Injection molding machines are essential and crucial industrial machines in manufacturing industry. They are the most widely used and numerous plastic molding equipment in rubber and plastic molding industry. With advances in industrial manufacturing technology and application of various modified plastics, development of injection molding machines is trending towards mechatronics and digitalization. Technology of injection molding machine equipment is also becoming increasingly advanced, the overall machine structure is moving towards a functional modular design. In the future, with application of digital information technology, injection molding machines will further evolve into intelligent and smart injection molding robots.
For technicians in injection molding industry, mastering some basic common injection molding machine troubleshooting and solutions will greatly improve machine's overall efficiency (OEE), significantly increase revenue of injection molding production. Below, we will explain common injection molding machine troubleshooting and routine maintenance solutions based on main functional modules of injection molding machine.
To master routine maintenance and solutions for injection molding machine troubleshooting, you must first have a clear understanding of machine's operating principles, including its electrical, hydraulic, and mechanical principles. Secondly, you must be adept at dissecting an injection molding machine. This involves analyzing its functions and operating principles based on following four key components. Specifically, you should diagnose machine's fault through observation, smell, questioning, and palpation to determine whether fault lies with electrical, hydraulic, mechanical, or motor components.

Injection molding machine components

 

As shown in diagram above, an injection molding machine typically consists of four major systems:
1. Industrial control and servo system, consisting of injection molding machine's main control computer with a human-machine interface, servo control drivers, servo motors, oil pumps, and sensor systems. This system can be considered brains of injection molding machine. If connected to cloud through IoT, edge computing and AI big data can be used to further upgrade injection molding machine to an injection molding robot.
Injection molding machine operator can set parameters on this system's user interface (touch screen or remote control terminal), providing control and feedback on operation of machine's actuators to produce desired plastic products.
2. Drying and mixing system, which serves as inlet for molding materials, includes a drying and feeding drum, along with associated sensors. Operator adds various plastic raw material pellets and additives to mixing drum. After thorough drying and mixing, they are then fed into high-temperature mixing drum.
3. Mixing and injection system includes mixing drum, a high-temperature heater reaching hundreds of degrees Celsius, a mixing screw, an injection nozzle, and associated sensors. Mixed plastic pellets undergo three mixing stages here: mixing, conveying, and melting. They are then injected into mold cavity at appropriate temperature and pressure.
4. Molding system primarily consists of mold and clamping mechanism. Mold is responsible for molding plastic product, while clamping mechanism is responsible for closing, demolding, and separating molded parts. This system is also the most critical component for quality of molded product. For example, mold design and manufacturing quality, clamping force, settings for mold cooling (mold temperature controller) and holding pressure parameters all directly impact molding process. Today, with advancements in mold manufacturing technology, precision of molded products is increasing. In-mold hot-cutting technology is increasingly being applied to molding of precision injection molded parts, such as mobile phones, medical devices, and precision electronic components.
Following diagram provides a clearer understanding of manufacturing principles of injection molding machines:

 

This type of fault typically occurs in servo sensor and servo drive programmer (displaying various error messages). As shown in diagram below,

 

When multiple alarms occur, as in diagram above, it's generally believed to be a problem with servo system or main control computer. In this case, servo drive or computer needs to be replaced. However, if a servo alarm occurs, in addition to inspecting servo control driver board, you should also check sensors associated with alarm and verify that main control computer is functioning properly. This process can be troubleshooted one by one.
Of course, main control computer with manual interface can also experience two other common faults:
Soft faults (rarely due to improper operator operation and parameter settings, resulting in injection molding quality issues or machine damage). This type of failure is the most common operational fault in injection molding, accounting for up to 70% of all failures, especially when switching to new injection molding products, requiring mold changes and parameter adjustments. The only way to resolve this type of failure is to continuously improve skills of injection molding machine operator or hire external injection molding experts. We will discuss technical methods for proper mold and machine adjustment in the future, so please stay tuned.
Second type of failure is a hardware failure of computer itself. This type of failure renders machine inoperable and requires replacement or repair of computer board, control panel, etc. When replacing main control computer, it is recommended to choose a mainstream, technologically advanced injection molding machine computer brand, such as Hongxun, to permanently upgrade precision machining capabilities of injection molding machine.
In addition, hydraulic system, consisting of motor and oil pump in this system, can also frequently experience following faults:

A. Proportional valve wiring on oil pump is loose or coil is burned. Check whether proportional pressure valve is energized.
B. Impurities are blocking oil port on proportional pressure valve on oil pump. Remove proportional pressure valve and remove impurities.
C. Pressure oil is unclean. Debris has accumulated on oil filter, preventing pressure oil from entering pump. Clean oil filter and replace pressure pump.
D. Oil pump is leaking internally due to overuse, internal wear, or impure pressure oil. Repair or replace oil pump.
E. Oil pump barrel, oil hose, or joints are leaking. Eliminate leak.
F. Oil control is stuck. Check oil control valve core for proper movement.

A. Power supply fuse is blown. Replace fuse.
B. Start button connector is defective. Replace or tighten it.
C. Overload relay has tripped. Replace it.
D. Main circuit is faulty. The breaker is broken. Replace it.
E. There is a humming sound, no rotation, or a missing phase. Check power supply. Other possible causes include: a faulty main pressure valve; a faulty amplifier board; a faulty motor or oil pump connection flange; or loose glue on servo valve solenoid coil. In these cases, inspect corresponding components.
Generally speaking, motor failure is a more serious problem. If repairs to three or more components fail, it's recommended to replace motor directly. This indicates motor has reached fatigue stage. If technician's motor repair skills are insufficient, forcing motor into service can lead to even greater problems.

A. Air or other oily debris is being drawn into suction side; repair suction-side joints.
B. Oil filter is clogged; clean filter.
C. Wear inside pump; inspect oil pump.
D. Oil level is too low; add more oil.
E. Oil pump is damaged, causing internal parts to wear and generate high heat at high speed.
F. Improper pressure regulation, resulting in prolonged high pressure in hydraulic system and overheating.
G. Internal leakage in hydraulic components, such as a damaged directional valve or seal, generates heat as high-pressure oil flows through a small space.
Similar to servo motor, if oil pump exhibits multiple simultaneous issues and remains unresolved, it's recommended to directly replace directional valve and pressure valve accessories. If noise problem persists, it's also recommended to simply replace these components. These components are inexpensive and can prevent new problems.

Poor drying of plastic pellets and high moisture content can seriously affect injection molding quality. Uneven mixing of pellets with fillers (such as flame retardants and talc) can directly impact product performance. Specific consequences include:
1) Uneven screw color mixing, difficulty in injection molding, and failure to feed material;
2) Black glue spots and carbon deposits on screw, barrel, or injection molding components;
3) Black spots or color mixing on injection molding components;
4) Water marks and bubbles on molded parts;
High moisture content in plastic raw materials can also cause corrosion damage to key molding components such as molding screw and barrel.

Mixing and injection molding is process of thoroughly mixing and melting plastic raw materials in barrel until injection molding. Molten state of material directly affects molding quality and efficiency, primarily influenced by heater temperature, screw design and operating pressure, nozzle design, related sensors and pressure devices. Common faults include:

A. Pre-molding end stroke switch is closed. Remove stroke switch block.
B. One-way throttle valve is stuck.
C. Material temperature is too low; increase it.
D. Pre-molding solenoid valve is stuck; remove and clean it.
E. Pre-molding pressure is too low; increase it.
F. Foreign matter has entered screw, causing it to become stuck. Disassemble screw and clean barrel.
G. Hydraulic or servo motor is faulty, bearings are stuck.

A. Back pressure is too high.
B. Insufficient cooling water at feed port, causing material to "bridge" inside feed port. Adjust water flow and remove any plastic blocks that are stuck to structure.
C. Material shortage: Add more material.
D. Screw breakage. Back pressure is too high. Lower back pressure.

A. Low injection pressure, slow speed: Increase injection pressure and speed.
B. Plastic heating temperature is too low: Increase temperature.
C. Nozzle is clogged; remove and heat for cleaning.
D. Injection time is too short.
E. Injection solenoid valve is stuck.
F. No power to injection solenoid valve. Check for electrical issues.

A. Uncontrolled temperature: Check contacts of heating contactor for sticking and thermocouple wires for loose or damaged wires.
B. Temperature fails to rise. Check electric air heater switch in electrical box to see if it is turned on, contactor and relay are engaged, and there is voltage. Check fuse and heating coil for burnout.
C. Check thermometer for damage.

A. Injection solenoid coil may be burned, or foreign objects may have entered directional valve, causing valve core to move. Clean or replace injection valve.
B. Pressure is too low. Increase injection pressure.
C. Injection temperature is too low. Adjust thermometer to raise temperature to desired level. If adjusting thermometer still does not raise temperature, check heater and fuse for burnout or looseness. If so, replace them immediately.
D. Injection combination valve opening wiring is loose or in poor contact. Reconnect combination switch wiring properly.

A. Injection molding back pressure is too high. Back pressure valve is damaged or improperly adjusted. Adjust or replace back pressure valve.
B. Insufficient water supply, resulting in excessively high temperature, hinders entry of rubber pellets into screws. Adjust water supply and remove bonded rubber pellets. Check water supply ring for blockage or water cooling.
C. No material in drop hopper. Add material.
D. Is melt barrel temperature too high?

A. Shooting platform travel limit switch is pressed by adjustment block. Adjust.
B. Coil of shooting platform travel solenoid valve may be burned or foreign matter may have entered directional valve, causing valve core to move. Clean or replace solenoid valve.
C. Check shooting platform's forward speed and pressure for improper adjustment.
D. Check I/O board and directional valve output voltage (DC+24V). If not, check corresponding output transistor or relay for damage.
E. Press shooting platform key and check if computer screen displays shooting platform front signal, other signals, or alarms.
F. Check pressure gauge for movement.
During injection molding process, problems often arise during critical phase of plastic injection. These problems often stem not from equipment failure but from issues with production process and operational parameters, making this a particularly important time for extensive debugging and inspection.