Using compressed air to save energy

How can we make plastic blow molding procedures more energy-efficient by reducing the amount of compressed air they use? Thanks to a research initiative sponsored by the German Foundation for the Environment (or DBU), the Chemnitz-based Steinbeis Transfer Center for Drive Engineering and Robotics in Engineering tackled this very question. Their efforts paid off: the Steinbeis experts developed a procedure that temporarily stores the high-energy exhaust air and feeds it back into the process.

Blow molding is a “highyield” way of manufacturing hollow objects out of plastic. It’s impossible to imagine how we would package our consumer goods today without these handy vessels, such as PET bottles. And that’s not all: plastic containers are gaining ground in technical areas.

In blow molding, compressed air up to 40 bar is used to expand the preform within the cavity to the object’s final shape. Once the plastic has cooled, the high-energy compressed air is released into the ambient air and the object is removed. One drawback of this procedure is the amount of energy it takes up – mainly due to the large amount of compressed air needed to shape the object.

Together with an SME named Drucklufttechnik Chemnitz, the Steinbeis Transfer Center (also based in Chemnitz) joined forces with the fluid technology research group at the Institute for Engineering and Plastics Technology (attached to the Chemnitz University of Technology). It was all part of a pilot project to engineer a way to reduce the amount of air blown during reshaping. The experts studied the process to establish a baseline for their solution. This solution would come into play at the end of the production process, where compressed air channeled into the ambient air would be stored temporarily and then reintroduced into the blow molding process.

The experts then got to work. With the formation process complete, pressure is now balanced between the tool and a separately attached pressure reservoir. This occurs as air is exhausted from the tool. Once the tool is fully exhausted, the finished object is removed. The compressed air inside the reservoir can be reused in the next cycle for another preformation process. To do this, the pressure is “rebalanced” between the tool and pressure reservoir, causing the preform to expand. Afterwards, the pressure reservoir is decoupled from the tool, and the object is worked into its final shape using fresh air.

To test the idea in practice, the researchers developed a prototype “efficiency unit”. Using suitable components, they configured the pneumatic circuit diagram of a typical blow molding machine to make it possible to insert the efficiency unit into the process – without modifying existing technology. Tests showed that up to 20 per cent of compressed air could be saved.

According to Forum PET, Germany produces nearly 13 million PET beverage bottles every year; the average bottle size is liter. If applied, this new procedure would reduce electricity use by about 9.63 million kWh. No wonder there are plans to apply lessons from the pilot project to a module that meets market demand.