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Choosing the Right Sublimation Transfer Paper
By Arnold van Houtum, Texo Trade Services
Very few manufacturers offer transfer paper for inkjet sublimation inks. Yet, choosing the right type of paper is not as easy as one might expect. Printers have to find the right balance between printability, productivity and behavior in the transfer press on the one hand, and the paper procurement costs on the other.
More issues need to be addressed too: How important is transfer yield? When do I need “high speed” paper? An entrepreneur who is considering his first investment in digital transfer printing is confronted with other dilemmas, too: Do I choose a water-based system, or should I consider oil-based or solvent-based inks? Which type of paper is preferable for transfers onto metal or tiles?
Why This Article?
Then we will provide a guide for water-based systems focusing on three aspects:
Water-based Systems versus Oil-based and Solvent-based Systems
Limitations of Oil-based Systems
When an oil-based ink is drying, the oil does not evaporate from the paper. At most the volatile fractions will evaporate. The remaining fractions facilitate the paper dye’s migration, causing colors to shift and details to lose their sharpness. Therefore, transfer paper printed with oil-based inks must be run through a transfer calender immediately after printing.
Promise of Solvent-based Systems
There is a very good reason why solvent inks have not played a bigger role in sublimation transfer printing. Solvents bring environmental and safety hazards, which printers thought they eliminated when they embraced water-based sublimation inks. Printers with a background in the textile industry are accustomed to water-based ink chemistry, which has its own environmental problems. To them, water-based transfer printing provides a clean and safe business model. The situation in the sign industry is quite different. Solvent ink is the preferred method for producing outdoor graphics on uncoated, non-absorbent media. Sign printers are fully familiar with health and safety requirements. To the sign industry, solvent-based dye sublimation is not equivalent with new health and safety hazards.
It is too early to make recommendations about solvent-based sublimation transfer printing because there are still technical hurdles. However, we will be following developments in this new technology closely.
Open Substrate (textile fabric) versus Closed Substrate (metal, ceramics)
Transfer papers for textile fabrics are specially designed to deliver the maximum amount of dye to the substrate (maximum “transfer yield”) during the pressing process, thus meeting the highest coloristic demands and allowing for the least ink consumption. Maximum-dye release occurs when carrier liquid and dye particles are not strongly absorbed into the paper and the dye particles are not bound to the internal paper structure. To achieve this, manufacturers have developed a closed film coating, which forms a gel with water from the printing ink. The barrier character of these coatings makes the papers less suitable for transfer to hard (closed) substrates. The vapor, generated when the paper is heated during transfer, can only escape in a lateral direction. This can easily cause damage to the print, which is visible in the cloudy vapor patterns in full colors and blurred contours.
For transfer onto hard surfaces, we recommend a transfer paper that allows the vapor to escape through the paper. This type of paper is similar to photo-quality inkjet papers, which dry immediately after printing. But there is a tradeoff. The dry paper surface is made possible by a special silicate-based coating, which draws the ink faster and deeper into the paper compared with the paper for textile fabrics. This results in a noticeably lower transfer yield. A second disadvantage of the hard-surface papers is that they cockle more easily due to the quick water absorption.
It should be noted that the hard-surface paper is popular with textiles and rigid surfaces in North America. Elsewhere textile printers generally prefer papers specially designed for soft substrates.
The Right “Basis Weight”
Criterion 1 Ink loading
Criterion 2 Difficult patterns
Criterion 3 Roll-to-roll or roll-to-sheet
Criterion 4 Printing speed
With the development of “high speed” or “HS” papers, manufacturers anticipate for the ever-increasing speed of the inkjet printers. These “high speed” papers, i.e. papers that dry considerably faster than the regular ones, meet the requirements of the latest generation of printers and even of tomorrow’s printers. It is especially noteworthy that the faster drying is not obtained at the expense of the transfer yield. The HS paper fulfills the highest coloristic demands. Many print shops use the same color profiles for “high speed” paper as they do for regular paper.
Regular transfer papers for open substrates typically show a gradual, ink absorption and dry relatively slowly. Transfer printers have adjusted to this phenomenon in different ways. In many cases, the slow drying does not pose a problem, even on fast inkjet plotters. Printers fall back upon a box of tricks to make sure that the paper is dry and, more importantly, smear-resistant before it is wound-up. The oldest and simplest trick is extending the paper path in the printer. This often is done by simply putting the rewind on the same side of the printer as the unwound, and by installing some extra rolls to guide the web underneath the printer. Sometimes, you see a separate winding stand at a short distance from the printer. Another solution is a heated plate under the printed paper that is sometimes combined with a heated plate before the printing heads. In many cases, an array of fans is used for additional convection drying. The above-mentioned tricks are so widely used that they are standard features on many printers.
After HS transfer paper made its appearance onto the market, ink manufacturers introduced similar “high speed” inks. All elements of transfer printing seem to be advancing in parallel with production speeds, be it the print heads, printers, paper or ink. We shall dedicate a separate article to the advances in sublimation inks sometime in the future.
The latest printer models are so fast that the limits of the various tricks are visible, particularly with print work done with high ink loading. HS transfer paper is the obvious solution because it halves the time needed for the image on the paper to dry and become smear-resistant. However, we always advise our customers to first make a detailed cost analysis. “High speed” paper costs about 10 cents per square meter more than the equivalent “standard” paper. Have all options been exhausted in reaching the desired printing speed? Does higher output justify higher media expense? Is it possible to accept a lower printing speed combined with lower media costs?
Color management — in other words things such as linearization, finding the proper settings and making and using ICC color profiles — forms the basis of a professional workflow. Color management guarantees a controlled process with reproducible output. This is a necessary condition for the optimal procurement of consumables to achieve quality improvement and cost reduction. Without good color management, any media or ink experiment becomes a risky venture, condemning the printer to use more expensive consumables.
The other technical matter is climate control or rather the lack of it. In the commercial printing industry controlling the temperature and the relative humidity of the air is taken for granted. Many litho printers know what temperature and relative humidity they need in the press room. This awareness is quite different in transfer printing. Sublimation transfer printers working with water-based inks know from experience that print heads become clogged easily in a dry environment. They are less aware of the negative impact of high relative humidity and of substantial changes in relative humidity on the paper’s productivity. Certain complaints about the paper at the end of each summer illustrate this.
When the outside temperature drops — but the heating system is not yet switched on — the humidity in the press room goes up, causing all sorts of problems with the printing paper. These problems and complaints disappear when the outside temperature drops further and the room-heating systems are on.
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