Closed-Loop Process Control Opens New Possibilities
Unsupervised blow molding has been a challenge for plant personnel despite sophisticated machine and process controls. Inherently, a process that exposes material to the plant environmental conditions changing in day/night and seasonal cycles seems to require at least occasional supervision. With a new product developed by Plastic Technologies, Inc. (PTI) of Holland, OH, USA and marketed by Agr*TopWave LLC, Butler, PA, USA this might all be subject to revision.
There are a variety of conditions that change in the course of an average day in the life of a blow molding machine. Preform temperatures vary depending on where they are stored and how long they been allowed to climatize before molding. Chiller water temperatures fluctuate depending on load and outside temperatures despite tight controls. Plant temperature and humidity may vary even if climate control is installed and most plants do not have even that. All this leads to fluctuations in reheating and stretching of the preforms. As a result, bottles vary in wall thickness distribution, requiring the experienced touch of processors to bring them back to the standard.
There have been a great many improvements in technology in the blow molder itself as well as upstream and downstream to alleviate common problems. Preforms are checked for neck ovality for example thus avoiding jams in the oven sections. Bottles are checked possibly on-line for a variety of defects and sorted out before hitting the palletizer or filler. Sophisticated packaging equipment allows operator-free bottle handling and even loading.
The missing link was the actual bottle wall distribution that could be monitored with infrared cameras but needed operator involvement when parameters went out of adjustment. PTI has now developed virtual prototyping software that can be wired into the oven section of the blow molder allowing a closed loop between bottle wall thickness measurements and oven control.
The heart of this system is a new approach to simulation of the reheating and stretching process. PTI has developed a system without the use of Finite Element Analysis, which is commonly used for this purpose. Instead, PTI developed a proprietary algorithm that is the cornerstone for independent modules for preform design based on bottle shape, virtual oven control, and predictive output for the blown bottle wall thickness distribution. Only with this algorithm is it possible to optimize the oven control when wall thickness measurements are fed into the system. An optimization routine can be tailored to run in different increments, using statistically cleaned real-time data of the wall thickness measurement program and bringing bottles ever closer to the pre-defined thickness parameters.
PTI markets the Virtual Prototyping software through SBA-CCI WorldPET of Jacksonville, FL, USA, which sells it as web-based, yearly access contracts. AGR markets the closed-loop control package besides its wall thickness measurement program but has not established pricing and other details.
Modern, high-speed blow molding machines with outputs measured in the tens of thousands of bottles require hefty investments and therefore must run as many hours as possible making high-quality bottles. Experienced processors able to make the fine adjustments for ever-lighter bottles may take years to train. The value proposition of a closed-loop blow molder control extends therefore beyond the possibilities of lights-out molding to everyday operation of lightweight containers under tight specifications. The ‘Black Art’ of blow molding may be one step closer to merge with today’s science allowing producers better control of quality and higher machine up-time. At press time no OEM had yet committed to the technology but that it may just be a matter of time before we see the first commercial systems in the market.