Films that wrapped a product are finally shrunk by the shrink tunnel. During the manufacture process, films will be oriented and retains a thermal memory. Stretch force was induced within the orientation. This stretch force dominates the maximum amount of shrink of the film. Shrink temperature is usually near the film melting points. The air velocity, conveyor speed and precise tunnel temperature are important to achieve optimum package appearance.
In order to shrink Polyolefin (POF) films, heat and air velocity are very important. Furthermore, direction and velocity of hot air flow is essential to shrink the POF films effectively. Therefore, a good quality shrink tunnel should have the following:
1. Air velocity and direction are adjustable
2. Controllable, Precise and stable temperature inside the tunnel
3. Adjustable conveyor speed
Beside the heat, air flow and conveyor, there are many variables that affect the package appearance. Therefore understand how the shrink tunnel control these variables is important. Those variables are:
1. Production Speed (PPM)
2. Tunnel Efficiency
3. Product Texture and Size
4. Shrink Film Type
Tunnel temperature controllers play an important role in shrinking process. Heat provides energy to influence the film to shrink. Too little heat will not induce enough shrinkage and too much heat will burn-through the film. The range of the thermostatic temperature controllers is directly related to the cost of tunnel. A consistence thermostats and controllers, factors that increase the tunnel price, will provide consistent and predictable packaging results.
Product temperature, shape, color and material will create potential problems during the shrinkage. A high quality of shrink tunnel has an air flow vent controller inside the tunnel. The air flow patterns could help to correct the potential problems. Air current will reduce the burn-through and dog ears by reducing the temperature of the hot air.
Conveyor speed not only controls the speed of the complete shrinkage in a tunnel, but also affects the degree of film shrinkage by controlling the length of time the film is exposed to the warm air.
A variety of tunnels conveyors are available for different shrink films. Dead rollers are suitable for Polyethylene (PE) films because PE film rely more on heat for shrinkage than air velocity. For PVC and PE applications, that are not dependent on air velocity for shrinkage, solid belt conveyors is suitable since it does not allow significant air flow beneath the belt. Live roller is most suitable for polyolefin films (POF) that require significant air current. Sometimes mesh conveyors made from Teflon or stainless steel are also employed since they are suitable for light or thin products that may slip between rollers.
To select a suitable shrink tunnel for a product. The production speed, shrinkage quality, chamber dimensions, tunnel opening size and the type of film to be used should be consider.
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