Inkjet; A Flexible Printing Strategy for High Quality Print, Unique Device 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 are using it, the integration into different industrial processes is a recent development.
The printing process sounds simple enough, a pattern of inkjet droplets 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)
But the implementation and operation of the 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.
The image above shows another sample of faulty inkjet printing (pixelation of letters)
This is our second article related to inkjet printing technology in an industrial environment. In the previous article, the components that make up an inkjet printing system are discussed as well as the challenges that shaped and plastic products offer for the printing process. Also, the impact of the ink selection on the printing process is reviewed in the previous article.
In this article we will first review the challenges of integrating an inkjet printing process in an industrial automated system. The first paragraph looks at the implementation of full circumferential (360°) print, additional requirements for curing (pinning) and difficult to print shaped products. The paragraph after that takes a deeper look at the challenges for integration into an industrial process, such as production speed, purging and cleaning of the inkjet system and covering need to ensure a reliable print process.
Interesting Product Challenges for Inkjet
When a circumferential print (print over 360°) is needed or when multiple colors are required, challenges for the integration of an automated inkjet system are introduced. We will look at these two aspects first and then at the major points for consideration when integrating this into an automated handling and assembly system.
Full Circumference (360°) Print
When the full circumference (360° around the product) is to be printed, the product may need to be rotated during the printing process. This introduces challenges for product handling and accurate positioning of the product (especially with multiple colors). With each color (CMYK) applied separately, the product must be aligned correctly to each individual printhead.
Image shows an item with a full circumference print
Pinning Units and Additional handling with multiple colors
Since inkjet inks have a low viscosity (comparable to water), the use of multiple colors introduces additional complexity. The inks can easily mix causing a “smeared” or “dripped” image. For this reason, each color is individually hardened (pinned). After each print step, the color applied is “pinned”. Pinning is similar to curing (full hardening) but normally uses low energy UV curing process (to avoid UV light entering the inkjet head).
This image shows an example of faulty inkjet printing due to incorrect timing
Apart from having to integrate the pinning units in the automated system, additional product handling may be required to apply the full circumferential (360°) print. Also, shielding of the UV light coming from the pinning unit is required to ensure no UV light enters the inkjet head (see also paragraph protection and covering).
The image below shows a flexible manufacturing system shuttle moving under an inkjet head. A cleaning tray can be seen just below the inkjet head.
This image shows an example of a pinning unit (left side) integrated in a hi-tech industrial inkjet printer
Inkjet Challenges with Shaped Products
Although inkjet printing is a contact free method of printing, it still requires a certain distance to the product. This is normally between 2 – 5mm. A larger distance does not allow the ink drops to be positioned correctly. Especially with shaped products, this may pose a challenge.
For some medical devices e.g., a syringe with a flange, the full area cannot be printed due to the shape limitations of the selected inkjet head (the flange may hit the inkjet head). The image below shows a syringe being printed where the inkjet head is positioned away from the flange to avoid a collision.
Rendered image of inkjet printing on a medical device (syringe)
Product handling becomes more complex when shaped products are to be presented to the inkjet system. The inkjet head needs to be aligned with the (complex) product due to the shape of thew product or position (start) of the print.
Inkjet System in Automated Assembly and Printed Systems
As listed in the previous paragraph, shaped products, especially requiring multiple colors, can be an engineering challenge. These challenges have a deep impact on machine design and operation. Some challenges that need to be considered for an automated and integrated inkjet solution are discussed in the next paragraphs.
Production Speed, Rotational and “On The Fly Printing”
When products are produced at high speed, there may not be enough time (in a single step) to apply the full print. There may simply not be enough time to rotate the product to create a complete print (full circumferential print).
This has a profound impact on the machine design where the product flow may be split into multiple lanes with additional handling units (adding engineering and operational complexity). This then provides the time to rotate all the products but requires a wide range of inkjet heads (for each individual lane).
Or an alternative print method is introduced where the print is applied in various steps (e.g., stitching;). The print is placed in different steps where only a part of the print is applied during each step (also called “on the fly printing”). Instead of a full rotation, the product is rotated slightly between the inkjet heads to present the next area which needs printing. Correct orientation of the product between each print step is very important.
Rendered image of a print in an “on the fly” movement, part of the full print is applied and the product is rotated to print the next step
Purging and Cleaning
For the inkjet head to operate correctly over a long period of time, it needs to be cleaned on a frequent basis. Part of this process can be automated where a small amount of ink (also referred to as purging) is pushed through the inkjet head. This avoids nozzle blockage and keeps the head clean. The ink released during the purging process needs to be collected to avoid spillage in the system (or undesired print on the system which may cause contamination).
The image below shows a flexible manufacturing system shuttle moving under na inkjet head. A cleaning tray can be seen just below the inkjet head.
This image shows a flexible manufacturing system shuttle moving under an inkjet head
To automate this process, a movable tray can be used which can be positioned under the inkjet head at dedicated intervals (this movement of the tray needs to be synchronized with the inkjet heads, printing machine and the process parameters).
The ink which is used during the purging process cleans the inkjet head and is subsequently collected in this tray. The printing system can run autonomously for an extended period of time with little or no support required from operators.
Protection & Covering
An inkjet head is a sensitive piece of equipment that must be protected to avoid damage. The head must be aligned with the products to be printed ensuring correct and high-quality images. The inkjet head should also be protected that, in case of a product misalignment or misposition, the inkjet head does not get damaged (and stops functioning or needs to be replaced).
Protective covering of inkjet head
Covering of the inkjet heads is also advised when UV curing inks are used. The pinning and curing station may be located close to the inkjet head but UV light should not enter the inkjet head. With correct protective covering in place a long life operation of the inkjet head is better secured.
These requirements add additional complexity to the machine design and operation but need correct implementation to provide stable inkjet operation over a longer period of time. Incorrect implementation may result in a poorly functioning inkjet printing process leading to missing, smeared or blurred images and in some cases to malfunctioning of the inkjet system altogether.
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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