In the plastics industry, twin screws are at the very heart of the extrusion process that transforms raw materials into rigid PVC pipe or profile. These highly engineered components are integral to transporting, compressing, mixing, heating, cooling, shearing, and pumping a variety of viscous substances through dies into highly structured products. As such, the screws are also the primary factors in production throughput and final quality.
Given the critical role that screws play in the extrusion process, when it inevitably comes time to replace them, many rigid PVC pipe or profile manufacturers continue to underestimate the impact of an optimized screw design. With the variability in raw materials, recipes, additives, and fillers utilized, screws are not off-the-shelf parts that can be simply “switched out” based on category of product.
An optimized design, on the other hand, is a consultative approach in which every parameter of the process is evaluated to create a customized solution that fits the application. When coupled with a replacement screw supplier with an extensive knowledge base of designs and an intimate understanding of extrusion processes, screw replacement morphs from a task into an opportunity for rigid PVC extruders to refine their process, resolve current processing and product quality issues, and even ensure that the next replacement is further in the future.
Optimizing rigid PVC production
Rigid PVC is often used to create pipe extrusion, but also profile for products such as vinyl click flooring. The ideal processing of rigid PVC involves screw design that enables heating the material very evenly to a temperature of approximately 150°C with all additives consistently distributed around the powder grain. The material/powder is then sheared and heated to a final temperature that allows optimal gelling and homogeneity in properties. The final process temperatures are 180°C to 200°C.
In this process, twin-screw extruders have two intermeshing identical screws encased in a matching barrel. The design can be parallel or conical. During twin-screw extrusion, the PVC/cPVC (chlorinated polyvinyl chloride) used to make pipe is conveyed, compressed, de-gassed, plasticated, sheared, kneaded, fused, and homogenized before it enters the die. Both parallel and conical screws are also used for processing uPVC (unplasticized polyvinyl chloride) and cPVC, utilized for doors and windows.
Among these screw types, conical has a large feeding area and simpler gearbox, but low to medium output due to limits on screw length. On the other hand, parallel twin screws are not limited in length. The difference is reflected in the key parameter known as the length-over-diameter (L/D), which is the ratio of the flighted length of the screw to its outside diameter.
“30 years ago, screws had about a 20 L/D, and 15 years ago about 30 L/D. Today many screws run 40 L/D,” says Shayan Moin, who has an M.S.E. in polymer engineering and is President of UniSol, an Ontario, Canada–based specialty polymer technical marketer with screw and barrel manufacturing expertise for plastic extrusion lines.
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|Optimizing screw design to suit the application can reduce wear while increasing productivity.|
Dealing with screw wear
Whether the screw is conical or parallel, wear life is an issue when extruding challenging materials. With rigid PVC, extruders generally require screw replacement in one to four years because of abrasive or corrosive fillers. “PVC contains fillers like talc or calcium carbonate that accelerate screw wear. So, some screws will need replacement in one to one-and a half years. Others with a good formulation running very carefully might last four years,” says Moin.
In most cases, the evidence of screw wear is clear, even if subtle. Screw wear can impact either processing or product quality. Symptoms include reduced throughput, increased use of electric heat, need for more additives, or the smell of burnt material during production. Abrasive wear may also cause backing-up in the main feed port, at side feeders, and in the die pressurization zone, all of which further reduces productivity.
Not addressing the problem of screw wear can be costly to extruders.
“While waste material for a rigid PVC pipe application is normally in the range of 1.5%, with worn screws the waste rate can go up to 10%,” says Moin.
Another sign of wear is the amount of time it takes the extruder to start up production for new pipe dimensions, which normally is one to two hours. “When the screw is worn out, it requires two to four times longer to switch over to start running another pipe dimension,” he adds.
When screw wear leads to burned material, substantial production downtime can be necessary for cleaning.
“If material burns, extruders may need to stop the production line every few days to clean the screws. Normally they never have to clean it. But when the screws are very worn, more cleaning is required and that costs more in labor,” says Moin.
However, extruders often manage screw or performance challenges by using additional additives. Given that 80% of costs are in the formulation, reducing the number and quantity of additives can reduce operating expenditures.
“Most of the time the manufacturer does not really need more additives — they just need to optimize the process with well-designed replacement screws,” says Moin.
Extending screw longevity, ensuring quality
Traditionally, screw manufacturers address wear issues through the selection of steel materials and by applying wear-resistant coatings. For parallel twin screws, bimetallic barrels and screws with tungsten carbide on the running surface can help with longevity. For conical screws, a similar approach can resist wear along with special cladding.
However, improving wear protection goes far beyond applying coatings. The better solution is to replace the worn screws with an optimized design that can reduce wear by up to 60%, according to Moin.
“Wear is the result of sudden increases of pressure in most cases, which causes some melt turbulence that damages the screw,” explains Moin. “Extruders can reduce wear with a proper design that eliminates these sudden pressure increases.”
For screw and barrel design and manufacture, UniSol partners with GermanTwinscrewS (GTS), a German manufacturer with decades of experience in the plastic extrusion field. Having manufactured and delivered tens of thousands of screws to customers in more than 100 countries, GTS has accumulated a database of materials, formulations, parameters, and OEM designs. In addition, the company has developed proprietary simulation software to optimize counter-rotating screw design based on specific raw materials, compounds, formulations, temperatures, and operating conditions.
“Ultimately, the information from the simulations is used to determine the melt pressure of the materials at different points along the screws. The melt pressure is key to designing the screw for superior wear protection, and to ensure proper melting and compounding of the raw materials,” says Ernest Krüger, CEO and founder of GermanTwinscrewS.
Alternatively, GermanTwinscrewS can often determine the specific stresses of an application by carefully inspecting the old, worn screws that will be replaced.
“By making careful measurements, we can calculate the energy the screw had to withstand in areas of high or low wear. From this, we calculate the optimal screw design for the new screw so it can reliably withstand the expected wear,” explains Krüger.
In addition to the design, the machining of the screw is also of utmost importance.
“Most screws on the market have up to a one-inch gap between different designs where the material collects. The material that remains in the gap can increase pressure, causing wear, or even burn causing quality problems,” says Krüger.
To extend longevity of the screws and improve product quality, GTS designs screws with continuous change (virtually no free cuts) to prevent material hang up.
“There is no cut between screw design changes, so the material flows directly from one channel to the next in the new design. There is no material hang up and no wait time, which prevents screw wear and polymer decomposition,” says Krüger.
In addition, to increase output in the pressure area for the last 100 to 250 mm of the screw design, GTS includes twice the numbers of “flights” to achieve a smoother pipe interior and reduce any pressure pulsation from the screws.
Improving productivity and profitability
For extruders seeking an optimized screw design, the increase in production is often significant. This is frequently achieved by increasing the length of the screws and barrels.
According to Krüger, production output can typically be increased as much as 25% by extending the screws from 32 L/D to 34 to 35 L/D and ensuring the gearbox has sufficient energy density. This simple adjustment makes parallel screws more economical, in general, for rigid PVC extrusion.
In addition, tailoring the screw design to the application also substantially reduces the cost of a rigid PVC formulation by increasing the use of fillers and minimizing necessary additives.
According to Krüger, rigid PVC formulations vary between manufacturers mainly in the parts per hundred (PPH) of stabilizer used and in the variation of the calcium carbonite filler. Increasing the amount of filler used reduces cost because filler is inexpensive. However, using too much filler is a problem since it can make rigid PVC pipe brittle.
According to Krüger, there are many applications where the percentage of calcium carbonate filler reaches 100 or even 150 PPH in pipe applications. Some applications, such as vinyl click flooring profiles may use up to a 300 PPH of calcium carbonate.
He notes that optimizing screw design can resolve this issue. “With optimized screws, extruders can use a higher percentage of filler and still get better homogeneity and maintain the same overall quality.”
In addition, he points out that since stabilizers are the most expensive part of the formulation, reducing its use while still stabilizing the rigid PVC material with the screw design can be very cost effective.
“With optimized screw design of parallel screws, rigid PVC extruders can have simpler formulations with a very low percentage of additives and a higher percentage of filler,” says Krüger. “As a result, rigid PVC extruders can reduce the cost of formulation and achieve much higher outputs.”
Precisely regulating temperature is also an important aspect of rigid PVC production that improved equipment design can facilitate. For many years, extruder manufacturers have utilized external oil temperature controls to maintain screw temperatures. However, GTS’s experience shows that closed-loop water temperature controls are even more effective.
According to Krüger, if there is too much heat in the front end of the process, it can be automatically transported to the feeding area in a closed-loop cycle. This saves a considerable amount of heating energy and lowers operational cost.
“The closed-loop system facilitates rigid PVC applications because these formulations are more sensitive. Also, there are many extruders who do not have the best formulation. The screws have to work with all of these formulations, so the closed-loop system is maintenance-free and very helpful,” he concludes.
Rigid PVC extruders may have accepted reduced screw wear life and output as a cost of doing business and adapted by using more additives to control their process. However, optimizing screw replacement design can be a more cost-effective way to achieve their performance goals. In this regard, working with a replacement screw expert to tailor the design to the application can significantly boost the bottom line.
About the author
Del Williams is a technical writer based in Torrance, CA. He writes about health, business, technology, and educational issues.