Screen Printing: Production

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March 17, 2015

If there is one sure-fire way to lose clients in this industry, it is to sell them finished garments that fade after five to 10 wash cycles.

As straightforward as the textile screen-printing process is (or seems to be) the number of undercured prints I see on a regular basis never ceases to amaze me. It is easy to see where a print was wrinkled when placed on the dryer belt, meaning it did not receive uniform heat. This also can result in a decaying print. I’ve also seen situations where an underbase was cured by the flash unit, but the overprint washes off due to poor intercoat adhesion (the two ink films not properly adhering to each other).

Here, I will review the components of plastisol textile screen-printing inks, how they react to themselves and heat, and how to properly determine the final cure of your print.

For the decades that plastisol (liquid PVC) has been used in textile embellishment, the formulation of the product has remained the same. The basic components that make up a screen-printing plastisol ink include:
• PVC resin
• Plasticizers
• Pigments
• Fillers
• Optical brighteners (optional)
• Dye-blocking agents (optional)
• Special-effects additives (optional)

All of these components are blended in customized formulations and sequences to produce a “balanced” finished ink that will meet the printer’s production requirements, as well as produce the desired effect for the end user.

The two primary components on which the ink is based are the PVC resin and the plasticizer. The PVC resin is introduced as a powdered compound, which is mixed with the properly proportioned plasticizer. These two critical components will not only determine how the ink will cure, but also its durability, elongation, hand and flashing characteristics.

Figure 1 shows how the ink reacts as it is exposed to heat over time. The plasticizers in the ink film are absorbed as its exposure to heat increases. To achieve a proper cure, all available plasticizers are absorbed by the PVC resin as it swells and takes in the plasticizer when exposed to heat. The plasticizers that remain in the ink film give the ink its pliability and elasticity.

FLASHING PARAMETERS
The flashing process is one of the many aspects of screen printing where more is not better. Flash units were designed to dry an initial ink film and allow a secondary ink film to be printed over it. The purpose of the flash unit is to gel or “dry” the ink film, not cure it. Many printers think hotter flashing means faster printing. Sadly, this is not the case. A plastisol only flashes as quickly as its formulation and components allow. Some inks flash quickly, while others require more time. In many cases, this depends on the plasticizers used in that particular formulation.

With most standard white plastisols, you should bring an underbase ink film to the 125˚F-225˚F range. Such a wide range is preferred because different products flash at different temperatures. Check with your ink manufacturer for its recommended flashing parameters.

One issue that easily can occur during curing is elimination of intercoat adhesion. In Figure 2, the light green area shows where the two respective ink films meet. Should the underbase’s surface reach 320˚F, the overprint will not fuse with it during the curing process. The underbase must be “gelled” so the two ink films can properly fuse in the dryer, thus ensuring a proper cure.

Flash units come in two basic formats: black panel and fused quartz. Black-panel flash units emit a relatively consistent heat temperature, whereas fused quartz units are triggered to emit temperatures from a few hundred to 1,400˚F in a few seconds. In either case, the objective is to bring the underbase ink film just to the gel state, where it simply is dry enough to overprint with minimal or no after-flash tack.

When initially testing a new white ink or flash unit, you should start at a point where the ink is not completely dried when emerging from the flash unit. Then, you can start increasing the heat settings in small increments until the underbase is properly dried and ready to overprint.

DRYER PARAMETERS
Textile screen-printing dryers come in three configurations: infrared, gas/forced air and combination forced air/infrared. Plastisols will work in all three formats. However, when printing with discharge or water-based inks, you will require hot forced air to drive the water or moisture content out of the ink film and fabric. From a production cost and efficiency standpoint, gas dryers allow for greater versatility in the use of different ink lines and typically perform more efficiently than other alternatives.

The objective of a dryer is to bring the entire ink-film thickness to the required cure temperature. I emphasize the entire ink-film thickness because not everyone considers the temperature of the ink where it contacts the fabric. The interface between the fabric and the ink also requires the correct curing temperature. Should this portion of the ink film not reach the required cure temperature, the result will be poor washability, cracking, and possible bleeding/dye migration on synthetics and 50/50 cotton/polyester blends. (see Figure 3)

One major consideration in determining if you are achieving and maintaining a proper cure is to know the temperature of the ink throughout its run through the dryer. There are three different methods for determining an approximate ink temperature when a printed garment emerges from the dryer. The first — and perhaps oldest — test method involves using Thermo Tapes. With this method, you apply heat strips to the surface of the garment being cured and the individual strips will turn black as the ideal temperature is reached. Once you have determined the point at which you have achieved a proper cure, the heat strips also act as a good quality control tool to ensure that your dryer is achieving consistent temperatures.

Second, and most popular, is the non-contact pyrometer, which is useful not only as a quality-control tool, but also for determining cool or dead spots on a flash unit’s infrared panels or within a dryer. This tool reads the ink film’s emitted heat energy, also known as emissivity. Although this is the most widely used tool for monitoring ink films and dryer temperatures, it still doesn’t indicate the temperature of the ink that is in contact with the fabric.

Finally, the donut probe also allows you to monitor the substrate’s temperature as it passes through the dryer and plot out the temperatures in multiple positions. Currently, this is the most accurate process for reading the actual temperature of the inside of the ink film.

A donut probe’s thermocouples allow you to place the wires directly into the ink film to read its actual temperature as it passes through the dryer chamber. The advantage here is that you can monitor and document the substrate’s temperature every five or 10 seconds as the probe passes through the dryer. Doing so on the left, center and right sides of the dryer allows you to plot a 3-D chart of your dryer’s history. Figure 4 shows what documentation would look like for a dryer with a 60-second retention time.

Once you have determined the proper dryer settings in relation to achieving the proper cure, you can use any of the previously mentioned tools to monitor and maintain them.

One issue that occasionally surfaces is when a print is exposed to excessive heat. This typically occurs when the ink film receives too much heat during the flashing process and then reaches the re-melting point in the dryer.

Remember, standard plastisols cure at 320˚F. Should an ink film reach above that temperature (typically 360˚F), the dried ink film will re-melt, resulting in the destruction of its durability.

The most common sign that an ink film is overcured or re-melted is blistering. This will happen with bleed-resistant inks where the bleed-resistant agents start to boil. The result is a cratering effect, where the ink film will look like the Moon’s surface under a magnifier. This is an example of the caution you must use when trying to reach the proper curing temperature, while also avoiding the re-melting point.

TEST METHODS
As our industry has never had an actual “how-to” reference on its respective processes, the method for determining proper cure tends to vary from among facilities. One of the greatest missteps I’ve seen involves determining the degree of cure by stretching the dried (and hopefully cured) ink film. Stretching an ink film simply reveals the elongation characteristics of that particular ink. You may have an ink with excellent elongation characteristics, but it still could be undercured.

This leaves one tried-and-true test method — the wash test — which will be repeatedly performed by the customer. This should be regularly conducted by screen printers to ensure the print is properly cured. When possible, printed garments should undergo as many as five wash tests.

When you have successfully wash-tested a garment, and know the dryer and flashing procedures are set, you can standardize those settings and use any of the aforementioned heat-measurement tools to ensure the process remains consistent. Conducting a proper wash test is insurance that the customer will not have issues once orders have left your facility.

Rick Davis is the southeastern regional sales manager for Triangle Ink Co. He is a 35-year veteran of the textile screen-printing and apparel manufacturing industries. His background includes plant design, management and troubleshooting, and he also is a member of the Academy of Screen Printing Technology. For more information or to comment on this article, email Rick at rickd5050@yahoo.com.

Hear Rick speak on screen-printing topics at the 2015 Imprinted Sportswear Shows (ISS). Individual seminars are just $25 if you pre-register: issshows.com.

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Suggested Reading

Like this article? Read these and other screen-printing articles at impressionsmag.com:
• “Screen Printing Tip of the Week: Curing Ink with a Heat Gun”
• “Screen Printing Tip of the Week: Low-Cure Additive”
• “How to Ensure a Print is Correctly Cured”