The best known and most commonly used technologies are HP Multi Jet Fusion, Selective Laser Sintering, and Colorjet. In this technology, powdery starting materials are selectively bound. This can be done by a chemical process (gluing) like the Colorjet process or by a thermal process (SLS → Laser; HP Multi Jet Fusion → Infrared). With the exception of metal printing (SLM), support structures are not necessary for these forms of additive manufacturing, since the object is completely embedded in the material powder.

Surface quality

The surfaces of the objects are of good quality. Due to the elimination of the support structures, the surfaces are largely homogeneous. Depending on the material (powder grains of ∅ approx. 60 µm ), holes/pores occur on the object surface. This means that even during grinding, no completely smooth surfaces can be produced. However, processes differ, e.g. laser sintering produces more porous parts than the HP Jet Fusion process (More details: Laser sintering vs. HP jet Fusion).

Image: surface roughness Laser Sintering & Multi Jet Fusion
Source: 3Faktur; EAH Jena

Finishing Options

Common post-processing methods are grinding, shot blasting or vibratory finishing (in particular SLS). This allows the surfaces to be smoothed, but the surface quality of liquid-based processes cannot be achieved. In laser sintering, creating an almost pore-free surface is only possible with great effort (vacuum infusions or a multi-stage process of surface filling with subsequent multiple grinding).

In the example below, laser-sintered parts were infiltrated with epoxy resin in a vacuum and then polished. A very good surface quality can be achieved by closing the pores. In most cases, however, this is practically not cost effective. If Ra values of 1 µm (+/-) are required, stereolithography or the Polyjet process are usually used.

Laser Sintering | PA2200 (raw); Ra 13,1 / Rz 90,5

Laser Sintering | PA2200 (blasted with Aluminum oxide); Ra 6 / Rz 38,7

Laser Sintering | PA2200 (vacuum infiltrated and grinded); Ra 1,3 / Rz 9,3

Image: Microscopic images of laser sintered parts
Source: 3Faktur; EAH Jena; The parts weere printed on a EOS P390 with the material PA2200.

Applications

All typical applications of rapid prototyping as well as end-use parts (additive manufacturing). Laser sintered and Multi Jet Fusion parts are relatively cheap, very resilient and of high quality, and therefore often used for fully functional parts in end-use products.

Layered structure (schematic)

• X – Y level: No homogeneous surfaces possible, smallest pores are created by the powder grains from of the raw material.
• Z plane: The same applies here as for the X-Y plane.
• Other characteristics: Pores can be partially “closed” by infiltration. Grinding only brings a limited increase in quality.

Sample prints

Design: Thingiverse | LuluPaw

Print sample Laser Sintering

The surface in laser sintering is slightly finer, but the layers of the 4.5 cm large owl are still visible.

There are a variety of technologies that use the extrusion method. The best known are FDM/FFF (filament 3D printing), special extrusion processes e.g. fiber-reinforced systems (carbon/glass fibers), syringe-based systems (Bio-3D printers, food printers, concrete printers). Support structures may be necessary depending on the geometry of the part. The process works according to the following principle: a thermoplastic material is discharged through an extruder and hardens during cooling.

Surface quality

The quality of the surfaces is rather low. The outer layers (Z-direction) are strongly corrugated, but they are almost pore-free (in industrial, high-quality printers) and can be reworked to a high-quality level. The underside is usually very smooth in places without support, on the upper side the surface roughness is comparable to other 3D printing technologies (see data below).

In places with support material the surfaces are partly strongly impaired. However, some printers use soluble (water or other chemicals like limonene) support materials, which allows for significantly better quality.

Image: Surface roughness FDM
Source: 3Faktur; EAH Jena

Finishing Options

For PLA and PA6/PA66, finishing is usually limited to the removal of support material. ABS can be reworked quite well and can be polished, blasted or tumbled.

FDM | ABS (raw); Ra 8,8 / Rz 88,4.

FDM | ABS (grinded); Ra 2,4 / Rz 28,3.

Image: Microscopic images of FDM printed parts
Source: 3Faktur; EAH Jena; The parts have been printed on an Ultimaker 3 with the material ABS (Ultimaker).

Applications

The most common applications for extruded plastic objects are functional parts. The objects are only suitable for the production of moulds or for painting with great effort.

Layered structure (schematic)

• X – Y plane: The surface is less corrugated, but the individual layers are visible (from the running direction of the print head).
• Z Layer: The layers can be differently pronounced, smooth surfaces in Z direction are possible by grinding, especially with ABS (red line).
• Other characteristics: Gaps between layers are possible due to the process → Reduction of tensile strength in Z direction.

Sample prints

Design: Thingiverse | LuluPaw

Sample print

The FDM technology produces clearly visible grooves. These can be reworked differently depending on the material. The PLA shown here can be post-processed to a limited extent. The ABS, on the other hand, can easily be mechanically and chemically refined.

The best known processes are Polyjet and Multijet Modeling (MJM). Light-curable polymers are applied to a work platform by a print head with several nozzles. The still liquid material is immediately cured by UV light. Support structures are always necessary for these procedures.

Surface quality

These technologies produce good to very good surfaces. The applied material droplets melt to form very low layer heights. This results in very homogeneous surfaces with barely perceptible layer thicknesses (15 – 30 µm). Here, too, support structures are necessary which impair the quality of the “affected” areas. This restriction no longer applies to 3D printers that use thermally soluble (3D systems) or water soluble (Stratasys, Keyence) support materials. By using material droplets, the deposit layers are also visible in the X-Y direction (direction of movement of the print heads).

Image: surface roughness Polyjet
Source: 3Faktur; EAH Jena

Finishing Options

Removing the support structures is necessary for “normal” Polyjet printers. With water / thermal-soluble support, post-processing is usually not necessary. The techniques used are in particular blasting and grinding, where very good surface values can be achieved (see figure below).

Polyjet | VeroBlack (raw); Ra 6 / Rz 29,1.

Polyjet | VeroBlack (blasted with aluminum oxide); Ra 3,7 / Rz 24,2.

Polyjet | VeroBlack (grinded); Ra 0,2 / Rz 1,7.

Image: microscopic images of Polyjet 3D printed parts
Source: 3Faktur; EAH Jena; The parts were printed on a Stratasys Objet 350 with the material VeroBlack.

Applications

Like Stereolithography, the surfaces are largely free of pores and can be further improved by repeated grinding. Therefore, the objects are also suitable for the production of original moulds in addition to the usual rapid prototyping applications.

Layered structure (schematic)

• X – Y plane: Minimal layers are created by the print head paths.
• Z Layer: Minimal noticeable layers present can be smoothed out by ablation methods (red line).
• Other characteristic: Layers are present at all levels (not shown).