Purging a plastic injection molding machine with resin or regrind for color changes or carbon removal often leads to costly waste and lost production time. Along with the right purging compound, there are best practices and procedures a business must follow to ensure the purging process is quick and thorough.
Purging is often necessary during color changeovers, a challenge that every molder must endure. These changeovers take valuable time, and cost money in labor and wasted material.
On the shop floor, the key for molders today is to minimize these losses.
Purging compounds are used in the injection molding process for eliminating air bubbles, un-melted resins, foreign contaminants, and degraded residues from the machine barrels. The process also has improved, meaning molders no longer have to use expensive virgin resins to purge.
As a run begins to wind down, operators can turn off material flow and colorant. The process settings should be maintained, and molding parts continued. Once the last part is done, the carriage can be retracted, and the hopper and colorant blender can be cleaned.
When starting the purging process, operators should make sure the injection unit is retracted and the screw and barrel are emptied. One to two-barrel capacities and purging compound should be put into the feeder. The nozzle and sprue bushing also can be cleaned.
The screw should be set completely forward, then the backpressure is increased to the maximum level.
Once the purging compound starts to come from the nozzle, the screw speed can be increased to the maximum safe level. When the compound is almost clean, the backpressure can be reduced.
The screw then should be retracted, and short, high-velocity injection shots should be performed.
If contaminants are still seen, put the same amount of purging compound into the feeder and repeat the process.
Finally, replace purging compound with the next resin to be run, at top pressure required and maximum safe screw speed, and the screw set completely forward.
A popular option today are single-dose compounds that allow operators to purge their machinery simply by dropping the packets into the feed throat/hopper of their machine.
Injection molders can realize several benefits after a purge, including fewer scrap parts, less carbon build-up in the barrel, elimination of color hanging up in the tool, and a reduction in color changing time and overall changeover time. Downtime also is reduced.