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I'm studying how industrial plastic film pallet wrappers work. The wrappers apply a certain amount of pre-stretch to the plastic film (by means of different rolls) before applying it to the object that has to be wrapped.

If the object to be wrapped isn't spherical, i.e. it has edges, then the plastic film is pulled more when it reaches an edge, and so there is an variation in how much the plastic stretches. I believe that this variation in stretching leads to the plastic film breaking when the pallet is rotating very quickly and subsequently has high pull on the film.

Are there techniques that make it possible to even out these variations and to make the tension the film is under during application more uniform?

Val
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  • It is unclear what you are asking. Could you add some more detail or maybe a sketch? – hazzey Feb 05 '16 at 14:20
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    Based on what I know about stretch wrapping parcels and other packages, I think what the question is about is a way to mitigate the stress concentrations that occur when wrapping object(s) with pointy, irregular features such as the edges and corners of a carton, but I can't say for sure. – DLS3141 Feb 05 '16 at 18:50
  • yes that's exactly it! – Val Feb 08 '16 at 16:01

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One way used to obtain more uniform loading when wrapping around square corners is to have the film run through a spring loaded dancer bar. When a corner pulls the film faster, the dancer bar rotates. Also, the dancer bar is connected to rotary potentiometer, which is wired to a DC SCR control board and speeds up the DC motor. This method provides control at a relatively lower control cost. A servo, encoder, and sensor system could provide very precise control for using high pre-stretch ratios, but may not be implemented in commercial systems as stretch wrappers are relatively lower cost machinery.

Greg Marsh
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  • sorry for asking, what's a "spring loaded dancer bar"? your idea looks interesting! – Val Feb 08 '16 at 16:02
  • The link below should show a spring loaded dancer bar. Also, the sales department of any major stretch wrapper manufacture can help with specific questions about your product and films, and also will have available pre-stretch retrofit units. https://www.google.com/imgres?imgurl=x-raw-image:///10163aec219a8bd587e0843e2d53e2692597d338f148535c91895da286dd5729&imgrefurl=http://www.converteraccessory.com/papers/tcpaper1.pdf&h=720&w=899&tbnid=1YuaIew6FhC0gM:&docid=XzRE0ht4nOKdfM&ei=jDW6Vu_kDMjXeLD4ncAL&tbm=isch&ved=0ahUKEwjv2sqcrevKAhXIKx4KHTB8B7g4ZBAzCCQoITAh – Greg Marsh Feb 09 '16 at 18:59