Fu­sed De­po­si­ti­on Mo­de­ling (FDM)

Fused Deposition Modeling (FDM) is one of the most used AM processes and has been developed by Stratasys. It belongs to the category of extrusion-based processes that use productiongrade, wire-shaped thermoplastic material. The material is melted and selectively deposited through a heated nozzle layer by layer. FDM is commercialized since 1991. Currently, the development focuses on new materials and material properties.

Using FDM technology, three dimensional objects of any shape can be built without restrictions on forming tools. The greatest advantages of the FDM process are the relative simplicity of the process and the availability of different materials. As the material is provided on spools, material changes can be made easily and no material loss occurs during the process. Parts are built with an accuracy of about +/- 127 ”µm and with only little warpage. The production time primarily depends on the volume of the parts to be fabricated. Due to the extrusion of the material, a seam line between layers exists resulting in parts having anisotropic properties.

Principle of Layer Generation

The FDM process uses wire-shaped thermoplastics furled on cartridges when delivered. In the machine, the material is partly melted and extruded in the nozzle. The material is applied to the building platform directly from 3D-CAD data. The nozzle moves to produce a profile of the part. The nozzle moves in x- and y-direction; the building platform moves in z-direction. Due to the thermal fusion, the material
bonds with the layer beneath and solidifies. Thus, a permanent bonding of two layers is formed. When the layer is finished, the building platform is lowered and the next layer is built on top. As the material hardens very fast, the complete model requires no further hardening. FDM needs a support structure for forming a base; especially for complex models with overhangs, two extrusion nozzles are often used. At the interface with the part a solid layer of support material is applied. Under this layer, roads with 0.5 mm and gaps with 3.8 mm are deposited.

Build Chamber Volume

The spectrum of FDM machines is wide ranging from small, lowcost machines to larger, more expensive machines that are adaptable and highly sophisticated. The FDM Fortus 400mc has a build chamber volume measuring 406/355/406 mm (x/y/z in mm) .

Build Time and Build-up Rate

The build time heavily depends on the amount of material in the part, the support material volume and build-up rate. The build-up rate is a function of layer thickness, road width and nozzle diameter.
In general, manufacturers indicate the build time to be 25-70% higher than for Stereolithography and Laser Sintering processes. Using Ultem 9085 with a T16 nozzle, 61 cm³/h can be produced.

Surface Quality and Accuracy

The surface quality is a function of the road width and layer thickness. As the contours of the passes of the extrusion and the single layers are still visible at the top, the bottom and the walls, FDM parts have the highest roughness. For instance, parts fabricated using 0.127 mm layers show a roughness of Ra = 12-14 µ”m. At a maximum part length of 127 mm, all available machines and materials are set to reach a precision of ±0,127 mm. For bigger parts the accuracy is set to be 0.1% (but at least 0.1 mm). According to the manufacturers’ specifications, the accuracy varies depending on the machine: for instance from to 0.37% to 0.6%. The accuracy of the FDM process is influenced by fewer variables in total than the accuracy of comparable processes, e.g. Laser Sintering.

Material

Build Material: A wide spectrum of advanced materials, providing special properties are available for being processed by FDM. ABS (acrylonitrile butadiene styrene) is the most used material; nearly 90% of all FDM prototypes are made of this material. Derivatives of the ABS, e.g. ABSplus and ABSi, are significantly stronger, translucent and available in different colors. These materials are widely used for medical and automotive applications. Polycarbonates (PC, PC-ISO) are applicable under greater forces and loads than ABS. New, high-performance polymer Ultem 9085 and Ultem 1010 (PEI) are characterized by high heat and chemical resistance, and are more solid and stiffer. In general, parts produced by FDM are applicable at high heat, in caustic chemicals, sterilization and under intense mechanical stresses.

Support Material: The so called break away support structures (BASS) and the water-soluble support structures (Water Works) are applied as support material that can be broken off or dissolved in water, respectively. As mechanical removal becomes obsolete using Water Works, it is especially suitable for small parts or for parts with regions that are difficult to access.

Post-Processing

No post-run operations are required in general, except the removal of support structures. BASS support structure is a brittle material to be removed without using any tools. Water Works dissolves in water. Machining/sanding is applied to reduce surface roughness.

Research initiatives

FDM technology is widely used for concept models and functional prototypes, but also for end-use parts and manufacturing tools within the aerospace, defense, automotive, medical industry, as well as business and industrial equipment, education, architecture and consumer-products. End-use parts and manufacturing tools – such as jigs, fixtures and tooling masters – are produced in low-volume production
Current research initiatives aim at better understanding the material, process and part properties.