In a column I wrote for the July 2011 issue, I discussed the value of conducting “forensic” examination of worn screws when they are removed for clues to design flaws. Severe wear areas are one of the most important things to look for when a screw is pulled, as they are often an indication of a design problem.
One of the most common causes of high wear is the effect of “wedging.” Wedging occurs when the transport rate of the screw exceeds the melting rate in the melting section of the screw. The melting section is the transition area between the feed and metering sections. This would apply to a conventional screw or a barrier screw.
Most extrusion screws are machined with what is called an “involute taper” in the melting section. An involute taper means the channel is a flat spiral rather than a true taper (or conical) spiral. As the polymer moves forward in the tapered channel, the melting rate needs to be great enough to keep up with the screw’s mass-flow rate or the channel will be completely plugged with solid polymer at some point. Since the screw channel is a spiral tapered ribbon, this plug will look like a wedge wound around a shaft.
It’s not hard to visualize that when the channel plugs with solids, it happens at a specific location and not along the entire channel. This causes the pressure to build rapidly at the point of the wedge, resulting in a force that pushes the screw to the opposite side of the barrel. This wedge is powered by the full force of the drive motor, which has enormous torque due to the gear reduction. Therefore, the force generated by the wedge is also enormous. The side force is so great that it will cause galling of the screw flights against the barrel on the opposite side of the screw. This results in what has been termed “adhesive failure” of the screw’s hard surfacing as it welds to the barrel under extreme pressure, and then is torn off as the screw rotates.
Crystalline polymers exhibit much more tendency to “wedge” than amorphous polymers. This is primarily because crystalline polymers do not soften appreciably until they reach their melting temperature. Also, they require additional energy at the melting temperature to break down the partially crystalline structure, meaning the temperature has to rise well above the actual melting point before they are able to absorb enough energy to break down the crystallinity and flow freely. This additional energy requirement is called the heat of fusion.
Amorphous polymers do not have any crystalline structure and simply soften gradually as the temperature increases. Consequently they are often soft enough to flow away from the area of a plug, reducing the pressure buildup, the resulting side force, and screw wear caused by adhesive failure or galling.
Some screws are made with what are called conical tapers, which tend to plug in a ring around the screw rather than in a more localized spot on one side. This still results in high pressure at the point of the plug, but since it tends to surround the screw, the pressure tends to keep the screw centered in the barrel, thereby avoiding the high side force and adhesive wear.
So why aren’t all screws made with conical tapers? The answer is that the machining cost is much higher, and the wedging effect can usually be mitigated by changes in the design of the screw, changes in operating conditions, or improved screw/barrel materials that resist galling. However the conical taper confirms the effect of the unbalanced wedge by distributing the localized pressure buildup. While more expensive, it is used on many very large screws due to their initial cost and cost of replacement.
The pressures that can be developed almost instantaneously by wedging are somewhat amazing. That’s because crystalline polymers are essentially incompressible and have a very high modulus right up to the point where they melt and begin to flow. Analysis of the worn areas on screws using simple beam-bending analysis and ignoring the strengthening effect of the screw flight indicates that the forces often exceed 100,000 lb on larger screws. This is enough force to bend the screw over a relatively short span, forcing it into the barrel even after the screw has worn many times the normal clearance between the screw and barrel. Depending on the area of the wedge it is likely that pressure builds to as much as 20,000 psi or even more.
The pressure is relieved almost as quickly as it builds. The high pressure accelerates the melt film elimination and therefore melting increases, allowing the plug to advance. However, it may plug again in a short distance, repeating the effect. This wear is usually found somewhere from the middle to the end of the compression section and often extends into the metering section because of the bending of the screw. Interestingly it can also be identified by a burr on the trailing side of the flight caused by the galling screw material being dragged across the flight as the screw rotates.
“Forensic” examination of screws can accurately identify wedging so that a cost-effective solution can be developed. Don’t discard or send screws in for rebuilding without a thorough examination.