Forming felt for sheet lines: how to choose and when to change
Choose the felt by the slurry the machine runs on (asbestos cement or fibre cement), the type of fabric (woven or needle-punched), the density in g/m² and the exact width and length of your machine. Change it when it clogs and drains poorly, stretches or starts to mark the sheet.
Updated
Contents
Forming felt is chosen by four things: what slurry the machine runs on, which fabric is needed (woven or needle-punched), which density in grams per square meter, and the exact size of your machine. The felt is changed not by the calendar but by its condition: it has clogged and drains poorly, stretched and runs off to the side, torn, or started to leave a mark on the sheet. A new felt is made to size, so it is better to keep a spare in stock.
What the felt does in the machine
In a Hatschek machine the felt is an endless belt on rolls. It passes over the wire-mesh cylinders and lifts thin layers of slurry off them. Then it carries these layers over the vacuum boxes, where the water leaves them, and passes them to the format drum. There the layers are wound into the sheet blank.
The felt does two jobs at once. It must pick up the layer from the cylinder and at the same time let water pass through. If the felt passes water badly, the blank comes out wet, and the layers hold to each other worse.
What the felt is made of
The base of the felt is a fabric of strong synthetic yarn, most often polyamide. It is woven in plain weave, less often twill or satin. The base is made flat and joined into a belt, or woven endless from the start.
A batt is often laid on the base: a layer of nonwoven fibre. It is attached by needle-punching: barbed needles pull the batt fibres through the base. This gives the needle-punched felt. The batt sits on the sheet side, sometimes on the machine side too. The material for it is polyamide, polyester, acrylic or blends of these.
How the felts in the catalog differ
| Item | Slurry | Fabric type | Density |
|---|---|---|---|
| 94047 | asbestos cement, roofing sheet and pipes | woven, plain weave | 1700 g/m² |
| 94047-1 | fibre cement | woven, plain weave | 1650 g/m² |
| 51291 | asbestos cement, roofing sheet | needle-punched | 1000 g/m² |
| 97049 | asbestos cement | needle-punched, synthetic fibre, pile on one side | 1800 g/m² |
The number in the first column is the manufacturing plant's part number, not a common standard. At another plant the same felt will be called something else. So when ordering it is not enough to give the number: you need the slurry, fabric type, density and dimensions.
Asbestos cement or fibre cement
A felt for asbestos cement and a felt for fibre cement are different items. Asbestos fibre and cellulose behave differently in the slurry, and the slurry clogs the fabric differently. If the plant has switched from one slurry to another, the felt is selected anew. Installing the old one "until it wears out" means testing it on your own sheet.
What the grams mean
Density in g/m² is the weight of one square meter of felt. The higher it is, the thicker and denser the fabric. A dense felt holds the layer better but drains water differently from a light one. Which density is needed depends on the machine, the slurry and the sheet thickness. The reference is the felt your machine already runs well on. Keep its tag or passport.
What to tell us when ordering
- Slurry: asbestos cement or fibre cement, and which product (corrugated sheet, flat, pipes).
- Width and length of the felt as an endless loop. Measuring from the old felt is risky: it stretches in service. Better to take the size from the machine passport.
- Fabric type and density, if known. The part number and tag from the old felt.
- Photos of the old felt from both sides: the weave and the pile are visible.
- Installation method: endless or with a seam.
If you are choosing a felt for the first time from a new manufacturer, it is reasonable to take one felt and test it on the machine before ordering a stock. When a sample is needed and when it is not is covered in a separate article.
When to change
The life of a felt depends on the slurry, machine speed, washing and tension. There is no general figure, and we do not give one. It is time to change when these signs appear:
- Clogging. The felt drains water worse, the blank comes out wetter, the vacuum has to be raised. Fine cement particles lodge in the fabric, and washing no longer returns it to normal.
- Stretching. The tensioner has reached its limit, the felt runs off to the side, folds appear.
- Mark on the sheet. The pattern of the felt or stripes show on the surface.
- Mechanical damage. Tears, worn spots, batt coming away.
- Uneven sheet thickness across the width, if the screens and rolls are in order.
The better the felt is washed in service, the longer it lasts. The washing and tensioning procedure is set by the manufacturer of the machine and the felt.
How to store the spare
Keep the felt in the plant packaging, dry, without sharp folds or heavy loads on top. The tag must show the part number, size and date of manufacture. A spare felt is one of the items worth always having in stock: without it the machine stands, and a new one is made to your size.
How we do it
- Forming felts and screens are made by the Xinhuibo plant, and its items are in the catalog. The size is made to the width of the customer's machine.
- By the tag, photo and dimensions of the old felt, we look for the item at the manufacturer. What we find, with the price, photos and specifications, we send before the order.
- We inspect the lot at the plant before shipment: part number, dimensions, lot, packaging.
What this article relies on
Below are the documents and materials we refer to, so that you can check what is said for yourself.
Reference materials
- Patent US 9415564 "Felt for forming fiber cement articles and related methods", background section: the construction of the felt for the Hatschek machine, base and batt materialspatents.google.com ↗
- taminota.com catalog: felts 94047, 94047-1, 51291, 97049 — part numbers, density and fabric type per the manufacturing plant's data.
The signs of wear are described from general operating practice, without an external source; the washing and tensioning procedure follows the instructions of the manufacturer of the machine and the felt.