By John P. Kaverman, Pad Print Pros
For all of its flexibility as a decorating process, people often overlook pad printing as a potential solution for transferring functional materials. Examples of functional materials that I’ve worked with include conductive and dielectric materials, adhesives (and activators), radiopaque materials, lubricants and dyes.
In a standard pad printing cycle, rapid physical changes occur in the ink film due to solvent evaporation. Optimally, these changes result in 100% transfer efficiency from the pad. Although functional materials often don’t behave like pad printing inks, it is possible to obtain consistent results in transferring specific weights and/or film thicknesses with functional materials.
Determining whether a given functional material can be efficiently transferred with pad printing involves process development: experimenting with variables such as different types of clichés, line screen frequencies, functional material characteristics, transfer pads, machine parameters and pre- and post-treatment processes.
This article addresses some of those variables, starting with clichés.
Cliché plates
Pad printing provides solutions for transferring very fine (narrow) lines, and/or specific weight (milligrams or even micrograms) of functional materials onto (or into) complex geometries which cannot be processed (at least not easily) using other methods such as screen printing, spraying or needle dispensing.
These solutions begin with the clichés used in the pad printing process. There are a variety of cliché materials to choose from, with the five most popular including photopolymer, thin and thick steel, hard anodized aluminum, and ceramic (see cliché material chart)
steel cliché.
Steel clichés
When pad printing a conventional ink, one can expect the dried ink film to be within a range of a few percentages are of the etch depth. For example, a straight etched steel cliché with an etch depth of ~ 25 microns will typically yield a dried ink film of between 6 and 8 microns (single print). By varying the etching parameters (Baume’, temperature, dwell/etching time, degree of agitation) in processing steel clichés, the etch depth can be adjusted and, theoretically, the resulting functional material’s deposit film thickness or weight.
Photopolymer clichés
The same theory can be applied when using photopolymer clichés, except with photopolymers producers can vary the line screen frequency to accurately control the amount of material that is deposited. In photopolymer clichés, two exposures are typically required. The first exposure is done with the image film. This first exposure, when done correctly, polymerizes or “hardens” the entire non-image areas of the polymer coating.
The second exposure is then done with a line screen. This second exposure creates a consistent pattern of peaks and valleys in the image area. When pad printing conventional inks, use a 120 line/cm – 90% screen. This means that for every square centimeter, there are 120 tiny openings in the film for UV light to travel through. The total surface area that those 120 dots comprise within that square centimeter is 10%. The remaining 90% of the surface area is opaque, meaning that UV light does not pass through.
Laser-etched clichés
For laser-etched materials, laser parameters, such as power output, etching speed and resolution can be adjusted to produce specific image profiles. Whereas the processing of chemically etched and even photo-engraved clichés involve chemicals, laser engraving does not. Laser engraving also is faster than other methods for producing clichés. Finally, one big benefit of laser engraving is the ability to easily vary the depth of different areas of the image.
Functional material characteristics
Viscosity and evaporation rate
The viscosity and evaporation rate of functional materials are two variables which need to be evaluated in determining whether they are transferrable using the pad printing process.
Assuming the functional material stays in the ink cup and does not flow out every time the doctor ring’s edge clears the edge of the image area, viscosity isn’t an issue. I’ve successfully transferred materials that were similar in viscosity to water… but only because they were able to adhere to and release from the transfer pad as required within the cycle.
In most cases, the evaporation rate of the thinner used to adjust viscosity proves to be the most important variable. Materials with organic solvents (like water or alcohol) are much more difficult to transfer than materials having faster evaporating organic solvents. In some cases, organic solvents can be introduced to aid in increasing the transfer efficiency of a functional material, while in other cases it is necessary to direct forced air at the cliché and/or the transfer pad to accelerate evaporation/change the surface tension of the functional material.
Transfer pads
Pad size
Sizing the transfer pad to a specific application requires a sufficient mass of silicone to be present to avoid image distortion. As the pad compresses onto the cliché and part, the pad body acts as a support for the image area. If the surface is not adequately supported, it will deform/distort. I recommend starting with a pad that has a contact area at least 25-30% larger than the image area. Of course, producers must be cognizant of the maximum pad height and printing force generated by the pad printing equipment. It doesn’t do any good if the pad used in process development won’t fit in and/or cannot be efficiently compressed in the production equipment.
Pad shape/contact angle
The shape of the pad determines the angle with which the pad contacts the cliché and the part. It is through the “rolling action” of the pad, as it compresses outward from its center point (or ridge), that air is displaced, creating an intimate contact between the surface of the pad and the cliché during pick-up, and between the pad and the part during transfer.
The ideal contact angle between the pad and the part during transfer is between 20 and 50 degrees. Lower contact angles should be avoided, as they invariably trap air. Medium angles (20-30o) are best suited for bold images on flat, smooth or convex transfer surfaces. (30-50o) are best suited to more finely detailed images on textured and concave surfaces. When printing on a radius or sphere, the higher the angle, the better.
Hardness/durometer
As is true in transferring standard pad printing inks, softer transfer pads typically pick up and transfer functional materials more easily, and in greater quantity by weight, than harder pads. Once the pad’s size and shape has been specified, experimenting with varying durometers, in combination with varying cliché etch depths, often produces results that are observable and quantifiable.
Machine parameters
Pneumatics are notoriously difficult to validate due to the inherent inefficiencies of industrial air delivery systems and resulting problems with the consistency of the incoming air pressure and volume. For this reason alone, the importance of using 100% electric, stepper or servo-driven pad printing equipment cannot be overstressed. This is especially true in industries such as medical device manufacturing, where process validation is required and ongoing.
Modern, stepper or servo motor-driven equipment affords the ability to independently program (and therefore quantify) stroke speeds and delays in all axes of motion, as well as pad stroke distances. Pneumatic pad printing equipment simply doesn’t offer a commensurate level of control or validation.
Pre- and post-treatment processes
Functional materials don’t normally adhere to materials that pad printing inks won’t adhere to. The substrate material’s surface energy must be sufficient for the transfer pad to release the functional material, and for the functional material to adhere (or at least stay where it is transferred, as in the case of lubricants).
Technical information on functional materials can be difficult to obtain and, unless you’re a chemist, understand. Aside from signing a bunch of non-disclosure agreements, it is necessary to develop (and maintain) working relationships with functional material manufacturers as well as manufacturers of any required pre- and post-treatment processes or equipment. Communication and transparency are key in creating a sustainable solution.
Summary
Pad printing may be a viable solution when other processes have proven difficult, inefficient or impossible to validate. Hopefully, this article has provided some insight as to whether pad printing is worth investigating.
John Kaverman has over 35 years of combined screen and pad printing experience in capacities including production, supervision, engineering, sales and technical support. He is founder and president of Pad Print Pros, an independent consulting and sales firm specializing in pad printing. Learn more at www.padprintpros.com.
