Causes of Damage to the Injection Molding Machine Hydraulic Pump

The pump is the “heart” of any hydraulic system. Therefore, at any time, when there is a problem with the hydraulic system, people will often blame the problem on the pump.

In fact, as far as the cause of pump damage is concerned, its rare that the problem lies in the pump itself. More precisely, when pump performance deteriorates, it is a symptom of other problems already lurking in the hydraulic system.

Ninety to ninety-five percent of the pump damage can be roughly summarized as follows:

1. Aeration

Aeration refers to a phenomenon in which air bubbles disperse in the hydraulic oil of the system.

This bursting effect occurs when compressed air bubbles in the oil system face the pump outlet pressure. And this cracking effect will cause the pressure side plate, wear-resistant side plate and other metal surfaces close to the cracking point inside the pump to peel off, resulting in extremely high temperature there.

When the aeration occurs, the pump will produce a loud crackling noise. The pitch of this noise increases with pressure, and excessive air entrainment can cause emulsification of the hydraulic oil (due to the large amount of water vapor in the air).

Moreover, the phenomenon of air compression in the hydraulic oil will cause problems such as unsmooth movement of various hydraulic components.

Possible ways to cause the aeration in the pump:

(1) Bad shaft seal.

(2) The junction between the oil tank and the pump inlet part leaks.

(3) The oil level is too low or the oil return pipe that returns to the oil tank is installed on the oil surface. That causesthe hydraulic oil in the oil tank to generate air bubbles, which will be sucked again by the pump before the air bubbles are eliminated.

Injection Molding Machine Hydraulic Pump (2)

2. Cavitation

Cavitation refers to a local gasification phenomenon that occurs in the fluid of hydraulic oil when the pressure is reduced below the saturated vapor pressure.

To put it more simply, when the hydraulic oil does not completely fill the space it should occupy. That is, there is a vacuum in the oil circuit, which will cause this cavitation phenomenon.

The noise caused by cavitation is roughly similar to the noise caused by aeration. The factors that cause cavitation phenomenon are:

(1) The matching problem between the pump and the electric motor causes the pump speed to be too high.

(2) The pump suction line is too long, or the suction port is restricted.

(3) The viscosity of the hydraulic oil is too high.

(4) The exhaust port on the fuel tank may beplugged.

(5) The filter screen of the pump suction port.

3. Contamination

Any foreign matter present in the circuit of the hydraulic system will have an adverse effect on the efficiency of the hydraulic components and the circuit. Contaminating foreign objects may be solid, liquid or gaseous.

Most foreign matter contamination will cause frictional movement between the workpiece and the tight clearance between the workpiece. And that will lead to accelerated wear and cracking of the workpiece.

Injection Molding Machine Hydraulic Pump (1)

The pollution sources in the oil circuit can be roughly divided into:

(1) Casting oil cylinder, oil block, inside of oil tank, pipeline joints, etc.

(2) The oil seal or O-ring is brittle and fractured.

(3) The hydraulic oil deteriorates under high temperature for a long time, resulting in oil pollution.

(4) The pump is stripped of substances on the metal surface due to aeration or cavitation.

4. Excessive heat

Excessive heat is a thermal phenomenon in which the fluid viscosity changes due to exceeding the specified upper limit temperature.

Excessive heat will cause accelerated oxidation of hydraulic oil and deterioration of hydraulic oil viscosity, leading to a chain reaction. Therefore, the true root cause of excessive heat must be eliminated and the problem must be effectively corrected.

Causes of excessive heat are:

(1) Overload operation and overpressure use.

(2) It is caused by aeration and cavitation.

(3) Caused by oil pollution.

5. Over-pressurization

Over-pressurization refers to the operating pressure greater than the design pressure of the pump.

Over-pressurization will subject the internal components of the pump to extreme forces and cause premature failure of the internal components.


All of the above reasons will leave their characteristic signs of damage. It is important to recognize and understand the messages these signs bring. In this way, the actual cause of the damage to the pump can be corrected.

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