Ink Jet Printing

• What types of ink jet printing technologies are normally used in industrial applications?


• How are characters formed?


• What factors should I consider in selecting an ink jet technology?


• What are some of the issues with the substrate?


• What is surface tension and surface energy?


• What are the advantages of using UV inks?


• What is different about UV-LED lamps?


• What are the print speeds of the various ink jet technologies?


• What is print resolution and how is it measured?


• What about food processing applications?


• What factors should I consider when qualifying an ink jet system?


• How do I begin the ink jet selection process?

What types of ink jet printing technologies are normally used in industrial applications?

Continuous ink jet (CIJ) printers run continuously by pumping a stream of ink through a combination nozzle, piezoelectric crystal, and electrode that breaks the ink stream into small electrically charged drops. The flight path of the charged drop is software controlled by an electrostatic deflector plate. The software control of the deflector plate determines if a drop is to be recirculated back into the ink reservoir or is used to form a character on the substrate. Ink formulations are typically based on MEK, alcohol, or acetone. For the most part, CIJ printers are used to print one to five lines of small characters on non-porous surfaces at very high print speeds.

Drop-on-Demand (DOD) printers only print when the printhead nozzle is energized. DOD printers have the widest range of printhead heights, print resolutions, and available inks. For printing on porous surfaces oil and water based inks are the most commonly used. For non-porous surfaces hot melt, solvent based, and UV cured inks are available.
The DOD printers can be further subdivided into valve jet and impulse jet. Valve jet technology uses a solenoid controlled plunger that "squirts" the ink drop on to the substrate. Both porous and non-porous inks are available. Ink formulations are based on ethanol, N-proponal, MEK, acetone, or waterbased. Valve jet printers are used to print one or two lines of large characters on porous and non-porous surfaces at moderate speeds. Valve jet printers have large drop sizes and long throw distances.

Impulse jet printers use either a piezoelectric crystal or heated element to pulse the ink much like an eye dropper. This group has the widest range of printer/ink options for printing on porous and non-porous surfaces. Ink formulations are based on oil, oil/glycol, alcohol, hotmelt, UV cured, and water based. Impulse ink jet printers have the highest print resolution but have limited ink throw distances.

How are characters formed?

Characters are formed by the printer software adjusting the ink drop path (CIJ) or nozzle energization (DOD) to the speed of the substrate past the printhead. It is these same parameters that limit printing speeds. There is a maximum rate of ink drop formation that limits how fast the substrate can be moving and still get an acceptable image. The substrate speed can be estimated and entered into the printer software. Linear encoders are an option for precise input of substrate speeds and are recommended for very small text and bar codes.

What factors should I consider in selecting an ink jet technology?

There a few things that will narrow your options very quickly. They are: substrate, print speed, print resolution, and food processing. This will get you through the Piezo vs. Valve vs. CIJ selection and will also eliminate from consideration many piezo solutions.

What are some of the issues with the substrate?

Whether you are printing on a porous or non-porous substrate is the beginning for selecting the appropriate ink jet printing technology. Each type of substrate will require a different ink chemistry. Oil, oil/glycol, and water based inks are typically used for porous surfaces. Solvent and UV cured inks for non-porous surfaces.

Some porous surfaces can be challenging. For example, corrugated surface parameters can change from porous to semi-porous to non-porous with each shipment of cartons.

The technical challenge is that the relative surface tension of the ink must be less than the surface energy of the substrate in order to provide good ink adhesion.

What is surface tension and surface energy?

Surface tension is a property of the surface of a liquid that causes it to "skin". It is measured in dynes/cm. Surface energy refers to the receptivity of a substrate to liquid "wetting" and is also measured in dynes/cm.

In order for wetting to occur the surface tension of the liquid - in this case ink - must be less than the surface energy of the substrate. If the surface tension of the liquid is higher than the surface energy of the substrate the liquid will not "wet" the substrate - this is what causes water to bead on a waxed car. Some non-porous surfaces may require pretreatment - corona discharge or heat - prior to printing to increase the surface energy and improve ink adhesion. Some treatments have a life so the surface energy may decrease over time.

Solvents have naturally low surface tensions - this is the reason they are used in inks for low surface energy non-porous substrates such as PVC.

UV inks are available for non-porous substrates and rely on surface tension to adhere to the substrate. The UV inks are basically liquid plastics that - when cured - become a plastic film.

What are the advantages of using UV inks?

UV inks are environmentally safe - they are solvent free with no VOC's. They dry instantly when exposed to the UV lamp.

They adhere to a wide range of substrates - from corrugate to wood to plastic film. Cured UV inks have superior lightfastness, very good scratch resistance, can withstand high temperature environments, can be low migration for food applications, and are resistant to many chemicals.

The fact that they don't cure until exposed to UV means that printhead open time is increased and maintenance is reduced. The printheads operate at a low temperature thereby significantly reducing energy consumption.

The inks can be shipped via UPS or other common carriers. Disposal of empty ink containers does not require any special handling or permits.

What's new is that UV inks have been developed to cure using the new UV-LED lamps.

What is different about UV-LED lamps?

UV inks have been used in the label printing industry for years. These inks are cured with a mercury vapor lamp. The mercury vapor lamps emit a potentially dangerous UV wavelength and generate a lot of heat.

The advantage of UV-LED is that the UV wavelengths at which they operate are safe. There is no exposure to the potentially dangerous UV wavelengths - the UV-LED lamps don't emit UV-B or UV-C wavelengths that are dangerous for the eyes.

Additional benefits of the UV-LED technology are: instant on/off; an operating life of tens of thousands of hours; and no ozone created as a by-product.

What are the print speeds of the various ink jet technologies?

What follows is a good rule of thumb for print speeds:

• CIJ - up to 3 lines of print to 1100 fpm
• Valve jet - to 300 fpm
• Impulse - to 350 fpm

What is print resolution and how is it measured?

Print resolution is the number of drops per inch (dpi). The higher the dpi the better images, small characters, and in-spec bar codes can be printed.

In most ink jet printer specifications both the vertical and horizontal dpi are given. The vertical dpi is fixed by the printhead design. The horizontal dpi is determined by the number of pulses per inch of printing at a given speed. This is why the vertical dpi is normally less than or equal to the horizontal dpi. Horizontal dpi can be misleading in the sense that a large number of pulses over a set distance can result in dot overlap with little improvement - if any - in image detail.

What about food processing applications?

There has been increased concern about the migration of inks through food packaging materials. In Europe, ordinances have been established and companies like Nestles have developed guidelines.

In the United States, food packaging and inking systems must be designed such that no chemicals migrate into the food product. Low migration inks are designed to comply with the Swiss Ordinance on Materials and Articles in Contact with Food (SR 817.023.21). No migration inks take the solution a step further in that the inks will not migrate to the food product.

What factors should I consider when qualifying an ink jet system?

Energy usage - many companies with sustainability initiatives will want to know how much energy is required to operate the printing system. Piezo ink jet systems typically have heated printheads and are sensitive to changes in printhead temperatures.

Start up/shut down procedures - what is involved? How do I recover from a loss of power to the printers?

Throw distance is the maximum distance from the printhead to the substrate that will result in an acceptable printed image. Can the printhead be mounted close enough to the substrate to insure good images? Must a material handling system be installed to maintain the substrate within the throw distance of the printhead?

Head open time is the maximum amount of time between print cycles before the ink dries in the print nozzle. Exceeding the head open time will result is missing dots. There is a tradeoff between fast drying non-porous inks and head open time. Not an issue with CIJ printers because the ink is continuously being recirculated. Determine if there is a head open time constraint and the symptoms? What happens when the head open time is exceeded and how do you recover?

Carton handling - if you are printing bar codes it is important that the printing surface not vibrate as it moves past the printhead. Belt conveyors are normally recommended for printing bar codes on cartons.

Solvent in inks - Volatile Organic Compounds (VOC's) can be released when printing with some non-porous inks. Will you need an exhaust system? As an alternative - can you use alcohol based or UV cured inks?

Ink refilling - what is the ink supply capacity and can it be replenished while the printer is operating?

Relation of printhead to ink supply - some impulse jet printers require that the ink supply be mounted at the same height as the printhead. Will this be an issue?

Cleaning procedures - many printers must be purged on a regular basis. How often must this occur? How is the printhead protected from dust?

Resistance to vibration and shock - some impulse printers will de-prime and lose print dots from vibration and occasional shocks. Insure the mounting hardware protects the printhead from shock and vibration.

How do I begin the ink jet selection process?

Start with the qualifying process as outlined above - this should reduce the number of vendors selected for further discussion. Prepare a sample of the substrate to be sent to the selected vendors for sample marking. As in any selection process tradeoffs may need to be made. If you have any additional questions either email us at sales@foxiv.com or call at 724-387-3500.