What is extrusion processing?
Extrusion is a continuous form of plastics processing that takes place in a grouping of equipment called an extrusion line. Unlike injection molding or similar processes that rely on repeated “cycles” of mold-filling and part-extraction, extrusion is a process that is ideal for producing products continuously, with high quality, and in very high volumes.
Extrusion creates product continuously through the constant rotation of the resin compressing screw. The process uses a die that molten plastic is pushed through to create a specific shape and thickness.
Typical extruded products include:
- Plastic films used in construction, agriculture, packaging or other applications
- Rigid or flexible plastic sheets, sheet-based construction or packaging products, thermoformed products
- Plastic siding, plastic lumber or plastic/composite decking materials
- Plastic “profiles” for window frames, housings, moldings, or decorative purposes
- Flexible plastic tubing for medical, industrial or domestic applications
- Rigid plastic pipe used for home and commercial plumbing, gas lines, and drainage pipes.
- Plastic filaments used in 3-D printers
What is the role of auxiliary equipment in extrusion processing?
Obviously, extrusion processing capabilities rely first on having an extruder that is sized and equipped to provide the right type and volume of plasticized polymer (extrudate) to a process, whether that process involves making film, tubing, pipe, profiles, or sheets.
But without the help of the auxiliary equipment that supplies them with raw materials and then helps process extrudate into finished products, extruders alone couldn’t produce anything.
Think about it. Auxiliary equipment is vital to:
- Accepting, storing, and moving the high volumes of raw material required to support large extrusion lines.
- Properly preparing and drying certain plastics that adsorb moisture that can cause quality problems for medical-grade or commercial applications.
- Reliably delivering materials to extrusion lines, without contamination or moisture regain, and mixing in precise levels of process regrind to minimize process material waste.
- Accepting and shaping hot extrudate into precise dimensions while maintaining consistent process speed.
- Managing cooling and heat transfer to maintain consistent extrudate shape and quality.
- Measuring, cutting, coiling and collecting a continuous flow of finished extruded product.
- Continuously collecting, granulating, and reprocessing scrap material from the process (e.g. film edge trim, thermoform blanks, sheet trim, or pipe scrap) into re-usable material.
Auxiliary Equipment that supports Extrusion Processing
Extrusion lines are typically spoken of as having two parts or sides: Upstream and Downstream.
On the upstream side is all of the auxiliary equipment that receives, mixes/blends, and delivers resin and ingredients to the extrusion machine.
The extruder melts the plastic and delivers a continuous, pressurized stream of extrudate to all of the downstream tools and equipment.
Downstream auxiliary equipment first forms, cools, and hardens the extrudate into its finished shape, then measures, cuts, and collects the finished products (e.g., rolls of plastic film and cut-to-length profiles, pipes, or sheets) so that they can be prepped or packaged for shipment.
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Upstream Auxiliary Equipment for Extrusion Processing
Material Receiving and Storage
Because extrusion is a continuous process that can support very high production, many extruders need to receive and store huge quantities of plastic resin. And, the most cost-effective way to buy bulk resin is by railcar or truckload quantity. It takes specialized auxiliary equipment, such as railcar- or truck- unloading systems, to receive and transfer such large quantities of resin over distances up to 1,000 feet, then lift it to fill tall material storage silos. These systems should also include proofing mechanisms to ensure that new resin is loaded into the proper silo, since putting tons of the wrong material into a silo causes enormous contamination problems. Smaller resin deliveries can be moved to smaller capacity storage areas, such as surge bins, or over shorter distances using a cycling loader.
To move material (virgin pellets, post-consumer recycled pellets, regrind) from in-plant storage locations to extruders and auxiliary equipment like dryers or blenders, processors rely on vacuum conveying systems. These systems, which draw their motive power from a vacuum pump, usually include a plant-wide network of conveying lines; material loaders, receivers, and valves; resin selection tables and line proofing systems, and automated controls.
To deliver value for an extrusion processor, conveying systems must not only move required amounts of material to the proper locations, but must do so without damaging it (i.e., pellet breakage, dust/fines, pellet smearing, angel hair, streamers) due to excessive or uncontrolled conveying speeds. This wastes valuable material and cost processors money in two ways.
First, this damaged material is often trapped in conveying system filters and must be disposed of as trash (just six pounds/day equals one ton of wasted material per year!). Secondly, when dust/fines or angel hair gets into the plasticating barrel of an extruder, it typically heats up and melts much faster than undamaged pellets, resulting in burned or blackened specks that can make extruded parts cosmetically unacceptable.
To convey materials in high volumes, but at “safe” speeds that prevent damage, consider upgrading from older dilute-phase conveying systems to a slower-speed, dense-phase conveying system like Conair Wave Conveying.
To support high throughputs of extruded product, many extrusion operations use centralized drying operations. But whether material drying is done with a large central dryer, a multi-hopper drying cart, a machine-side dryer or a mobile drying unit, this auxiliary equipment is about much more than removing moisture.
Of course, consistent moisture removal is essential for processing any hygroscopic resin (i.e., PET, polycarbonate, ABS, nylon, etc.). That is because internal moisture in these resins can cause voids, discoloration, holes, or structural weakness in injection molded parts, resulting in diminished performance and appearance properties and, ultimately, scrap.
The other great advantage of consistent drying—for all resins—is that it “homogenizes” the material, enabling you to feed your extruder a steady flow of material that’s at a constant temperature. When you consider how hot material can get sitting in a silo during the summer—or how cold it could be in the winter—taking the opportunity to pre-dry and homogenize incoming material can make a big difference in part quality all year around.
Mixing and Blending
The appearance, performance, strength and quality of extruded products depends on feeding the extruder a consistent blend of raw materials that can include virgin pellets, pelletized post-consumer material, film or process scrap regrind, colorant, and/or performance- or property-enhancing additives. Delivering such blends consistently and continuously is the job of feeding, mixing, and blending equipment.
Blenders typically hold multiple ingredients in top-mounted hoppers or feeders, using digitally controlled “recipes” to measure each ingredient with a gravimetric weighing system before releasing it into a central mixing chamber for batch preparation. Simplicity, precision, repeatability and consistent reporting are essential to good blending operations, since even small variations in ingredients can change input costs and affect extruder output and quality.
Reusing the large amounts of process scrap produced in some extrusion lines can pose a serious blending challenge. For example, film extrusion lines continually produces so much “edge trim” they require in-line granulation systems that capture and process film scrap immediately (see “Downstream equipment” below). But because film scrap is very light relative to resin pellets, it doesn’t mix uniformly when reintroduced for processing in a conventional blender. To overcome this problem, film extrusion lines often use specialized blending equipment that can mix high-percentages of film regrind uniformly with virgin material to help ensure consistent extruder output.
The extruder itself is the last element of the upstream process. It accepts the blended raw materials through a feed throat, melts them into a homogenous mix, then pushes the melted material (“extrudate”) under pressure through external tooling—the start of downstream extrusion processing.
Extruders are available in a huge range of output capacities and types to accommodate different materials. Single-screw extruders are typical for pellets while twin screw extruders are more for powder materials. Sizes range from very small extruders that produce fine filaments, medical swab sticks or small-diameter, thin-wall tubing, to very large, high-output machines that can process thousands of pounds of material per hour into plastic-composite lumber and decking, or thick-walled plastic pipes several feet in diameter.
Every extruder relies on a master extrusion control that regulates not only its operation and output, but the speed of the downstream equipment. In particular, the extrusion control regulates the speed of the downstream puller equipment, which gently grips and draws the stream of extrudate from the extruder, through the tooling, measuring and cooling equipment, then on to cutting and final processing. Fine-tuning the speed and throughput of every element of the line through the master extrusion control is absolutely essential to maintaining the dimensional consistency and physical characteristics of every extruded product.
“Upstream” Auxiliary Equipment involved in Extrusion Processing
Process Requirement or Task:
|Material, ingredient, or batch storage
|Truck or railcar unloading systems
Large storage bins
|Vacuum conveying systems:
Loaders, Receivers, Valves
Material Selection Tables
Mobile conveying systems
|Material drying, pre-heating or dehumidifying
Mobile or mini systems
|Material weighing, ingredient dispensing & blending
Downstream Auxiliary Equipment for Extrusion Processing
Downstream extrusion processing begins when hot extrudate leaves the extruder. There are several major types of downstream equipment in a typical tubing, pipe or profile extrusion process, including:
Calibrators and dies
A sequence of these specialized tools accept the hot extrudate stream exiting the extruder and gradually shape the hot, moving plastic into a near-finished shape. In many cases, these tools extend right into the cooling equipment.
Vacuum and Cooling tanks
Immersion or water-spray cooling tanks transfer heat out of the extrudate as it forms so that it can cool, shrink, and harden into its finished dimensions. Cooling equipment varies based on the application: Larger extrusions, including pipe and decking, often use spray-cooling, while smaller flexible tube products use immersion cooling, often in vacuum cooling tanks. Drawing a vacuum in the cooling water helps hollow extrusions, like flexible tubing, to maintain their internal dimensions.
Temperature Control Units (TCUs)
TCUs support the cooling process, providing a steady flow of “tempered” water to cooling and spray tanks so that they can maintain a constant coolant temperature and a consistent rate of heat transfer as extruded product moves continuously through the cooling process. TCUs manage coolant temperature using an internal, thermostatically controlled circuit that can automatically heat or chill the circulating coolant as needed. Multiple TCUs and larger cooling circuits may be served by higher-capacity portable or central chilling units which provide much higher levels of chilling capacity.
In-Line Measurement Systems
Many extrusion lines utilize a series of instruments that measure extrudate dimensions in real time, then feed the dimensional information into the master process control. This allows for real-time adjustment of extruder output, or line speed as needed to produce size changes, adjust for variances, or maintain production consistency. Should product fall out of tolerance, these systems can identify “bad” segments and use positioning information to digitally mark and then cut out reject segments further down the line.
A “puller” is a motorized device that uses rotating belts or cleated tracks to pull extrudate through downstream equipment at a precise, finely adjustable speed. Extrusion line speed – and the speed of the puller – is regulated by the master extrusion control based on real-time readings from an in-line extrusion measurement system. Minute adjustments in puller speed, especially during forming and cooling processes, enable fine adjustment of dimensional characteristics and features (e.g., wall thickness, internal diameter) of the extruded product as needed to maintain consistency. Though their function is basically similar in every extrusion application, pullers vary widely in size, power and construction based on the size and type of extrusion being produced.
Sometimes integrated with pullers as part of puller/cutter units, cutters are used to cut extruded products – lumber, window profiles, housings, films, sheets — to the required length. Because extrusion lines are moving, many cutters operate on moving tables, synchronized to line speed and to digital measuring data from in-line measurement systems. There are two basic types of cutters:
- “Displacement” cutters make a single clean cut that displaces the extruded material on both sides. Displacement cutters may use a single knife or a rotating wheel of knives for smaller or softer extruded products, such as flexible tubing. For larger or thicker walled extruded pipe, planetary knife cutters rotate around the circumference of the extrusion, deepening the cut until it is completed. Displacement cutters are preferred when clean, particle free cuts are required.
- Saw-type cutters utilize a circular saw blade, housed in a moving table that makes cuts by removing a small strip of extruded material. Saw cutters are extremely fast and versatile, able to cut virtually any rigid extrusion in any size: sheets, pipes, decking, and profiles.
This equipment collects finished extrusions (cut-to-length products, rolls of film or flexible tube) and stages them for labeling, banding, or packaging prior to shipment.
Granulators and scrap-reprocessing equipment
As noted earlier, some high-volume extrusion processes, such as film extrusion, produce significant amounts of scrap material continuously, while others produce occasional scrap resulting from changeovers, interruptions, or variances that result in rejected products. In any case, size-reduction equipment, specifically granulators, are essential to minimize process waste and maximize your raw material investment. Granulating scrap parts or films, from edge trim to thermoforming blanks, sheet trim, or off-spec profiles, enables “closed loop” reclaim of virtually every bit of material that enters your plant.
Extrusion processors often need specially adapted granulation systems to accommodate the unique sizes, types and scrap volumes typical of their processes. Here are a few examples:
- Film extrusion processors use continuous granulation systems equipped with blowers to direct “ribbons” of lightweight edge-trim into granulator feed throats. Because lightweight film scrap quickly accumulates in volume, regrind is automatically conveyed back to material feeding and blending systems for immediate re-use.
- Thermoform extrusion processors may also require continuously-running, automatically fed granulation systems to prevent the accumulation of blanks from cluttering or delaying thermoform production.
- Plastic sheet processors typically need granulators with extra-wide feed throats and cutting chambers, enabling manual or automatic feeding of wide, narrow plastic sheet scrap.
Whatever the process, it is essential to size granulation systems and accessories to deal with scrap volumes effectively without process disruption. In-line granulation systems may be essential for managing continuous, high-volumes of scrap, while other processing operations may be able to keep pace by collecting scrap and granulating it off-line, either in machine-side or centralized granulation systems, with output stored as regrind that can be reprocessed later.
Downstream Auxiliary Equipment for Extrusion
Process Requirement or Task:
|Extrusion calibration and sizing
|Calibrators and dies
|Extrusion cooling/heat transfer
Immersion cooling tank
Vacuum cooling tank
Spray cooling tank
Temperature Control Unit
|Process equipment cooling for extruders, motors, hydraulics, pumps, or drying systems
Forced-draft or adiabatic cooling towers
Recirculating water systems
|In-Line Measurement Systems
|Finished product handling
|Take-off equipment (unloaders, racks)
Automatic coilers for film or flexible tubing
|Scrap granulation & reprocessing
Machine-side, operator fed granulators
Centralized granulation systems
The extrusion process would not be possible without the use of multiple types of auxiliary equipment upstream and downstream from the extruder.
In performing all of these vital roles, properly functioning auxiliaries address and eliminate a wide range of material, temperature, and process inconsistencies that can cause many common extrusion problems. Thus, they make a tremendous contribution to continuous, high-quality production.
To learn more about auxiliary equipment needed to support extrusion or for help in developing an extrusion line, you can contact Conair here.