Although they are commonly referred to in same context and are used for some of the same applications, there is a considerable difference between knitted and woven metals. As their name implies, knitted metals are knitted into a mesh structure in much the same way as stockings or sweaters. The structure of woven wire, on the other hand, is usually simpler, consisting of interwoven strands of wire. Whereas weaving usually produces a symmetrical mesh, usually with square openings and parallel wires, knitting produces an asymmetrical mesh of interlocking loops.
Knitted Metals
An important advantage of knitting is that it produces a mesh of interlocking loops each of which acts as a small spring and provides resiliency. Because of this resiliency, knitted metals are usually able to withstand greater loads and deflections without being permanently deformed.
Knitted wire also has both a large surface area and a high percentage of free space. Thus, knitting permits construction of a mesh using wire with a maximum surface area and with interstices of almost any desirable size, regardless of wire diameter. Fine wire, for example, has been knitted into a mesh with as few as three to five openings to the inch.
The free volume of knitted wire can be controlled between 50 and 98%, depending on the interstice size, and regardless of the wire size used. Even when the wires are widely spaced to produce a potential volume of 98%, the structure retains its shape; a similar spacing with woven wire would result in a shape that would be almost impossible to handle.
Another important property of knitted metals is their ability to maintain their dimensions during temperature cycling. When the meshes are slightly stretched in every direction and expansion occurs, the sides of the loops are merely forced closer together without changing overall dimensions or the plane of the surface.
Knitted metals can be produced in wire diameters of 0.01 to 0.6 mm in such materials as steel, copper, brass, aluminum, stainless steel, and various nickel alloys including Monel. In fact, almost any metal that can be drawn into wire can be knitted. Thus, knitted parts can be made in a wide range of strength, corrosion-resistance, wear-resistance, electrical-shielding, and heat-resistance properties.
The asymmetrical mesh of knitted parts is advantageous for electronic shielding because the continuous-loop structure apparently causes induced currents to cancel themselves. Apart from this application, however, the biggest use for knitted wire is to remove one material from another. Thus, it can be used to separate two phases of the same material, to separate two immiscible liquids, or to separate a solid from a liquid or a gas. The degree of separation can be controlled by varying the compression used to form or shape the knitted part, and by controlling the size and shape of the wire and the size of the loops.
A good example of this kind of application is a mist eliminator in which the knitted mesh separates the liquid phase from the gas phase. Gases bearing droplets from such processes as distillation, evaporation, scrubbing, cleaning, or absorption are passed through built-up layers of knitted fabrics up to 152 mm thick. The droplets collect on the loops and slowly run to the bottom where they accumulate. Liquid particles as small as 5 to 20 |im can be separated with efficiencies as high as 98 to 100%. Although small eliminators are usually made in one piece, eliminators can be made up to 8.4 m in diameter by building up layers of crimped mesh.
Knitted metals are also used in many other applications where their special structure and properties are useful. Fuel line filters, for example, of knitted wire are resilient and do not require precise machining for sidewall fit. Gaskets of knitted wire have excellent conductivity and sufficient resiliency to produce tight joints on uneven surface, thereby preventing radio-frequency leakage. Heat dissipation sleeves of knitted metal for subminiature glass tube envelopes provide high cooling efficiency. Knitted metals also provide good shock and vibration control when used as mountings for airborne and industrial equipment.
Woven Metals
Woven wire cloth is used in a wide range of applications for grading materials, filtering, straining, washing, guarding, reinforcing, and decoration. A variety of meshes in different materials and sizes is available to meet these applications.
Like knitted wire, woven wire cloth can be produced in almost any metal that can be drawn into wire, including carbon and stainless steel, copper, brass, bronze, nickel, Monel, Inconel, and aluminum. Plain steel wire cloth is one of the most economical and generally used materials. However, it may require a protective coating to prevent rusting.
Corrosion can be prevented by using tinned or galvanized wire; tinned wire is preferred for handling food products, galvanized for all other applications. Where required, the cloth can also be provided with a protective electroplate of cadmium, chromium, or tin. Phosphate coatings and paints can also be used.
Where severe corrosive conditions are encountered, the weave can be made from such materials as stainless steel, monel, phosphor bronze, and silicon bronze; some of these materials are available only in limited sizes.
Abrasive resistant steels are available for applications where abrasive materials have to be handled. Many grades of stainless can also be used to withstand high temperatures, e.g., 347 stainless (up to 760°C), 309 and 310 (up to 871°C), and 310 stainless as well as Inconel and Nichrome (up to 1003°C).
Woven wire cloth is widely used for filtering and straining, particularly in automotive and aviation applications for carburetors, air screens, and oil and fuel strainers. They are also used to grade materials, and for wire baskets, insect screening, and safety guards.
