Probably the most precious milliseconds in the plastics industry

Netstal (Näfels, Switzerland) develops, produces and sells machines and complete systems for the injection molding industry with a focus on medical technology, thin-wall packaging as well as closures and PET preforms for the beverage industry. Control systems from Keba are included.

The injection molding process is highly automated and the machines run around the clock. The machines are also located in regions where power interruptions can occur or where the stability of the power grids is not always given. Power interruptions can be expensive and time-consuming - so Netstal set out to find a solution to deal with such interruptions in PET production in a cost- and material-saving way.

Trends in the PET industry

The history of the PET bottle (PET stands for "polyethylene terephthalate") started in the late 1960s. The international 'career' of the PET bottle began when Coca Cola introduced a two-liter bottle in the USA in 1978 - at that time still equipped with a bottom tray made of a different material.

Today, about 1.5 trillion beverage containers of different sizes and made of different materials are produced every year - one third of them (500 billion units) are beverage containers made of PET. 80% of these are produced for water and carbonated sweet drinks. It is precisely for this market that Netstal has designed a new series and launched it on the market in 2020: the PET-Line - a line which, by the way, has been specially designed to be able to process recycled PET(rPET) without any restrictions.

Due to the fact that the recycling concept of PET containers is very mature and widely used, the SUP (Single Use Plastics Directive) has not questioned the PET approach, as it is the case for cotton swabs or straws, for example. In addition, PET containers can be made from 100% recycled PET (no virgin material needed) - the demand for recycled PET is extremely high - greater than the supply.

PET beverage packaging/containers are soaring, with the PET market for beverage containers growing at an average rate of about 4% per year. PET is also increasingly being used in food packaging, e.g. for the production of fruit and vegetable trays. Stefan Kleinfeld, Product Manager at NETSTAL: "During the Corona pandemic, many events and meetings around the world were cancelled. People could not go on vacation. As a result, there was a massive slump in demand for PET containers for beverages - especially for small bottles (< 1 L), which make up the lion's share of PET packaging (about 80%). Now the market is picking up again - and there is also investment in new equipment again, the order situation is very good."

Netstal injection molding machines are ahead of the game when it comes to energy requirements, speed and user-friendliness. For example, the latest generation systems require 10-15% less energy than competitor systems for comparable processes and conditions. Netstal systems can be put into operation after just a few days of training - the reason for this is the Netstal Smart Operation approach: each system can be controlled with just four operating buttons. In addition, the PET-Line systems are currently the fastest on the market: with a lock-to-lock time of 1.9 seconds, they offer the fastest movement time and the highest output - in other words: maximum utilization.

In 1981, the company launched its first PET system. It also sells its injection molding machines for the PET industry in markets and regions where the stability of the power grid is not always given or where power fluctuations can occur. And these are definitely lucrative markets, such as countries in South America, South-East Asia, etc.. - Tendency increasing. In order to be able to produce in a controlled manner even in such regions in the event of a power failure, for example, Netstal set out to find a solution. The engineers developed the theoretical model themselves, while KEBA Industrial Automation GmbH took care of the practical implementation.

Tension field: ever more power vs. unstable power grids

The new PET line has a high degree of electrification. Manuel Hausammann, Control Systems Engineer at Netstal: "We work with a nominal output of up to 240 kW - the output and output have increased more and more over the years in injection molding. However, there are still many regions with unstable power grids. This combination is particularly tricky. Power outages or interruptions happen all the time, and you have to be prepared for that. However, as a machine builder, you can make sure that scrap and the effort required to restart the line are minimal."

When an injection molding system stops abruptly, it means the cycle cannot be completed. The worst case: when the interruption occurs during the injection process, but the cavity is not yet completely filled with plastic. This results in so-called "short shots" - half-finished PET preforms. These have to be removed by hand, which can break the mold or damage the coating. There can be up to 144 short shots in the event of a cycle interruption - in this case, the production downtime due to cleaning and restarting is between two and three hours.

Ending the cycle in a controlled manner

Netstal developed a function to bridge the time between power interruption and the end of the cycle. Hausammann: "The goal was to end the current cycle in a controlled manner without external energy - similar to shutting down the system at the end of production. And to do so in such a way that the plasticizing and the mold remain clean and the preforms from the last cycle are not scrap."

The solution, which was given the product name Cycle Guard, essentially consists of two components: a very fast electrical energy storage system for the short time immediately following the power outage, and a hydraulic accumulator for the larger amount of energy needed to demold the plastic parts and shut down the line. The idea behind Cycle Guard: when the mains supply fails, the energy stored in the hydraulic accumulators is called up to keep the plant running. To harness this energy, the delivery direction of the hydraulic pump must be changed. This takes about 100 milliseconds. To bridge this time and provide the missing energy, the Energy Storage System of the KeDrive D3 from Keba is used.

The power failure is detected and within just one millisecond the electrical energy storage system reacts and replaces the feed from the grid for 10 to 20 msec with a power of up to 300 kW. At the same time, all electric axes perform a quick stop, and the injection process is not interrupted. The braking energy of the motors is absorbed by the energy accumulator, which then stabilizes the intermediate circuit of the drive system until the hydraulic pump operates as a generator and provides energy. These first 20 milliseconds are essential for the function of the Cycle Guard.

There are therefore two storage systems in the Cycle Guard function:

A very fast electrical energy storage system for reliably bridging short power failures or power fluctuations in the millisecond range. This consists of an energy storage system with high-performance electrolytic capacitors and an energy manager that regulates the power flow and monitors the energy storage system. The system is optimized for high power requirements with minimal response time, a small installation space and easy integration into the drive system. By using storage capacitors, large load cycles with high frequency are not a problem.

A slower hydraulic energy storage system that uses a pump and a hydraulic accumulator - this stores about 100 times more energy than the above system. This system therefore ensures that longer interruptions can be bridged. In the event of longer power interruptions, the current cycle is controlled and safely terminated. Preforms are cleanly finished and demolded. After mains return and a short system check, production can be quickly restarted.

The challenge is to coordinate these two storage systems in such a way that the power failure is bridged in every case of malfunction and the cycle is completed in a controlled manner. This task is performed by aXos 9, the injection molding machine control system.

"Cycle Guard offers enormous advantages to manufacturers using injection molding processes, says Wolfgang Kapp, Sales Manager Switzerland at Keba. "The system permanently monitors the power supply and, in an emergency, provides sufficient energy to stop production in a controlled manner. Cycle Guard requires no batteries or accumulators and is therefore particularly low-maintenance and cost-effective to operate."

Cycle Guard is an option at Netstal and an investment that quickly pays for itself - depending on the product, after less than ten bridged power outages. Currently, the supplier delivers about 40% of PET lines with this option.

A long-standing partnership of equals

Netstal and Keba have been working together for over 14 years. Netstal particularly appreciates the proximity to the development team and the support provided by Keba Switzerland. Hausammann: "The cooperation is uncomplicated and efficient. We have a direct line to people in development and product management - there is a solution to almost every question or requirement. Due to the very good cooperation during the introduction of the new axle controller generation and their extensive previous knowledge in the field of energy storage, we also relied on our partner for the energy storage development. Thanks to the advanced simulation capabilities in both companies, the development of the suitable product was possible in a very short time."

Netstal is currently planning with Keba to convert the Elion and Elios product ranges to Keba KeDrive D3 drive modules. "We have also already started talks for future projects," says Hausammann.