Maintenance of PET preform injection system: A core measure to ensure efficient and stable production

2025-08-29 09:51:08

The PET preform injection system, as a key pre-equipment for plastic bottle production, its operational stability directly affects the quality of preforms and production efficiency. Scientific maintenance should cover the core components of the system (such as the embryo injection machine, molds, and auxiliary machines), and combine preventive maintenance with fault detection to extend the service life of the equipment and reduce the risk of downtime.

I. Maintenance of Core Components: Daily and regular maintenance of the embryo injection machine

The embryo injection machine is the core of the system. Special attention should be paid to the four major modules: the injection molding unit, the clamping mechanism, the hydraulic system, and the electrical control system. Maintenance should be distinguished between "daily inspection" and "regular in-depth maintenance".

1. Maintenance of injection molding units (directly affecting melt quality and injection accuracy

Daily inspection

Barrel and heating ring: Check daily whether the heating ring is in close contact with the barrel (looseness may cause temperature deviation), and clean the oil stains and plastic residues on the surface of the heating ring. Observe through the temperature control system whether the temperature in each area is stable (the deviation should be ≤±2℃). If there is any abnormality, check the thermocouple or the temperature control module.

Screw and anti-reverse ring: Each time the material is changed or the machine is shut down for more than 8 hours, the screw should be manually turned (or the machine should be started for idling) to prevent the PET melt from cooling and solidifying, adhering to the screw. Observe whether there is any "material leakage" or "flash" during injection. If so, it may be that the anti-reverse ring is worn and needs to be replaced in time.

Regular maintenance (every 3 to 6 months)

Disassemble the screw, backstop ring and nozzle. Use a special cleaner (such as PET-specific machine cleaning material) to remove the residual melt. Check if there are any scratches on the screw ridges and if the sealing surface of the backstop ring is worn. Worn parts need to be replaced (it is recommended to keep spare parts of the same model).

Lubricated screw bearings: High-temperature grease (suitable for PET processing temperature, usually with a temperature resistance of ≥250℃) is used to ensure smooth screw rotation and prevent injection pressure fluctuations caused by bearing jamming.

2. Maintenance of the clamping mechanism (ensuring clamping accuracy and concentricity of the preform)

Daily inspection

Guide pins and guide sleeves: Check the surface daily for oil stains and plastic debris. Wipe them with a clean cotton cloth and apply special grease (avoid using ordinary grease to prevent contamination of the mold). Observe whether there is any "abnormal noise" during mold closing. If there is, it may be due to wear of the guide pins or parallelism deviation, and adjustment or replacement is required.

Mold locking cylinder/servo motor: Check whether the cylinder seal is leaking (hydraulic oil), whether the servo motor terminal is loose, and whether the cooling fan is operating normally (to prevent motor overheating alarm).

Regular maintenance (every 6 to 12 months)

Check the uniformity of the clamping force: Through the built-in "clamping force testing function" of the equipment, check whether the forces on the four pull rods are consistent (the deviation should be ≤5%). If they are not consistent, adjust the pull rod nuts to avoid mold deformation due to uneven force.

Replace the vulnerable parts of the clamping mechanism: such as the sealing parts of the clamping cylinder and the contacts of the limit switch, to prevent the clamping speed from decreasing or false signals from being triggered by seal failure.

3. Hydraulic system maintenance (Ensuring stable power and reducing energy consumption

Daily inspection

Hydraulic oil level and cleanliness: Check the oil tank level daily (it should be between the "lowest - highest" mark), and observe whether the oil is turbid (normally it should be light yellow and transparent). If it turns black or contains impurities, filter or replace it in time (it is recommended to replace the hydraulic oil every six months, using 46# anti-wear hydraulic oil).

Hydraulic pump and pressure gauge: Listen to the operating sound of the hydraulic pump (normally it should be a stable humming sound; abnormal sounds may indicate wear inside the pump or air intake), and check if the pointer of the pressure gauge is stable. If the fluctuation is too large, check whether the relief valve or check valve is blocked.

Regular maintenance (every 6 months)

Clean the hydraulic oil filter element and the oil tank: Disassemble the return oil filter element and the suction oil filter element, and blow them back with compressed air (or replace them with new filter elements) to remove the sediment impurities at the bottom of the oil tank. Check whether there is any leakage at the hydraulic pipeline joints. If there is any leakage, re-tighten or replace the sealing ring.

4. Maintenance of electrical control system (to avoid program disorder and signal failure)

Daily inspection

Control cabinet and terminal blocks: Check daily whether the cooling fan of the control cabinet is operating and whether there is moisture or dust inside the cabinet (especially in a humid environment, a dehumidifier needs to be installed). Gently pull the key wires (such as motor power lines and sensor signal lines) with your hand to prevent loosening and poor contact.

Sensors and actuating elements: Check whether the position sensors (such as mold closing position sensors and injection end point sensors) are blocked by plastic debris, and clean the sensor probes. Test the emergency stop button and the interlock function of the safety door to ensure that the equipment can stop immediately after being triggered.

Regular maintenance (every 12 months) :

Backup equipment program: Back up the parameters of the embryo injection machine (such as injection speed, holding time, and temperature setting) via a USB flash drive or dedicated software to prevent program loss. Check whether there are any error records of the PLC and servo driver, clear historical fault codes and analyze the causes of high-frequency faults.

Ii. Maintenance of Key Accessories: Molds, drying Systems and cooling systems

1. Maintenance of preform molds (directly determining the quality of preforms, requiring meticulous care)

Daily maintenance (each mold opening/mold change) :

Cavity and core cleaning: Wipe the surface of the cavity and core with a soft cloth (or special sponge) to remove residual PET debris. Do not use hard scrapers (to avoid scratching the cavity and causing surface defects of the preform). Check if there is any "burnt material" at the gate. If so, gently sand it with fine sandpaper (800# or above).

Lubrication of the ejection mechanism: Apply food-grade grease to the ejection pins and tubes (it must meet FDA standards to avoid contaminating the preforms) to ensure smooth ejection and prevent the preforms from sticking to the mold or cracking during ejection.

Regular maintenance (every 20,000 to 30,000 cycles) :

Disassemble the mold for deep cleaning: Use an ultrasonic cleaner (with water temperature of 50-60℃ and a neutral cleaning agent) to clean the cavity and core, removing stubborn oil stains and carbides. Check the cavity for wear and scratches. Minor scratches can be repaired by polishing (using diamond polishing paste with a particle size of ≥1000#).

Replace the vulnerable parts of the mold, such as the guide column sealing ring, gate sleeve, and ejector pin spring. Especially for needle valve type hot runner molds, it is necessary to check whether the needle valve core is stuck, test the uniformity of the hot runner nozzle temperature (the deviation should be ≤±3℃), and replace the heating wire or thermocouple if necessary.

2. Maintenance of the raw material drying system (PET moisture absorption can cause the preforms to turn yellow and develop bubbles. It is necessary to ensure the drying effect)

Daily inspection

Dryer Hopper: Check daily whether the raw materials in the hopper have lumps (moisture can cause lumps). Observe the "dew point temperature" on the dryer display screen (it should be ≤-40℃, otherwise the drying is not sufficient). If the dew point is too high, check whether the molecular sieve has failed.

Fan and heater: Listen to the sound of the fan running and check whether the heater is heating up normally. Clean the air filter (located at the air inlet of the fan) to prevent dust from clogging and causing a decrease in air volume.

Regular maintenance (every 3 months)

Replace the molecular sieve: Replace the molecular sieve inside the dryer based on the processing capacity of the dryer (for example, once every 10 tons of PET raw materials processed), or when the dew point remains persistently high. When regenerating molecular sieves, ensure that the regeneration temperature (usually 160-180℃) and time (4-6 hours) meet the standards to avoid incomplete regeneration.

Cleaning the hopper: Disassemble the hopper, blow off the residual raw materials with compressed air, and check if there is any wear on the inner wall of the hopper. If so, repair it (to prevent the growth of bacteria due to residual raw materials, especially when producing food-grade preforms).

3. Maintenance of the cooling system (including mold cooling and equipment cooling, which affects the molding cycle and equipment lifespan)

Daily inspection

Cooling water pipeline: Check if there is any leakage at the water pipe joints and observe the flow meter display (ensure that the cooling water flow in each cavity of the mold is uniform, with a deviation of no more than 10%). If the flow in a certain path is too small, it may be due to pipeline blockage and needs to be cleared in time.

Chiller: Check the water temperature of the chiller (mold cooling is usually set at 15-20℃, equipment cooling at 25-30℃), observe whether the condenser is frosted and the evaporator is dirty. If so, blow it off with compressed air or clean it with a special cleaner.

Regular maintenance (every 6 months)

Clean the cooling water circuit: Add a descaling agent (such as citric acid solution) to the water circuit, circulate for 1-2 hours, and then rinse with clean water to remove the scale in the pipeline (scale can cause a decrease in heat dissipation efficiency and prolong the cooling time of the preform).

Check the chiller compressor: Listen to the operating sound of the compressor, measure whether the working current is within the rated range, and check the refrigerant level. If it is insufficient, replenish it (the corresponding model of refrigerant, such as R410A, should be used).

Iii. Maintenance Management and Precautions

Establish a maintenance ledger: Record the time of each maintenance, components, models of replaced parts, fault phenomena and handling results, which is convenient for tracing the operating status of the equipment and predicting the replacement cycle of vulnerable parts in advance (for example, the typical lifespan of a screw is 500,000 to 800,000 cycles, and inventory can be prepared in advance based on the ledger).

Targeted maintenance: Adjust the maintenance frequency according to the Production environment - In high-temperature and high-humidity areas (such as the rainy season in the south), the frequency of dehumidification of the electrical control cabinet and inspection of the molecular sieve of the dryer should be increased. When producing high-transparency preforms (such as beverage bottles), it is necessary to enhance the standards for mold cleaning and screw maintenance to prevent impurities from causing a decrease in the light transmittance of the preforms.

Safety Operation Specifications: During maintenance, the main power supply of the equipment must be cut off and the hydraulic system must be shut down. When disassembling molds or screws, special lifting gear should be used (to avoid collision damage). When touching heating components (such as barrels and hot runners), high-temperature resistant gloves should be worn to prevent burns.

Through the above-mentioned stratified and periodic maintenance measures, the sudden failures of The PET preform injection system can be effectively reduced, ensuring the stability of preform production (such as weight deviation ≤±0.2g, eccentricity ≤0.1mm), while extending the overall service life of the equipment (usually from 5 years to over 8 years), and lowering the long-term production cost.