Quick answer: A filter press seals in two places: between neighbouring plates (the plate-to-plate seal, made by the raised sealing face of the plate pressing on the filter cloth) and at the feed and filtrate ports (made by O-rings or moulded gasket rings). Most “gasket” problems are really a sealing-face problem: a worn or deformed plate edge, a cloth that has crept or wrinkled, or too little closing force. Gasketed (caulked) plates cost more but give a drip-free press for hazardous, hot or valuable liquids; non-gasketed plates weep a little by design and are fine for most sludge and mineral duties.
Where a filter press actually seals
People searching for a “filter press gasket” are usually looking for a part that does not exist as a single item. A recessed chamber filter press has no continuous gasket between plates. The seal is made by the plate’s raised rim (the sealing face, typically 25–60 mm wide depending on plate size) clamping the filter cloth against the rim of the next plate under the closing force of the hydraulic cylinder. The cloth itself is the sealing medium, which is why cloth thickness and edge treatment matter so much.
Elastomer parts appear in four places:
- Feed port and filtrate port O-rings or gasket rings on each plate, sealing the cloth around the corner and centre holes.
- Membrane plate seals where the squeeze diaphragm is welded or clamped to the plate body, and the air or water inlet fitting seals into the plate.
- Caulked gaskets on gasketed plates, where a rubber cord is pressed into a groove around the sealing face and the cloth is sewn or caulked into the same groove.
- Manifold and valve seals on the feed pipe, filtrate outlets and cloth wash header.
Knowing which seal has failed tells you the fix. Liquid running down the outside of the plate pack is a plate-to-plate problem; a wet spot only at a corner is a port seal; filtrate turning cloudy from one plate is a torn cloth, not a gasket at all.
Gasketed versus non-gasketed plates
The industry splits chamber plates into two families, and the choice is made at the time of purchase.
| Feature | Non-gasketed (standard) plates | Gasketed (caulked and gasketed) plates |
|---|---|---|
| Seal between plates | Cloth clamped between plain sealing faces | Rubber gasket in a groove, cloth caulked into the groove |
| Leakage at the edge | Slight weeping is normal and is collected by a drip tray | Essentially drip-free at rated pressure |
| Cloth change | Fast: cloth is hung over the plate or fixed by a barrel neck | Slower: cloth must be caulked into the groove with a tool |
| Cloth cost | Standard | Higher, because cloth is sewn with a caulking cord |
| Typical uses | Municipal and industrial sludge, mining, sand washing, ceramics | Chemicals, pharmaceuticals, food and beverage, hot filtrates, hazardous or odorous liquids, high-value filtrate |
| Plate cost | Base | Higher, more machining |
For most wastewater and mineral duties the weep from a non-gasketed press is a few litres per cycle, harmless, and caught by the drip trays that are standard on our chamber filter presses. Gasketed plates are worth the extra cost when the filtrate is hot, corrosive, flammable, smelly, or when the product in the filtrate has value and every litre lost off the edge is money. See the wider plate comparison in our plate selection guide.
Gasket materials and where each one fits
Port O-rings, membrane seals and caulking cords are made from a small set of elastomers. Choose by the filtrate, not by habit.
| Material | Continuous service temperature (typical) | Good against | Weak against | Typical filter press use |
|---|---|---|---|---|
| EPDM | −40 to +120 °C | Water, dilute acids and alkalis, steam, ozone, most sludge liquors | Mineral oils, fuels, aromatic solvents | Default for wastewater, mining, municipal sludge |
| NBR (nitrile) | −30 to +100 °C | Oils, greases, fuels, aliphatic hydrocarbons | Ozone, ketones, strong acids, weathering | Oily sludge, machining coolant, edible oil, biodiesel |
| FKM (fluoroelastomer) | −20 to +200 °C | Acids, hydrocarbons, solvents, high temperature | Ketones, hot water and steam above about 150 °C, cost | Chemical and pharmaceutical presses, hot acid leach filtrates |
| Silicone | −60 to +200 °C | High and low temperature, food contact grades available | Abrasion, tearing, oils, steam | Food and beverage where a food-contact grade is required and abrasion is low |
| PTFE-encapsulated | −60 to +200 °C | Almost all chemicals | Low elasticity, needs a clean and rigid groove | Aggressive solvents and oxidizers |
The same logic applies to the plate body. Polypropylene plates are the norm and tolerate most sludge liquors up to about 80–90 °C; hotter or more aggressive duties move to glass-filled PP or to other polymers. The gasket must survive at least as long as the plate around it, so match the two.
Why the seal fails: the five usual causes
When a press starts leaking between plates, the elastomer is rarely the first suspect. Work through these in order.
1. Closing force too low
Every plate size has a rated closing force, and the hydraulic system must hold it during the whole cycle, including the pressure peak at the end of feeding. If the pump has worn, a relief valve is set low, or the accumulator has lost its pre-charge, the pack relaxes as feed pressure rises and the seal opens. Check the closing pressure gauge against the nameplate before touching anything else. Our article on hydraulic faults and fixes lists the tests.
2. Cloth wrinkles, creep or build-up on the sealing face
A fold in the cloth on the sealing face is a leak path the width of the fold. So is a smear of dried cake left on the rim after cake discharge. Presses that run without cloth washing accumulate a hard rim of solids over weeks, the plate pack grows by a few millimetres, and the closing force is spent on crushing cake instead of sealing. Wipe or wash the sealing faces, and set the cloth so it hangs flat with even tension. Our guide to barrel neck, latex edge and drape-over cloths explains which fixing suits which plate.
3. Worn or deformed sealing faces
Polypropylene creeps under sustained load and abrasive slurries erode the rim. After thousands of cycles the faces are no longer flat, and the pack leaks at the same plates every time. Lay a straight edge across the rim; if daylight shows, or the rim is visibly rounded, the plate is due for replacement. Uneven closing, such as a pack that has been repeatedly closed with one plate missing, deforms plates faster than any process does. Our article on why plates crack or leak covers this in more detail.
4. Feed pressure above the plate rating
Standard chamber plates are rated for a feed pressure that is fixed by the plate design, and membrane plates carry a separate rating for the squeeze pressure. Running the feed pump against a nearly full press at a pressure above the rating forces filtrate out between the faces and, over time, bows the plate. The fix is a pressure relief on the feed line and a control that stops feeding on pressure or on filtrate flow, not a thicker gasket.
5. Port seal wear
Feed port O-rings see the most abrasive flow in the press: every litre of slurry passes through them. On mineral and sand duties they wear into an oval and the cloth around the port tears. Inspect them at every cloth change and keep a set in stock; they are cheap and a torn port cloth ruins a cycle.
Fitting and replacing seals
Port O-rings simply press into their groove; clean the groove first and check that the ring is the right cross-section, because a ring that stands too proud is cut by the next plate. Caulked gaskets need a caulking tool and patience: the rubber cord and the cloth edge are pressed into the groove together, working around the plate in stages so the cloth stays flat. Membrane plate seals are best left to the plate supplier; a leaking diaphragm is usually a sign of over-pressure or a foreign object, and a repair without addressing the cause fails again.
Keep a written log of which plates have been re-sealed and when. Leaks that return to the same plate after a new gasket point to the plate, not the gasket, and the plate should be moved to the end of the pack or replaced. Our spare parts guide lists what to hold in stock for each press size.
Specifying seals when you buy a press
Tell the supplier what is in the filtrate, its temperature and its pH, and whether the filtrate is going to a drain, being recovered, or is hazardous. That determines gasketed or non-gasketed plates, the elastomer for port and membrane seals, and whether drip trays and a closed filtrate manifold are needed. For a press that will run several different products, choose the elastomer for the worst case rather than the most common one; changing every port O-ring on a 100-plate press is a day’s work.
Related reading: recessed plate vs membrane plate, filter press troubleshooting, filter press cloth selection guide.
Frequently asked questions
Does a filter press have gaskets between the plates?
Standard recessed chamber plates do not. The seal is the filter cloth clamped between the raised rims of neighbouring plates. Only gasketed (caulked) plates have a rubber gasket in a groove around the sealing face. All plates have small O-rings or gasket rings at the feed and filtrate ports.
What gasket material should I use for a filter press?
EPDM is the default for water-based sludge and mineral slurries. Use NBR for oily filtrates, FKM for hot or strongly acidic and solvent-bearing liquids, and a food-contact silicone or EPDM grade where the filtrate is a food product. Match the gasket to the plate material so both last the same service life.
Why is my filter press leaking between the plates?
In order of likelihood: closing force below the rating, cloth wrinkles or cake build-up on the sealing faces, worn or deformed plate rims, feed pressure above the plate rating, and finally a damaged gasket or port seal. Check the closing pressure gauge and clean the sealing faces before replacing any parts.
Is a small amount of leakage normal?
On non-gasketed plates, yes. A light weep along the plate edges during the pressure peak is expected and is caught by the drip trays. Streams of liquid, or leakage from the same plates every cycle, are not normal.
When are gasketed plates worth the extra cost?
When the filtrate is hot, corrosive, flammable, odorous or hazardous, when it must be recovered because it has value, or when the press runs indoors where drips are unacceptable. Most municipal and industrial sludge duties do not need them.
How often should port O-rings be replaced?
Inspect them at every cloth change and replace any that are cut, ovalled or hardened. On abrasive mineral duties that can be every few months; on soft biological sludge they often last as long as the cloth. Keeping a full set in stock costs little compared with a lost cycle.
Tell us the filtrate, its temperature and pH, and we will recommend the plate type and seal material for it: ask for a plate and seal recommendation.
References: ASTM D2000 — Standard Classification System for Rubber Products in Automotive Applications (the classification used to specify EPDM, NBR and FKM compounds), ISO 3601-1 — Fluid power systems, O-rings, inside diameters, cross-sections, tolerances and designation codes.
This article is part of our guide to filter press plates.