This positioning is determined and influenced by:

• Ink, drop volume, drop formation and drop fly
• Precision of media transport
• Media itself
• Geometrical and electronic precision of print heads
• Precision of the print head in the print carriage
• Carriage speed

Temperature and temperature changes have a great influence on the ink property, influencing the viscosity and thus drop size, formation and fly time. Also, temperature has a significant influence on the precision of media transport.

In the analogue printing industry the influence of ambient conditions in the printing and media storage areas is well known and air conditioning within the printing industry is a challenging task. Digital printing, using similar (if not the same media) faces similar issues.

Temperature changes have quality related influences on the media.

Whether paper, fabric or polymer films, media is ‘living’ and has changing dimensions.

Media stretching by tensile force

The length of the media (and thus the transport step) is, in addition to temperature, influenced by the tensile force of transport. (See SP&DI magazine - Jan 07 for diagram)

A standardised stretch force of the printing equipment may result in different transport steps on different media. An offset length, which equalises the different stretching qualities of the media, overcomes this problem.

Media stretching by temperature

UV inks are cured by UV lamps which, apart from providing UV light, emit heat and subsequently heat the media (although cooling systems are installed). Whereas ambient temperature changes (morning v afternoon) may vary considerably in southern regions but generally change between eight to 10ºC in a four to six-hour period, the temperature changes caused by the UV lamps occur in a much shorter time. The influence of such changes is even sharper when the media is stored at a different temperature than the printing area. This combines the effects of ambient temperature and the UV lamps.

The reaction of the different media to changes of temperature is totally different: some media, like PVC films, expand, whilst other paper based media, such as blueback paper, shrink at a rate of 2mm/m length for a temperature change between 21 and 31°C. This is due to the water content in the paper which evaporates at higher temperatures. Storage, media thickness, media density and media quality all influence the sensitivity of the media in respect of temperature. Particularly, PVC or polycarbonate films are very sensitive to temperature change. Thus, some media purchased in Asia may not be the bargain first considered but could result in a serious quality issue.

What is interesting to note is the fact that for nearly all media the temperature point of 31°C is a critical point. (See SP&DI magazine - Jan 07 for diagram)

Influence of temperature on the printing plane

As the media is stretched over a metallic printing plane in order to ensure a smooth surface during printing, the UV lamps heat the metallic plate. This expands the print plane and also heats the surface of the media. The influence of continuous printing on the print plane is shown in the diagram below. A cooling system for the print plane and a choice of material which is less heat absorbing, may overcome this problem. (See SP&DI magazine - Jan 07 for diagram)

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Source: Durst PROCESS

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