3D high-speed rotation printing process revolutionizes the production of prostheses

The use of 3D printing in the production of prostheses and orthoses for orthopaedics is expedient because they are usually unique pieces. The drive specialist, JAT Jenaer Antriebstechnik, is also involved, along with additional companies, in the still ongoing research project from the pro-O-light network, which wants to make this procedure more efficient and cost-efficient.

In orthopaedic first aid, the rapid and cost-efficient production of prosthetic elements is a huge problem because they are practically all unique fabrications. The pro-O-light network is taking on the challenge of improving this situation in cooperation with the Mittweida University and several partners from the machinery and electrical sector.

In the past, the manufacturing process for prostheses using the conventional procedures was tedious, complex and accordingly expensive. The production of head orthoses is one example: They are used primarily in helmet therapy when treating skull deformations and asymmetries in infants during the first few weeks of life.
Another application is a head orthosis used as a stabiliser after an accident. To date, these head ortheses were manufactured manually. The 3D printing process is already opening up new production options in other industries with respect to batch sizes of 1. With a completely novel 3D printing process, a head orthosis is produced without the tedious process of making plaster moulds which has to be performed several times on infants because their heads are still growing, and a significantly lighter orthosis with an open, comfortable structure is created.

Initial situation

In the practical environment of orthopaedic technology, it was quickly determined that the established additive printing processes with only one printing head are not cost-effective because the production process is very time consuming and did not meet the expectations of medical professionals. The solution to the problem was to develop a printer that uses a rotating work table and up to 4 printing heads working simultaneously. Thanks to this novel approach, extremely high work speeds can be achieved. Since the printing heads only work unidirectionally, a specific printing head jet geometry can be created. At an average use of four printing heads, a prosthesis can be printed in under 60 minutes.

Drive technology plays a crucial role in this process. After careful research, FGMD GmbH, which is responsible for the network management and project coordination, chose Jenaer Antriebstechnik to be its partner. The local proximity and the broad portfolio and high level of expertise in drive technology were decisive factors when making the choice.

The mechanical layout of the 3D printer

To achieve the high production speed, it was necessary to use several printing heads simultaneously at average travelling lengths. Since, however, these would hinder themselves via their mechanical guides during the printing process, a new technology had to be developed. This meant not positioning the workpiece on a static table or a table that could move in the XYZ direction, as usual, but to rotate it during the printing process. Since the printing head jet is aligned at a tangent to the workpiece due to the geometries, significantly higher printing speeds can be achieved.
By continuously rotating the workpiece, the travelling lengths of the printing heads are reduced to a minimum. The combination of the radial movement of the printing heads, the rotation of the workpiece and an additional lifting mechanism on the work table allow for the practical production of spatially free-form objects.

Requirements for the drive unit

The research project offered JAT the opportunity to expand the business segment to include an innovative and novel 3D printing process. Thus, JAT had the opportunity to expand the range of products and services for 3D printing solutions, since there is a high potential to adapt the development for other systems.

To achieve the required process speed while printing the prostheses, a special printing head drive concept is required. The novel printing method will make it possible to increase the process speed by a factor of ten. This requires the simultaneous use of several heads and an additional increase in the printing speed by developing a printing head with new a jet geometry.

One of the greatest challenges was the interaction between the printing heads and their drives in this composition. The printing heads, contrary to the current state-of-the-art, do not have to move to their working positions consecutively and complete the printing process there; instead, they position themselves simultaneously according to the individual geometry which prevents the printing heads from colliding. They are thus positioned on the respective tangent at the right time and the right amount of printing material is extruded during this dynamic process. The constantly changing scope, varying speeds for the respective printing head, the associated calculation of different material quantities and the dependency of these components are a completely novel, but necessary approach to production.

To achieve the cost objective, the Ecostep drive system was selected. It is particularly suitable for price-sensitive applications because it works with a multi-pole servomotor with feedback in the form of an encoder. This achieves a significantly higher dynamic and operational safety in comparison to the drive technologies without closed bearing controllers because there is no risk that the motor will lose its position. In addition, the drive controller offers a high level of connectivity to superordinate systems like a PLC. Therefore, interfaces like CANopen, EtherCAT, PROFIBUS, PROFINET, Ethernet, RS485, RS232 are available. A Siemens SPS S7-1500 with a TIA portal can be used for the 3D printing project. The Ecostep drives were activated via PROFINET.

The realisation of the drives

The mechanical layout of the 3D high-speed rotation printer requires a total of twelve Ecostep drives. Spindle drives are used for the horizontal and vertical linear Y and Z axes, which are responsible for flexibly positioning the extruder. The motor is an Ecostep motor from the 23S21 series with an absolute value encoder feedback unit. The same drive type positions the rotary table as well. Four 17H13 Ecostep motors with incremental encoders are used to transport the material into the printing heads specially developed for this purpose. These three axes are linearly interpolated. This algorithm is integrated into the Ecovario D114 servo amplifiers. Since this servo amplifier is a compact, double output power amplifier, six amplifiers control the twelve motors.

To achieve the best possible printing result with respect to precision and speed, a high level of track accuracy of the drives is indispensable. Especially in such highly dynamic applications, the Ecostep drives show off their strengths, because even when load fluctuations occur, a precise adherence to the specified track is ensured thanks to the position feedback. Because, in Ecostep drives, analogue to the classic, but expensive servo drives, both the speed and the position are regulated.

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