What should you pay attention to when using PET preform injection molding machine?

In the beverage packaging industry, PET preform injection molding machines are key equipment in the plastic packaging industry chain, and their operating status directly impacts the quality and production cost of the final product. PET (polyethylene terephthalate) has become a mainstream choice for beverage packaging due to its excellent transparency, gas barrier properties, and recyclability. However, producing high-quality PET preforms requires not only precision injection molding equipment but also scientific operating methods and a comprehensive maintenance system.

1. Pre-production Preparation

Raw Material Handling: PET raw materials must be strictly kept dry (dew point ≤ -40°C). Dry them at 150-180°C for 4-6 hours before use to avoid contamination. Ensure the conveying pipes and hoppers are clean.
Mold Inspection: Confirm that the mold temperature control system is functioning properly (typically, the mold temperature is maintained between 10-40°C), check that the mold cooling water channels are unobstructed, and verify the working condition of the hot runner system's heating elements.

2. Production Process Control

Temperature Management: Barrel temperature is controlled in stages (rear section 240-260°C, middle section 260-280°C, front section 270-290°C). Nozzle temperature is controlled within the range of 270-285°C, and the melt temperature should be maintained at 280±5°C.
Process Parameters: Injection pressure is controlled between 80-140 MPa, holding pressure is 30-50% of the injection pressure, injection speed is controlled in multiple stages (first fast then slow), and cooling time is adjusted according to preform wall thickness (usually 15-30 seconds).
Quality Control: Regularly check preform weight (fluctuation should be ≤0.5g), observe preform mouth flatness and thread integrity, and test preform transparency and crystallinity.

3. Equipment Maintenance

Daily Maintenance: Clean mold parting surfaces and guide pins every shift, check hydraulic oil level and temperature (recommended to maintain 45-55°C), and lubricate moving parts. Regular maintenance: Check the heating coil and thermocouple monthly, replace the hydraulic oil filter quarterly, and overhaul the hydraulic system annually.

4. Safety precautions for using PET preform injection molding machines

(1) Equipment operation safety specifications

Personal protective equipment requirements: Wear high-temperature resistant gloves (capable of withstanding temperatures above 300°C) when handling hot molds and preforms, wear a scalding mask or goggles to prevent injury from molten material splashing, wear anti-static work clothes and non-slip safety shoes, and wear a bund-up and work hat for long hair.
Pre-startup inspection: Confirm that all safety door interlocks function properly, check whether the emergency stop button is sensitive and effective (functional tests should be performed monthly), verify that the hydraulic system pressure gauge displays normally (working pressure is usually ≤140 bar), and ensure that the mold area safety grating is unobstructed and responsive.

(2) Risk prevention and control of high-temperature operations

Heating system safety: The barrel heating area is controlled in sections. If the abnormal temperature exceeds 300°C, the machine should be shut down immediately. A clear high-temperature warning sign should be set up in the hot runner area (reflective warning tape is recommended). When the working temperature of the mold surface exceeds 60°C, a heat insulation protective cover should be installed. Melt spraying protection: No one is allowed to stand in front of the nozzle. Protective baffles are set up. Dangerous areas are marked on both sides of the moving track of the shooting platform (yellow warning lines are recommended). Special long-handled tools must be used to clean the nozzle residue.

(3) Protection of mechanical moving parts

Safety of the clamping unit: The mold thickness must strictly match the mold capacity of the injection molding machine (retain a 10% safety margin). The lubrication points of the clamping mechanism are checked daily (high-temperature grease is recommended). The low-pressure mold protection pressure is set to 10-15% of the normal value (usually 5-15 bar). Ejector system protection: The ejector stroke setting must retain a 5mm safety margin. A photoelectric protection device is installed in the robot pick-up area. Safety support bars (load-bearing capacity ≥ 2 tons) must be used during mold maintenance.

5. Common Problems of PET Preform Injection Molding Machine

(1). Silver streaks (bubbles) appear on the product

Possible causes: Insufficient drying of raw materials (PET is easy to absorb moisture, and the moisture content must be ≤50ppm). Excessive melt temperature causes degradation. Excessive injection speed causes air to be drawn in.
Solution: Check the drying system (it is recommended to dry at 150-170℃ for 4-6 hours). Reduce the melt temperature (usually the PET melt temperature is controlled at 270-290℃). Adjust the injection speed and inject in stages (slow first and then fast).

(2). Yellowing or black spots on the preform

Possible causes: Thermal degradation of raw materials (too long residence time or too high temperature). Impurities remain in the screw or barrel. Local overheating of the hot runner system.
Solution: Clean the screw and barrel, and check whether there is carbonized material in the hot runner. Reduce the melt temperature and shorten the residence time (to avoid long shutdown without material discharge). Regularly replace the filter (if any).

(3). Gate wire drawing or tailing

Possible reasons: Nozzle temperature is too high. Insufficient holding time or too short cooling time. Unreasonable mold gate design.
Solution: Lower the nozzle temperature (5-10℃ lower than the melt temperature). Extend the holding and cooling time (to ensure that the gate is completely solidified). Check whether the gate is worn or poorly polished.

(4). Unstable preform weight

Possible reasons: Too low back pressure leads to uneven melt density. Fluctuation of injection volume (screw wear or check valve failure). Unstable hydraulic system pressure.
Solution: Increase back pressure (usually 3-10MPa). Check whether the screw and check valve are worn and replace if necessary. Calibrate the injection volume and pressure sensor.

(5). Uneven preform wall thickness

Possible reasons: Uneven mold temperature (cooling water channel blockage). Unbalanced injection speed or pressure. Mold core offset.
Solution: Check the mold cooling system (it is recommended to control the mold temperature at 10-40℃). Adjust the injection speed curve to ensure balanced filling. Correct the concentricity of the mold core.