Innovating Inkjet Technologies for Plastic Products
by Scott R. Sabreen, The Sabreen Group
and Dene Taylor, Ph.D., SPF-Inc
The three process parameters for high print quality are polymer substrate, inkjet printer and ink and pretreatment compatibility.
The demand for digital inkjet printing on 3D plastic products is increasing exponentially. Application challenges to achieving robust operations are the optimal ink chemistry-printhead design, compatibility between the ink and polymeric substrate, and curing. There are important process factors combining polymeric surface compatibility when extending inkjet into new opportunities.
Inkjet printing is far more complex and delicate than analog printing. Inkjet requires the nozzles to fire precisely sized drops with exact accuracy. High quality inkjet printing systems must simultaneously integrate printheads, fluids, electronic controllers, pretreatment and cure. All of these items must work together to produce the intended results. Most companies investing in inkjet technology desire to decorate multiple substrates. Since there are often substantial chemical and physical differences between plastics, even within the same polymer family, it becomes challenging to print on all materials. Thus, a technical printing paradox challenge exists which is illustrated picturing a peacefully resting feline under a stable three-legged stool.
There are three main input process parameters each represented by a leg; polymer substrate, inkjet printer and ink and pretreatment compatibility. All elements must be stable to achieve excellent print quality. Changing any one of the process legs has an unbalancing effect requiring modifications to the others. Printing on contoured plastic geometries by inkjet is further complicated as there are relatively few OEM printer/printhead choices. Each OEM limits certified inks developed for a limited range. Through understanding the intricacies of inkjet printing and surface science enables unprecedented capabilities and results.
UV-curable Inkjet Inks
Ultraviolet (UV)-curable inkjet ink has been widely adopted for printing 3D plastic products. UV-curable inks dry instantly, bond directly to a limited number of plastics, do not emit solvents and are available for some OEM platforms. Extending the range to "tough-to-bond" polymer substrates requires custom formulating expertise to meet the stringent requirements for decorating and printing. Formulators need to know the unique chemistries that provide fluids able to be micro-jetted, the curing mechanisms that ensure rapid hardening, as well as the physical and chemical properties of the resultant ink, so the job is more complicated than for any other ink type.
The greater part of any dry ink is the binder – it traps the color in place, protects it from abrasion and bonds to the surface. The bonder is typically a polymer. UV ink printing differs from other types because the polymer is formed during curing by chain reaction of monomers and oligomers. Monomers are low viscosity liquids so they also function as the liquid ink carrier, and eliminate the need for water or solvent – that is why UV cure inks are 100 percent solids and ideal solvent ink alternatives. Oligomers, larger reactive molecules, have multiple chemical functionalities, and are critical to properly building the binder.
Free-radical inkjet UV inks dominate the market due to their relatively low cost and the extensive availability of monomers, generally acrylates. These offer a range of polymers with desirable performance characteristics from durable abrasion resistance to flexibility. A drawback to free radical polymerization is its inherent sensitivity to oxygen in air, so short chain lengths are often short. Although the reactions may be very rapid cross-linking effectively ceases when the UV light is gone. Consequently formulations have high photo-initiator contents. While free radical inks account for the vast majority of UV-cure inkjet ink consumption, cationic UV-cure inks are emerging for sophisticated needs.
Pigments and Color Pallets
UV inks use only pigment colorants, but the range is broad and not a serious limitation. Formulating and ink-making developments enable manufacturers to offer white (W), which is especially useful as a base on dark colored substrates or as a background on clear. Inkjet primary colors cyan, magenta, yellow and black (CMYK) together offer a large color gamut because ink layers can be thick. However, being based on pigments they are at least partially opaque and they mask colors below. The intensity does not fully translate into the secondary colors, which excludes a number of Pantones colors popular with major brands. Analog printing uses specifically formulated spot colors. They could be available in this market, especially with 6, 8 and 10 print-head machines, but they are not popular because it is far more difficult to change out a spot color in an inkjet printer than in flexographic or screen printing. It also conflicts with the ability of digital to switch jobs with consecutive images. An alternative is to employ extended gamut ink sets with intense inks of hue intermediate between CM & Y. Red, green and blue (RGB), or orange, green and violet (OGV) are used in label and advertising printing to extend the gamut and cover most bright specified intense brand colors. These colors can also be custom formulated for 3D plastic products.
Additives are essential in all ink formulations. Inks for printing plastics commonly contain adhesion promoters. Jetting and droplet formation, obviously central to inkjet, require careful balances of viscosity modifiers and surfactants. If not correct the ink can mist, or wet out on the head, neither of which are satisfactory. These same compounds also control ink wetting and flow on the substrate – i.e., adhesion and dot gain. They are generally optimized for a particular surface chemistry, but there are limitations. For example, an ink that will wet a low surface energy substrate will leak from the printhead. For that reason, many surfaces must be treated to put them in the range of ink functionality.
Surface Pretreatments – Ink/Plastic Substrate Compatibility
Inkjet inks have low viscosity and low surface tension which creates adhesion bonding challenges on many polymeric substrates such as acetals, polyolefins and polyurethanes. These types of chemically inert plastics are hydrophobic and not naturally wettable. Consider a single liquid fluid droplet on a flat solid surface at rest (equilibrium). The angle formed by the solid surface and the tangent line to the upper surface at the end point is called the contact angle; it is the angle (x) between the tangent line at the contact point and the horizontal line of the solid surface. See Figure 1.
The speed of the printing press can also impact ink’s effective surface tension. An ink that statically measures 25 dynes of surface tension could behave dynamically like an ink with 40 dynes of surface tension on a high speed press. The actions of inkjet print heads and print systems on the fluids they dispense can significantly impact the way fluid components realign during dispensing and interaction with the print surface and other ink or coating layers. As the frequency at which inkjet print heads can eject drops increases to higher levels and as the speed of inkjet printers and presses increases with single pass full-width arrays, surface tension interactions of inks, coatings and substrates present additional challenges.
Digital UV inkjet is revolutionizing printing and decorating of 3D plastic products. Inkjet is a complex, multi-variable process. There are significant challenges for printing on tough-to-bond polymers which extend beyond OEM standard inks and printers. Low viscosity inks jetted on low surface energy plastics are chemically and physically incompatible and frequently require pretreatment to solve adhesion problems. While UV curing technology has significant benefits, improperly cured inks are hazardous. Understanding the intricacies of inkjet printing and material surface science enables custom solutions to meet the demands of new applications.