Optimize replacement screw design to maximize throughput, product quality

By Del Williams 

Single and twin screws are at the heart of the extrusion process that transforms raw materials into pellets, sheets, pipes or profiles. These highly engineered components are integral to transporting, compressing, mixing, heating, cooling, shearing and pumping viscous substances through dies. As such, the screws are also the primary factor in production throughput and final product quality. 

Given the critical role that screws play in the extrusion process, when it comes time to replace them, many manufacturers underestimate the impact of an optimized screw design. With variability in the raw materials, recipes, additives and fillers used, screws are not off-the-shelf parts that can be simply “switched out.”

Shayan Moin, who has an MSE in polymer engineering and is president of UniSol, an Ontario-based supplier of specialty polymers with expertise in screw and barrel manufacturing, talked about the advantages and benefits of utilizing expert screw design tailored to the application to cost-effectively improve longevity, quality and productivity.

What is optimized screw design? How is it accomplished?

Moin: Optimized screw design 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 extruders to refine their process, resolve quality issues and even ensure the next replacement is further in the future.

How frequent is screw replacement typically for abrasive materials?

Moin: Although manufacturers of products made of PE or PP may only need to replace screws every 10 years or so, those extruding more abrasive or corrosive materials often must change screws every two to three years.

PVC, for example, contains fillers like talc or calcium carbonate that accelerate screw wear.  Wood and plastic composites are particularly corrosive. Even recycled raw materials can contain impurities that damage screws and shorten the usable life.

What are signs of screw wear? What problems can it cause?

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 reduce productivity.

How can screw wear best be resolved?

Moin: While the feed rate can be reduced as a temporary solution, the better solution is to replace the worn screws. Instead, extruders often manage the situation by using additional additives. Given that 80 percent 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. For one extruder, we replaced both the screws and barrel and we were able to reduce the additives from 1.3 percent to less than 1 percent. This represented a 30 percent savings and, given the high price of additives, paid for the replacement in less than a year.

How are wear issues addressed by an optimized screw design? 

Moin: Traditionally, screw manufacturers address wear issues through the selection of materials and by applying wear-resistant coatings. Although these techniques extend the life of the screw, optimizing the design to fit the application can reduce wear by up to 50 to 60 percent.

Wear is the result of sudden increases of pressure in most cases, which causes some melt turbulence that damages the screw. 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 more than 100 countries worldwide, GTS has accumulated a database of materials, formulations, parameters and OEM designs. In addition, the company has developed proprietary simulation software to optimize co-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.

Does the machining of the screw impact longevity? 

Moin: In addition to the design, the machining of the screw is also of utmost importance.

Most screws on the market have up to 1-inch gap between the different mixing elements where the material collects. The material that remains in the gap can increase pressure, causing wear, or even burn, causing quality problems.

To extend longevity of the screws and improve product quality, GTS designs screws with no gaps between mixing elements, to prevent material hang up.   

There is no gap 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.

What are some real-world benefits of implementing optimized screw design? 

Moin: For extruders that seek out an optimized screw design, the increase in production is often significant. For one extruder, the original OEM screws were designed to produce 4.5 tons per shift, which over time had decreased to 3.5 tons. With new replacement screws, production immediately doubled to 8.5 tons.

Even details like the location can be a factor in productivity because humidity and ambient temperature affect the extrusion process.

An extruder in Toronto purchased equipment from a company in Florida. Given that Toronto is considerably less humid, when the screw and barrel were replaced, we eliminated one vent port. With one vent port instead of two, they were able to increase output up to 35 percent.

An optimized screw design also widens the processing window, which means more flexibility in the use of materials with different melt indices, as well as the ability to use a wider range of additives and stabilizers.

The design can also be used to alter product characteristics to increase profitability, such as with PVC foam.

Reducing PVC foam’s density is important because it is sold by the length, not by weight.  When the foam density is lower, it can still be sold at the same price while using less material — which is more profitable.

For one foam manufacturer, the goal was to reduce the density from above 50 to 48 without changing the physical and mechanical properties of the final product.

Why should extruders consider optimizing screw design today?

Moin: Although plastics extruders may have accepted the rather short lifespan of screws as a cost of doing business and adapted by using more additives to control their process, optimizing the design of replacement screws 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.

Contact:  

UniSol Inc., Toronto, 855-274-3004, [email protected], www.unisol-inc.ca

Del Williams is a technical writer who wrote this on behalf of UniSol.