PREFORM PRODUCTION PETplanet Insider Vol. 23 No. 06/22 www.petpla.net 34 In-depth characterisation of the blowing behaviour of preform materials Same conditions, same result? Growing environmental awareness has seen the bottle industry increase its uptake of sustainable materials, such as recycled PET, bio-materials and additives. However, these materials can significantly affect the behaviour during blowing of the final bottle, so it is imperative that their influence on the performance of preforms during stretch blow moulding (SBM) is fully understood and quantified. Blow Moulding Technologies (BMT) offers its material characterisation service to accurately, and efficiently, gather complex data for prospective material compositions and preform designs. Full-field deformation mapping BMT has developed a custom lab-based stretch-blow machine with bespoke instrumentation to investigate the influences of the many variables associated with the bottle blowing process. First, a unique speckle pattern is applied to each preform before it is loaded onto a universal gripper and transported to the heating station by a 3-axis robotic arm. Heating by way of an oil bath or infrared is offered as part of the service to isolate the investigation into either the mechanical or optical properties, respectively, of the preform. An investigation covering both heating methods will offer a comprehensive insight into the forming behaviour of the specific material/ preform configuration. This would then allow for informed changes to process parameters for optimisation of the resultant bottle, as well as garnering an extensive data pool that could be used for calibration and fine-tuning of a characteristic material model. Following the heating stage, the preform is then transported to the blowing station and subjected to a wide array of operator-defined parameters in a single-stage, pre-blow procedure. Two high-speed cameras stereoscopically record the preform deformation during blowing and, when used in conjunction with the patterned preform, three-dimensional Digital Image Correlation (DIC) develops a full-field, transient strain map of the blown article, across its entire length. DataRod, BMT’s instrumented stretch rod, allows for simultaneous measurement of cavity pressure and reaction force. This combination of real-time process outputs produces principal stress-strain data mapped along the entire preform edge, facilitating indepth analysis into local hoop and axial stresses as a function of process set-up. The ‘free-blown’ article is then ejected from the machine, allowing for visual inspection from the user. Case Study – recycled material An investigation into the blowing behaviour of a 100% rPET preform with one of virgin resin compared the preform geometry, temperature profile and blowing parameters for both. The resultant contour plots demonstrated that the rPET blown article appears larger in volume from around 100ms onwards. Differences in material behaviour become even more apparent when probing the hoop and axial stressstrain behaviour. The true stressnominal strain data, extracted from a point approximately halfway down the length of the evolving preform wall, shows significant differences between the two sets of preforms. The onset of strain hardening, known as Natural Draw Ratio, is shown to be delayed by around 14% in the hoop direction during the blowing of the rPET preform, compared to its virgin counterpart, in the same processing conditions. This confirms the difference in blowing behaviour previously observed and further implies an unavoidable difference in thickness distribution that will ultimately affect future performance characteristics if unaccounted for. Conclusion The material characterisation service provided by BMT offers a systematic assessment of new material and preform configurations and provides detailed insights into preform blowing shape and global stress-strain behaviour. With the drive for increasing adoption of bio-based and recycled materials for the SBM market, this service is a valuable screening tool for early stage preform design and material selection. www.bmt-ni.com Differences in material behaviour virgin PET versus rPET
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