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Extrusion Extrusion is a process used to create objects of a fixed cross section (geometry), cross-sectional profile by pushing material through a Die (manufacturing), die of the desired cross-section. Its two main advantages over other manufacturing pro ...
is a plastic deformation process in which
raw material A raw material, also known as a feedstock, unprocessed material, or primary commodity, is a basic material that is used to produce goods, finished goods, energy, or intermediate materials/Intermediate goods that are feedstock for future finished ...
(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 Ram pressure is a pressure exerted on a body moving through a fluid medium, caused by relative bulk motion of the fluid rather than random thermal motion. It causes a drag (physics), drag force to be exerted on the body. Ram pressure is given in ...
, 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

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 Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content is specified or required for chromium, cobalt ...
sheet is 3mm whereas same sheet of
aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
can be extruded into minimum sheet thicknesses of 1mm. A variety of materials such as Carbon steel, aluminium,
titanium Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
,
magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...
, ABS and
PVC Polyvinyl chloride (alternatively: poly(vinyl chloride), colloquial: vinyl or polyvinyl; abbreviated: PVC) is the world's third-most widely produced synthetic polymer of plastic (after polyethylene and polypropylene). About 40 million tons o ...
etc. can be manufactured via extrusion processes.


Profile shape

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.


Wall thickness

When deciding the wall thickness of any extrusion profile, strength and
cost efficiency Cost efficiency (or cost optimality), in the context of parallel computer algorithms, refers to a measure of how effectively parallel computing can be used to solve a particular problem. A parallel algorithm is considered cost efficient if its as ...
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).


Corner Radii

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 if possible

Solid Profiles can reduce die costs and are often easier to produce.


Fewer cavities in hollow profiles

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.


Profiles with deep channels

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 In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is the line segment or ...
at the opening of the channel, and a full radius at the bottom, width-to-height ratios could rise to 1:4.


Heat sinks

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.


Surface Finish

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,
flange A flange is a protruded ridge, lip or rim (wheel), rim, either external or internal, that serves to increase shear strength, strength (as the flange of a steel beam (structure), beam such as an I-beam or a T-beam); for easy attachment/transfer o ...
s 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.


Symmetry

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.


Length tolerances

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.


Extrusion ratio

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.


References

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