Prestretch

What Is Prestretch ?

• Prestretch is the process that allows stretch film to be stretched before (pre) being applied to a load.
• The film threads past two rollers (Primary and Secondary) in which the second rollers' rotational speed is faster than the first, thus stretching the stretch film between the two.

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

How Is Film Prestretched ?

• Film is prestretched when it is elongated mechanically between the two film carriage rollers due to the gearing ratio and diameter of the rollers.
• As the surface of the secondary roller is rotating faster than the Primary the film gets pulled between them and stretches.