Plastic extrusion is a high-volume production process in which raw plastics are melted and formed into a continuous profile. Extrusion produces goods such as pipes/tubes, weatherstripping, railings, deck railings, window frames, plastic films and tarpaulins, thermoplastic coatings, and wire insulation.
This process begins by feeding plastic materials (pellets, granules, flakes or powders) from the hopper into the barrel extruder. This material is gradually melted by the mechanical energy generated by turning the screw and by the heater set along the barrel. The liquid polymer is then forced into the dice, which forms the polymer into a hardened form during cooling.
Video Plastics extrusion
Histori
The first precursor of modern extruders was developed in the early 19th century. In 1820, Thomas Hancock invented a rubber "masticator" designed to reclaim the remnants of processed rubber, and in 1836 Edwin Chaffee developed a two-roller engine for mixing additives into rubber. The first thermoplastic extrusion was in 1935 by Paul Troester and his wife Ashley Gershoff in Hamburg, Germany. Shortly after, Roberto Colombo of LMP developed the first twin screw extruders in Italy.
Maps Plastics extrusion
Process
In plastic extrusion, the raw compound material is generally in the form of nurdles (small beads, often called resins) which are fed the gravity of the hopper mounted on top to the extruder barrel. Additives such as UV dyes and inhibitors (in liquid or pellet form) are often used and can be mixed into the resin before arriving at the hopper. This process has much in common with plastic injection molding from the point of extruder technology although different in that it is usually a continuous process. While pultrusion can offer many similar profiles in continuous length, usually with additional reinforcement, this is achieved by pulling the finished product from the dice instead of extruding the melted polymer through the dice.
The material enters through the feed throat (opening near the back of the barrel) and in contact with the screw. The spinning spinning (usually rotating at eg 120 rpm) forces the plastic beads forward into the heated barrel. The desired extrusion temperature is rarely the same as the temperature regulated by the barrel due to viscous heating and other effects. In most processes, the heating profile is set to a barrel in which three or more independent PID-controlled heating zones gradually increase the temperature of the barrel from the back (where the plastic enters) forward. This allows the plastic beads to melt gradually as they are pushed through the barrel and reduce the risk of excess heat which can cause degradation in the polymer.
The extra heat is caused by intense pressure and friction that occurs inside the barrel. In fact, if the extrusion line runs a certain material fast enough, the heater can be switched off and the melting temperature is maintained by pressure and friction only within the barrel. In most extruder, the cooling fan is present to keep the temperature below the specified value if too much heat is generated. If forced air cooling proves insufficient then a throated cooling jacket is used.
At the front of the barrel, the liquid plastic leaves the screw and moves through the display packet to remove contaminants in the melt. The screen is reinforced by a breaker plate (thick metal pieces with many holes drilled through it) because the pressure at this point can exceed 5,000 psi (34 MPa). The package plate assembly/screen breaker also serves to create back pressure in the barrel. Back pressure is required for uniform mixing and proper polymer mixing, and how much pressure can be "adjusted" by varying the composition of the screen packet (number of screens, wire size, and other parameters). The combination of breaker plates and screen packs also eliminates the "memory rotation" of liquid plastic and creates, "longitudinal memory".
After passing the plastic breaker plate breaker enters the die. Dying is what gives the end product its profile and should be designed so that the liquid plastic flows evenly from the cylinder profile, to the product profile form. An uneven flow at this stage may produce products with undesirable residual stress at certain points in the profile that may cause warping after cooling. Various forms can be created, restricted to continuous profiles.
The product must now be cooled and this is usually achieved by pulling the extrudate through a water bath. Plastic is an excellent thermal insulator and therefore difficult to cool quickly. Compared to steel, the plastic delivers heat 2,000 times slower. In pipes or extrusion pipes, the sealed bath tub is followed up with a carefully controlled vacuum to keep the newly formed and still liquid tubes and pipes from collapsing. For products such as plastic sheeting, cooling is achieved by pulling through a set of cooling rolls. For very thin films and layers, air cooling can be effective as an initial cooling stage, such as in blown film extrusion.
Plastic extruders are also widely used to reprocess recycled plastic waste or other raw materials after cleaning, sorting and/or mixing. This material is usually extruded into a suitable filament for cutting into bead or pellet stock to be used as a precursor for further processing.
Screw design
There are five possible zones in thermoplastic screws. Because terminology is not standardized in the industry, different names may refer to this zone. Different types of polymers will have different screw designs, some not combining all possible zones.
Most screws have these three zones:
- Feed zone (also called solid carrier zone): this zone inserts resin into the extruder, and channel depth is usually the same across zones.
- Melting zone (also called transition or compression zone): most polymers are melted in this section, and channel depth is getting smaller.
- Metering zone (also called melting zone): This zone melts the last particle and mixes with a uniform temperature and composition. Like feed zone, channel depth is constant throughout this zone.
In addition, the drilled (two stage) screw will have:
- Decompression zone. In this zone, about two thirds down the screw, the channel suddenly becomes deeper, which reduces the pressure and allows trapped gases (moisture, air, solvents, or reactants) to be pulled out with a vacuum.
- Second measurement zone. This zone is similar to the first metering zone, but with greater channel depth. It serves to suppress the melt to get it through the screen resistance and die.
Often, the screw length is referenced to its diameter as L: D ratio. For example, the 6-inch (150 mm) diameter screw at 24: 1 will be 144o, 12in (12ft) in length, and at 32: 1 the length is 192Ã,¼ ). The ratio of L: D 25: 1 is common, but some machines go up to 40: 1 for mixing more and more output on the same screw diameter. Two-stage screws (vented) are usually 36: 1 to account for two extra zones.
Each zone is equipped with one or more thermocouples or RTDs in the barrel wall for temperature control. The "temperature profile" ie, the temperature of each zone is very important for the quality and characteristics of the final extrudate.
Unique extrusion material
Special plastic materials used in extrusion include but are not limited to: polyethylene (PE), polypropylene, acetyl, acrylic, nylon (polyamide), polystyrene, polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS) and polycarbonate.
Off type
There are different types of dies used in plastic extrusion. Although there are significant differences between die type and complexity, all dies allow advanced extrusion of melt polymers, as opposed to non-continuous processing such as injection molding.
Blown film extrusion
The making of plastic films for products such as the longest shopping bag and tarpaulin is achieved using a blown film line.
This process is the same as the usual extrusion process to death. There are three main types of dies used in this process: annular (or crosshead), spider, and spiral. The annular death is the simplest, and depends on the melting polymer channeling around the entire dice cross section before it dies; this can produce an uneven flow. Spider dies consists of a central mandrel attached to an external die ring through a number of "legs"; while flow is more symmetrical than in annular dies, a number of weld lines are produced that weaken the film. Spiral dies eliminates the problem of weld lines and asymmetrical flow, but by far the most complex.
Melt cooled somewhat before leaving to produce a weak semi-solid tube. The diameter of the tube is rapidly expanded through the air pressure, and the tube is pulled upward with a roller, stretching the plastic in both transverse directions and drawing. Drawing and blowing causes the film to be thinner than the extrusion tube, and also preferably aligning the polymer molecular chain in the direction that sees most plastic strain. If the film is drawn more than blown (the diameter of the final tube close to the extrusion diameter) the polymer molecule will be very aligned with the direction of the draw, making a strong film in that direction, but weak in the transverse direction. A film that has a diameter larger than the extruded diameter will have more force in the transverse direction, but less in the direction of the draw.
In the case of polyethylene and other semi-crystalline polymers, such a cooling film crystallizes on what is known as the frozen line. As the film continues to cool, the film is drawn through several sets of yarn rollers to flatten it into a flat pipe, which can then be rolled or slashed into two or more sheets of rolls.
Extrusion sheet/movie
Extrusion sheets/films are used to extrude plastic sheets or films that are too thick to blow. There are two types of dies used: T-shaped and coat hanger. The purpose of this dies is to reorient and guide the melt polymer flow from a single rotation output from the extruder to a thin, flat planar flow. In both types of die ensure constant, uniform flow across the cross sectional area of ​​the die. Cooling is usually by pulling through a set of cooling rolls (calendars or "cold" rolls). In sheet extrusion, this roll not only provides the required cooling but also determines the thickness of the sheet and the surface texture. Often co-extruding is used to apply one or more layers above the base material to obtain certain properties such as UV absorption, texture, oxygen permeation resistance, or energy reflection.
The general post-extrusion process for plastic sheets is thermoforming, in which the sheets are heated to soft (plastic), and are formed through the mold into new shapes. When a vacuum is used, it is often described as a vacuum. Orientation (ie the ability/density available from sheets to be drawn to a mold that can vary in depth from 1 to 36 inches) is very important and greatly affects the formation of cycle times for most plastics.
Pipe Extrusion
Extrusion tubes, such as PVC pipes, are manufactured using very similar molds such as those used in blown film extrusions. Positive pressure can be applied to the internal cavity through the pin, or negative pressure can be applied to the outer diameter using a vacuum suction to ensure the correct final dimensions. Lumens or additional holes can be introduced by adding the corresponding inner strands to the dice.
Multi-layer tubing applications have also existed in the automotive, pipe & amp; heating industry and packaging industry.
Exceeded extrusion coating
More than jacketing extrusion allows for the application of the outer layer of plastic to the existing wire or cable. This is a typical process for isolating cables.
There are two types of die tooling used to coat the cable, tubing (or jacketing) and pressure. In the jacketing tooling, the melted polymer does not touch the inner wire until just before the lips die. In the pressure tool, contacts melt the inner wire long before it reaches the dead lips; this is done at high pressure to ensure good melting adhesion. If intimate contact or adhesion is required between the new layer and the existing wire, the pressure tool is used. If undesired adhesion is required, use the jacketing tooling.
Coextrusion
Coextrusion is the extrusion of several layers of material simultaneously. This type of extrusion uses two or more extruders to melt and produces stable volumetric throughput from different viscous plastics to a single extrusion head (die) that will extrude the material in the desired shape. This technology is used in all the processes described above (blown film, overjacketing, tubing, sheet). The thickness of the layer is controlled by the speed and relative size of the individual extruder that sends the material.
In many real-world scenarios, one polymer can not meet all application requests. Compound extrusion allows the mixed material to be extruded, but the coextrusion maintains the separate material as different layers in the extruded product, enabling proper placement of materials with different properties such as oxygen permeability, strength, stiffness, and wear resistance.
Extrusion layer
Extrusion coatings use blown or printed film processes to coat additional layers onto rolls of paper, foil or film. For example, this process can be used to improve the paper characteristics by coating them with polyethylene to be more resistant to water. The extruded layer can also be used as an adhesive to carry two other materials together. Tetrapak is a commercial example of this process.
Extrusion compound
Extrusion blending is a process that mixes one or more polymers with additives to produce plastic compounds. Feed may be pellets, powders and/or liquids, but these products are usually in pellet form, for use in other plastic forming processes such as extrusion and injection molding. Like traditional extrusions, there are different sizes of machines depending on the application and the desired throughput. While single or dual screw extruder can be used in traditional extrusion, adequate mixing requirements in extrusion combinations make the twin screw extruder all but mandatory.
Extruder type
There are two sub-types of twin screw extruders: co-rotating and counter-rotating. This nomenclature refers to the relative direction of each rotating screw compared to the others. In joint-rotation mode, the two screws rotate clockwise or counter-clockwise; in counter-rotation, one screw rotates clockwise while the other rotates counter-clockwise. It has been shown that, for certain cross-sectional areas and intermeshing rates, the axial velocity and mixing rates are higher in the twin extracts that rotate together. However, the accumulation of pressure is higher in the reversible extruder. The screw design is generally modular in various delivery and mixing elements arranged on the shaft to allow for quick reconfiguration of process changes or replacement of individual components due to wear or corrosive wear. Engine sizes range from 12 mm up to 380mm [12-Polymer Mixing by James White, pages 129-140]
Benefits
The great advantage of extrusion is that a pipe-like profile can be made of any size. If the material is flexible enough, the pipe can be made long and even rolled on the roll. Another advantage is the extrusion of pipes with integrated couplers including rubber seals.
See also
References
Bibliography
- Todd, Robert H.; Allen, Dell K.; Alting, Leo (1994), Manufacturing Process Reference Guide , Industrial Press Inc., ISBN 0-8311-3049-0 < span> .
Source of the article : Wikipedia
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