THERMOFORMING | MATERIALS


Thick sheet with crystal clarity


Separate to the conference, SML says calendering is critical for producing extra-thick APET sheet for thermoforming large cups with a one-litre (32 ounce) capacity. “If a glass-like quality is required for thermoforming APET sheet in the range


from 1,200 to 2,000 microns, manufacturing becomes relatively demanding,” said Rupert Becker, product manager at SML. For high transparency, intrinsic viscosity (IV) of the melt is a key to successful


production. A highly viscous melt is essential for making crystal-clear, extra-thick APET sheet. Using raw materials with an IV value higher than 0.78 g/dl – combined with pre-drying and single screw extrusion – minimises IV loss for the melt through- out production. A slanted roll stack allows the viscous melt to stabilise before entering the nip. Disturbances or interruptions can be largely excluded. This contributes towards the formation of highly transparent APET sheet. To prevent scratches, dots or dents on the sheet surface, the surface of each roller – from roll stack to winder – must have a premium surface. Crystal clear, extra-thick APET sheet also needs a relatively dust-free environment, as it has a propensity for static charges. Otherwise, dust will be sucked towards the sheet during production – which can create scratches. “Dust might seem to be a tiny issue, but all the measures mentioned above are null and void if the production environment is not relatively free of it,” said Becker.


C-PET light sheet was also processed on a Kiefel KMD thermoforming machine, in two different ways: one with a half hot/half cold tool, which has a lower output; or with a hot forming tool and a cold BFS tool – which has higher output because it uses the whole of the forming area.


Crystal benefits John Mara, technical director of Amfine Chemical, explained how new beta-nucleating agents can be used to produce superior grades of PP – for applications such as thermoformed cups and containers.


“Demand is increasing for home delivery


packaging – and there is a trend towards thinner packaging from the perspective of resource saving,” he told delegates. There are two crystalline forms of PP: the alpha form is more stable and has higher density and high flex modulus. The beta form, though less common, has higher elongation and impact strength. “In PP products, the fraction of alpha crystals


predominates over beta crystals,” he said. “Beta- nucleating agents can be used to increase the fraction of beta crystals, thereby modifying properties.” The properties include higher ductility, lower yield and higher elongation and impact strength.


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These help to improve the draw (stretch) ability of extruded sheet. “Beta-nucleating agents can also widen the


processing window, allowing sheet to be uniformly drawn – minimising thickness variation,” he added. The company’s NA-B99 nucleating agent was blended with PP and tested against a conventional rival. Firstly, it showed high nucleation ability from a low loading and exhibited higher beta-crystal formation. Impact strength more than doubled, and elongation at break showed a large improvement. “NA-B99 shows high beta-crystal formation from a low loading amount,” said Mara. “It enhances impact resistance and elongation of HPP/ICP.” n The next Thin Wall Packaging conference takes place on 26-27 November 2024 in Cologne, Germany. For more details, contact Rebecca Weir (rebecca.weir@amiplastics.com) on +44 (0)117 314 8111.


CLICK ON THE LINKS FOR MORE INFORMATION: � www.perstorp.com � https://polyolefins.grupaazoty.com � www.synthomer.com � www.lyondellbasell.com � www.kiefel.com � www.sml.at � www.amfine.com � www.amiplastics.com


March 2024 | FILM & SHEET EXTRUSION 25


IMAGE: SML


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