Inkjet; A Flexible Printing Strategy for High Quality Print, Unique Part Identification and Batch Customization
AAE (Grauel) pushes technical boundaries. We provide high tech printing & assembly solutions. We support smart printing and manufacturing equipment with Industry 4.0 technologies and solutions. This series of articles provide background information on how these developments are supported.
Inkjet printing is frequently used by consumers as it is relatively cheap, reliable and very flexible. Even though the technology is around since the 1950’s and so many customers using it, the integration into different industrial processes is just now becoming more common.
The printing process sounds simple enough, a pattern of inkjet droplet is positioned onto a substrate*1 without making contact. When different color droplets are used and combined, high-quality images can be generated.
*1: the base material that images will be printed onto (we will use the terms product and material in the article as well)
Smeared amber (incorrect application of white ink on plastic product)
But the implementation and operation of inkjet technology into industrial environments is very complex. It requires a profound understanding of the complete process to ensure it is integrated correctly and provides the operational quality required.
This article consists of two (2) parts. In this part we will review various aspects of the inkjet process. Two main types of inkjet technologies are available commonly known as continuous inkjet and Drop on Demand (DOD). This is explained in the first paragraph. The parts that make up an inkjet system (focus on DOD inkjet mainly) are dealt with after that. The difference between single pass and multiple pass inkjet is described in the following paragraph.
With so many aspects having an influence on print quality, an overview of the aspects that influence the printing process are listed in this paragraph. With a dedicated focus on printing on (shaped) plastic products, the typical aspects that need to be taken into consideration are dealt with in this paragraph. The importance of the correct ink selection (and the effect it has on the full printing process) is dealt with in the last paragraph.
The next article deals with the integration of the inkjet printing process into an industrial process.
Continuous (CIJ) vs Drop on Demand (DOD)
Industrial inkjet printing systems, and the industrial inkjet printheads can be categorized in two main categories being continuous inkjet (CIJ) and drop on demand (DOD).
Continuous vs DOD
The continuous inkjet technology (CIJ) ejects ink droplets continuously from the printhead (hence the name). The inkjet droplets are either deposited on the substrate or returned to be re-used.
The Drop On Demand (DOD) Inkjet technology only provides ink droplets when required. A pulse is created in the print head and a specific droplet is generated. The droplet is commonly generated by either a thermal or a piezo method.
Continuous inkjet printing (CIJ) is primarily used for coding and marking of products and packages. DOD is typically used where decoration is needed using multiple colors for high-quality images.
For reference purposes we include two examples of each inkjet printing type (using black as the print color).
DOD Inkjet executed by Grauel
The image above shows a single-color inkjet print using DOD technology. The image below shows a single-color continuous inkjet image. The continuous inkjet image is more “pixelated”. But the great benefits is that this type of inkjet images can be applied extremely fast.
Example of continuous inkjet technology executed on a cable
In the rest of the article, we will focus on DOD (Drop On Demand) Inkjet technology mainly.
Inkjet System (DOD) Overview Integration
An (DOD) inkjet system is made of various components that all need to work together. The configuration may vary a little depending on the process in which it is used but a general overview of an industrial application is shown below:
- Print head (single color)
- Header tank (supply ink to inkjet head)
- Head Interface Board (HIB, controls inkjet head pulses and electronics)
- Print Management Board (PMB, interfaced between production line and other electronics)
- Encoder (pulse train from production line, speed verification)
- Bulk Tank and overflow (main storage of inks)
- Pump and filter (provide ink to header tank(s))
- Computer (HMI and controls)
Overview of (DOD) Inkjet System
The main components are listed above which make up an (industrial) inkjet system.
The configuration shows a drop on demand, single pass inkjet system. Before we move on to the major factors that influence image quality, we will first look at the difference between multiple pass and single pass inkjet print.
Single Pass or Multiple Pass Inkjet Printing
As the name implies, a single pass print produces the print image in a single “pass”. In a multiple pass system, on the other hand, the substrate (product) is presented multiple times to the inkjet heads to obtain the complete image.
In order to provide a full range of colors, an inkjet system normally uses four (4) basic colors: Cyan, Magenta, Yellow and Key (black). Using these basic colors in different amounts and proportions used as dots on the substrate (product), different colors can be created. Sophisticated software (e.g., Print Management Board and software drivers) convert an electronic image into precise instructions that are sent to the print head.
The four basic colors are shown below for illustrative purposes.
Header Tank with CMYK colors (Key being black)
In a multiple pass print system, the full print can be obtained using a single row (as show above) inkjet heads. Large print on products can be made using just the four (4) basic color in a single position (single row of 4 inkjet heads). The benefit of multiple pass inkjet print is that it offers a more economical choice than single pass inkjet. But the disadvantage is that it requires more time than single pass print.
Automated industrial systems often handle complex product shapes. Having to present a (shaped) product multiple times to an inkjet head increases automation complexity. When running at high speed, the time needed for multiple pass inkjet print may not be available. This is one of the reasons why single pass inkjet print is often used in industrial processes running at high speed or handling complex shapes. The downside of single pass inkjet print is that multiple inkjet heads (at various positions) are needed to print larger areas.
We will review single pass print (and drop on demand) throughout this article.
Inkjet, how difficult it could be…
At a first glance, the components that make up an inkjet system are limited which should make it relatively easy to control the print process (also with consumer inkjet in mind).
But there are a lot of factors that influence print quality, especially when high quality, high print speed and alternative substrates (plastics) are introduced.
An overview of the factors having an influence on image quality is shown below. This has been taken from a presentation by Debby Thorp of GIS during the Inkjet Printing conference in Orlando, February 2015 (used by permission).
Different factors effecting image quality
The graph shown above can be a bit overwhelming, we will review the major aspects below. The presentation by Debbie Thorp is a great read and highly recommended. In this article, however, we will place the emphasis on the challenges that shaped plastics products provide when requiring high quality print in an industrial environment.
Printing on plastics
Most plastics have a poor tendency to bond to other materials because of their inherent inert chemical structure. Without any treatment prior to the printing process, the ink can easily be wiped off or “smears” on the substrate (product). For this reason, the substrate is often pre-treated to increase the tendency to bond to the ink.
The effect of pre-treatment can also be regarded as the “wettability.” The contact angle between the inks and substrate is important for the ink to adhere correctly (a smaller angle indicates better adherence).
Wettability (left image shows better wettability)
This “wettability” is also expressed as the surface tension or surface energy, measured in dynes/cm. This must be greater than the surface tension (energy) of the ink for correct adherence. A smearing effect can be observed in the image below which does not contribute to a high-quality print.
This image shows what happens in case of incorrect timing.
Plastics and Dyne levels
Polyethylene (PE), polypropylene (PP) and polythene (as an example) have a (natural) surface tension of 30 dynes/cm. This needs to be altered to be a minimum of 38 dynes/cm, preferably 42 or 44 dynes/cm (or higher) to allow ink to adhere correctly.
To verify the Dyne level of the substrate, test fluids are used. The test fluid is applied to the substrate and the operator observes if the fluid does not break up in droplets, normally in a 2 second period. When the ink holds its integrity (does not break up) the next higher-level fluid should be used to test. This is repeated until the highest value is found (see also ASTM standard-2578).
The correct Dyne level can be established through this test procedure. An image of test fluids is shown below (with the corresponding Dyne levels).
To achieve the correct Dyne level, various pre-treatment methods are available. The most common methods include Corona, Flame and Plasma treatment. The three types are shortly explained below:
Note: different pre-treatment methods such as primers and alcohol are also available but have dedicated uses and not discussed in this article.
Corona treatment is a surface modification technique that uses a low temperature corona discharge plasma to impart changes in the properties of a surface. Corona is generated by the application of high voltage to an electrode. Materials such as plastics, cloth, or paper may be passed through the corona plasma curtain to change the surface energy of the material.
Corona treatment being applied
Fuel gas is pre-mixed with air, and the resulting flame is directed at the surface to be treated. The resulting chemical process (in the gas) break the long-chain molecules in the substrate (plastic) and attach themselves to the break points, resulting in polar point charges on the surface.
Flame treatment being applied
Plasma is a gas that is electrified charged with freely moving electrons in both the negative and positive state. When plasma comes into contact with solid materials like plastic and metal, its energy acts on the surfaces and changes important properties, such as the surface energy.
Plasma treatment being applied
The suitability of a specific treatment type depends on various factors such as substrate (material), shape of the product, print speed, integration into the production process, safety requirements, etc.
Selection of the correct pre-treatment method is often done through testing and validation where the suitability of the treatment in combination with the printing process is verified.
The selection of a suitable ink for the inkjet system and the products is extremely important. Incorrect ink selection may have major consequences for the image quality, product integrity (ink contamination in medical devices) and even the complete design and operation of the automated system.
Image of missing ink and smeared ink on parts of the product
A lot of factors determine which ink is suitable for a specific process and substrate (product). For example, will the ink be used on a container that includes medicines or foodstuff and will be plastic barrier be thick enough to avoid contact? Or is the ink used on (medical) containers that need to be resistance to extreme temperatures or dedicated chemical post printing treatment (e.g., sterilization).
Incorrect selection of an ink may jeopardize the full project or even cause harm to human health. It may prove to be extremely difficult to change the ink once the inkjet print system configuration has been determined.
We will review some of the most important aspects to consider when selecting an ink for an automated inkjet print process.
Print Head Suitability
Because industrial inkjet printheads are precisely manufactured with ultra- fine nozzles, the composition of the ink must be matched carefully to the printhead. This is to ensure the ink can be jetted reliably and consistently.
Compatibility of the ink with the inkjet head includes verification of the viscosity, surface tension, drop formation etc. which all needs to be checked against the specification of the inkjet (head) manufacturer. This ensures correct jetting performance, avoids clogging and ensures system functionality.
Ink Droplet formation
The formation of the correct droplet shape is important in an inkjet system. Normally a pulse is generated which creates a (single) droplet that then travels down to the substrate (product). When the required droplet shape is not achieved (e.g., satellite forming, ink not released on time, etc.), the image quality and print head operation cannot be maintained.
Ink Delivery Systems and Software
The first parts that are in contact with or control the flow of ink are the (drive) electronics and ink reservoir(s). See paragraph “Inkjet System (DOD) Overview Integration” for all components.
Ink is pumped from the bulk reservoir to the header tank but when air or gas is included, the ink flow will be disturbed, leading to a low-quality image (or malfunctioning inkjet head). Incorrect settings of vacuum and pressure may also lead to a problematic ink flow. The ink flow itself must be constant for the inkjet head to operate correctly. The ink reservoir system performs a quiet but extremely important task.
Electronics and software are used to power the actual print process. This ranges from pulse train verification (production line speed or rotation), controlling the ink pattern (e.g., droplets, waveforms, etc.), compensation for product shapes and also purging (cleaning) times. These settings are part of the project and defined for each inkjet system.
Image of incorrect placement of (and even missing) ink in some areas leading to low quality image
There is a wide range of ink types available including aqueous, latex, eco-solvent, solvent, UV-curable and dye-sublimation inks. Each of these types can be used for specific substrates (material) or production processes.
To print on plastic products, a UV curing ink is often preferred as it allows for immediate hardening using UV lighting. Especially when running at high speed, this is a great advantage (see also next paragraph).
A solvent or water-based solution, on the other hand, is better suited where (direct) contact with food, medical, and pharmaceutical products is anticipated.
Whatever ink basis is used, the ink will need to go from a liquid state to a dry or solid state. Since most inkjet inks have a low viscosity, the chances of smearing increases when the ink is not (quickly) hardened. This is certain a point of attention for round or shaped products or products moving at high speed.
UV curing inks are commonly used for plastic products where the inks harden when the ink is exposed to UV light. Both mercury lamps and (LED) UV lamps are available for the UV curing process, but LED UV curing systems may require different ink as LED light operates at a narrow wavelength (which may not cure the full ink as this needs a broader wavelength). This depends on the ‘photoinitiators’ used in the ink).
This image shows a high speed Inkjet printer (running at > 550 parts per minute) with an integrated curing system.
A critical point of attention for inkjet systems is the shielding of the inkjet head. UV light entering the nozzles of the inkjet head may cause the ink to harden inside the head. This may seriously damage the inkjet head (even beyond repair). For industrial processes, protective shielding needs to be included in the design.
The integration of an automated inkjet printing process into an industrial environment requires more attention to ensure high quality print and reliable operation. See the next article for more information on this topic.
Post Print Conditions
Medical devices and laboratory consumables may be subjected to “unusual” conditions which adds additional requirements to the ink. Medical devices may undergo a sterilization process (gamma or steam) which may impact the adherence properties (steam in particular may cause the ink to come loose).
But also, chemical resistance (to alcohol and cleaning agents), which may be used in laboratory conditions, are important to consider when the ink is selected.
The ink used should be resistant to these post printing processes. When the printed parts are used in these conditions, they are often (severely) tested to ensure the ink remains on the products even in these unusual conditions.
These Cryo Vials are used at extremely low temperatures and need to be resistant to cleaning agents.
Regulation & Direct Food Contact
Another aspect to consider, especially when printing medical devices or food stuff, is regulation. This is no general guideline that can be followed such as an “FDA Approved Ink.” or “EU approved Ink”.
Image of Inkjet printing technology used on food items.
Switzerland is currently the only European country with specific legislation that regulates food packaging inks (but these requirements do not apply when printing directly onto food stuff).
In general, there are various government agencies around the world regulating food safety. Additionally, major food and medical device manufacturers have developed detailed requirements (a well-known one is the Nestlé Quality Requirements for Vendors of Raw and Packaging Materials). Contact your ink supplier when defining ink for your projects. Incorrect selection may jeopardize the full project or even cause patient harm.
Stay tuned for more, coming up next month! In the meantime, please consider following Grauel, a brand of AAE, on LinkedIn for weekly updates and extra content.
Inkjet; A Flexible Printing Strategy for High Quality Print, Unique Part Identification and Batch Customization, by Ivo Brouwer – Business Developer Production Automation at AAE b.v.
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