Printing is an essential part of many businesses and industries. To ensure that the printed images and designs look their best, it is important to use clean substrates. Cleanliness is a large factor in printing and coating, as debris on substrates can cause printing defects.
What is a Substrate?
A substrate is a material used to support an image or design when it is being printed. Common substrates include paper, fabric, vinyl, and other materials. These substrates must be prepped and cleaned before they can be used for printing. This helps ensure that the image or design being printed will be of the highest possible quality.
What is a Clean Substrate?
Clean substrate is any substrate that has been thoroughly cleaned and prepped before being used for printing. This helps ensure that the image or design being printed will be of the highest quality. Cleaning the substrate helps to remove any dirt, dust, or other particles that could affect the print quality. Additionally, it can help to reduce smudging, fading, and other imperfections that can occur when printing on a dirty substrate.
One common type of defect is a hickey. These unwanted specks, surrounded by an unprinted halo, are caused when foreign material. dirt, hardened ink, or other matter displace ink in a transferred image print area. These defects when detected, cause press shut-downs, wasted stock as prints are discarded, and result in time-consuming wash-ups.
The Quality of Substrates
Quality in substrates typically goes beyond cleanliness and takes into account other factors that affect press runability. These include smoothness, dimensional stability in terms of freedom from shrinking, and warping, and an absence of other objectionable defects.
Many different varieties of substrates are being used to satisfy the requirements of print customers. There continues to be high demand for traditional coated and uncoated paper and paperboard substrates including SBS, (bleached pulp, white throughout, C1S clay coated for smoothness, and often C2S backside coated), FBB, (three-ply, double clay coated for smoothness), CUK (coated unbleached kraft), also manufactured as SUS and CNK, CRB (coated recycled board aka, clay coated news), CCNB (clay coated news back board) and UKB (uncoated kraft boxboard). Specialized substrates can include substrates with special properties provided by double and triple coatings, barrier coatings, corona treatment, 90+ brightness levels, and metalized film lamination.
The term synthetic paper describes substrates made from petroleum-based synthetic polymers, which look and feel like paper. The benefits of synthetic papers are water and tearing resistance. Unique to these is Teslin® a single layer, uncoated, polyolefin-based substrate. Other synthetic papers, often clay coated, may be composed of thermoplastics including commonly used, (BOPP), (PET), (PP) and (HDPE). Certain of these exceptionally smooth cast films perform with high quality for the printer. Specifically, calcium carbonate-reinforced polypropylene (PP) films are offered as high-performance, print substrates.
High visibility reflective metalized papers and films are producing eye-catching high-quality graphics for print markets. Laminated to board, metalized substrates provide the “LOOK”, and surface coatings allow excellent print acceptance.
Why should a printer choose film over paper?
Films offer conformability, water & chemical resistance, a clear no-label look, and increased run speed. Dominant markets are shrink-sleeves and in-mold labeling. Film labels are a necessity for durable goods, automotive, aerospace, electronics, etc. Consumer products, beverage, personal care, and cosmetics are also benefiting.
A Paper Mill’s Finishing Processes Matter
When we consider paper and paperboard substrates, the producing paper mill has a direct influence on printing press performance. On press quality issues such as poor ink coverage, holdout and mottle frequently can be related to paper smoothness, or better put, the lack of. Smoothness is produced by the mechanical processing of repeated ironing of the web. Simply, the more a mill irons the web the smoother the result.
Mill terms for the ironing process can include dry or wet stacking and calendaring. In practice, a wet stack adds moisture to the web after the hot press section has removed excess water to allow effective ironing to occur. Further smoothening is accomplished using dry stack ironing after wet stack ironing. Extended nip techniques, used in the paper-making process prior to web drying and ironing, also contribute to smoothness. Seeking maximum smoothness, a paper mills’ final finishing processes can utilize hot and soft calendaring (better) or gloss calendaring.
Substrate Smoothness is a Factor
Smoothness is also a product of coatings applied to substrates. Typical clay coating formulations are applied by either air-knife or blade-knife processes. The blade-knife technique is thought to produce a smoother surface because it applies an even, level coating layer regardless of the web’s smoothness. The air-knife coating process, on the other hand, follows the surface of the web, undulating over any hills and valleys.
Many variations in clay coating formulations are used by paper mills with viscosity, strong binders, and high solids of leading importance. Poor coating processing can produce coated substrates with surface scratch and streak defects, or unevenness that compromises subsequent printing and top coating.
Dimensional Stability in the Paper Sheet
Dimensional stability is important since a paper sheet that wants to warp or shrink is a nightmare to print on. A smooth, stable, and printable sheet results from a proper blend of short and long wood fibers. Long fibers produce a strong stable, but a rough sheet, while short fibers produce a weaker, but much smoother sheet. Moisture content can also affect dimensional stability and needs mill, production, and storage control.
The overall benefits of running a clean sheet have led many printers to install cleaning systems on sheeters, and all varieties of presses. Plants that have adopted cleaning systems generally report much improved reductions in rejected and wasted printed stock resulting from hickeys and time-consuming wash-ups. Less wash-up downtime also results in the ability to maintain better color consistency, and improve productivity, not to mention overall quality.
A clean substrate not only leads to benefits in the quality of printing but also in the quality of overcoating. Whether one is an aqueous coating, UV coating, or EB coating, none will cover and hide printing defects. Debris of any sort will show up as distinct visual defect points, as thin coating film forms tent-like over a debris particle.
Cleaning Systems used by Printers
Cleaning systems vary with some using stationary brushes and others rotating brushes to loosen debris. All loose particles and embedded material can be removed and collected. High and/or low-viscosity air flows, usually combined with vacuum, are used to remove, and collect debris. Both single and two-side cleaning systems are made to clean the entire surface, and edges of a substrate. Units mounted on printing presses can effectively lift and remove debris providing a clean sheet, or web for printing and coating.
Remember, when considering the cleanliness of substrates, clean substrates lead to improvements in print quality, more consistent color, and reduced contamination of ink trains, dampers, blankets, and plates. Further, and taking it a step further, clean substrates and quality printing provide the required foundation necessary for eye-catching top coating results.
Benefits of Using Clean Substrates
Using clean substrates for printing can improve the overall quality of the image or design being printed. Clean substrates can help to provide sharper and more vibrant colors, as well as a smoother finish. Additionally, it can help to reduce smudging, fading, and other imperfections that can occur when printing on a dirty substrate. This can help to ensure that the finished product looks professional and high-quality.
Using clean substrates for printing can help to ensure that the image or design being printed looks its best. This can help to improve the overall quality of the finished product and ensure that it looks professional. Clean substrates can also help to reduce smudging, fading, and other imperfections that can occur when printing on a dirty substrate. For these reasons, it is important to use clean substrates whenever possible.
