Industry experts in materials, machinery, and compounding provide their insights on best practices and techniques to keep your compound twin-screw extruder running normally.

Twin-screw extruder (TSE) has long been the machine of choice for mixers due to its excellent performance and customizability. They process almost all resins and allow different additives and fillers to be combined to achieve different particle sizes and shapes, and have an impressive array of possible characteristics.

Although there are countless combinations of possible end products, some methods of achieving these products can also cause pollution problems and many low flow or low pressure areas throughout the barrel. In a continuous process like extrusion, contamination can adversely affect the bottom line. Cleaning in extrusion is often more challenging than other processes, and TSE faces these challenges on a larger scale because the system is much more complicated than their single-screw brothers.

This article will study some specific challenges of cleaning in twin-screw extrusion. Conversion, pollution, screw drive, and extended downtime are the four major processing problems faced by composite plant engineers. Here, Asaclean purification experts will share their best practices of over 40 years of comprehensive experience. In addition, we will listen to the first-hand experience of the machinery manufacturer Leistritz Extrusion and Asaclean's sister company Asahi Kasei Plastics (one of the largest composite manufacturers in North America). They will identify the common processing problems they encounter most often.

The correct cleaning procedure helps to maintain the normal operation of the compound twin screw.

Tip 1: Do not use production resin, recycled materials or additives to clean the extruder: Production resin is not designed to clean the machine, nor can it replace commercial cleaning compounds. Cleaning the extruder with the next resin or recycled material is time-consuming and wasteful, and it is inefficient in removing color and carbon pollution. David Krueger, a process engineer at Asahi Chemicals North America (APNA), has spent his career in composites, and he knows that this seemingly cost-saving decision may backfire. "In my past work, I have seen many processors try to use low-melting PE instead of engineering resins, including nylon, styrene, and ABS. These resins inevitably decompose and form a large amount of carbon deposits. You can try to use fillers to Help clean the barrel, but if the base resin tends to form carbon, it will happen no matter what you fill. Avoid headaches and embrace the use of purifying compounds."

"Clearing the compound allows you to have a consistent conversion time, no matter what you are mixing."

Lenny Gutierrez, Asaclean’s purification expert and sales representative, added that he has been working on solving the contamination problem of composite manufacturers for many years: “Twin screw end users are very concerned about the customer’s product specifications. Any possible process variants, especially from previous runs Pollution or accidental degradation of materials can lead to process interruptions and production delays due to product failure.” Krueger pointed out another headache of trying to use production resin for cleaning-unpredictable downtime. "Even if you don't talk about pollution, if you use resin or recycled materials, the conversion time will be very different. If you are mixing olefins, it may take 20 minutes to clean the barrel. For other resins such as styrene and nylon, carbon deposits increase This may result in an extended cleaning time. It may take 3-4 hours to clean the barrel alone. The purge compound allows you to have a consistent transition time, no matter what you are mixing."

When you use recycled resin or virgin resin for cleaning, the material will form another layer on any existing resin, color deposits or carbonized material in the barrel and on the screw. Over time, these layers become additional sources of pollution, making purification more difficult. Once your machine resumes production, the carbonized material will eventually break away and contaminate your product. Twin screw extruders cannot withstand material or carbon contamination. Fortunately, these problems are easily avoided by choosing the right purification compound for your specific needs.

Tip 2: Ask yourself these questions to choose the purification compound that best suits your needs: The introduction of a purification program can easily help compound manufacturers reduce downtime by 50-75%. But when cleaning twin-screw extrusion equipment, there is no one-size-fits-all process or solution to remove compounds. Carbon accumulates in dead corners of the machine: mixing parts, molds, downstream equipment, etc. These are low-flow areas that can benefit from chemical removal of compounds. However, if you are dealing with frequent disassembly, you may also need a strong cleaner.

Without the correct purification compounds and procedures, you will be at risk of increased waste rates. However, determining the appropriate level of purification compound is not only about eliminating color and carbon pollution, but also about the impact on your bottom line. Adopting a holistic approach will help you increase efficiency and profitability. When you contact a purification expert for a free consultation, they will consider several key considerations in order to select the correct grade and quantity of the best purification compound for your system. Some of these issues include:

 • What is the size of your extruder?

 • What resin are you running?

 • What is your processing temperature?

 • What is the intended use of the removal compound?

 • What problem are you currently experiencing?

The answers to these questions are critical to making an informed decision on the best course of action. Using the right cleaning agent can effectively eliminate all pollution and increase uptime and productivity, thereby helping you maximize operational efficiency. If you understand the impact of each problem, you have the ability to choose the right solution to effectively clean the extruder.

Both mechanical and chemical cleaning types can help twin screw processors to keep their machines pollution-free. The mechanical cleaning compound does an excellent job of safely scrubbing the surface of the screw and barrel. However, the cleaning performance is mainly limited to the surface through which the material actually flows at the appropriate pressure. For twin-screw machines that use simple screw types and mixing elements, mechanical purge can work perfectly.

"If you use resin or recycled materials for cleaning, the conversion time will be very different." 

However, for proper cleaning in low-pressure areas (such as near vents) or hard-to-reach areas (found on more complex screw mixing elements), chemical cleaning will produce better results. Chemical cleaning compounds achieve cleaning by measuring the chemical reactions contained in the machine during soaking. The gas produced by the chemical reaction will quickly enter the hard-to-reach areas. These areas are considered to be low pressure or low flow (usually called dead zones or dead spots), otherwise they may not be directly and physically scrubbed through the mechanical stage. Gutierrez explained: "A general understanding of how the material passes through a specific screw type, while determining the high and low pressure flow paths, can help processors decide which type of purge is best for their specific twin screw settings."

President and General Manager Charlie Martin noted that Leistritz Extrusion has conducted extensive testing to determine the best cleaning options for its customers. Various TSEs have been studied to determine the most optimized purge technique with the goal of determining the minimum amount of purge material required and the minimum purge time. The methods analyzed are: overflow feeding (filling the hopper, TSE screw determines the rate); metering feeding (separate feeder and TSE screw have independent speeds); and plug feeding (purge charging is introduced intermittently TSE). It has been determined that plug feed is generally the fastest and most effective in cleaning the system, requiring significantly less material to become significantly clean.

Taking these suggestions into consideration, the purification procedure of the extrusion process will vary according to the grade you use. If you are not sure how to clean the extruder most effectively, it is best to contact your cleaning compound supplier directly.

Tip 3: Practice preventive cleanup: Many processors only use cleanup compounds when absolutely necessary. However, the use of this type of removal strategy will result in reduced profitability and efficiency. This usually happens when the processor is running within a tight deadline or when the clearing budget is limited. APNA's Krueger also pointed out that sometimes processors push the limits because they believe their screws are "self-cleaning." Self-wiping screws will not significantly remove contaminants.

Twin screws interact, but don't confuse their self-wiping properties with self-cleaning. Photo: Leistritz Extrusion

Krueger explained: "Even if the twin screw is designed to be'self-wiping', the screw design does not have any significant effect on the removal of contaminants. Shearing generates a lot of heat, and pollution will occur anyway." If your factory is dealing with these situations For any of these, please consider working with a purification expert to conduct a cost-saving analysis so that you can show the management the cost of the decision.

Adopting preventive purge practices can not only save your facility from machine downtime, but also reduce scrap rates, customer rejects, and production line downtime. You can't wait until the last minute to run the purification compound, because by then you are already plagued by pollution problems or color streaks. Twin screw extruders cannot withstand this method. Experts agree that conversion is the biggest problem facing composite manufacturers. If you need faster conversion, clearing the compound is an easy option. Regular use of cleaning agents can prevent contamination from the beginning.

 The screw design does not have any significant effect on the removal of contaminants, even if the twin screw is designed to be "self-wiping".

Usually, only one to two buckets of cleaning compound are needed to prevent contamination. Preventive clearance saves time and money by effectively using capacity and preventing unplanned downtime. There are three main benefits of implementing a preventive maintenance plan and decontamination process in your facility:

 • Prevent color accumulation and carbon pollution;

 • Reduce excessive cleaning time and material waste at the end of production operation;

 • Limit the frequency of screw pushing, and minimize the time and effort required to pull and manually clean the screws.

Once your factory is recognized and your production line workers understand that proactive is more beneficial than passive, you will start to see immediate savings.

Tip 4: Make sure you are optimizing your screw drive to make it easier to disassemble: If conversion is the biggest challenge for the mixer, then screw drive follows closely behind. Sometimes, product changes require you to replace screw components. If the screw is not in the ideal disassembly state, this may be time-consuming. Asaclean has several large composite customers who use multiple resins or the same resin with different additives. Usually mixing different resins requires different screw designs, or the mixer needs to pull the screw to ensure that the processed resin does not mix. Most polymers inherently tend to adhere to the surface of the screw, and these layers not only provide challenges for screw cleaning but also for screw removal.

A typical mixer might pull the screws four to five times a week. Those who produce color masterbatches may pull screws every day. Some factories may carry duplicate screws, but these screws may cost tens of thousands of dollars in additional capital expenditures. For most mixers, pulling the screw and cleaning it may be the only option to resume production. This process can be labor intensive and very costly, as downtime can cost 1,000 to 1,200 USD/hour.

Gutierrez of Asaclean said that it is not uncommon for the extruder to struggle with 8-12 hours of screw drive. Weddell agrees, adding that “the buildup on the threads is difficult to remove.” You will want to use a grade that does not require baking in an oven, wire brush, torch, etc. to clean the screws. These secondary processes also increase the possibility of injury or accidental damage to expensive screw components.

There are three main reasons why the mixer pushes the screws:

 • Mixers may require them to be used to ensure 100% polymer cross-mixing;

 • Conversion requires different screw designs;

 • Normal purging cannot remove serious pollution.

Although spirals may be required in your plant, this does not mean that they require time-consuming and troublesome profits. There are purification compounds specially designed for screw pull/push. APNA's Krueger said that using Asaclean's EX Grade pull screws can save the processor 75-90% of downtime.  

Tip 5: Know your screw design and mixing area and pay close attention to the machine settings: TSE is modular and offers unlimited combinations of screw sections and different mixing areas, which also exacerbates some of these cleaning challenges. Many machine settings and mixing factors require close attention to maintain consistent melt temperature, melt flow, mixing, and shear. This leaves a lot of room for pollution to occur. On production lines with additional feed streams, the margin of error will increase exponentially. Once the contamination begins, it is almost impossible to stop it without active intervention—usually by pushing a screw or using a scavenging compound.

Although spirals may be required in your plant, this does not mean that they require time-consuming and troublesome profits.

Both twin-screw and single-screw extrusion face the common challenges of low-pressure processes. These processes do not have the same options for controlling pressure and turbulence as the injection molding process. "Many twin-screw machines also rely on proper ventilation, which helps achieve the proper mixing, fusion, or chemical reaction required for the final product. These vents can provide lower pressures in those specific mixing areas and may cause It is more difficult to remove the product correctly," Gutierrez pointed out. "Please pay attention to these areas and pay close attention to your processing volume."

In order to maximize cleaning performance, please follow the machine settings recommended by the supplier. Settings are very important. If the settings are not correct, your purification compound will not activate properly and will minimize your gains. Using the correct settings will activate your chemical cleaning for the best cleaning performance.

Tip 6: Super engineering resins and heat-sensitive resins need special attention: as mixers create new resins that can withstand more extreme conditions, they need to be careful about the possibility of contamination. For "super engineering" resins such as PEEK, polyetherimide, PEKK, and polysulfone, scrap and downtime have a devastating impact on profitability. Jerry Weddell, Asaclean's business manager, said that one of the common challenges he faced with the mixer was the conversion with temperature fluctuations. "For super engineering resins, you need to use grades designed for challenges. Some grades can reach 790 F, but this is not all. We always recommend that you gradually lower the temperature when changing to a resin with a lower processing temperature range. When you start your next run, it can avoid black specs."

Gutierrez added that low-temperature resins have similar concerns about carbon. He said: "When processing heat-sensitive resins such as PVC or EVOH, please pay attention to degradation and contamination. These resins are easily degraded. It is important to control the process temperature because high temperatures can cause degradation, and lower temperatures can cause excessive shear. "For materials with lower heat sensitivity, the residual polymer left in the machine will degrade over a long period of time, but for PVC, degradation will occur immediately. This is a very serious processing problem.

In order to work effectively, the cleaning compound does require heating and sealing, so it is important to get the heat required to clean it well, but not to burn any residual resin that may still be left in the barrel. It is important to perform the initial cleaning at the processing temperature of the resin. Do not increase the temperature in the barrel until all PVC has been removed. For heat-sensitive materials, additional steps are indeed required, but they can be cleaned very effectively using this method.

Tip 7: Seal your machine with heat-stable compound during each shutdown: In addition to regular and preventive maintenance cleaning, use heat-stable cleaning compound for temporary or extended shutdown and sealing, especially on weekends or holidays. During shutdowns without purging, oxidation will occur, causing pollution, delays in start-up, and prolonged machine downtime. These levels of purification create an "air-tight" environment in the barrel and promote further cleaning during shutdowns.

Krueger explained, “Idle lines usually require a bank temperature of about 200 degrees Celsius (392 degrees Fahrenheit). When you use conventional processing resins for sealing, you usually encounter oxidation problems at startup. This is what you see in the extrusion plant One of the most avoidable processing problems. If you are not running 24/7, a good shutdown cleanup is a good solution."

Weddell said that once customers start to seal up before the holiday closes, they are almost all converts. “Most of my employees contact me on July 4th or a few weeks before the Christmas holiday to make sure they have inventory before the break. None of them want to deal with the horrible shift that started on Monday.”

About the author: Tom Hanvey is the senior marketing manager of Asahi Kasei Asaclean Americas. He joined the company in early 2016 and is responsible for the sales of Asaclean purification compounds in North and South America. He is also the co-chair of Asahi Kasei's North American Marketing Committee and a member of PLASTICS' Plastic Future Leaders Group (FLiP). Contact: 973-257-1999; thanvey@asaclean.com; asaclean.com.

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