Convection heating eliminates common shrink-wrapping defects – AND much rework


In today's competitive market, where perfect work done "just in time" is the expected norm, the cost for rework, setup time, downtime, materials, and downright ugly product coming out of a shrink wrap systems is starting to get attention. Based on research with hundreds of users, we found that many problems centered around appearance defects such as dog ears, wrinkles, fish eyes, burn-outs, and hazing, caused largely by variations in tunnel temperature and air turbulence.

Tunnel can create or cure a multitude of problems

After several years of studying shrink wrapping technology and correlating users' observations, a common denominator emerged: shrinking the film uniformly is key to the process. In fact, a uniform shrink job can make even a poorly wrapped pack look good, but even a well-wrapped and sealed pack will come out looking sloppy after a poor shrink job. "Uniform" shrink generally means that the entire film bubble reaches the same temperature at about the same time, without the bubble being "blown" into contact with the package before it has a chance to shrink naturally.

Most users find it very difficult to achieve this with a forced-air tunnel. Forced air creates an unstable environment in the tunnel, then relies on the operator's ability to determine settings that produce a good pack. With its heat source outside the tunnel chamber, there is an inevitable time delay (beyond operator control) between when the element cycles on/off and when the effect is felt in the chamber. So, the temperature inside constantly varies ±7-8°F around the setpoint, even in a well "tuned" tunnel. Variations of as much as ±15°F are common, and during these peaks and valleys, bad packs are often produced. Air currents in a tunnel can also cause a film bubble to contact the package prematurely, freezing the film and causing dog ears and wrinkles. Dog ear is caused by non-uniform shrinking – as the center of the bubble starts to shrink first and faster, the dog ear areas lag behind and thicken so quickly that they never shrink completely.

Convection cooking is perfect for food – why not shrink wrap?

If the objective for a shrink tunnel is uniform "cooking," then what is considered best-technology in the food-preparation industry? That's the thinking that led to the development of convection heating for shrink tunnels. Convection cooking was developed because it subjects the product to a perfectly uniform, constant temperature, top to bottom in the oven, from entrance to exit. No hot spots, no cold spots, no air blasts to blow cheese off the pizza.

Shrink convection tunnels are shorter and work at a lower operating temperatures than conventional designs. In practice, convection produces ideal results on packages of any shape that can be passed through the chamber, even poorly wrapped packages.

Convection tunnels can even do things that are impossible with forced-air designs. For example, a package can actually be re-shrunk, if it does not quite shrink completely on a first pass. This is simply not possible with a forced air tunnel, because once the film has been pushed against the pack by the air stream, it remains frozen there. Convection tunnels are also so uniform in temperature that they can even shrink a pack if the film bubble develops a hole. In a forced-air tunnel, if a hole develops in the film bubble during the shrink process, whatever the position the film was in at that time is how it will finish shrinking.

The convection tunnel starts with a chamber shape designed to promote uniform heat and air circulation throughout. There are no inlets or discharge ports for air. Heating elements are inside the chamber, exposed beneath the transport belt, with low-speed propeller fans below the elements. Thus, the packs are exposed to some direct radiant heat as well as the hot air circulating in the chamber. The fans gently circulate hot air throughout the chamber to transfer heat efficiently and evenly to the film bubble, while never "pushing" on the bubble. Air circulation is the minimum required, ensuring quiet operation, too.

Heating elements inside the tunnel allow very quick response to temperature change. The convection tunnel holds its setpoint temperature within ±2°F, and can be operated around 280°F for polyolefin film, which is about 70°F less than a typical forced-air tunnel would use for the same film.

Setup for a convection tunnel is intuitive. The operator sets the conveyor speed slightly faster than the wrapper speed, then determines the optimum temperature by observing the behavior of the film bubble through a unique ViewWindow – just set a temperature that causes the film bubble to peak and start shrinking as the pack passes over the riser bar.

Custom results with commodity films

New machinery technology IS the answer for shrink packagers, not custom films. Users can enjoy the cost advantage of using commodity films, while still achieving efficiency and quality that, until recently, were unattainable or required special films.

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