The requirements are clear: Injection molding machines should produce high quantities quickly and efficiently in consistently high quality. In particular, the clamping and injection units are crucial for this. This article explains how you can optimize these two central axes in your machine. In addition to the selection of the optimum motor concept, the most important parameters are the drive topology used and drive-integrated software features.
Synchronous direct drives offer optimum properties for use in the clamping unit: They have a high system efficiency because they eliminate the need for mechanical transmission elements, which has a positive effect on the energy efficiency of the machine. In addition, high torque density and overload capacity combined with optimum torque of inertia provide for an ideal combination of dynamics and compactness.
Another advantage: The hydromechanical drive train of the clamping unit can be reduced or completely eliminated, depending on the machine size. The very good acceleration properties of the direct drives ensure low cycle times, reduce unit costs, and increase productivity during operation.
At the same time, the machine can be designed to be more compact, saving floor space. The reduced number of wearing parts optimizes the entire manufacturing process: Hoses, belts, and even the gearing are no longer required, preventing the contamination of end products in the tool space with oil or dirt particles.
In times of climate change, the carbon footprint of machines and products is coming into ever greater focus. For example, an enormous amount of energy can be saved through recuperation during braking. This increases the system efficiency of the machine and improves the energy balance of producers. At the same time, it also reduces the noise level, which brings significant relief to the employees in the plant.
Depending on the requirements of the application, direct drive technology can also be implemented with a special bearing concept developed specifically for toggle kinematics. In this case, the high-torque motor has a special A-side bearing to absorb the acting axial forces, both on the tension and pressure sides, directly in the motor. This means that clamping forces of up to approx. 5,000 kN can be realized in combination with a toggle by employing a purely electric drive train. This eliminates the need for an additional, elaborate machine bearing. The robust motor bearing has a relubrication feature to ensure a long service life and very high availability. The shaft and flange design allows the spindle unit to be easily connected. This allows optimal integration into the machine with a minimal footprint.
Baumüller’s DST2 high-torque motors are available with this option in various sizes. In the size 135, maximum torques of up to 1,160 Nm are possible. The two sizes 135 and 200 achieve peak torques of 330 Nm to 4,450 Nm at speeds up to 2,000 rpm. Other axis heights are also available.
Baumüller offers two motor concepts for injection axes: Direct drives and servo motors with a special bearing concept. Both solutions in combination with a high-performance servo controller achieve very a good repeat accuracy, thereby making an important contribution to ensuring the high quality and precision of injection molded parts.
Energy-efficient synchronous direct drives boast a high system efficiency due to the elimination of mechanical transmission elements. This has a positive effect on the energy efficiency of the machine. In addition, high torque density and overload capacity combined with optimum torque of inertia provide for an ideal synthesis of dynamics and compactness. This allows cycle times to be reduced, making the machine footprint significantly smaller.
Direct drives are available in various mechanical designs to enable optimum integration into the machine with a minimal footprint. The available solid or hollow shaft versions allow, for example, the connection of a spindle screw drive. This is ideal for the injection unit, for example.
As an alternative to direct drive, a highly dynamic synchronous servo motor combined with a rack-and-pinion gearbox can also be used for the injection unit, for example. The DSD2 series is characterized by an excellent ratio of torque to torque of inertia with high overload capacity, enabling very high accelerations to be achieved. A specially developed bearing concept for the gearbox connection ensures compensation of any radial and axial forces. This enables optimum and compact integration into the machine. The robust motor bearing has a relubrication feature to ensure a long service life and high availability. Sizes 100 and 132 achieve peak torques of 110 Nm to 1,080 Nm and speeds up to 6,000 rpm. Other axis heights are available for optimum scalability.
Multiturn encoder systems that are scalable in resolution and robustness can be configured for the various synchronous motors. As a result of the “counting” of the motor revolutions, the absolute position can be determined beyond 360° by using multiturn encoder systems. This makes it possible, for example, to measure the absolute position along the travel range for spindle or rack-and-pinion drives.
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The fact that the two main axes work quickly, precisely, and energy-efficiently is largely thanks to the high-performance drive train and its integration into the drive topology of the entire machine.
During braking, the motors of the clamping and injection units are operated as a generator. This means that the kinetic energy of the system is converted into electrical energy. The servo drive feeds the supplied energy into the DC link system and distributes it to the other motor-driven axes. This results in an improved energy efficiency of the entire system and allows energy-efficient operation.
Optionally, energy can be fed back into the power supply by a regenerative feedback unit or a capacitance module can be integrated into the DC link for intermediate storage of excess energy. In the event of a power failure or during clamping unit braking operations, this module can store energy and make it available again as required. Furthermore, after a power failure or malfunction, it is possible to move the drive axes to a defined position depending on the available residual energy and the energy requirements. This prevents damage and accelerates a machine restart.
The “oversampling” function in the b maXX 5000 servo controller enables a highly precise sine-cosine evaluation of the encoder signals. With this function, the sine-cosine periods provided by the encoder are evaluated intelligently and therefore with great precision. This high-quality signal processing in the servo controller makes precise and dynamic movements possible even for encoder systems with a low number of sine-cosine periods – making the drive system more economical.
If the encoder fails during a clamping operation of the injection mold, the closure may close uncontrollably. This can cause major damage to the cost-intensive mechanics of the injection molding machine, among other things. Depending on the damage caused, this may result in a prolonged and costly production downtime. The patent-pending Smart Protection function “error response to encoder breakage” was developed to solve precisely this problem. The controller firmware in the servo drives checks the status of the encoder with microsecond precision. In the event of an encoder error, it automatically switches over from closed-loop control to U/f operation. The system is then immediately braked and brought to a standstill in accordance with the individually set parameterization of the controller. At the same time, information can be sent to the control unit via the field bus to shut down the machine in a controlled and synchronous way. This way, the clamping process is terminated without damage and machine comes to a stop.
In larger injection molding machines, the movement of the injection axes is often carried out using two gantry axes. This allows the force to be divided uniformly and controlled very precisely through fast communication. If one axis fails and the other continues to move, the injection axis will jam. This may cause mechanical damage, potentially resulting in high repair costs and production losses. The “gantry function with synchronous error response” prevents jamming. Even in the event of errors, both axes always act synchronously, i.e. performing the same movements at the same time. This happens automatically after an error is detected and without delay. As soon as the error has been eliminated, the system can be operated in gantry mode again. This helps prevent damage to the mechanics.
For the holding pressure phase, the option “PWM frequency switchover” to 2 kHz is available. This functionality extends the service life of the IGBT units and thus that of the servo drives. Moreover, the reduced IGBT losses enable an extended torque or pressure holding duration. This smart quality function allows holding pressure times to be increased and the accuracy and precision of very delicate and complex injection molded parts, in particular, to be optimized even further.
Energy-efficient, precise, and dynamic motor concepts are available for the clamping and injection units. Solutions with direct drive technology or rack-and-pinion gearbox each have their particular strengths, depending on the requirements of the application.
The electrification of these axes offers many advantages, such as the elimination of hydromechanical transmission elements in direct drive technology, high efficiency levels, compact design, and shorter cycle times. The right choice of drive topology also plays a decisive role when it comes to high performance and higher energy efficiency. Intelligent servo drive functions protect the system and also extend its service life.