This is not only a drive

Possibly the most valuable milliseconds in the Plastics Industry.

Netstal (Näfels, Switzerland) is a developer, manufacturer and seller of machines and complete systems for the injection molding industry, in particular medical engineering, thin-walled packaging as well as caps and PET preforms for the beverage sector.

The injection molding process is highly automated, and the machines are running 24/7. Some of the machines are located in regions where power outages can occur and where the reliability of the power grid is not necessarily a given. Power outages can be expensive and time-consuming, so Netstal started looking for a solution for PET manufacturing that would be able to handle such outages in a cost-effective manner while affording protection from damage.

Trends in the PET industry

Today, about 1.5 trillion beverage containers in different sizes are produced annually, using a variety of materials—about one third of them (500 billion) are PET beverage containers. 80 % of these are manufactured for water and carbonated sweetened beverages. This is exactly the market for which Netstal designed their new series launched in 2020: PET-LINE, a system tailored in particular to the processing of recycled PET (rPET) without limitations.

Netstal launched their very first PET system in 1981. The Swiss-based company sells their injection molding machines for the PET industry also in markets and regions where the reliability of the power grid is not necessarily a given and where fluctuations in the power supply can occur. This includes some very lucrative markets, such as countries in South America, Southeast Asia, etc., and this is a growing trend. Because they need to be able to continue production in a controlled manner even if a power outage occurs, Netstal started looking for a solution. Engineers at Netstal developed a theoretical model; the practical implementation was done by Keba Industrial Automation GmbH.

Conflict: ever-higher power needs vs. unreliable power grids

The new PET-LINE comes with a high degree of electrification. Manuel Hausammann, Control Systems Engineer at Netstal: “We work with a nominal power of up to 240 kW – power and output in injection molding have been increasing more and more over the years. But there are still many regions where the power grids are unreliable. This combination is rather precarious. Power outages or disruptions happen time and time again, you just have to be prepared. As machine manufacturers, what we can do is to make sure that the rejects rate and the effort for restarting the machine are minimal.” If an injection molding machine stops abruptly, it means that the cycle cannot finish. The worst case is if the disruption occurs during the injection process while the cavity it not yet entirely filled with plastic, resulting in “short shots”, half-finished PET preforms. They need to be removed by hand, which can destroy the mold or damage the coating. One disrupted cycle can produce up to 144 short shots – with production losses of two to three hours due to the cleaning and restarting effort in such a case.

Controlled completion of the cycle

Netstal has developed a feature that bridges the time between the power outage and the end of the cycle. Hausammann: “Our goal was to finish the current cycle in a controlled manner without any external power supply—similar to shutting down the machine at the end of production— while keeping the mold and the plasticizing unit clean and making sure that the preforms made during the last cycle don't turn into rejects.”

The solution was given the name Cycle Guard and includes two main components: an ultra-fast electric energy storage unit for the brief time immediately following the power outage, and a hydraulic storage unit for larger amounts of energy for unmolding the plastic products and shutting down the machine. The idea behind Cycle Guard: if the supply from the power grid fails, the energy from the hydraulic storage unit will be used to continue the operation of the machine. In order to make this energy usable, the direction of the hydraulic pump must be reversed. This takes about 100 milliseconds. Keba’s KeDrive D3 energy storage system is used to bridge this time and provide the missing energy.

The power outage is detected and the electric energy storage system responds within just one millisecond: for 10–20 ms, it replaces the grid power supply with a power supply of up to 300 kW. Simultaneously, all electric axes perform a quick stop, which does not interrupt the injection process. The braking energy produced by the motors is taken up by the energy storage unit, which then stabilizes the DC link of the drive system until the hydraulic pump starts working as a generator and provides energy. These initial 20 milliseconds are essential for the Cycle Guard feature.

This means that Cycle Guard includes two storage systems:

• An ultra-fastelectricenergystoragesystemfor the reliable bridging of brief power outages or power fluctuations in the millisecond range. It consists of an energy storage unit with high-performance electrolytic capacitors and an energy manager that controls the power flow and monitors the energy storage unit. This system has been optimized for high power requirements with the shortest possible response time, small size and easy integration into the drive system. The use of storage capacitors ensures that the system can handle high load cycles at high frequency without problem..
• A slowerhydraulicenergystoragesystem consisting of a pump and a hydraulic storage unit. It stores about 100 times as much energy as the system described above. This system ensures that longer disruptions can be bridged. In case of longer power outages, the current cycle is completed in a safe and controlled manner. The injection of the preforms is completed, and they are then unmolded. After the grid power supply has been restored and after a quick system check, production can restart quickly.

The challenge is to match the configuration of these two storage systems so precisely that in any scenario, the power outage can be bridged and the cycle can be completed in a controlled manner. This fine-tuning is the job of aXos 9, the controller of the injection molding machine.

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