Feed Screws and Barrels

Proper Screw Wear Evaluation

Proper Screw Wear Evaluation

The first step to optimizing throughput, increasing screw life and saving money.

Proper inspection and wear evaluation of a plasticizing screw can be of great benefit.

Most people will measure the wear on the flight tops to determine if there is sufficient

wear to adversely affect screw performance. If so, they will either have the screw rebuilt

or replaced, however, there is much more to consider. It is correct to look for flight top

wear, but where are the tops worn most? What kind of wear is evident? Is the surface of

the flight tops uniform all the way down the flighted length? Is there a burr on the corners

of the flight tops? If so, is the burr all the way down the flight tops or on just a few of

them? Is the burr the same all the way around the circumference of the flight? Is there

damage on the body of the screw? Is there erosion on the face or body of the screw? Let’s

examine some of these conditions and see what they reveal. With proper inspection and

screw wear evaluation, adjustments can be made to eliminate or reduce some of this

wear, increase screw life and produce more parts of better quality.

First, let’s take a look at flight top wear. Regardless of whether it’s an aluminum

or titanium screw, we all know the tops are going to eventually wear, but are the flight

tops worn evenly down the entire flighted length? Years ago, flight tops would often

wear more towards the discharge end of the screw. These days, however, with all the

different engineered resins that are run, we see more wear on the flight tops back in the

feed and transition sections of the screw. If this is the case, it may be time to make a

change or two. The first place to look is the heat profile you have been running. You will

likely see this area run with much less wear if you wet the material out sooner by running

a reverse profile. You may also want to examine your screw profile. The feed section of

your screw may be too short, causing the flight tops to be forced into the barrel wall by

the non-wetted out material leaving the feed section and going up the transition section.

This side loading is what can create burrs on the flight tops.

If you have galling or burrs on the flight tops for most of the flighted

length, you may want to look at your purging methods or start up and shut down

methods. Running the machine with no material along the flighted length of the screw

will allow metal to metal contact of the barrel wall and the flight tops. This contact can

cause the flight tops to get hot and expand reducing clearance. This reduction in

clearance may in turn cause galling of the flight tops and even the barrel wall. Whenever

you need to run the machine void of material, you should always use the lowest possible

screw RPM.

If you have a burr on the flight tops or galling that isn’t totally around the

circumference of the flight tops, there are two things to consider. First, the screw may be

bent. A bent screw will contact the barrel wall on the high side of the bend, thus wearing

or galling the tops on one side only. Second, and probably more often the culprit, the

screw could have been manufactured improperly or poorly rebuilt. Before the flight tops

are ground to final size, either in the manufacturing or rebuilding process, the screw

should be straightened to the body diameter. This will insure that the screw body and

flight tops are on the same centerline. If you have a depth micrometer, you can check for

this by measuring the height of the flights all the way around their circumference of the

screw. That measurement should be the same within .002 of an inch all the way around

the screw. Be sure you check the flights in the feed section and metering sections of the

screw as well. In the transition section, the body diameter increases with every

revolution, so you cannot easily check body run out here.

Another important evaluation is the condition of the surface of the screw. Is it

extremely smooth and polished looking, as if it were new? If so, this would indicate that

the surface of the screw is compatible with the material you are running and how you are

running it. However, if the screw has root erosion or if it is pitted in the flow path, some

adjustments may be in order. Pitting often results from a reaction of the material the

screw is manufactured from or coated with to the material in process. A nitrided screw

surface, for example, can negatively react to material that may be somewhat acidic. This

may result in specks, splay, etc. in your product. The best solution in this case may be to

reduce screw RPM or anything you can do to minimize the time material sets dormant in

the barrel. When material isn’t moving in the barrel, it has more time to affect the metals

it encompasses. The more you keep the material moving in the barrel, the fewer problems

you will encounter.

In the case of root erosion, you may need to elevate your rear temperatures or again,

adjust your RPM’s. You could also have the screw chrome plated, but this adjustment

will require removal of the screw and out of shop time. Root erosion is most often seen

with chrome-plated screws as the plating is usually only a 001. or 002. of an inch thick. It

also can appear on nitrided screws. It is almost never seen on solid tool steel screws.

Increasing your temperatures in the zones in front of the eroded area can often alleviate

this issue. This area is typically at the end of the feed zone and into the transition section

of the screw. Again, wetting the material out before this eroded area will make the

material more lubricious and less abrasive. Reducing RPM’s if possible, will also help.

Another option would be to strip the chrome from the screw and have the screw nitrided.

Nitriding is a much deeper process and usually a better choice of surface treatment in an

abrasive environment; however, this will require some out of house time for the screw.

We recommend solid tool steel screws whenever possible. They may cost more money

initially, but the pay back is many times over that of a surface-coated screw

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Concor Tool and Machine, 9665N Concor Rd., Hayward, WI, 54843

715-435-3756

www.concortool.com