Machinery & Equipment: Maintaining the Edge

For many processors, screws and barrels are much like air — necessary, but not worth the bother of thinking about. Yet, as Ralph Torning, vice president sales and marketing, Wexco Corp. observes, even with a properly specified and installed screw and barrel configuration, you will be faced with this central fact of life: The performance of your plasticating barrel and screw will deteriorate with use and time. Proper monitoring and maintenance of these critical internal components can reduce the rate of this deterioration.

WHEN GOOD SCREWS GO BAD

How important is screw wear and maintenance to your operation?

“A screw’s performance can be the determining factor in whether a process is profitable or unprofitable,” says Jim Frankland, vice-president, Progress Precision Inc., noting deteriorating screw performance over time gradually results in an increasing loss of productivity, as well as declining quality. One of the main reasons for this loss is that as a screw O.D. wears, output declines for a given screw speed, and melting rate decreases due to poorer wiping of the barrel. Raising screw speed can compensate for the output loss, but causes the temperature to rise, therefore lengthening cycle time.

Chris Barr, field service manager, Glycon Corporation, recommends tracking process changes in order to establish a baseline against which any output loss can be measured. Processing times, temperatures, pressures and scrap rates should be accurately logged once a shift as part of a screw/barrel maintenance program. If a production monitoring system automatically records process data, a report should be created daily highlighting key wear-related parameters for easy monitoring.

When process data indicates an output loss of 5 to 10%, screw and barrel wear is a high possibility. In order to confirm wear, a physical inspection of the screw and barrel should be performed, Barr advises. Once the screw is pulled, a flight micrometer or gauge block should be used to measure the flight diameter at every turn. Barrel I.D. should be measured with a bore gauge. The readings can be compared to the OEM print specifications, but a general rule-of-thumb is that the screw diameter should equal the barrel I.D. times 0.998, with a tolerance of +/-0.002 in.

A visual inspection of the screw should also be performed to check for cracks in the root or O.D., heavy wear or score marks, chipped flights and burn or chips on the screw drive.

The decision to replace or refurbish a worn screw is primarily a financial one that balances the cost of lost production, labor and new equipment versus the cost of declining production and quality. Frankland suggests the use of the cost calculation sheet at left (Figure A) as an aid to help processors determine when it is time to remove the screw for repair or replacement — i.e. when it costs more to leave the screw in than to replace it.

AN OUNCE OF PREVENTION

As with other types of machinery, some of the most important factors affecting the performance and lifetime of your screw and barrel are the decisions you make before the equipment is installed on your machine.

One critical prerequisite for good performance and long wear life, says Wexco’s Torning, is barrel straightness. While processors assume the straightness of the barrel is a given, barrel makers may apply different interpretations of the requirement for straightness. Most manufacturers abide by the standards set by the Society for Plastics Industry, however some manufacturers adhere to their own tighter straightness tolerances. If you’re retro-fitting, research to ensure you purchase a high-performance barrel from a reputable, experienced barrel manufacturer.

Selection of the bore lining material is another critical decision. Nitriding is the most common type of barrel bore treatment. Nitriding is effective when the resins processed are only mildly abrasive and not corrosive. Many machine builders select nitrided barrels as standard, off-the-shelf equipment, however, high-performance requirements or more severe processing conditions may mandate lining materials that can exceed the protection provided by nitriding, says Torning.

An alternative to nitriding is bimetallic barrels. While more costly, bimetallic barrels feature a hard, anti-corrosive protective layer in the bore. The layer is applied using a “spun-cast” manufacturing process, involving high heat and centrifugal forces. Common bimetallic barrel types are iron/boron and nickel/cobalt. Wexco offers five different inlays or bore lining materials, which provide additional protection against a variety of corrosive or abrasive materials. While a selection chart (Table 1) can be an aid in choosing the barrel and inlay most appropriate for the type of materials being processed, it generally makes sense for a processor to discuss their specific application directly with the barrel manufacturer before placing an order.

Proper design of the feed screw plays an important role in reducing the amount of screw wear, notes Klaus Melisch, vice-president, P.E.M. Feed Screws. The melting characteristics of the particular resin and the expected pressures are the critical design-influencing factors. Melisch says the melting section should be optimized for the resin and application. A compression ratio that is too high, or a transition section that is too short can force the solid bed to the barrel wall and screw surface, creating abrasive wear on the screw root, in front of the active flight and on the barrel.

Choosing the right material for the construction of the screw is a key in extending its service life, according to Frankland. The base screw metal and facing materials need to be matched to the types of polymer systems to be extruded or injection molded, as well as to the material of the barrel.

About 90% of the plastic processing screws made in the U.S. each year are made from carbon steels, primarily 4140 steel. In general, both extrusion and injection screws made from 4140 steel are also made with a surface hardening of the wear surfaces.

In the group of widely-used commodity resins, vinyl polymers are the most corrosive. The corrosion is caused by the release of chlorine, which under high heat and pressure conditions in the barrel reacts with water to form hydrochloric acid. Chrome plating is commonly used to combat corrosion, although in some cases the extreme conditions generated by processing vinyl require multiple layers of chrome and nickel, or a solid stainless steel barrel. Chrome or nickel plating may also be needed for screws used to process fluoropolymer-based resins or resins containing halogenated flame retardants or abrasive fillers. Table 2 gives a list of common screw materials and their relative corrosion and wear resistance.

The screw material must also be compatible with the surface of the barrel. The tendency of materials to gall or weld is generally reduced by using dissimilar metals for the screw and barrel, and by increasing their relative hardness. As wear is inevitable regardless of metal type or hardness, one strategy employed to facilitate repair is to let the screw be sacrificial to the barrel. This is because it is easier to remove and change the screw than the barrel. This strategy sometimes necessitates using a material for the screw that provides less than optimum wear resistance.

Ensuring your screws and barrels are designed correctly for the resins you intend to run, and maintained within specified production parameters, will lengthen the life time of these vital components.

Figure A: Quick Estimate of Economics of Screw Replacement*

Note: The cost of the replacement screw is divided by a useful life of 6000 hours before wear starts to affect production, and labor to change screw is assumed available at no cost.

* Courtesy of Milacron Inc.

Table 1: Resin Material Wear Environments*

*Courtesy of Wexco Corporation

Table 2: Screws Material Wear Resistance*

*Courtesy of Milacron Inc.