There is a chapter on grammages in the manuals that come with the machines in which RISO provides various recommendations, all of the machines in RISOs lineup can print on papers from 46 g /sqm up to sheets with a grammage of 210 g /sqm.
If you do not need to print with two colours or do not mind printing difficulties, thicker paper can also be used. In some tests, it was possible to effortlessly print on paper with a grammage of up to 350 g / sqm. In this case, the volume of the paper is more important: it should be rather low. High-volume paper in particular can obstruct the paper feeder. The paper feeder, of course, should be at an adequate setting with exactly the right amount of pressure. An increased wear and tear of the colour drums or transport rollers could not be determined but could still be possible. The needles in the machine carrying the sheets from one printing drum to the next and finally to the paper ejection, however, are posing a risk: if the paper is too thick, the needles scratch over the fresh ink and draw a streak orthogonally to the print direction over the freshly printed sheet that is very visible depending on the darkness of the colour. The thicker the paper, the more the riso will start to struggle as the sheet of paper inside the machine is transported from the first to the second print drum. Thus it cannot be ruled out that the paper way of the machine may be damaged as a result.
Much greater inconsistencies and pass inaccuracies between printing drum 1 and printing drum 2 must therefore be expected (especially in the direction of pressure)–some readjusting is definitely needed when trying to work with these heavy weights grammages. Thus it is not uncommon that the second colour drum needs to be moved to the left by about 0.5–1.0 mm using the print position adjustment keys to “catch” the very inertial sheet of paper and adjust the printing speed.
For very thin paper, i.e. ones with about 70g / sqm, it is recommended to hold the papers in place with ones hands so that the paper feeder (even when properly set) does not feed too many sheets at once, even though it is indicated in the riso manuals that these sheets are suitable for printing. This is simplified by using interval printing, in which case the risograph does not feed a sheet of paper every time the colour drum moves, still it could try to feed multiple sheets at once due to the printers strength of the paper feed being too big. Making sure that thin paper is not too wavy for it to go through the machine properly is of utmost importance. Thin paper in particular tends to cause jams, can damage the master foil and soil the inside of the machine. Especially contamination on the inside of the machine can cause the back of papers to be soiled during printing, rendering them unsuitable for a double-sided use. Therefore, thin sheets in particular should randomly be examined on both sides after printing in order to avoid having to reprint an entire edition; depending on the speed that the edition is printed it might help unexperienced users to have a helping hand while printing.