Injection molding machines are the most widely used molding equipment in mold industry. They inject molten plastic into a mold, where it condenses and forms various shapes. They are preferred process for producing plastic products, and are used in a wide range of industries, including food, electronics, aviation, chemicals, automobiles, and manufacturing. Their structure typically consists of an injection mechanism, hydraulic system, clamping mechanism, heating and cooling systems, lubrication system, and electrical control system. Hydraulic system provides power for various operations of injection molding machine and meets pressure, speed, and temperature requirements of each component. Hydraulic control system and external structure are shown in Figures 1 and 2. This article summarizes five diagnostic methods for common hydraulic system faults in injection molding machines: cancellation method, blocking method, parameter measurement method, field test method, and empirical method. These methods provide reference for troubleshooting related equipment.

 

Figure 1 Injection molding machine hydraulic system

 

1-Mold working structure 2-Barrel and plastic injection mechanism 3-Base
Figure 2 Injection molding machine hydraulic working system

Cancellation method is to observe whether symptoms disappear after canceling certain links or components in hydraulic system. If they disappear, it proves that there is a fault at that location. If they do not disappear, further investigation is required until fault is determined.
For example, if hydraulic pump of power element in hydraulic system of injection molding machine makes abnormal noise after being powered on, a preliminary judgment should be made first. It may be that oil suction resistance is too large, causing cavitation. In order to confirm whether this hypothesis is correct, oil pump filter is removed to reduce oil suction resistance and observe whether pump makes abnormal noise. At this time, noise disappears. This means that filter may have a lot of carbon foam, impurities, foam and dust on the surface after long-term use. In this case, not only should filter be replaced, but cause of these blockages should also be found. Excessive carbon deposits are caused by excessive temperature and pressure during system operation. Hydraulic oil grade should be adjusted in time. Excessive dust is caused by poor sealing. Find location of obvious leakage and replace or adjust sealing ring. Excessive foam may be caused by low liquid level in tank or displacement of oil suction pipe. Observe oil tank level gauge in time and open oil tank to observe internal structure when appropriate. In terms of electronic control, hydraulic system often has a more complex control circuit. If there is a problem with circuit, it will inevitably cause hydraulic system to work abnormally. When analyzing fault, you can find a way to cancel some links in control circuit and then observe whether symptoms change.

Blocking method is a method of determining fault location by artificially blocking a certain position and observing pressure change. When using this method, it is usually necessary to first understand composition of hydraulic system of equipment, then reasonably select blocking position according to fault phenomenon. After blocking position, observe changes in pressure and flow, find fault point after a systematic analysis. This method can quickly determine location of fault, and principle is easy to understand. However, this method requires a lot of disassembly and assembly work, often requires dismantling an entire circuit for measurement. Some injection molding machines often use hollow copper tubes for their hydraulic circuits, which increases workload of detection, requires special blocking tools and circuit connection components.
From injection molding machine hydraulic system shown in Figure 1, it can be seen that there are many parallel hydraulic circuits. Multiple parallel hydraulic circuits complete corresponding working processes (such as mold closing, injection, pressure holding, etc.). Each parallel circuit is composed of many components. Therefore, when pressure drops during operation of injection molding machine or injection pressure is insufficient, it is difficult to find location of fault point. By isolating relevant circuits or components, scope of investigation is narrowed. Since hydraulic pump is a common power component of each parallel circuit, when pressure or flow drops, pump is first isolated from the other parts, pressure and flow output of pump are examined separately. After confirming that pump is fault-free, isolate part of circuit and inspect another part of circuit, thereby continuously narrowing scope of inspection until fault point is found.
As shown in Figure 3, fault detection method using blocking method is explained:
(1) First, block points A and B to seal oil circuit, loosen relief valve 6, press button to start equipment, adjust set value of relief valve 6, and quickly increase pressure. If pressure gauge 4 shows normal pressure, it means that pump 2, pressure gauge 4, relief valve 6 are all fault-free.
(2) Open point A and block point B. At this time, hydraulic cylinder 8, reversing valve 3DT and its closed pipeline are tested. Pressing relay 1YJ closes IYJ, energizing reversing valve 3DT. Hydraulic cylinder 8 begins to extend until its piston rod presses against that of cylinder 7. Due to load-dependent nature of system pressure, pressure should gradually increase. However, if pressure fails to reach its intended position, this indicates a fault in reversing valve 3DT, hydraulic cylinder 8, or oil pipeline. This narrows fault location to these three points, and remaining investigations only focus on these three locations.

 

Figure 3: Circuit Fault Detection Using Intercept Method

Hydraulic system failures are diverse, and their causes are often result of a combination of related factors. However, they are often caused by a failure in a specific component or location. Accurately identifying fault location and prescribing appropriate remedy can quickly resolve system problem.
Use of injection molding machines in environments with relatively high dust levels and high temperatures during equipment operation often results in faults caused by excessively high oil temperatures and excessively fast liquid flow if cooling cycle is not in place. Parameter measurement methods are used to identify components that cause faults during operation, determine location and cause of fault, and take decisive measures to troubleshoot faults. Figure 4 shows diagnostic steps of parameter measurement method.

 

Figure 4 Diagnostic steps of logical parameter measurement method
Detector is main tool of parameter measurement method and consists of two parts: a data processing system and a parameter test system. Structural principle of parameter test system is shown in Figure 5. As can be seen from figure, from input stage to output stage of oil circuit, system pressure, flow rate, temperature are fed back through eddy current sensor and preamplifier circuit. Abnormal conditions of these three parameters are quickly found through feedback results, and relevant fault location is determined based on abnormal state value.

 

Figure 5: Test System Structure

Detecting hydraulic faults through field testing is the simplest and most effective method. This requires technicians to record relevant data while equipment is in good condition. When a fault occurs, comparing fault data with original data allows immediate understanding of current fault status and extent of damage.
For example, for an auxiliary vane pump in an injection molding machine that has been used for a long time, pump's output flow rate can be measured under different load pressures, recorded, and compared with normal pressure-flow curve to determine wear of pump. Table 1 shows pressure-flow relationship of a vane pump under different conditions. Data in table shows that as system pressure increases, flow rate of a worn pump decreases more significantly than that of a normal pump.

 

Table 1: Pressure-Flow Relationship Data for a Vane Pump
In table, p—set system pressure; Q1—normal output flow rate; Q2—wear output flow rate

Empirical method is to directly use human sensory organs to inspect, identify and judge location, phenomenon and nature of equipment fault during operation. Through direct sense of maintenance personnel's eyes, ears, nose and hands, combined with investigation and comprehensive analysis of equipment operation status, purpose of making an accurate judgment on equipment condition and fault situation is achieved.
5.1 Visual diagnosis - looking
(1) Observe whether there are bubbles and discoloration in working oil in oil tank;
(2) Observe oil leakage at sealing parts, pipe joints, hydraulic components and various mounting joints;
(3) Observe whether equipment has shaking, crawling and uneven operation.
5.2 Auditory diagnosis - listening
(1) High-pitched and piercing whistling sound;
(2) "Hissing" or "gurgling" sound, other oil and air leakage sounds;
(3) Sound of electromagnet moving iron core being stuck and unable to operate;
(4) "Gurgling" sound produced by inhaling air;
(5) Sharp and short friction sound, etc.
5.3 Taste diagnosis - smelling
Inspector relies on sense of smell to identify whether there is any abnormal odor, such as burning smell when insulation of electrical components is damaged or short-circuited, and can also determine whether there are ants, flies and other decaying objects in oil tank.
5.4 Tactile diagnosis - touch
(1) Used to touch pump housing or hydraulic oil to determine whether hydraulic system has abnormal temperature rise based on degree of hotness and coldness;
(2) Used to touch moving parts and pipes to see if there is vibration;
(3) Used to touch hydraulic cylinder to see if there is creeping when running slowly.

This article summarizes five diagnostic methods based on common faults of injection molding machine hydraulic system. Each method has its own strengths, required working environment and technical requirements also change with changes in working conditions, but diagnostic principles are basically same. With continuous advancement of science, hydraulic equipment is gradually developing towards intelligence, integration, modularization and servoization, which also puts higher requirements on fault diagnosis methods and diagnostic technologies. We will continue to track new technologies for hydraulic fault diagnosis and propose more practical technologies and methods.