A proper shutdown procedure for a vertical injection molding machine, especially for overnight stops or material changes, is a critical step to prevent thermal decomposition (scorching) of the plastic resin. If operated incorrectly, residual resin heated in the barrel for an extended period can lead to carbonization and discoloration. This will cause a high number of defective products at the next startup and can even damage the screw and barrel.
The Proper Overnight Shutdown (Purging) Procedure
To prevent resin from decomposing in the high-temperature barrel, we strongly advise following this purging procedure when planning an overnight shutdown:
- Close the Hopper Slide: First, shut the slide at the bottom of the hopper to stop new plastic from entering the barrel.
- Continue Purging (Empty Shots): Continue to perform injection cycles (air shots) to extrude all remaining resin from the barrel.
- Turn Off Heaters: As soon as you confirm the barrel is empty (no more plastic is being extruded), immediately turn off the heaters for the injection cylinder and nozzle.
- Shut Down Main Power: After the heaters are off, you can safely shut down the machine’s main power, preparing it for the next use.
Why is Purging Necessary?
If the purging procedure is not performed before shutdown, the high-temperature barrel (often over 200°C) will “bake” the remaining plastic overnight. This leads to thermal decomposition and carbonization. These carbonized particles will then mix with new material during the next day’s startup, causing severe quality issues like black spots, yellowing, or color streaks.
Preventive Measures and Material Selection
- If Scorching Has Already Occurred: If the plastic has already decomposed or burned inside the barrel, the only solution is to completely turn off the machine. After it cools, you must disassemble and thoroughly clean the nozzle, screw, and barrel.
- Material Selection: A preventive measure is to choose resins that possess high thermal stability, as they are less prone to decomposition during extended heating.
Special Considerations During Machine Maintenance
When production is suspended for short-term maintenance (e.g., dealing with a stuck mold), a full shutdown is not necessary.
In this situation, do not use the machine’s power to purge the remaining plastic. The correct procedure is to immediately set the barrel heater to a low holding temperature, such as 150°C. This temperature is just high enough to keep the plastic in a semi-molten state (preventing it from solidifying and “seizing” the screw) but low enough to minimize the risk of thermal decomposition, thereby protecting both the material and the equipment.
How Does MINHUI Help Simplify This Operation?
MINHUI’s vertical injection molding machines are equipped with high-precision PID temperature control systems and intuitive user interfaces. Our control system can precisely execute your holding temperature commands (such as 150°C) and ensure there are no significant fluctuations. Furthermore, our user-friendly design makes purging and maintenance operations simpler and faster, reducing the risk of operational errors at the source.
A proper shutdown and maintenance procedure is the cornerstone of guaranteeing product quality and extending equipment lifespan. Following the steps above will help you avoid costly material waste and machine downtime.
