Cleaning Screws and Barrels
Contamination in a high output process like injection molding can create hundreds or even thousands of bad parts before anyone even notices, putting just-in-time orders in jeopardy, potentially shutting down a customer’s line. And once contamination begins, it is almost impossible to stop without aggressive intervention – namely, either a screw pull and clean, or use of a commercial purging compound. And, an unplanned downtime event like pulling a screw due to contamination, will wreak havoc on operations – labor, planning, logistics, material variances, etc...
The best way to combat contamination is not to make it in the first place. Using a purging agent at regular intervals will keep contamination from beginning, and it typically only takes 1-3 barrels full of purging compound to prevent or correct contamination. Most operations would much rather have a short planned purging related downtime event, than an unplanned (or even planned for that matter) screw pull.
When using a mechanical purge, the following procedures will generally rid a screw and barrel of contamination. To start, an operator should keep the barrel full of material and slowly add purging compound right behind. Once the purging agent has displaced the production resin, increase back pressure to the maximum safe amount. Do the same with screw speed while keeping the screw in the forward position. Continue purging until the purge pile is free of contamination. Using high pressure and velocity are critical to the process, because they promote scrubbing action inside the barrel and on the screw surfaces. If the contaminant is not yet removed, a good rule of thumb is to add another half barrel full of purge and execute a series of high velocity air shots. This should displace any contaminants that might be hung up in the check ring and nozzle areas. Once the purge pile is clean, the purging compound should immediately be displaced by the production resin. As soon as the purge pile is free of the purging compound, resume production.
When using chemical purges, it is generally best to follow the above procedure, with the exception of soak time. The operator should chase the production resin with the chemical purging agent, until purging compound is observed in the purge pile. Then, stop the screw and let the material soak – typical soak times are 10-15 minutes, depending on the supplier and grade. This added soak time allows the purge to expand into negative flow areas. Some suppliers also recommend varying the screw speed to provide so mechanical agitation.
In most cases, material and color changes can also be dramatically shortened using a purging compound. Many companies still use the old method of changing color or material – just push the next material behind it, and keep running until good product is produced. However, operators using an effective commercial purge to eliminate residue of the first material or color will gain a competitive advantage due to greatly reduced changeover times and material reductions. Most purging compounds are designed to remove stubborn carbon and color deposits on the root and flights of a screw, as well as in dead spots within the screw/barrel assembly that virgin resin simply cannot address. Just imagine a standard 1 hour color change reduced to 10 or 15 minutes using a purging compound! The reduced scrap and downtime will typically more than pay for the price of the purge. Just contact any purging compound supplier – their technical sales reps should be able to answer your technical questions as well as assist you with a cost anlysis.