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 die moves relative to the container.
Contents
Rule based analysis of extrusion process would help to determine set of rules essential which are essential to take care while designing a product or even during cost estimation of the product. Some of the rules are discussed below
Material
Material of the profile to be extruded plays an essential role in determining the various process parameters and limitations of the process. For example, minimum thickness of extruded carbon steel sheet is 3mm whereas same sheet of aluminium can be extruded at minimum sheet thickness of 1mm. Variety of materials such as Carbon steel, aluminium, titanium, magnesium, ABS and PVC etc. can be manufactured via extrusion process.
Profile shape
Extrusion process can extrude sheets with a high variety of profile shapes but it is essential to take care of the profile features to ensure products feasibility and strength.
When deciding the wall thickness of profile, strength and cost efficiency are two main factors. Though Uniform wall thickness is most easy to manufacture, however when necessary wall thickness can easily be varied within a profile. If changes in the wall thickness are unavoidable, make them gradual rather than abrupt. Both thick and thin cross sections should be avoided as material tend to flow faster where thicker sections occur giving rise to distortions in the extruded shape.
Hence for an extrusion process wall thickness varies from 1mm(aluminium) to 32mm(PVC).
Extrusion processes cannot achieve razor sharp corners without additional fabrication. Corners should be rounded and a radius of 0.5-1mm is often sufficient enough. As far as possible sharp tips should be avoided as the tip can easily become wavy and uneven. Tips should be therefore also be rounded.
Solid Profiles reduce the die costs and are often easier to produce.
Often a variety of hollow profiles are very difficult to produce hence a hollow profile should be replace by two telescoping profiles, to ease the manufacturing of the product. In many cases reducing the number of cavities in a hollow profile makes it easier to extrude, this also increases die stability.
For profiles with pockets or channels, there is a basic rule that the width to height ratio should be approximately 1:3. This ensures that the strength of the die is not jeopardised. By using a large radii at the opening of the channel, and a full radius at the bottom the ratio can be increased to 1:4
Use of cooling fins on profiles greatly increases the heat dissipation area. This can be further increased by giving the fins a wavy surface. Where there is forced air-cooling longitudinally along the profile, it is better to leave the fins smooth. This helps to avoid the problem of eddy formation. An undulating surface increases the heat dissipation area of fins.
Surface Finish
During an extrusion process it is essential to take care of the surface finish of the exposed surfaces of the product. As a general rule, the narrower the exposed surface, the more uniform its finish. 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 by some design changes such as rounding the 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 tolerances are designed in to the required length of extrusion. 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 possible if lengths are cut off-line. A typical length tolerance for UPVC would be +/- 1mm on a 500mm length.
Extrusion Ratio
It is the ratio of the cross sectional areas of 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. It may net 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 ling to handle practically with even the shortest billets the press can extrude.