• Pre-stretch is the process that allows stretch film to be stretched before (pre) being applied to a load.
Why Prestretch ?
• STRENGTH
As you stretch the film , the tensile strength of the film increases so it is significantly (2X) stronger once you pass the point of permanent deformation.
• FILM MEMORY
The memory of the film wants to return it to its initial form . However the more you prestretch the lower the memory . (which is why Bi-stretch did not work)
If the film is applied to a load with no film force, minutes later you can observe that the film has tightened due to the film elastic memory as it tries to return .
It is this film memory (like an elastic band on your hand) that differentiates stretch film from other means of unitizing . Settling that occurs during shipment can loosen other methods of unitizing where stretch film memory takes up the slack and continues to secure the load .
Film Memory is the single most important reason for prestretching film!!!
• FILM ECONOMIES
The cost of consumables (film) is reduced drastically compared to wrapping a load with film that has not been prestretched ie. Handwrap film.
Payback of capital expenditure of 6-12 months is easily obtainable.
• STRETCH CONSISTENCIES
Stretching using tension or a brake (post stretching) does not provide uniform stretch and thus is not able to repeat its performance day after day .
This can lead to intermittent load failures
As prestretch is more controllable than post stretch it is typically more desirable to maximize prestretch and minimize post stretch
• Reduced amount of film used, so film costs are greatly reduced
• Requires less revolutions to wrap a load, saving time, more efficient
• Less packaging film means less waste to clear and dispose
• Prestretch films are recycable
• Prestretched film is strong, consistent and stable
