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Multi-color Pad Printing with Automation
by Christopher Connell, ITW Trans Tech

Technology Feature

At some point, every pad printing operation must rise to the next level of difficulty by taking on jobs that demand technical expertise beyond the single-color, manual-load comfort zone. The risks are significant but the benefits to your customers can be substantial. Surely you would like to be considered for those high-volume, automated projects that come along– especially multi-color ones. But how can you conquer the technical pitfalls, be competitive with other methods, and still show a decent profit on your bottom line? Here are a few guidelines to consider.

Fast, Good, Cheap; Choose Any Two
Of course, choosing a fully automated system versus a semi-automatic, or even versus a manual one, all depends on your customer’s application and goals. If the volume is high enough and speed is critical, full automation is in order. With a production run in several millions of units within a few months time, a manual operation is out of the question. Feeding blank parts into a machine, pad printing each with one or more colors, and then outfeeding the finished parts would be impossible without at least some form of mechanical assistance – if those high production goals must be met. So the choice is driven by the numbers: total run times cost per unit equals production budget. And, don’t forget the time factor. You may find a way to produce the job with just as good quality and cheaper, but if you blow your deadline, the result won’t be acceptable to your customer.

When is a Manual-Load Perfect and When is it just too Slow?
Before moving forward, consider carefully whether you should stick with a manual operation or move up to an automated pad printing system. If the part is too large or irregular to be sorted and oriented by a feeder bowl or conveyor, or if a pick-and-place robot cannot easily grasp the part, manual is the preferred method, at least for the part-loading section.

Another consideration when pad printing larger parts that are continually lifted and pushed into a fixture is operator fatigue and repetitive physical stress. These ergonomic questions also may impact your decision to keep a manual load and unload situation versus seeking an automated alternative. Finally, the key factor is speed. If your production goal is beyond what can be humanly achieved under normal working conditions, it may be time to look into adding some form of mechanical device to boost that piece rate to a more profitable level.

Options for Feeding Parts into Automation
Vibratory feeder bowls are especially useful for basic orientation and sorting of bulk parts prior to dropping or inserting them into an infeed track. The constant vibration keeps the bulk parts in motion while gravity tends to orient them into a heads-or-tails position. A sensor eye detects any improperly positioned parts and signals an eject device to return them to the bowl. The remaining parts are pushed or inserted into the final feed track upon which they are advanced individually by an indexing device to the pad printing machine.

Turn to the Dial
Using a dial indexer (also called a rotary table) can boost production of either a manual or semi-automatic operation. With multiple positions, holding from four to twelve identical fixtures, the output can be increased dramatically. Once a part is loaded into the fixture, it is indexed to the next position where various operations can be performed prior to printing, such as pretreatment and ‘securement’, as well as post-print inspection, curing, and off-loading.

Dial indexers are the dependable workhorses of automated systems, due to their efficient circular arrangement of components and their highly repeatable accuracy. The end result is increased production within a compact area, while maintaining good registration on multi-color applications. By using an indexer in a semi-automatic system, your operator can gain a time edge by loading every fixture and running the machine continuously at top speed to achieve maximum output.

Programmable Linear Shuttles and Variations
Another auxiliary device that is popular for multi-color pad printing is the linear shuttle. The shuttle advances a single fixture along a straight-line path at exactly programmed intervals, receiving a separate image or color at each position. Up to seven colors have been successfully applied in sealed ink cup systems using this device, depending on the size of the part and the image requirements. The benefit of using a shuttle in this way is its ability to hold tight tolerances on multi-cup systems with a single fixture. The disadvantages are its relatively slow speed and manual load and unload.

A technical variation on the shuttle that can extend its capabilities is the addition of a rotating fixture. This rotation is done by incorporating an actuator that responds to a programmed command and, by turning the part in increments of 90 degrees, allows the print heads to apply an additional image or set of images. This is especially useful for producing custom golf ball decorations that demand tight, multi-color registration.

How Tolerant is too Tolerant?
For most applications, an acceptable tolerance for multi-color images is two thousandths of an inch - that’s 0.002”. Anything more than that allows the fine lines and halftone screen dots to blur. The result is just not a visually acceptable product. Any pad printing system worth its money should be able to hold two “thous” or better.

Hold It Right There
A word to the wise: don’t cut corners on that holding fixture - it may be the weakest link in your chain of quality. The worst place to save money is by buying second-rate tooling. If your fixture is too loose or too tight, it will directly impact your quality by producing inconsistent prints and increased wastage. Investing in the best materials and workmanship available will save you money in the long run.

High-Volume Conveyors
Two other reliable auxiliary devices that engineers often specify for multi-color pad printing applications are the racetrack conveyor and over-under conveyor. Both are preferred for long runs where tight registration and durability are required. The racetrack carries ten to twelve pallets, each with its own part-holding fixture, in an oval-shaped path. The operator manually loads each fixture opposite the printing station. As each pallet moves along the racetrack oval, it makes a 180-degree turn to face the printing machine, receives the image or images, and then proceeds around to return to the unloading station. Racetracks usually employ barrel cam drives to accomplish their positive-stop positioning, critical for low-tolerance, multi-color jobs.

A comparable but much larger device is the over-under conveyor with its single-row, vertical loop of pallets, with as many as 48 separate fixtures. This unit also uses barrel cams to advance and hold the pallets in exact position. Over-under conveyors are recommended for fully automatic systems that incorporate multiple printing units and other auxiliary equipment. Such pad printing machines may include infeed conveyors, pick-and-place robots, curing units, sorting devices, and vision inspection.

Four-color Process and More
Certainly, the most dramatic and vivid print jobs are the four-color images that appear on consumer items, giftware, and sporting goods. These show off the flashy graphics and colorful artwork that adorn every available product surface of mass merchandise. Pad printing has certain advantages in decorating those uniquely shaped products by applying images to three-dimensional surfaces with minimal distortion.

However, not all process color jobs can be directly printed on any shade of material. When the substrate is colored or transparent, it may require an undercoat or background of white to allow the semi-transparent cyan, magenta, and yellow inks to properly reflect their true colors. Thus, the job necessarily becomes a five-color project with white and black finishing the look.

There are far many more examples of multi-color jobs that are not process color. These may have several spot colors or even many layers of artwork. Frequently, the high-gloss finishes that are seen on electronics and cell phones are achieved by multiple layers of metallic inks with a final clear coat. Several custom-matched spot colors can also be applied at widely-spaced intervals by using multiple ink cups and special pad assemblies.

Another example of multi-color applications are large, single-color images that are broken into smaller parts. By carefully dissecting a large artwork into segments, each piece of the whole image can then be inked by a separate cup and, in turn, each part reassembled into the complete image.

Tips and Pitfalls
Multi-color pad printing comes with its own set of challenges and difficulties. The correct formulations of inks and thinners, always important in single-color jobs, become even more critical with two or more. When in doubt, read and follow the ink manufacturer’s instructions printed on the can.

Ambient air conditions also should be monitored and controlled to ensure consistency from start to finish. And don’t forget your pad blowers; they speed up multi-color jobs by helping ink transfer and release properly. By flashing off the solvents, pad blowers allow your operation to run at peak rate.

Cure is another area that requires careful consideration. With so much ink being transferred to a part, adequate curing and part handling are especially important. A good rule of thumb is to apply moderate heat for as long as possible without damaging the part. Though the ink on a recently printed part may feel dry to the touch, it isn’t truly cured for 48 hours unless forced heat is used.

The Added Value of Expertise
Before leaping into automated pad printing, first consider this choice: trial-and-error guesswork versus calling an expert. Buying a product without ongoing support is a simple business transaction; entering into a partnership is an interaction. The latter involves relying on trusted expertise compared to do-it-yourself learning. You could choose to make a one-time purchase compared to beginning a long-term relationship of confidence. The real question is: what is your time worth? Or to put it another way, what is it worth to avoid mistakes and wasted time?

Likewise, why invest your time and money in reinventing the wheel when there are experienced wheel designers, specialists, and inventors who have already researched and developed lots of perfectly good wheels (and pad printing machines) for companies just like yours? Go ahead; ask them for directions. They have ‘been there’ and ‘done that’. And ask to see their samples and videos. Inquire. Discover. Get a quote. Collaborate.

Christopher Connell is Marketing Coordinator for ITW Trans Tech in Carol Stream, Ill. Chris has been publishing ITW’s quarterly newsletter PADPRINT since 1990 and can be reached at (630) 752-4000.