Mark Vruno

Can you help me build a quality process for producing polybags?
August 2, 2007

Sponsored by

Question: I am looking for a list of quality control points for a poly bagger. I have recently purchased this equipment and want to build a quality process for producing this product.

—Greg Fredwest

Answer: Most of us are very familiar with conventional finishing systems such adhesive binders, folders, cutters and stitchers. In determining both the quality of the machine and the end product, we have some well-defined bindery standards that we can rely on. But what about a polybagger? Well, here’s what you need to know:

Pick Your Machine Carefully
Many printers and mailers may confuse shrink-wrapping systems and polybagging machines. Shrink-wrapping machines are widely used both in packaging and printing environments, but they are designed to work with shrink packaging films such as polyolefins and PVCs, not polyethylene, which is what makes a polybag. They also lack the feeders used for magazines and catalogs.

Is This Really A Polybagger?
Polybaggers typically form the polybag by using the magazine, book or catalog itself as the forming agent. Polyethylene film is formed into an unsealed tube around the media itself. Film is overlapped on top of the media, and continuously sealed with a rotating hot wire or block. This is called the longitudinal seal. A “walking knife” then cuts and seals the film between each magazine, catalog or book. This is a heated stainless-steel or aluminum knife mechanism with a heating element inside. The knife opens and moves in synch with media which is encased in the film. The knife closes and seals between media at the ends. It moves with the media for a short time. This is the “dwell” which is necessary to complete the seal. The front product is also accelerated as the seal is being made. This forces the melted edges of the upper and lower film layers to “bead” together to make a strong seal.

Lightweight but Strong
It’s a complex process in which sealing temperatures must be controlled as machine goes from jog to run speed and back again. True polybagging systems are designed for this. The main sealer or “cross sealer” must be especially strong, BUT… light, as this mechanism is accelerated to great speeds. The mechanism for opening and closing this seal bar is usually mechanical (cammed), with pneumatic cylinders which act as buffers for the force of the stroke. Sealing temperatures will range from 200 to 400° C. Avoid designs which activate the seal bar entirely with pneumatic cylinders, these will be unreliable at higher speeds.

Who Makes ‘em?
The manufacturers polybagging machines are Sitma, Buhrs, CMC (represented in the U.S. by Böwe Bell + Howell) and Norpak. In addition, used and rebuilt machines are available. Most polywrapping machines consist of a collating—gathering section with either rotary gripper or shuttle feeders for the media, and the polywrapper itself. In addition, there will be modules that permit inkjet addressing or paper label application, either before or after the media is wrapped.

In addition, USPS will penalize polybagged media which has excess “selvage.” This is an excess of film at both ends of the package. If a machine cannot achieve no more than ½” at each end of the media, beware.

Don’t Forget the Film
This may be the most important part of the process. The overwhelming majority of polybags are made with Low Density Polyethylene Film (LDPE), which ensures good sealing qualities at moderate temperatures. The film is sold in roll form. A typical roll used for magazines will be 19.5” wide. Thickness is critical, with the average film thickness gauge running from 0.9 to 1.25 mm.

Test for QC
With poly being a petroleum-based product, prices have risen. Many printers will “down gauge” the thickness of the film to reduce cost. (Printers typically pay for the film used.) Thinner films may present sealing difficulties in both the longitudinal and cross seals, and may also limit cycling speed. A well-sealed polybag will have a thick “bead” running through the film overlap on the center seal, an even seal line on the cross seals. Testing is the only real way to quality control, and this is done simply by pulling on the seals to see at which point they give. The more force required, the better. Film used for mailing must be ‘automation approved’ by the USPS. In general, avoid films using recycled content; they may have uneven sealing qualities.

Posted by Mark Vruno on August 2, 2007 | Comments (1)