Extrusion screw feeding/bridging

Please let me know how can a Nitrided screw surface for PVC extrusion changes its color after one single use of 8 hours.

I used Scotch Brite and it was fine, but in your case it may actually be too harsh. Let us know.

Thanks,
Monster Galaxy

I agree on the screw cleaning portion. Scotch Brite may be tad too harsh for them.
Regards,
http://www.supplyways.com

Feed issues on a 1" machine can be daunting in general, especially at lower RPM. Different manufacturer's feed throat designs work better than others.

I would think the cleaning methodology is somewhat excessive. A good pyrolysis oven will usually oxidize at about 900°. Most screws are usually 4140 with the flights flame hardened, and 1200° is in the area of stress relieving/tempering for the metal.

I would stay away from wire wheels and scotch brite as well. If you must go for the polished look, coratex with a clean rag may suffice. Polymer bead cleaning (compressed air bead blasting) works as well.

Sometimes a deeper feed section may help gain a little better solids conveyance.

Re Ron R,
I'm not surprised at Surlyn, more surpised that nylon sticks (unless it's not the usual formulations of nylon 6 or 66).
Screw cooling works if the problem is sticking to the screw, rear barrel temp (RBT) reduction may work if the problem is self-lubrication and slip of pellets at barrel wall.
These are two separate issues. Slippery pellets like nylons and HDPE may benefit from lower rear barrel temps (but not too low or the same thing will happen and you'll need to raise RBT).
Screw cooling will help for sticky stuff like Surlyn (really a functionalized polyethylene), but many small extruder screws aren't bored. With these, too, particles may be too big and thus cause erratic feed, as well.
Better throat cooling will help if the feed is bridging in the throat/hopper region.

ALG, Forum Moderator

All the extruders we use with nylon or surlyn have had feed issues. The most surefire way to prevent this is to install screw cooling. This will prevent the pellets from sticking to the screw and will also reduce heat transfer when the screw is stopped or idled. Most screws are bored for this if not reducing the feed zone heats is the best option.

Tombayford, thank you for your comment. You explained it very well. I couldn't have done better myself.
Allan

Extrusion is a transfer of energy from the motor—and sometimes the heaters—to the cool plastic, thus converting it from a solid to a melt. The entering feed is cooler than the barrel and screw surfaces in the feed zone. The barrel surface in the feedzone, however, is almost always above the melting range of the plastic. It is cooled by contact with the entering particles, but is kept hot by the conduction of heat backward from the hot front end, as well as by controlled barrel heating. Even when the front end is kept hot by viscous friction and no barrel heat input is needed, the rear heaters may need to be on. The most important exception is the grooved -feed barrel, used almost exclusively for HDPE.
The screw root surface is also cooled by the feed and is insulated from the barrel wall by the plastic feed particles (and the air between them). If the screw suddenly stops, the feed stops too, and the screw surface gets hotter in the feedzone as heat travels backward from the hotter front end. This may cause sticking of the particles to the root, or bridging.
The above is an important principle to keep in mind regarding extrusion. It should help save money, yield higher quality products, and use equipment more efficiently for the benefit of writing jobs.

Peter, you didn’t mention the grade or softness of the material that you are using.

As to cleaning, the best advice is to use a good purge material to keep the manual cleaning to a minimum. Wire wheel brushes, even brass, will cause more wear than purging. Scotchbright is great, but again, depending on the grade, you could be damaging the screw. I might be concerned with the burn off oven causing some warping of the feed screw; you might want to check the top of the flights for wear indicating a slight twist or bend.

Typically, sticking to the screw in the feed section is caused by too high a rear zone temperature setting, or as Alan mentioned, “sitting” at elevated temperatures. If you must run the rear zone high then follow his suggestion for starve feeding at startup and run the barrel dry before stopping the screw.

A good HDPE purge followed by a 90 Shore A durometer or higher PVC will push out the Pebax and leave your screw almost perfectly clean. As will most purchased purging compounds, though they do tend to have a strong odor.

Hope that helps

~L

Typical cause of sticking to screw root is stopping screw while full. Heat conducts back from output end and root gets hotter, no material moves.
Other contributory causes include excessive preheating (entering particle surfaces at maximum adhesive temp), rough root surface, and unusually adhesive materials such as some olefin copolymers. These causes indicate their own solutions.
Cooling the screw root works, but is rare with small screws as the central hole needed weakens the screw and risks screw breakage.
Starve-feed extrusion may help, too, in which the feed is dropped onto the screw which is turning fast enough to take it away, but that means poorer mixing further downstream.
Particle size is a separate problem with small extruders, as too-big particles make for irregular feed, but that is typically overcome with a gear pump.
I am seldom worried about wear unless I can show that output per rpm is declining and I have to run much faster (more heat development) to catch up. However, with a small extruder, that may just mean the heaters are on less. Monitor melt temp to be sure.
ALG, Forum Moderator