This past fall, Krones AG shook up the bottling market with a newly developed PET stretch blow molding machine. The third generation of the Contiform series features astounding production output, increased precision and freedom in the stretch-forming process, all while still reducing power consumption significantly. The bottling specialist achieved this development leap by switching from pneumatic and belt drives to an electric direct drive solution and an integrated drive solution based on B&R technology. In anticipation of the projected demand, Krones will be implementing assembly line production for the new Contiform machines – an absolute first in the company’s history.
With up to 36 blow molding stations, the brand new Contiform 336 rotary stretch blow molding machine can achieve an output of 81,000 PET containers per hour. The bottle preforms are heated to a temperature of 100 to 120°C in an infrared linear oven and then guided by a starwheel into the aluminum blow mold. As soon as the mold closes, a stretch rod is inserted in the neck of the preform to stretch it axially while applying up to 40 bar of blowing pressure. This presses the sides of the preform against the interior wall of the mold to shape the bottle, which is then collected by an output starwheel.
This method is used to produce around 300 billion bottles per year, and with this number growing rapidly, bottling plants are demanding more and more performance from their machines. Still, with environmental awareness and energy prices both increasing steadily, all this performance must not come at the expense of sustainability.
Each station handles 2,250 bottles/hour
With this latest revamp of the Contiform series, first introduced in 1997, the main challenge was balancing the top priorities of maximum output and minimum power consumption. Krones mastered this challenge in expert fashion. They were able to further raise the bar set by the previous generation, boosting the output of each blowmolding station from an already impressive 2,000 bottles to a record volume of 2,250 bottles per hour.
To achieve this jump, the engineering team led by Dieter Finger, head of electrical development at the Krones Plastics
Technology division, had to do some pretty heavy innovating. «Until now, belt drives have been sufficient for driving the various wheels,» he states. «With the new Contiform, however, we’re dealing with speeds where the belt stretch inherent in this solution keeps us from achieving the precision needed for a smooth transition between the blowing wheel and the transfer wheel.»
The engineers solved this by replacing the belt drives with direct drives. The third generation of Contiform machines now uses torque motors that provide up to 2,300 Nm of torque. «These are not only much more precise than belt drives,» explains Finger, «their sophisticated power management system allows them to share power in the event of an outage or emergency stop, which means they can quickly bring the machine to a stop without losing synchronization.»
These motors are controlled using Krones’ SDC (Servo Drive Control) solution based on B&R technology, which the company has used across multiple divisions for over five years. The SDC solution allows an unlimited number of axes to be coupled via POWERLINK in a wide range of topologies. The inverters come from B&R’S ACOPOSmulti series, with an X20 CPU acting as the controller.
Electric direct drives supplant pneumatics
The great leap in performance seen in the new Contiform generation isn’t explained by the torque motors alone. Only by combining this innovation with the move from pneumatic to electromagnetic control of the stretch system were the Krones engineers in a position to achieve their formidable output. In place of pneumatic drives, tubular linear motors now control the movement of the stretch rods in the blow stations.
«Unlike conventional flat or U-channel linear motors, this design has a permanent magnet slider rod that moves inside a stator tube, which holds the drive coils and a Hall sensor,» explains Krones’ electronics expert. «The tubular linear motor eliminates the belts and play associated with standard stretch rod drive solutions. Since the force is translated directly into extending the stretch rod we can also get by with much smaller bearings, resulting in a motor that is now greaseless and maintenance-free.»
This custom motor was developed by NTI (LinMot) according to Krones’ specifications. «The specifications included that the motor provide a maximum force of 2 kN and achieve a top speed of 2 m/s, adds Finger. «We also needed positioning precision of 0.2 mm while also being able to integrate the motor seamlessly into the rest of our drive solution.»
Tubular linear motor with Hall encoder on a standard ACOPOS drive
In order to satisfy these specifications, the motor was designed for operation on a standard three-phase ACOPOS drive from B&R with a DC bus voltage of 800 V. The B&R ACOPOS drive also evaluates and linearizes the signal from the Hall sensor. «The programming involved here is not to be underestimated,» Finger explains. «Yet B&R again showed their flexibility and commitment by modifying the standard ACOPOS firmware. As a result, we now have company-standard tubular linear motors that combine very homogeneous movements with minimal power consumption.»
For the inverters controlling the linear motors, Krones selected B&R’s ACOPOSmulti65 with IP65 protection together with a B&R X20 CPU to provide control for the blowing module. To provide valve control and monitor the blowing pressure, IP67 I/O modules are connected to the controller’s second POWERLINK interface. Finger is well aware of the clear advantages this brings for machine manufacturers. «Installing this inverter directly on each stretching station enables us to produce the entire station in advance, test it and parameterize it. We also only need one supply line for each station. This not only fits right in with our modular approach, it also fully supports the assembly line production strategy we’re using for the first time on these machines.»
Electric stretching: More output, faster changeovers, more flexible processing
Machine operators also profit from the switch to electric stretching, which not only boosts output, but also gives the operator considerably more freedom to adapt the stretching process. Finger illustrates what this means in practice: «First of all, the bottle type can be selected with the push of a button, eliminating the time-consuming task of manually switching out heads. The push of a button is also all it takes to modify the movement profile of the stretch drive. As a result, when switching from a small volume bottle to a large volume bottle, it is no longer necessary to switch out the curve.»
But they didn’t leave it at that. The curve can even be freely configured to divide the stretching process into phases
with different speeds. The operator can use the additional control freedom to produce the same bottle quality from cheaper preform material or to achieve more complex bottle designs that used to be impossible to create with the necessary quality.
B&R ACOPOS: DC bus supply up to 70 meters
What’s at least equally important for the operator is that the new generation consumes significantly less power. This was achieved in part by eliminating the pneumatic drives in the stretching stations. Another contribution comes from the ability of B&R’s solution to extend the DC bus supply up to 70 meters. Krones took advantage of this feature to couple the DC bus supplies of the machine’s stretcher modules and drive modules. In the event of a power failure, power can be diverted from the blowing wheel to bring the system to a controlled stop and move the stretch rods to a safe position. «If we didn’t have our entire drive system from a single supplier, we would have to work with a variety of DC bus voltages.This would require DC/DC converters, which have given us problems in the past,» recalls Finger.
Power is fed back into the DC bus even during normal operation. The energy generated by braking the stretch rods (the compressed air in the bottles shoots them back to their starting position at up to 650 N) can be utilized by other stations, allowing each station to use an average of only 250 watts.
Other measures implemented by Krones to further optimize power consumption include the newly developed intermediate blow stage and a perfected internal compressed air recycling system, as well as further reductions in dead space volume. All in all, the amount of compressed air required by the machine was reduced by a third.
The advantages of Krones’ new series of blow molding machines are clearly evident. It comes as no small surprise, then, that the new generation of Contiform machines was met with heavy demand even before its official release, or that it was a smash hit at the 2011 Krones Customer Day.