Multi-material 3D printing
Multi-material 3D printing is the additive manufacturing procedure of using multiple materials at the same time to fabricate a workpiece. Similar to single material additive manufacturing it can be realised through various 3D printing methods such as fused filament fabrication, selective laser sintering, stereolithography and inkjet. By expanding the design space to different materials, it establishes the possibilities of creating 3D printed objects of different colours, that is multi-colour printing, or with different material properties like elasticity or solubility.
History
One of the first multi-material 3D printers for consumers, Fab@Home, became publicly available in 2006. The concept was quickly adopted by the industry followed by many consumer-ready multi-material 3D printers.Multi-material 3D printing technologies
Fused filament fabrication (FFF)
describes the process of continuously extruding a line of thermoplastic material to form a three dimensional model. The FFF process supports a variety of materials reaching from biodegradable ones like PLA to PETG, ABS and engineering-grade materials like PEEK. This technology additionally allows for the use of flexible materials like TPU.Some possible solutions to realise a multi-material filament printer are:
- Single-nozzle:
- * Filament changer: For using different materials or filaments in the same nozzle. Some well-known commercial examples are Prusa Multi-Material Upgrade 3 and Bambu Lab's Automatic Material System, and some open-source alternatives are Enraged Rabbit Carrot Feeder and BoxTurtle for Voron Design.
- Multi-nozzle:
- * Tool changer: One dedicated printhead per material or filament, with a mechanism for switching between the multiple printheads. Each printhead has its own hotend, as well as an extruder in the case it is a direct-drive extruder. Commercial examples include Bambu Lab Vortek, Prusa XL and Snapmaker U1, while open-source alternatives include for example Stealtchanger and Tapchanger.
- * Nozzle-changer: A single printhead can swap between different nozzles, thereby using the same hotend and multiple materials or filaments. A commercial examples includes Bondtech INDX.
- Separate gantries: Multiple printheads can run independently, like for example independent dual extruder in the case of two printheads, or independent quad extruder in the case of four printheads.
- Combinations of the methods above can allow for even more combinations of filaments, at an additional complexity
Single-nozzle design
Multi-nozzle design
The multi-nozzle design features a separate nozzle for each material. The nozzle can either be mounted on the same print head or on independent print heads. For this approach to work the different nozzles have to be calibrated to the exact same height relative to the print surface to circumvent the interference of an inactive nozzle with the printed object. Such a design reduces the amount of waste material during the printing process significantly compared to a single-nozzle design which does not use the previous material as infill or to print another object.Stereolithography (SLA)
is the process of solidifying a photopolymer with a laser layer by layer to form a three-dimensional object. To realize multi-material prints with this technology, one can use multiple reservoirs for different photopolymers. A major problem with this approach is the removal of the not yet polymerised material as the print may contain cavities filled with the old material, which should be emptied before the next material can be used. The photopolymer resins used for SLA can have highly different physical properties, generally being more brittle and having a lower heat deflection temperature. The SLA standard resins come in different colours and opacities. Besides the engineering grade materials like ABS-like or PP-like resin, there exists bio-compatible resins used for medical applications and flexible resins.Material jetting
The process of material jetting, often also called inkjet 3D printing, is similar to the 2D inkjet printing procedure. The print head consists of multiple small nozzles which jet droplets of photopolymers on demand. Each nozzle can extrude different materials, which allows for the creation of multi-material parts. The droplets of material are then immediately cured using an ultraviolet light source mounted to the printhead. In contrast to the filament FFF printing process, a layer is not formed by moving the printhead along a pre-calculated path, but by scanning the layer line by line. For example, the Stratasys J750 allows for full colour prints. The materials supported by the material jetting printing process are similar to the ones of the SLA process, and hence share similar properties. Additionally there have been advances in the field of material jetting metals by suspending nano metal particles in a fluid. After the removal of the support material the printed object has to be sintered to create a final metal part.Binder jetting
A binder jetting 3D printer uses particles of a fine-grained powder, which are fused together using a binder, to form a three-dimensional object. In principle, it consists out of two separate chambers: One functions as a reservoir for the powdered material, the other one as the printing chamber. To fabricate a layer of an object, a blade pushes the material out of the reservoir and spreads it over the printing surface to create a thin layer of powder. A print head similar to the one found in a 2D inkjet printer then applies the binder to the layer to solidify and bind it to the previous one. Although binder jetting does not allow for multi-material support, there exist printers, which feature a second print head to apply pigment to the layer after the binder to allow for full-colour prints.Workflow
Designing
Designing a three dimensional object is the first step in the workflow of 3D printing. This design process can be supported by software. Such computer-aided design software is capable of creating, managing and manipulating different 3D geometric figures while giving the user feedback through a graphical interface. Most CAD programs already support the annotation of a geometric figure with a material. The combination of different geometries then forms a single multiple material object. However, not all file formats support the annotation of materials together with the geometry of the object.Slicing
is the process of splitting a 3D model into layers to transform them into a sequence of G-code instructions. These instructions can be processed by a 3D printer to manufacture the corresponding model in either a bottom-up, top-down or even left to right manner. Before generating the instructions, support structures can be added to connect overhanging sections of the model to either the printing surface or other parts of the model. The support structures have to be removed in a post processing step after the print has finished.The slicing process for multi-material prints differs depending on the hardware used. For fused filament based machines, instructions for changing the material have to be added. This comes with multiple computational challenges such as handling two print heads at the same time without them interfering with each other, or clearing the melting chamber from the previous material. For stereolitography based multi-material prints, the slicing software has to handle the additional degrees of freedom arising from the possibility of moving the print from one resin tray to the next one. The slicing procedure for material jetting printers involves the generation of multiple bitmap images representing the voxels of the object.