MATERIALS | STRETCH & SHRINK FILM Model behaviour
At the equivalent event – held late last year in the USA – Dow presented details of how pallet stability can be modelled using finite element analysis (FEA). The technique allows greater understanding of
Above: Carbonated soft drinks are a typical load carried on pallets
products. The machinery was originally developed for thermoforming – in order to determine the look of the final product – but has also been applied to operations such as biaxial film stretching. There are several challenges to determining how a ‘stretched’ product will end up: tests on ‘real’ assets are often difficult, because the relationships between the various processing parameters (such as temperature and applied force, for instance) are complex.
Using numerical solutions can help to overcome
some of these difficulties, he said. However, they require accurate material characterisation. The results from IVV’s experiments can help to deter- mine real material characteristics more accurately – which can then be used to plug into computer- ised simulation models. IVV has developed a machine that applies
Right: Heavy items – such as concrete blocks – require extra-strong pallet wrap to ensure stability
stretching forces more evenly, by improving on previous designs. Existing designs use mechanical clips or grippers to hold the edges of a piece of sheet or film, and then apply a stretching force. The downsides of this method include: damage to the material; non-homogenous application of stretch- ing forces; and insufficient force measurement (only by torque). The main way that IVV overcome these problem was to clamp the film using silicone tube ‘grippers’. Two such grippers are placed either side of the film, and inflated in order to clamp it in place along its length. This makes for a more regular force on the film when it is stretched, and reduces the chance of film damage. In one example, the researchers stretched a 200
micron PET film using both methods. Using the ‘old’ technique, the final thickness ranged between 20 microns (in the middle) and 22 microns (at the edge), and generated around 3cm of edge trim. Using the new IVV method, film thickness was consistent at 24 microns, and required an edge trip of less than 1cm.
24 FILM & SHEET EXTRUSION | July/August 2018
www.filmandsheet.com
stretch wrap film deformation mechanics, said Pavan Lalavala, associate research scientist at Dow Chemical in the USA. The modelling attempts to simulate ‘real world’ tests such as tilt testing, on the performance of films made using Dow’s Elite and Elite AT resins. The FEA model makes certain assumptions, including: the pallet is rigid; corrugated boxes stacked onto its have perfect shape; and the initial stretch film configuration is a tube, with uniform thickness from top to bottom. Stacked boxes can be modelled in either ‘column’ form (square boxes) or ‘interlocking’ (in which oblong boxes are stacked alternately for higher stability). The FEA model also performs a virtual ‘tilt test’, in which the pallet is ‘tilted’ through a typical 15° -- and its maximum displacement measured.
n AMI’s next Stretch & Shrink Film conference takes place in Florida, USA on 13-14 November 2018. For more details, contact Alexandra Fish (
alexandra.fish@
ami.international) on +44 (0) 117 314 8111. A full line-up of the speaker programme is available here.
CLICK ON THE LINKS FOR MORE INFORMATION: �
www.ami.international �
www.braskem.com �
www.sabic.com �
www.estl.be �
www.ivv.fraunhofer.de �
www.dow.com
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