PREFORM PRODUCTION PETplanet Insider Vol. 26 No. 03/25 www.petpla.net 20 Mastering preform heating for rPET success Beyond the surface based on an article by David McKelvey, Head of Strategic Partnerships and Innovation, and Jude Cameron, Project Lead at BMT The use of recycled PET (rPET) in injection stretch blow moulding (ISBM) has been steadily increasing. However, batch-to-batch variations in material quality and colour create production challenges, often leading to defects, downtime, and the need for constant adjustments. Temperature plays a critical role in ensuring bottle quality, yet traditional monitoring methods fall short. BMT’s Thermoscan addresses this by precisely measuring both internal and external preform temperatures, enabling better process control and consistency. While most ISBM systems monitor input variables, such as oven temperature, the key determinant of success is the actual temperature profile of the preform - both along its length and through its wall thickness. The infrared (IR) heating process, central to ISBM, presents inherent complexities due to the interplay of multiple heat transfer mechanisms: Radiation: Heat transfer through the preform sidewall via IR lamps. Convection: Cooling effects from airflow within the oven. Conduction: Heat transfer through the preform wall thickness. During the heating process, the goal is to transfer heat energy to the core of the preform efficiently, rather than merely heating its outer surface. IR heating achieves this by enabling attainment of higher temperatures within shorter timescales, thereby expanding the process window. However, this approach results in a more fluctuating temperature profile through the preform sidewall - one that cannot be adequately assessed through external surface measurements alone. For instance, it is well-established that the internal temperature of the preform should exceed that of the outer surface, as the inner layer undergoes greater stretching during the ISBM process. When the temperature difference between the inner and outer surfaces exceeds 5 °C, it can lead to excessive shearing within the preform wall, increasing the risk of defects in the final product. Compounding this difficulty is the influence of ambient temperature within the production environment, which can fluctuate significantly across seasons. For example, during winter, preforms begin heating from a much lower ambient temperature compared to summer, necessitating adjustments to the process. Consequently, many facilities implement season-specific blow moulding recipes to ensure consistent product quality. A standard ISBM oven setup highlights the complexity of precise preform heating, with multiple lamp banks in both penetration and distribution sections, each containing several IR lamps. Operators can individually adjust the power of each lamp, as well as set the overall oven power and target temperature. Despite this extensive control, the majority of ISBM machines only provide a single-point temperature measurement for the preform, leaving the actual temperature profile along its length and through its wall thickness unknown. This scenario places a significant burden on process technicians, who must navigate over one hundred adjustable parameters to achieve consistent bottle quality. The lack of direct measurement of the preform’s internal temperature profile further complicates this task, emphasising the need for more advanced measurements to address the challenges posed by rPET variability. BMT says that its Thermoscan provides precise measurement of both the internal and external temperature profiles of a preform. During the process, the preform is heated within an IR oven, ejected, and subsequently transferred to the Thermoscan for analysis. Figure 1 presents a sample dataset where the preform setpoint was 120 °C; however, the actual measured temperature distribution deviated from this target and exhibited spatial variation along the preform length. Specifically, the peak internal temperature recorded was 116 °C at 15 mm from the neck support ring, whereas the internal temperature at the midpoint of the preform was measured at 110 °C - substantially lower than the intended 120 °C setpoint. This non-uniform temperature distribution is intentionally engineered to achieve the desired material distribution in the final bottle. The actual temperature profile results from the power settings of individual IR lamps, which are typically not directly monitored. By capturing and analysing the preform’s temperature profile, manufacturers can conduct in-depth evaluations and optimise IR heating processes, particularly for critical applications such as aseptic filling or preforms with thick sidewalls.
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