Expanded Metal

Chapter 1: What is Expanded Metal?

Expanded metal is a sheet metal mesh. It is made by stretching a slitted sheet of malleable metal. The orientation of the slits is perpendicular to the direction of the stretch. The result of such stretching is an array of holes on the slits – to give a mesh form.

The concept emanates from the idea that metals change size – expand and contract – when subjected to various conditions. The most natural of these changes arise from the chemistry of these metals and the behavior of their atoms. This brings about the development of expanded metal.

Expanded metal is distinct from, and competes with, the following:

Perforated Sheet

Perforation is the punching of holes, in this case, into a metal sheet. The result of such perforation is a (metal) perforated sheet. In many cases, sheets that can be perforated can also be expanded. Perforation is a subtractive process – the punching of holes implies the removal of material. This comes as a disadvantage as the removed material is regarded as wastage. Perforation does not subject the perforated metal sheet to stretching, at least not necessarily. One of its advantages is the feasibility of very small openings and drainage casings for various applications. It can also have shapes that are not feasible with other meshing techniques – this comes in handy if the mesh is intended for decorative purposes.

Computer equipment often comes fitted with perforated metal sheets as part of the housing.

Woven Mesh

In this meshing process, metal wires are bought together more like threads of a piece of cloth. The weaving is held together by friction together with the rigidity of the wire, otherwise there is no bonding between individual wires. The edges tend to unravel if not bound. Screens for sieving sand are often woven.

Welded Mesh

An array of straight wires is placed on top of another, with the two arrays being perpendicular to one another. A welded joint is placed wherever any two wires meet. This mesh is usually used when large spaces are left. Finer meshes are less convenient to make because of the substantial number of welds required per unit area.

Welded mesh is commonly used in concrete reinforcement.

Chapter 2: Material Types of Expanded Metal

Technically, all malleable metals can be expanded. However, many of them cannot be viably expanded at an industrial scale. Some of those materials are discussed below.

Copper Expanded Metal

Traditionally, copper is not the first-choice structural material, that is, if the considerations are purely structural. Also, copper is relatively more costly, especially when rated per unit volume. Volume is a particularly important parameter since it influences the eventual dimensions of the expanded metal sheet. Copper’s high specific weight counts as a demerit where weight is of concern, such as in rigging.

The usage of copper comes when there are more aspects of significant concern other than structural concerns. Such include:

• Corrosion resistance – essential when the intended application exposes the expanded metal sheet to moisture or other reactive environments.
• Electrical conductivity – copper is a better electrical conductor than most metals that are available on an industrial scale.
• Appearance – where the mesh is intended for decorative purposes, copper can be a good choice.
• Malleability – the malleability of copper makes it easier to cut and stretch, often into more complex shapes.

Expanded Aluminium

Structurally, Aluminium has a fair number of applications. It is fairly strong and relatively lightweight. It is also a good conductor – good enough to be the main material used in overhead power cables.

Aluminum is malleable, making it easier to work than steel (when cold). It is also corrosion resistant. Its high melting point and good thermal reflectivity make it a good fit for high temperature application.

Expanded Steel

Steel is the most common structural metal. It has the highest strength amongst the metals listed in this compilation – at least for as long as it is not corroded. There are several types of steel:

• Expanded Mild Steel

  This gives a strong and cost-effective solution. It is often hot dipped into molten zinc for galvanizing.

  
  

• Grating Metal Sheet

  This expanded metal sheet is produced from higher gauge metal sheets, usually of mild steel.

• Stainless Steel Expanded Metal

  Stainless steel is less susceptible to corrosion than mild steel and performs better for high temperature applications.

Chapter 3: Manufacturing Process of Expanded Metal

The manufacturing process of expanded metal involves various steps. These steps will be detailed below.

Sheet Metal Selection

The first step is the selection of sheet metal. The choice made is influenced by the respective application for which the expanded metal is required.

The following are among the properties considered when choosing the metal:

• Rigidity
• Corrosion resistance
• Strength
• Appearance
• Specific weight
• Toughness
• Flexibility
• Elasticity

Apart from the chemical (and structural) nature of the metal, another important aspect is the dimensions.

Dimensions of Metal Sheet

The aspects considered to be dimensions of the metal sheet include:

• The total area covered by the metal sheet before expansion
• The thickness of the metal sheet before expansion
• The flatness and squaring
• Size of each cut, hence the size of each hole
• Mesh dimensions
  
  
• For diamond meshes, the following is usually considered:
  • Long way of diamond – the larger diagonal. The long way of diamond is the dimension along which rows are defined.
  • Short way of diamond – the shorter diagonal. The short way of diamond is the dimension along which the die moves when stretching a piece of metal.
  • The long way of diamond and the short way of diamond become equal when a square mesh is being produced.
  • Long way of opening – the larger diagonal of the opening
  • Short way of opening – the smaller diagonal of the opening
• Extent of flattening (if any), and leveling
• Total area covered by the sheet after expansion
• Thickness of the sheet after expansion
• Distance between the cuts hence the thickness of the strands. This is controlled by controlling the speed of the feed during the cutting and stretching process during expansion.
• Before expansion, the metal sheet is cut to size. The cutting is often done with an automatic guillotine. The desired pattern determines the size of the expanded metal sheet.

Cutting and Stretching Metal

Cutting is the process that initiates openings on the expanded metal. It determines the eventual dimensions of the openings. The hole cutting and stretching can be two separate processes or can be accomplished in one operation. The latter is more common.

A die is used, which defines half a row for the entire width of the sheet. The metal sheet is placed flashing with the inner edge of the die, which also flashes with the platform the sheet will be resting on. The metal sheet is then fed underneath the die at a rate equal to the desired thickness of each strand. The die comes down, cutting and stretching the sheet in one move (to make the first half row), and goes back up.

If the shapes that are to be on the expanded sheet metal are all the same size and shape, the same die will be used in the entire expansion, in which case:

• The die moves sideways the distance equal to the maximum dimension of each shape along that axis.
• The sheet is fed again.
• The die comes back down, cutting and stretching the second half of the first row, simultaneously cutting the first half of the second row.
• The next cut and stretch complete the second row, simultaneously cutting the first half of the third row, and so on.

If the shapes to be developed are of different sizes or shapes, two different but complementing dies are used. They alternate in descending to create the shapes.

This manner of cutting and feeding gives an expanded metal sheet with a raised surface, that is, a ridged surface. Sheet metal with a raised surface is also called standard expanded metal.

The coolants and lubricants used by the presses during the expanding process often remain on the surface of the expanded material. These may be left there, especially for expanded metal that is not intended to have any other finishing done post expansion (and flattening). Such chemicals may (inadvertently) assist the expanded metal fight corrosion.

The said chemicals may also be undesirable when finishing is to be done on the expanded metal and/or where appearance is a factor. It follows that they can be removed – mainly by chemical means such as using a detergent.

Shapes of Openings

Diamond shape – this is the most common shape for expanded metal sheets.

Hexagonal shape – also a common shape. It is a geometrical extension of the diamond achieved by elongating the bond along the plane that is perpendicular to the direction of the feed. Compared to the diamond shape, the hexagonal shape is more difficult to execute bond shearing on.

Square shape – a variation of the diamond shape where the diagonals are equal.

Louvered – the metal is pressed to give a louvered finish. This type of opening usually gives the least expansion, that is, an increase in total outer dimensions of the metal sheet.

Decorative – many other shapes can be custom made to meet certain decorative and architectural requirements.

Strand Thickness

For any given die shape, which is to imply that for any given shape and size of the openings, the thickness of the strands influences the overall percentage of open spaces on the expanded metal sheet. The strand thickness (for a given die) also determines the eventual ratio of the overall area of the expanded metal sheet to that of the solid sheet from which the former was made.

Strand thickness is, in turn, determined by feed rate during the stretching process. A higher feed rate yields expanded metal sheets with a smaller percentage of open space.

A single sheet can have sections with varying strand thickness. This is achieved by adjusting the feed rate as may be required.

Flattening Process

This process gives the expanded metal a flat surface finish, where such is desirable. Often, the cutting and stretching process leaves a ridged surface or a rippled finish on the expanded metal. Such a surface is also known as a raised surface. The raised finish is desirable in some cases, but not in others. It is in the latter case where flattening is required.

Expanded material that is not flattened is often referred to as standard expanded metal, as it comes out of the expanding machine. Flattening is achieved by cold rolling the standard expanded. The compression from the cold rolling makes the expanded metal thinner and flat. As the cold rolled expanded metal sheet gets thinner, it gets longer along the direction of movement of the sheet through the roller.

The said lengthening of the expanded sheet metal respectively stretches the openings of the sheet. The stretching of the openings will be along the direction of lengthening of the entire sheet itself. Thus, the openings may appear elongated, or widened, as compared to the standard expanded sheet equivalent.

Metal Leveling

This process should not be confused with flattening. Flattening removes the ridged profile on the surface of the expanded metal. Leveling ensures that the sheet levels when placed on a flat and level surface. Flattening flattens the surface, while leveling flattens the entire sheet. The property of being level is spatial – it concerns the three-dimensional geometry of the expanded metal. It is a measure of how well corresponding points in different rows fit into the same plane.

A level expanded metal sheet is one that is free from waves and buckles. Since eliminating those waves and buckles completely is impractical to achieve consistently, tolerances are set to determine what is acceptable.

Chapter 4: Surface Finishing and Shearing of Expanded Metal

This chapter will discuss surface finishing and shearing of expanded metal.

Surface Finishing of Expanded Metal

For many applications, the expanded metal sheet is used as it comes from the roller, or in many cases from the stretching machine, without any other work on the sheet.

However, many other applications require the expanded metal sheet to undergo additional surface finishing. These include the following:

Finishing Through Painting

This is often the most economical and cost-effective way to finish the expanded metal sheet. The paint may be applied for appearance or corrosion protection. For the latter, the paint will have to remain intact for protection to subsist. Small gaps in it allow corrosion to spread underneath the otherwise sound looking paint coating. Amongst the coatings listed herein, painting is the least costly, has the least effectiveness as a corrosion barrier, and has the greatest requirement for re-doing.

Powder Coating

This process often yields similar results to those from painting – albeit using plastic instead. This process is typically more durable than painting. A spray gun may be used to apply the powder onto the metal sheet, with the powder often being electrostatically charged. The powdered sheet is then heated to make the plastic bond with the metal surface.

Galvanizing Process

This process involves submerging the expanded metal sheet into molten zinc. This gives it a coating of zinc. Such coating protects the mesh from corrosion. Given the temperatures at which this process is feasible, it gives a limit to the dimensions of expanded metal that can be galvanized. Very fine meshes cannot be galvanized for this reason.

Anodizing Process

This is an electroplating process. The name anodizing denotes the polarity of the expanded sheet metal (to be coated) during electrolysis. The sheet metal is coated at a much more granular level than with the rest of the coating techniques. Also, the bonding of the coating to the sheet metal is better than other methods. The process is fairly costly over and above the cost of the plating material. The apparent unpopularity of this method emanates from its cost implication.

Anodizing is usually common with iron-based metals (which are prone to corrosion). This process circumvents some of the dimension limits that arise from the galvanizing process since it can be carried out at much lower temperatures.

Shearing of Expanded Metal

Shearing cuts the expanded sheet to size. This is the process used to attain smaller expanded metal sheets from a larger sheet or reduce the size of a given sheet and/or alter its shape. The shearing is done with shears. Shears are a set of metal blades or some mechanism such that the blades come together, like a scissors. The shears can be manual or mechanized. When the blades come together on an expanded metal sheet, they exert enough shear forces to tear the expanded metal apart.

There are several types of shearing. These will be detailed below:

• Side Shearing

  This is the shearing along the long way of diamond

• Bond Shearing

  Bond shearing tears the sheet exactly on the edge, such that there are closed shapes only at either side of the tear. It matches the shapes.

  
  

  Raised expanded metal should be bond sheared along the bonds between rows.

• Random Shearing

  Random shearing does not necessarily cut along shape edges. It, therefore, leaves open shapes and irregular patterns at the edges. This process leaves out spiked strands that can be sharp and piercing. It follows that random sheared expanded metal should have U-edging to facilitate better handling. Framing can also be done as an alternative to U-edging.

  U-edging is the use of a U-shaped strip of metal to cover the edge of a sheet of expanded metal.

• Centerline Shearing

  This is shearing such that there is a line of symmetry on the expanded metal sheet.

• Balance Shearing

  This is done to provide a matching section, such that the order of the mesh appears continuous.

• End Shearing

  This is shearing along the short way of a diamond.

• Random Shearing

  This is cutting across strands at points other than the bonds.

Expanded Metal Meshes

For expanding metal, the most common shape is the hexagon – diamond. For a given sheet, it is most common to use the same shape and size of openings throughout the sheet. The diamond shape gives a four-sided opening with the strands bonding at the corners of that shape.

As the difference between the diagonals of the opening becomes smaller and smaller, the diamond shape approaches a square (geometrically, the square is a special case of a diamond, where the diagonals are equal). In such cases, the mesh is actually called a square mesh.

The hexagon need not be regular, but the length of the sides defines the dimensions and strength of the bonding.

Chapter 5: Quality of Expanded Metal

Tolerances of expanded metals should be maintained for:

• Sheet thickness
• Strand width
• Dimensions of openings
• Coating thickness

Generally, expanded metal should be free from:

• Burrs
• Broken strands
• Laminations
• Welded strands
• Slivers

Tolerances in Expanded Metal

Expanded metal can be made with varying tolerances. These will be detailed below.

Camper Curve

This is the bow or curve in the sheet. It may be inconsequential if the expanded metal sheets are to be rolled for packaging. However, it might be undesirable in some applications which require more planar material. This is especially the case if the material required is of a higher rigidity, which would leave residual stresses in the structure.

It is often very rare to produce expanded sheet metal with zero camper. It follows that tolerances are placed for given applications. Corrective action would be required if the camper obtaining is outside the requisite tolerance, otherwise the material is used satisfactorily.

The curve usually develops along the edge that is along the direction of the feed (during the cutting and stretching stage of the expanding process). The flattening process also usually involves rolling. The process of removing (or reducing) campers usually also removes waves and buckles, yielding a more level product.

Squareness Property

This is the property of being shaped like the rectangle – or its special case, the square. The important features are having 90° angles and straight edges. It follows that the diagonals of such geometry are equal.

Squareness is a planar property – it is satisfied by only one plane. This is the plane seen when the view is from above when the sheet is lying on the ground. An expanded sheet may have the required squareness without necessarily all the other spatial and geometrical requirements, such as being level. Conversely, an expanded metal sheet may not square because of certain spatial developments, such as camper and buckles.

Deviations from squareness often arise from the metal sheet used, which is rarely perfect square itself though it is often produced with very tight tolerances. They can also arise from the defects of the die used.

If the intended application allows, re-squaring is done by random shearing the expanded metal sheet along appropriate lines. Border shearing often yields no re-squaring effect and stretching the shorter diagonal often yields non-straight edges.

Parallel Sides

This characteristic is often required, especially if the metal sheet used to expand has parallel (opposite) sides. It may be noted that sheets that square always have parallel sides, though it is not necessarily the case that sheets with parallel sides do square (the case in point being the parallelogram). Also, sheets without parallel sides are never square. Deviation from having parallel sides may be caused by campers, among other causes.

In some applications, the expanded metal is intentionally produced with non-parallel sides. One way of producing such expanded metal from a square solid sheet is to have a die with varying sizes of “teeth.” For example, the teeth get smaller across the width of the sheet. Such applications are very uncommon though.

Taper Sides

This is the deviation from having parallel sides, particularly conspicuous when such an eventuality is undesirable. It may be noted that a sheet with tapered sides is not square.

Expanded Metal Customization

In many cases, the expanded metal sheet still must be worked on as it comes out of the expanding machine. Such work involves cutting, bending, welding, bracing, and many more. In many contexts, the said work is not routinely part of the production process partly because of the scale at which each unique customization is required. Some applications would also require different gauges of expanded metal complimenting each other.

Thus, customization involves the tasks that are done on the expanded metal sheet, usually at the request of a specific customer. Such tasks include:

• Shearing to fit specific dimensions
• Shearing to fit specific frame shapes
• Shearing to produce a tear or cut without removing any material or isolating any pieces of the original expanded sheet metal
• Bending the sheets
• Joining sheets together
• Rolling to specific curvatures, including affecting various curvatures on the same sheet

Chapter 6: Applications and Advantages of Expanded Metal

This chapter will discuss applications and advantages of expanded metal.

Applications of Expanded Metal

Expanded metal has a wide range of applications. These include:

• Metal Platforms

  Many platforms that are required to be used at considerable heights use expanded metal mesh. This is done to utilize the advantage of light weight. An example of such application is in rigging, be it for the transport industry, oil industry, telecommunications industry, etc.

• Machine Guards

  The mesh can be placed over rotating shafts and parts of machinery. It is lighter than solid guards and equally effective in protecting machine operators.

• Drainage and Ventilation

  Expanded metal can be put as part of a structure to allow for drainage and/or ventilation. It can be put on top of drainage trenches such as those on roadsides, those along or across walkways, and those on factory floors etc.

• Protective Screens

  Expanded metal mesh can be used to make protective screens for building windows and vehicle windows, such as for cash-in-transit vehicles.

• Walkway Applications

  Certain bridges use expanded metal for their pedestrian walkways to reduce the weight of the deck, but maintain structural performance. Many other suspended walkways use expanded metal mesh for similar reasons.

• Use in Barriers

  Expanded metal mesh can be used to make barriers. This is ideal if light, air, and sometimes water are required to pass freely. Such barriers are used in aquariums, buildings, tunnels, etc.

• Fence Applications

  Expanded metal can be used for protective fences, demarcating yards, making cages, and other animal enclosures.

• Gabion Walls

  Expanded metal can be used to make gabion walls. It is more suitable that welded mesh if a finer aggregate of stones is used.

• Equipment Ventilation

  Expanded metal mesh can be part of covering for equipment. Using the mesh can be ideal for cases where neither using a solid sheet nor leaving the area uncovered would be acceptable.

  Such equipment whose covering includes sections of expanded metal include generators, tractor (engines), earth moving machinery engines, air and water pumps, etc.

• Uses for Decoration

  Expanded metal mesh can be used for architectural reasons. The mesh can be included in gardens, building openings, building walls, roofs, guard rails, demarcation walls (as distinct from gabion walls), etc. The much finer meshes can be used to make jewelry.

• Metal Mesh Trays

  An array of trays may be made using expanded metal mesh. Such trays can be used in agriculture, for example, in the harvesting and transporting of green tea from the fields.

• Recreational Uses

  Some recreational parks put expanded mesh over steep and high cliffs to provide thrill and entertainment.

Advantages of Expanded Metal

As much as expanded metal has demerits, such as its larger surface area per unit length, which exposes it to corrosion and chemical attacks, the merits outweigh the demerits. Some of the merits of expanded metal include:

• Relatively lighter, compared to a solid sheet of the same outer dimensions and the same material
• Decorative
• Allows light and ventilation
• Amount of light can be varied by varying strand thickness
• It yields up to three times the original solid sheet metal size
• It has no material wastage
• Does not require joining any material – it is built from a single sheet of metal
• Easy to cut (shear) as compared to the solid metal sheets
• It can be recycled
• Allows drainage and fluid flow

Conclusion

It can be reasonably inferred that the need for expanded metal products will persist into the distant future. The combination of (relatively) light weight, high strength, and moderate cost will continue to place a distinct attractiveness on expanded metal products. The general trends of structural and industrial development also seem to agree – amongst other things, the requirement for civil structures to become increasingly taller will persist, with it the need for platforms, barriers and ornamentation.