Effects on a Material
Perhaps this is something you currently struggle with in your flexographic printing operation—certain jobs don’t tend to wind into a high-quality finished roll, while others do just fine. Comparing the printed designs and/or ink formulations for the jobs that process well versus the ones that don’t might be where the answer lies.
When certain inks are deposited onto a flexible web, a small increase in overall thickness results where the printed regions exist. If the design is such that there are significant areas of printed and unprinted material, this imperceptible increase in thickness in the printed regions is magnified once multiple layers are stacked on top of one another as the winding roll increases in diameter. These areas can appear as bands (often referred to as gauge bands) across the face of the roll where the printed areas exist. The application of web tension and lay-on nip loading during the winding function can promote permanent deformation in these localized regions once wound. These damaged areas then show up as bagginess in the web once the roll is unwound into the next manufacturing process and can wreak havoc in a slitter/rewinder.
Careful consideration must be given to the layout of the printed areas to help minimize this effect, but caliper variation will always be present to some level of severity. It is important to review your printing designs and sample materials with your machine supplier to make sure the machine is properly prepared with the features necessary to process these materials.
Coating properties can also impact the design requirements of a slitter/rewinder. Coatings with low coefficients of friction (COF) require special web handling and winding solutions to ensure successful processing. For example, at high operating speeds, lay-on rollers are required for low COF materials to reduce the amount of air that becomes trapped between the wound layers, in order to produce a more tightly wound roll.
Depending on the level of caliper variation, individual lay-on rollers are very often necessary to provide the proper levels of nip load to each individual slit package. The combination of the amount of print coverage and the COF of the substrate are two critical parameters that play into the level of sophistication, and overall cost, of the lay-on roller and web handling systems required on the machine.
Factoring the Challenges of Short Runs & Setup Changes into a Slitter/Rewinder
Many just-in-time (JIT) manufacturing operations are faced with short runs and resulting setup changes, which can be overcome with the correct equipment. This begins with looking at the type of unwind for the slitter/rewinder.
To maximize uptime and minimize waste, efficiently loading new rolls and having an accurate means of splicing the start of a new master roll to the tail of an expiring roll is a must. This is especially true when attempting to align the printed web with the slitting tooling to ensure accurate cuts. A slight shift of the incoming web can result in slit edges out of alignment with the printed image, leading to scrap material. A splice table offers assistance to the operator in maintaining the proper alignment between the expiring roll and the new material. It can also assist the operator in preparing a smooth, low-profile splice that has a better chance of making it through the slitting section without disturbing the knife setup or causing a web break.
There are several unwind styles to choose from, but most fall into two basic categories: shafted or shaftless. As a rule, shafted unwinds are less expensive but may require additional shaft handling equipment depending on shaft weight. They can also have lower roll weight capacity due to limitations in the shaft’s ability to carry the load safely. Alternatively, shaftless unwinds eliminate the need to handle shafts and can have telescoping support arms that adjust to various roll widths. Designs that feature lift-from-the-floor operation can conveniently acquire a parent roll from a roll cart or pallet, or simply from the floor, eliminating the need to handle shafts and possibly saving time.
The next focus should be on the frequency of slit width setup changes and lay-on roller size changes required. Slitter sections can be supplied in various levels of automation, from none at all to fast and highly accurate fully automatic systems. Once again, knowing your machine throughput requirements and understanding how much time is required for a setup change using each type of slitting system will help you determine their returns on investment. Some of these systems come with a high initial investment cost but the long-term productivity savings can be significant, and the setup accuracy and repeatability is superior to a completely manual system.
The same holds true for the various lay-on roller systems available. Different lay-on roller lengths and coverings may be required to cover the complete range of finished packages being processed on your machine. Having a convenient way of changing out these rollers and getting the machine back in production without producing a lot of scrap rolls should be everyone’s goal.
Substrate Sensitivity to Scratching or Wrinkling
The last thing a flexographer wants is a machine that damages the printed and/or coated material that was just processed. Roller coverings and construction must be carefully considered to prevent such damage. This is particularly important at higher operating speeds where roller inertia can hinder the web’s ability to accelerate or decelerate the rollers, causing relative motion between the web and the roller surfaces. This relative motion can lead to scratches or web tracking errors that can ruin a perfectly good web very quickly and sometimes without immediate notice. Once again, careful attention to detail can ensure a slitting operation with minimal material waste.
In summary, flexographers can benefit by addressing these discussion points with their equipment suppliers prior to making a final decision. A slitter/rewinder should not merely be chosen based on its width, diameter or speed capability. Use these guidelines to narrow your search to equipment that will not only process your current materials, but also has the added flexibility and durability to serve your needs for years to come.
The buyer must also have a solid understanding of the economics involved to make the right choice on how much automation or additional tooling to consider, with a return on investment that meets internal guidelines and offers the flexibility to convert new products in the future.
About the Author: Joe Connelly is a mechanical engineering graduate from The University of Rhode Island and has been involved in automated assembly and web handling equipment design and technical sales support for more than 33 years. He is currently product manager – winding and slitting for Parkinson Technologies, where he has worked for more than 22 years in various capacities, including equipment design, engineering management and applications engineering management. Joe can be reached at 401-762-2100, ext. 309, or [email protected].