Embossing

In addition, hot embossing is a dry process, which is essential for patterning polymers susceptible to degradation by solvents, water, or other chemicals, whereas many polymer materials such as some biodegradable polymers are not compatible with the solvents and/or the developers used.

From: Comprehensive Biotechnology (Third Edition) , 2011

Nanostructured Polymer Materials and Thin Films

D.J. Lipomi , ... G.M. Whitesides , in Polymer Science: A Comprehensive Reference, 2012

7.11.4.3 Step-and-Flash Imprint Lithography

Embossing using hard molds in various forms has been used commercially on the submicrometer scale for decades in the production of CDs, DVDs, diffraction gratings, and holograms. 1 Chou and co-workers were the first to apply this approach to the fabrication of sub-100-nm structures for microelectronic applications – nanoimprint lithography 149,150 – which is reviewed in Chapter 7.13 by Carter. The most common hard molds are made from quartz, silicon, and metals, with minimum feature sizes of 20   nm for quartz and 10   nm for silicon. 1 Molds are prepared by first patterning a resist film on the flat substrate by e-beam or photolithography, and then modifying the exposed regions using RIE, wet etching, or electroplating. The use of transparent quartz molds has enabled the most successful form of nanoimprint lithography to date: SFIL.

SFIL has been developed over the past decade by Willson and co-workers: 151,152 it replaces the photomask in traditional photolithography with a quartz mold, and replaces complex optical steppers with molding tools. Figure 8 (a) summarizes the procedure used. The prepolymer is irradiated and cross-linked through the transparent mold. Release of the mold reveals the relief structure in the cross-linked polymer. A residual layer ('scum') connects the molded features in the imprint resist to one another. A breakthrough etch removes the scum layer and completes the pattern transfer step. The unconnected features of imprint resist can then direct further elaboration of the substrate. 153 Figure 8 (b) shows an example of a pattern produced by SFIL. The transparent mold facilitates alignment and registration. SFIL proceeds at ambient temperature, low applied pressures (<10   kPa), and avoids the baking and solvent-processing steps of photolithography. The accuracy of alignment in SFIL is as high as ±10   nm (3σ), 154 and it can produce patterns with 20-nm half-pitch. 152 SFIL thus has potential for incorporation into fabrication of computer memory, and perhaps microprocessors (probably initially in the back plane); it certainly has more than sufficient resolution for use in optical systems. 153 An advantage of SFIL for efficient manufacturing is the ability to produce multiple layers of relief in a resist in a single imprint. This characteristic could reduce the number of steps required to produce vias and other complex structures in three dimensions.

Figure 8. Step-and-flash imprint lithography (SFIL). (a) Schematic summary of the process used. (b) SEM images of structures bearing two layers of relief, but patterned in a single step of SFIL.

 Reproduced with permission from Heath, W. H.; Palmieri, F.; Adams, J. R.; et al. Macromolecules 2008, 41, 719–726. 155 Copyright 2008, American Chemical Society.

Challenges to overcome for SFIL include deposition of cross-linked resist on the mold (fouling), which can cause irreversible adhesion of the mold to the substrate. Passivation of the surface of the mold using fluorinated silanes can promote release of the mold from the pattern, but fewer than 100 uses per mold is typical. 1 Selectively cleavable cross-linking groups containing acetals enable the mold to be de-fouled by a simple acidic wash. 151,155

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Nanostructured Polymer Materials and Thin Films

N.R. Hendricks , K.R. Carter , in Polymer Science: A Comprehensive Reference, 2012

7.13.3.1.1 Thermal nanoimprint lithography

TNIL (sometimes called hot embossing lithography (HEL)) was the first reported NIL technique described by Chou et al. in 1995. 17,23,24 The basic principles of TNIL have origins in HEL (e.g., LIGA) with the difference being that the process is performed at the sub-100   nm scale. The process of TNIL requires a hard mold, with predetermined nanoscale features, to be brought into contact with a film of thermoplastic polymeric material on a substrate by means of external force (controlled temperature and pressure). The mold embosses the polymeric layer and displaces material three-dimensionally during the imprint due to the high pressure exerted on the mold during the imprinting procedure, usually from low pressures of tens of psi up to thousands of psi. Typically, a residual polymeric layer remains at the region where the mold depresses farthest toward the substrate, which prevents the hard mold from making contact with the underlying substrate. Depending on the intended application of the patterned material, the residual polymeric layer may need to be removed via a subtractive etch process such as an anisotropic oxygen plasma RIE process to expose the underlying substrate for further processing. Figure 4 depicts the process of pattern transfer and subsequent RIE for TNIL. A further variation of TNIL is known as capillary force lithography (CFL). 25–27 The distinction of CFL is that a template containing small pores or channels is placed into contact with a solid thermoplastic polymer or liquid polymer layer. Upon heating, the viscosity of the polymer decreases and strong capillary forces cause the polymer to fill the cavities. Upon cooling, the mold is released. Many UV-assisted nanoimprint lithography (UV-NIL) processes, described in the next section, rely on capillary forces to fill the mold, so the distinction of CFL as a separate lithography technique is blurred.

Figure 4. The TNIL process: (a) hard mold is impressed into a thermoplastic polymer layer that has been applied to a substrate; (b) the mold is embossed into the polymer layer at elevated temperature and pressure; (c) the mold is released from the polymer layer with pattern transfer (note the small amount of residual material); (d) the residual layer is removed by an anisotropic plasma etch process.

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Polymers in Biology and Medicine

S.A. Hacking , ... A. Khademhosseini , in Polymer Science: A Comprehensive Reference, 2012

9.23.5.5.2 Patterning to guide tissue development and assembly

Scaffolds that support cell growth can be fabricated from polymers using a variety of methods including printing, fiber spinning, embossing, and casting. Such scaffolds are macroscale examples of patterned polymers. Scaffolds fabricated from polymer comprised of certain natural materials offer additional advantages such as the incorporation of cell adhesion sites and degrade into components that can easily be processed and excreted by the body.

Patterning approaches need not be complicated to be effective. In an elegant model, L'Heureux et al. 169 cultured human cells on a mandrill to generate tissue-engineered blood vessels. Fibroblasts were extracted from the skin of patients undergoing cardiovascular bypass surgery. Harvested cells were cultured under conditions that produced relatively robust cell sheets. Vessel-like structures were fabricated by wrapping and maturing cell sheets around a rod-like Teflon-coated support. The tissue-engineered blood vessels were implanted in non-human primates and they showed complete tissue integration at 8 weeks. The vessels maintained a patent lumen with no signs of stenosis, thrombosis, or mechanical failure. Tissue analysis of harvested specimens revealed a confluent endothelium and a smooth muscle layer within the graft.

The requirement for the production of large-scale tissues with high local precision presents yet another challenge, namely the availability of efficient or even feasible methods of fabrication. Two approaches commonly employed to create organized, cell-laden, and structured materials are the top-down and bottom-up approaches. 170–172 Common to both approaches is the need for precision in some form of patterning generation or reproduction.

From the macro- to the microscale, the 'top-down' approach directs the many features of relatively large scaffolds as is common with bioprinting techniques. Jakab et al. 173 used a modified ink-jet printing system to deposit a variety of cell types onto a collagen-based scaffold. In one experiment, cells derived from embryonic chick heart tissue were printed in discrete spots and fused after 70   h incubation to form a thick asynchronously beating tissue graft. In another experiment, ring-like structures consisting of small spots of Chinese hamster ovary cells were printed onto a collagen-based scaffold. Repetitive printing of the same shape in the same location resulted in the generation of a 3D structure similar to a tube.

In contrast to the 'top-down' approach, a 'bottom-up' approach to tissue fabrication may be achieved by the assembly of repeating subunits in a predictable and directed manner to form larger functional and organized structures. In this regard, self-assembly may be defined as the preferential organization and linkage of micrometer-scale structures by interactions based upon shape, chemical moieties, or interfacial energy.

Bottom-up assembly has been achieved by using self-assembled precursors to generate larger, composite cell-laden structures. These precursors can be created in a number of ways, such as self-assembled aggregation, microfabrication of cell-laden hydrogels, creation of cell sheets, or direct tissue printing. Once formed, these precursors can be assembled into larger units resembling tissues by random assembly, directed assembly, or stacking of layers. By creating modular tissues with microarchitectural features resembling native physiology, bottom-up tissue engineering aims to provide more guidance on the cellular level to direct tissue morphogenesis.

In this regard, self-assembly or directed assembly presents potential benefits for the efficient generation of tissues of clinically relevant size. First, since organs contain billions upon billions of cells, self-assembly presents a means to rapidly generate organized tissue-like structures with little manual manipulation. Due to the nano/microscale nature of the repeating building block, self-assembly provides a means to generate structures that can be differentiated, validated, and matured in vitro before assembly and implantation. Finally, self-assembly provides a means to generate structures within structures by combining micro- and macroscale assembly.

In terms of tissue engineering, self-assembly has different meanings at different scales which can be defined as the molecular, cell, and tissue level. At each level, the potential degree of organization and control of self-assembly can vary widely. For example, larger tissues may simply be formed by assembling aggregates of cells. Alternatively, they may also be formed by the self-assembly of complementary shaped structures each containing different cells and each intended to support a different function of the structure.

While promising, the controlled or directed self-assembly of microscale tissue components such as microgels offers certain advantages. In terms of materials for fabrication, hydrogels are well suited for both tissue engineering and self-assembly. This is a direct result of their biocompatibility, high water content, ability to support the sequestration and diffusion of growth factors, and the relatively facile production of cell-laden microgels of well-defined shapes and properties by simple and rapid techniques such as casting. 170,172,174,175 Du et al. 170 demonstrated that self-assembly of cell-laden hydrogels can be achieved by controlling surface tension, shape, and assembly conditions. Square (400   µm   ×   400   µm   ×   150   µm), cell-laden microgels were fabricated by the UV polymerization of PEG under a photomask. Hydrophilic microgels were placed in a hydrophobic solution and agitated and it was found that agitation rate, time, and surfactant concentration all influenced the extent of self-assembly and organized structures up to 1000   µm in length were achieved. The efficiency of self-assembly based upon shape was also demonstrated with lock-and-key-shaped cell-laden microgels. Following assembly, microgels were further cross-linked to maintain their secondary structure.

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Plastics Overview

Michel Biron , in A Practical Guide to Plastics Sustainability, 2020

2.1.5 Hybrid Materials

Hybrid materials are not a clearly defined material category but result from a design method that associates, by integrating them closely, one or more polymers on the one hand and, generally, one or more other materials, which provide one or more functionalities difficult or impossible to obtain with only one polymer.

The dividing line between hybrid materials and associated materials is rather fuzzy. This definition does not regard as hybrids, for example, those polymers joined after their manufacture onto structures of metal or concrete. On the other hand, overmolding on structural and functional inserts is regarded as hybrid.

The hybrid techniques often associate polymers and metals and combine the benefits of the two material classes. Metal provides the rigidity and the overmolded reinforced plastic keeps the shape of the metal and adds numerous functionalities.

There is also a growing interest in the association of elastic polymers, which assume sealing or damping functionalities, with rigid plastics or composites that have the structural role. One of the materials can be overmolded on the other, or the two materials can be comolded.

By associating simple and inexpensive plastic processes (e.g., injection molding) with simple and inexpensive metal processes (stamping, embossing, bending), the polymer/metal hybrids allow the integration, thanks to the plastic elements, of the maximum number of functionalities: mountings, fastening points, fixings, cable holders, housings, embossings, eyelets, clips, and so forth. This leads to:

•

the elimination of the assembling stages of the suppressed components

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the reduction of the dimensional defects of the assembled components

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the avoidance or reduction of welding operations that can cause metal deformations

This principle, in more or less complex versions, is applied to:

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the front ends of cars such as the Audi A6, Ford Focus, and VW Polo

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footbrake pedals in metal/plastic hybrid

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wheels of planes in hybrid metal/composite EP/carbon

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car doors

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frame hull (MOSAIC project) in hybrid composite/aluminum.

Inversely, the polymer can sometimes provide structural functions, while the metal ensures a role not easily assumed by the polymer:

•

For high-pressure air tanks, a hybrid design gives the best results: a thin metal liner ensures sealing and is used as a mandrel to make the envelope by the filament winding technique. The ArF or CF ensures mechanical resistance. The weight saving is 30%–50% compared to all-metal tanks and the costs are optimized.

•

The engines of the Polimotor and Ford projects are hybrid composites of PF/GFs and EP/GFs with combustion chambers, cylinders, and pistons in metal. This permits the direct contact with hot combustion gases that the polymer could not withstand. The composite provides the rigidity of the engine.

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Certain incinerator chimneys are used in hybrid stainless steel with an inner lining in sandwich resin/GFs with a core in foamed PUR.

The materials associated with the polymers can also be concrete or wood, for example:

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azurel structural panels for individual construction, developed by Dow, made of wood and expanded PS

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rigid elements for the modular design of dwellings made of hollow structures of GF-reinforced UP filled with concrete

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Rubber, Natural

Stephen T. Semegen , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

III.A.2 Smoked Sheets

Ribbed smoked sheets (RSS) are prepared much as is pale crepe. However, even speed rollers are used, with a final embossing roll to provide the characteristic ribbed surface. The rib pattern is intended to provide increased surface area for a faster rate of drying.

The ribbed sheets are placed on racks mounted on trolleys, then placed in a smoke-house, then a drying chamber. The total cycle takes 2–4 days, with entry temperature at 40   °C and exit temperature at 60   °C. Most estates convert 80–85% of their total crop into RSS.

Smoking is done with wood fires. The smoke provides the characteristic odor of RSS. Some people feel that the cresols, phenols, and other components of the smoke confer antioxidant properties to the rubbers. Most probably, some antiseptic effects are gained. Indeed, if the smoke is eliminated, air-dried sheets result. These are becoming more popular, with their amber or light brown color.

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MICRO TOTAL ANALYTICAL SYSTEMS

T. McCreedy , in Encyclopedia of Analytical Science (Second Edition), 2005

Micromachining Methods

The choice of machining method is somewhat material dependant, however, wet chemical etching in conjunction with photolithography is a highly versatile method suitable for both glass and silicon, and can be readily employed in most laboratories. Molding and hot embossing are machining methods reserved for polymers and permit the replication of hundreds of devices from a single master; this can be of significant importance where disposable devices are required. More specialist fabrication techniques include lamination, laser ablation, and dry etching methods using plasmas and reactive ion beams. While all the above methods require some degree of specialist facilities, the latter methods tend to be more specialist and will not be discussed further.

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PAPERMAKING | Tissue Grades

P. Parker , in Encyclopedia of Forest Sciences, 2004

Visual Appearance

The visual appearance of the product, both in its roll form and as individual sheets, is an important marketing tool. This is especially true for the consumer distribution channel, but is playing an increasingly important role in the AFH distribution channel as well. The visual appearance can be affected in a number of ways. Until recently, the primary way was to treat the product during the converting process to impart a distinctive pattern on each sheet. Embossing, passing the sheet through a nip in which the opposing rolls contain a pattern, is a conventional way to impart a pattern on the sheet. This embossing could also be used to increase the bulk and absorbency of the sheet. Recently, manufacturers have developed ways to put patterns directly on the fabric used in the forming and/or drying processes.

Tissue products can also be printed to provide a distinctive visual appearance as well. Since the products are designed to absorb water (and other materials), high-resolution printing is difficult, as the ink tends to spread on the surface and creped tissue is not flat, thus destroying a crisp print pattern. Printed products tend to be slightly higher in price and are used primarily for special occasions.

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Medical Biotechnology and Healthcare

K. Li , ... B. Cui , in Comprehensive Biotechnology (Third Edition), 2011

5.04.1 Introduction

Nanoimprint lithography (NIL) 1,2 is a next generation solution for low-cost, wafer-scale nanopatterning with demonstrated resolution down to 2 nm. It is based on mechanical deformation of a material called resist. Besides ultrahigh resolution, it also offers high throughput and low cost and is considered as the most promising nanolithography technique for applications other than integrated circuit. In a standard NIL process, an imprint mold or master is used to physically pattern a thin polymer layer prespun on a substrate such as a silicon or glass wafer; and the polymer layer can itself be a functional material or act as an etching mask or liftoff layer for transferring the imprinted pattern into the underlying substrate or another material. Due to the high fidelity of the imprint process, all features, including defects and surface roughness, are replicated precisely and that makes initial mold quality a critical issue. Master molds made by electron beam lithography can be expensive, especially over large areas; however, the possibility of massive replication by NIL significantly alleviates this initial capital investment. Cost reductions can also be gained by using alternative mastering technologies such as interference lithography that can produce periodic structures with period down to 50 nm over wafer-scale areas.

The choice of imprint resist and the way it is set or cured before the master mold is released divide the NIL technique into two broad classes: thermal NIL and ultraviolet (UV)-NIL, which are both used regularly. In thermal NIL (also called hot-embossing NIL) ( Figure 1(A)), a thermoplastic polymer is embossed by the master above the polymer's glass transition temperature, allowing the molten polymer to flow and fill the mold pattern. The applied pressure is maintained during cooling to below the glass transition temperature, thus producing a solid replica. UV-NIL (Figure 1(B)) uses UV-curable resist which allows for room-temperature processing in applications where the underlying substrate is sensitive to embossing temperatures and/or temperature cycling. Imprint pressure can also be significantly lowered or even reduced to zero by using liquid resist formulations that have low viscosity and low surface energy before cross-linking. In UV-NIL, the molds are typically transparent to UV and visible light, enabling alignment of mold patterns to existing substrate features. Because the entire substrate–mold sandwich is not heated as a unit, UV-NIL is amenable to step and flash where a small die is imprinted in series across a larger wafer, much like a conventional photolithography stepper.

Figure 1. Schematic of nanoimprint lithography (NIL): (A) thermal NIL: (B) UV-NIL; and (C) replication molding.

Besides thermal and UV-NIL, other variations of NIL have been developed such as reverse NIL and replication molding (Figure 1(C)) where the liquid prepolymer is cast onto a mold without external pressure, and then cured in place by either thermal or UV cross-linking. The cured material can then be peeled off, transferred to a secondary substrate or even onto a previously patterned layer to build up a three-dimensional (3D) nanoscaffold. Casted structures can be used directly as a functional substrate or as a new negative mold for imprinting, thus greatly increasing the lifetime of the original master mold. Furthermore, variations within each class allow for flexibility in downstream processing. For example, multilayer resist structures are frequently used in both thermal and UV-NIL in order to facilitate pattern transfer to the substrate.

As already mentioned, the imprinted polymer layer can be a functional material or (if the layer is thick enough for handling) even the substrate itself, depending on the final application. This type of processing is particularly useful for plastic biosensing devices where the packaged sensors may be widely distributed or even disposable so that low per-unit fabrication cost becomes important. In such a situation, further pattern transfer is unnecessary; instead, subsequent surface modification steps or surface coatings provide functionality. It is also possible to imprint quasi-3D structures directly or imprint continuously in a roll-to-roll fashion.

In the field of biosensor technology, NIL's core competency is cost-efficient patterning at relevant length scales such as the wavelength of an interrogating light source for plasmonic sensing. This cost-efficient and high-throughput technique opens new avenues for nanobiosensors that often require many samples in order to probe the variability in biological systems. In the field of tissue engineering, NIL (hot embossing) provides a low-cost and high-throughput route for the fabrication of micro- and nanostructured plastic substrate for contact guidance of cell growth. Unlike photolithography that can also pattern polymers by lithography followed by pattern transfer via etching, hot embossing creates a pattern inside a polymer within a single step. It is suitable for patterning a broad range of thermoplastic polymers including those that are biodegradable and biocompatible. In addition, hot embossing is a dry process, which is essential for patterning polymers susceptible to degradation by solvents, water, or other chemicals, whereas many polymer materials such as some biodegradable polymers are not compatible with the solvents and/or the developers used.

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LAB-ON-A-CHIP TECHNOLOGIES

U. Bilitewski , in Encyclopedia of Analytical Science (Second Edition), 2005

Substrate Materials

Early reports were based on devices produced by etching and bonding of silicon, as these technologies were well established from the fabrication of microelectronic devices. However, electroosmotic transport of liquid requires the application of electric fields (see below) and, thus, suitable substrates have to have a high electric resistance. Moreover, the combination with optical detectors is facilitated by glass as substrate material. Thus, etching processes and bonding procedures were also established for glass substrates.

Polymers are discussed as less-expensive alternatives to glass, in particular with respect to mass production of structures. Injection molding, hot embossing, wire imprinting, and laser ablation are among the technologies that were established to produce required capillary structures in PC, polystyrene, PMMA, poly(ethylene terephthalate), poly(vinyl chloride), or cellulose acetate. Major problems are related to bonding a cover plate to the structured substrate to close the capillaries tightly, without deforming capillary dimensions. Solutions range from adhesive tape lamination to gluing, and the application of heat or pressure.

Besides these rigid polymers, PDMS is widely used in recent studies, because structures are easily fabricated without the need for specialized equipment and PDMS shows good adhesion to a number of materials, e.g., glass, facilitating the tight closure of channels. Compared to glass, polymers are often more hydrophobic, which hinders an easy filling of structures with aqueous solutions and leads to an increased adsorption of hydrophobic compounds on capillary surfaces. Thus, fabrication conditions have to be chosen resulting in hydrophilic surfaces, or surfaces are treated after processing, such as by modification with charged polymers or by treatment with plasma or with SO3 vapor from fuming sulfuric acid (Table 1).

Table 1. Influence of the material and fabrication technology used for the production of microfluidic chips (labs-on-chips), and of surface pretreatments or additives to the running buffer on the electroosmotic mobilities

Material Fabrication process Pretreatment μosm (10−4  cm2  V−1  s−1)
Fused silica 6.2–10.2
Borosilicate glass Wet chemical etching 2.2–4.2
Hydrophob. silan. 2.0
Protein adsorption 1.5
PDMS Molding 0–6.4
oxidation 4–8.2 (pH 9.0)
Polybrene addition −4.29 to −1.94
Dextran sulfate addition 2.47–3.69
MES addition 2
SDS addition 4.5–6.5
PET Imprinting 4.3
Polyallylamine −1.8
Polystyrene sulfonate 4.2
Laser ablation 4.4–6.3
PC Molding 0.7
UV irradiation 2.7
Hot embossing 7–8 (pH &gt;8)
SO3 treatment 7–8 (pH &gt;6)
Laser ablation 2.8–5.3
Tween addition 0.6
SDS addition 2.8
Imprinting 3.0
PMMA Imprinting 1.3–2.5
LIGA 1.9
Hot embossing 1.8–2.2
SDS addition 2.8
Co-polyester Imprinting 4.3
Polystyrene Imprinting 1.8–2.5
Polyallylamine −1.3
Polystyrene sufonate 4.1
Laser ablation 4.47
Cellulose acetate 4.74
PVC Laser ablation 3.53–5.24
Zeonor plastic Hot embossing Oxidation 1.1
Ceramic (LTCC) Mechanical milling 3–4.2

PDMS, poly(dimethylsilozane); PET, poly(ethylene terephthalate); PC, polycarbonate; PMMA, poly(methyl methacrylate); PVC, poly(vinyl chloride); LTCC, low-temperature cofirable ceramics; MES, 2-morpholinoethanesulfonic acid; SDS, sodium dodecyl sulfate.

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PACKAGING, RECYCLING AND PRINTING | Printing

P.D. FlemingIII, in Encyclopedia of Forest Sciences, 2004

Relief Printing

Relief printing was the basis for the original printing press, as invented by Johann Gutenberg. This is based on raised letter type (Figure 1). Gutenberg's characters were molded out of lead, a metal, which was used until only recently. This process came to be known as Letterpress and was the basis for all newspaper printing until only recently.

Figure 1. Illustration of raised image as used in relief printing. Source: © H. Kipphan, Handbook of Print Media, Springer 2001.

The letterpress process is still used today for die cutting, numbering, perforation, scoring, hot-foil stamping, and embossing. Letterpress inks are relatively high in viscosity to assure its even distribution as it passes through the multiple rolls ( Figure 2) of the inking system. To distribute the ink better, some or all of the rolls oscillate.

Figure 2. Illustration of multiple rollers used for letterpress printing. Source: © H. Kipphan, Handbook of Print Media, Springer 2001.

Letterpress has evolved into flexography (flexo), which uses a flexible plate, which contains a raised image area of a cured photopolymer. The plate is wrapped around a cylinder and is used to put ink on the substrate. A typical flexo print station is illustrated in Figure 3.

Figure 3. Typical arrangement of rollers for flexographic printing. Courtesy of the Flexographic Technical Association.

Flexo is the fastest-growing conventional printing process, especially in packaging, such as corrugated containers and flexible films. It has also made significant advances in publication printing, particularly newspapers. Because the quality of flexo printing has improved so much, it is now used extensively for process color printing, as well as spot color, on a wide variety of substrates. It is used extensively for printing tags and labels, many in full process color.

Flexography was originally called 'aniline' printing because of the aniline dye inks originally used in the process. These were made from coal tar and were banned from food packaging by the Food and Drug Administration because of their toxicity. Other coloring agents were developed that were safer, but the name aniline printing persisted. Because the name carried bad connotations, the name was changed to flexography in 1951.

Flexo plates are flexible and imaged in relief, a natural outgrowth of letterpress printing. The origin of these plates was in rubber stamps, which were formed in Bakelite molds that had been pressed with lead type. Thus, the original plates for aniline printing were made of molded rubber.

The first aniline press was built in 1890 by Bibby, Baron and Sons in Liverpool, UK. It used water-based dye inks, which were not chemically bleed-proof. In 1905 C.A. Holweg built an aniline press as a tail-end unit on a bag machine and patented it in 1908. The ink metering on these presses was crude until 1938, when the anilox roll was introduced. This employed a mechanically engraved copper-coated roll with controlled cell sizes (Figure 4). The idea grew out of gravure printers laying down coating from a uniform cell-engraved roll. The anilox uses this process to coat the raised surfaces on the plate.

Figure 4. Cells in anilox roll. Courtesy of the Flexographic Technical Association.

Anilox rolls are coated with chrome to prevent corrosion and wear. The original aniline inks gave way to ones based on polyamide resins. These stable, fast-drying inks enabled web speeds to increase from 45–230   m   min−1.

The 1980 Clean Air Act led to more extensive use of water-based inks in flexo. Water-based inks are now used extensively for printing on paper-based substrates. The quality is now approaching that of lithography and is even impinging on gravure.

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