Clean edges and surfaces
Clean edges and surfaces and their impact on the printing process can be simply defined in terms of:
• Their ability to be free from dust and fibrous debris particles that may detach and adhere to the printing press plate or blanket, thus causing premature stoppages for wash-ups.
• The presence of such material interfering with the print impression may cause break-up of the screen definition, apply ink in non-printed areas, or cause spots in solid prints (hickies).
• Such stoppages adversely impact on press efficiency and in extreme situations can cause immediate rejection of the material being processed.
The ability to provide paperboard with clean edges and surfaces is a prime consideration when the printer is assessing material for optimum process efficiency. Any loose particles in the form of solid or fibrous material, etc. on the paperboard edges or surfaces may transfer to the printing blanket, plate or cylinder during the printing operation and result in print defects. In extreme cases exceptionally large pieces may even result in damage to the press blanket in offset litho.
Edge cleanliness is as important as surface cleanliness, since debris can break away during sheeting, stacking or printing operations and become attached to the printing blanket in offset litho. Edge debris can sometimes be characterised by the build-up of debris on the press blanket in the contact areas adjacent to the sheet edges.
It is possible to identify different types of edge and surface debris. Some types of surface debris are clearly not edge related. It is also possible to identify the difference between slit (machine direction) or chop (cross direction) edge debris.
Edge- and surface-related debris can usually be categorised under the following headings:
Edge debris (slit and chop):
• loose fibres
• fibre clumps (usually chemical pulp in origin)
• shive (usually mechanical pulp in origin)
• edge debris (see above)
• coating particles
• miscellaneous process particles
• miscellaneous contraries.
Debris of the above type is not necessarily differentiated by the printer, whose prime requirement is for press efficiency with any excessive presence of the above being unacceptable. The build-up of unwanted particles on the press blanket and printing plate will gradually result in a break-up of the screen definition, print spots in non-printed areas or cause hickies and other blemishes in text and solid prints.
Once debris has built up on the printing press there is no alternative except to shut down the press and clean the plates and blankets. This not only incurs lost time but also increases waste when the press is restarted and colour balance restored.
The minimisation and elimination of the above debris types is therefore an essential factor for good press performance and the faithful reproduction of the print design, which may ultimately impact on the consumer’s decision whether or not to buy.
Roller and marked sector for dust evaluation.
Click to enlarge.
While the need for the paperboard manufacturer to provide clean and debris-free products is recognised, much has been done in recent years to eliminate dust and debris etc. immediately prior to the printing operation. The development of vacuum extraction systems, usually in combination with ionising bars to eliminate static, has led to enormous improvements to the length of print runs.
This does not, however, mean that there is no room for further improvement. The printer knows what improvements have been achieved by the introduction of these systems, but the emphasis is still on the paperboard supplier to provide clean material so that the printer can maintain the expected high levels of through-put.
As far as the paperboard production process is concerned, the introduction of extensive vacuum- and dust-removal systems at all stages of the finishing process has become an important element in achieving the ultimate goal of a totally clean product. Such vacuum systems are commonplace and sometimes used in combination with antistatic.
It is, however, important to stress that as with on-press systems the paperboard must be clean to start with in order to obtain the optimum ultimate performance. In this respect it is essential that careful attention is paid to knife design, quality, and alignment and to the adjustment of chop to ensure that unwanted debris is not generated in the first instance.
||Particles with one straight edge and one torn edge. Other fibrous material and coating may also be attached.
||Hard bundles of unseparated mechanical fibres with a characteristic cross-grained structure.
||Fibrous, wavy trim material from the slitting process.
||Flat pieces of coating, generally quite small, that have, for example, become detached from build-ups on rolls in the coating section of the paperboard machine.
||This debris can originate from a variety of materials used in the paperboard manufacturing process, e.g. synthetic rope fibre, calender debris.
In all processes emphasis must be placed on the slitting and sheeting operations with the optimum application of only the best debris removal systems. With today’s modern slitting and sheeting operations, which are able to offer a minimum of manual handling, a further reduction in the generation of unwanted dust and debris in the final product has been made possible.
During subsequent die-cutting and creasing operations the generation of dust and debris (or swarf) must also be avoided by use of the correct and properly set tooling. Sharp, properly set and maintained knives are, therefore, a prerequisite for well-cut, clean edges.
Characteristics of clean edges and surfaces
Assessment of clean edges and surfaces
An optimum paperboard product is one that is totally clean. The amount of debris must therefore be almost
zero to assess a paperboard as good with respect to clean edges and surfaces.
There is no officially recognised method for the measurement of edge or surface cleanliness.
Despite the absence of a recognised test procedure it is possible to quantify the cleanliness of both edges and surfaces. In simple terms the visual assessment of slit and chop edge quality can provide an early indication of a potential problem.
It should be noted, however, that visible edge debris does not necessarily indicate loose edge debris. Where a potential problem is indicated, further examination is necessary to identify the likely impact on press operation.
Quantification of loose edge debris can be achieved by the vacuum removal of material from pallet edges.
This is done by using a specially adapted vacuum head which incorporates a black filter cloth supported by a wide mesh holder. Material removed can then be compared visually using a rating scale.
The presence of surface debris is most easily quantified by rolling a soft polyurethane roller on the paperboard surface and physically counting the number of particles removed over a specified area.
Both edge and surface assessment methods need to be performed under carefully controlled conditions and repeated to obtain statistical significance.
Material removed can be further classified and quantified according to type by microscopic examination. The table on the previous page describes and illustrates some of the types of particles referred to in the first part of this section.
As far as the printing operation is concerned the performance of a paperboard is measured by the time which elapses between blanket washes, or the length of print runs. This can, however, depend on the detail of screen definition required and also on the positioning of any obvious blemishes, which can sometimes be removed without a complete blanket wash or stoppage.
Different paperboard ply constructions
Solid Bleached Board has the advantage that loose fibres and dust are least likely to be generated as a result of the use of long, strong chemical fibres and also pulp and surface treatments. Folding Box Board, where mechanical pulp is sandwiched between chemical pulp layers, also utilises pulps and surface treatments to minimise debris generation but does have a greater potential for debris release.
FBB therefore requires close attention to slit and chop quality.
During the die-cutting and creasing operation, different types of paperboard will react differently. Strong high-density paperboards require sharp, well-set knives to cut through but give very clean, well bound edges with low risk of dust and debris. Stiff but lower density paperboards are less sensitive to knife sharpness, but because the fibres are less bound the risk of debris generation is higher and therefore knife sharpness remains crucial. Many recycled fibre boards have a high mineral content. This makes them difficult to cut efficiently whilst maintaining good quality edges.
Impact on loose particles
The optimisation of the finishing operation and the efficiency of debris removal systems have the greatest impact on the ability to achieve clean edges and surfaces. However,
the paperboard construction can also play a role. Construction features that may affect the degree of bonding or fixing
of fibres within the paperboard sheet include:
• the chemical/mechanical pulp ratio
• the fibre length or fibre treatment
• the surface and “forming end” treatment processes.
However, in general the key to the prevention of debris is still to pay close attention to the slitting and sheeting operations where the potential for debris generation is at its highest.