The seal pressure is critical during the sealing cycle. It can be controlled by three common methods:
1. Manual L-sealers with no magnetic hold down
Seal pressure is applied by the operator who pushes down the sealing bar until the sealing cycle complete. A buzzer light is installed to indicate a completed cycle. The disadvantage of this method is the seal pressure may too weak and uneven due to man-made mistake. Sometimes operators will release the seal bar before complete the sealing cycle and the seal pressure will inconsistence due to different operators use the machine.
2. Manual L-sealers with magnetic hold down
When the operator brings the seal bar down, magnetic force will hole the seal bar against the sealing pad. The seal bar will rise up when electronically released by the machine timer. Under this magnetic clamping system, the dwell time is accurately controlled. A uniform pressure during dwell is achieved by this magnetic clamping system although different operators control the machine.
3. Air-actuated automatic machines
In this method, air pressure is employed to hold the seal bar against the pad at a proper pressure. Sometimes a pneumatic arm is used to press the bar against the pad, in some cases, the sealing bar is press down by electronic magnets.
An even seal pad is important for the machine to apply proper seal pressure. Seal pad become uneven may due to excessive use or poorly designed equipment. A flat seal pad is necessary in order to achieve proper pressure. Furthermore, the seal bar alignment is also need to be even for apply proper pressure.
Center-folded film will be able to utilize on shrink wrap machine by passing an adjustable inverted film forming plow. Semi-automatic and intermittent automatic L-sealer and automatic side seal equipment usually employed this system.
The seal height position is adjustable by changing the take-away conveyor height or the seal bar height. A centered seal height is important to achieve a properly shrinking of the film. Proper seal height is at the center of the product. Seal height adjustable wrapping machines have height flexibility for a range of products. E-shrink L-type vertical Automatic Sealing Machines series are able to adjust the seal height position.
Film clamps is a device that hold the film in position during the sealing cycle.
A power take-away conveyor is normally after the seal bar and automatically carries the product to shrink tunnel after sealing is complete.
A film separating rods is used to separate the center-folded film. Winding of the film may cause residual static that holds the central-film together. This rod will improve film operability and machinability.
Perforation of the films (air holes on the film) is important too. When the product is surrounded by the shrink films, some air will remaining inside the package. Once it is exposed to hot air, the air inside the package will start to expand and causes the film to “balloon” around the product. Air venting which allow air escape from the package is necessary to ensure proper shrinkage. Too much residual air, unreleased due to not enough air hole, could cause the seals burst or may leave air trapped within the package. When the air cools down or escape from the film, the film will appear a relaxed or loose appearance like an incompletely shrink. In the other cases, when there are too many air evacuation holes or poor seals occur, air escape too rapidly may have ineffective shrink. Some wrinkles will appear on the film.
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