Before the First World War, most of Jämsänkoski paper was exported to Russia. When exports there stopped at the end of the war, markets for the paper had to be found in Europe. The demands of the new markets were higher than in Russia and reforms had to be implemented to improve the quality of the paper. PK 1 of Patalankoski paper mill was stopped in 1925 and the manufacture of paper was centred at Hovilanhaara mill.

Jämsänkoski focused on cellulose-rich fine grade papers The manufacture of newsprint ended in 1929. The paper machines were converted to electricity at the beginning of the 1930s. In spite of the slump, Jämsänkoski paper sold moderately well. Exports were made both to the Soviet Union and Western markets. Almost normal amounts of fine grade paper were produced during the war years on two machines, although most of the men were at the front and paper was only exported to Germany's allies.

After the war it was decided to shift the focus of Jämsänkoski production from pulp to paper. The circumstances of the company paper production changed when Myllykoski broke away from United in 1952. United needed a newsprint mill to replace the one lost. Following enquiries and consultations, it was decided to build a new mill at Kaipola in Jämsä.

All through the 1950s, small improvements were made to the old paper machines of Jämsänkoski. During the building of Kaipola the other company mills had to wait their turn. Many kinds of paper were produced at Jämsänkoski, but gradually it focused on printing and writing papers, and board. The most important export countries were the Soviet Union and West Germany.

The company Board decided to build a new paper mill for a fine grade papermaking machine at Jämsänkoski at the end of the 1950s. At that time other paper companies were also increasing their capacity; when the Jämsänkoski machine was started up in 1960, eight new papermaking machines became operational in Finland. PK 3, made by Wärtsilä-Yhtymä, was at that time the largest fine grade papermaking machine in continental Europe, with a working width of over four metres. Jämsänkoski paper production almost tripled.


At the turn of the 1970s engineer Niilo Hakkarainen took over as CEO of United. In 1974, PK 4 started up at Jämsänkoski, using the company's own bleached pulp. The space for the new machine had been ready as early as the mid-1960s, but due to lack of demand its purchase had been delayed. The old paper machines, PK 1 and PK 2, which had been working since the beginning of the century, were sold to India.

The 1970s was a period of energy crisis and international economic recession, which was also reflected in the paper industry. Production was reduced and staffing cuts made. Jämsänkoski decided to change over to using thermomechanical pulp as the new raw material of paper.

JÄPRO, the Jämsänkoski project, was launched at the turn of the 1980s. The project included the building of a new paper mill, a thermomechanical pulp mill, and a peat burning plant. The river Jämsänjoki was diverted into a 640 metre long tunnel beneath the industrial site, and the factory housing the new papermaking machine rose on the former river bed. PK 5 started up in October 1981, producing SC gravure printing paper. The machine used thermomechanically pulped spruce as raw material.

Jämsänkoski's PK 4 started up in July 1974. PK 4 was also modified to use thermomechanical pulp. As a result of further development, the machine gained coating units, and the manufacture of wood-rich print papers started in 1985. PK 3 was renovated to produce facing paper for stickers. The achievement of three million tonnes of paper production was attained at Jämsänkoski in May 1985.