After our previous post on Piezo and thermal print head technology we have had several of our customers stop in and ask about the working principles of the Piezo inkjet print head. In fact one of our largest customers, a siding contractor, places posters printed with this technology for window advertisements because they are durable and resistant to the elements.
Diving into the working principles of how these work can be summarized at a very high level of a deformation mode of either: squeeze, bend, shear, or push.
The squeeze type of piezo print head works by heating a metal plate which transfers heat to the nozzles on two sides to create pressure gradient which forces the droplets out of the print head.
The bend and push methods are similar and work by heating a plate which transfers heat to a reservoir of ink. This increase in pressure again will force the ink out of the print nozzle. The main difference is that bend will apply to the center of a reservoir, while the push will apply to the bottom of a reservoir.
The shear method has separate cavities split by metallic plates. These vertical plates are heated and create a pressure gradient to force the ink out of the print head.
When choosing between the different methods, print head manufacturers have to decide whether or not the ink can withstand different temperatures and pressures, because that is the main difference in applications between these methods.
How Piezo Print Heads Function
A commonality between each of these methods of piezo printing is that the heating of metal plates around the nozzles heats the ink to a point where the ink creates a pressure gradient in the print head. The above examples only differentiate as to where the heat is applied and where the pressure gradient originates. All of the piezo printing technologies creates a pressure gradient so as to expel the ink out of the nozzles at each burst of heat applied.
After the heat is applied in the form of an electric voltage, a drop is fired. A wave of ink equal to the expelled is then reflected back at the reservoir to create a wave of positive pressure, which is then amplified by the second waveform that is created when the next droplet is fired out of the print head.
An actuator for each piezo print head is the electrical element that applies the voltage across the channel.
An electric voltage on a piezo heating element enlarges a channel to generate a large negative pressure in the printing lines and the print head itself. After reflection at the reservoir, this becomes a positive-pressure wave. The positive-pressure wave is amplified by the second slope of the driving waveform to obtain a large positive-pressure peak at the nozzle, which fires a droplet. There are many mathematical equations that can adequately describe this nature, but what can be taken away from this type of printing is the following:
- The charging of a piezo element will enlarge the cross sectional channel, which initiates the negative pressure wave. The negative pressure wave will propagate through the print head and bounce back to the front of the nozzles.
- The reservoir acts as an opene ended channel in order to send a return wave to the front of the nozzles.
- Upon discharge of the piezo printing element amplifies the wave thus increasing the positive pressure.
- The channels are designed to handle a large incoming positive pressure peak at the nozzle.
- The above combine together to release a droplet of ink from the print head.
When conceptualizing inkjet technology, it is incredibly easy because it is essentially a digitally controlled ejection of drops of fluid from a print head onto a film or substrate. The manner in which the ejection happens, varies in two primary ways. The first being industrial inkjet, and is most commonly referred to as CIJ or Continuous Inkjet, where the second way to classify it is drop-on-demand inkjet. Variants of each of these styles are numerous and generally are only initiated for unique solutions.
Continuous inkjet ejects drops continuously, where the droplets are directed directly to the substrate, or in some instances to a collection tank for recirculation, recycling, or reuse. Drop-on-demand only produces and ejects droplets when required.
Continuous inkjet is stigmatized in the printing industry as a primitive or amateur type of printing. Continuous inkjet printers account for roughly 10% of the printer sales in North America in 2016. This is primarily and almost exclusively used in the marking and coding of products and packages, where high volumes and fast line speeds are critical to manufacturing process performance. CIJ utilizes a pump which pushes fluid from a reservoir through small nozzles to produce a continuous stream of droplets between (50kHz and 175kHz) via vibrating piezo print head crystals. The print quality and resolution is determined by how the droplets are charged through an electrostatic field, and deflected using a magnetic plate or field. The charge on the droplet, the magnetic field strength both determine where the droplet lands on the substrate, or if not charge is applied, the droplet will be directed to reflow through the reservoir conserving and recycling the unused ink. One of the unique benefits of CIJ printing is the fact that the droplets are pumped through the system at speeds often ten times faster than other methods which enables the substrate to be printed upon from a further distance away. Finally, the benefit of using a solvent based ink, allows for compatibility with a wider range of substrates including those with high surface energies.
The main drawback of CIJ is that most people consider this technology to be bad for the environment due to solvent being driven off in the curing process. While, this may be true, often times the alternative method of printing these labels or specialty signage have a far worse impact on the environment.
In the above image, pressure (red arrow) is applied to the heating plate (orange outline) in order to force the droplet of ink out through the print head nozzles.
DOD inkjet is by far the most common and broadest classification of inkjet printing. This is most commonly used by companies such as Hewlett-Packard, Fujifilm, Lexmark, and Canon. As the large brand names suggest, these are most commonly used in consumer products such as desktop printers, but in the recent history has been proving to be useful in industrial printing as well. DOD print heads such as, thermal, piezo, and electrostatic use large pulses of pressure, with the differentiating factor being how the pressure is generated. Many companies may consider MEMS to be a unique method of drop-on-demand printing, but in the end it is still a subcategory based upon piezo or thermal architecture.
DOD droplets can be formed by rapidly heating a resistive element in a small enclosed chamber containing the ink. In this scenario the heated element is increased to 350°C to 400°C, which will cause a thin film of ink above the heater to vaporize. The vaporization causes a bubble to create a pressure pulse that forces a droplet of the ink through the print heads nozzle. Upon relaxation, the small chamber is then backfilled with more of the ink which will then be jetted through the nozzle onto the substrate as well.
The greatest advantages of DOD inkjet are the ability to create extremely fine droplet sizes which translate to resolutions observed greater than 1000p. Additionally, the high nozzle density due to the creation of droplets via pressure allows for more compact devices, lower print head prices, and reduced production costs. The main disadvantage is most commonly stated to be the limited inks available for vaporization. Each of the components in an inkjet droplet must be able to withstand high temperatures during vaporization without being adversely impacted upon solidification on the printed surface.