Plastic processors often clean screws wrongly, causing serious and expensive damage. A common tool used as part of the cleaning procedure is an acetylene torch that unfortunately, affects the metallurgical properties of the base metal of the screw and closely machined screw tolerances. During manufacturing of tool-steel screws, the base metal is heat treated to increase its hardness. If heat from an acetylene torch is used to remove plastic from the root of the screw, it will undo the annealing of the base metal, reduce the yield strength of the steel at that point and increase the likelihood that the screw will break because of torque.
If the area where the torch was applied turns a permanent blue, it means heat has caused metallurgical changes to the base metal and reduced the wear resistance in that area of the screw. Occasionally, torching has caused delamination of the metal wear coating from the base metal of the screw. Heat from an acetylene torch also causes the metal to expand on the side of the screw where it is applied, which in turn causes the screw to bend. Another big source of damage to screws during cleaning is using steel screw drivers, scrapers or pries bars to remove plastic.
Tools and cleaning materials for "pristine" cleaning of a screw and barrel are few and simple: high-heat gloves, safety glasses, a brass putty knife, brass wire brush, copper gauze, stearic acid flakes, an electric drill, cotton rags, a round brass wire brush about the ID of the barrel that is mounted on a rod as long as the barrel.
A more efficient alternative to blow torching is to purge and clean the screw immediately after use, while it is still evenly heated from production. In high-pressure applications like wire and cable and blown film, commercial purging compounds can be used. In low-pressure applications, a fractional-melt (0.35-MI) HDPE may be sufficient as a purging material instead.
For larger extruders, it may be more practical and economical to use a commercial purging compound for product changeovers, without pulling the screw. The first step in purging the screw is to close off the flow of the polymer being processed, by closing the slide gate at the bottom of the hopper. Screw speed needs to be reduced to 15-25 rpm and run at this speed until melt stops flowing from the end of the die. All barrel zones should be set to about 200°C. Once the barrel zones have reached that temperature, purging can begin. Depending on the type of extrusion process, it may be necessary to remove the die or head tooling to reduce risk of over-pressurizing the end of the extruder. The screw should be turning at 15 to 20 rpm. In a low-pressure die application, the die can remain on the extruder during purging until there is a complete change from the processing material to the purging material.
Once the die has been completely purged, stop the screw so that the die can be removed and the end of the screw exposed. Once the die is removed, the screw can be restarted and run at about 10 rpm to allow the remaining purge material to be pumped out.
Once the purging material has stopped extruding from the screw, the screw needs to be removed from the machine. For an extruder with a screw cooling system, this apparatus of hoses, rotary union, and piping needs to be removed before the screw extractor mechanism can be attached to the extruder gearbox. Using the screw extractor, the screw should now be pushed forward until 4 or 5 turns of the screw are exposed for cleaning. Clean the purge material from the channels of the screw using the brass putty knife and brass wire brush. When the purge material has been removed from the exposed portion of the screw, push another 4 to 5 turns of the screw forward using the screw extractor, and continue the cleaning process.
The screw can eventually be pushed down most of the length of the barrel. When the large amounts of purge or HDPE have been removed with the brass putty knife and wire brush, stearic acid flakes need to be sprinkled onto the root of the hot screw. The copper gauze is to be used to remove any remaining residue. After the entire screw has been polished with copper gauze, a final clean-up to "pristine" condition needs to be done using a soft cotton rag to ensure that there is no contamination in the next production run. Once the screw is completely cleaned, it can be set aside until the barrel has been cleaned, or placed on a screw rack. If the screw is stored, it should be sprayed and wiped down with light oil, such as WD-40 or PB Blaster to prevent rusting.
Chrome-plated screws are less likely to rust during storage and don't need oiling.
Cleaning the barrel is much easier than cleaning the screw, but just as important. With barrel temperatures still set at 200°C, the barrel is ready to clean. A wire brush is to be wrapped with copper gauze. Before inserting the brush and gauze assembly into the bore of the barrel, throw in a handful of stearic acid. Stearic acid can be sprinkled over the copper gauze before pushing the brush/gauze assembly into the barrel. Once the brush/gauze assembly is inside the barrel, the electric drill is to be used to rotate it while moving it in and out until it moves easily. Additional stearic acid may be needed to be added before the bore is thoroughly cleaned.
When the brush/gauze assembly is removed from the barrel, push a bundle of cotton rags back and forth inside the length of the barrel to remove any purge or stearic acid residue. When the rags have been passed back and forth several times and return totally clean, barrel cleaning is complete. The entire screw and barrel assembly are pristine and ready for the next production run.
Many purge materials are commercially available. It is the processor's job to identify the appropriate purge to use with the resin being processed. Purge materials work by one of four mechanisms:
• Mechanical purge
• Abrasive mineral filled material
• Chemical purge to break down resins and contaminants
• Hard resin filled with surfactants
Mechanical purges are stiff materials, normally polyethylene (PE) based, containing cleaning and release agents. Fractional melt flow high density polyethylene (HDPE) functions very well as a purge material over a wide temperature range. Low viscosity abrasive mineral or glass filled materials will force most resins out of an extruder while scouring the screw, barrel, and die. Cast acrylic is an abrasive purge, as it does not completely melt in the extruder. Due to the high melt viscosity of cast acrylic, it is better to remove the die prior to purging the screw. Typical amounts required are 5 to 10 lbs/inch of screw diameter. Once cast acrylic is in the barrel, it has to be purged out or the screw pulled and mechanically cleaned. The third purging material uses a surfactant that penetrates and loosens residue on the screw, barrel, and die, dispersing it in the melt. The surfactant is mixed with a melt flow resin that is 0.1 to 0.3 times that of the original melt to provide maximum purging effectiveness. The fourth material is a chemical purge that attacks the material left in the barrel.
This may be through plastication of the resin in the barrel, lowering the viscosity and making it easier to force out of the barrel, or the additive may actually cause polymer depolymerization into lower-molecular-weight components. Mechanical purging commonly uses a fractional melt flow HDPE. The stiff material pushes the resin being purged out in front of the HDPE. Fractional melt flow HDPE is cheap, works well, and has a wide processing temperature range. After the resin being purged is out of the extruder, the barrel temperatures can be lowered and more HDPE added to remove the earlier purge material.
As the screw is being pulled from the extruder, any residual HDPE can be brushed off the screw with a brass wire brush. Alternatively an electric or air-powered brass wire brush can be used in cleaning. Several commercially available materials are produced as purge compounds based on mechanical purging
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