The choice of flexo printing packaging board

Flexographic printing technology has made great progress in recent years, and the packaging industry has also benefited from it. Using flexo-printed folding cartons, the cost is lower and the printing patterns are more abundant. However, choosing a suitable paperboard is like facing the “first chicken or egg first” problem. Is the selection of materials determining the printing process or the printing process determines the choice of materials? This is a complicated issue. This article will analyze the factors to consider when making this choice.

We first analyze this issue from the properties of paperboard to discuss the four most commonly used paperboards in the industry today: single bleached sulphate board, coated unbleached kraft paper, coated recycled board, and folding carton board.

Single Bleached Phosphate (SBS) Cardboard

SBS paperboard has always been the most expensive type of paperboard, made by bleaching chemical pulp from hardwood (deciduous forest). These short and thin fibers make the surface of the paperboard very smooth, which is also the guarantee of obtaining high-quality printing effects. So SBS cardboard is widely used in the high-end market, such as cosmetics packaging, pharmaceutical packaging, packaging of audio and video products and tobacco packaging.

At present, there are 28 paper machines in the world that produce SBS paperboard, which has an annual production capacity of about 5 million tons. This figure accounts for 15% of the world's coated paperboard supply. As North America has a large number of fast-growing deciduous forests, which can ensure the supply of hardwood fibers, most of the world's SBS board is produced in North America.

All North American mills use the Fourdrinier paperboard papermaking machine to produce single-layer SBS paperboard, that is, single-layer fibers with a thickness of 10 to 24 pts, and then coated on one or both sides. The stiffness of the SBS cardboard ensures good printing, folding and embossing performance. The printer is very satisfied with the printing suitability, homogeneity and no-scrap properties of the SBS paperboard.

Coated unbleached kraft paper (CUK)

Coated unbleached kraft paper is also known as coated kraft paper (CNK) or single unbleached kraft paperboard (SUS). CUK paperboard is generally less expensive than SBS paperboard. This type of cardboard is slightly smaller, with a total of 12 paper machines worldwide, producing 3.5 million tons per year and accounting for 10% of the total coated paperboard supply in the world. CUK paperboard is made using the Fourdrinier paperboard paper machine, using unbleached chemical pulp, consisting of two or more layers of fibers. The fibers used in CUK cardboard are mainly coniferous fibers. These fibers are very strong and suitable for durable packaging. The outermost layer of CUK cardboard uses a thin layer of broad-leaved forest fiber noodles and multi-layer coating to improve the surface smoothness, and can achieve higher printing quality through a variety of printing methods, including flexographic printing.

The stiffness of CUK cardboard is similar to that of SBS cardboard, both of which are considered by the US Food and Drug Administration (FDA) as materials that can be in direct contact with food.

CUK board thickness is generally 14~30pts, especially suitable for frozen food packaging. Repeated freezing/thawing can reduce the performance of the board, resulting in a decrease in barrier properties and mechanical protection. CUK board with higher wet strength is more suitable for packaging of bottles and cans that need to be frozen.

Coated recycled board (CRB)

CRB cardboard is also known as clay-coated newsprint, double-layer paperboard, grey-white paperboard, and the like. CRB paperboard is the largest paper board in the industry. The annual output of 265 paper machines in the world reaches 18 million tons, accounting for 55% of the global supply. The design of CRB paperboard paper machines is quite different. Most of them are older and slower roller type machines. The cardboard produced has a high degree of stiffness, and the stiffness of the spun yarn is 3 to 4 times that of the crossbar. This also means that the elasticity of this kind of cardboard is slightly poor in flexographic printing.

Some of the recycled paperboard is a composite machine or a new type of Fourdrinier paperboard machine. The stiffness is similar to that of the original paperboard.

CRB paperboard is mixed with various kinds of recycled fibers, and then made by multi-layer coating. The thickness is generally 14~40pts. The main raw materials used are: recycled waste newspapers, old corrugated boxes, recycled SBS cardboard, CUK cardboard, and office waste paper. CRB cardboard is a multilayer structure that can be divided into outer noodles, cardboard cores and back or bottom noodles. According to different physical properties, the various layers of the cardboard may be the same material or different materials, and the back may be brown, white or semi-bleached pulp. The outermost layer is the layer of fibers below the coating, and generally recycled bleached hardwood fibers are used.

Because the waste paper recycling channels used for the production of recycled cardboard are different and the quality is different, the quality of the recycled paperboard is not as stable as the original paperboard. In the papermaking process, although major impurities such as plastics, waxes, metals, and rubber have been removed, the strength, cleanliness, and uniformity of the paperboard are inevitably affected.

CRB paperboard is a low-cost product and is generally used only for packages that do not require high quality, strength, and cleanliness.

Folding carton board (FBB)

This type of cardboard is very widely used in Europe and other regions, but it is not very common in North America. FBB board accounts for 20% of the global supply of paperboard. The world's 47 paper machines produce 6.5 million tons annually. Most of the FBB paperboard is made from mechanical pulp with a high slurry rate. This means that the lignin in the fiber was not removed during the pulping process and was finally made into a slightly yellowish pulp. From an economic point of view, papermakers prefer mechanical pulp because mechanical pulp has twice the slurry rate of chemical pulp.

Of course, this is at the expense of strength, and the internal strength of the FBB board is relatively low (such as the degree of tear, the bonding between the fiber layers, etc.). Depending on the quality and type of paint used, a higher quality printed surface can be obtained.

FBB board thickness is 14~28pts, which can be used instead of SBS board in areas where SBS board is not very popular. It is suitable for use in packaging where the printing quality and stiffness are required to be high and the durability requirements are somewhat lower, such as cosmetic packaging, pharmaceutical packaging, etc. Due to the high lignin content, FBB paperboard is not suitable for long-term outdoor use because the quality of paperboard exposed to strong light will drop rapidly.

Cardboard printing performance requirements

Let's take a look at what type of cardboard is the best quality for flexographic printing. The performance parameters to be analyzed here include smoothness, absorbency, uniform smooth surface, cleanliness, surface strength, internal bond strength, dimensional stability, and fade resistance. Finally, we look at the cost of various types of cardboard.

Smoothness Today's flexographic printers use closed-ink systems, ceramic anilox rolls, and photopolymer plates to print finer, more-resolution text than ever before. Flexo printing can use FM screening or mixed screening techniques for illustrations. The effect can be comparable to offset printing. Cardboard's macro smoothness (using the Sheffield Smoothness Tester) and micro smoothness (which can use Parker's printed surface roughness instrument) are all closely related to flexographic printing. The higher smoothness is SBS cardboard and some advanced FBB cardboard. The smoothness of the CUK board is also good, and the CRB board is relatively poor.

Absorption As with offset printing, flexographic printing requires control of the ink's absorbency in order to ensure the accuracy and fidelity of the printing. The ideal printing surface is porous, allowing water to penetrate but retaining pigment particles on the surface. Higher ink content inks require less surface absorption. The more absorbent is SBS cardboard and some FBB cardboard.

Surface Uniformity and Stability The prominent feature of flexographic printing is light press printing. In order to get the best image resolution, we must control the dot gain by controlling the printing pressure. Although dot gain is also affected by factors such as the anilox roll, plate, press type, parameter settings, and ink type (solvent ink or water based ink), the uniformity of the paperboard thickness is also very important.

In this respect, the best paperboard is SBS paperboard, followed by CUK paperboard and some FBB paperboard.

Cleanliness Today's flexographic plates are more susceptible to impurities or other dirty spots than in the past, such as those caused by paper-wool, so the surface strength of the paperboard is higher and cleaner to reduce the tendency of the paper to bend around the plate cylinder. Fall off. The higher cleanliness is SBS cardboard and CUK cardboard.

Surface Strength One of the recent advancements in flexographic printing is the development of high pigment inks. The ink layer printed using this ink is thinner than ever, which can greatly reduce the amount of ink used. However, on the other hand, a thinner printing ink layer also imposes requirements on the surface strength of the paperboard, and the surface strength of the paperboard must be large enough to prevent the ink layer from being picked up from the surface of the paperboard during printing. The higher surface strength is CUK cardboard, SBS cardboard and some FBB cardboard.

Internal Bonding Because the speed of presses is now very fast, the requirement for cardboard is not only that the surface strength is high, but also that the internal bonding force is high to prevent the delamination of the cardboard in high-speed printing. Cardboard with better internal bonding is CUK cardboard.

The dimensional stability of plant fibers is very sensitive to moisture. Fortunately, web materials are not as easy to feed and transport as paper sheets in printing and processing. However, the dimensional stability of the material is critical to achieving good print quality. In order to maintain dimensional stability, the cardboard cannot be dimensionally changed due to lateral shrinkage or stretched due to printing and processing. Relatively speaking, virgin pulp paper is generally not very sensitive to moisture, so its dimensional stability is also higher. SBS cardboard, FBB cardboard, and CUK cardboard have higher dimensional stability. The fade-free cardboard coating technology is now very mature. Indiscriminate pigments, synthetic resin adhesives and special additives are now widely used. Even so, bleached fibers may still be yellow when exposed to UV light.

SBS paperboard and FBB paperboard are most yellowish over time. However, CUK cardboard has advantages in this regard.

If you want to say the relationship between the price of cardboard and the suitability of printing, there is no doubt that the price of cardboard with good printing performance will also be relatively high. Although CRB cardboard has the advantage of lower price, its quality and printing performance are often not very good. The lower price of cardboard will result in lower production efficiency and unstable printing quality.

Uncoated cardboard uses flexographic printing. Although uncoated kraft paper and recycled cardboard can be printed, the effect is not ideal. If the cardboard is uncoated, surface smoothness can be improved by selecting short fibers, calendering, and surface finishing. In addition, the internal sizing of the paperboard is also very important for controlling ink absorbency in order to avoid ink bleeding and excessive dot gain.

Make the right choice

The last thing to say is that various types