If the stack temperature is too low, the process will be slowed and the chance of set-off increased. Too high a temperature can also cause set-off because the ink resins melt: it will cause embrittlement, static and sheet shrinkage as well. Do not make the mistake of thinking that the more heat you apply, the more efficiently the unit will function.

Be particularly careful with short run work-and-turn jobs. If you back-up the sheets too quickly, say after only 20 minutes, there may still be an appreciable level of residual heat in the stack; and if you do not reduce the drier's operating

temperature to compensate, this will lead to too high a temperature after the second pass.

INKS

Although, unlike ultraviolet, the infra-red process does not demand specialised inks. some attention does need to be paid to ink selection. Generally, inks should be of the quicksetting type and of the 'short oil system' breed. This means that

the oil content should be low, and the resin and solvent content high. Should any reduction of the ink be necessary this should be done using a gelled solvent type reducer - not linseed or standard oils.

Since a large part of the effectiveness of this process stems from speeding the penetration of the vehicle into the substrate, the substrate needs to be reasonably absorbent.

It is worth experimenting with different spray powder settings and grades You may well be able to operate without any spray powder at all. However, try this with caution and, certainly on heavier stock, you will need some. Whatever the level. do not use sugar-based spray powders because they tend to turn the infrared unit into a giant toffee bar! (Despite warnings, engineers are called out year to rebuild infra·red heads for just this reason.).

MAINTENANCE

The maintenance required by infra-red units is quite minimal. but do check once a week that all the lamps are working. Remember, they are not like ultraviolet lamps: they do not gradually deteriorate, they either or they don't.

The press operator should also make sure that. when lubricating the chains and bars, the correct grease and oil are used as specified by system's manufacturer; otherwise grippers can dry out and this will lead to an increase in dropped sheets.

Once six months the infra-red head should be taken out, the bus-bar covers removed and any excess spray-powder cleaned out. At the same time, the lamps and reflectors should be cleaned with alcohol.