The process and idea of punching seems simple, but it requires precision in the set-up.
Tooling tolerances and specifications are quite complicated, precise, and can be expensive. Therefore, the correct training and set-up of the punching units is critical to the life span of the tooling and punching units.
Only trained personnel should be in charge of the punch-changing process. Why? Incorrect set-up is expensive. Also, there are SAFETY factors to consider if punching units are handled and used incorrectly by an untrained operator.
The punch explained
A punch is comprised of a male and female die. The female die is the extraction side and can be a bar or sprocket hole. The male punch cuts through the substrate into the female die.
There are various types of punches; from rotary sprocket punches to punch bars with different shapes for different applications.
The punch units will have an extraction unit with a vacuum to get rid of the chad waste (fragments sometimes created when holes are made in a paper, card, or similar synthetic materials) to allow for clean and consistent punching. A die can also be used for punching.
Use compressed air to help eject the chads. Although this can be noisy and seems like a waste of air, things can get very messy if you don't have compressed-air extraction.
An air-eject cavity die is also a viable option since it can be re-sharpened and depending on your use, it can be cheaper than using a punch bar.
Punching can also be done offline for printed cups on a specially designed machine. An older flatbed letterpress machine can also be used, but this is more for sheetfed not narrow-web roll-to-roll presses.
There are other ways to punch your jobs whether they are on or offline. It's best to talk with the technical support staff at the company supplying your punch to decide what's best for your job.
Punch bar units
A punch bar differs from a sprocket punch as these will cut out shapes for different reasons, such as for an eye-mark to know when the next ticket is ready and print each ticket correctly. They’ll cut specified shapes out across the web, unlike a sprocket punch that will go along the web path.
A punch bar unit and punch bars are precisely engineered tools and are expensive. They should only be set up by a designated and trained operator or maintenance personnel.
If correctly installed, they can produce 25 million and more tickets. If the units and/or bars are damaged during set-up, they will need to be re-sharpened or even re-purchased.
If there’s no backup set, it can take six to eight weeks just to make your set and deliver them.
Punch sample: unit and punch bar set examples
Punching with dies
Using flexible or flatbed dies for rotary punch is another viable option.
This can be used for punching holes, shapes for sprocket use boarding passes, tickets, and any other required application. An air nozzle will be needed to help eject the chads, and if there is no extraction unit, things will get messy. This may seem like a cheaper process, but it is not without issues.
Kane Marsh is Regional Printing Instructor Asia Pacific with MPS Systems Asia, providing training on MPS' narrow web flexo presses and specialized label applications. Kane has direct print experience on multiple presses in a range of printing technologies including flexo, gravure, offset, screen and a variety of applications.