Cleaning a Screw and Barrel of an Injection Molding Machine
March 17, 2014
Cleaning a screw and barrel can be a full day's job, even when conducted by two or more operators. It can be done by removing parts or injecting a purging compound. It is not always necessary to remove the screw and barrel from the machine, however it may be more cost effective than using a purging compound. The following will guide you in either method of cleaning.
Using a Purging Compound
When using a purging compound, the first step you want to take is closing off the flow of the resin that's being processed by pushing the slide gate at the bottom of the resin hopper closed. Next, slow down the speed of the screw's rotation by about 15 to 25 rpm until the polymer stops flowing from the end of the die and/or out of the machine's nozzle. The molding machine's barrel zone temperature should be around 200º C (400º F) to begin the purging process. Fractional melting High Density Polyethylene (HDPE) is used as a purging compound. The size of the screw determines how much HDPE is necessary for purging without overloading.
With a team operation, the second operator should be observing the head pressure gauge to make sure that the system isn't over pressured and monitoring the screw to ensure that it's rotating at 15-20 rpm consistently. Continue with the purging process, if the die remains on the extruder, until a total change in the extrudate from the processing resin to the purging resin is noticed.
Once the die has purged completely, the screw's rotation can be completely stopped so that the die on the extruder or the end cap can be removed, exposing the end of the screw. The screw can then be restarted and rotated to 10 rpm, allowing the remaining HDPE purging resin to be pumped out of the rest of the screw.
Removing the Screw and Barrel
Once the remaining HDPE is pumped out of the screw, you will need to remove the screw. After removing the rotary union, hoses and piping from the back of the extruder, you can attach the screw extractor mechanism to the extruder gearbox. The screw can then be pressed down most of the way down the barrel by the screw extractor. After that, you will need to push out the screw about 4-5 turns to expose it for cleaning. You can use a copper wire brush and a copper putty knife to scrape and remove the excess HDPE from the screw. Repeat this process of turning the screw outward and removing the HDPE until the entire screw is exposed and all the large amounts of HDPE is removed. For removing the HDPE residue after that, you can sprinkle stearic acid on the root of the hot screw and scrub using brass gauze, which also polishes it. A final swipe from a cotton cloth will complete the cleaning process of the screw.
To clean the barrel, you can coat the bore of the plasticizing barrel in stearic acid and use a round wire brush wrapped in copper gauze (which can also be covered in stearic acid) to push into the bore. Rotate this brush with an electric drill until it can easily move in and out of the barrel bore. You might need to add more stearic acid before you're done. Once that step is complete, a bundle of cotton rags can be used to swab the length of the inside of the barrel bore and remove and of the HDPE or stearic acid residue. Once you have swabbed the barrel bore completely clean with the bundle of rags, the process is finished.