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Methods of Compounding
There are four basic methods used for compounding plastics with their additives: dry mixer, batch mixer, continuous mixer, and screw extruder. The selection of the method determined by the condition of the material, the volume of end product required, and the sensitivity of the material to breakdown during shear. The compounding process includes two stages: (1), mixing the materials and (2) forming the mixture into pellets, sheets, rods, or lumps.
Dry Mixers
Dry mixer are preferred for powders, plastic pellets with fibers, and plasticizer with PVC. Powders are dry, finely ground plastics and their ingredients. Plastic are often mixed with glass, carbon, or metals fiber to increase plastic materials strength and stability. PVC plastic is rigid and must be mixed with lubricating additives called plasticizer to make it flexible. There are four types of dry mixers: high speed impellers, ribbon mixers, paddle mixers and drum tumblers. The high speed impeller mixer, illustrated in Figure 4-1, is used for dry blending powdered resins, such as PVC with its plasticizer and other additives.
The mixer consists of an enclosed container with a higher speed impeller mounted at the bottom. An impeller is similar to the rotating blades of a helicopter. The impeller is capable of 80 to 3600 revolutions per minute (RPM). Mixing is fast with materials blended within 2-4 minute cycles. Heat generated during the mixing process must be drawn off through a cooling jacket to retard decomposition of the material and to block the development of lumps. There are two types of high volume/low intensity mixers. The ribbon mixer illustrated in Figure 4-2 and paddle mixers illustrated in Figure 4-3, are used to disperse fibers into plastic pellets.
Control of the mixing process is important in these mixers since fibers tend to clump together if the mixing process is too long. The capacity of low intensity mixers are in the hundreds of pounds. The large volumes are feed to extruder compounder. The normal cycles for the low intensive mixing of plastic with glass fibers usually takes from 10 to 20 minutes.
The fourth type of dry mixer is the drum tumbler illustrated in Figure 4-4. Drum tumblers are popular with molding operation for the dispersion of powdered colorants with the plastic pellets. In a drum tumbler, plastic pellets (with approximately 1 to 2% colorant) are placed in a 250 gallon paper drum and tumbled dry for approximately 30 minutes. Drum tumblers are used when the additives disperse easily throughout the mixture without tending to lump or cluster together.
Batch Mixers
The high intensity batch mixer, illustrated in Figure 4-5, is used for the processing of high viscosity plastics. This plastic is used for the production of vinyl film, sheeting, and vinyl-impregnated cloth. The batch mixer are used to feed callendering process. The callendering process consist of large multiple rolls than flatten the mixed dough like plastic into flat sheets. The thermoplastic materials of PVC, vinyl acetate, polyethylene, polypropylene, ABS and impact modified polystyrenes are the most often used. The thermoset materials of melamine and urea are also batch mixed and then rolled into flat stock for grinding into molding powders.
Batch mixers consist of an enclosed steel chamber with internal rotating and intermeshing blades. The rotating blades smear the material against the container wall while it shears and kneads the material at the center. After the cycle is complete, the glob of mixed material is then discharged, ready for callendering into sheets or processing into molding powder or pellets. A variation of the batch mixer is the Banbury mixer developed in 1916. The Banbury mixer uses two spiral shaped rotors and can be easily fed material through the top. When material is discharged the batch is complete and exits through a door at the side. The combination of smearing and intensive working produced by batch mixers results in a highly homogeneous mix. Batch mixers produce material in 2-4 minutes. Batch mixer range in sizes with machine capacities from 2 to 150 pounds.
Continuous Mixers
A continuous mixer, illustrated in Figure 4-6, is similar to the Banbury mixer. The material is continuously fed into a hopper end while mixed and transported, by intermeshing irregular screw device to the opposite end. The mixing takes place between the rotating screws and the chamber walls as the material is kneaded between the two screws and scraped off the wall surface. The amount and quality of the material can be controlled by adjusting the rotating speed of the screws, and the amount of material allowed to leave the chamber. The advantage of the continuous mixer over the batch mixer is its ability to provide a continuous mix of material with optimum dispersion. The continuous mixer is capable of production in quantities from 1,000 to 10,000 pounds per hour. The continuous mixer's significantly larger outputs provides the manufacturer of PVC film with higher product rates not possible with batch mixers.
Single Stage Screw Extruders
Screw extruders are the process tool used in compounding, injection molding, injection blow molding, extrusion, and extrusion blow molding. Screw extruder consist of a long auger screw that fills the opening of a steel tube. Plastic and additives that are placed at one end of the screw are mixed and melted when the screw turns. Understanding the mixing and melting capabilities of screw extruders is important in understanding compounding technology. They play a critical role in over 70% of the compounded plastic material. Screw extrusion technology is used as the plastic mixing and melting device in four more plastic processes. These four processes consume over 80% of plastic materials. These four process will be covered under process technology in the second half of this text.
A single screw extruders, illustrated in Figure 4-7, is classified by the ratio of the length of the screw to the inside diameter of the barrel. Standard extruders have 30 to 1 ratios. A 1" diameter extruder screw would be approximately 30" long. The size of extruders range from small laboratory models with 1/2 inch diameters to large production extruders with 8" diameter barrels. As the size of the extruders increase so does the ability to produce mixed plastic materials.
Extruder outputs range from a few pounds an hour up to 1,000 pounds per hour. Larger extruders are used to produce 24" diameter PVC pipe. The standard compounding extruders are 3 to 4" in diameter with output ranging to 500 pounds per hour. The output of an extruder varies with the melt viscosity of the plastic material. A 3.5" extruder can produce 400 pounds per hour of compounded polycarbonate; however, this drops to 350 pounds an hour when working with higher viscosity of polyphenylene oxide plastic.
The screw extruder functions in the same way as a friction pump. The principle of a friction pump can be demonstrated by placing a long board in water and then drawing the board out of the water. The friction between the water and the board's surface will hold some water on the board. This water can be removed by allowing it to drip off. The board can be turned into a rotating pump by twisting the board into the shape of a cork screw. The twisted board can be placed in a tube and laid on the bank of a water pond; the end in the water will act as a pump. By rotating the screw shaped board in the tube, water held at the bottom will be forced to move up the board due to friction between the board and the water. Turn the board fast enough and a large quantity of water can be moved out of the pool and lifted to irrigate crops. This principle was first discovered and used by the Egyptians. A plastic extruder compounder works with the same principle; where a steel auger screw represents the twisted board and the plastic represents the water.
The extruder used in compounding is enclosed in a steel tube with two small openings at both ends. One opening on top allows plastics and additives to enter. The second opening at the opposite end shapes the melted and mixed plastic into rods. Plastic in the form of pellets, powder, or granules enter through the small hole in the top of one end of an enclosed steel tube. This is illustrated in Figure 4-8. The steel tube, called a barrel, is wrapped with heating elements to maintain the barrel at the melt temperature of the plastic. A steel auger screw, with an expanding root diameter, fills the center of the tube. The plastic is pumped from one end of the screw to the other by rotating the screw. The rate at which the plastic moves from one end of the extruder to the other is controlled by the speed of the screw.
The screw is divided into three section. These section are easily identified by changes in the root diameter of the screw. Figure 4-9 illustrates a standard extruder screw and its three sections. The first section the root diameter remains constant and the material is mix, this section is called mixing. The second section the root diameter begins to expands compressing the material, this section is called the compression section. The final section the root diameter if full expanded and remain constant. In the section the material completely mixed and is called the metering section. The three section of the screw mix, compress and homogenize the plastic between the screw flights and against the walls of steel tube.
Extruder Mixing Principle
The rotation of the screw, the friction at the barrel wall, and the forward movement of the screw produce a three way internal mixing of pellets and additives. The mixing process takes place as the friction between the hot wall of the barrel and the plastic melts the outside of the plastic pellet. Rotating the screw rolls and tumbles the mixture back into the center of the space between the screw flights. The leading edges of the screw flight scrapes the melted plastic off the wall of the barrel and collects it into an expanding pool. This process of mixing is illustrated in Figure 4-10. The mixing takes place in all three sections of the screw: dry mixing in the first section, compression and melting in the second section, and complete homogenization in the metering section. The geometry of the screw, the root diameter, the angle of the screw flights change to alter the mixing intensity.
Extruder Screw Geometry
The mixing action within the extruder is controlled by screw geometry, the speed of the screw, and the back pressure provided by the constriction of the melt as it passes out the end of the barrel. The geometry of the screw significantly affect plastic melting and mixing. The angle of the screw flights can be changed to reduce the shearing force on the plastic. The length of each section of the screw changes for different materials. When the length of the compression section is increased, this reduces the shear intensity on the plastic. Many plastic materials are shear and heat sensitive. Shear sensitive materials have complex molecules that will decompose if they are compressed too quickly. Polycarbonate is one of these plastics and is called a shear sensitive material. The metering section of the screw can be lengthened to improve the homogeneity of temperature and color additives. There are general purpose screws designed for extruding low melt general purpose plastic materials, and there are specialty screws for compounding shear sensitive materials and for the mixing of abrasive additives.
Mixing Section
The plastic material and their additives are introduced into the mixing section at one end of the screw. In the mixing section, the screw's root diameter remains constant. The root of a screw is the center rod that the screw flights are attached and wrapped around. The root diameter increase in size from the mixing section to the metering section on an extruder screw. The root diameter accounts for approximately one third of the space between the barrel wall. As the screw rotates, plastic and additives are softened and mixed. The small constant root diameter in the mixing section mixes the plastic with additives without compressing them. This allows the irregular shaped pellets and additives to soften and settle into open spaces, becoming more compact, and mixing uniformly.
Compression Section
As the mixing ends, the compression section begins when the screw's root diameter gradually increases. The gradual increase in the screw's root diameter plus the rotating screw pushes the plastic pellets and additives against the wall of the barrel and screw flights. This compression intensifies the shearing and mixing action of the screw and completes the melting of the plastic. The compression section account for one third of the center of the screw. The length of the compression section can be shortened to increase the shear and mixing of the material or it may be lengthened to decrease these forces.
Metering Section
In the final stage, called the metering section, the root diameter is at its largest and remains constant from here on. The metering section intensifies the mixing action and provides for homogenization of the plastic melt with its additives. When the plastic leaves the extruder, it emerges with physical and thermal homogeneity, ready to be shaped by a strand die.
Two Stage Compounding Extruders
The high shear rates induced by the extruder's compressive section and the abrasive properties of certain additives require that a compounding extruder screw be constructed in two stages with six sections. Figure 4-11 illustrates a two stage extruder screw. The two stage screw is similar to two single stage screws attached end to end.
The first stage allows for an initial low intensity melting of the plastic with its additives. During the first stage of the compounding process, the heat from the barrel and the shearing action of the screw turns moisture into steam. Short molecules of plastic are decomposed into gas volatiles. These must be drawn out of the plastic melt prior to forming into strands Between the two stages the root diameter of the screw returns to the narrow diameter of the mixing section. In the narrow section between the two stages the mixing pressure is zero. In this section between the two stages any moisture and gas molecules generated in the first stage will be drawn off through by a vacuum center vent, see Figure 4-8.. The is an opening in the barrel, located between the first stage and second stage of the screw. A vacuum is drawn on the vent. Gas in the form of volatiles and steam are removed into a sump.
In the vent area plastic tends to build up on the cylinder wall and then decompose blocking the vent's ability and contaminating the material. Any moisture or volatile that remains will damage the final mixing of the plastic or be deposited in the final compounded pellet. Any leftover moisture and volatile gas contaminates the final plastic part and significantly reduce physical or chemical properties.
The second stage of compounder extruder provides a second opportunity for the introduction of shear sensitive additives. At the end of the final metering section, a one inch thick steel screen, called a breaker plate, restricts the plastic flow and provides back pressure on the mixing action. The size of the screen determines the amount of back pressure and contributes to the intensity of the mixing action. For color additives a fine wire screen is added, called a screen pack, between the breaker plate and the end of the metering section to assure color homogenization.
Twin Screw Extruders
The efficiency of the mixing action of the single screw extruder compounder depends on maintaining higher friction between the wall of the barrel and the material than between the materials and the screw surface. If the friction is higher on the screw surface, plastic will stick to the screw surface, decompose and clog the mixing process. In a single screw, extruder's plastic pastes, flakes. Sometimes low bulk density materials tend to stick to the screw surface. To overcome these restrictions, the twin screw extruder was developed. The twin screw extruder has two screws that are arranged side by side and intermesh. The intermeshing action of the two screws constantly self-wipes the screw flights. It is impossible for plastics to stick to the screw surface. The cross section of the barrel, Figure 4-12, is in the shape of a figure eight. The two screws rotate forcing the materials to form a figure eight pattern. The positive pumping action of the intermeshing screws allows for the compounding of all forms of plastic materials not possible with a single two stage extruder. |
