Contamination in a continuous process like extrusion takes a profitable piece of business to unprofitable in no time at all. And, once contamination begins, it’s almost impossible to stop without aggressive intervention – namely, either a screw pull or use of a purging compound.
The following are four tips for effectively purging your extruders:
1. Clean The Screw For Extruders
An unplanned machine downtime event, like pulling a screw due to contamination, wreaks havoc on operations, from labor and planning to logistics and unhappy customers.
The best way to combat color and carbon contamination is to avoid letting it build up in the first place. Using a purge compound at regular intervals prevents contamination from beginning. And, it typically only takes one to three barrels capacities of purging compound to prevent contamination. Most operations would much rather have planned purging-related downtime than an unplanned (or even planned for that matter) screw pull.
2. Conduct A Screw Pull Of Extruders
If a screw pull is mandatory or required, using a purging compound to clean and then pull the screw is the safest, most efficient way to get the job done. Using a glass-filled grade of purging compound speeds up the process even further. A cleaned screw is easier to pull, and the result is significant labor savings in terms of manually cleaning the screw when it comes out – some purging compounds just peel right off.
A good technical sales representative should be able to assist you in determining the best temperature range and purging techniques for a specific grade when pulling a screw. It is important to request specific procedures for this from your purging compound supplier.
3. Prevent Contamination With Sealing At Machine Shutdowns
To prevent contamination or deposit formation over a weekend, or really during any extended machine shutdown, use a purging compound to clean and seal your extruders.
To seal an extruder with a purging compound, first purge out the production resin, follow the basic purging instructions. If using a glass-filled grade for maximum cleaning, displace it with an un-filled grade for safe sealing. Empty the barrel.
Once the extruder is clean, feed the recommended amount of unfilled grade of purging compound at a slow screw speed. Then, shut the heats off, leaving the screw full. When ready to start back up, turn the heats on, and when at the recommended temperature extrude the purging compound with the next production resin.
4. Follow High-Temp Extrusion Procedures
At ASACLEAN, we have three high-temperature grades (SX, PF, PX2). Purging procedures are grade specific, but typically come with the following high temperature purging instructions: sealing, pre-maintenance (screw pull) and transitions from high-heat resins to other resins.
As an example, the following are PF Grade extrusion purging procedures:
1. Upon completion of the production run, extrude the high heat resin from the machine, emptying the barrel. Purging must begin as soon as possible. Thoroughly clean the hopper and feed throat.
2. Feed about one system capacity (barrel and die) of PF Grade into the hopper. Start purging at a slow screw speed. After the PF Grade starts coming out of the extruder, increase the screw speed to a moderate level. Several times during the purge, stop the screw and allow the PF Grade to sit for about 30 seconds, then resume screw rotation. If resin contamination is still visible in the purge pile, repeat this step until clean.
3. After the machine is clean, and you’ve extruded the PF Grade from the machine, empty the barrel, as far as possible, and start feeding the next resin to be processed. Displace the PF Grade with the next resin following the same purging procedures above.
4. After all of the PF Grade has been displaced, begin production using normal processing conditions for the resin.
Please note: If processing temperatures are at or near the maximum recommended temperature for PF Grade, a low-to-medium screw speed is recommended. Screw speed can be increased at lower temperatures within the recommended range to maximize cleaning.
Taking apart your extruder to clean it requires extensive manual labor and wasted production time. By following the above purging procedures, and using the right purging compound for the job, you could minimize cleaning time to less than one hour a per day.
Ready to learn more about proven methods for achieving effective purges to improve your bottom line?
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