1. General questions
Additive manufacturing (in short, AM) or 3D printing, is the production of a three-dimensional object from a CAD model or a digital 3D model. Upon its development in the early 1980’s, 3D printing was exclusively used for the creation of prototypes. Therefore, early on 3D printing was also called “rapid prototyping”.
3D printing established itself rapidly, as it reduced the time for developing the prototype of a product from several weeks to just a few days or, even, hours.
While initially only plastic was used as a base material, by mid 1990 the technology had evolved to a point, where it also allowed the sintering or melting of different types of metals. At that time the automated methods that created products by “adding” metal would be called “additive manufacturing”. Today, the terms 3D printing and additive manufacturing are totally interchangeable.
The 3D printing technology is still used for prototyping. But in recent years it has expanded into actual manufacturing of small lots of production parts. Since 3D printing requires no set-up times and allows the fast production of small quantities of components, it is increasingly employed as a method for “on-demand manufacturing”. In other words, it allows producing a component at the exact time it is needed in the manufacturing process. We can expect that within the near future additive manufacturing will even be used for volume production of standard components.
Besides the benefits for “rapid prototyping” and “on-demand manufacturing” 3D printing offers another key advantage: It allows the creation of extremely complex shapes that cannot be created by casting, forging, etc. This helps not only to reduce the number of components in a product but also cut its weight. For example, 3D printing enabled GE to reduce the number of parts for its LEAP jet engine fuel nozzle from 20 to just 1 and to reduce its weight by 25%.
The multitude of different 3D printing methods available in the market can be quite confusing. However, all additive manufacturing technologies have one thing in common: They create parts/components by depositing material layers on top of each other, one layer at a time.
Where they differ, is how these material layers are created:
In the field of plastic 3D printing there are printing systems that are using a liquid resin as base material; others are using extruded thermoplastic filaments, whereas another group of printing systems uses a powder bed. In each case the plastic material is liquified by heat – for example, a laser -- to form one material layer and then be hardened quickly. This process is repeated, layer by layer, until the product is completed.
3D printing of metal parts uses mostly a bed of powder of the respective metal. But industrial-grade metal filaments, for example 316L stainless steel, are also used. Like with plastic, layers are created, one after another by melting, sintering or binder jetting. Once a material layer has cured, the next layer is created.
The following table provides an overview over the various 3D printing systems currently available in the market:
- Polyjet – Works like an inkjet printer. But instead of jetting ink, Polyjet printers are jetting tiny blobs of liquid plastic to form one layer at a time.
- DLP – Digital Light Processing: This 3D printing technology uses a light source – a digital light projector screen – to cure a liquid photopolymer resin.
- SLA or Stereolithography: The predecessor of DLP. With the SLA method a laser has to cure the resin in a “point-to-point” technique. SLA is much slower than DLP.
- CLIP – Continuous Liquid Interface Production: A photochemical process using light and oxygen to build a part from a reservoir of UV curable resin.
- FDM – Fused Deposition Modeling: A plastic filament, extruded through a nozzle, melts while gradually being deposited on a build platform, one layer at a time
- FFF – The same printing method as FDM
- HP MJF – Multi Jet Fusion: MJF uses an inkjet array to selectively apply fusing and detailing agents across a bed of nylon powder. One layer of new material and agents is printed on top of a previous layer that is still molten – so that both layers fuse completely.
- SLS – Selective Laser Sintering: A laser is used to sinter a layer of powdered plastic material into a solid structure.
- LFAM – Large format additive manufacturing: As the name suggests, LFAM is used to produce very large parts. The process uses pellets fed from an extruder, which can be mounted to a X/Y gantry or a multi-axis robot.
- SLM – Selective Laser Melting / DMLS – Direct Metal Laser Sintering / DMLM – Direct Metal Laser Melting: A 3D printing process, where a laser melts the metal powder to create one layer of the component at a time
- EBM – Electron Beam Melting: Instead of a laser EBM uses a high-power electron beam as heat source to melt layers of metal powder, which are then fused together
- Binder Jetting: A liquid binder is selectively applied to join powder particles, layer by layer. The “green” parts must be cured and then sintered in a high temperature furnace.
- FDM/FFF – Fused Deposition Modeling: This technology cannot only be used with plastic but also with specialized metals, for example, with Ultrafuse stainless steel 316L. Parts are printed from 90% of the specialized metal and 10% polymer filament. The printed parts are in a green state und must undergo a de-binding process to remove the polymer binding agent followed by a sintering process.
3D printing is no technology for mass production of standard components; not yet anyway! However for prototyping and production of small volumes of customized designs it offers numerous advantages. Here are some:
- Speed - This applies to prototyping as well as small volume production. All it takes is uploading a CAD model to the printer to create the physical part, which is a matter of a few hours. Design optimizations can be implemented and tested in a very short time
- Customization – Products can be easily customized. For example, in the medical field teeth aligners can be tailored to fit the teeth of an individual patient. Or, with the CT scan-to-CAD technology CT scans of cranial plates can be directly translated into CAD models, which are then uploaded to a 3D printer to produce an artificial cranial plate just fitting the one single patient.
- Freedom of design – 3D printing allows the creation of extremely complex work piece designs not possible with traditional manufacturing methods (bionic structures, cavities, undercuts) . This helps not only to reduce the number of components going into a product but also to reduce its weight (topology optimization, lightweight design). This has been impressively demonstrated in the field of jet engines.
- On-demand manufacturing – Since 3D printing is a single step manufacturing method, it requires no work-in-process inventories but allows producing a component at the exact time it is needed in the manufacturing process (spare parts on demand).
- Material savings / Waste reduction – Subtractive manufacturing like milling, drilling, grinding, etc., produces a significant amount of waste. Additive manufacturing generally just uses the material required to build a component. Any surplus material like support structures and residual powder can be easily recycled.
- Low costs – For prototyping and small volume production of customized designs 3D printing is extremely cost effective and handily beats conventional prototyping and manufacturing methods.
- Risk reduction – 3D printing allows the easy and quick verification of a prototype before having to invest in expensive tooling for manufacturing. This reduces the risk of producing a faulty product with, potentially, significant financial losses.
- Functional integration – Intelligent functional integration drastically reduces the number of components, as many functions can already be integrated during production. This translates to lower assembly and logistics cost savings.
While in the beginning 3D printing was limited to all kinds of plastic materials, nowadays nearly any material can be utilized.
There is a huge selection of plastic materials available for 3D printing:
- Liquid resins: For example, epoxy and acrylate
- Filaments: Generally thermoplastics, for example, PLA, ABS, PEI, PPSU, PETG and ESD PEKK
- Powder: Powdered thermoplastics, for example, polyamides (nylon), PC and PEI
- Pellets: Carbon fiber-, glass fiber- or mineral-reinforced resins like ABS, PC, PEI and PPS
Probably the fastest growing segment of the additive manufacturing industry is in the field of metals allowing the use of
- Aluminum (e.g. AlSi10Mg, AlF357)
- Titanium alloys (e.g. Ti64ELI)
- Inconel (e.g. IN718, e.g. IN625)
- Carbon and stainless steel (e.g. 316L, 17-4 PH)
- Maraging steel
- Precious metals (e.g. gold, silver platinum)
Today even ceramic products like specially designed bath tiles can be 3D printed from a ceramic powder consisting of ultra-fine particles of alumina and silica. Of course, after the printing step the “green” products must then be fired in a kiln.
Another fast growing market segment is the combination of different materials. For example, a PLA base material mixed with cork and wood dust gives the parts a real wooden look and feel.
Modified existing and new materials are rapidly coming into the market. Therefore, the above materials list does not claim to be complete but only provides a cross section of the materials available today.
When 3D printing was first introduced in the market, it was exclusively utilized for prototyping purposes across many industries who develop and make consumer and industrial goods. Its main purpose was to reduce the lead time and cost of developing new parts and devices, which to date could only be done with subtractive production methods like CNC milling, turning and precision grinding. While prototyping is still the predominant application for 3D printing, in recent years this technology has evolved into a manufacturing method for the production of small lots of custom-engineered components.
Because of its many advantages (faster product development, greater design flexibility, easier customization, creation of complex geometries, shorter lead times) additive manufacturing is increasingly used in industries like:
- 3D printed custom components like special seats
- Tooling - 3D printing allows creating tooling at a fraction of the cost of conventional production methods
- On-demand manufacturing of spare parts, especially for vintage cars
- Volume production of standard components. This applies especially to luxury vehicles, where production runs are relatively small.
- Fuel nozzle for the GE LEAP jet engine. The number of parts for this component could be reduced from 20 to 1
- Cabin interiors, for example, wall panels, lattice structures, spacer panels, etc.
- 3D printed antennas
- On-demand manufacturing of spare part
- Medical & dental engineering
- 3D printing of patient-specific prosthetics for legs, arms and hands
- Implants required for reconstructive surgery like cranial plates
- Joint replacements like knee and hip
- Molds for teeth aligners
- Crowns and bridges
- Industrial goods
- On-demand production of tooling
- Bearings, heat exchangers, brackets,…
- On-demand production of spare parts
- Consumer goods industry
- Spectacle frames
- Thermoplastic soles for running shoes
- Customized shaver handles
- Custom jewelry
- Civil engineering
- Printing of models of individual buildings or, even complete neighborhoods
- Musical instruments
- Printing of complete instruments like saxophones
- Other industries
- Fire arms, tooling, robotics, soft sensors and actuators, etc.
AM has moved way beyond just prototyping. It is being widely used for the creation of customized parts like teeth aligners, implants for reconstructive surgery, etc., but also for the production of small volumes of standard components, for example, in jet engines and airplane fuselages.
With great improvements in the speed of industrial 3D printers and the availability of more materials, 3D printing is about to become a viable technology for the volume production of standard parts with production volumes of up to 100,000 pieces annually.
Most of the time 3D printed components contain certain imperfections and require further refinement, when coming out of the 3D printer. In conjunction with 3D printed components the following imperfections must be dealt with:
- 3D printed parts are built up on a build plate, layer by layer. The first layer adheres to this plate, and once a part is completed, it must be separated from the plate. This can sometimes be problematic, because, especially in case of metal printing the components are essentially welded to the build plate.
- The printing of complex components with pronounced “overhangs” requires the integration of so-called support structures, which prevent the component from sagging or collapsing during the printing process. After completion of the print, these support structures must be removed.
- The surface of components produced from plastic or metal powder with the MJF, SLS, SLM, DMLS or EBM method is usually covered with residual powder. Frequently, the powder can even be sintered onto the component surface. Any loose and sintered-on powder must be completely removed from the component surface.
- Depending on the printing method and the material, the components can have a very high initial surface roughness of Ra = up to 25 µm as compared to cast and forged parts with a roughness of Ra = 3 – 8 µm. Frequently, the layer-upon-layer building process creates a so-called “stair stepping” effect making the high initial surface roughness even more pronounced. Most of the time this is not acceptable and must be corrected for downstream manufacturing or final use of a component.
- Sometimes, especially in case of consumer goods, the 3D printed parts must be dyed with a special color.
The various steps required to refine a 3D printed part are defined under the term “Post Processing”.
Generally, post processing involves the following tasks:
Removal of the 3D printed component from the build plate. For metal parts this may require the use of a bandsaw or wire EDM. For plastic components frequently a spatula may be sufficient.
After unpacking, excess material must be removed from the build plate so that the plate can be re-used for the next print.
- Removal of support structures
After completion of the print any required support structures must be removed without damaging the actual component. The supports of plastic components can be separated chemically, mechanically or a combination of the two. The supports of 3D printed metal parts can be removed electrochemically, mechanically, or a combination of the two.
- Powder removal (de-powdering)
This only applies to powder based printing technologies like MJF, SLS, SLM, DMLS and EBM. Frequently, residual powder can just be blown-off from the component surface by air. However, if the powder is sintered to the surface, somewhat more aggressive cleaning methods must be applied, for example, shot blasting, but also mass finishing.
- Surface smoothing, polishing
Surface refinement can be one of the more challenging post processing tasks. Especially, if a part with an initial surface roughness of Ra = 40 µm must be polished to Ra = 0.1 µm or lower. Surface smoothing may require multi-stage processes. For example, an initial cut-down can take place in a shot blast machine, followed by a smoothing and polishing stage in a mass finishing system.
- Dyeing of 3D printed component with a specific color
Placing a color on 3D printed parts requires a special dyeing process that offers a big color scale, high water resistance, good abrasion resistance, scuff resistance and, above all, no color fading.
The short answer is no! The applicable post processing technologies depend on the used printing technology, the printing material and the component type.
There are definitively post processing technologies that can fulfill more than one task. For example, shot blasting can be used for powder removal and the initial smoothing of extremely rough surfaces. It might even be usable for removal of light support structures or unpacking. But this depends entirely on the condition of the raw 3D printed component, the printing technology and the printing material. Likewise, mass finishing can sometimes be utilized for de-powdering or removal of light support structures. But its primary strengths are deburring/edge radiusing, surface smoothing and polishing.
Determination of the right post processing technology, or better, mix of technologies, will most likely require a series of processing trials. It might even demand a review of the initial product design and selected printing method. For example, a minor design change might help minimizing the support structure requirements. Or, choosing a different printing method might reduce the initial surface roughness of the work pieces.
Therefore, the above question should be re-phrased as follows:
What is the most effective and most economic combination of post processing technologies for a given component or group of 3D printed components?
Shot blasting and mass finishing are by far the most versatile surface refinement and surface improvement technologies available in the market. As such they play a key role in numerous post processing tasks for 3D printed components. Their applications range from unpacking (de-powdering) to high gloss polishing.
Shot blasting is a technology for general surface cleaning, surface homogenization and surface preparation for coating or painting. In its special form, known as “shot peening”, shot blasting is also used to improve the fatigue life of components by inducing a compressive stress in the component surface.
In conjunction with additive manufacturing shot blasting is also applied for reducing the initial high surface roughness of a work piece coming out of the printer.
Shot blasting is mainly used for the following post processing tasks:
After having been separated from the build plate, powder-based AM components created with the SLS, Multi Jet Fusion, SLM, DMLS or EBM technology can be covered with loose and sintered-on powder residues. Shot blasting is an excellent method to clean the components and remove all residual powder.
- Surface homogenization:
Shot blasting creates a highly homogeneous, even surface profile. Such a profile can be required for functional reasons, but also for cosmetic purposes.
- Initial surface smoothing:
Shot blasting is often utilized for the initial cut-down of the high surface roughness of 3D printed components. With fine blast media shot blasting can achieve surface roughness readings of Ra = 0.5 to 0.8 µm. For further refinement of these readings, for example, a high gloss polish, subsequent mass finishing stages will be required.
- Surface preparation for coating or painting:
To ensure a good bond between the substrate and the coating or paint, the work piece surface must be textured. Shot blasting is an excellent tool for surface texturing.
Mass finishing is a universally applicable technology for deburring/edge radiusing, cleaning (for instance, de-oiling), surface grinding, surface smoothing and high gloss polishing. It can be used for any work pieces, irrespective of their shape, size and the material they are made of.
Mass finishing allows not only the treatment of external surfaces but also the finishing of internal work piece passages, undercuts, drilled holes, etc.
The following post processing tasks for AM components can be handled by mass finishing:
After printing AM components may have to undergo a machining operation, which leaves coolant and/or oil on the work piece surface. Mass finishing is completely removing these pollutants.
- Deburring/edge radiusing:
The printing process or a subsequent machining step may leave burs or sharp edges. Mass finishing not only removes the burs but also rounds sharp edges.
- Surface smoothing:
For functional and cosmetic reasons the work piece surface must sometimes be extremely smooth, some work pieces even require a high gloss polish. Mass finishing can reduce the surface roughness of a raw 3D printed component from Ra = 25 µm down to below 1.0 µm. Depending on the hardness of the material – cobalt chrome or titanium is tougher to treat than aluminum – this may require a two- or, even, a three-stage process.
- High gloss polishing:
With dedicated media the work piece surface can be further improved to a high gloss polish with surface roughness readings of Ra < 0.1 µm.
Absolutely! Consideration of all post processing aspects during the design phase is essential for the overall functionality and the cost efficiency of a 3D printed component. This “integrated” approach is an absolute pre-condition for the success of a product.
For example, the design must ensure that the thickness and the number of support structures are kept as small as possible. Likewise, the designer should explore the possibilities of making the support structures from a different material, which might facilitate their removal. It might even be possible to select a printing method that does not require any support structures.
The final surface finish must also be a consideration during the design phase. If the 3D component in question must have a polished surface with an extremely smooth surface, it might be worthwhile to choose a printing method that creates parts with a much lower initial surface roughness than others. For instance, Metal Binder Jetting produces much smoother initial surfaces than laser sintering or laser melting methods. And it has the additional advantage of not requiring any support structures.
The required mechanical properties and dimensional accuracy of a component must also be a consideration during the design phase: For example, the downside of Metal Binder Jetting is that it offers poorer mechanical strength, does not allow the creation of geometrically intricate parts and provides less dimensional accuracy with potential problems for post processing. The laser melting method (SLM, EBM) produces the most dense and, therefore, the strongest parts. But this can make the surface finishing of the printed components a bit more challenging.
These few examples show, how important it is to make post processing and printing method a major focus already during the design phase. Just preparing a CAD drawing and sending it to the printer is not sufficient and could lead to disastrous results.