Runnability and efficiency
Good runnability comprises the different factors that let you run your job efficiently through the press and finishing equipment with low down time and low material waste. Multi-ply paperboard has many important features that support cost-effective printing and finishing operations, as well as the total quality image of the finished products. The prime building block for efficient converting of the paperboard is a product which behaves consistently in both printing and post-press converting. Consistency makes the product predictable, which means in many cases a shorter make-ready. This reliability will also help maintain expected production rates from batch to batch year in and year out.
In-feed and operation
How fast you can set up the in-feeder and how well the substrate runs are factors that affect the total economy of a print- or converting job. The factors that mainly determine how quickly the job can be set up and processed are efficient feeding, flat sheets, dimensional stability, and dust-free stock.
Rough surface enabling mechanical
interlocking and friction.
Smooth surface with local changes due to
a chemical-like attraction. Click to enlarge
The main paperboard properties that affect consistent feeding from pallet to pallet or reel to reel are friction, uniform thickness and paperboard flatness/shape (the last of which is described in the following section). The friction originates from several different sources but the main factors are the surface chemistry and the surface topography.
The sheets may adhere to each other due to electrostatic attraction. This is mostly applicable to thinner paper but may occur with lower grammages of paperboard. The best way to prevent electrostatic charges from building up is not to let the paperboard dry out too much.
If the surface is somewhat rough you risk mechanical interlocking between the sheets, which will obstruct the feeding. On the other hand, a too smooth surface will create a larger contact area between the sheets, which might enhance the interlocking caused by surface chemistry. This effect could be compared to the force that makes it hard to separate two glass plates that are stuck together.
Uniform thickness is of importance in several operations. When feeding paperboard into a printing press or finishing equipment the double-sheet control function can engage if too large variations in thickness occur. This will result in a machine stop and lost efficiency. In a saddle stitching operation a large thickness variation of a thick paperboard cover can, when a very thin insert paper is being used, also trigger the control function which secures the correct number of pages collected. The thickness can also influence the print result in that a sudden drop in thickness can minimise the contact pressure between printing cylinder and substrate. This will lead to variations in print quality and losses in efficiency and quality.
The flatness of the sheet affects the press speed and sometimes even the print results as well. Flatness irregularities are described as twist or curl. Both will cause difficulties in feeding and running the press or finishing machine. The best way to avoid twist or curl is to maintain the original moisture content of the paperboard (please refer to the chapter “Handling”).
Flatness is a crucial property of paperboard and will affect the efficiency of operations throughout the entire conversion chain. During the paperboard manufacturing process, control of curl and twist is a very complex operation. It starts in the forming of the multi-ply structure at the forming end of the board machine, where the following factors contribute to minimising built-in stress in the sheet:
- Fibre composition in the respective layers of the multi-ply construction
- Fibre orientation
- The content of small particles ("fines")
- Controlled removal of water and rewetting
- Web tension in the drying section of the paperboard machine
Optimising these factors will produce a flat sheet and will minimise the risk that uneven tensions caused by hygrodimensional (moisture-related) changes could occur later in the printing or finishing processes, causing distortion of the sheet.
In multi-step processing (or even in multi-colour printing) it is important to use a substrate with excellent dimensional stability. The substrate is exposed to many different forces that might stress the structure in such a way that the sheet changes its dimensions. The forces might come from hygro-expansion due to moisture exposure or from mechanical stress imposed on the sheet during either the printing or finishing operations. Different paperboard types are more or less prone to distort due to humidity changes or mechanical stress.
To achieve efficient operation of the printing or finishing machine you need a substrate free from dust or fibrous debris. Minimising dust or debris (mainly coming from the sheet edges) is achieved by controlling the pulping process and by optimising the retention of smaller fibres.
- Clean edges depend to a large extent on the fibres’ ability to bond with each other. Fibres from chemical pulp are flexible and interlace easily with other fibres to form a strong network. In the case of mechanical pulp, the refining process plays a key role in increasing the binding abilities of the fibre network by providing larger contact areas between the individual fibres. Fine-tuning this process ensures a well bonded network.
- With the help of retention chemicals the small parts of fibres (“fines”) are kept inside the network. This provides more contact points in the fibre network and helps to bind the fibres together. Fibre retention is essential for both mechanical and chemical pulp.
To achieve undisturbed run in the printing or converting machine you need a substrate free from dust or fibrous debris.
Paperboard features that are especially important for good ink application are good surface strength and good ply bond.
Good surface strength
In offset litho printing the inks have a very high tack. To resist the forces in the printing nip and to prevent coating picking, the coating must be well attached to the baseboard surface. A good bond between coating and baseboard is promoted by the following paperboard properties:
- To provide a strong bond to the coating, the fibres in the baseboard surface should adhere well to each other. If the fibres are not sufficiently bound together the coating can come off, taking some of the top fibres with it as well (this is known as coating picking). The good bond between the fibres is promoted by good fines retention.
- Uniform surface sizing of the baseboard also helps to create a strong base for the coating. Weaker spots may otherwise be torn off in the printing nip.
- Good internal bonding within the coating layer is a vital factor. This is not only an internal coating issue, but can also be influenced by the baseboard. Variations in the baseboard surface porosity may cause binder migration in the coating layer and thereby variations in the internal strength.
Uneven surface sizing penetration will result
in locally weaker areas.
Local variation in the distribution of coating
components can be due to variations in
surface porosity either indirectly due to
baseboard density or directly due to uneven
surface sizing distribution.
Click to enlarge images.
Good ply bond
Good ply bond is primarily applicable to offset litho printing, since the tacky inks impose a high force on the substrate. This force is a combination of pulling and shearing in the exit of the printing nip and may cause delamination. Unlike coating picking, which may occur in the interface between the coating layer and the baseboard surface, delamination occurs within the baseboard structure, either within a ply or between different plies, often close to the surface.
Delamination is very undesirable during the printing process, so the ply bond must be sufficiently strong to ensure this does not happen. However, delamination is necessary during the creasing, folding and embossing operations, so the ply bond cannot be too strong either. Therefore the properties of the baseboard must be very well balanced.
Good ply-bond depends on the forming of a strong and elastic network inside the baseboard. This is influenced by the fibre characteristics and the formation of the sheet. The use of virgin fibres with different tensile strength, stiffness, shape and bonding abilities affects the strength of the final fibre network. The key to producing a fibre network with the right strength is to use virgin fibres together with controlled sheet forming on the paperboard machine.
A denser fibre structure promotes strength.
Click to enlarge images.
Quick turn around
To achieve high print speed and good economy it is important to be able to turn the sheet around and print the reverse side as soon as possible after the first print run. The most important factor for quick reverse side printing is ink drying. Too much ink or the wrong pH in the fountain water may decrease the drying speed.
Since different ink types are designed to dry in different ways, the drying process is influenced by various surface properties of the substrate. It is therefore vital to match the ink type carefully to the absorption properties of the paperboard. With offset ink for coated surfaces, the ink must set fast enough to enable the drying process. Smaller pores in the coating absorb the low viscosity part of the ink oils more quickly and the total pore volume will affect the speed of ink setting.
The phase of ink drying. Absorption of the
vehicle enables oxidation.
Modern inks often set and dry due to a
combination of absorption and evaporation.
Click to enlarge images.
Click to enlarge.
To minimise the risk of set-off, fast ink setting and drying are crucial. The required absorption properties correspond to those mentioned above. Other paperboard properties that influence set-off are surface smoothness and low density.
- Surface smoothness involves correctly matching the surface properties on both the print side and the reverse side. With a rough surface, set-off from the “peaks” in the surface topography may occur. In contrast, with a very smooth surface, the large contact area between the sheets may increase the risk of set-off. It is therefore extremely important to match the ink type carefully to the surface smoothness of the paperboard.
- A high density substrate, such as a thick, wood-free paper, will result in higher pressure on the underlying sheets in the delivery stack compared to a high bulk paperboard. Given the same height of delivery stacks in a sheet-fed printing press, a high bulk substrate such as paperboard decreases the risk of set-off.
Runnability in general
In addition to the factors discussed above (flatness, dimensional stability and lack of dust) good runnability in the various post-press operations basically depends on good rub resistance and no powdering.
Good rub resistance
It is essential for an ink or varnish film to be durable and preserved through the finishing operations and when handling the finished product. Factors that influence rub resistance are ink type and surface abrasiveness.
- The best way to achieve good rub resistance is to choose an ink with a somewhat higher wax content. Here, too, it is very important to match the ink type properly to the absorption properties of the substrate surface.
- A smooth surface contributes to a good rub resistance by having fewer peaks in the surface topography thereby helping to avoid loss of ink or varnish in the finishing operations. Surface abrasiveness is also influenced by the coating formulation and choice of coating pigment.
The ink pigments should be well bound to the surface and not come off during the finishing operations. Once again, this requires carefully matching the ink type to the substrate absorption properties. The surface should not absorb the ink binder, which would leave the pigments somewhat less bound to the surface. This can occur when using a substrate with too high or fast ink absorption, a problem that is primarily controlled by coating pore size and total pore volume.