(Editor's note: Thanks to John Pizzo, technical service & development engineer of Sun Plastech Inc., who offers the following "tips of the trade" on his specialty, purging of your processing machinery. This is an updated version of an article we published in 2010. We welcome non-commercial Tips from processors, mold makers and suppliers.]
It is important to keep in mind that there is a number of purging compound categories in which products fit, based upon their method of cleaning action. For example, there are mechanical-type purging compounds, and chemical-type purging compounds, to name the two most common. While some of the following tips can be used universally across purging compound categories, the following tips are particularly recommended for mechanical-type purging compounds.
For years my company, which supplies the Asaclean brand of purging compound, has recommended three basic, yet crucial tips for purging injection molding machinery:
1) Use maximum safe screw speed while purging (and while displacing the purging compound)
2) Use maximum safe backpressure while purging (and while displacing the purging compound)
3) Keep the screw completely forward while purging
Using high screw speed and backpressure ensure greater agitation and "turbulence" so that the purging compound's scrubbing / cleaning capabilities are maximized. Keeping the screw completely forward ensures that the agitation of the screw (along with the purging compound) is focused at the front end of the machine, where color and carbon deposits often tend to be a problem (nozzle, check-ring).
Nozzle / check ring cleaning
After completing general purging procedures for cleaning the screw and barrel, we recommend charging the machine with a small quantity of purging compound (less than half-barrel capacity) and performing short, high-velocity injection shots. This action blasts purging compound into tight areas and dead spots that material would not reach otherwise, and helps purge these areas too. Mixing nozzles are notorious for having color hang-up and degraded material hang-up (due to design and greater shearing).
Shutdown / sealing
Aside from regular and preventative-maintenance purging, we also recommends using a heat-stable purging compound for temporary or extended shutdowns and sealing. During a shutdown, air and oxygen in the barrel will cause any residual polymer to degrade and carbonize, delaying startups and resulting in extended machine downtime and scrap. A heat-stable purging compound will not degrade, will create an "airtight" environment in the barrel, and will even promote further cleaning during the shutdown.
Purging compound suppliers have different procedures when it comes to purging hot runners (some of these compounds may not even be appropriate for cleaning hot runners, so first check with your supplier). One such as ours can be used using in either a closed-mold or open-mold method, depending on the resin(s) processed, mold design and cleaning difficulty. In fact, "moldability" of a purging compound is an important characteristic because closed-mold purging often provides better, more thorough cleaning of the hot runner system.
In general, it is recommended that hot runner temperatures, particularly gate temperatures, be raised in order to help loosen any deposits while purging. Short high-velocity shots are also helpful when purging using open-mold procedures.
For valve-gated hot runners, we recommend shutting off all the gates (pins in closed position) and purging troublesome gates individually. This allows for greater pressure to be delivered to each gate, which translates to greater cleaning (notably with a mechanical-type purging compound).
Want more purging tips plus slides to go with them? View and hear our archived webinar on "The 10 Principles of Purging", and download the slides to that presentation, all for free; you find it right here.