In the Mix Continuous Compounding Using Twin Screw Extruders

Medical Plastics and Biomaterials

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

Polymer compounds are used for an extremely wide variety of molded and extruded medical components and units. Such compounds are comprised of a foundation resin that's thoroughly blended with other components offering specific beneficial properties relating to this end product-for example, impression resistance, clearness, or radiopacity.

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

An important type of plastics processing machinery known as a twin-screw extruder can be used to combine fillers and additives with the polymer in a continuing manner, so the compound will perform as required and achieve the desired properties. Factors including the selection of corotation versus counterrotation, screw design parameters, and downstream-pelletizing-system and feeder-system configurations are all important design conditions for an effective compounding operation employing twin-screw equipment.

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

THE COMPOUNDING PROCESS

Compounding extruders are accustomed to mix together several materials into a homogeneous mass in a continuous process. This is achieved through distributive and dispersive combining of the many components in the compound as required (Figure 1). In distributive mixing, the components will be uniformly distributed in space in a uniform ratio without having to be divided, whereas dispersive mixing requires the breaking down of agglomerates. High-dispersive mixing requires that significant energy and shear participate the process.

Compounding extruders perform number of basic features: feeding, melting, blending, venting, and producing die and localized pressure. Various types of extruders can be used to accomplish these goals, including sole screw, counterrotating intermeshing twin screw, corotating intermeshing twin screw, and counterrotating nonintermeshing twin screw. The sort and physical type of the polymer substances, the real estate of any additives or fillers, and the degree of mixing required will have a bearing on machine selection.

Twin-screw compounding gadgets are primarily focused on transferring heating and mechanical energy to provide mixing and different support functions, with minimal regard for pumping. Various operations performed via this sort of extruder include the polymerizing of latest polymers, modifying polymers via graft reactions, devolatilizing, blending different polymers, and compounding particulates into plastics. In comparison, single-screw plasticating extruders are designed to minimize energy type and to improve pumping uniformity, and are inadequate to perform highly dispersive and energy-intensive compounding functions generally.

Among the typical process parameters that are managed in a twin-screw extruder operation are screw speed (in revolutions each and every minute), feed rate, temperatures along the barrel and die, and vacuum level for the devolatilization plant. Usual readouts contain melt pressure, melt temperature, electric motor amperage, vacuum level, and materials viscosity. The extruder electric motor inputs energy in to the process to execute compounding and related mass-transfer capabilities, whereas the rotating screws impart both shear and energy so as to mix the factors, devolatilize, and pump.

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

Twin-screw extruders work with modular barrels and screws (Figures 3 and 4). Screws will be assembled on shafts, with barrels configured as ordinary, vented, side stuffing, liquid drain, and liquid addition. The modular characteristics of twin-screw models provides extreme process overall flexibility by facilitating such improvements as the rearrangement of barrels, making the length-to-size (L/D) ratio much longer or shorter, or modifying the screw to match the specific geometry to the mandatory process task. As well, since wear is quite often localized in the extruder's solids-conveying and plastication section, only specific components may need to be replaced during preventive maintenance procedures. By the same token, expensive high-alloy corrosion- and abrasion-resistant metallurgies can be employed just where protection against use is needed.

SCREW DESIGN

The center of any twin-screw compounding extruder is its screws. The modular dynamics of twins and the decision of rotation and degree of intermesh makes practical thousands of screw design variables. On the other hand, there are some similarities among the various screw types. Forward-flighted factors are accustomed to convey components, reverse-flighted elements are used to create pressure fields, and kneaders and shear factors are used to mixture and melt. Screws can be made shear intensive or much less aggressive using the number and kind of shearing elements built-into the screw program.

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

Channel-low shear. The combining price in the channel in a twin is comparable to that of a single-screw extruder, and is lower than found in the other shear areas significantly.

Overflight/tip mixing-substantial shear. Located between your screw suggestion and the barrel wall, this place undergoes shear that, by some estimates, is as much as 50 times higher than in the channel.

Lobal pools-huge shear. With the compression of the material entering the overflight location, a mixing-price acceleration happens from the channel, with a effective extensional shear effect particularly.

Intermesh interaction-high shear. This is the mixing region between your screws where the screws "wipe," or nearly wipe. Intermeshing twins are obviously more shear-intensive in this area than will be nonintermeshing twins.

Apex mixing-substantial shear. This can be a region where in fact the interaction from the next screw affects the material mixing rate. Mixing factors can be dispersive or distributive. The wider the combining element, the considerably more dispersive its action, as elongational and planar shear results occur as substances are pressured up and over the land. Narrower mixing elements tend to be more distributive, with substantial melt-division rates and considerably less elongational and planar shear (Figure 5). Newer distributive combining elements allow for various melt divisions without extensional shear, that may be particularly ideal for mixing temperature- and shear-sensitive materials (Figure 6).

Single-screw extruders contain the channel, overflight, and lobal blending regions, but not the apex and intermesh ones. Because single-screw units lack these high-shear regions, they are generally not suitable for high-dispersive mixing. They are adequate often, however, for distributive combining 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 used and independent to optimize compounding efficiency. The four high-shear regions are independent from the amount of screw fill basically. Accordingly, at a given screw acceleration, as throughput is improved, the overall mixing decreases, since the low-shear channel combining place tends to dominate the four independent high-shear areas. If the extruder quickness is held regular and the throughput is normally decreased, the high-shear regions will dominate more, and better mixing will result. The same principle applies to corotating and counterrotating twins, each of which gets the same five shear regions.

In a traditionally designed counterrotating intermeshing twin, the top velocities in the intermesh area are in the same direction, which outcomes in an increased percentage of the supplies passing through the high-dispersive calender gap region on each turn. New counterrotating screw geometries will be less reliant on calender gap blending, and make use of the geometric freedom that's inherent in counterrotation to employ up to hexalobal mixing element, when compared with a bilobal aspect in corotation.

The top velocities in the intermesh region for the corotating intermeshing twin are in opposite directions. With this construction, materials are generally wiped from one screw to the different, with a comparatively low percentage getting into the intermesh gap. Materials have a tendency to extrusion systems follow a figure-eight pattern in the flighted screw areas, and most of the shear is normally imparted by shear-inducing kneaders in localized regions. Because the flight from one screw cannot clear the various other, corotation is bound to bilobal mixing elements at standard trip 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 suggestions can be designed for one mode on the other.

FEED SYSTEMS

Single-screw extruders are flood-fed machines generally, with the solitary screw speed determining the throughput amount of the machine. Because twin-screw compounders aren't flood fed, the outcome rate is determined by the feeders, and screw quickness is used to optimize the compounding performance of the process. The pressure gradient in a twin-screw extruder is certainly controlled and stored at zero for much of the process (Figure 7). This has substantial ramifications in regards to to sequential feeding also to immediate extrusion of something from a compounding extruder.

The selection of a feeding system for a twin-screw compounding extruder is really important. Components may be premixed in a batch-type mixing system and volumetrically fed into the main feed interface of the extruder. For multiple feed streams, each materials is separately fed via loss-in-excess fat feeders into the main feed slot or a downstream area (top or side feed). Each set up has advantages with respect to the product, the average manage size, and the type of the plant procedure.

When premix is feasible, a percentage of the entire mixing job is accomplished to the products appearing processed in the twin-screw extruder prior. The result could 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 due to 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 elements into the extruder. Loss-in-weight feeders accept a placed point and start using a PID algorithm to meter resources with extreme precision (normally <¡À0.5%). They are typically employed when elements segregate, when there are bulk density fluctuations of the feedstock, when a product is being extruded straight from the compounder, or when any additional factor exists that can lead to inconsistent metering. The feeders are interfaced with SPC/SQC operations readily. Multiple-component feed streams will be the better choice for larger-volume commodity production runs often.

The pressure gradient linked to the starved-fed, twin-screw extruder facilitates feeding downstream from the main feed port. Generally, there is near-zero pressure for much of the procedure. The localized pressure depends upon the screw style, 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 perhaps via twin-screw part stuffers for an array of other materials, in filler loadings seeing as high due to 80%. This separation of the process tasks combined with targeted introduction often effects in significantly less barrel and screw dress yourself in with abrasive elements and in a better-quality product.

DOWNSTREAM SYSTEMS

After the material passes through a filtering device, the merchandise emerging from the extruder should be converted into an application that can be handled by fabricating equipment. This consists of selecting a downstream pelletizer-generally a strand-cut normally, water-ring, or underwater system.

In strand-trim systems, the molten strands are cooled in a water trough and pulled through a water stripper by the draw rolls of the pelletizer. The pelletizer uses both top- and bottom-powered rolls, which feed the strands to a helical cutter. Die-face or water-band pelletizers slice the strands on or close to the die deal with with high-speed knives. The pellets are then conveyed right into a slurry discharge, which is pumped right into a dryer where the pellets happen to be separated from the normal water. In underwater pelletizers, the die deal with can be submerged in a water-stuffed housing or chamber, and the pellets happen to be water quenched.

Sometimes, users desire to extrude a item like a tube, film, sheet, or perhaps fiber directly out of your compounding extruder, bypassing the pelletizing operation thereby. This involves conflicting process goals often. For instance, to optimize compounding efficiency, the twin screws are likely to be managed in a starved method at excessive speeds, with a zero pressure gradient along a lot of the barrel. This can result in inconsistent or low pressure to the die, that is unacceptable for extruding something. If the screws happen to be run slower or stuffed more, pressure can be attained and stabilized but at the expense of an excellent compound. Gear pumps or takeoff single-screw extruders are sometimes attached to leading 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 weighed against those for a stand-alone compounding procedure. The takeoff gear pump or solitary screw becomes the expert device, with extruder and feeder speeds adjusted to that of the pump to keep a constant inlet pressure. A PID control algorithm is definitely developed that communicates with the feeder(s) and considers the residence period from the feeder through the extruder-generally about 1 minute. Each product operate on the machine will generally require a fair sum of development effort in regards to to the pressure control function.

Advantages connected with in-range extrusion from a good twin-screw compounder include the polymer having one-less heat and shear background, which often results in improved end-product properties, the elimination of pelletizing, the avoidance of demixing that may occur in the single-screw method, and the ability to fine-tune a formulation on-line to get quality assurance.

CONCLUSION

There are plenty of critical design conditions that a medical manufacturer should think about when installing a compounding system. They are influenced by the elements being processed, the specific end market where the product will be used, the common run size, and the nature of the plant where in fact the equipment will be located. Upstream downstream and feeding program options are no less important than the choice of counterrotation or corotation, or the shear intensity used in the screw design. Because many subtle dissimilarities are present between competing twin-screw settings, a user's own choices also enter the equation. All alternatives is highly recommended before a decision is simply finalized carefully.

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