision.
Conversion to Injection Pressure: Output torque (T) of servo motor is proportional to current (I) (T ∝ kt * I, where kt is motor torque constant).
This torque is converted into an axial force (F) that propels screw forward through a transmission mechanism such as a ball screw or synchronous belt. For a ball screw, F = T (2π η) / P, where P is screw lead and η is efficiency.
Finally, this axial force is divided by screw cross-sectional area to calculate and display injection pressure.
Features: Direct drive, indirect but precise calculation: Although calculated, servo system's extremely precise measurement of current and position, along with mechanical efficiency and high rigidity of drivetrain, results in an injection pressure value that closely approximates actual pressure acting on melt, with extremely high repeatability.
No intermediate medium: Unaffected by hydraulic oil temperature, cleanliness, or leakage, resulting in a smooth pressure curve and fast response.
Energy efficiency: Motor only consumes current when torque is required, avoiding continuous overflow losses in hydraulic system.