Rethinking Machine Direction Sealing

Rethinking Machine Direction Sealing

Nearly all products in the flexible packaging relm employ a seal, many of which include a machine directional (MD) seal. The seal, which is an integreal part of the package, plays a vital role in the reliability of the package as a whole. Yet today, despite advances in die technology, processing and new resin blends, the majority of machine directional sealing is done with old technology which is often unreliable at best.

In fact because of this, many film converters are hesitant to even design a package with a MD seal and thus limit the flexibility of their existing extrusion lines or converting equipment. Beyond gaining flexibility in the product size range and added production of running two up,processors can improve the blow up ratio during extrusion which improves film properties – an added advantage of doubling the layflat size and then running two‐up production.

DR Joseph has an alternative to the traditional approach of machine direction sealing; a unit called the Seal‐Cut. The unit improves seal stength, relability and appearace. The Seal‐Cut is designed for applications where seal strength, appearance and reliability standards were not previously obtainable with traditional hot knife slit sealing.

The innovative approach of this unit reverses the traditional slit seal process by first sealing, and then cutting. Seal temperatures are drastically reduced with this design, which results in a clean seal with minimal gauge gain for perfect roll geometry. Seal temperatures can be lowered with the units design thanks to the much longer dwell period (roughly 11 inches).

The low seal temperatures are the key to this units improved performance. Often hot knife systems are run above the degradation temperature of polymers, which damages and embrittles the seal. A common misconception is that more heat provides a more complete seal in the process. The case study and chart to the right illustrates this fallicy.

By avoiding high temperatures which damage the polymer structure, the result is a stronger seal that you can trust meaning no more returns from customers due to failed seals.Because of the long seal dwell period, the effective seal temperature window is larger than a hot knife, meaning materials that are typically hard to seal, can now be reliably sealed, such as CaCO3 rich blends, PA and TPEs. Another benefit is the possibility to seal at high speeds (up to 1000 feet per minute), or seal thick films (up to 6 mil).

By avoiding the high temperatures, polymer does not “pool” at the seal edge and thus gauge gain is reduced, improving roll geometry and therefore avoiding roller damage and issues converting downstream.

With bow rollers built into the unit, the film web is guided through the optimal sealing process: the incoming bow roller eliminates wrinkles in the web to avoid sealing through folds or wrinkles in the web, and the outgoing bow roller is adjustable to optimize the amount of web separation after the sealing process to prevent the web from sealing back together. Adjustability is key as different thickness film and different web widths will require slightly different bow roller settings to achieve the best sealing conditions.

Having a reliable sealing solution designed for challenging applications can allow film producers to target new markets and new opportunities. It builds in flexibility to a blown film line allowing new width formats to be produced on larger lines, and also improve productivity when running 2 or 3‐up production.

A substantial amount of time, money and effort is put in to the products that we produce, from the latest die technology to new resin blends. The seals that literally hold together our product should not take backstage to any other aspect of the completed product. Rethinking the possibilities and improving seal quality can greatly improve an integral part of what we do.

Learn more at: https://www.drjosephinc.com/slit‐seal‐film‐sealing/