One trend in the plastics industry is the use of electric servomotors on the plasticizing axis on injection molding machines that are otherwise hydraulic. This offers significant energy savings due to the higher efficiencies of electric devices compared to hydraulically operated solutions. Some additional benefits of the electric replacement of hydraulic motors are:
- Higher Productivity: Allows for parallel movement of the axis along with shortened machine cycles, resulting in higher productivity
- Higher Quality: The parallel operation can be used to extend the plastisizing time at a given machine cycle, leading to improved thermal balancing of the melt and therefore producing higher quality parts
- Reduced noise levels
- Precise reproducible metering
The major challenge when designing an electric motor for hydraulic machines is to fit the envelope size provided in the machine. The energy density of hydraulic motors defines existing mechanical machine interfaces. Consequently, it was necessary for Moog to create a special, customized electric motor design.
Other design challenges involve the need for continuous power requirements in conjunction with the restricted envelope size. The Moog solution features water-cooling to optimize the heat dissipation. In comparison to convection-cooled servomotors, the unit will dissipate the losses up to 3 times better. Standard off-the-shelf solutions are cooled at the motor housing, leading to bulky mid sections.
The Moog solution integrates the cooling within the standard motor flange dimensions. The long and slender design allows machine integration thereby avoiding changes to the existing machine structure. The mechanical interface of the device allows for direct replacement of the hydraulic motor. The envelope of the system is further reduced using a single stage gearbox to match the ability of the motor running at fairly high speed to the low speed requirement of the axis. Power requirements as function of 2*Phi*Torque*Speed can cost-effectively be generated by the motor speed.
The life expectancy of the system is strongly related to the bearing life and quality of the gearbox lubrication. The placement of the cooling between the gearbox section and the motor in the Moog solution results in higher life expectancies through lower thermal temperatures of the lubrication, which is directly related to the lubrication life. Keeping the gearbox temperature low will reduce the wear of the shaft seals due to limited internal gearbox pressure levels. The sealing life, the contamination of the lubrication and the overall gear life are related. The calculation of the operating life of the system is targeted to the expectations of injection molding machine manufacturer. With that goal in mind, Moog eliminated the redundancies of motor and gearbox bearings in favor of a single integrated architecture.
In addition to the technological benefits of the electric solution described above, the Moog electric screwmotor provides extended machine availability and ease of integration.
Author
Thomas H. Czeppel is the manager responsible for the development and application of solutions using electro-mechanical technology in Europe. He has worked for Moog for 11 years in engineering and application functions in Germany and the US. He studied Precision-Engineering in the University of Esslingen and currently pursues an MBA at the SIMT in Stuttgart.