As companies look to cut costs, the ability to shutdown or startup a line quickly becomes more and more significant. In response to shifts in customer demand and general pressure to cut labor, many traditionally 24/7 job shops are now shutting down parts or all of their production over weekends. Most thermoplastic resins, when left in the barrel for even a short period of time, are likely to degrade and cause product contamination. Running engineering grades raises the likelihood of this occurring. Many technicians are taught to turn down the heats when a line goes down, but degradation will even occur if resin is left in the barrel when the heats are shut off completely – there is simply no way to cool that resin down quickly enough to avoid degradation (see ABS example below). To prevent contamination or deposit formation over the weekend or for any extended shutdown (maintenance, lack of packaging materials, lack of orders, etc...), a purging compound should be used to clean and seal injection units.
Sealing an injection unit with a purge compound is normally a pretty simple process. The operator should keep the barrel full of production resin to start. Wipe down the hopper and add the appropriate amount of purging compound. When using mechanical purges, it is standard to run the screw slowly until the purge begins exiting the nozzle. Then, crank up the back pressure and screw speed to their respective safe maximum values. Once the purge pile is free of contamination, immediately turn the heats off. The barrel must be left completely full of purging compound so that no air gets into the barrel, as air will cause oxidation and contamination will shortly follow. Upon startup, get the heats up to operating temperatures. Once the purge is adequately heat soaked, add production resin and purge out purging agent. Once the purge is free of purging agent, begin production contaminant free.
Some chemical purge manufacturers recommend running the barrel clean of the purging compound prior to shutdown. Otherwise, the process of sealing an injection unit is the same when comparing mechanical and chemical purging compounds.
*Remember that all purging compounds are different and it is critical to match the application with the proper type and grade of purging agent. It is also extremely critical to find a purge that works well at the production resin processing temperature to avoid degradation and long purging times.