May 1, 2008
By definition, extrusion is “the act or process of shaping by forcing through a die.” When pelletizing or specialty-forming a product, one must give consideration to the advantages of extrusion. The various methods of extrusion include ram, pellet mill, and screw type, and “force” is applied by piston, rolls, or screw (auger), respectively.
A ram extruder is a piston inside a cylinder. The piston must be retracted to charge the hopper before it can be extended into the barrel. Once charged, material is moved by the piston or plunger into the barrel and compressed against the die, creating pressure. Ram extrusion is intermittent due to this basic functionality.
A pellet mill typically employs two rolls inside a cylinder. The rolls are attached to an arm that rotates on the axis of the cylinder, and this causes the rolls to rotate against the cylinder’s inner diameter where the die plates are mounted. Product is introduced to the cylinder and forced through the dies with this rolling action. Pellet mills deliver continuously, but equipment size becomes an issue when significant throughput is required.
Screw-type extrusion incorporates an auger inside a feed hopper, which continues into a barrel. When material is introduced to the hopper, it is conveyed by the screw into the barrel and compressed against the die, creating pressure. It differs from the ram in that the screw rotates continuously to create a constant flow of material. The continuous nature of screw-type extrusion offers maximum productivity through the inherent design. However, many factors must be considered in order to maximize the extrusion process.
Sizing and OptionsWhen selecting equipment, it is important to know the throughput requirements, space constraints, input power requirement and profile specifications. The equipment manufacturer will also require product information and may request a sample of material for laboratory testing. This information is crucial in order to determine the appropriate equipment size, feed requirements, and screw and die design.
Once the extruder is sized, drive options should be taken into consideration. An overall compact design will afford more flexibility, whether designing a new facility or fitting a new line into an old facility, and various drive options add to this flexibility. A gear motor is the most compact and requires less maintenance than belts and sheaves, but space constraints sometimes dictate design. Where D/C used to be the standard, A/C motors and inverter controllers are now more common as constant torque is available at a fairly low cost in ratios ranging from 3:1 to 10:1. Though rarely required, ratios up to 1000:1 are available at a slightly higher premium.
Ancillary equipment as it relates to the desired end product also needs to be considered. In some pelletizing applications, it is sufficient to simply extrude onto a conveyor belt. The strands will eventually break into lengths that are acceptable. If size requirements are more stringent, it is necessary to cut the extrudate. Various means of cutting are available in forming applications, including die face, guillotine, traveling cutters, etc., and all are dependent on the specific application.
A multiple hole plate is commonly used in pelletizing (see Figure 1), while a forming die can range from a very simple, solid cylindrical shape, such as that used in electrical insulator pugs, to an extremely complex monolithic shape like those of automotive catalytic converters (see Figure 2). More complex shapes are not only more costly to manufacture but also require improved flow properties that sometimes necessitate the use of extrusion aids.
Die design is as important to the quality of the extrudate as it is to throughput of the extruder. Open area, land area, and material of construction directly relate to back pressure, and in turn, product density and screw efficiency. Open area refers to the actual cross-section of the die and is typically expressed in ratio to the cross section of the screw. The land area of the die is the portion that forms the shape of the product. Material of construction will affect pressure through friction. Carbon steel, for example, has a much higher coefficient than that of acetyl plastic.
Flow characteristics of the extruded material also impact screw and die design. A product with the consistency of bread dough will flow quite easily through any orifice. However, a material that is more like corn meal will be more difficult to process. Extrusion aides ranging from petroleum to soy to clay are available to increase a product’s lubricity and ease the extrusion process.
Design ConsiderationsThe overall design of the extruder with respect to how the material passes through is vital. For example, the buildup of residual material in the feed hopper is a common problem in the extrusion pelletizing of certain catalyst carriers. The buildup is harmless until it dries out, breaks loose and causes die plugging, which is costly because it reduces throughput and eventually requires downtime for die removal/replacement.
One solution to die plugging is to utilize an improved feed hopper design that includes counter-rotating feeders to keep the material from bridging and from building up on the hopper walls (see Figure 3). These feeders are configured above the screw in a manner that directs material into the screw efficiently, and they are designed to be virtually self-cleaning.
Another important factor is the clearance between screw and barrel. As clearance increases, the actual volume delivered per revolution decreases, along with die pressure. Conversely, a drop in die pressure can be an indication of increased clearances. It is important to monitor wear to ensure maximum efficiency. Tighter clearances mean higher efficiency, and maintaining a tight tolerance is crucial to maintaining output. Tighter tolerances, along with other design improvements, can be implemented to prolong the life of the barrel and screw. The advantages are maintained productivity, less downtime, reduced rebuild costs and less die plugging.
Temperature is important to extrusion for a number of reasons relative to each product or process. When designing cooling channels, it is therefore imperative to utilize as much surface area as possible and create flow patterns that transfer heat more efficiently. This is achieved through improved barrel, barrel jacket and screw design. Giving more attention to DT, dead spots and turbulence can greatly increase cooling efficiency.
Self-Monitoring SystemsIn the past, much of what determined the performance of an extrusion system were the individuals who controlled the process. The operators would determine the consistency of the mix, feed levels, extruder speeds, etc. based on their particular knowledge base and level of experience. Although some of these individuals deserve much commendation for the results they consistently obtained, others might not achieve the same quality or performance, thus creating a perpetual QC headache.
It is often better to limit human involvement and utilize a system that is self-monitoring and features the ability to adjust itself accordingly. Today’s electronics make these systems both practical and affordable. For instance, a mechanical feeder can be supplied that operates on an electronic feedback loop. Once the desired output is achieved, the extruder speed is set and the feeder adjusts itself to maintain a desired hopper level. The advantage is a more steady extrusion rate, more stable die pressure and consistent product density.
For more information regarding extrusion, contact Diamond America Corp., 151 E. Miller Ave., Akron, OH 44301; (330) 535-3330; fax (330) 535-3327; e-mail email@example.com; or visit www.diamondamericacorp.com.