3D composite builds become a reality

Stratasys’ ‘Robotic Composite 3D Demonstrator’ represents a coming together of three distinct advanced manufacturing and materials processing sectors and communities in composites, additive manufacturing and automation, and opens up some very exciting potential technology developments in the interrelated realms of material, process, and product.

About five years of serious development have gone into the various technology threads which come together in this demonstration. There are innovations here in the manipulation system as well as in material and processing.

The demonstrator provides a physical and control architecture for the use of multiple additive and subtractive technologies in order to create a finished part in one cell. Because fused deposition modelling (FDM) is so flexible in terms of the types of engineering thermoplastics that can be extruded and because it is possible to extrude in any direction, it is an obvious choice for an additive technology to utilise in such an architecture. Compared to conventional FDM printers, it is able to deposit this material along true 3D toolpaths, enabling the machine to print from the inside out rather than from the bottom up.

The robotic composite 3D demonstrator uses a physical architecture which is drawn from existing industrial automation capability and a development of the basic Stratasys FDM printing head. The industrial automation aspect has been enabled through a strategic technology development partnership with Siemens. The equipment configured for the demonstrator comprises a standard 6-axis anthropomorphic Kuka robot with the workpiece mounted on two additional rotational axes. The control architecture is built around the industry standard Siemens Sinumerik.

As the part during its build process is held on the additional axes and is presented to the processing head in a flexible orientation, it is possible to always be depositing the fused material in a preferential orientation and using the previous build and gravity to provide the necessary support. Consistent build orientation also brings with it a high degree of repeatability and this has a knock-on effect for all of the quality aspects of material processing: critical in the highly engineered parts that Stratasys has in mind.

The process is fundamentally the classic Stratasys FDM thermoplastic additive manufacturing technique, which is already being used extensively in some 100,000 3D printers worldwide. This drives a filament through a liquefier. Some components within this have been modified to accommodate the abrasive nature of carbon fibres. The composite deposition process has also been adapted with a screw extruder to alternatively handle the composite feedstock as pellets.

Carbon fibre deposition

The IMTS Chicago demonstration utilised a carbon fibre-filled polyamide, or nylon within a ribbed dome geometry. Stratasys has screened a number of other resins in its development portfolio, as well as some of the more specialised custom materials that it has done for customers in the past. These include PEEK and PEKK materials which are very strong candidates for high value composite applications. For fillers, the most significant work has been done with carbon fibre and carbon nanotubes, but they have also worked extensively with glass and mineral filled materials.

Fibre lengths, at this stage are in the order of a few hundred microns, which is substantially longer than can be achieved in any powder-bed process today, and with a fibre volume fraction up to 40% the resulting strength is highly valuable in a number of structural and metal-replacement applications. Stratasys has future intentions of depositing continuous carbon fibre, which will provide the strength required for more critical structural applications. The process is compatible with the use of recycled carbon fibre within a higher value application than would be normal for randomly orientated fibres.

At the heart of all of this is the ability to provide a high degree of alignment of the fibres, unlike other possible methods of processing with chopped fibres, such as injection moulding or powder bed additive processes. There are two processes which contribute to this, firstly starting with roughly aligned fibres within a thermoplastic filament which then translate some degree of that order into the part, and also the high shearing rates which occur during the material transfer by screw extruder within the classic FDM process.

The design freedom brought about by multi-axis build in the absence of support tooling has the capability of manufacturing structurally efficient parts.

Apart from the inclusion of structural fibres, there are a number of elements of the demonstrator process where Stratasys has increased speed including the speed of deposition, work on axis motion, elimination of discrete layer changes through continuous toolpaths, and the elimination of supports. They have demonstrated more than a 10x increase in deposition rate over industry leading systems today for FDM, and have shown feasibility in taking the deposition rate up 100x through further optimisation of the motion platform for speed.

By using a continuous, multi-axis toolpath, the process is no longer stopping at the end of each layer, switching to print support material for the next layer, moving the build platform by a single layer height, and then starting the process all over again. Because of the multiple axes, there is no need to support the material. The part is continuously reoriented so that on the occasion where support would be needed, gravity provides it. The elimination of all this non-printing time in the commercial processes today has an equally dramatic effect to the extrusion speed improvements. Many geometries are 10x faster, purely on the elimination of the start/stop/support switching time. Some parts that require a large amount of support see even more significant improvements. Considering not just print time, but workflow time, support removal is one of the most significant steps in the workflow for high performance materials, so not only does the demonstrator equipment print the part dramatically faster, but hours of support removal have also been eliminated.

Ultimate design freedom

The ability to create truly 3D composite builds, with a high degree of freedom in achieving specific fibre alignment, provides a potentially highly useful capability for both additive manufacturing and composite sectors. The demonstrator simultaneously provides an extension of additive manufacturing into much more high-performance structural materials, and within the field of composites, a method for achieving freeform build.

It is possible to envisage this manufacturing method sitting within a hybrid value chain of more conventional composites manufacturing, providing some of the high geometrical complexity features within a value chain of more conventional composites manufacturing. The capability of the design process is now going to have to catch up.

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