The roll coming from film extrusion, and from printing where needed, is still a long strip of film. The cutting and sealing step turns this strip into individual bags. The bag’s length, closed edges, handle shape and mouth design all take shape at this step.
The basic principle is simple: the film is sealed to itself with heat and pressure, then cut to size. But which edge is sealed, how the handle is opened and how the bags are stacked vary with the type of bag being produced.
- Film roll Tubular or flat film is fed into the machine from the unwinder at a balanced tension.
- Sealing bar The heated bar presses the film and seals the layers to one another.
- Cutting blade The sealed film is cut at the set bag length.
- Draw rollers In each cycle the film is drawn forward by one bag length.
- Stack Finished bags are counted and collected on top of one another.
Feeding the machine and tension
The cutting and sealing machine unwinds the roll and draws the film forward step by step. In each cycle the film advances by one bag length, stops, is sealed and is cut. Because this intermittent motion constantly strains the film, the machines have dancer rollers and brakes that balance the tension.
If tension is too high, the thin film can stretch just after sealing, before it has cooled, and the bag length changes. If tension is too low, the film creases as it goes under the sealing bar and the seam wrinkles. A constant film width along the roll and evenly wound edges are also very important at this step; defects from the previous step show up here.
How is a seal formed?
Sealing relies on polyethylene softening with heat and fusing under pressure. The heated sealing bar presses two layers of film for a short time; when the bar lifts, the cooling layers have joined into a single piece. To reduce sticking to the bar surface, most machines use heat-resistant coatings or tapes.
Three variables are adjusted together for a sound seal seam: temperature, dwell time and pressure. If the temperature is low or the time short, the layers do not fully fuse and the seam separates easily. If the temperature is too high, the film thins and wrinkles around the seam or tears right next to the seal line.
The surface of the sealing bar also affects the result. A flat, wide bar leaves a wider seam, while a thin, hot wire or blade seals and cuts at the same time. The right setting depends on the film’s thickness and raw material. LLDPE- and LDPE-rich films generally seal over a wider temperature range, while for thin HDPE-rich films the setting window can be narrower. So when the raw material or thickness changes, the seal setting is checked again too.
Bottom seal and side seal
Most bags are closed by one of two basic methods. In bottom sealing, the tubular film is closed with a single seam perpendicular to the machine direction; the seam forms the bottom of the bag, while the sides are the tube’s own folds. T-shirt bags, garbage bags and many gusseted bags are produced this way. Since there is no seam on the side edges, the load rests on the bottom seam.
Side sealing, on the other hand, uses film that has been opened or folded along one edge. The sealing bar comes down between two bags; in a single stroke it closes the right edge of one bag and the left edge of the next while also making the cut. In this method the bottom of the bag is the fold line. Side sealing is common for flat bags, zip-lock bags and some courier bags.
Which method is chosen depends on the load the bag will carry, the mouth and handle design and the position of the print on the bag. On printed film, the cutting point is aligned with the design by sensors that read print marks on the film.
Handle cutting, punching and extra operations
After sealing and cutting, extra operations follow depending on the bag type. For T-shirt bags, stacks of folded bags are cut with a punch to create both the handle openings and the mouth opening. On reinforced bags, an extra layer of film can be sealed to the mouth edge, and the grip is opened by punching.
- Perforation: On garbage bags supplied on a roll, a row of holes is made next to the seal line so that the bags tear off easily.
- Zipper profile: On zip-lock bags, the interlocking profile is either extruded together with the film during film extrusion or produced separately and sealed onto the film during converting; in cutting and sealing the profile must be aligned correctly with the bag mouth.
- Adhesive strip: On courier bags, a closure strip is applied to the flap.
- Vent holes: On some packaging bags, small pinholes are made to let the air inside escape.
Common sealing problems
- Opening seam: The layers have not fully fused; temperature, time or pressure may have been insufficient.
- Tearing beside the seam: Excessive heat has thinned the film around the seam; under load, the bag separates exactly along this line.
- Wrinkled or wavy seam: Film tension is uneven or the sealing surface is not clean.
- Length variation: The bags do not come out the same length; the draw system, sensor setting or film stretch is checked.
Some of these problems stem from the film itself. On a film with uneven thickness, however good the machine setting, the seam can remain weak in places. That is why solving sealing problems also involves going back and looking at the film extrusion step.
Counting, stacking and packing
The cut bags are counted and stacked at the machine outlet. T-shirt bags are often gathered into packs; garbage bags are wound into rolls; other bags are packed in set quantities. Accurate counting is important for stock control by both the user and the manufacturer.
The packing method also takes into account how the bag will be used. Packs with hanging holes may be preferred for T-shirt bags pulled one by one behind the counter, neatly stacked packs for a business with automatic filling, and easy-open rolls for household use. Including information such as product size, quantity and production batch on the pack label makes later checking and tracing easier.
Edge trims and defective bags produced at this step are separated and sent for recovery. In the next step, the finished bags are checked to see whether they meet expectations for size, thickness and sealing.