John Albrecht identifies the key variables affecting direct-to-garment digital print quality and shares some tips on how to control those variables to achieve the best possible results
Maybe you are just starting to research direct-to-garment (D2G) digital printing, or perhaps you’re an industry veteran looking for the best way to produce thousands of short run images. You might be looking to reduce your overhead, to turn more jobs each day, or perhaps you need the ability to produce detailed full-colour prints. Whatever your situation, you will need to optimise the D2G variables that affect print quality. We asked John Albrecht, head of sales of Kornit direct-to-garment products at US supplier SPSI, to advise on those variables affecting pretreatment, the shop environment, and the shirt itself.
Pretreating is key to producing ongoing, high quality D2G prints. Broadly speaking, there are two ways of pretreating: inline and offline. In almost all cases, inline pretreating [the process used by Kornit print machines] consists of applying pretreatment and printing ink, wet-on-wet. Offline pretreating involves curing the pretreatment prior to printing with the ink.
Offline pretreating requires the operator, or a second staff member, to add the pretreatment to a shirt, dry it, and then load the pretreated shirt onto the printer for printing. You can spray the pretreatement onto the shirt manually, within a machine, or transfer it via paper. Each method has its advantages in regard to cost and/or consistency.
Offline pretreatment is generally used whenever white ink is printed on a shirt. White or light shirts may or may not require pretreatment based on the desired end look: using pretreatment on whites and lights is widely considered to give a more vivid finished print,but at extra cost.
With machine-applied spraying you need to determine the proper amount of spray to use. You must then determine if you are going to pretreat only the area on which the design will be printed, or the entire shirt. The former option demands pretreating immediately prior to printing; pretreating the entire shirt allows you to pretreat in advance.
Consider the following when pretreating offline:
- You can cure the pretreatment using a heat press (30 to 45 seconds) or through a conveyor dryer. Space savings and lower cost are the advantages of using a heat press, while a conveyor dryer is likely to increase productivity. Keep in mind that if the pretreat is not uniform, or not present in the print area on a dark shirt that requires white ink, the result will be scrap.
- If a separate department is pretreating shirts (up to a couple of weeks in advance), then, in most cases, the garments’ entire image area will need to be pretreated. This can save time during printing; however, the cost of pretreatment will be greatly increased due to the larger volume of pretreat required to cover the larger area of fabric. Pretreating the entire area in advance will allow a single operator to load and cure, leading to increased production and quicker turnaround times, but you’ll also need to factor in the higher cost of employing separate members of staff to pretreat shirts and to operate the D2G printer. Offline pretreating tends to require much higher handling and labour costs compared with inline wet-on-wet pretreating. But since offline pretreatment is cured prior to the print, overall cure time will be reduced compared to inline wet-on-wet pretreat curing.
- Other offline pretreating considerations include the final look of the print. You may need to explain to the customer that the square outline latent on the shirt after heat pressing should disappear after the first washing.
Inline wet pretreating allows the operator to pull a shirt straight from the box, load it onto the printer, and then press the print button; the printing press will carry out both the pretreating and printing of the ink inline. Just as with offline, it is critical to apply a uniform spray, and to determine the correct amount of spray to be used based on the shirt.
Consider the following when pretreating inline:
- Inline wet pretreatment requires that the pretreated area be only slightly larger than the actual image. Registration of pretreatment is less of a concern since the shirt is not moved, and the software incorporates the pretreatment area with the art (a set percentage over image size).
There is no additional staff required when using inline wet pretreatment.
- There is no additional staff required when using inline wet pretreatment.
- The cost to pretreat is greatly reduced as a function of the smaller size of the area being pretreated and the nature of the unique wet-on-wet process.
- The inline wet pretreat completely evaporates in the curing process, leaving no need to wash a garment before wearing it. You will, however, need to factor in a larger dryer that’s capable of evaporating both the wet pretreat and the ink after printing when pretreating wet-on-wet inline.
The printshop environment is of primary importance to the quality of your digital prints: it is essential to control humidity, temperature and cleanliness.
Humidity A humidity level of 45 to 70 per cent should be maintained to achieve repeatable quality for consistent D2G printing. There are several ways you can do this: perform your D2G printing in a controlled room; add humidifiers to the area when the humidity is low; or install humidity systems within some of the larger printers.
Humidity will affect printers with smaller picoliter openings more so than printers with heads that have much larger openings (especially with white ink), and is key to consistent jetting, filtering, and de-gassing.
Temperature Like humidity, temperature plays a role in consistent jetting. Digital ink is water-based and needs to avoid becoming frozen in transit. D2G printers will also see quality issues when temperatures in the work area exceed 32°C (90°F).
Cleanliness This is another important consideration when working with D2G printers. Keep in mind that there are hundreds, and even thousands of nozzles per printhead on these printers. Screen printing adhesive, lint, dirt and other environmental contaminants will greatly affect the consistency of print quality. Be sure to keep the printers away from outside loading doors and other high traffic, less clean areas of the shop.
Some manufacturers suggest replacing printheads every year while some say heads will last five or more years; either way, do not let an unclean environment be the reason you have to go through the high cost of head replacement.
Shirt consideration is critical to high quality D2G printing. The factors to be aware of, and which will have the most marked effect on ultimate quality, are: absorption rate, tightness of weave, polyester content, and dischargability.
Absorption This affects the amount of pretreatment required, as well as the colour and brightness of the final print.
Often, the absorption rate from one brand of T-shirt is significantly different to that of another brand. In order to best control this variable, it is advisable to use as few manufacturers as possible, settling on tried and trusted products and dialling in set parameters.
Pretreatment acts as a carrier for the ink and helps maintain the colour of the print on the surface of the fibres. Some shirts will require a higher percentage of pretreat compared to others. You will save time and product by determining this in advance. Check with your shirt suppliers, as well as the press supplier, to determine the proper settings to optimise pretreatment on particular shirts.
Tightness of knit This is another critical variable that directly affects print quality. Ring-spun shirts tend to have a tighter, smoother surface for the digital ink to adhere to. Like absorption rate, you will need to adjust your pretreat parameters to accommodate the specific knit of the shirts you are using to control the brightness and quality of your prints.
Polyester content Some D2G printers can produce acceptable prints on white polyester material (see Images August 2014, ‘Pretreatment for Polyesters’, page 38). You will encounter many similar challenges when D2G printing on dark polyester T-shirts as you would with traditional screen printed T-shirts. Dye migration remains an issue. Screen print ink manufacturers have chased this migration by adding bleaches to the white ink for decades. White inks for digital printing have no dye blockers.
Currently, the only way to bypass this issue, to some degree, is to use a process that cures the ink at a very low temperature while trying to maintain washability. Some D2G manufacturers have had limited success with dark polyester fabrics, although, from a productivity standpoint, there are still improvements that can be made. (See ‘Printing on polyester revisited’, Images December/January 2015, page 46.)
Discharge ink Some printers use a discharge ink when D2G printing on dark cotton T-shirts in order to achieve bright colours and prints with virtually no hand or feel to them. If you go down this route, work with your shirt supplier to identify the best cotton shirts in terms of their dischargability.
This is an extract from an article published in the SGIA Journal and is reproduced with the kind permission of the Specialty Graphic Imaging Association: www.sgia.org
John Albrecht is a 30 year veteran of the garment decorating industry. He owned and operated a screen print supply company and has spent the past 20-plus years at SPSI, a one-stop-shop for screen printing and other textile related equipment, in various leadership and sales capacities. He currently heads the sales of Kornit direct-to-garment products at SPSI. John is a longtime volunteer at SGIA.