Keeping web tension constant for tissue production
A growing market worldwide for ‘single use disposable’ paper napkins and towels is being met by Omet which supplies printing units to upgrade product lines. Its customers are converters from small companies which purchase the paper from mills and the large converting plants owned by a paper mill. The mill supplies to the converter master reels with 220 cm diameter and a multi-ply of 1-4 layers are wrapped around the reel without connection between them.
Some Omet tissue machines are equipped with two unwinders and the two webs merge in the infeed station. Its four-colour Central Impression (CI) printing units are mainly used for napkin and handkerchief presses if more than four colours are required. Omet installs up to four CI units in line especially if floor space is limited and printing on both sides is requested. While a large amount of napkins are still printed with one or two colours, often with an continuous pattern, a logo or brand name, requests for multi-colour print is growing steadily.
Omet machines can have up to three embossing stations. The steel embossing cylinder with engraved relief pattern works against a flat steel, rubber covered compressible or steel cylinder with paper and wool layers wrapped around which allows the embossing cylinder to punch a ‘matrix’ into it. Another technology is the point to point connection whereby two engraved relief steel cylinder rotate against each other; when the relief points meet they compress the web layers together - often the contact point are covered by glue. This method allows the creation of flush napkins and the formation of soft ‘cushions’.
Paper tissue experiences a stretch factor of 10-12% during printing, embossing and folding which must be considered when the printing form is made and during set-up of the folder. The most important fact during tissue production is a constant web tension. This can be corrected in the unwinder and infeed, printing units, folding group and web guide rollers. The reel stand works with periphery outside unwinding which reduces web tension tolerances because the reel is driven with a constant pressure. The central axial unwinder is driven trough the reel shaft and the web volume tends to increase with the reducing reel diameter.
Napkin lines are equipped with 1 or 2 folding stations. The towel machine has up to 11 folding units across the web width - the web can be cut into 11 ribbons. The web separation allows wider machines to print different jobs simultaneously, provided the run length is identical, because the exit of folding units can be separated. According to requirements the folding stations are equipped with former cones, plough fold, cross fold cylinders, folding pockets and chopper folds. Omet builds tissue machines with standard modules which allows them to be tailored to meet the customer’s requirements, permitting proven machines to be built at reasonable prices.
Today most machines on the market are equipped with a standard tower unwinder with a reel quick change of about 3-4 minutes. In order to meet the need to reduce downtime, Omet has developed an automatic non-stop unwinder for the napkin press which eliminates the stop and restart of the press and the connected paper waste with multicolour jobs.
