Additive Processes

Binder jetting operates in a similar fashion to a powder bed process where a powdered material is raked out over a build volume in a thin layer. Instead of being consolidated with a heat source, a binder is jetted across the surface and the powdered material binds together. Once the first layer is completed, the build platform lowers and a fresh layer of powdered material is raked across. The print head is similar to that of a 2D printer, and thousands of droplets can be jetted at a time, making binder jetting a relatively quick and simple process.

There are many materials that can be used in binder jetting, these are:

• plastics, such as nylon
• metals
• sand
• ceramics

Similar to SLS, the part is supported in the build volume and does not need support structures to keep it intact. In the case of metals and ceramics, the binder must be burnt out in an oven to reach sufficient strength. Typically, part density is lower with binder jetted parts than with those built via thermal (laser of electron beam) processes. Another option available on ExOne binder jetting machines is to infiltrate the oven-baked part with another metal in an additional thermal process. This results in a part that is a matrix of materials (both the original metal and the infiltrate material). Desktop metal is another provider of binder jetting for metal, however their binder jetting machine has not yet been publicly released.

Plastics such as nylon undergo a UV cure in-situ, this occurs immediately after every layer has been jetted. As a result, the part exiting the build volume does not need any further processing. Both voxeljet and HP have binder jetting machines that process polymer materials. HP plans to release a metal binder jetting system in 2020.

ExOne and voxeljet both have printers that will print in sand. Sand printing is useful for inexpensive model making, but has the strongest value proposition in printing moulds and cores for casting. Sand printing can be used to avoid pattern-making for low volume moulds.

Blown powder systems can be divided into two categories – directed energy, and gas dynamic spray (or ‘cold spray’). Both work with a spray of metal powder towards a surface, but have different bonding mechanisms.

Directed energy systems typically use a laser to consolidate sprayed metal powder particles in layers. These systems can be used for creating free-form objects (an object constructed layer by layer using the directed energy AM method) or can be applied as a coating or built up as an addition or feature to an existing part or substrate (often traditionally manufactured, such as repairing a turbine blade). Some directed energy systems are contained within a sealed, inert atmosphere hence are suitable for reactive materials, such as the Optomec LENS system. Other directed energy systems such as the DMG MORI Lasertec incorporate a milling/machining tool, so that a built part can be machined in-situ. Trumpf has an offering that employs a shield of inert gas over the built part, enabling larger build volumes as it is not restricted to a sealed environment.

Gas dynamic spray (or cold spray) does not use a thermal source for consolidating powder; instead the cold spray system subjects metal particles to supersonic speeds by feeding a heated powder and gas stream through a de Laval nozzle. Upon exiting the nozzle the metal particles are projected towards a substrate at supersonic speed; they bond via kinetic fusion energy with the substrate and each other. As this is not a thermal process, reactive materials can be utilised in the absence of an inert atmosphere. It is important to note that the lack of thermal processing means that the properties differ to that of a thermally created part (i.e. laser powder bed, or casting/forging processes).

Commercial suppliers of cold spray systems exist in Germany (Impact Innovations) and Japan (Plasma Giken), however Australia is home to packaged systems. Spee3D offers a cold spray-based machine which is intended to be used as a replacement for low volume cast parts. Titomic offers a large format cold spray and robotic system that offers build volume flexibility.

Digital Light Processing (DLP) is a type of Stereolithography (SLA) however the one major difference is that is uses a more conventional light source such as an arc lamp or light emitting diodes (LEDs), rather than a UV light source. Additionally, this light source can work in tandem with an array of micromirrors so that the light source is applied to the whole layer at once, making printing speeds quicker than UV light SLA technology.

DLP usually has the light source directed upwards into the photopolymer vat so that the object being built is ‘grown out’ of the photopolymer vat, rather than being built inside the vat. This limits the size of these printers as they can only operate with a small amount of resin in the vat. Some well-known companies that sell DLP printers are Formlabs, 3D Systems, Carbon, XFAB, and SprintRay.

Electron beam powder bed systems use metal powder as a feedstock, similar to laser powder bed systems, but the powder used is slightly larger in diameter, being approximately 50 microns. The powder is raked across the build platform, and an electron beam melts each layer according to a 3D CAD file. The melted layer solidifies and joins to the layer below, forming a solid object. Once this layer is completed, the build platform drops by the exact depth of the new layer of powder to be raked over (approximately 30 microns). The fresh powder is raked over the existing layers and the process repeats again.

Unlike laser powder systems, electron beam systems operate under a total vacuum. This ensures that there is very little oxygen in the build environment, making it ideal for reactive materials such as titanium. The electron beam has a greater energy density than a laser beam, and for this reason can process larger particle sizes. The larger particle size has the effect of causing a rougher surface finish than a laser processed object; this can be advantageous in circumstances where friction at the surface is required (such as for bone in-growth), but can be a detraction for applications requiring a smooth surface finish (such as a bolt thread).

Similar to laser systems, electron beam systems require support structures for angles over 30-45 degrees, however electron beam systems do not require the structures to be anchored to the build plate; therefore parts may be nested (built on top of each other) in a build volume. Unlike laser powder bed systems, electron beam built parts generally do not require additional heat treatment. This is due to the elevated temperature of the build chamber during processing. Currently GE Additive (Arcam) sells electron beam powder bed systems, with newer entrants Freemelt and JEOL also releasing EBM systems.

As the name suggests, material extrusion involves a material, usually a semi-liquid polymer being extruded through a heated nozzle onto a platform. Either the nozzle head or the build platform moves in the x-y-z direction to guide the layer path. Each layer solidifies before another layer of extrudate is applied, and the process is repeated to form a final object.

This technology is the most recognisable form of 3D printing, and many desktop/consumer printers are material extrusion printers. The names for this technology can vary, the most widely recognised name is Fused Deposition Modelling (FDM) however this is a proprietary name held by Stratasys. Another name is Free Form Filament (FFF) which is the generic term for material extrusion.

As with other AM processes, material extrusion technologies require support structures for overhanging segments. For Stratasys printers, a water-soluble support material is printed in and washed off upon completion of the build. Alternatively support structures can be built in and snapped off manually.

The most common materials for entry-level material extrusion printers are ABS and PLA. Stratasys has developed additional materials that are engineering grade thermoplastics, suitable for industrial applications. Objects made from FDM material extrusion are both strong and durable, but commonly show a layered effect on the surface, so need sanding to obtain a smooth finish. This layered surface effect is common for all material extrusion-made parts.

More recently, polymers have been combined with other materials such as metal powders or carbon fibre and are extruded as a mixed product to either remain in state (with carbon fibre), or get burnt out in a furnace (as binder does with metal parts made with this method). Desktop Metal and Markforged both sell machines using this type technology.

Material jetting works in a similar way to 2D printing whereby semi-liquid material is jetted onto a surface via an inkjet. There is usually a UV curing step once each layer has been jetted. Print heads can contain multiple inkjet heads which allows a material jetting printer to print different materials at the same time. This may be support structures, but can also be materials of different colour and/or properties, enabling graded material parts.

Materials are typically photopolymers or wax-like materials and are suitable for use as investment casting patterns. Material jetting is a very precise type of 3D printing and it returns parts of a good surface finish, detail and accuracy. Companies that use a material jetting technology include 3D Systems, Stratasys, Mimaki, and Solidscape.

Selective Laser Melting (SLM) is a common AM technology for processing metals; it is also referred to as powder bed fusion, direct metal laser melting, or selective laser sintering. Laser powder bed systems use metal powder as a feedstock. The powder is raked or rolled across a build platform, and a CO₂ laser melts each layer according to a 3D CAD file. The melted layer solidifies and joins to the layer below, forming a solid object. Once this layer is completed, the build platform drops by the exact depth of the new layer of powder to be raked over (approximately 30 microns). The fresh powder is raked over the existing layers and the process repeats again.

Laser powder bed processes consist of approximately 75% total metal machine sales worldwide. Common machine manufacturers of metal laser powder bed systems include: EOS, SLM Solutions, GE Additive (Concept Laser), 3D Systems, and Renishaw. Aurora Labs, an Australian-based company also produces a laser powder bed system.

Some advantages of laser powder bed systems are good surface finish and a high level of detail and complexity. A disadvantage can be slower build speeds, and the need to apply a heat treatment post-build (this depends on application). Laser powder bed systems require a ‘support structure’ to be built into the object if the build angle exceeds 30-45 degrees. This support structure is built from metal and anchors the part to the build plate (although in some instances anchors another part of the object). This support structure must be removed once the build is completed.

Selective Laser Sintering (SLS) is very similar to Selective Laser Melting (SLM) except it refers to polymer materials, not metals. SLS fuses polymer powder particles, thermally bonding them together such that they form a complete part of significant strength. The process starts with a layer of powdered material spread on a bed with a recoater or spreader bar, then a laser fuses that layer of powder according to the input 3D CAD file. Once the layer is completed, the build platform lowers and a new layer of powder is spread over the existing layer. The process is repeated until the 3D file build is complete. Any unused powder can be recycled back into the system for reuse, however polymers degrade when exposed to heat, so can only be recycled a finite number of times.

Typically polymers are referred to for SLS processes, and nylon is a common SLS material. Nylon parts made via SLS are strong and durable enough to be used in end-use applications such as on planes, in cars, and other industrial applications. Polymer parts made via SLS do not require any support structures such as metal systems require, as they sit supported in the surrounding material.

SLS systems can be sold as small, desktop style of printers, or larger industrial printers. Companies such as Formlabs, Sinterit, and Sintratec all sell desktop variety SLS printers, whereas companies such as EOS, 3D Systems, and Prodways sell the larger industrial printers.

Stererolithography (SLA) was the first 3D printing process to be commercialised (it was commercialised by 3D Systems in 1988). SLA uses an ultra-violet (UV) laser directed into a vat of photopolymer resin. The resin is heated to a liquid state and the UV laser is directed down onto a platform that is submerged in the vat of resin. The laser solidifies the resin layers according to a 3D CAD file containing the shape data. Once the layer is completed, the platform moves down and a new layer of photopolymer resin is spread over the prior layer. This continues until the final part is complete and can be removed from the vat.

SLA is similar to other 3D printing processes in that it requires a support structure for overhanging sections that need to be manually removed. The final part is then submerged in a bath to wash away excess resin, and it is often also subjected to a curing step in a UV light oven to strengthen and stabilise the material. Depending on the intended use, the part may then be sanded and/or painted.

SLA is a very fast and cost-efficient prototyping and modelling technology. SLA produces parts with excellent dimensional accuracy and surface finish; as a result SLA is a popular choice for model-making and can also be used in pattern-making. 3D Systems and XYZprinting and others sell SLA systems.

Wire fed systems use a metal wire as a feedstock and a thermal source to melt the wire. Freeform objects can be built by adding layer on layer of melted wire. This process is much like the traditional welding process, but instead of using it as joining technology, it is used to make whole objects.

The thermal source for wire fed systems can be any of electron beam, arc/plasma, or laser. All energy sources require a shielding gas or a sealed, inert environment for processing; the gas type will change depending on the material being processed (e.g. argon is used for titanium). The heat source can dictate which materials are best to process, for example, aluminium is not well-suited to electron beam wire fed processes as it is prone to evaporation in a vacuum environment.

Wire fed systems build what is considered a ‘near net shape’ part. This means the additively manufactured part will still need machining down to the final part shape. As a result, there is some wastage associated with wire fed processes, however wire fed processes have value for large format parts made with high value material that would ordinarily be machined, as machining incurs significant waste. For this reason, the technology has strong relevance to aerospace and defence. Other applications that demand high value materials such as offshore oil and gas processing are also relevant.

Using a wire feedstock has advantages over powder systems, as wire is a commonly traded mill product and is therefore widely available in multiple alloys. The systems are inherently simpler than powder fed systems, and therefore require less development effort for new materials, or changing between materials.

AML Technologies in Adelaide owns and operates a wire-arc wire fed system. This is based on WAAMâ technology licensed from Cranfield University in the UK.  Norsk Ti has also developed a plasma wire fed system and has certified aerospace parts for Boeing.