Shrink Packaging Machinery
Shrink Tunnel

A shrink tunnel is used to shrink films that have been sealed around a particular product. Oriented film retains a thermal memory. The maximum amount of shrink which can be achieved is directly related to the degree of stretch which is induced during orientation. All oriented films shrink at temperatures near their melting points. Precise tunnel temperature, air velocity, and conveyor speed control are necessary to achieve optimum package appearance
without scorching.

Polyolefin films require both heat and air velocity for shrinkage. Additionally, the direction and velocity of air in a shrink tunnel must be controlled to shrink polyolefin films effectively.

Because there are many variables related to both product shape and material, and film characteristics, it becomes important to understand the shrink tunnel controls that affect those variables:

• Package Size
• Type of Film
• Package Color and Texture
• Ambient Conditions
• Tunnel Efficiency
• Conveyor Speed
• Tunnel Temperature
• Air velocity
• Production Speed (packages per minute)

Adjustable air temperature controllers provide heat energy that is necessary to influence the film's thermal memory (shrinkage). Too little heat will not induce enough shrinkage. Too much heat will cause film burn-through. Directly related to the cost of a machine (quality) are the range of thermostatic temperature controllers. More expensive tunnels have more consistent thermostats and controllers that produce consistent and predictable packaging results.

Adjustable air flow vents control the flow of the warm air both above and below a product, as well as the sides, where product temperature, materials, size, color and shape create potential shrinkage problems. Air flow patterns help to correct potential shrinkage errors.Air velocity controls adjust the volume of warm air (energy) that makes contact with the film. Air turbulence may allow the temperature to be reduced so that burn-through can be reduced. Also, it may eliminate unsightly dog ears that may exist on larger packages.

Air velocity may be controlled by adjusting the dampers for each blower or by adding more blowers to a particular tunnel. Air velocity varies from tunnel to tunnel as does price and quality. Adjustable conveyor speeds not only control the speed at which a product completes shrinkage in a shrink tunnel, but also affects the degree of film shrinkage by controlling the length of time the film is exposed to the warm air (energy).

A variety of tunnel conveyors are available for shrink tunnels. Dead rollers are utilized for polyethylene films that rely more on heat for shrinkage than air velocity. Solid belt conveyors do not allow significant air flow beneath the product and are often used for PVC and polyethylene applications. Some manufacturers provide heated belt conveyors for polyolefin films. Live rollers are most often used for polyolefin films that require significant air turbulence. Mesh conveyors (teflon or stainless steel) are also utilized by some manufacturers and are optimal for light, unstable or thin products that may slip between rollers.

The optimal shrink tunnel for a product should consider the packaging quality required, the tunnel opening size (width and height), chamber size (internal dimensions), conveyor length, production speeds desired and the type of film to be used.

Return to Packaging ABC