Screw and Barrel Wear in Plastic Processing: Causes, Types, and Predictive Maintenance

In today’s highly competitive global market, plastics processors can gain an advantage by incorporating relatively simple practices into their operations. In injection molding, extrusion, or blow molding, the plasticizing unit is critical to production efficiency and final product quality. The main components of the plasticizing unit—the screw and barrel—should be designed to provide maximum melting efficiency at the original clearance between the barrel bore and the screw flight OD. These clearances are typically small (0.001 in. per side per inch of diameter) to achieve good heat transfer and shear rate.

Once the screw begins rotating inside the barrel, wear begins. Wear may occur on the screw flights, the barrel surface, or both. The harder the material being processed—whether abrasive fillers in the polymer or metal particles “mixed in” that do not belong in the melt stream—the faster wear occurs and the more the original clearance increases. As clearance increases, melting efficiency decreases. When melting efficiency at the original settings drops, many operators tend to compensate by increasing screw speed, adjusting barrel temperatures, increasing back pressure, or some combination of these adjustments.

Operators make adjustments to avoid unmelted material, higher melt temperatures, or higher scrap rates. Often these changes are gradual and go unnoticed until they significantly affect part quality or productivity (see Figure 1).

Figure 1 Once the screw begins rotating inside the barrel, wear begins on the screw flights, the barrel surface, or both.

When plasticizing rate, productivity (whether measured in pounds per hour or parts), or scrap rate reaches unacceptable levels, the process engineer may begin looking for the cause. Unfortunately, there may be no quick fix at that point unless a spare screw or barrel is available. If replacement parts are on hand, the changeover may take about a day. If not, the line will have to be shut down or run at reduced efficiency until replacement components are found and installed.

Types and Causes of Wear

So how can this dilemma be avoided? Let’s first identify three types of wear.

1. Adhesive wear. This occurs when metal surfaces (the screw flight surfaces and the barrel bore) contact each other. Extremely high stresses are generated when surfaces briefly touch, bond, and then break apart. Evidence of adhesive wear may be burrs or “rollover” on the trailing edge of the flight, or hard-facing material on the flight top being scratched or missing. The barrel bore at the location of screw damage should also be inspected for scratches or wear.

2. Abrasive wear. This is probably the most common type of wear in plastics machinery. It can be minimal when processing unfilled or low-viscosity polymers. As particles are conveyed down the screw and compressed against the barrel wall, shearing action causes some degree of wear on both the screw flights and the barrel wall. Abrasive wear becomes severe when processing resins containing fillers, additives, or contaminants from regrind, or even when processing high-viscosity polymers. Hard materials tend to scrape material away from softer surfaces. Therefore, hard facing in the clearance area typically uses wear-resistant alloys. Glass- and mineral-filled resins can also cause screw root wear, usually at the rear of the channel, before the additives are wetted and mixed with the molten plastic.

3. Corrosive wear. Corrosion is the gradual destruction of material by chemical attack. Most atoms on the same metal surface become oxidized, damaging the entire surface. Damage to the metal surface actually makes the surface more susceptible to mechanical wear mechanisms. Corrosion is most common when processing PVC or fluoropolymer families and can severely attack components with a high-iron base matrix.

See the table below for wear mechanisms in the plasticizing unit, and let’s take a deeper look at some of the main causes of wear.

• Straightness, concentricity, and alignment: When preparing to install a new or rebuilt screw, carefully check the straightness and concentricity of the screw. If the old screw is available, it is best to compare overall length, shank, and drive configuration. Next, if the screw is to be installed in a new barrel (or the existing barrel is available and clean), slide the screw into the barrel and rotate it by hand (if possible) to ensure straightness and concentricity. The screw should rotate freely in the barrel without interference or binding. On extruders and most blow molding machines, the machine should be aligned before running new parts. Plastics processing machines have been aligned for more than 220 years, and during that time we have seen very few machines that did not require alignment when checked.

• Poor screw design: When the capacity of the melting section of the screw is less than the amount of unmelted material conveyed by the feed section, the channel can become blocked with unmelted material. The resulting pressure actually forces the screw against the barrel on the opposite side, causing wear and damage to the screw flights and the barrel bore surface. This condition is commonly called solids wedging (see Figure 2). Solids wedging often occurs when a screw originally designed for a specific resin is run on a different polymer with different melting rates.

Figure 2 When the capacity of the melting section of the screw is less than the amount of unmelted material conveyed by the feed section, the channel can become blocked with unmelted material. The resulting pressure actually forces the screw against the barrel on the opposite side, causing wear and damage to the screw flights and the barrel bore surface. This condition is commonly called solids wedging.

• Incompatible metals: Generally, softer materials wear more easily when in contact with harder materials. Therefore, it is important to consider the screw material, the hard-facing material on the screw flights, and their compatibility with the barrel liner material. Alloys with lower tungsten carbide content may be easier to apply and finish, but not surprisingly, they offer poorer wear resistance. They are also more prone to wear, especially when run against some of the hardest liner materials. Today’s bimetallic barrels offer excellent wear resistance. By using centrifugally cast alloys with high tungsten carbide content, barrel manufacturers are able to provide products with long service life.

• Abrasive fillers, additives, and foreign objects: As polymer and plastic applications become more specialized and demanding, abrasive fillers and additives require improved wear and corrosion protection. Resins containing glass fibers, minerals, flame retardants, stabilizers, and other additives require consideration of their abrasive and corrosive wear effects during extrusion or molding. Some of the most aggressive fillers can reduce screw/barrel service life by 50% or more. If the screw and barrel are manufactured with appropriate protection, wear life can be extended and abrasive wear greatly reduced.

Another common cause of screw and barrel wear and damage is foreign objects—everything from screwdrivers to grinder blades to nuts and bolts seems to find its way into the hopper of plastics machinery. If not caught by screens or magnets, these materials can cause severe damage to the screw and barrel. Hard-facing alloys that come loose or delaminate from the flight area when the alloy is not properly bonded, or that have already cracked and loosened, can also cause problems as they travel down the screw.

Fine-Tuning and Diminishing Returns

When screw and barrel wear or damage occurs, process engineers typically make small adjustments to screw speed, barrel temperature, or back pressure. These incremental changes usually consume more energy and affect part quality. The initial effect may be small, but at some point these minor “adjustments” will have a significant impact on production. This may be increased cycle time, poor mixing, or higher scrap rates. This point can be called the “point of diminishing returns.”

Ideally, we want to identify this point—the limit at which machine adjustments can successfully minimize the loss of productivity or quality—and be able to predict when the screw/barrel should be rebuilt or replaced. Whether this point is defined in machine hours, parts produced, or scrap rate, maintenance personnel can plan to replace the screw/barrel in the future and run to that point. This will optimize the process while reducing costs. In addition, if we can identify this point, we can supply replacement parts and schedule installation without interrupting production.

In the future, through digitalization, we hope to be able to use sensors to accurately measure screw and barrel wear and then predict when parts should be rebuilt or replaced to maintain optimum productivity. Until that technology is perfected, plastics processors can chart performance related to screw and barrel wear by linking production efficiency to screw and barrel measurements (Figure 3).

Figure 3 When the capacity of the melting section of the screw is less than the amount of unmelted material conveyed by the feed section, the channel can become blocked with unmelted material. The resulting pressure actually forces the screw against the barrel on the opposite side, causing wear and damage to the screw flights and the barrel bore surface. This condition is commonly called solids wedging.

Unfortunately, especially on large machines, regularly measuring the screw and barrel may be impractical. However, if adjustments to operating parameters are recorded and charted based on changes in productivity, reference points can be established where screw and barrel wear is indicated by its relationship to output, controller setpoints, energy consumption, and final product quality.

With good record-keeping, downtime for screw and barrel replacement can be predicted with relative accuracy. Component life can be optimized, and “run-to-failure” and unexpected downtime can be avoided.

When installing a screw or barrel:

• The screw should be designed for the specific application.

• The screw and barrel should be manufactured from compatible alloys.

• The screw and barrel should be made from the wear-resistant materials best suited to the application.

• Before installation, the dimensional accuracy of the screw and barrel should be carefully checked.

• If possible, the alignment of the screw and barrel should be checked.

Proper material selection, precision manufacturing, and correct installation are therefore essential to maximizing screw and barrel life and maintaining stable production efficiency. Choosing an experienced manufacturing partner is equally important.

About SUPERIOR MACHINERY

Established in 1992, SUPERIOR MACHINERY has been engaged in the rubber and plastic molding machinery industry for more than 30 years. With an increasing market share, our company has established itself in the field of high-quality fully hardened screws and bimetallic barrels that are highly resistant to wear, corrosion, high pressure, and high speed. We always look forward to supporting your business with our reliable products!