by Melissa Donovan
Direct digital printing to plastic is challenging, but that doesn’t make it impossible. Variables such as low surface energy, the variations of plastic, how it is manufactured, and environment it is placed in all play a role when it comes to ensuring ink initially adheres to the surface and remains there for the intended duration. Pretreatments—whether executed inline, near line, or offline of the printing process—represent one way to help enhance adhesion. Wipe-on primers and coatings are another option to consider.
Shown: IDS provides a range of digital inkjet solutions—both dedicated flatbed and rotary style for cylindrical printing—that are able to print to plastic.
Today’s ink sets are also advancing at a rapid rate. Their chemical configurations make it less necessary for outside help like pretreatments, but aren’t quite there yet.
Identifying Issues
When direct printing to plastics, the surface energy will make a difference. This is affected by the plastic manufacturing process. All plastics are not created equal.
“Plastics are ubiquitous, but that doesn’t mean they are all the same. Each material type and mold process can present digital printers with a very different surface to print on—things like surface energy, mold release chemicals, and other surface contaminants create adhesion and wetting issues for any print process, digital or analog,” shares Kenneth Stack, executive chairman, Engineered Printing Solutions (EPS).
Plastic versatility is an advantage and disadvantage. There are seven main types of plastic, says Jessica Makrinos, marketing manager, Inkcups, each with its own set of pros and cons, its own flexibility, and its own particular surface energy, which affects adhesion. “In addition to this, because plastic is used in so many different ways, each individual use case also warrants careful consideration before selecting pretreatment, inks, and equipment.”
“The key factor is ink compatibility with the surface, which is dictated largely by the surface energy of the plastic. Some plastics, like polypropylene (PP) have very low surface energy, which translates to low adhesion. Others, such as ABS, have substantially higher surface energy, which leads to exceptional adhesion. Dyne testing is a common way to get an idea of how ink may adhere to a particular surface, even before printing,” says Lon Riley, founder/CEO, The DPI Laboratory.
James Bullington, CTO, LSINC Corporation, notes that other olefins besides PP like polyethylene (PE) and high-density PE (HDPE) also have low surface energy, which makes them incompatible with inks without pretreatments or primers. On the other hand, “co-polyesters can print like a dream with fantastic adhesion, but one has to watch the static and its impact on ink mist. Styrenes and expanded polystyrenes are highly susceptible to static generation that must be mitigated.”
“Static can attract dust or distort the ink spray path—especially in inkjet printing, degrading print quality,” cautions Bas Buser, global market segment manager printing applications, Plasmatreat GmbH.
A plastic’s manufacturing process may present challenges. “HDPE, PP, or PET plastics for example, depending on method of manufacture, may have mold release present in a range of ratio mixes that can adversely impact print adhesion. Parts in a common lot may even have more or less mold release present depending on when they were produced in a run,” suggests Samantha Solomon-Crump, research and development (R&D) manager, Innovative Digital Systems (IDS).
“Different plastics, even within the same family, can vary significantly based on the supplier, colorant compounds, glass-fill/mineral fill, polymer resin melt flow, and processing aids. Many companies printing on plastics do not have a definitive material specification and procure based on best pricing,” add Erik Kiel, president, 3DT LLC and Scott Sabreen, president/owner, The Sabreen Group.
Plastic is also able to take on many shapes. “Often I am presented with a plastic object that requires use of specific print settings in regard to controlling speed, direction, and dot size to maximize the surface area of an odd shape that can be printed accurately,” says Emilio Rangel, UV product manager, Mutoh America, Inc.
Also, consider thermal sensitivity. “Many printing processes—especially digital and thermal transfer—involve heat. Plastics can deform or melt under high temperatures, limiting the choice of printing methods,” explains Buser.
“Add in issues like expansion and contraction under temperature shifts as well as chemical resistance and it creates challenges for both adhesion and long-term durability. Some products choose these difficult-to-print plastics because of their specific ability to resist staining or marking,” notes Hugo Gonzalez, senior segment specialist, Mimaki USA, Inc.
Combat Challenges
There are a number of ways to remedy the issue of poor surface adhesion—among other problems that may arise.
The solution to adhesion problems can occur one of two ways—from within or outside the printer system, recommends Gonzalez. “From within, ink chemistry can be improved and more compatible curing systems built like low-energy curing lamps. Hardware can also include precision ink drop placement, size, and shape that may optimize the way the ink is cured.”
Mimaki takes a multi-layered approach. The UJF Series offers a variety of ink types along with jetted primer that is printed only in the required print area reducing waste and expanding the types of printable material types. Mimaki also employs low-energy UV LED lamps that reduce the possibility of warping temperature-sensitive materials.
From outside of the printer system, pretreatment equipment is introduced to remedy adhesion problems. Plasmatreat is an international leader in the development and manufacture of atmospheric pressure plasma systems for the pretreatment of substrate surfaces.
“The system we recommend and suggest depends on a number of process parameters like production speed, size of the treatment area, product shape, and chemical composition of the product,” shares Buser.
Makrinos says Inkcups constantly works to develop inks and equipment in response to challenges. “Sometimes, that means innovations in pretreatment, sometimes it sparks ideas for equipment updates, and sometimes it leads to the development of new inks, but it always entails keeping the end use of our customers’ products in mind.”
When EPS sells a system, it needs to ensure the customer can successfully print on the complete range of materials that led to its purchase. “We take this responsibility very seriously and offer our customers access to many different forms of surface pretreatment inline with our printers. Both surface pretreatment and curing are critical to the print process for UV inkjet,” explains Stack.
Noting that adhesion is challenging to plastic when working with its water-based printers, O&PM Europa BV developed its own set of primers in response. “With more than 20 recipes we guarantee adhesion on any surface. If none of these work, we can adapt the recipe to the properties of the substrate,” explains Dursun Acun, sales director, O&PM.
While product advancements continue to address issues, Riley admits that education is the most important factor. “The DPI Laboratory designs its systems and compatible inks to adhere to a wide range of surfaces, but we cannot plan for every application. Educating customers about ink adhesion considerations during the buying phase arms them with the information they need to make the right decisions and create long-lasting prints on their products,” he says.
Similarly, Kiel and Sabreen believe “education is important for companies in order to make the best purchase decisions not only for the types of pretreatment, but for long-term project success. Specific to inkjet printing machines and prospective procurement, companies too often do not conduct adequate testing of samples only to learn that certain inks and UV LED pinning lamps do not produce sufficient power/energy density to achieve full cure. Radiometers are important to determine proper UV dosage. Undercure and overcure conditions result in adhesion/abrasion failure. Inkjet printers require exact setup and maintenance.”
Testing makes all the difference, agrees Rangel. “I keep a variety of adhesion promoters to test on plastics. There are quite a few good choices that usually do the trick. Mutoh also prides itself in its R&D teams’ ability to maximize adhesion properties out of the box so to speak.”
“R&D plays a role in taking on a new plastic product line in your catalog,” admits Solomon-Crump.
Pretreatment Recommendations
Pretreatment is a common solution to ensure proper ink adhesion to a plastic surface, indirectly it also improves long-term durability. Prevalent options include corona discharge, plasma treatment, wet chemical (solvents and primers), and flame treatment. They may be placed inline, near line, or offline from the printer; or applied manually.
According to Kiel and Sabreen, gas phase “glow discharge” plasma surface pretreatments are generally necessary to achieve acceptable ink adhesion and abrasion resistance. These surface pretreatments increase surface energy and improve the wetting and adhesive properties of polymer materials. Chemical primers are sometimes used instead of gas plasma processes.
Kiel and Sabreen say the industry uses a variety of gas-phase surface oxidation pretreatment processes, including low-pressure, cold-gas plasma; electrical or corona discharge; flame plasma; and low-temperature, voltage-free atmospheric plasma.
“Each method is application specific and possesses unique advantages and potential limitations. Each of these processes is characterized by its ability to generate gas plasma– an extremely reactive gas consisting of free electrons, positive ions, and other chemical species. In the science of physics, the mechanisms in which these plasmas are generated are different but their effects on surface wettability are similar,” explains Kiel and Sabreen.
3DT offers ten different surface pretreatment systems that include corona and plasma pretreatments. Each system is designed for adaptability and can be configured to meet the exact requirements of unique applications. The engineering team at 3DT collaborates with customers through every stage of system development, including comprehensive application development, dyne testing, design, engineering, as well as system integration.
Its new HeliDyne corona treatment is designed to produce strong bonding on wide, thick materials for printing, gluing, coating, and laminating applications. The system is engineered with numerous pin electrodes, which generate a broad corona discharge that increases surface tension on challenging materials without the heat damage typical of conventional treatment.
Additionally, unlike most systems, HeliDyne handles materials up to 200 inches wide and two inches thick in a single pass, according to 3DT. It can be integrated inline or paired with a conveyor.
Plasma pretreatment systems can integrate within the inkjet printer as well, note Kiel and Sabreen. 3DT also offers these. “When pretreatment heads are mounted onto flatbed systems there are weight considerations for the balance of the reciprocating printhead. For single-pass inkjet printing, there are other design considerations. Pretreating rigid parts is much different than pretreating thin, flexible banners. For example, polyolefins are often selected for both rigid and thin material products, however the amount of heat applied is an important factor that determines which of our plasma technologies is best.”
As a manufacturer of plasma treatment systems, Plasmatreat advises inline placement of its treatment, but ultimately it depends on several process parameters, admits Buser. Choosing inline assures that between the plasma treatment and printing process there is no room for new contamination of the plastic surface. However, the size of a product or technical limitations might make offline treatment the best possible choice.
LSINC accommodates jettable primers in its printers. “To maintain production rates, place portions of the pretreatment upstream of the printing process. Any process that raises the temperature of the media must be far enough upstream to ensure that it is at the proper temperature to prevent impacts to the image quality,” advises Bullington.
Propane flame treatment is often used at IDS to combat plastic adhesion challenges. “Propane burns off releasing agents and other physical contaminants purifying the surface while also opening the ‘pores’ of the material prepping it for printing. This is what is referred to as ‘surface activation,’” explains Solomon-Crump.
Typically, IDS’ auxiliary PyroBond or PyroTrack flame treatment systems are recommended. “These systems provide the capability to propane flame and/or Pyrosil flame materials as an offline solution. The PyroBond system is a four-station rotary system used for drinkware while the PyroTrack conveys flat goods such as coolers. Both of these systems would be placed near line to create a work station in conjunction with the printing process,” recommends Solomon-Crump.
If choosing to work with corona discharge, plasma treatment, wet chemical (solvents and primers), or flame treatment systems, the surface energy of the plastic is changed, “by breaking bonds on the chemical level. By doing this, it gives the ink anchor points to adhere with the plastics. These effects are generally temporary, so printing needs to occur soon after application or treatment. These systems can be placed inline with the printer to improve production and increase the effectiveness of the treatment,” says Gonzalez.
Another avenue is wipe-on adhesion promoters or UV LED curable jettable primers that allow for registered-to-print surface treatment, notes Solomon-Crump.
Usually a simple cleaning with alcohol, then a wipe down application of the promoter that provides optimal adhesion characteristics is suggested when working with Mutoh printers, comments Rangel.
The level and type of pretreatment depends on the plastic as well as the intended life of the product. “For instance, printing onto a frisbee or a cooler demands very different adhesion standards to a squeezable sports bottle or a Tritan picnic cup,” shares Makrinos.
“The pretreatment solution of choice is dictated by the substrate to be printed, the machine of imprint, and even the print environment,” concurs Solomon-Crump.
Stack says EPS customers use everything from corona treatment to plasma treatment to flame. “It really depends on the materials and configuration of parts, the type of surface we are dealing with, and most importantly the customer’s end user performance requirements.”
Riley cautions that some surface preparations can be aggressive, so testing is suggested. “We first recommend testing to ensure any pretreatments affect adhesion sufficiently, but do not negatively affect the appearance of the end product. Most treatments can be applied in manual or automated fashion, our recommendations rely largely upon our clients’ volume and processes.”
Ink Negates Pretreat
Ink advancements change the way we print. It is possible that in the future, such ink chemistries will make pretreatment prior to printing to plastic unnecessary.
Currently, Solomon-Crump says ink chemistries formulated for very specific plastic applications can negate the use/need for a pretreatment. “If an ink manufacturer is given a specific product to which they need to adhere ink on, or meet a specific regulatory requirement, it provides brackets for the expectations of the ink formulation. For example, an HDPE squeeze bottle may require a different flexible and low-migration ink solution with certain pretreatment requirements that varies greatly from the requirements of a rigid PET cooler or ABS automotive good. So pretreatments may or may not be needed depending on the substrate of choice, its post-imprint functional use, and the adhesion requirements to the end user. This directly translates to the formulation of the ink itself.”
Numerous advances in inks have improved adhesion, UV in particular, notes Stack. “Higher viscosity inks allow formulators to improve ink films and add adhesion promotors that before were limited by lower viscosity. While improved photoinitiators improve curing.”
“In general, I feel there are industry wide ink improvements, but there is such a range of plastic variability that pretreatment processes are a constant necessity. I feel they will continue to be a necessity because printers generally want to be able to print to a range of materials and it is unlikely that one ink will encompass a large range of dyne levels,” believes Gonzalez.
Kiel and Sabreen say the simple answer is no—ink chemistries will not negate the use of pretreatment. “While more types of inks are offered, printing on plastics requires a wettable surface and oxidative chemical functionality. Too often, companies limit their focus on surface wetting and not the polar functional groups introduced onto the surface.”
Successful Adhesion
Achieving successful adhesion to plastic means understanding the type of plastic being utilized as well as the application’s end use.
Nov2025, Industrial Print Magazine
