Analysis of the root cause of the hydraulic pressure drop of the injection molding machine
2025/06/04 By Topstar

When hydraulic pressure drops in an injection molding machine, it can result in incomplete mold filling and inconsistent part dimensions. Therefore, our operators need to understand the root cause of the hydraulic pressure drop and take corresponding measures. Then, we will analyze the root cause from the perspective of the injection molding machine manufacturer. First, we will examine how hydraulic oil quality and contamination can lead to pressure drop. Next, we will discuss the wear and internal leakage of hydraulic pumps, which can impact the maximum pressure output. Next, we will assess whether valve failure is a significant factor in the unstable pressure curve. After that, we will analyze and investigate the causes, including cylinder seal wear and pipeline leakage.
Hydraulic oil quality and contamination of the injection molding machine
First, one of the most common reasons for hydraulic pressure drop in injection molding machines is the poor quality of the hydraulic oil. When your injection molding machine has been in use for an extended period, the hydraulic oil will degrade due to oxidation, thermal decomposition, and contamination, resulting in an increase in viscosity and a reduction in flow. When the viscosity of hydraulic oil exceeds the manufacturer’s specifications, pump efficiency decreases, and the pressure cannot reach the required set point. Additionally, particulate contaminants such as metal chips, dust, and moisture in the workshop can clog the tiny passages of servo valves and proportional regulators, resulting in limited flow and pressure loss under load.
To address this, we can perform an oil analysis every two weeks, check ISO cleanliness codes and monitor water content to less than 200 ppm. Additionally, maintaining an oil temperature between 40°C and 60°C optimizes viscosity. At the same time, manufacturers should equip hydraulic oil tanks with heat exchangers or coolers to dissipate the heat generated during long production runs. If the analysis reveals elevated water or acidity levels, technicians must replace the oil entirely and flush the system.
Wear and internal leakage of hydraulic pumps in the injection molding machine
In injection molding machines, the pump is their “heart,” converting mechanical energy into hydraulic pressure. However, internal wear affects gear pumps, vane pumps, and piston pumps alike, and as operators use them over time, their volumetric efficiency and mechanical efficiency gradually decrease. As the pump wears, the gap between the piston and the cylinder or between the gear and the housing will expand, causing the fluid to bypass under pressure. This internal leakage prevents the pump from building full pressure, resulting in a pressure drop that can make the set clamping pressure unattainable or cause the injection speed to be unstable under high back pressure.

To diagnose pump-related pressure problems in an injection molding machine, measure the pump outlet pressure with a calibrated meter and compare it to the expected rated output. If the pump cannot reach this set point, there is most likely an internal leak. Disassembling the pump to inspect the vanes, pistons, and ports for wear can also confirm the diagnosis. Volumetric efficiency can be restored by replacing or re-machining worn parts. Proactively monitoring pump health and addressing wear issues can avoid incidents of sudden drops in hydraulic pressure.
Valve Failures and Control Issues
In an injection molding machine, hydraulic valves regulate the flow of fluid to the clamping unit, injection cylinder, and servo actuator. Valve failures are also a common cause of hydraulic drops. For example, a worn spool in a directional valve may allow an internal bypass, preventing the machine from reaching the desired clamping force. Similarly, if the spring tension of a pressure relief valve weakens, it may rupture prematurely, causing a drop in pressure throughout the system. Proportional valves rely on precise solenoid currents, and if wear damages the internal orifice or a short occurs in the coil windings, the valve may leak, which causes inconsistent pressure regulation and erratic injection speeds.
To troubleshoot valve-related pressure problems on hydraulic injection molding machines, first, confirm that the electrical signal from the proportional valve is consistent with the specified value under nominal conditions. Next, use a digital multimeter to measure the actual coil resistance. If the reading deviates from the manufacturer’s specified range, it indicates that the coil has sustained damage. You can also disassemble the valve body and visually inspect the sealing surface to check for scratches or contamination on the valve core. In this way, you can gradually troubleshoot valve failures and maintain regular operation.
Cylinder seal wear, pipeline leaks and system lags.
Worn cylinder seals or leaks at pipeline joints also cause a drop in hydraulic pressure in injection molding machines. During repeated pressing cycles, dynamic seals on injection cylinders and clamping cylinders wear due to friction. Even minor leaks can cause the system to be unable to maintain maximum pressure under load, resulting in slow clamping or inconsistent holding force during injection. Similarly, external pipeline leaks at joints, hoses, or quick connectors can cause fluid to escape before it reaches the actuator, thereby reducing the available pressure.

We need to combine visual inspection and pressure decay testing. First, shut down the injection molding machine and visually inspect the hoses and joints for signs of leaks. Next, use a hand pump to pressurize each circuit while isolating the valve; monitor the pressure decay every 60 seconds. A significant pressure drop (more than 5%) indicates a leak. For cylinders, dye penetrant or ultrasonic leak detectors can be used to pinpoint internal seal damage. Crimped stainless steel hoses, with appropriate bend radii and torque fittings, can minimize external leakage.
Analyze the root cause for targeted optimization.
The hydraulic pressure drop in an injection molding machine can be caused by multiple interrelated factors, including hydraulic oil quality degradation, pump wear, valve failure, seal leakage, and other issues. By analyzing each component of the injection molding machine, the root cause of pressure loss can be identified, and targeted corrective measures can be implemented. Using real-time monitoring, precise component inspection, and data-logged diagnostic curves, hydraulic pressure drop problems can be effectively addressed to improve consistency in production cycle time and extend equipment uptime.
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