Printing Process
Basics
Printing techniques are the textile processes by which a thickened dye mixture or print paste is applied to the fabric. These modern processes include flatbed screen, rotary screen, or engraved copper roller printing. Printing is further classified as either dyestuff printing or pigment printing.
Dyestuff printing, also known as wet printing, has five distinct processing steps. These include print paste formulation, printing, drying, fixation, and afterwashing. Afterwashing is performed to remove thickener, unfixed dye, and chemicals after the fixation step.
Pigment printing incorporates a binder with the pigments and therefore requires no fixation or afterwashing. A typical process sequence would be to formulate the paste, print the pattern, dry, and then cure the binder.
Regardless of the system used, the printing process should flow continuously from printing through drying.
Preparation of the Print Paste
The specifics of the print paste formulation depend on the fiber content of the fabric, the colorant system used, and to some extent, the type of printing machine employed. The typical ingredients found in most paste formulations include dyes or pigments, thickeners, sequestering agents, dispersing, or suspending agents known as surfactants, water retaining agents known as humectants, defoamers, catalysts, and hand modifiers. In addition to these ingredients, pigments require a binder or resin system to fix the pigment and may include adhesion promoters.
The most important ingredients of any print paste formulation are the colorants and the thickener system. The dyes used for cellulose fibers, specifically cotton, rayon, and lyocell or Tencel are reactive, vats, naphthyl, and directs. Reactives dominate the dyes used for printing these fibers because of their wide shade range bright colors, good wash fastness, and availability. Vat dyes are also used for textile printing. They usually have very good overall color fastness properties but have a limited shade range. They are mainly available in deep colors such as yellows, burgundies, violets, blues, and greens which makes them a good choice to produce camouflage material. Vat dyes are also known for their light pastel shade fastness.
When printing with cotton dyestuffs, thorough afterwashing is essential for good crock fastness and wash fastness. If the printed fabric is a blend, then a combination of different dye types in the print paste will be necessary. For example, a cotton polyester blend would require reactive dyes for the cotton and disperse dyes for the polyester. These would also require different color fixation conditions, therefore the dominant type of colorant for these blended fabrics is pigment systems.
Pigments are not dyes but are colored particles glued to the surface of the fabric. They can color all fibers in the blend the same shade with a single coloring. Once printed, fixation of the pigment requires dry heat for a defined amount of time. The colorfastness of pigments directly depends on the type and amount of the binder system employed.
Binders can be water-based, known as latex, or solvent-based. These two types of binders vary widely in their stiffness. While greater amounts of binder will improve color fastness, it may negatively affect the hand of the printed fabric. A balance must be achieved between the desired feel of a fabric and an acceptable level of color fastness.
Hand modifiers, most specifically softeners are often incorporated with pigment printing formulations. While these additives may improve certain aspects of the print, they may also interfere with the binder and should be used with caution.
Some drawbacks of pigment prints include poor crock fastness, especially on deep shades, and stiffening of the fabric so that it takes on a rigid feel. Their wide shade range as well as the flexibility and simplicity of processing male pigments an extremely popular choice for both blended and 100% cotton fabrics. The thickener system is the next crucial component of print paste.
The purpose of the thickener system is twofold; first, the thickener gives print paste the proper viscosity or flow characteristics so the color can be applied uniformly and evenly, second, it holds the color in place so that one color paste can be applied adjacent to another without the colors bleeding together. The thickener applied with the pigment system becomes part of the pigment binding polymer and cannot be washed out. With dyes, the thickener also holds the color in place after drying until the printed fabric goes through a fixation process such as steaming.
During fixation, the dye transfers from the thickener and diffuses into the fiber. The thickener is then washed off the fabric before any chemical or mechanical finishing is performed.
Other chemical products can be added as needed to the print paste. These include sequestering agents, surfactants, humectants, defoamers, and hand modifiers. Sequestering agents bond with metallic contaminants in order to prevent interference with the printing dyes or auxiliary chemicals. Calgon is the most well-known sequestering agent. Surfactants are additives that allow chemicals of dissimilar nature to mix. They are used in print paste as dispersing agents, suspending agents, and/or wetting agents.
Water retaining agents or humectants are additives that prevent premature water evaporation or skim over from print paste. Additionally, they often absorb moisture from the air to keep dried print paste from cracking and shedding off the fabric before fixation.
Defoamers are materials added to natural thickeners such as sodium alginate and starch-based print paste to eliminate unwanted bubble or foam formation during the mechanical action of the mixing and/or printing process. Unwanted foaming can result in uneven or light print color. Defoamer additives must be monitored to ensure against adverse effects on final print quality.
Printing of the Fabric
There are several fabric printing techniques currently being used. Among these processes are rotary screen, flat bed, engraved roll, digital ink jet, and transfer printing. Both fabrics and garments can be printed with direct or indirect systems. In direct systems, the print paste is directly printed onto the substrate. Indirect processes make use of the print paste being applied to a block or roller engraved with a pattern that transfers the color to the substrate. Another indirect method is to apply print paste to a release paper, dry the paste and then heat transfer the pattern to the substrate. Each of the printing techniques is discussed in greater detail in the printing section of this program.
Drying of the Printed Fabric
After printing the fabric, the paste is dried to prevent accidental smearing of the print design and color migration.
For continuous drying, a tenter frame, a conveyor dryer, or a looped dryer is used. The tenter frame gives precise control of the width of the fabric. The conveyor only allows for support of the fabric. Loop dryers were designed for extended drying time.
Fixation of the Printed Fabric
Depending on the layout of the printing plant, the printed fabric may go through the fixation process immediately following drying or it may be held to go to fixation later. The type of colorant and production issues of the printing operation dictate the choice.
Fixation can be accomplished through several different heating mechanisms such as atmospheric steaming, high-temperature steaming, or curing, also known as baking.
For dyes, fixation normally incorporates an atmospheric steamer with specified moisture content and a nominal temperature of 212 degrees Fahrenheit or 100 degrees Celsius. With certain dyes, an auxiliary chemical may be necessary as an extra additive to the print paste. For example, reactive dyes require additional alkali for complete fixation. In the case of vat dyes, reducing agents are necessary.
For pigments on all fibers and disperse dyes for polyester or nylon, only high temperatures are necessary. For the binder to polymerize and form a film trapping the pigment on the fabric, the water must be removed from the printed fabric. This is usually accomplished with dry heat in a curing oven or a finishing tenter frame oven. The key issue for pigments is reaching an appropriate temperature for the particular synthetic binder which is usually between 300- and 325 degrees Fahrenheit or between 150 and 163 degrees Celsius. Disperse dyes require energy in the form of heat to move from the surface of the fiber into the core of the fiber where they are trapped. This energy can be in the form of dry heat or superheated steam.
Afterwashing
When printing with dyestuff, the printed fabric is thoroughly washed, then dried after fixation. This step is necessary to remove the thickener, alkali, and other components of the print paste left on the fabric surface after fixation. If not removed, these components could interfere with subsequent finishing processes. Pigments are often printed on finished fabrics so afterwashing is not necessary.
