In today's highly competitive global market, plastic processors can gain an advantage by incorporating relatively simple practices into their operations. In injection molding, extrusion or blow molding, the plasticizing device is the key to production efficiency and final product quality. The main components of the plasticizing device, namely the feeding screw and barrel, should be designed to provide maximum melting efficiency at the original gap between the inner diameter of the barrel wall and the outer diameter of the Screw Barrel . These gaps are usually small (0.001 inches per side, per inch diameter) to achieve good heat transfer and shear rates.

Once the screw starts to rotate in the barrel, wear begins. Wear may occur on the screw threads or the surface of the barrel, or both. The harder the processed material, whether it is the abrasive filler in the processed polymer or the "mixed" metal particles that are not part of the melt flow, the faster the abrasion and the increase in the original gap. As the gap increases, the melting efficiency will decrease. Due to the reduced melting efficiency at the original setting, many operators tend to compensate by increasing the screw speed, adjusting the barrel temperature, increasing the back pressure, or some combination of these adjustments.

Resin drying is done correctly
Welcome to this collection of content dedicated to resin drying, which contains tips, techniques, and best practices to help your materials handlers improve their skills in this critical and often misunderstood area.

Adjustments are operators trying to avoid longer cycles, unmelted materials, higher melting temperatures, or higher scrap rates. Usually, these changes are gradual and will not be noticed until they significantly affect part quality or productivity (see Figure 1).

The impact of screw wear on plastics
Once the screw starts to rotate in the barrel, the screw threads or the surface of the barrel or both will begin to wear. As the melting efficiency at the initial setting decreases, many operators tend to make adjustments to avoid longer cycles, unmelted materials, higher melting temperatures, or higher scrap rates. Usually these changes are gradual and will not be noticed until they significantly affect part quality or productivity.

When the plasticization rate, productivity (whether in pounds per hour or parts), or reject rates reach unacceptable levels, process engineers may begin to look for reasons. Unfortunately, it may not be possible to repair quickly at this time unless there are spare screws or barrels around. If replacement parts are available, they can be replaced in about a day. Otherwise, the production line will have to shut down or run at reduced efficiency until replacement parts are found and installed.

Types and causes of wear
So how to avoid this dilemma? Let's start by identifying the three types of wear.

1. Adhesive wear. This happens when the metal surface (the propeller plate and the inner surface of the barrel) is in contact. When the surfaces briefly touch and weld together, and then break, extremely high stresses are generated. Evidence of adhesive wear may be burrs or "rolling" on the rear edge of the rear wing, or the hard surface material on the top of the rear wing is scratched or missing. Also check the inner diameter of the barrel for scratches or wear. The damage on the screw is located in the barrel.

Adhesive wear is the adhesive wear of plastic processing screws when the metal surface (screw thread surface and the inner surface of the barrel) contact.
2. Abrasive wear. This is probably the most common type of wear in plastic machinery. In the case of handling unfilled or low viscosity polymers, it can be minimal. When the particles are transported down the screw and pressed on the barrel wall, the shearing action will cause a certain degree of wear on the screw thread and barrel wall. When processing resins with fillers, additives, or contaminants in recycled materials, or even when processing high-viscosity polymers, abrasive wear can become severe. Hard materials tend to remove material from softer surfaces. For this reason, wear-resistant alloys are commonly used for surface hardening during flight landings. Before the additives are wetted and mixed with the molten plastic, glass and mineral-filled resin can also cause wear at the screw root, usually at the back of the channel.

Abrasive wear in plastic processing screws When particles are transported down the screw and pressed on the barrel wall, the shearing action will cause a certain degree of abrasive wear on the screw threads and barrel wall.

3. Corrosion and wear. Corrosion is the gradual destruction of materials by chemical attack. Most of the atoms on the same metal surface are oxidized, damaging the entire surface. The breakdown of the metal surface actually makes the surface susceptible to mechanical wear mechanisms. Corrosion is the most common when processing polymers in the PVC or fluoropolymer series and can severely attack components with high-iron matrix.

Corrosion in screws Wear corrosion is the gradual destruction of materials by chemical attack.
Please refer to the table below to understand the wear mechanism of the plasticizing unit, and let us understand some of the main causes of wear.

Wear of screw and barrel in plastic
• Straightness, concentricity and alignment: When preparing to install new or rebuilt screws, check the straightness and concentricity of the screws carefully. If old screws are available, it is best to compare the total length, shank and drive configuration of the two. Next, if you want to install the screw 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 jamming. On extruders and most blow molding machines, the machine should be aligned before running a new part. The calibration of plastic processing machines has a history of more than 20 years. During that time, we have seen very few machines that do not require calibration during inspections.

• Poor screw design: When the capacity of the melting section of the screw is less than the amount of unmelted material conveyed in the feed section, it may cause the channel to be blocked by the unmelted material. The resulting pressure actually forces the screw to rest on the barrel on the other side, causing wear and damage to the screw threads and the inner diameter of the barrel. surface. This situation is commonly referred to as solid wedging (see Figure 2). When screws originally designed to process a particular resin are run on different polymers with different melting rates, solids wedging usually occurs.

The influence of poor screw design in plastics
When the capacity of the melting section of the screw is less than the amount of unmelted material conveyed in the feed section, it may cause the channel to be blocked by the unmelted material. The resulting pressure actually forces the screw to rest on the barrel on the other side, causing wear and damage to the screw threads and the inner diameter of the barrel. surface. This situation is commonly referred to as solid wedging.

• Incompatible metals: Generally, softer materials are more prone to wear when in contact with harder materials. Therefore, it is very important to consider the screw material and the surface hardening material of the screw thread and its compatibility with the barrel lining material. Alloys with lower tungsten carbide content may be easier to apply and finish, but not surprisingly, they have poor wear resistance. They are also more prone to wear, especially when using some of the hardest barrel lining materials. Today's bimetal barrels have excellent wear resistance. By using a centrifugal casting alloy with a high content of tungsten carbide, barrel manufacturers can provide products with a long service life. Then, it becomes important to specify a screw that not only has high hardness but also runs compatible with the barrel lining material.

• Abrasive fillers, additives, and foreign bodies: As the applications of polymers and plastics become more specialized and demanding, abrasive fillers and additives need to improve wear and corrosion protection. Resins containing glass fibers, minerals, flame retardants, stabilizers and other additives need to consider their impact on the wear and sometimes corrosion wear of these materials during the extrusion or molding process. Some of the most aggressive fillers can shorten the life of the screw/barrel by 50% or more. If the screw and barrel are properly protected during manufacturing, the wear life can be prolonged and abrasive wear can be greatly reduced.

Another common cause of wear and damage to screws and barrels is foreign objects-everything from screwdrivers to grinder blades to nuts and bolts seems to be able to get into the hopper of plastic machinery. If not caught by screens or magnets, these materials can cause serious damage to the screw and barrel. Case-hardening alloys loosening or delamination from threaded connections where the alloy is not properly joined, or cracks and loosening appear, can also cause problems when they move down the screw.

Compensation and diminishing returns
When the screw and barrel are worn or damaged, process engineers usually make small adjustments to the screw speed, barrel temperature, or back pressure. These increments usually consume more energy and affect part quality. The initial impact may be small, but at some point, these small "adjustments" will have a measurable impact on production. This may increase cycle time, poor mixing, or higher reject rates. This can be referred to s "point of diminishing returns" or "point of diminishing returns".

Ideally, we want to determine this-the machine tuning successfully minimized the limit of productivity or quality degradation-and be able to predict when the screw/barrel will be rebuilt or replaced. Regardless of whether this point is defined in terms of machine hours, parts produced, or scrap rate, maintenance personnel can plan to replace the screw/barrel and run to this point in the future. This will optimize the process while reducing costs. In addition, if we can determine this, we can provide replacement parts and arrange for installation without interrupting production.

In the future, through digitization, we hope to be able to use sensors to accurately measure the wear of the screw and barrel, and then predict when parts should be rebuilt or replaced to maintain optimal productivity. Before this technology is perfected, plastic processors can map performance related to screw and barrel wear by correlating production efficiency with screw and barrel measurements.

Schedule maintenance to avoid screw and barrel wear
Figure 3 When the capacity of the melting section of the screw is less than the amount of unmelted material conveyed in the feed section, it may cause the channel to be blocked by the unmelted material. The resulting pressure actually forces the screw to rest on the barrel on the other side, causing wear and damage to the screw threads and the inner diameter of the barrel. surface. This situation is commonly referred to as solid wedging.

Unfortunately, especially on larger machines, measuring screws and barrels on a regular basis may be impractical. However, if the adjustment of operating parameters is recorded and graphs are drawn based on changes in productivity, a reference point can be established where the wear of the screw and barrel is indicated by the relationship with the output, controller set point, energy consumption and final product quality.

The layering of screws in plastic processing, from screwdrivers to grinder blades to nuts and bolts, seems to be able to enter the hopper of plastic machinery. If not caught by screens or magnets, these materials can cause serious damage to the screw and barrel.
Through good record keeping, the downtime caused by screw and barrel replacement can be predicted relatively accurately. The life of components can be optimized, and "operation failures" and unplanned downtime can be avoided.

When installing the screw or barrel:

• The screws should be designed for the application.

• The screw and barrel should be made of compatible alloys.

• The screw and barrel should be made of wear-resistant materials most suitable for the application.

• Before installation, carefully check the dimensional accuracy of the screw and barrel.

• If possible, check the alignment of the screw and barrel.