Understanding filtration › 3. Filter media

CHAPTER 3.4

Seams, necks and fastenings

A filter fabric only becomes a filter press cloth after fabrication: cutting, seams, feed neck, fastenings. These details determine how the cloth sits, how well it seals and, often, where it will fail.

Nicolas Ract · 7 min read · October 2026

Two ways of holding a cloth on a plate

The non-gasketed plate: the draped cloth

On non-gasketed recessed chamber plates, the cloth is rectangular and extends beyond the edges of the plate. It is draped over the top and held by grommets along the outer edge; this is the simplest shape to fit and replace [1]. Sealing is then achieved by clamping the fabric between the plate edges, which is why a coating on the edges is sometimes worthwhile (see chapter 4.3).

The grooved plate: the caulked-in cloth

On plates described as "caulked and gasketed" (often referred to by the English acronym CGR), the cloth is cut as an octagon with a central eye, double-stitched with a cord incorporated around its periphery; this cord is then driven into a groove in the plate. This construction gives an almost leak-free seal [1]. Fitting is done with a small hand-held pneumatic hammer [2].

The trade-off is more demanding fabrication: the cloth must match the groove exactly. Field experience: a corded cloth that is too large forms folds; too small, it pulls on the cord seam; in both cases, leakage or failure follows.

The feed neck

The principle

On a centre-feed plate, the slurry enters through an eye in the centre of the plate. The cloth must cover both faces of the plate while letting the slurry through. The classic solution connects the two faces of the cloth with a cylindrical section, a fabric sleeve sewn on one side to one face and on the other side to the other face; to fit the cloth, one of the faces has to be passed through the eye of the plate [3]. In practice, one face is rolled up, passed through the eye, then unrolled on the other side, aligning the feed and filtrate holes [4].

The weaknesses of the sewn neck

The patent describing this construction lists several drawbacks: sewing the sleeve takes a long time, a cloth that has hardened in service is often impossible to remove without cutting it, and if only one face is damaged, the whole cloth has to be discarded [3]. A cloth manufacturer adds that the needle holes of a sewn neck are a possible path for leakage or product bypass, and that fabricating a sewn neck can introduce folds or twisting [5].

Field experience: the neck is the area where the slurry arrives fastest and where the fabric works hardest on each cycle. Localised wear around the eye, a frayed sleeve or an unstitched neck are among the most frequent defects found when examining a used cloth (see chapter 4.1 and chapter 4.2).

Variants

To avoid seams in the most heavily stressed area, manufacturers offer welded or moulded necks in thermoplastic elastomer, necks coated with heat-treated synthetic rubber, rigid plastic necks fixed to the cloth, or clip-in necks that allow each face to be fitted separately but require modification of the plates [5]. Each has its field of application; the choice is made with the manufacturer, depending on the product and the plate.

The case of corner feed

On corner-fed plates, the eye is not in the centre and the cloth must let the slurry pass through to the chamber. This requires a precisely fabricated cloth neck or a rubber neck; according to one manufacturer, any reduction in the feed cross-section can create a differential pressure between chambers and damage the plates [5].

Fastenings

A single filter often combines several systems: one replacement guide, for example, describes a cloth tied at the top with ties, then closed along the sides and bottom with hook-and-loop strips, and a support under-cloth attached in the same way [4].

Seams and their weak points

• Needle holes: each stitch passes through the fabric and can let product through [5].
• The sewing thread: field experience, it must withstand the same temperature and chemistry as the cloth, washes included (see chapter 3.1). A thread less resistant than the fabric makes the seam give way before the cloth.
• Folds and extra thickness: a seam adds thickness; placed on a sealing edge or a plate edge, it can hinder closing or wear through flexing.
• Edges: a cut edge frays if it is not protected. The selvedge, an uncut woven edge, naturally resists fraying; elsewhere, a hem or coating does this job.

Field experience: when several cloths fail at the same point of the fabrication (neck, cord seam, grommet), ask the supplier to review the fabrication before changing fabric. The fabric is not always to blame.

Measurements for ordering a cloth

For corner-feed plates, one manufacturer cites three essential dimensions: the plate thickness, the thickness of its central part (the core) and the feed diameter [5]. Field experience: the full list to prepare is longer:

Field experience: the safest approach is to send the supplier a clean used cloth, together with a dimensioned drawing or photo of the plate. Replacement plates from another manufacturer, or old drawings, may no longer match the actual filter; measure on site. See also chapters 6.1 (preparing a shutdown) and 6.4 (specification).