"Shut Down and Seal" is a term in the plastics industry that refers to the common act of shutting down a press or a line for a period of time and using a purging compound to seal the machine so that startups will be easier and contain less degraded resin once the processor is ready to run product. Shut down and seal is often an overlooked variable of a good purging program.
Most processors we speak with have had to deal with black specs on startup. Whether that be for injection molding, extrusion, blow molding, blown film, etc. Shut down and seal really is a variable that spans every type of plastics processing and throughout all industries associated with plastics manufacturing.
Most commonly, processors will try to empty out the barrel so there is little melted plastic remaining in the barrel as possible. Once the barrel is empty the processor can kill the heats and follow through with the rest of the shutdown procedure. The problem is, you can’t really empty out a barrel without a complete teardown. There will always be residual plastic left between the flights of the screw. That material will remain there while the machine goes through the cool-down process at shut down as well as the entire heat-up process at start up. This is the material that will degrade during those cycles and cause black specs once the machine is back up and running. When a processor empties a barrel (as much as possible) air will fill the empty void where the melted plastic used to be. With air comes Oxygen and that allows oxidation to occur which leads to material degradation. That’s where those pesky black specs come from at start-up when you try and run parts on a Monday morning or after maintenance shut down.
The good news is that oxidation can be avoided by using a non-filled grade of purging compound to seal with during shutdowns. So far, we have talked about the barrel, but shut down and seal can also be used for molds, dies, and downstream equipment on extrusion applications.
Here is how it works:
When the press or line is ready to be shut down a non-filled grade of purging compound is used to purge out the previous resin. Once the purge pile is clean (free of previous resin and contamination) the machine is ready to be sealed. With the barrel full of clean purging compound the heats can be turned off and the purge left to sit until it’s time to start the machine back up. The purge seals the barrel, mold, die, etc by not allowing oxygen to enter the system due to the barrel being left full. Asaclean’s purging compounds that are unfilled are much more thermally stable than a typical resin so the purge is engineered to be able to withstand the cooldown and heat-up process.
At start-up, once the heats are up to production set points the purge can be flushed out of the system and followed by the production resin. With this procedure, the processor can be confident that there is no degraded material left behind on the screw, mold, or die once the purge is out of the system and the production resin is reintroduced. Black specs at startup can be a thing of the past and could potentially save companies thousands by reducing or eliminating the downtime and bad parts usually associated with machine startups.
Sealing with Purge isn't the only way to reduce downtime. Learn more about how to reduce production downtime and protect your profits with a purging compound.
Jeremy Cooley is an Asaclean Technical Service Representative & Purging Expert living in South Carolina. He's worked in injection molding & purging for over 15 years, and frequently presents at national plastics trade shows and conferences.