By Olivia Cahoon
Part 1 of 2
Adhesion promoters enhance ink’s bond when direct printing to metal objects. Applied by hand or with an automated system, primers and promoters enable a variety of benefits, including improved product durability and adhesion to difficult metal types.
Before considering an adhesion promoter for direct to metal printing, manufacturers should know that it entails additional steps and costs.
Grounds for Application
It’s common for print providers and manufacturers to use an adhesion promoter when direct printing to metal objects. Products that require a promoter or primer are generally handled often or use a specific metal type.
Typically, adhesion is the first concern for UV printing to any object, especially metal objects like drinkware. “Given that items such as metal cans and water bottles are handled quite often, it is important that the printed image is durable enough to withstand the constant abuse,” comments Chuck McGettrick, sales manager, digital inks, Marabu North America. Other reasons for requiring a primer depend on the manufacturing process’ oils or contaminants and the metal’s surface energy.
Metal types also dictate adhesion levels. Aluminum typically only requires cleaning due to a thin layer of aluminum oxide that helps facilitate adhesion. On the other hand, stainless steel has a smooth surface that lacks functional groups. Alex Folloso, manager metal decorating R&D, INX International Ink Co., says that for this reason, stainless steel requires some type of chemical pretreatment or method to roughen the surface. “Generally oxide and hydroxide groups on the metal surface are essential to react with functional groups of the coating resin.”
In addition to surface roughness, stainless steel oil levels dictate the adhesion and chemical resistance of coatings. According to Folloso, too high an oil level creates a low surface energy and causes the coating to de-wet, while a rougher surface enhances adhesion.
A common misconception for direct metal printing is that the object’s shape affects its printability. While the substrate’s shape provides challenges for certain decoration application methods, it does not have a bearing on the substrate’s printability in terms of adhesion, shares Craig Smith, president/CEO, Innovative Digital Systems.
According to Adam Tourville, account manager, inks, Avery Dennison Digital Ink Solutions, the one exception to using an adhesion promoter is if the metal is coated. If so, he says a primer is typically not needed, however this does depend on the coating type.
The Application Process
Adhesion promoters are applied to metal objects in a variety of ways and differ depending on the product. Application methods include wiping, spraying, roll coating, and jetting with the use of rags, spray bottles, and printheads.
Some adhesion promoters are applied manually by hand with a lint-free material while others are applied through an engineered inline process. For digital printing technologies, Smith says jettable primers are available, which allow for registered application. “Depending on artwork and machinery, primers either are printed in the same pass as CMYK plus white, or they may require an additional dedicated print, which increases the overall application time for a given product.”
The application process is generally immediate but also depends on the type of promoter/primer.
For example, solvent-based promoters applied by wiping or spraying evaporate almost immediately after application. On the other hand, flame treatments are exposed to the substrate very briefly to change the substrate’s surface area. “Both of these processes take a matter of seconds to complete and once applied the substrate is ready for printing,” comments McGettrick.
Time and Cost
While beneficial to the metal object’s finished appearance and durability, adhesion promoters still require additional steps and costs. However, the added time and price point does not detract from the benefits.
Typically, the advantage of better adhesion outweighs additional costs or time. “The additional step will be an extra cost because you are buying this consumable and it will extend the process slightly, but many operators take this into account,” explains Jessica Makrinos, marketing specialist, Inkcups.
Without an adhesion promoter, the imprint has insufficient adhesion. Additionally, most applications are done within the cycle time of an imprint, says Smith. The increased time comes from the additional print pass if a jettable primer is needed as a sublayer to the CMYK plus white print. “The added cost of time and consumables is something that each decorator must keep in consideration when evaluating the printing process,” he offers.
In short runs, the cost of this additional step is minimal when compared to the cost of other decoration methods. Additionally, Smith says the additional expense allows for a durable, multi-color imprint, which can be marketed and sold as added value to the end user.
Manufacturer Concerns
Before considering an adhesion promoter for direct to metal printing, manufacturers should be aware of testing methods, safety data sheets, and how added steps affect the production process.
Manufacturers should understand of the product’s end use and if the adhesion promoter complies with federal regulations and does not present health risks to the end user. “For example, a substrate that may come in contact with skin or something ingestible may need to pass certain tests before it is deemed safe for print,” offers Smith.
To avoid health-related challenges it is helpful to review safety data sheets prior to utilizing promoters. “These documents explain what the promoter is made of and if it aligns with the restricted safety list,” comments Makrinos.
After reviewing data sheets, manufacturers should properly test the adhesion promoter on the substrate to ensure it works for the customer. Tourville recommends manufacturers use ASTM methods for testing and to also simulate the environment that the substrate will be in, such as dishwashers, hot environments, wet environments, or salt water. “Every application is unique and therefore every precaution should be taken to make sure that the printed output is going to work for the end users.”
It’s also important to test the workflow for each substrate to ensure proper adhesion, as all substrates perform differently and require varying amounts of primer for proper adhesion, says Chase Pender, marketing manager, Supply55, Inc. “Spending time to dial in the workflow prior to running production will eliminate potential problems.”
Printing That Lasts
Adhesion promoters are essential for direct printing to metal objects. While some manufacturers are wary of the added steps and expenses associated with an adhesion promoter, these small issues don’t detract from the benefits.
Part two of this series provides a selection of adhesion promoters for metal objects in the industrial space.
Click here to read part two of this exclusive online series, Direct to Metal.
Apr2019, Industrial Print Magazine