Versatile twin-screw systems can be used for compounding, devolatilization, or reactive extrusion-with the ultimate end products which range 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 equipment. Such compounds are composed of a bottom resin that is thoroughly mixed with other components that provide specific beneficial properties associated with this end product-for example, influence resistance, clarity, or radiopacity.
Twin-screw extruder with gear-pump front side end and profile system,.
An important kind of plastics digesting machinery referred to as a twin-screw extruder is used to mix fillers and additives with the polymer in a continuing manner, so that the substance shall perform as required and achieve the required properties. Factors like the selection of corotation versus counterrotation, screw design and style parameters, and downstream-pelletizing-program and feeder-program configurations are important design standards for a successful compounding procedure employing twin-screw equipment.
Single-screw extruders are commonly 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 is to melt and pump the polymer to the with reduced mixing, die and devolatilizing. The use of a single screw for such applications minimizes energy input into the process; such systems are in lots of ways the exact reverse of a compounding extruder, that is a high-energy-input device.
THE COMPOUNDING PROCESS
Compounding extruders are accustomed to mix together two or more materials into a homogeneous mass in a continuing process. This is completed through distributive and dispersive mixing of the various components in the substance as required (Figure 1). In distributive blending, the components are uniformly distributed in space in a uniform ratio without being broken down, whereas dispersive mixing involves the wearing down of agglomerates. High-dispersive mixing necessitates that significant strength and shear be part of the process.
Compounding extruders perform a number of basic works: feeding, melting, mixing, venting, and growing 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 sort and physical type of the polymer resources, the houses of any additives or fillers, and the degree of mixing required will have a bearing on machine selection.
Twin-screw compounding equipment are primarily focused on transferring warmth and mechanical energy to provide mixing and different support functions, with minimal regard for pumping. Numerous functions performed via this kind of extruder include the polymerizing of latest 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 suggestions and to maximize pumping uniformity, and are generally inadequate to perform extremely dispersive and energy-intensive compounding functions.
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. Common readouts involve melt pressure, melt temperature, engine amperage, vacuum level, and materials viscosity. The extruder motor inputs energy in to the process to execute compounding and related mass-transfer features, whereas the rotating screws impart both shear and energy to be able to mix the factors, devolatilize, and pump.
Twin-screw compounding extruders for medical applications can be found commercially in three settings: corotating intermeshing, counterrotating intermeshing, and counterrotating nonintermeshing (Figure 2). Although each possesses certain attributes which make it suitable for particular applications, the two intermeshing types are usually better fitted to dispersive compounding.
Twin-screw extruders employ modular barrels and screws (Figures 3 and 4). Screws will be assembled on shafts, with barrels configured as plain, vented, area stuffing, liquid drain, and liquid compounding extruder addition. The modular character of twin-screw devices provides extreme process versatility by facilitating such alterations because the rearrangement of barrels, making the length-to-diameter (L/D) ratio much longer or shorter, or modifying the screw to complement the specific geometry to the required process task. Also, since wear is typically localized in the extruder's solids-conveying and plastication section, only specific factors may need to be replaced during preventive repair procedures. By the same token, expensive high-alloy corrosion- and abrasion-resistant metallurgies can be employed only where protection against put on is needed.
The cardiovascular system of any twin-screw compounding extruder is its screws. The modular character of twins and the decision of rotation and amount of intermesh makes feasible an infinite number of screw design variables. Nevertheless, there are several similarities among the various screw types. Forward-flighted factors are accustomed to convey substances, reverse-flighted elements are accustomed to create pressure fields, and kneaders and shear elements are used to blend and melt. Screws could be produced shear intensive or fewer aggressive based on the number and kind of shearing elements built-into the screw program.
You can find five shear regions in the screws for any twin-screw extruder, regardless of screw rotation or amount of intermesh. The following is a brief description of every region:
Channel-low shear. The mixing price in the channel in a twin is similar to that of a single-screw extruder, and is leaner than in the other shear areas significantly.
Overflight/tip mixing-great shear. Located between your screw hint and the barrel wall, this location undergoes shear that, by some estimates, is as much as 50 times greater than in the channel.
Lobal pools-big shear. With the compression of the material entering the overflight place, a mixing-pace acceleration happens from the channel, with an especially effective extensional shear impact.
Intermesh interaction-large shear. Here is the mixing region between your screws where in fact the screws "clean," or wipe nearly. Intermeshing twins are more shear-intensive in this area than are nonintermeshing twins obviously.
Apex mixing-high shear. This can be the region where in fact the interaction from the next screw affects the material mixing rate. Mixing elements could be distributive or dispersive. The wider the combining element, the considerably more dispersive its action, as planar and elongational shear results occur as materials happen to be forced up and over the land. Narrower mixing elements are more distributive, with large melt-division rates and significantly less elongational and planar shear (Figure 5). Newer distributive blending elements allow for many melt divisions without extensional shear, which may be particularly ideal for mixing high temperature- and shear-sensitive materials (Figure 6).
Single-screw extruders possess the channel, overflight, and lobal combining regions, but not the apex and intermesh ones. Because single-screw units absence these high-shear regions, they are generally not ideal for high-dispersive mixing. They are adequate often, however, for distributive mixing applications.
Practically 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 can be used and independent to optimize compounding efficiency. The four high-shear regions are independent from the amount of screw fill basically. Accordingly, at confirmed screw swiftness, as throughput is increased, the overall mixing decreases, because the low-shear channel combining location tends to dominate the four independent high-shear areas. If the extruder rate is held regular and the throughput is going to be decreased, the high-shear areas will dominate more, and better blending will result. The same principle applies to corotating and counterrotating twins, each of which has the same five shear areas.
In a designed counterrotating intermeshing twin traditionally, the top velocities in the intermesh region are in the same direction, which results in an increased percentage of the materials passing through the high-dispersive calender gap place on each turn. New counterrotating screw geometries will be less reliant on calender gap combining, and take advantage of the geometric liberty 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 construction, materials are generally wiped in one screw to the additional, with a comparatively low percentage entering the intermesh gap. Materials tend to follow a figure-eight design in the flighted screw areas, and most of the shear is undoubtedly imparted by shear-inducing kneaders in localized regions. Because the flight in one screw cannot clear the various other, corotation is bound to bilobal mixing elements at standard airline flight depth.
The aforementioned comparison of corotation and counterrotation can be an extreme oversimplification. Both types are great dispersive mixers and may perform most tasks similarly well. It is only for product-particular applications that definitive suggestions can be designed for one mode over the other.
Single-screw extruders are flood-fed machines generally, with the solo screw rate determining the throughput cost of the device. Because twin-screw compounders aren't flood fed, the result rate depends upon the feeders, and screw quickness is used to optimize the compounding productivity of the procedure. The pressure gradient in a twin-screw extruder can be controlled and maintained at zero for a lot of the procedure (Figure 7). This has substantial ramifications in regards to to sequential feeding and to direct 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 product and volumetrically fed into the main feed port of the extruder. For multiple feed streams, each material is individually fed via loss-in-excess weight feeders into the main feed port or a downstream location (top or area feed). Each setup has advantages with respect to the product, the average operate size, and the nature of the plant procedure.
When premix is feasible, a percentage of the overall mixing work is accomplished prior to the materials being processed in the twin-screw extruder. The total result can be a better-quality compound. Outputs can also be increased, since the screws could be run more "filled" weighed against sequential feeding. Many functions do not lend themselves to premixing because of segregation in the hopper and other related problems. A premix operation is often appealing 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 factors into the extruder. Loss-in-pounds feeders accept a established point and utilize a PID algorithm to meter elements with extreme precision (normally <0.5%). They are employed when materials segregate typically, when there are mass density fluctuations of the feedstock, whenever a product is being extruded directly 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 are often the better decision for larger-volume commodity production runs.
The pressure gradient linked to the starved-fed, twin-screw extruder facilitates feeding downstream from the main feed port. Generally, there's 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 for example barium sulfate.
Downstream feeding can be accomplished through injection ports for liquids, and into vents or via twin-screw part stuffers for a wide range of other materials, in filler loadings seeing that high as 80%. This separation of the procedure tasks coupled with targeted introduction often results in less barrel and screw utilize with abrasive components and in a better-quality product.
After the material passes through a filtering device, the products emerging from the extruder should 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 program.
In strand-cut 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 major- and bottom-powered rolls, which feed the strands to a helical cutter. Water-ring or die-deal with pelletizers cut the strands on or close to the die face with high-speed knives. The pellets are then conveyed right into a slurry discharge, which is pumped into a dryer where in fact the pellets are separated from the drinking water. In underwater pelletizers, the die face is submerged in a water-loaded 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 from the compounding extruder, thereby bypassing the pelletizing operation. This calls for conflicting process goals often. For instance, to optimize compounding proficiency, the twin screws are likely to be operated in a starved approach at big 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 packed more, pressure could be stabilized and gained but in the expense of an excellent compound. Equipment 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 tend to be more complex compared with those for a stand-alone compounding procedure. The takeoff equipment pump or single screw becomes the grasp device, with extruder and feeder speeds adjusted to that of the pump to keep a constant inlet pressure. A PID control algorithm is normally developed that communicates with the feeder(s) and takes into account the residence period 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 in regards to to the pressure control function.
Advantages associated with in-line extrusion from a good twin-screw compounder include the polymer having one-less shear and heat background, which results in improved end-product real estate often, the elimination of pelletizing, the avoidance of demixing that may occur found in the single-screw procedure, and the capability to fine-tune a good formulation on-line in support of quality assurance.
There are various critical design issues that a medical manufacturer should consider when installing a compounding system. They are influenced by the components being processed, the precise end market in which the product shall be used, the common run size, and the nature of the plant where in fact the appliances will be located. Upstream feeding and downstream system options are no less important than the selection of counterrotation or corotation, or the shear strength found in the screw style. Because many subtle variations exist between competing twin-screw modes, a user's own tastes also enter the equation. All alternatives ought to be carefully considered before a decision is definitely finalized.