In the Mix: Continuous Compounding Using Twin-Screw Extruders

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Medical Plastics and Biomaterials

Versatile twin-screw systems can be used for compounding, devolatilization, or reactive extrusion—with the end products ranging from pellets and fibers to tubes, film, and sheet.

Polymer compounds are used for an extremely wide range of molded and extruded medical components and devices. Such compounds are composed of a base resin that is thoroughly mixed with other components that provide specific beneficial properties relating to the particular end product—for example, impact resistance, clarity, or radiopacity.

Twin-screw extruder with gear-pump front end and downstream profile system,.

An important type of plastics processing machinery known as a twin-screw extruder is used to mix fillers and additives with the polymer in a continuous manner, so that the compound will perform as required and achieve the desired properties. Factors such as the choice of corotation versus counterrotation, screw design parameters, and feeder-system and downstream-pelletizing-system configurations are all important design criteria for a successful compounding operation employing twin-screw equipment.

Single-screw extruders are commonly used to make products such as catheters and medical-grade films from pellets that have already been compounded. The primary function of these extruders is to melt and pump the polymer to the die, with minimal mixing and devolatilizing. The use of a single screw for such applications minimizes energy input into the process; such systems are in many ways the exact opposite of a compounding extruder, which is a high-energy-input device.

THE COMPOUNDING PROCESS

Compounding extruders are used to mix together two or more materials into a homogeneous mass in a continuous process. This is accomplished through distributive and dispersive mixing of the various components in the compound as required (Figure 1). In distributive mixing, the components are uniformly distributed in space in a uniform ratio without being broken down, whereas dispersive mixing involves the breaking down of agglomerates. High-dispersive mixing requires that significant energy and shear be part of the process.

Compounding extruders perform a number of basic functions: feeding, melting, mixing, venting, and developing die and localized pressure. Various types of extruders can be used to accomplish these goals, including single screw, counterrotating intermeshing twin screw, corotating intermeshing twin screw, and counterrotating nonintermeshing twin screw. The type and physical form of the polymer materials, the properties of any additives or fillers, and the degree of mixing required will have a bearing on machine selection.

Twin-screw compounding devices are primarily dedicated to transferring heat and mechanical energy to provide mixing and various support functions, with minimal regard for pumping. Various operations performed via this type of extruder include the polymerizing of new polymers, modifying polymers via graft reactions, devolatilizing, blending different polymers, and compounding particulates into plastics. By contrast, single-screw plasticating extruders are designed to minimize energy input and to maximize pumping uniformity, and are generally inadequate to perform highly dispersive and energy-intensive compounding functions.

Among the typical process parameters that are controlled in a twin-screw extruder operation are screw speed (in revolutions per minute), feed rate, temperatures along the barrel and die, and vacuum level for the devolatilization plant. Typical readouts include melt pressure, melt temperature, motor amperage, vacuum level, and material viscosity. The extruder motor inputs energy into the process to perform compounding and related mass-transfer functions, whereas the rotating screws impart both shear and energy in order to mix the components, devolatilize, and pump.

Twin-screw compounding extruders for medical applications are available commercially in three modes: corotating intermeshing, counterrotating intermeshing, and counterrotating nonintermeshing (Figure 2). Although each has certain attributes that make it suitable for particular applications, the two intermeshing types are generally better suited for dispersive compounding.

Twin-screw extruders use modular barrels and screws (Figures 3 and 4). Screws are assembled on shafts, with barrels configured as plain, vented, side stuffing, liquid drain, and liquid addition. The modular nature of twin-screw units provides extreme process flexibility by facilitating such changes as the rearrangement of barrels, making the length-to-diameter (L/D) ratio longer or shorter, or modifying the screw to match the specific geometry to the required process task. Also, since wear is often localized in the extruder's solids-conveying and plastication section, only specific components may have to be replaced during preventive maintenance procedures. By the same token, expensive high-alloy corrosion- and abrasion-resistant metallurgies can be employed only where protection against wear is needed.

SCREW DESIGN

The heart of any twin-screw compounding extruder is its screws. The modular nature of twins and the choice of rotation and degree of intermesh makes possible an infinite number of screw design variables. However, there are some similarities among the various screw types. Forward-flighted elements are used to convey materials, reverse-flighted elements are used to create pressure fields, and kneaders and shear elements are used to mix and melt. Screws can be made shear intensive or less aggressive based on the number and type of shearing elements integrated into the screw program.

There are five shear regions in the screws for any twin-screw extruder, regardless of screw rotation or degree of intermesh. The following is a brief description of each region:

Single-screw extruders possess the channel, overflight, and lobal mixing regions, but not the intermesh and apex ones. Because single-screw units lack these high-shear regions, they are generally not suitable for high-dispersive mixing. They are often adequate, however, for distributive mixing applications.

Virtually all twin-screw compounding extruders are starved-fed devices. In a starved twin-screw extruder, the feeders set the throughput rate and the extruder screw speed is independent and used to optimize compounding efficiency. The four high-shear regions are basically independent from the degree of screw fill. Accordingly, at a given screw speed, as throughput is increased, the overall mixing often decreases, since the low-shear channel mixing region tends to dominate the four independent high-shear regions. If the extruder speed is held constant and the throughput is decreased, the high-shear regions will dominate more, and better mixing will often result. The same principle applies to corotating and counterrotating twins, each of which has the same five shear regions.

In a traditionally designed counterrotating intermeshing twin, the surface velocities in the intermesh region are in the same direction, which results in a higher percentage of the materials passing through the high-dispersive calender gap region on each turn. New counterrotating screw geometries are less dependent on calender gap mixing, and take advantage of the geometric freedom that is inherent in counterrotation to employ up to a hexalobal mixing element, as compared to a bilobal element in corotation.

The surface velocities in the intermesh region for the corotating intermeshing twin are in opposite directions. With this configuration, materials tend to be wiped from one screw to the other, with a comparatively low percentage entering the intermesh gap. Materials tend to follow a figure-eight pattern in the flighted screw regions, and most of the shear is imparted by shear-inducing kneaders in localized regions. Because the flight from one screw cannot clear the other, corotation is limited to bilobal mixing elements at standard flight depth.

The above comparison of corotation and counterrotation is an extreme oversimplification. Both types are excellent dispersive mixers and can perform most tasks equally well. It is only for product-specific applications that definitive recommendations can be made for one mode over the other.

FEED SYSTEMS

Single-screw extruders are generally flood-fed machines, with the single screw speed determining the throughput rate of the machine. Because twin-screw compounders are not flood fed, the output rate is determined by the feeders, and screw speed is used to optimize the compounding efficiency of the process. The pressure gradient in a twin-screw extruder is controlled and kept at zero for much of the process (Figure 7). This has substantial ramifications with regard to sequential feeding and to direct extrusion of a product from a compounding extruder.

The selection of a feeding system for a twin-screw compounding extruder is extremely important. Components may be premixed in a batch-type mixing device and volumetrically fed into the main feed port of the extruder. For multiple feed streams, each material is individually fed via loss-in-weight feeders into the main feed port or a downstream location (top or side feed). Each setup has advantages depending on the product, the average run size, and the nature of the plant operation.

When premix is feasible, a percentage of the overall mixing job is accomplished prior to the materials being processed in the twin-screw extruder. The result can be a better-quality compound. Outputs may also be increased, since the screws can be run more "filled" compared with sequential feeding. Many processes do not lend themselves to premixing because of segregation in the hopper and other related problems. A premix operation is often desirable for shorter-run, specialty high-dispersion compounding applications, such as those with color concentrates.

Loss-in-weight feeding systems are often used to separately meter multiple components into the extruder. Loss-in-weight feeders accept a set point and utilize a PID algorithm to meter materials with extreme accuracy (normally <±0.5%). They are typically employed when materials segregate, when there are bulk density fluctuations of the feedstock, when a product is being extruded directly from the compounder, or when any other factor is present that can lead to inconsistent metering. The feeders are readily interfaced with SPC/SQC operations. Multiple-component feed streams are often the better choice for larger-volume commodity production runs.

The pressure gradient associated with the starved-fed, twin-screw extruder facilitates feeding downstream from the main feed port. Generally, there is near-zero pressure for much of the process. The localized pressure is determined by the screw design, facilitating downstream feeding of liquids or fillers such as barium sulfate.

Downstream feeding can be accomplished through injection ports for liquids, and into vents or via twin-screw side stuffers for a wide range of other materials, in filler loadings as high as 80%. This separation of the process tasks combined with targeted introduction often results in less barrel and screw wear with abrasive materials and in a better-quality product.

DOWNSTREAM SYSTEMS

After the material passes through a filtering device, the products emerging from the extruder must be converted into a form that can be handled by fabricating equipment. This normally includes selecting a downstream pelletizer—generally a strand-cut, water-ring, or underwater system.

In strand-cut systems, the molten strands are cooled in a water trough and pulled through a water stripper by the pull rolls of the pelletizer. The pelletizer uses both top- and bottom-driven rolls, which feed the strands to a helical cutter. Water-ring or die-face pelletizers cut the strands on or near the die face with high-speed knives. The pellets are then conveyed into a slurry discharge, which is pumped into a dryer where the pellets are separated from the water. In underwater pelletizers, the die face is submerged in a water-filled housing or chamber, and the pellets are water quenched.

Sometimes, users wish to extrude a product such as a tube, film, sheet, or fiber directly out of the compounding extruder, thereby bypassing the pelletizing operation. This often involves conflicting process goals. For instance, to optimize compounding efficiency, the twin screws are most likely to be operated in a starved manner at high speeds, with a zero pressure gradient along much of the barrel. This can result in inconsistent or low pressure to the die, which is unacceptable for extruding a product. If the screws are run slower or filled more, pressure can be gained and stabilized but at the expense of a quality compound. Gear pumps or takeoff single-screw extruders are sometimes attached to the front of the twin-screw compounder and used to build and stabilize pressure to the die.

The controls associated with attaching a front-end takeoff are more complex compared with those for a stand-alone compounding operation. The takeoff gear pump or single screw becomes the master device, with feeder and extruder speeds adjusted to that of the pump to maintain a constant inlet pressure. A PID control algorithm is developed that communicates with the feeder(s) and takes into account the residence time from the feeder through the extruder—generally about 1 minute. Each product run on the system will generally require a fair amount of development effort with regard to the pressure control function.

Advantages associated with in-line extrusion from a twin-screw compounder include the polymer having one-less heat and shear history, which often results in improved end-product properties, the elimination of pelletizing, the avoidance of demixing that can occur in the single-screw process, and the ability to fine-tune a formulation on-line in support of quality assurance.

CONCLUSION

There are many critical design issues that a medical manufacturer should consider when installing a compounding system. These are influenced by the materials being processed, the specific end market in which the product will be used, the average run size, and the nature of the plant where the equipment will be located. Upstream feeding and downstream system options are no less important than the choice of counterrotation or corotation, or the shear intensity used in the screw design. Because many subtle differences exist between competing twin-screw modes, a user's own preferences also enter into the equation. All alternatives should be carefully considered before a decision is finalized.

Charlie Martin is sales manager at American Leistritz Extruder Corp. (Somerville, NJ), which supplies twin-screw extruders and systems to the medical device industry.

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