Additive manufacturing using MPA technology
For more than 10 years now we investigate in the development of new additive manufacturing techniques and processes. The MPA technology developed by Hermle Maschinenbau GmbH is a versatile tool for generative creation of large-volume components offering a widespread field of applications. It is especially in the field of injection moulding and die casting tools where the MPA technology can play to its strength. One of its prominent features is the possibility to complement precast blanks with additively applied components. The results are hybrid-manufactured components entering new dimensions in additive manufacturing with weights up to several hundred kilos and dimensions larger than 500mm in diameter.

MPA = Metal-Powder-Application
Hermle MPA technology stands for a thermal spray process where metal powder particles are compacted layer by layer to massive solids. To do so, the powder particles are accelerated to very high speed by means of a carrier gas and then deposited on the substrate via a laval nozzle. Material build-up rates exceeding 200 cm³ per hour and a powder jet diameter of several millimeters predestine our MPA technology for additive application of comparatively large volumes on rather large component surfaces.

Additive manufacturing and milling combined
The MPA material application unit is integrated into a Hermle 5-axis machining center of type C-40. The combination of MPA material application and milling in just one machine results in a hybrid additive manufacturing process. The machine's workspace allows for part dimensions exceeding 500mm in diameter.
 
Layer-based alternating material growth
MPA technology is generally designed to grow material on geometrically accessible planes of a component. If additional materials are to be integrated or embedded, the materials are applied in layers as far as the relevant contours of the other material components are accessible for machining. The contour accuracy of the components depends on the milling method and can be adapted to individual requirements. Now we start the application of the respective other material onto the prepared component contours and subsequently mill it into shape. With this alternating scheme, massive components made of two or more materials can be created.

Filling material for channels and cavities
A special filling material allows for the creation of inner hollow areas, channels and undercut contours. This material is water soluble and flushed out at the end of the manufacturing process to expose the inner geometry. Subsequent heat treatment optimizes the component's microstructure and ensures the component has the desired hardness.
 
Manufacturing process explained on a demonstration marble run
The part shown in the upper left image was created for demonstrational purposes. It contains a 3.4m long channel (18x18mm) serving as a marble run as well as an embedded copper component which, by milled out openings, shows Hermle's company logo.

Part dimensions: 520 x 520 x 140 mm, total weight approx. 270kg, of which about 40kg where applied with MPA technology.
 
The image sequence illustrates the main steps of the manufacturing process:
  1. Milling of the marble run channel into the prepared blank.
  2. Filling of the channel with a water-soluble filling material.
  3. Application of a layer of 1.2344 hot-working steel. Milling of the copper component contours.
  4. Application of pure copper onto the pre-milled surface and subsequent milling of the copper part's shape.
  5. Application of a second layer of 1.2344 steel. Milling of the copper inlay openings and dissolving of the remaining filling material in the channel.
Layer build-up with MPA technology
Powder particles from the nozzle as well as the substrate surface undergo heavy plastic deformation on particle impact. In particular the impacting surface grains are getting highly stretched by local pressures exceeding 10 GPa and temperatures up to 1000°C. Within this process, a binding contact surface is created between powder particle and the subtrate. The subsequent plasticity of colliding particles during kinetic spraying leads to an isothermal heating of the whole component.
Base material: metal powders
Metal powders are the base material for additive manufacturing with Hermle MPA technology. The physical properties of the applied metal layers and the transitions between materials must meet the high mechanical and thermal requirements defined by the operational environment of the manufactured component. Precise adjustment of the process parameters is therefore essential for each metal powder used. The properties of the resulting material microstructure are determined in extensive test series. In addition to tension and compression tests, information about particle and layer adhesion, porosity and material inclusions is derived from the examination of grindings in an optical microscope.
Currently available materials for MPA technology:
  • hot-working steels  1.2344,  1.2367,  1.2083
  • cold-working steel  1.2333,  1.2379
  • stainless steels  1.4404,  1.4313
  • invar  1.3912
  • pure iron, pure copper, bronze…
MPA Studio
For the MPA manufacturing process we designed and developed our own CAD/CAM software named MPA Studio. It is used to create programs integrating both the additive material application and the milling paths into one process. The software allows the customization of the component geometry and the layer-by-layer analysis required for the additive manufacturing. The ability to simulate the complete process as well as quality assurance functions for checking the finished component make the software a flexible and versatile tool for the use of MPA technology.