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The infra-red unit is usually located between the delivery grippers of the press; all colours are printed wet-on-wet and heated just prior to being stacked in the press delivery.

The drier will normally be infinitely variable in terms of power output with reduced print-width setting and to be able to be switched on and off automatically by connection to the impression contacts of the press Most applications will require

a drier rated up to maximum power density of 96 Kw/sqm or in cases of very high speed carton presses, up to 144 Kw/sqm. For a typical 1,020 mm wide press, these figures correspond to a system rated at 24 Kw and 36 Kw respectively.

The ink drying process can be split into two parts. The first stage occurs in the press delivery; as the sheets pass under the lamps, any fount solution in the ink film evaporates and the viscosity of the solvent and light oil part of the ink vehicle decreases. This means that there is a faster penetration into the substrate. The risk of set-off in the stack is now reduced as there is also a partial skinning of the ink film's surface at this stage.

The second stage of process takes places within the pnnted stack. The resin oil component of the ink, which is used for pigment-binding, undergoes an accelerated oxidation/polymerisation change because of the increased temperature.

Having the correct stack temperature is crucial and the infra-red system should come supplied with a stack probe: do not ignore this, experience should be enough set the level of without constantly to it.

Just what the temperature should be depends on the type and weight of the stock you are printing. It should be lower with lighter stock; for example, paper will probably a temperature of between 31 and 33 deg C, while for cartonboard it is from 38 to 42 deg C.

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