Filter paper or nonwoven? What gsm does not tell you about microns ➔ plus the five dimensions to measure on your belt filter. Read the guide!
Key points at a glance
Coolant filter media selection usually runs into the same three questions. What is this material actually called and what is it made of? Which quality suits the solids…
In enquiries, tenders and machine documentation the word is almost always paper. In our product names the word is fleece. Neither is a material specification, and that is…
This is the single most useful thing on this page.
Three questions, in this order
Coolant filter media selection usually runs into the same three questions. What is this material actually called and what is it made of? Which quality suits the solids load? And which dimensions does the machine need?
The first question sounds trivial and is not, because the words used in the market do not describe the material. The second is the one most often asked wrongly, because a filtration rating gets read out of the fabric weight. The third is the easiest and is still answered wrongly most often, because one dimension gets forgotten. This guide takes all three.
Filter paper or filter fleece? Start here
In enquiries, tenders and machine documentation the word is almost always paper. In our product names the word is fleece. Neither is a material specification, and that is worth knowing before you compare anything.
Filter paper in the strict sense is made from cellulose fibres formed into a web in a wet process. A nonwoven is made from fibres bonded mechanically, chemically or thermally, without the papermaking stage. Materials such as polypropylene and polyester can be processed into a nonwoven; they cannot be made into paper at all.
Both web materials are in the market for belt filter systems, and both run on the same machines. That is why the terms are used interchangeably and why nobody in a machine shop is being careless when they say paper. What matters practically is wet strength, material compatibility and structure, which is where the two differ.
So if your documentation says filter paper, that does not automatically mean a nonwoven cannot be used. It is worth checking against a sample and the dimensions.
One caution the other way round: filter fleece on its own means something different in English and usually leads to aquarium pads. Where it appears on our pages it always carries coolant, belt filter or machining with it.
Filter paper gsm vs micron: what the number does and does not say
This is the single most useful thing on this page.
Fabric weight in grams per square metre describes how much fibre mass sits on a given area. It is a material figure and nothing more.
It is not a filtration rating. No micron value can be derived from 20, 30, 50 or 150 gsm. What a media actually separates in service arises from fibre structure, particle size distribution, solids concentration, belt speed and above all from the filter cake that builds up while the machine runs.
It is not a thickness either. The clearest evidence sits in our own range: the polypropylene media P20 weighs 20 gsm and measures 0.27 mm. The viscose media V30 weighs half as much again and measures only 0.25 mm. Chemically bonded viscose is built more compactly than a polypropylene spunbond.
What fabric weight does do is rank qualities within one material family. The V30 and V50, same raw material and same bonding, can sensibly be compared at 30 against 50 gsm. Across material boundaries that comparison does not work.
The one figure in our range that does describe a pore dimension is the 54 µm wet pore size of the P150, measured to BS 3321. It is a laboratory characteristic of the material, not a cut-off for your machine.
What the filter cake has to do with it
This explains why every data sheet in this product family states that the actual filtration rating cannot be guaranteed.
Freshly indexed media is not the filter. It is the carrier. As soon as the first solids land on it a layer forms, and that layer takes over a growing share of the separation work. It holds back particles that would have passed through the bare media, and at the same time it increases the resistance to flow.
That is exactly what a belt filter is designed around. It lets the cake grow until the differential pressure or the level reaches a threshold, then indexes fresh media forward. How long that cycle lasts and how clean the filtrate is depends on the interplay of media, fluid, load and machine control.
Which is why a trial roll is not a fallback. It is the only method that produces a reliable answer.
Material: polypropylene, viscose or polyester?
| Material | Bonding | In the range | Typical consideration |
|---|---|---|---|
| Polypropylene | spunbond | P20 | when the fluid argues for PP and a light, open quality is enough |
| Viscose | chemically bonded | V30, V50 | the usual starting point for water-miscible coolants in machining |
| Polyester | hydroentangled | P150 | fine grinding swarf and high mechanical load |
Material comes first because it is the hardest thing to correct later. What decides is the medium, the concentration, the temperature and the contact time. There is no blanket resistance statement for any of the three materials; it is always a case-by-case assessment.
30 gsm against 50 gsm: the difference in numbers
This is the most frequent comparison question in the group, and it can be answered cleanly because both qualities were tested to the same standard at 2 millibar.
| Feature | V30 | V50 |
|---|---|---|
| Fabric weight | 30 gsm | 50 gsm |
| Thickness | 0.25 mm | 0.36 mm |
| Air permeability at 2 mbar | approx. 5,300 l/m²/s | approx. 3,600 l/m²/s |
| Maximum tensile force, MD/CD | 80/13 N/5 cm | 125/25 N/5 cm |
The V50 is 44 per cent thicker and passes about a third less air. The cross-direction tensile force very nearly doubles, which counts under a heavy filter cake as the belt pulls the media forward.
What follows: the V50 offers more resistance to flow and tends to hold back more. What does not follow: a particular cut-off or a longer roll life. More material takes up more, but a denser media closes faster. Which effect wins in your machine is shown by the trial.
Why the air permeability figures are not a ranking
Four figures appear next to each other in the category comparison, and sorting them is tempting. It misleads.
The P20 and the P150 were tested to DIN 53887, the V30 and the V50 to DIN EN 9237 at 2 millibar. Different methods with different pressure differentials give numbers that are not on the same scale. Only V30 against V50 and P20 against P150 compare directly.
And in no case is an air figure a liquid throughput. Air and coolant behave differently in a nonwoven, if only because of viscosity and wetting.
Measuring a filter roll: the five dimensions at the machine
Five figures are enough to order a replacement coolant filter roll. Take them once, write them on the machine, and every future order is a matter of minutes.
Roll width. Measured across the running direction. It has to match the filter area and the guides. Standard widths are 500, 710 and 1.000 mm.
Running length. The length of media on the roll. Our viscose and polypropylene qualities come on 100 m rolls, the thicker polyester media on 50 m rolls. Calculate your requirement in metres, not in rolls.
Core diameter. The coolant filter roll core diameter is the inside diameter of the tube the media is wound onto. Measure it inside the empty core, not outside, or the wall thickness goes into the figure. A 70 mm core is documented for the P20 and the P150; for the two viscose qualities it is not documented and we check it before you order.
Maximum roll outside diameter. The space in the machine’s magazine. This is the value most often forgotten and it matters most with thick media.
Unwind direction. Whether the media runs off over the top or under the bottom. On asymmetrically built media it can matter for filtration, and for the guiding in the belt it almost always does.
Identifying an existing roll
If you have taken over a machine, or the previous material was never documented, four things get to an answer fastest.
A photograph of the roll label. It often carries the quality designation, the width, the length and a part number, even if it comes from another supplier.
The measured dimensions from the list above.
Details of the fluid and the contamination: which coolant, which workpiece material, which process, how high the solids load.
A sample piece if possible. Around half a metre is enough to place the material, the bonding and the fabric weight.
With those, we either narrow it down to a standard quality or tell you it needs a special solution.
When a trial roll is worth it
In four situations no data sheet replaces the trial.
When changing quality, for example from V30 to V50. On a new machine with no documented history. When the workpiece material, the cutting data or the utilisation has changed. And when the fluid changes.
A trial roll runs over a full maintenance cycle. Watch the indexing behaviour, the roll life until the change, the appearance of the filtrate and the state of the media as it comes off. Four observations that together say more than any material table.
Frequently asked questions about choosing coolant filter media
Answers to the most common questions about this topic.
Does a higher gsm mean finer filtration?
Within one material family it suggests higher retention, but it does not guarantee a cut-off. Across material boundaries the comparison is not valid at all: the viscose media V30 is thinner than the polypropylene media P20 despite the higher weight.
What do the designations V30 and P150 mean?
The letter stands for the raw material, V for viscose and P for the plastics qualities, the number for the fabric weight in grams per square metre. V30 is viscose at 30 gsm, P150 is polyester at 150 gsm.
How do I measure the core diameter?
Inside the empty core, across the winding direction. Do not measure outside, or the wall thickness is included. A 70 mm core is documented for the P20 and the P150.
Can I get this media cut to size?
The four qualities are roll stock in fixed standard widths. For contoured parts and cut shapes we cut technical textiles by laser. Widths outside the standard three we check on request.
Is filter fleece the same as filter paper?
No. Filter paper is classically cellulose formed in a wet process; a nonwoven is bonded mechanically, chemically or thermally. Both are used in belt filter systems, and which one suits is decided by the requirement.
How do I tell that my media is closing too quickly?
The machine indexes at ever shorter intervals, consumption per shift rises, and the media coming off is blinded across its whole area rather than evenly loaded. The cause may be too dense a quality, but it may equally be a higher solids load or missing pre-separation.
What does the advice team need from me?
Roll width, length, core diameter, maximum roll outside diameter, unwind direction, machine manufacturer, the fluid in use and the type of contamination. A photograph of the roll label and a sample piece speed the placement up considerably.
Still unsure which filter or next step fits?
Send us the medium, dimensions and application. We will help you decide whether a standard product is suitable or whether the case needs a custom solution.
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