Purging the blow molding machine on the shop floor will ensure efficient performance and a better end product.
While there are some differences between standard and custom blow molders, the blow molding design is similar enough that there is no real impact on the purging process. Even when it comes to extrusion blow molding versus injection blow molding, purging is a key part of the production process.
The purging compound used for extrusion blow molding machine is a specially engineered commercial-grade product. The compound is comprised of a base resin with various additives mixed in so that not only is the machine purged but also the die, screw, and barrel. However, a purging compound has other purposes. It can be used to clean a hot runner mold and removes carbon and color deposits.
A popular option for blow molders today is single-dose compounds that allow operators to purge their machinery simply by dropping the packets into the feed throat/hopper of their machine.
Depending on the type of compound used on the shop floor, the dies and screws may not be left in place during the purging process.
In addition to choosing the correct grade and type of compound, follow all directions carefully. That way, the compound can fight contamination from carbon and color.
To start purging, increase the temperature of the die between 35 and 55 degrees Fahrenheit, followed by extruding any remaining resin from the system.
Next, the feed throat and hopper are cleaned. In the event that the extrusion machine is badly contaminated, be sure to remove the screen pack to prevent an issue with plugging and remove the die.
Slowly feed the purging compound into the hopper, initially using the lowest screw speed. When material comes out from the head, increase the speed to the maximum yet safe level. Periodically, stop the screw altogether so the compound can sit for several minutes. Then, simply set the screw speed back to maximum.
Operators should do the purge at two times the capacity of the barrel. An operator may find that more may be needed based on the condition of the machine.
To ensure an efficient and effective purge, the material should rise, bubble or swell.
Once all of the resin, color, and/or contamination is eliminated, set the extrusion machinery parameters to normal, make sure the die is in place, and add a new screen pack.