Date Published: 2007-12-01
Direct-to-Garment inkjet devices - a technology update.
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Direct-to-garment digital printing has come of age in 2007, and if the rumours are to be believed, it is set to make further technological leaps ahead over the next 12 months. So now is the time to review current digital printing technology, understand how it works, appreciate what different systems have to offer, and prepare for the growing influence that direct-to-garment systems will have on the industry’s operating practices. Johnny Shell provides the timely advice and information… |
The basics
Advantages and disadvantages
Many find the biggest advantage of digital technology is its ease of use in producing vivid, full colour photographic images. Gone are the challenges associated with producing a four-colour process, photographic print that actually looks good and generates a profit when using screen printing. Digital equipment also has less space requirements and is a lowcost investment when compared with screen printing, which occupies a larger area of the factory floor and potentially tens of thousands of pounds in equipment costs.Let’s not forget,either, the labour and material costs associated with making, cleaning and reclaiming screens, plus the solvent usage and mess that is generated. Digital produces little mess, requires no screens, uses little solvent and could save facilities time by avoiding the set-up delays or downtime associated with screen printing. Digital also thrives when variable data is a job requirement. Names, numbers and design variations all are easily handled with digital technology. However, as with any technology, digital has its disadvantages. For large run lengths, digital is slower than screen printing, and can cost a company more to produce a job. The break-over between digital and screen depends on each shop, but reports of printers using direct-to-garment devices to produce thousands of prints are becoming quite common. Obviously, the lead-time with a high-volume order will be a considerable factor when choosing the most appropriate printing technology. Digitally printed ink costs also are considerably higher than those for screen – up to double or quadruple the cost per print compared with screen printing. Digital also is not capable of producing the necessary abrasion resistance required for athletic printing, nor is it capable of printing special effect inks, such as glitter or gels. Matching Pantone or custom-spot colour can also be challenging because direct-to-garment devices use ink sets based on cyan, magenta, yellow and black (CMYK). While many Pantone colours are achievable in the CMYK colour space, a considerable percentage of the Pantone range is not.
Dark Matters
With most available direct-to-garment devices, printing on dark shirts with white ink entails additional tasks of pre-treating the garment, extra print passes, and more regular maintenance procedures. Pre-treating a garment involves using a solution that is applied through a high-volume, low-pressure (HVLP) spray gun or via automatic-spray equipment, (which may be included on board the printer or as a standalone unit). The application consistency and pre-treatment’s repeatability, when applying it manually with a spray gun, is very important. Applying too much will affect how well the image holds up in a laundry cycle. Applying too little will cause the white ink to fall down into the dark fibres of the shirt, instead of sitting on top to build the necessary opacity. The gun’s spray pattern can also affect the resulting print, so keep the nozzle properly adjusted and cleaned to avoid unwanted spurts. On the majority of systems using white ink will virtually double the total print time for each shirt, as white ink tends to be printed separately in the initial print pass, (which deposits the underbase). A second print pass deposits the colour, and sometimes a highlight white. The big advantage of using a white ink-compatible printer is having the capability to decorate any colour of fabric. But this has to be weighed against the aforementioned disadvantages: longer print times, increased ink cost as compared with a digital print on a light fabric, and added work- flow and maintenance steps. Another issue with white ink is the settling properties. One might think white ink is no different than any other used in inkjet printers. In reality, white ink presents many challenges for developers, especially when printing on fabric. Developers quickly realised that only certain white pigments would provide the necessary performance and opacity needed for their printers. Titanium dioxide is the most widely used white pigment because of its brightness, high-refractive index and effectiveness as an ‘opacifier’.However, titanium dioxide pigment is harder to keep suspended in a solution because of its weight. If the pigment falls out of suspension, it is difficult to return the ink to its original, homogenised state. The most common cause of this ‘falling out’ is when a printer is not operated for quite some time. Most printers offering white ink have adequate movement that is naturally generated by the machine to keep the pigment in suspension. If the machines are not operated for more than a few days, however, the pigment will begin to fall out of suspension. Formulators are regularly introducing newer versions of white ink that inhibit pigment separation, so it is a problem every leading company faces. Another potential problem with titanium oxide is its aggressive action on piezoelectric print heads. White ink requires a bit more involvement from the operator to ensure the device will function properly. A proper maintenance schedule should be in place and followed to extend the life of the print head. All inkjet printers will malfunction if they are not maintained and cleaned on a routine basis, no matter who manufactured the device.
Discharge and digital
As mentioned, many direct-to-garment devices are capable of printing white ink. However, one manufacturer’s approach was to adopt a technique that has long been used in screen printing. The Mimaki GP604D uses discharge ink, which contains a chemical agent that works to remove the dyes in the shirt and return it to its pre-dyed state. Colour can then be printed in the discharge areas. Discharge produces a print with a much softer feel as compared with screen or digitally printed dark garments. To achieve the best results with discharge ink, garments must be made of naturalfibres (100% cotton) and dyed using a reactive dye. Many suppliers brand their garments as dischargeable, but it’s always good to test them. Also, it’s important that the garments not be over-dyed as the results can be unsatisfactory. Over-dyeing occurs when fabric is re-dyed to another colour. This often happens because of a shortage of a certain fabric colour or, in many cases, because quality control rejected the fabric colour. The rejects are then over-dyed with black. When using discharge with over-dyed garments, the discharge ink may discharge the secondary black dye and reveal a phantom colour underneath it. The digital discharge ink also contains formaldehyde. However, the formaldehyde content is used in extremely small quantities, and washes out of the shirt in the first washing. In the past, people were wary because of the formaldehyde’s presence, but much progress has been made to make this ink as safe as possible.Print heads
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| Printheads are the lifeblood of any device. Shown here are the printheads from an Epson 2200 based unit (left) and the printhead from a Brother GT451. All colours are jetted through the Epson head, while the Brother unit requires one head per colour | |
The print head is the heart of an inkjet printing system. It contains either a single channel (jets only one colour) or multiple channels (jets more than one colour), and causes an image to be transferred to the substrate. Piezoelectric drop-on-demand (DoD) print heads are used exclusively in direct-to-garment devices. As referred to above, piezoelectric print heads use crystalline materials, which have the ability to deform when high-electric fields are applied. As the material deforms under electrical voltage, an acoustic-pressure wave is generated, which affects the ink volume in the pressure chamber of the head. This wave moves through the ink in the reservoir and toward the nozzle, eventually causing an ink droplet to be ejected. One aspect of digital printing that every user should understand is that the print head is a consumable and will need to be replaced. The print head’s life expectancy will vary based on the device, how frequently it is used and how well it’s maintained. Plan to replace at least one head per year, even if a head does not actually fail. Replacement costs for print heads will vary among manufacturers, but the average cost is £150 to £500. Most heads fall in the lower-end of the range. Industrial heads, manufactured by Spectra and Xaar, fall in the upper price range. Lower priced print heads will need to be replaced more frequently while more costly print heads will require less frequent replacement.
Print platforms
There are several important distinctions between the different digital devices that you should know about and understand. The most important, perhaps, is the type of print engine or print heads used. A print engine is the underlying device that has been modified into a garment printer. Many current devices are based on an Epson print engine or Epson print heads. Epson has been manufacturing print heads ever since its first affordable piezoelectric head was introduced in 1993. In 1997, Epson introduced a print head design that dramatically simplified the fabrication process and reduced overall consumer costs. This enabled engineers to work on miniaturising nozzles and improving print resolution and quality, bringing the price point of a piezo-based printer within the budget of many consumers. Epson is well known for the image quality and resolution its products produce, and its components are used in many small- and wide-format inkjet devices,which has further expanded the company’s reputation. Devices based on Epson print engines use pre-existing document printers, which are modified to print on apparel using water-based pigmented textile ink. Common Epson print engines used in direct-to-garment devices include the R1800, 2200, 7800 and 4800, which are used in printers such as the T-Jet 3, Flexi-Jet and DTG Kiosk. Other device manufacturers integrate a pre-existing Epson print head into their device and source the remainder of the printer components. These devices include the Mimaki GP604 and Anajet FP-125. The last device group falls into what I call the ‘ground-up builds’ – that is, printers that have been designed using available components and a mere concept. All printers offered by Kornit Digital and Brother are considered ground-up builds. Printers in this category are robustly engineered and come at a higher price compared with Epson-based machines offering equal capability and print size.Workflow: light versus dark
Many devices allow for printing on white or light shirts by using a standard set of inks and a print driver. This method allows the user to choose the printer in the print dialogue box, similar to selecting a particular printer on an of. ce network. When using this method, the user creates an image within a preferred graphic application and selects ‘Print’. As standard ink sets such as CMYK or CMYKlclm are used, the image is accurately reproduced on the garment with little operator intervention. However, when printing to dark garments, white ink is introduced and requires workflow adjustments. |
| Many devices offer the capability to print the same image to either light or dark garments. Digital prints on darks are in big demand, but dark garment printing often requires a pre-treatment stage, can pose technological challenges with regard to ink formulation and can dramatically slow production. |
Curing
Properly curing the ink ensures a durable image that will withstand common laundering practices. If the ink is not properly cured, the washfastness will suffer greatly. Cure temperatures are specific to the ink being used, so be sure to follow the manufacturer’s recommendations. Generally, cure temperatures hover around 176.7°C (350°F), with cure times ranging from 30 to 60 seconds or more. (The manufacturer may suggest cure times of two minutes or more when printing with white ink.) Variability in the degree of cure can be caused by several factors, especially when printing with white ink. Common considerations to determine proper cure are the number of print passes, amount of white ink coverage and total ink coverage in the image. A wash test is recommended to identify any problems in the degree of cure.Maintenance: it’s not plug ‘n’ play!
The chief factor that many owners tend to underestimate is the amount of routine maintenance that is required to keep direct-to-garment inkjet printers functioning. All manufacturers have daily, weekly and monthly routine maintenance schedules for their devices, which should be strictly followed. Poor maintenance can cause machines to malfunction, possibly causing misprints. Furthermore, parts may need to be replaced prematurely. Common maintenance tasks include cleaning the capping station and encoder strip, greasing guide rails, replacing print heads and servicing the waste ink tank. The capping station on many devices is used to collect waste ink while the heads are being cleaned. The capping station also may serve to seal the head when the machine is turned off to hinder ink drying in the print head’s nozzles. Epson-based machines also include a wiper blade near the capping station, which periodically needs to be replaced.
Future technology developments
SCREEN PRINTING TASK COMPLETION TIME | |
| Task/Consumable | Completion Time |
| Separations | 60 minutes |
| Print Film Positives | 20 minutes (5 min per film) |
| Register, Expose and Develop Screens | 15 minutes |
| Dry, blockout and tape screens | 30 minutes |
| Setup, print, teardown, reclaim | 60 minutes |
| Total estimated time | 3 hours, 5 minutes |
DIRECT TO GARMENT INKJET COMPLETION TIME | |
| Task | Completion Time |
| RIP file | 2 minutes |
| Print time of 45 seconds per shirt | 18 minutes |
| Cure time of 40 seconds per shirt | 16 minutes |
| Total estimated time | 36 minutes |
| The approximate time required for printing the same full-colour design on 24 shirts using direct-to-garment digital and screen equipment. On shorter runs, the digital approach offers signficant time and cost savings. The break-over between digital and screen will vary from shop to shop, but reports of printers using direct-to-garment devices to produce thousands of prints are becoming more common. | |
Many direct-to-garment inkjet manufacturers are making improvements to print speed, ink chemistry, overall durability and user friendliness of their machines. Modifications and upgrades are constantly released to address workflow or functionality issues, and include on-board pre-treatment applicators, magnetic stirrers to keep pigment suspended in bulk ink systems and evacuation fans to draw dust and lint away from the print head. Manufacturers also are incorporating digital devices in traditional screen printing presses, marrying the two technologies into a single unit. The next disruptive technology to impact digital inkjet printing will be single pass devices capable of printing an image in just a few seconds. Using Micro-Electro Mechanical Systems (MEMS), future print heads will be cheaper to make, have smaller ink orifices and feature much larger nozzle arrays to increase resolution and print speed. Print heads traversing back and forth will be replaced by a full-width print head array to produce full-colour printing at lightning speeds in a single pass. Direct-to-garment inkjet is definitely here to stay. The installed base of direct-to-garment machines is estimated to be approximately 3,500 units worldwide, and the adoption rate is expected to continue rising. Many users claim profits generated with their machines enabled them to pay off the device in as little as three months. These same users also state the technology has opened new revenue streams for their businesses, allowing for additional market share in their regions. As technology advances, direct-to-garment inkjet printers will become ever more profitable options for garment decoration. These options are currently available, but make sure to research all the devices that fit your business model. Be sceptical of print speeds and specifications stated by manufacturers. Time the printers, and determine how long prints really take. You should also evaluate and determine the speed at which the printer produces a sellable product for your market. Some perform quite well at 360 dpi while others do not. Determine if adequate coverage is achieved in one print pass or two. Ask the manufacturers questions about warranties and service level agreements as well as references from customers that bought the printers. Get customers’ feedback on the level of satisfaction they have with the device, the service received and any learning curves associated with the hardware or software. This article is based on a feature that appeared in the SGIA Journal, July 2007, Volume Eleven, Third Quarter issue and is reprinted with permission. © 2007 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.
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