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Extrusion is a plastic deformation process in which raw material (billet) is forced to flow by compression through the die opening of a smaller cross-section area. The extrusion process is divided in two basic types: direct extrusion and indirect extrusion. In direct extrusion the billet is pushed through the die with ram pressure, whereas in indirect extrusion a die moves relative to the container.
Rule based analysis of extrusion process would help to determine a set of rules essential for consideration while designing a product, or even during cost estimation of a product. Some rules are discussed below.
Material of the profile to be extruded plays an essential role in determining process parameters and potential limitations of a process. For example, minimum thickness of extruded carbon steel sheet is 3mm whereas same sheet of aluminium can be extruded into minimum sheet thicknesses of 1mm. A variety of materials such as Carbon steel, aluminium, titanium, magnesium, ABS and PVC etc. can be manufactured via extrusion processes.
Extrusion processes can extrude sheets into a high variety of profile shapes, but it is essential to consider profile features, to ensure product feasibility and strength.
When deciding the wall thickness of any extrusion profile, strength and cost efficiency are two main factors. Though Uniform wall thicknesses are most easy to manufacture, wall thickness can easily be varied as necessary within a profile. If changes in the wall thickness are unavoidable, make them as gradual rather than abrupt variations. Thick with thin cross sections should be avoided, as material tends to flow faster where thicker sections occur, giving rise to more expected distortion in an extruded shape.
For an extrusion process wall thickness may vary from 1mm (aluminium) to 32mm (PVC).
Extrusion processes cannot achieve sharp corners without additional fabrication. Internal corners should be filleted with a minimum radius of 0.5-1mm, and sharp external edges should be rounded as those tips can easily become wavy and uneven.
Solid Profiles can reduce die costs and are often easier to produce.
Varieties of hollow profiles are often very difficult to produce, but a hollow profile can be replaced by two telescoping profiles, to ease product manufacturing. In many cases reducing the number of cavities in a hollow profile makes it easier to extrude, which can also increases die stability.
For profiles with pockets or channels, a basic rule is that the width to height ratio should be approximately 1:3. This ensures that the strength of the die is not jeopardised. When using larger radii at the opening of the channel, and a full radius at the bottom, width-to-height ratios could rise to 1:4.
Use of cooling fins on profiles greatly increases areas for heat dissipation. Surface area can be further increased by giving any fins a wavy surface. An undulating surface increases heat dissipation area of any fins. However, where there is forced air-cooling longitudinally along the profile, it can be better to leave fins smooth. This helps to avoid a problem of eddy formation. [1]
During an extrusion process it is essential to consider the surface finish of exposed product surfaces. As a general rule, the narrower an exposed surface, the more uniform its finish becomes. Webs, flanges and abrupt changes in metal thickness may show up as marks on the opposite surface of an extrusion, particularly on thin sections. The marking of exposed surfaces can be minimized with design changes such as rounding transitions, to reduce the chance of opposite-side streaking. [2]
Symmetry provides for more balance forces and helps avoiding over stressing areas of the extruding die. Hollow areas within the cross section, in particular, should be balanced. [3]
Some waste tolerances are often included in a required extrusion's length. It can be difficult and expensive to cut a perfect length during production, as metals or thermoplastics expand and contract at different temperatures. Greater accuracy is often possible if lengths are cut off-line. A typical length tolerance for UPVC might be +/- 1mm (0.2%) on a 500mm total length. [4]
Extrusion Reduction ratio is the ratio of the cross sectional areas in the shape of the die opening to that of the container through which the billet is pushed. A large-diameter billet pushed through a very small die opening has a high reduction ratio, and it may sometimes not be possible to extrude such a part. Ratios of 75:1 are common, though difficult.
The solution, however, for a difficult ratio shape is to make the part on a press with a smaller container. Another option is to use a multihole die that lets a number of profiles extrude simultaneously. They also come in handy for small shapes that are too long to handle practically, with even the shortest billets a press can extrude. [5]
A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.
A die is a specialized machine tool used in manufacturing industries to cut and/or form material to a desired shape or profile. Stamping dies are used with a press, as opposed to drawing dies and casting dies which are not. Like molds, dies are generally customized to the item they are used to create.
Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex cross-sections; and to work materials that are brittle, because the material encounters only compressive and shear stresses. It also creates excellent surface finish and gives considerable freedom of form in the design process.
Drawing is a manufacturing process that uses tensile forces to elongate metal, glass, or plastic. As the material is drawn (pulled), it stretches and becomes thinner, achieving a desired shape and thickness. Drawing is classified into two types: sheet metal drawing and wire, bar, and tube drawing. Sheet metal drawing is defined as a plastic deformation over a curved axis. For wire, bar, and tube drawing, the starting stock is drawn through a die to reduce its diameter and increase its length. Drawing is usually performed at room temperature, thus classified as a cold working process; however, drawing may also be performed at higher temperatures to hot work large wires, rods, or hollow tubes in order to reduce forces.
Blow molding is a manufacturing process for forming hollow plastic parts. It is also used for forming glass bottles or other hollow shapes.
Punching is a forming process that uses a punch press to force a tool, called a punch, through the workpiece to create a hole via shearing. Punching is applicable to a wide variety of materials that come in sheet form, including sheet metal, paper, vulcanized fibre and some forms of plastic sheet. The punch often passes through the work into a die. A scrap slug from the hole is deposited into the die in the process. Depending on the material being punched this slug may be recycled and reused or discarded.
Magnesium alloys are mixtures of magnesium with other metals, often aluminium, zinc, manganese, silicon, copper, rare earths and zirconium. Magnesium alloys have a hexagonal lattice structure, which affects the fundamental properties of these alloys. Plastic deformation of the hexagonal lattice is more complicated than in cubic latticed metals like aluminium, copper and steel; therefore, magnesium alloys are typically used as cast alloys, but research of wrought alloys has been more extensive since 2003. Cast magnesium alloys are used for many components of modern automobiles and have been used in some high-performance vehicles; die-cast magnesium is also used for camera bodies and components in lenses.
Plastics extrusion is a high-volume manufacturing process in which raw plastic is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weatherstripping, fencing, deck railings, window frames, plastic films and sheeting, thermoplastic coatings, and wire insulation.
6061 aluminium alloy is a precipitation-hardened aluminium alloy, containing magnesium and silicon as its major alloying elements. Originally called "Alloy 61S", it was developed in 1935. It has good mechanical properties, exhibits good weldability, and is very commonly extruded. It is one of the most common alloys of aluminium for general-purpose use.
Honeycomb structures are natural or man-made structures that have the geometry of a honeycomb to allow the minimization of the amount of used material to reach minimal weight and minimal material cost. The geometry of honeycomb structures can vary widely but the common feature of all such structures is an array of hollow cells formed between thin vertical walls. The cells are often columnar and hexagonal in shape. A honeycomb shaped structure provides a material with minimal density and relative high out-of-plane compression properties and out-of-plane shear properties.
Bar stock, also (colloquially) known as blank, slug or billet, is a common form of raw purified metal, used by industry to manufacture metal parts and products. Bar stock is available in a variety of extrusion shapes and lengths. The most common shapes are round, rectangular, square and hexagonal. A bar is characterised by an "enclosed invariant convex cross-section", meaning that pipes, angle stock and objects with varying diameter are not considered bar stock.
Impact extrusion is a manufacturing process similar to extrusion and drawing by which products are made with a metal slug. The slug is pressed at a high velocity with extreme force into a die or mold by a punch.
AlBeMet is the trade name for a beryllium and aluminium metal matrix composite material derived by a powder metallurgy process. AlBeMet AM162 is manufactured by Materion Corporation Brush Beryllium and Composites.
Roll forming, also spelled roll-forming or rollforming, is a type of rolling involving the continuous bending of a long strip of sheet metal into a desired cross-section. The strip passes through sets of rolls mounted on consecutive stands, each set performing only an incremental part of the bend, until the desired cross-section (profile) is obtained. Roll forming is ideal for producing constant-profile parts with long lengths and in large quantities.
Plastic forming machines, or plastic molding machines, were developed on the basis of rubber machinery and metal die-casting machines. After the inception of the polymer injection molding process in the 1870s, plastic-forming machines were rapidly developed up until the 1930s. With the gradual commercialization of plastic molding equipment, injection molding and extrusion molding became the most common industrialized processes. Blow molding is the third-largest plastic molding method after the injection molding and extrusion blow molding methods.
Extrusion in food processing consists of forcing soft mixed ingredients through an opening in a perforated plate or die designed to produce the required shape. The extruded food is then cut to a specific size by blades. The machine which forces the mix through the die is an extruder, and the mix is known as the extrudate. The extruder is typically a large, rotating screw tightly fitting within a stationary barrel, at the end of which is the die.
A die in polymer processing is a metal restrictor or channel capable of providing a constant cross sectional profile to a stream of liquid polymer. This allows for continuous processing of shapes such as sheets, films, pipes, rods, and other more complex profiles. This is a continuous process, allowing for constant production, as opposed to a sequential (non-constant) process such as injection molding.
Extrusion is a metal forming process to form parts with constant cross-section along its length. This process uses a metal billet or ingot which is inserted in a chamber. One side of this contains a die to produce the desired cross section and the other side a hydraulic ram is present to push the metal billet or ingot. Metal flows around the profile of the die and after solidification takes the desired shape.
Extrusion process can be done with the material hot or cold, but most of the metals are heated before the process, if high surface finish and tight tolerances are required then the material is not heated.
Friction extrusion is a thermo-mechanical process that can be used to form fully consolidated wire, rods, tubes, or other non-circular metal shapes directly from a variety of precursor charges including metal powder, flake, machining waste or solid billet. The process imparts unique, and potentially, highly desirable microstructures to the resulting products. Friction extrusion was invented at The Welding Institute in the UK and patented in 1991. It was originally intended primarily as a method for production of homogeneous microstructures and particle distributions in metal matrix composite materials.
Covema srl was a historic Italian company specializing in the design of plastic processing machinery, based in Milan, via Fontana 1. Founded in 1953 by the Terragni brothers, it also included the companies Corima spa, GBF spa, GBF iberica, RIAP srl, FIRS spa, Covepla Spain, Italproducts srl, Omam spa, TPA srl, AGRIPAK srl, Floraplant srl, Interfinance SA, Technical Die spa, Covema SAE. The technology that Covema has developed since the 1950s is merged into Agripak srl based in Milano and managed by the sons of Marco Terragni: Fabio Terragni (president), Patrizia Terragni and Massimo Terragni.
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