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For some, undemanding, applications, a simple felt can provide suitable filtration performance, without any form of strengthening. However, their low tensile strengths, and the ease with which fibres can become detached, make simple felts unattractive for most filtration purposes, and some mechanical (or chemical) strengthening is required.

Needle punching is the most common mechanical strengthening technique, which originated in the 1880s with natural fibres, but it is only since about the early 1970s that it has come into prominence because of its suitability to the processing of many synthetic fibre felts. A thick ‘ batt ’ of several layers of carded fibre is assembled, and then compressed into a denser structure by punching with an array of special barbed needles reciprocating at speeds up to 2000 strokes/minute, and moving perpendicularly to the felt layer. With perhaps 100 or more needle penetrations per cm2, the effect is to entangle the fibres in the thickness of the felt, and to reduce the thickness substantially, to a degree that is controlled as desired. Punching can be from one side of the felt, or from both sides simultaneously, which improves the uniformity of the felt.

Needlefelts are used extensively as bag filters for the filtration of dusts and gases, because of their above-average collection efficiency. Common applications include the cement industry, steel and aluminium plants, spray drying, coal grinding, sand
blasting, the food industry, detergent manufacturing, ship unloading, pneumatic conveying and hot gas filtration processes where metal fibre felts and ceramic fibres are used. Some typical applications for filter fabrics of various kinds are shown in
Table 2.5 with their key characteristics.

Most felts are mechanically strengthened by needling, but an alternative, and more specialized, technique employs a set of high pressure water jets to fix the fibres in place – a technique known as hydroentanglement. Hydroentangled felts are also said to be spunlaced.

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