What a filter cloth is actually made of, and what each choice does
This page is the vocabulary a filter cloth specification runs on: the polymers, the yarn forms, the weaves, the finishes, the seams and the edges — and what each of them decides.
How a cloth is specified
Selection runs backwards from the slurry, not forwards from a catalogue.
- The liquor sets the polymer. Chemistry and temperature decide which polymers are even candidates. Everything after this is chosen inside that envelope.
- The solids set the yarn and the weave. Particle size distribution and whether you need clarity or release decide the yarn form and the weave together — they are one decision, not two.
- The machine sets the dimensions, the seam and the edge. Width, developed length, joint type and edge treatment are not preferences. They are whatever your machine is built for.
- The failure sets the correction. How the last belt died tells us more than the purchase order did.
The figures here are industry reference — polymer properties published for the materials themselves, dimensional limits imposed by the machines, and the test methods the industry uses. They are not our product specification, and they are not a quotation. The numbers that end up on your confirmation come from your machine, your slurry and the cloth you are replacing.
Polymers
The first decision and the one that constrains all the others. The temperatures below are the continuous figures the industry works to for filtration fabrics; construction and finish shift them, which is why they are given as approximate.
| Polymer | Continuous, °C | Acid | Alkali | Hydrolysis | Abrasion | Relative cost |
|---|---|---|---|---|---|---|
| PolypropylenePP | up to ~90 | Excellent | Excellent | Excellent | Good | Low |
| Polyester (PET)PET | up to ~150 | Good | Limited | Limited | Excellent | Low |
| Polyamide (Nylon 6 / 6.6)PA | up to ~110 | Poor | Good | Moderate | Excellent | Medium |
| Polyphenylene sulfide (PPS)PPS | up to ~190 | Excellent | Excellent | Excellent | Good | High |
| Polyvinylidene fluoride (PVDF)PVDF | ~120°C continuous / short excursions to ~140°C | Excellent | Moderate | Excellent | Good | High |
| Polytetrafluoroethylene (PTFE)PTFE | up to ~250 | Excellent | Excellent | Excellent | Moderate | Very high |
| PolyethylenePE | up to ~80 | Excellent | Excellent | Excellent | Moderate | Low |
Polypropylene
Where it belongs. The default where the liquor is chemically aggressive at both ends of the pH scale and the temperature stays moderate. Caustic and strong acid duties both sit inside its envelope.
What to watch. Temperature is the constraint, not chemistry. It also creeps under sustained load, which matters on a long tensioned belt.
Polyester (PET)
Where it belongs. The most widely used industrial filter cloth polymer. Good dimensional stability under tension and very good abrasion resistance — which is why it dominates belt duties.
What to watch. Alkaline hydrolysis. PET loses strength in hot alkaline liquor, and the rate climbs with temperature and concentration. This is the single most common mis-selection in filtration.
Polyamide (Nylon 6 / 6.6)
Where it belongs. Chosen when abrasion dominates everything else and the liquor is neutral to alkaline.
What to watch. Acid attacks it. It also absorbs moisture and changes dimension with it, which complicates a tensioned belt.
Polyphenylene sulfide (PPS)
Where it belongs. Where the duty is both hot and chemically hostile and cheaper polymers have already failed.
What to watch. Oxidising conditions at high temperature shorten its life. The cost step over PET or PP is large, so it has to be justified by a duty, not by a preference.
Polyvinylidene fluoride (PVDF)
Where it belongs. Strongly oxidising and halogen-bearing liquors, where PET and PP are both out.
What to watch. Strong alkali is its weak side — the opposite of the usual assumption that a fluoropolymer resists everything.
Polytetrafluoroethylene (PTFE)
Where it belongs. The chemical last resort, and the best cake-release surface available.
What to watch. Low tensile modulus and high creep make it difficult on a tensioned endless belt, and the cost is in a different bracket. Rarely the right answer for a vacuum belt.
Polyethylene
Where it belongs. Cold, chemically aggressive duties where even PP is more than the job needs.
What to watch. The lowest temperature ceiling of the group. Rarely specified for belt filters.
Polyester in hot alkaline liquor. PET is the default industrial filter cloth polymer and it is excellent at most of what a belt filter asks of it — but it hydrolyses in hot alkali, and the rate climbs with both temperature and concentration. A cloth that is entirely correct on one alkaline duty can fail quickly on another that is only slightly hotter. How that failure presents, and what to do about it →
Yarn forms
Monofilament
A single continuous smooth filament.
What it gives you. The best cake release and the easiest cloth to wash back. Filtrate tends to be less clear than a multifilament of the same rating.
Typically. Vacuum belt duties where blinding and wash-back dominate.
Multifilament
Many fine filaments twisted into one yarn.
What it gives you. Higher tensile strength for a given thickness and finer particle retention. Holds more solids inside the yarn bundle, so it blinds more readily.
Typically. Where filtrate clarity matters more than wash-back.
Mono + multi combination
Monofilament in one direction, multifilament in the other.
What it gives you. A working compromise — release behaviour closer to monofilament, retention and strength closer to multifilament.
Typically. Very common on belt filters carrying a wide particle distribution.
Staple / spun
Short fibres spun into a hairy yarn.
What it gives you. The finest retention and a felt-like surface. The hardest to wash back and the most prone to blinding.
Typically. Seen on pressure filtration rather than on vacuum belt duties.
Weaves
Four filter-cloth weave structures
01 Plain weave 1/1: plan is a one-over one-under checkerboard; section shows symmetrical interlacing. 02 Twill weave 2/2: plan shows a stepped diagonal; section shows the 2/2 diagonal float step. 03 Satin weave 4/1: plan shows long surface floats; section shows a 4-yarn uninterrupted float. 04 Double-layer (DLW): section through a dual-layer cloth with a fine filtration top face, binder yarn, and a coarse drainage bottom layer.
Plain
One over, one under.
What it gives you. The most stable and the most rigid. Tight, uniform pores; lower permeability for a given yarn.
What to watch. Least forgiving of dimensional movement; can blind faster on fine solids.
Twill
The crossing point steps sideways each pick, giving a diagonal rib.
What it gives you. More flexible and more permeable than plain at the same yarn count, with better cake release.
What to watch. Slightly less dimensionally stable than plain.
Satin
Long floats, few crossing points.
What it gives you. The smoothest surface of the three, so the best cake release and the easiest to wash back.
What to watch. Floats snag and abrade; retention is coarser at a given yarn count.
Double layer / multi-layer
Two or more interlaced fabric layers woven as one.
What it gives you. A fine filtering face carried on a coarse structural back — high strength with fine retention, and drainage channels inside the fabric.
What to watch. Thicker and heavier; more expensive to weave. The usual answer on heavy belt duties.
Finishes
Finishing is not cosmetic on a filter cloth. Heat setting in particular is what stops a tensioned endless belt changing length on the machine.
Heat setting
The woven fabric is taken through a controlled thermal cycle under tension.
Why it matters. Locks in dimensions so the cloth does not shrink or grow in service. On a tensioned endless belt this is not optional — an unset cloth changes length on the machine.
Calendering
The fabric passes through heated rollers under pressure; one side or both.
Why it matters. Flattens the surface, reduces permeability in a controlled way, and improves cake release. How hard it is calendered is a selection decision, not a finishing detail.
Singeing
Surface fibres are burned off.
Why it matters. Used on spun yarns to clean up the surface. Not usually relevant to monofilament belt cloth.
Chemical / surface treatment
Anti-blinding, hydrophilic or release treatments applied to the finished fabric.
Why it matters. Targets a specific failure — blinding or cake adhesion — after weave and polymer have already been settled. Not a substitute for either.
Seams
The joint is where the belt carries its load and where it loses its vacuum seal. It is a specification decision, and your machine dictates it. The same four constructions are set out in the filter cloth specification guide, in the order a specification is built rather than as a list.
The four filter cloth seam types
Clipper: rows of metal hooks on each cloth end interleave, and a pintle wire passes through the channel they form. PAD or welded: the two cloth ends are skived and overlapped, then thermally fused into one thickness. Spiral: a helical coil is fixed to each cloth end, the two coils mesh, and a pintle wire passes through. Sewn overlap: the two cloth ends overlap and are joined by rows of stitching.
Clipper (metal hook)
Interlocking metal hooks crimped onto both cloth ends, joined by a pintle wire.
What it gives you. Fitted and replaced on the machine without dismantling it. The fastest change-out.
What to watch. The joint is a discontinuity: it leaks vacuum as it passes the box, and it is where most belt failures start. Hook material has to suit the liquor.
PAD / welded overlap
The two ends are overlapped and thermally fused.
What it gives you. A smoother, quieter joint that seals better than a clipper and carries load more evenly.
What to watch. Made off the machine or with a portable press; a change-out takes longer.
Spiral
Spiral coils on each end interlaced and pinned.
What it gives you. Flexible, drains through the joint, distributes load along the full width.
What to watch. Adds an open line across the belt — not appropriate where the joint must seal.
Sewn overlap
The ends are overlapped and stitched.
What it gives you. Simple and repairable in the field.
What to watch. The stitch line is the weak point and a leak path. Least common on vacuum belts.
Edge treatments
The edge is the highest-stress zone on a belt filter. A belt that keeps failing at the edge while its body is sound is an edge-treatment conversation, not a cloth conversation. The specification guide covers the same treatments next to what to measure on the old cloth before choosing between them.
The five filter cloth edge treatments and seal wear zones
01 Cut and thermally sealed: baseline heat-fused selvedge. 02 Folded or hemmed: selvedge doubled back for stiffness. 03 Reinforced edge band: separate bonded wear tape protects weave. 04 Coated or sealed band: polymer impregnated for airtight vacuum seal. 05 Guide strip profile: keyed V-profile on underside for tracked machines.
Cut and sealed
The cut edge is thermally sealed against fraying.
Why. The baseline. Adequate where the machine edge seal is in good condition.
Folded / hemmed
The selvedge is folded back and secured.
Why. Adds thickness and stiffness at the edge where the seal rubs.
Reinforced edge band
A separate band is bonded along the selvedge.
Why. For machines that chew edges — the usual answer when a belt keeps failing at the edge while its body is sound.
Coated / sealed band
A polymer band is applied to the edge.
Why. Seals the edge against vacuum bypass as well as protecting it mechanically.
Guide strip / tracking profile
A profile bonded to the underside to run in the machine guide.
Why. Only where the machine is built for it. Fitting one to a machine that is not is a common and expensive mistake.
Material Compatibility Pre-Screen
Preliminary screening of polymer chemical resistance against slurry chemistry, acidity/pH, and operating temperature. Weave, permeability, seam, and finished dimensions are determined by slurry testing and mechanical machine layout.
Polymer Screening: Neutral / Mineral Duty
This preliminary screening evaluates polymer chemical resilience only. The following must be confirmed by slurry testing and mechanical machine review:
- Weave pattern: Determined by cake release, filtrate clarity, and blinding resistance (e.g. satin vs twill vs double-layer).
- Air permeability: Tested at stated ΔP per ISO 9237 based on slurry particle size distribution.
- Seam type & metallurgy: Dictated by roller wrap diameter, operating tension, and chloride/acid corrosion risk.
- Edge treatment: Determined by machine tracking guide geometry (EPDM vs ultrasonic seal).
- Finished dimensions: Governed by physical machine nameplate and verified belt travel length.
Educational Cake-Filtration Sensitivity Calculator
Explore cake filtration sensitivity based on user-provided specific cake resistance and slurry viscosity. Medium resistance (Rm) and filter cloth specifications require physical slurry filtration testing.
Full parameter reference
Every parameter that ends up on a filter cloth confirmation, what it governs, where the industry reference comes from, and what we need from you to fix it for your order.
Cloth construction
| Parameter | What it governs | Industry reference | How it is set for your order | What we need from you |
|---|---|---|---|---|
| Polymer Polymer property | The chemical and thermal envelope. Everything else is chosen inside whatever the polymer allows. | PP / PET / PA / PPS / PVDF / PTFE / PE — see the polymer table for the envelope of each. | Chosen from your slurry chemistry and temperature. | Slurry name, pH, temperature, and any cleaning chemistry. |
| Yarn form Polymer property | Cake release, wash-back behaviour and how readily the cloth blinds. | Monofilament / multifilament / mono+multi / staple. | Chosen from particle size distribution and whether release or clarity dominates. | PSD or a d50, and which of the two is hurting you today. |
| Weave Polymer property | The balance between filtrate clarity, permeability and resistance to blinding. | Plain / twill / satin / double layer. | Chosen with the yarn form as one decision, not two. | Same inputs as yarn form. |
| Fabric weight g/m² Set per order | A proxy for how much material is in the structure — strength and life, and freight. | Follows from polymer, yarn and weave; not specified independently. | Falls out of the construction. | Nothing directly — it is an output. |
| Thickness mm Set per order | Whether the cloth clears the machine geometry, and how the seam sits. | Measured per ISO 5084 / ASTM D1777 under a stated pressure — the pressure matters. | Follows from the construction; checked against your machine clearances. | Thickness of the cloth you are running now, if you have it. |
Filtration behaviour
| Parameter | What it governs | Industry reference | How it is set for your order | What we need from you |
|---|---|---|---|---|
| Air permeability l/dm²·min or CFM Test method | The headline number everyone compares — how open the cloth is. | Commonly measured per ISO 9237 (at a stated pressure drop, often 200 Pa) or ASTM D737 (Frazier). A value with no stated pressure and no stated unit is not comparable to anything. | Targeted from the duty, then confirmed on the finished lot. | The value and the test pressure for your current cloth, if it is on a datasheet. |
| Pore size / retention rating µm Test method | The particle size the cloth is aimed at holding. | For a woven cloth this is a nominal rating, not an absolute cut-off — woven media do not have a single pore size, and the cake quickly becomes the real filter. | Targeted from PSD and required filtrate clarity. | PSD, and your filtrate spec if you have one. |
| Filtrate clarity target mg/l or NTU Set per order | Whether the downstream process accepts the filtrate. | Set by your process, not by the cloth. | Used as an acceptance target. | Your current figure and your required figure. |
| Cake release behaviour Set per order | Discharge, carry-over, and whether cake releases cleanly without blinding. | Driven by surface finish and yarn form more than by rating. | Tuned through weave, calendering and surface treatment. | Photographs of the discharge point and a description of the carry-over. |
Dimensions
| Parameter | What it governs | Industry reference | How it is set for your order | What we need from you |
|---|---|---|---|---|
| Belt width mm Set by the machine | Fit. A belt that is wrong on width is scrap. | Rubber-belt HVBF machines commonly run to about 4.5–4.8 m. The machine sets it. | Confirmed per enquiry. | Measured width of the old belt, or the machine width. |
| Developed length / circumference mm Set by the machine | Whether the belt tensions correctly on the machine. | Set entirely by your machine and its take-up range. | Confirmed per enquiry. | Developed length of the old belt, measured along the running direction. |
| Filtration area (pressure filters) m² Set by the machine | The equivalent of width×length for a tower or chamber machine. | Set by plate size and plate count. | Confirmed per enquiry. | Plate size and number of chambers, or the machine model. |
| Dimensional tolerance mm Set per order | Whether the belt tracks. Out-of-square is a tracking fault before it is anything else. | Squareness matters as much as the nominal dimension. | Agreed with the order. | Any tolerance your maintenance standard requires. |
Fabrication
| Parameter | What it governs | Industry reference | How it is set for your order | What we need from you |
|---|---|---|---|---|
| Seam type Set by the machine | Change-out time, vacuum loss at the joint, and where the belt will eventually fail. | Clipper / PAD or welded / spiral / sewn overlap. The machine dictates which. | Matched to what the machine is set up for. | A close-up photograph of the existing joint. |
| Seam strength % of fabric Test method | Whether the joint or the fabric is the weak link. | A joint is always weaker than the body; the question is by how much. | Stated with the confirmation. | Nothing — this is ours to state. |
| Edge treatment Set by the machine | Edge life and vacuum bypass at the seal. | Cut and sealed / hemmed / reinforced band / coated band / guide profile. | Matched to the machine edge seal and to how the last belt failed. | Photographs of both edges of the old cloth and of the machine edge seal. |
| Guide / tracking profile Set by the machine | Whether the belt runs true on a machine built for a guide. | Only fitted where the machine has the matching guide. Fitting one otherwise causes the fault it was meant to prevent. | Only on confirmation that your machine takes one. | A photograph of the underside of the old belt and of the guide rail. |
Operating envelope
| Parameter | What it governs | Industry reference | How it is set for your order | What we need from you |
|---|---|---|---|---|
| Continuous service temperature °C Polymer property | Which polymers are even candidates. | Approximate continuous figures by polymer: PE ~80, PP ~90, PA ~110, PVDF ~120 continuous (short excursions to ~140), PET ~150, PPS ~190, PTFE ~250. Finish and construction shift these. | The polymer is selected so your duty sits inside its envelope, not at its edge. | Normal and peak slurry temperature. |
| pH range pH Polymer property | Chemical attack, and for PET specifically the hydrolysis risk. | PP and PTFE span effectively the whole scale; PET is limited on the alkaline side, PA on the acid side. | Polymer chosen against your measured pH, including during cleaning. | Operating pH and cleaning pH — they are often different and the cleaning one is often the killer. |
| Chemical exposure Polymer property | Long-term strength loss that does not show up as abrasion. | Specific species matter, not just pH — fluoride, chloride, oxidisers and solvents each behave differently. | Assessed per species, not per industry label. | What is actually in the liquor, including trace species you consider minor. |
| Abrasion exposure Set per order | Mechanical life where the slurry is hard and angular. | Driven by particle hardness and angularity, not by the mineral the plant is named after — silicate gangue is often the abrasive. | Addressed through yarn, weave and surface, and by pointing out the machine-side causes we cannot fix. | A mineralogical breakdown or assay if you have one. |
Machine interface
| Parameter | What it governs | Industry reference | How it is set for your order | What we need from you |
|---|---|---|---|---|
| Machine family Set by the machine | Everything. The cloth form for one family does not fit another. | Rubber-belt HVBF / vertical tower press / indexing or tray vacuum / drum, disc, leaf, chamber press. | Confirmed before anything else is discussed. | Machine brand and model, or a nameplate photograph. |
| Cloth form Set by the machine | How the cloth is made up — endless belt, zigzag loop, cut panels. | Endless belt for rubber-belt HVBF; endless low-elongation zigzag for tower presses; panels for chamber presses. | Follows the machine family. | Nothing beyond the machine. |
| Elongation under tension % Polymer property | Whether the cloth loop stays within the machine take-up range in service. | Low-elongation construction is a defining requirement on tower presses. Heat setting is what holds it. | Controlled through construction and heat setting. | Take-up range of the machine, if known. |
Turning this into a quotation
None of the above becomes a specification until it meets your machine. Send the machine and the old cloth and these choices stop being a menu and start being an answer.
Filter Cloth Specification Guide → Fifteen pages covering this same ground end to end. It is the methodology edition: everything in it is industry reference — published polymer properties, machine-imposed limits and public test methods — and none of it is our mill's own confirmed data. A parameter edition follows when the mill confirms it.