The Compounding Process |
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The effective use of compounding has made it possible for plastic materials to replace metal in high strength products. For example, Nylon plastic is commonly filled with 40% glass fibers and used in the molding of large valves for sprinkler systems. The glass filled nylon costs approximately $40 compared to a brass valve that costs $340. Glass filled polycarbonate and Acetal have been produced in cams, levers, and wheels effectively replacing metal parts in clocks, kitchen appliances, car accessories, bicycles and electronic appliances. The most dramatic breakthrough has been the graphite, glass bead, glass fiber and carbon fiber filled polyphenylene oxide (PPO) plastic. This compounded plastic has replaced aluminum die cast parts in copiers, printers, plotters, camcorders, and video players. The use of this compounded grade of PPO has allowed these consumer items to be produced in higher qualities which makes them available to more people.
The compounded grade of PPO used in the manufacture of chassis is a good example of the advantage of plastics and additive blends. A plastic chassis part for a copier device must meet multiple service requirements to be effective. The part must be rigid and remain flat while the paper or plastic tape moves across the surface. It must have long term stability because any change in dimension will affect the repeatability and accuracy of the copies. Any medium moving across a good insulator will build up static electricity on the insulator's surface. Static electricity can curl paper and short out the sensitive electronic circuit boards. The plastic chassis, which is an excellent insulator, must provide some means for removing the build up of static electricity on its surface. The surface of the part must have dry lubricating qualities so that the paper or plastic tape slides easily across the surface. All these requirements can be met with a stiff polyphenylene oxide (PPO) plastic compounded with additives. The specialty compounded grade of PPO meets these demanding requirements. Polyphenylene oxide is a stiff metal like plastic material. By adding 10% glass fibers its dimensional stability is assured. Ten percent more carbon fiber can be added to reduce static electricity. For the carbon to be an effective conductor of static electricity it must be exposed to the surface of the part. To aid the carbon fiber migration to the surface during molding 2% silicone is added to the surface of the plastic pellet. Static build up is a direct result of friction between the plastic part and the paper or tape. To reduce the friction Teflon, in amounts of 5%, are added to the plastic mix. Teflon acts as a dry lubricant. Since PPO is a stiff material and difficult to mold glass bead are added to act as a processing lubricant. PPO in a natural state is brown in color yet the addition of 4% graphite will produce a dark black plastic. To better understand the compounding process, we will follow PPO from dry mixing, through extrusion, palletizing, and quality assurance. Dry Mixing A compounding line, illustrated in Figure 4-13, begins with dry mixing the plastic with its additives in a 500 pound capacity paddle mixer. The paddles are turned for about 30 to 90 seconds or until the mix is homogeneous. If the mix is too long, it will develop clumps of glass, while too short a mix will produce an unequal dispersion of glass fibers. Glass fibers are a 1/4" long and provided in bundles. These bundles must be broken up and the finer glass fibers evenly dispersed throughout the dry mixer with the plastic pellets. Carbon fiber, which is very brittle, is added and mixed for 10 to 20 seconds. If the process were longer the brittle carbon fibers would break into to short, ineffectual pieces. Other additives such as Teflon, an internal lubricant, is premixed with a graphite colorant (the color of carbon black) and then added to the paddle mixer. If Teflon powder is not fully dispersed white chunks may form; the process work like this: The mixer has a trap door in the bottom which, when opened, introduces material into the hopper feed tray. The tray then transports the material by (gravity and mechanical vibration) to the extruder hopper loading tray. Since the hopper loading tray is horizontal, and the material will not move to the hopper until vibration is introduced into the tray, the vibration's speed increases the rate of the materials to the hopper. The hopper resembles a large funnel which feeds dry mixed materials into the extruder screw. Silicone, a process lubricant, is added in quantities that equals 2% of the weight of the mixed material. The silicone is not added during the extrusion process, but is applied to the surface of the pellet after extrusion and pelletizing. Silicone added during the compounding process reduces the mixing effectiveness of the compounder. One method for adding silicone is completed by placing the pellets and silicone in a large 250 pound paper drum and tumbling the drum for approximately 30 minutes. The Extruder Compounder The extruder compounding process accepts material from the dry mixer into its hopper and forces the material past a two stage auger screw, then past a vent, through a breaker plate, and then out a multiple strand die. The dry mixed materials are loaded into the extruder hopper by a vibration tray. By increasing or decreasing the speed of the vibration, the operator controls the amount of materials delivered to the hopper. The compounding extruder is the same as the two-stage single screw extruder discussed earlier. The extruder mixes and compresses the material twice with a vented at the middle. Figures 4-8 and 4-9 illustrates a two stage compounding extruder. A breaker plate, also called a screen pack, Figure 4-14, is attached at the end of the extruder. The breaker plate is constructed of steel. The screen is formed by drilling several holes through the steel plate. The breaker plat add pressure to the flow of plastic by constricting the larger extruder melt stream though the small screen holes of the screen pack. Additionally, an adjustment valve is located between the breaker plate and extruder. By controlling the opening of the valve, back pressure is increased on the melt stream. The back pressure holds the plastic in the extruder longer increasing the mixing time. The combination of the valve and breaker plate control the flow of plastic out the extruder. A die is attached to the front of the breaker plate to form 10 strands of plastic with 3/16 " diameters. In order to accomplish the homogenization of this special grade of PPO several problems must be overcome. Glass has a tendency to clump at the opening of the hopper just before falling into the screw. Clumps do not mix well with the plastic and can result in clumps of glass imbedded in the plastic strands. During the compounding process, clumps of glass, carbon, and decomposed plastic build up between the screw flights and the vent area. These clumps are deposited in the melted strands, form into pellets, and if undetected may eventually become part of a part of a plastic part. The clumps will partially restrict the opening in the strand die which decreases the diameter of the finished pellets. Clumps of glass also build up and clog the breaker plate and screen pack. The intensive shearing action of the screw can also break the individual fibers into shorter lengths. Carbon fibers also suffer from fracture. Stainless steel and Kevlar fibers tend to fold. The pressure at the die head, which influences the intensity of the mixing between the screw flights, is maintained between 1,500 to 2,000 pounds per square inch. The hopper opening, vent, die head pressure, and die openings must be constantly monitored to assure the quality and consistency of compounded plastic.
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