Metering equipment, twin-screw extruders and downstream systems must all work together to control the characteristics of the final part and manage the mixing process.

The co-rotating intermeshing twin-screw extruder mixes the polymer with fillers, additives and modifiers to impart the required properties. The twin-screw extruder (TSE) can be used for research and development purposes and can process batches as small as 50 grams and perform full-scale production at speeds of more than 50,000 kg/h. The inherent characteristics of continuous mixing with twin-screw extruders make these equipment unmatched by commercial products and high-tech precision products.

The twin-screw extruder utilizes a unique screw-to-screw flow, which is effective, fast, and efficient, and can achieve dispersion and distributed mixing. TSE is the most commonly used continuous mixing device for countless plastic compounds with countless properties. Some need to be dispersed and mixed, and some only need to be distributed and mixed.

The rotating twin screw transfers the shear force and energy from the motor to the process. In the TSE process part (barrel and screw), the material undergoes a series of unit operations-feeding, melting, mixing, exhausting and pumping-which together constitute the "process experience". The segmented characteristics of the TSE screw, combined with the controlled pumping and wiping characteristics of self-wiping, co-rotating, and intermeshing screws, allow users to sequentially match the geometry of the screw and barrel with the expected process tasks.

TSE is a complex machine that allows various operating conditions. Control parameters include screw speed (up to 1000 rpm), feed rate, process temperature and vacuum. Melt temperature, melt pressure and motor load are monitored and controlled to ensure consistent product quality. The control includes PLC and HMI screens with data acquisition, trend and recipe management functions.

Focusing on output rate/rpm, screw design and components, and viscosity management, you can troubleshoot the twin screw process to achieve a high-quality, uniform melt without degradation.

Figure 1 The concept of "mixing experience" in a twin-screw co-rotating extruder. 

TSE provides the unique flow required to achieve dispersion and distributive mixing quickly and efficiently. Dispersive mixing means that large particles are reduced (rocks turn into sand), while distributive mixing means that the components are spatially distributed without being worn (that is, glass microspheres are mixed without destroying them). The efficiency of dispersive mixing depends on the strong force exerted by the viscous polymer matrix on the aggregate additive, and is related to the stretching and planar flow field induced/applied almost relentlessly by the mixing element. Distributed mixing relies on the repeated rearrangement of trace component additives (liquid or solid) while avoiding shear flow with high energy-consuming shear stress.     

TSE provides the unique flow required to achieve dispersion and distributive mixing quickly and efficiently.

Many formulations require one or two mixing mechanisms. The goal is to achieve uniform mixing with minimal degradation (by wise selection of screw elements to minimize energy consumption flow), which is sometimes easier said than done.

Figure 2 The TSE has four high-transmission zones and one low-transmission zone (channel) for mixing.

Almost an unlimited number of screw elements can be used. However, there are only three basic types of elements: flying, mixing, and partitioning. The flying components send the material through the barrel mouth, mixer and pump out of the TSE. The partitioning element isolates the two operations in the extruder-for example, to induce melt sealing before vacuum exhaust. Based on the grading of the screw elements on the high-torque spline shaft, the screw design can be shear-intensive or passive design according to the recipe requirements.   

"Kneader" is the most common type of TSE mixing element. The wider the kneading element, the more dispersed it is, because when the material in the channel is forced up and over the platform, the effects of stretch mixing and plane shear will increase. In contrast, narrow kneading elements result in distributive mixing by promoting effective melt split rate mixing with minimal dispersion impact, which helps to mix heat/shear sensitive materials with minimal degradation/wear. The kneading elements can be arranged in forward pitch (not too aggressive), neutral or reverse pitch (most aggressive). High-liquid phase mixing usually benefits from special high-split-rate distribution elements that prevent the "clustering" of liquid in the screw.

Figure 3 A wider kneading disc is used for dispersive mixing, while a narrower disc is used for distributive mixing.

Depending on the formulation requirements and sensitivity, the screw design can be shear-intensive or passive.   

The gap between the kneader tip and the barrel wall or the shear rate in the flyover area is usually called the peak shear rate, which can be a useful benchmark for troubleshooting mixing failures (and predicting degradation). The peak shear rate is calculated as follows:

Peak shear rate = (π × D × n) ÷ (h × 60), where

Therefore, for a TSE with a 77.5-mm OD screw and 0.55-mm across the kneader gap at 600 rpm, the calculation is:

(3.14 × 77.5 × 600) ÷ (0.55 × 60) = 4424.5 sec-1

This peak shear rate calculation is undoubtedly an over-simplification of the "mixing experience" in TSE, because it ignores the extensional flow mixing and the vertex and meshing effects, which may be relatively more obvious. In any case, the peak shear rate is easy to calculate, making it a very useful daily tool and benchmark.  

Another useful formula that should be tracked for existing compound operations is specific energy (SE), which is the amount of power that the motor inputs into each kilogram of processing. SE is a benchmark used to confirm that the process and mixing between batches are the same, and is calculated in two steps:

Application power: KW (application) = KW (motor rated value) ×% torque × rpm operation/maximum. rpm × 0.97 (transmission efficiency)  

Specific energy = KW (application) ÷ kg/hr

A lower SE means less mechanical energy is used per kilogram of processing, while a larger SE means more energy use. Maintaining SE records is very important, especially for troubleshooting. For example, if the product always runs at an SE of around 0.25, and then suddenly changes to 0.20 or 0.35, it means that something has changed (hardware, process conditions, or materials), so the mixing quality must be checked because it may also be different.

The premix can be metered into TSE through a volumetric feeder. Loss in weight (LIW) feed streams are often used to accurately meter multiple feed streams entering the TSE process section. The pressure gradient in the TSE process section is determined by the selection of the screw, and is zero at all stages of the process section, which enables the material (that is, filler or fiber) to be introduced downstream through the side filler or packer. Syringe pumps are used for liquids.

Different forms of materials, such as pellets, granules, powders, fibers and liquids, are metered into TSE. These extruders are in a hungry state, causing most of the processing space between the screw and the barrel to be partially filled. The output rate is set by the feeder, and the screw speed is independent, and is used in conjunction with the rate to optimize the compounding efficiency. To a large extent, the residence time (RT) in TSE depends on the feed rate. The residence time distribution (RTD) is then determined by the screw speed. The RTD inherent in the TSE mixing system helps to "balance" the smaller feeder fluctuations.

Figure 4 Speed ​​and screw speed work together to determine the residence time and residence time distribution in the TSE rotating in the same direction.

The relationship between the feed rate and the screw speed is combined with the screw design to adjust the mass transfer/mixing characteristics of the process. Since feed rate and screw speed are inseparable from mixing, it is insightful to understand the five screw areas inherent in any TSE:

 1. Channel: The mixing rate in the channel is similar to that of a single-screw extruder, and is lower than other TSE shear zones

 2. Flying over the gap: between the top of the screw and the barrel wall, the material in this area is subject to obvious plane shearing, which is easy to calculate.

 3. Expanding the mixing zone: When the material transitions from the channel to the flyover gap, an extremely effective mixing mechanism appears here, with a stretching/spatial acceleration effect. (There is no simple formula; it requires computer modeling to calculate). 

 4. Vertex (up/down): The upper and lower vertex areas are where the material "feels" the second screw, and where the compression/expansion and directional flow field affect the mixing.

 5. Engagement: A small amount of limited material passes between the screws and produces a strong shearing effect. As the output decreases or increases (at a constant screw speed), the material spends more or less time in the mixing zone, so the "mixing experience" will be affected. RT can be less than 10 seconds or as long as 10 minutes. The typical RT range is 20 seconds to 2 minutes, and the reaction process may (and requires) a longer RT.

As the screw speed decreases or increases (at a constant rate), there is a significant impact on the RTD and "mixing experience". Increasing the screw speed will widen the RTD, and reducing the screw speed will tighten the RTD.

Viscosity also plays a role in affecting the shear stress/mixing in the TSE for dispersive mixing. The calculation is as follows:

Shear stress = peak shear rate × viscosity

Higher viscosity will increase shear stress, thereby increasing dispersion and mixing. In the early stages of the TSE process part, during the melting process, the viscosity (and shear stress) is at its maximum, which helps to achieve dispersive mixing. In the latter part of the TSE process, the viscosity decreases and the shear stress is relatively low.

The twin-screw barrel is modular and uses inner holes for cooling. Each barrel section (usually 4:1 L/D) uses a PID temperature controller to set the temperature set point. The temperature profile is combined with the screw design for strategic management of melt viscosity (shear stress) and "mixing experience". 

Higher viscosity will increase shear stress, thereby increasing dispersion and mixing.

For example, a higher temperature set point in the melting zone may soften the resin and reduce the dispersion and mixing effect early in the process. The introduction of materials (and lower viscosity) downstream after melting can avoid unnecessary wear and degradation of shear-sensitive materials. The combination of increased or decreased cooling and screw design is a useful tool to achieve the desired mixing effect.

Please note that due to the relatively high heat transfer efficiency, the ability to manipulate temperature and viscosity is significantly more pronounced for smaller TSEs. Compared with the larger TSE, in the smaller TSE, the ratio of the heat transfer surface to the volume of the processed material is larger, so the efficiency is higher. This is why most processes do not scale by volume.

Figure 5 Controlled pressure gradient facilitates continuous downstream unit operation. Viscosity management adjusts the mixing effect.

Premix: Premix formulation can be prepared in a batch mixer, and then TSE can be metered in. To determine whether the premix is ​​feasible, you can mix a small batch and put it in a glass jar and shake it well; if the recipe does not separate, then a single feeder may be used for the mixture. The use of powdered resin helps to minimize the separation effect in the hopper.

Premixing should now also be considered as part of the "mixing experience". The use of premix can usually achieve higher TSE flux; by carefully designing the melting zone to avoid compaction/agglomeration effects, exposure to high shear stress in the melting zone is particularly beneficial for dispersion mixing. Examples of products that benefit from pre-mixed systems include: masterbatches, flexible PVC formulations, and alloys/mixtures. Small samples are ideal for premixed TSE configurations.

Multiple feed streams: The feed stream can be metered into the TSE feed throat through the LIW feeder. The particles and additives are usually metered into the main feed port and/or at different downstream locations. Depending on the batch size and formula, some pre-mixing may still be required.

Sometimes the additive particle compression phenomenon during the melting process can produce agglomerates that are difficult to disperse later in the process, so it is better to meter into the melt and manage the residence time exposure and "mixing experience" of different feed streams. The inherent pressure gradient of the under-supply TSE makes it possible to introduce material downstream after the melting step.

A side filler is a device that is usually integrated with TSE to introduce fillers, fibers, and other materials after plasticization. Like TSE, the side filler uses a twin-screw auger to "push" the material into the TSE process section to avoid entering the high-shear melting zone. This is why side fillers are preferred to minimize glass fiber abrasion and processing shear sensitive fillers, such as flame retardants or chemical blowing agents.

Various liquid injection streams are also common, including environmental and heating. The reactive extrusion process (ie thermoplastic polyurethane) is usually processed on the TSE system.

Tandem system: SE connects a series of unit operations in series to continuously modify, mix and devolatilize polymer formulations. The length/diameter ratio of co-rotating, intermeshing TSEs is mechanically limited to approximately 60:1 L/D, which limits the number of unit operations that can be performed. There is a trend to arrange multiple extruders (single and co-rotating/counter-rotating TSE) together in a single system to promote L/D of 100:1 or higher, and to integrate different screw diameters and speeds The extruder. Examples might be:

 • Mix polymers/additives and form a homogeneous melt in TSE, then inject supercritical CO2 and pump into a cooled single screw extruder to make foam products.

 • Remove 25% of the formula in 60:1 TSE, then pump into the crosshead TSE, where the filler is mixed, and then pump into the gear pump and downstream tableting operations.

 • Recycling post-consumer waste (ie PVB in safety glass) and dispersing residual glass, removing moisture and metering into the second TSE, where additional compounding operations are performed.

The opportunities offered by tandem extrusion systems are largely untapped.

The twin-screw compounding system can be part of a pelletizing and direct extrusion system for the manufacture of sheets, films, profiles or fibers. Front-end equipment includes screen changer, gear pump, diverter valve and various cooling and sizing equipment. In any case, the main goal of TSE is to mix the formula quickly and efficiently without degradation.

The melt filtering device is specially used to remove undispersed components and clean the melt before the mold. Ultra-fine filtration is possible, but it always leads to increased pressure and temperature (which may lead to degradation). The design of the front-end components should minimize the pressure, have a streamlined melt flow path, and be as short as possible to minimize the melt residence time at high temperatures.

It is important to realize that a well-mixed melt can degrade after TSE. An acceptable melting temperature for one product may not be acceptable for another product. For example, the RT in the underwater pelletizing mold takes only a few seconds before quenching the melt, which will allow higher melt temperatures without degradation. This is significantly different from RT in sheet or film front-end adapters and components including gear pumps, screen changers, and molds, where RT may be several minutes.

For different products, the same formula may also have different mixing requirements. For example, thin structures (ie films or fibers) will have a higher dispersion mixing threshold than thick-walled products (sheets or molded parts). In extreme cases, it may be necessary to perform post-mixing after granulation to average and minimize batch variance. This can also be seen as part of the "mixed experience". 

As we all know, the co-rotating twin-screw extruder is a powerful and flexible continuous mixing device that can mix countless formulas to make the plastic products we see and use every day. 

Development will continue to expand the scope and improve the quality of products manufactured through TSE, leveraging the unique geometric capabilities inherent in two interacting screws. Metering equipment, twin-screw extruders, and downstream systems should all work together to control the characteristics of the final part and manage the "mixing experience." 

About the author: Charlie Martin is the president and general manager of Leistritz, a major supplier of twin screw extrusion equipment for compounding, devolatilization, direct extrusion, pharmaceutical and other applications. Martin has worked in the extrusion industry for more than 30 years, is a member and former chairman of the SPE extrusion department, and has published dozens of papers at various technical conferences on various extrusion topics around the world. Contact: 908-685-2333; cmartin@leistritz-extrusion.com; leistritz-extrusion.com.

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