Archive for : March, 2016

Printing on Textured Surfaces

Pad Printing on Textured Surfaces

Textures vary according to their individual depth, the degree of draft on the sidewalls, and the frequency of peaks and valleys for a given surface area. Some textures are more difficult to print successfully than others, and there are certain textures that are simply impossible to cover completely.

The following diagram illustrates the three variables mentioned above. Dimension A represents the depth of the texture, B represents the draft angle of the sidewalls, and C represents the frequency. (Typically, the deeper the texture, the higher the angle of the sidewall to allow the part to release from the mold.)

textureB&W

The Experiment

In an attempt to find a correlation between these variables and printability, a series of sixty-odd black ‘visual texture standard’ plaques, provided by Mold-Tech, was printed as a test. The plaques were molded out of ABS (Acrylonitile-Butadiene-Styrene). An automotive-approved, two component, white ink-thinned 15 percent by weight; a steel cliché with an etch depth of .001”; and a 60 durometer (Shore scale A) transfer pad were used. Each plaque was single-printed at one end and double-printed on the other, then allowed to dry per the ink manufacturer’s recommendation.

Each single- and double-printed image was visually inspected for coverage under a uniform, non-directional (unfocused) light source at a distance of 18” for approximately ten seconds (per Ford Motor Company visual inspection procedure for automotive interior parts). Acceptance or non-acceptance was determined by the presence of any visible defect or void resulting from insufficient coverage of the texture.

The results indicate that the frequency of the texture plays a significantly larger role in achieving an acceptable print than the depth of the texture or the angle of the sidewalls. One texture having a depth of .0055” and 8 degrees draft was successfully single- and double-printed, whereas another texture that was only .0015” with 2.5 degrees draft couldn’t be printed successfully at all. The difference was the number of peaks and valleys in the texture within the image area. The obvious conclusion: the higher the frequency, the lower the likelihood of acceptance.

The data from this experiment only covers a limited number of known textures in a sea of millions. Unless you’re considering printing a part that came from a mold textured by Mold-Tech, it doesn’t help much. So how can you determine if your texture is printable? Print it yourself.

Determining Printability

In experimenting, it is important to use a machine, cliché, ink formulation, transfer pad, and cycle time that accurately represent what you plan to do in production. For example, a machine that generates more compression than the machine you intend to use for production may compress the pad further into the texture, resulting in an acceptable print that your production machine can’t recreate. The type of cliché, the size and depth of the image also need to be as real as possible, as does the ink formulation (i.e., ink : hardener : thinner ratio). Finally, use the same pad shape and durometer as will be used in production to conduct the test. Set up the test machine to run with production settings for speed and compression. When starting out, set the compression on the cliché and the substrate at the minimum amount necessary to pick up and transfer the image.

If you can’t get an acceptable print using the process you prefer, there are several variables to change. The easiest thing is to try a harder pad. Contrary to what logic would dictate, a harder pad penetrates the texture further than a soft pad does before the ink releases.

If a harder pad doesn’t do the trick, try slowing the speed with which the pad compresses on the substrate. This can make the displacement of the air in the valleys of the texture more efficient, leaving less potential pinhole-causing air under the ink film. Allowing the pad to dwell on the surface of the substrate for a few seconds may achieve the same results. Some machines, such as those with a programmable stepper motor, can be programmed to do this. Other machines require that the pad be over-compressed on the part, resulting in the machine actually stalling out. This can result in image distortion or worse yet, undue wear and tear on the machine, the pad, and the part being printed. In the event that the pad doesn’t work regardless of the durometer being used or how it is compressed, perhaps you need to experiment with one having a different angle – the steeper the angle, the better.

On finely grained textures, if you fail to cover the texture with a single pass, the chances of covering it with a second pass aren’t very good. This is because the thickness of the ink layer only makes the voids (or valleys) that much deeper than they were initially. When this occurs, revert to ‘bridging’ the texture, rather than continue trying to fill it in.

 Bridging Textures

Bridging textures can be achieved by changing how the ink film releases from the pad. If you are lucky, this can be achieved by simply using less thinner, or by slowing down the machine so as to allow more solvents to evaporate from the ink film while it is on the pad. This increases the tackiness of the ink, making it leave the pad in favor of the substrate sooner. When the ink releases sooner, it adheres more to the peaks and less down the sidewalls and into the valleys of the texture. In the event that you can’t sacrifice the speed, you can try directing some low velocity airflow to the surface of the pad in between image pick up and transfer. The increased airflow accelerates evaporation of the solvents. If you’re double printing, you also may wish to direct some air at the surface of the part to dry the first hit a little before the second pass.

Bridging textures can result in the dried ink film having less mechanical resistance, especially when the texture is a deep one. Since the ink is really only adhering to the peaks of the texture, there are tiny voids under the ink film in between. Ink that becomes brittle can fracture at these points more easily. Therefore it is important to keep the end use of the part being printed in mind when you’re deciding to bridge or not to bridge.

Finally, some textures are just plain impossible to cover completely. In these cases, it is necessary for the texture to be modified to make it printable. This typically isn’t up to the decorator; it is up to the people manufacturing the parts. If you’ve conducted some of the simple experiments outlined in this article, you’ll be better able to communicate the reasons why the texture in question isn’t printable, and your findings will undoubtedly be an asset in determining what the texture should be.

Email or call John Kaverman for a ‘Texture Test Chart’ showing the results of more than 65 texture tests determining printability given normal, average operating parameters.

padprintpro@gmail.com

1-517-467-5340

Options and Accessories for Productive Pad Printing

Pad printing is an effective method of decorating parts of all kinds; from ad-specialty items such as pens and golf balls to industrial products like faceplates and propane tanks. Numerous add-ons and custom options are available to make this already productive process even more efficient. Some are designed to enhance parts handling, while others address potential bottlenecks in the printing workflow. Let’s take a look some of these beneficial technologies.

Multiple colors

Most manufacturers offer standard vertical machines in one through four or six-color configurations. Multicolor open systems can have multiple cliches, or one cliche in a split inkwell. Independently adjustable cliches are more desirable than having all of the colors on a single cliche. The benefit of independent cliches is simply the ability to more easily absorb potential image-to-image and/or image-to-part-location problems.

Multicolor closed systems can also have single or multiple cliches. A few manufacturers offer small (60-mm) multicolor ink cups, commonly referred to as split ink cups, that allow you to print two or three colors. The limitation is that the colors must be side by side, unless your machine can pick up once and then stroke the print two or three times. In that instance, you can shuttle the part to print colors on top of one another. The other limitation is that split ink cups are difficult to manufacture, and thus are expensive.

 

indexer

             Indexers

The four types of indexers are pneumatic, electronic, stepper-motor driven, and manual. Pneumatic indexers are less expensive and more popular than the other automated shuttle systems. Pneumatic indexers can have multiple positions by means of either multiple cylinders or a single, more expensive cylinder equipped with magnetic brakes. Be very careful to avoid any fluctuation to air pressure when working with parts of a critical nature on pneumatic indexers.

Electronic indexers have programmable, servo-driven motors. These indexers are more expensive than their pneumatic counterparts and are somewhat hard to find. Stepper-motor-driven shuttles can be programmed to travel a desired number of steps between prints. Stepper-motor and electronic indexers will typically last longer than pneumatic indexers. Manual indexers are used for low-volume jobs where the registration of colors is not critical.

 

 

 rotary table     Rotary tables

Rotary tables for parts placement can also be electronic, stepper-motor driven, or pneumatic. Accuracy and price are determining factors. Electronic and stepper-motor-driven rotary tables are more expensive than pneumatic ones, but they can move weight more accurately. Rotary tables can be of just about any reasonable diameter, allowing anywhere from two to 12 or more fixtures to be attached.

Some manufacturers have modular rotary systems with up to four independently adjustable machine-mounting stations. Depending on the application, one, two, three, or all four machines can be used simultaneously, turned off individually, and even rotated 180º on their mounts to operate alone. This allows more than one job to run at the same time.

 

Hot-air dryers

Hot-air dryers are common on racetrack conveyors and rotary tables. Even though most pad-printing inks dry to the touch within a few minutes, most people prefer to have the additional drying equipment, especially if they are using two-component ink or printing multiple colors with a lot of coverage, or at high speeds.

 

racetrack conveyer

Racetrack conveyors

Racetrack conveyors are standard equipment on many multicolor machines. Racetracks are pneumatically driven in most cases and can have several nests or fixtures. If necessary, cams can be attached to allow the nests to rotate in between stations for printing multiple sides on a given part. These systems are rarely used outside of the pad-printing industry.

 

 

Walking beams

Walking beams are a mechanical means of moving parts from one print station to another. Walking beams are limited in that they usually require that the part be picked up, moved over, and located against a stop of some sort for each printing operation. In most multicolor operations, it is not recommended that the part be moved in this fashion.

 

over-under conveyorOver-under conveyors

Over-under conveyors are usually chain driven or precision-link mechanically driven. In most cases, parts simply fall off the nests into a container at the end of the line, or are transferred to separate conveyors for subsequent operations.

A few manufacturers use the bottom of under-over conveyors for secondary operations. One in particular has under-over conveyors that present the part at a 30º angle after printing, rather than upside-down. This allows for easier access for post-print operations such as assembly.

 

Air blasts

Automated closed-cup machines can have thinner-metering systems added to continuously add thinner to the ink cup. These systems simply drip a predetermined amount of solvent into the top of the ink cup at regular intervals. Unless the ink cup has some feature that allows the thinner to be mixed into the ink, the thinner just sits on top of the ink, having very little effect on viscosity.

 

pad shuttles

          Pad shuttles

Pad shuttles can be a less-expensive alternative to the purchase of a larger machine in some applications. Using a split pad, a pad shuttle can, in some cases, print an image that would otherwise exceed the cliche or ink cup’s maximum image area. For example, let’s say you have two images of the same color that must print with their respective centerlines being 45 mm apart. When etched separated by this dimension, the two images don’t fit within your machine’s maximum image area. If these two images fit when separated by, for example, 30 mm, then you could etch them that way, pick them up with two pads butted together, and then shuttle the two pads apart to achieve the desired 45-mm separation prior to image transfer.

Pad shuttles can also be used to print two images on two different sides of the same part when both images, etched side by side, are picked up at the same time by two separate pads. After image pick up, the pads move along the X-axis to position the first print. After the first print, the nest rotates while the pad shuttles into position for the second print. In order for this to work, your machine needs to be able to pick up once and print twice, and you must have both a pad shuttle and a rotating nesting fixture. You can also print two colors this way if you have a two-color ink well on a one-color machine.

 

Nesting fixtures

All too often nesting fixtures are an afterthought in tooling up for a job. In pad printing, it is necessary for the part to be properly supported, especially at critical stress points and areas where the forces of pad compression are greatest. The nests also need to be ergonomically correct and where several nests are necessary, within a few thousandths of an inch of being exactly the same. In some applications, the type of pad to be used must be taken into consideration when designing the nests.

In the interest of saving time and money, it is sometimes tempting to go to the tool maker down the street to have your nests built. That is fine, provided that your tool maker has experience dealing with pad printing. On several occasions, I have seen quality problems result from a lack of pad-printing-process knowledge in the design of nests, especially on automated systems.

Pad-printing-equipment manufacturers should know how to design and build nesting fixtures correctly. Let them build your nesting fixtures, at least until you become proficient enough to be able to educate the tool maker you use.

Of course, not all parts are critical enough in nature to warrant having someone build your nests. Some parts require little more than double-sided tape or a lump of modeling clay as a nest. A lot of people make their own nests for one-up printing jobs by using automotive body filler. You can find or build a container, mix the filler, pour it into the container, then place your part in at the angle you want. (Spray the part first with a light lubricant so you can get it out of the filler after it cures.) After 30 minutes or so, the filler is cured hard enough for uneven surfaces, tight corners, and rough edges to be sanded away for a reasonable, consistent fit.

Safety guards

Operator safety is everyone’s concern. Some pad printers don’t require any safety equipment, while others require complex enclosures or light curtains. Most automatic machines come with standard safety guards or shields that are effective and don’t interfere with the efficient operation of the printer.

The potential for productivity

If you can develop a way to hold a part steady and transport it, chances are good that the product is a suitable candidate for pad printing. Consider the types of jobs you currently accept, think about the types of projects you’d like to handle, and involve the equipment manufacturer in these processes. In the end, you’ll come to appreciate that pad printing is both a precise and flexible method of decorating.

About the author

John Kaverman is national sales manager for Innovative Marking Systems, Lowell, MA. He holds a degree in printing technology from Ferris State University and has more than 20 years of combined experience in the pad-printing and screen-printing industries. Kaverman welcomes comments and questions and can be reached by e-mail at john@padprinters.com.

Editor’s note: This article is an excerpt from John Kaverman’s Pad Printing Technical Guidebook. You can purchase the book by visiting the ST Online Bookstore at http://www.stmediagroup.com/st-bin/quikstore.cgi
or by calling 513-421-2050.