Multi material 3d printing

3D printing with several filaments

Extremely long service life and outstanding resilience due to combined materials

What is multi-material printing?

Multi-material printing uses two different filaments to make one component in a single work step. This combines the advantages of two or more different materials in one component.
The filament that igus developed specifically for mechanical engineering is called «tribofilament». Filaments known by this name have outstanding, unique wear resistance specifications and very good coefficients of friction. However, other filaments have also been specially optimised for strength and rigidity. Multi-material printing can combine tribofilaments with other materials within a component so as to supplement the high wear-resistance with other mechanical and functional specifications.

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NEW: Rigid multi-material partner for iglidur® i190 and for structural components

igumid P190 fibre-reinforced filament

The new igumid P190 fibre-reinforced 3D printing filament was developed specifically as a partner material for iglidur® i190 for manufacturing especially strong components that are also low-friction in the multi-material printing process.

As a multi-material composition, it combines the wear resistance and sliding properties of the iglidur i190 tribofilament and the special stiffness of igumid P190 in a single component. The impressively high flexural strength (237 MPa) and flexural modulus (11.5 GPa) (printed flat, print lines aligned according to optimum strength, filling direction optimised), also makes igumid P190 suitable for structural component production.

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Multi-material 3D printing with up to four materials

Rapid production of multi-functional special parts in a single work step

A four-component printer allows tribofilaments to be combined with up to three other filaments. This gives rise to multi-functional special parts that are printed in 3D and thus manufactured quickly and economically in a single work step.

One possible combination is intelligent bearings for predictive maintenance: a fibre-reinforced housing, tribofilament at stress points and integrated sensors to warn of bearing overload.
Other application areas include abrasion-resistant components with integrated seals.

Do you have an application that requires multi-material components or questions about the igus multi-material 3D printing? We would be happy to help you implement your project.

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What kinds of multi-material combinations are there?

How and with what materials can iglidur tribofilaments be combined?

Many materials are compatible and can be combined in one way or another in two-component printing. Similar filaments (such a materials with the same base polymer or similar processing temperatures) can be firmly bonded: the materials meld with one another, connecting the component’s two «phases». This no longer functions when the difference between processing temperatures is too great: then one of the two materials may not remain dimensionally stable during processing or may even be subjected to damaging temperatures. To be absolutely certain of the connection, an interlocking design in which the two phases are combined so that they cannot be separated by non-destructive means is a good option.

Mechanical properties

Tribofilaments are combined with highly rigid or flexible materials. This allows the construction of very strong components — including ones made with the lightweight construction method — combined with the tribological advantages of iglidur filaments. The combination of iglidur filaments and flexible materials works very well in the manufacture of gripper fingers if the gripping surface has to provide a good grip, but the moving components have to slide.

Injection moulding combined with 3D printing

Moulded, 3D printed components, make it possible to implement geometries that are only possible with 3D printing in combination with the larger choice of available materials in the area of injection moulding, as all iglidur materials can be selected.

3D printing with smart functions

Multi-material printing allows speedy manufacture of wear-resistant polymers with an integrated sensor layer. This is how extremely durable special parts are created which issue a warning before overloading occurs or when the wear limit is reached to enable predictive maintenance. Find out more about 3D isense.

Where are two-component parts used?

Deflection lever

A component for deflecting forces; its great rigidity protects the joint from interfering influences. The bearing points operate especially smoothly and are durable and wear-resistant. A good solution here is a combination of a carbon-fibre-reinforced filament such as igumid P150 and a tribologically optimised material such as one of the igus iglidur tribofilaments.

Gripper

The dynamic requirements dictate that the gripper element be as light as possible, but with great flexural strength, and grip securely but gently. While the gripper’s body is manufactured with a fibre-reinforced filament, a flexible filament whose high coefficient of friction ensures a secure grip can be used for the gripping surfaces.

Customer application: two-component gripper in the packaging industry for cosmetics products

Lead screw support block

A shaft’s bearings should compensate for any angle errors and dampen shocks. The iglidur® plain bearing element can be encased with a flexible filament such as TPU with a Shore hardness of 95 A.

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Multi-material 3D printing

Upgrades for the greatest possible material and colour flexibility

A multi-material or multi-colour upgrade for your 3D printer allows you a flexible choice of colours and materials and thus infinite potential and more complex prints. By using different filaments, you can create more aesthetic and realistic 3D prints, create functional parts, and print models with breakaway or removable support structures.

Multi-Material Upgrades: 1 product

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BIQU Dual Colour Upgrade Kit, B1

• Dual printhead
• Bowden extruder
• Filament holder

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All You Need to Know About Multi-Material 3D Printing

Multi-material 3D printing is, as the name implies, 3D printing several materials in one continuous printing process. It is not the same as multi-color printing, where one material is printed in several color variations. Although the visual result of multi-material printing can be that it contains several colors, the basic aim of printing with multiple materials is to achieve a combination of their properties. Different materials have different characteristics, for example they could be conductive, hard, soft, transparent, chemically resistant, etc. and are therefore particularly suitable for specific applications. In some cases, however, it is necessary to combine the characteristics of different materials in one object in order to fully exploit its application, hence the existence of solutions for multi-material 3D printing.

Indeed, multi-material 3D printing is particularly useful when you want to combine two complementary material properties in one workpiece so that it is, for example, partially rigid and flexible. Multi-material 3D printing also makes it possible to print support structures in a different material that can be easily removed during post-processing. Examples of this include PVA and HIPS. Furthermore, in terms of post-processing, multi-material 3D printing can greatly reduce this, as end parts made of several materials can be produced in one go, making the assembly of individual, separately printed parts unnecessary (thus saving time).

Efficient multi-material 3D printing depends on the technology and hardware used. Although there is a lot of experimentation and constant progress in the field of multi-material 3D printing, not every process is suitable for printing multiple materials at the same time. Currently, multi-material 3D printing is possible with SLA, for example by using several vats with different materials, but also with material jetting (which by design is particularly suited for printing with multiple materials at once). In the latter, printheads are equipped with several nozzles that jet different materials using inkjet technology.

However, FDM is currently one of the most productive and advanced solutions for multi-material 3D printing especially for those who are new to additive manufacturing. Compared to other printing processes, FDM is cost-effective, simple and allows the processing of various thermoplastic polymers and composite materials. Depending on its configuration, an FDM printer can already print various materials. However, depending on the hardware version, there are a few things to consider, which we will discuss below. This guide therefore focuses on multi-material 3D printing for extrusion processes.

Although multi-material printing is possible with various technologies, it is most advanced with FDM processes (photo credits: Printing with Novojet technology from Quantica / Quantica)

The Multi-Material 3D Printing Process

Like any printing process, multi-material 3D printing starts with the design. Nowadays, most common CAD programs support the labeling of parts with different materials or the assignment of geometries to different materials. For successful multi-material 3D printing, it is then important to set the correct slicer parameters, as the printer needs to know when each material is to be printed. In some cases, corresponding instructions must be integrated into the G-code. However, the exact settings depend on which printer configuration is used for printing; whether it is a printer with one nozzle, whether there are several nozzles or an add-on. Let’s take a closer look at the different possibilities!

Single Printhead

It is perfectly possible to implement multi-material 3D printing with a standard FDM printer. However, if the printer has an extruder with only one hotend and one nozzle, manual material changes must be made. This requires the printer to interrupt printing as soon as a new material comes into play. Corresponding pauses can already be embedded in the G-code. Depending on the slicer, this step can be very simple or quite complicated.

In general, printing with several materials on a standard printer is very time-consuming and labor-intensive, even when small parts are involved. The process becomes more difficult if several materials are used per layer and the materials do not alternate layer by layer.

The different materials have to be taken into account as early as the design process and during slicing (photo credits: UltiMaker)

In addition to manual filament changing, there are already printers that provide for printing with multiple filaments. One example, which is mainly used in multi-color printing, is a mixing hotend. However, if this system is to be used for printing with several materials, the respective printing temperatures must be very close to each other – in the optimum case identical – to avoid printing errors.

If you use a printer with a Bowden extruder, you can use a classic Y-splitter to combine two different materials. With the Bowden system, the materials are used and extruded alternately, but are always “ready” so that the eponymous Y shape is formed. This approach is also used for two-color printing and can be extended to a dual Y-splitter for multiple extruders.

In addition to these hardware features, you also have the option of upgrading your printer with add-ons to print with multiple materials. With its MMU1, MMU2 and the latest MM3 multi-material upgrade kits, Prusa has created add-ons that allow printing with up to five different filaments. The MMU is simply attached to a single-material printer and a splitter-based Bowden extrusion system is used to print with multiple materials.

Prusa3D offers the possibility to print multiple materials with its Multi Material Upgrades (MMU) (photo credits: PrusaDd)

Another add-on is the Mosaic Palette. This additional device for FDM printers of all kinds cuts the filaments to size and reassembles them into a single, precise print strand. The filament can then be printed in a single pass without changing or pausing. There are now also suppliers who equip their printers with additional systems for multi-color or multi-material 3D printing. With its AMS (Automated Material System), Bambu Lab has created a system that no longer limits the user to printing with a single filament and allows for versatile, aesthetic and functional prints.

That being said, multi-material prints are much easier to achieve if there are several hotends in one print head. In this case, there is a corresponding nozzle for each material so that no material residue from the previous material clogs the nozzle. There are already single print heads with two to four hotends. However, these make the print head heavier, affecting the printing speed.

This print head has four filament feeders (photo credits: Harvard John A. Paulson School of Engineering and Applied Sciences)

Multiple Printheads

If the multiple nozzle approach for material extrusion is extended to several printheads, the variety of materials can be further increased. This approach also delivers better results, as the different materials do not come together in one heating block in the hotend. A well-known example of this is IDEX printers (Independent Dual Extruder).

With these printers, there are dual heads, each with a hotend, which are able to move independently to each other. The separate paths of the respective filaments prevent clogging or blockages in the nozzle and some of the most common errors in multi-material 3D printing can be ruled out in advance. However, an IDEX printer can only print two materials at the same time. These are often the base material and the support material.

One tool that pushes the limits of the dual extruder is the tool changer. This tool changer can change heads during operation. These are not only print heads, but also those that can cut, mill and drill (for hybrid manufacturing). In the case of printheads, however, this makes it possible to print different materials with several print heads, similar to IDEX printers.

IDEX printers are equipped with two independent extruders and can therefore process two filament types in one print (photo credits: Raise3D)

Applications, Benefits and Limitations

The core objective of multi-material 3D printing is to give an end part different properties. These can relate to the appearance, the material properties or the texture. For example, an end piece can have both glossy and matt parts or be equipped with filling materials.

In terms of its material properties, a multi-material print can be hard, heat-resistant, flexible and much more by combining the different materials that provide these properties. The surface can also be created specifically and have a positive effect on post-processing as well as the feel. By eliminating the need to piece together individual components in multi-material prints, a tedious post-processing step is even eliminated altogether.

The different, combined material properties also open up various application possibilities in a wide range of sectors. Multi-material 3D printing is used for microfluidic chips or for grippers in robotics and soft robotics, where flexible, soft and hard, durable parts are required. Multi-material 3D printing is also increasingly being used in medicine. Materials such as TPU (soft) and carbon fiber composites (hard) are often combined for 3D-printed prostheses. Success is not guaranteed with multi-material 3D printing, as caution is required to achieve the desired results.

Multi-material prints are used in many areas when different properties are required in a workpiece, for example hard and soft (photo credits: UltiMaker)

When designing the print object, the question arises as to how the different materials should interlock, for example whether they should overlap or join together in a zipper system. It is important to create both a form-fit construction and a “fabric-fit” construction. Some filaments do not combine well due to their different properties. If the processing temperatures are too far apart, for example, it will be difficult to process them together. Even if the materials are similar, like adheres better to like. To artificially improve adhesion, interface layers can be used to better bond the layers together.

The material properties also determine the printing properties. Different materials require different settings in the printer in terms of nozzle temperature, print bed temperature, speed and retraction. All these parameters must be taken into account when combining two or more materials. These regulations also have the highest error rate and therefore, depending on the hardware used, there are a few things to keep in mind. For example, if you are using a single extruder with only one hotend from whose nozzle all the materials used are extruded, it is necessary to ensure that the materials do not mix there. Otherwise, a backlog can occur and, in the worst case, the nozzle may have to be replaced. This is particularly the case if unusual materials such as wood or metal are used for printing. If several extruders are used, dripping from the unused nozzle can become a problem; this phenomenon is also known as oozing.

Manufacturers of Printers for Multi-Material 3D Printing

There is now a wide range of printers for multi-material 3D printing, including desktop solutions, industrial systems and various add-ons and upgrades, as outlined above. One of the first multi-material printers was the Fab@Home, which was launched in 2006. At the maker level, Prusa, Bambu Lab and Flashforge stand out with their solutions, as well as Raise3D and UltiMaker. The Flashforge Creator 4, the Prusa XL with its up to five materials and the E2 from Raise3D are particularly noteworthy.

Industrial solutions are offered by Modix with the Modix Big-120Z, OMNI3D with the Factory 2.0 and Factory 2.0 NET and WASP with the Delta WASP 2040 Industrial X, among others. AIM3D also focuses on multi-material 3D printing at industrial level and even combines pellets and filaments in its printing systems. Whichever solution you choose to implement successful multi-material 3D printing, it is essential to consider the material properties and printer settings in advance.

Bambu Lab’s AMS allows different filaments to be processed for multi-color or multi-material printing (photo credits: Bambu Lab)

Do you use multi-material 3D printing? Let us know in a comment below or on our LinkedIn , Facebook , and Twitter pages! Don’t forget to sign up for our free weekly newsletter here , the latest 3D printing news straight to your inbox! You can also find all our videos on our YouTube channel.

*Cover Photo Credits: Harvard University

Centrifugal multimaterial 3D printing of multifunctional heterogeneous objects

There are growing demands for multimaterial three-dimensional (3D) printing to manufacture 3D object where voxels with different properties and functions are precisely arranged. Digital light processing (DLP) is a high-resolution fast-speed 3D printing technology suitable for various materials. However, multimaterial 3D printing is challenging for DLP as the current multimaterial switching methods require direct contact onto the printed part to remove residual resin. Here we report a DLP-based centrifugal multimaterial (CM) 3D printing method to generate large-volume heterogeneous 3D objects where composition, property and function are programmable at voxel scale. Centrifugal force enables non-contact, high-efficiency multimaterial switching, so that the CM 3D printer can print heterogenous 3D structures in large area (up to 180 mm × 130 mm) made of materials ranging from hydrogels to functional polymers, and even ceramics. Our CM 3D printing method exhibits excellent capability of fabricating digital materials, soft robots, and ceramic devices.

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Introduction

Additive manufacturing, also known as 3D printing, is an advanced manufacturing technology to create complex 3D objects for a wide range of applications 1,2,3,4,5,6 . Beyond the conventional techniques of 3D printing with single material, it is desired to develop multimaterial 3D printing capability to manufacture heterogeneous 3D object where the volumetric elements (“voxels”) with different properties and functions can be precisely arranged in 3D space 7,8,9 . Only a few multimaterial 3D printing systems offer such capability by either selectively ink-jetting multiple micro droplets that are cured through photopolymerization 7,8 or developing a multinozzle printing head that generates continuous multimaterial filaments by high-frequency material switching 9 . However, the diversity of printable materials is constrained by the special rheological requirements of these techniques. The feature size and the size of multimaterial transition zoom are also limited by the manner of selectively depositing materials through printing nozzles (Supplementary Fig. 1).

Digital light processing (DLP) 3D printing is a high-resolution fast-speed additive manufacturing technology that forms 3D structures through digitalized UV irradiations that convert liquid photocurable resin to solid 3D structure. DLP can print a variety of materials ranging from hydrogel 10,11 , elastomer 12 , rigid polymer 13 , metal 4,14 , ceramic 4,15 to even functional materials that can be either shape changeable 16,17 , electric conductive 18,19 or self-healable 20,21 . Recent efforts have been made to significantly improve DLP’s printing resolution 4,14 , speed 22 and building size 23 . Despite the recent explorations on realizing multimaterial 3D printing capability for DLP 24,25,26,27,28,29,30 , most of the multimaterial switching process requires direct contact of solid wiper 24,25,26 or fluidic flow 27,28,29 onto the printed part which constrains DLP-based multimaterial 3D printing to small building size, limited available materials, slow speed, severe material contamination, and low function integration.

Here we report a DLP-based centrifugal multimaterial (CM) 3D printing method to generate large-volume heterogeneous 3D objects with multiple properties and functions through precise control on the spatial arrangement of each material voxel. Using the CM 3D printing system, we can directly fabricate a large-volume octet truss structure (155 × 108 × 57 mm) where the white and black units are alternatively arranged in space (Fig. 1a). As shown in the zoomed-in images, the CM 3D printing system is able to realize nearly zero material contamination during the multimaterial switching process so that the white and black units could be clearly printed. The CM 3D printing system can print more than two materials. Figure 1b presents a printed octet truss structure consisting of four colors (Supplementary Fig. 2) where the layers of white, black, light green and transparent units are stacked from bottom, and the units with four colors are alternatively placed in the top layer. The zoomed-in images confirm that the transitions between different materials are sharp, and no apparent material contaminations can be found. More importantly, the CM 3D printing system is suitable to print a wide range of materials with distinct properties and functions (Supplementary Fig. 3). Figure 1c presents a printed blood vessel system where the red blood vessels are embedded into a transparent hydrogel matrix. As time proceeds, the red “blood” in the blood vessels gradually diffuse into the matrix. Figure 1d shows a Kelvin foam structure where a soft polymer layer is sandwiched by two hard polymer layers (Supplementary Movie 1). Figure 1e demonstrates a printed Miura-origami sheet where the hard polymer panels are connected by the shape memory (SM) polymer hinges which allows the flat Miura-origami sheet to be programmed to a 3D shape (Supplementary Movie 2). Figure 1f presents a flexible ionic conductive (IC) octet truss consisting of an IC elastomer (ICE) core surrounded by the nonconductive soft polymer part (Supplementary Movie 3). Moreover, the CM 3D printing system is also capable of printing multiple ceramics. Figure 1g demonstrates a two-material Kelvin foam structure. After sintering process, the structure made of ceramic-polymer precursor is converted to a pure ceramic structure (Young’s modulus: 122.37 GPa, Supplementary Fig. 4). As summarized in Fig. 1h, the Young’s modulus of the materials (Supplementary Fig. 5) that are used in Fig. 1c–g spans in about seven orders of magnitude.

Results

Working principle of CM 3D printing system

Figure 2a illustrates the setup of the large-area CM 3D printing system that adopts “bottom-up” projection approach where digitalized UV light is irradiated from the UV projector, which is placed below the printing platform that moves vertically to control the thickness of each slice. Between the printing platform and UV projector, there is a glass plate that supports two or more polymer resin containers and moves horizontally to deliver a needed resin for the corresponding slice. More importantly, we add a rotating motor that spins the printing platform to remove residual resin sticking on the printed part during multimaterial switching (Supplementary Fig. 6). Figure 2b depicts the procedure to print a two-material octet truss. After a slice of the black part is printed in Step I, the printing platform lifts up from the black resin container (Step II). In Fig. 2c, it can be clearly seen that the black residual resin is sticking onto the printed part. In Step III, the rotating motor spins the printing platform to remove the residual resin. Figure 2d shows that the residual black resin is completely removed due to the centrifugal force. Then, in Step IV, the printing continues to complete the white part. Detailed printing and multimaterial switching processes can be found in Supplementary Movie 4. In contrast, if the spinning is not applied to the printing platform, both the printed structure and resin containers are badly contaminated (Supplementary Movie 5 and Supplementary Fig. 7). Moreover, as shown in Supplementary Fig. 8, the CM 3D printing system can even print multimaterial structures with all the channels are perpendicular to the centrifugal force direction. In Supplementary Fig. 9, we schematically illustrate the details on the process of removing residual resin via centrifugal force.

Inspired by mammals who dry themselves through body-shaking (Fig. 2e) 31 , we develop the CM 3D printing system that removes the residual resin during multimaterial switching by spinning the printed part with a high angular speed (ω = 1000~10,000 rpm). Compared with previously reported methods 24,25,26,27,28,29,30 , the method in this work avoids the direct contact between the printed part and the solid wiper 24,25,26 or fluidic flow 27,28,29 , and thus is applicable to print multimaterial structures with much greater area. As demonstrated in Fig. 2f, the residual resin on a printed part with a large area (180 × 130 mm) can be quickly removed within 10 s by spinning the printing platform with ω = 6000 rpm. Moreover, the proposed approach can remove residual resins with a wide range of viscosity. In Fig. 2g, we carried out experiments (details can be found in Methods) to investigate the effects of spinning time and speed on the thickness (h_R) of residual resin with viscosity measured at shear rate of 0.1/s ( \(_\) ) ranging from 0.065 to 6 Pa·s (Supplementary Fig. 10a). h_R decreases dramatically as the spinning proceeds (Fig. 2h). A higher ω leads to a faster drop in h_R (Supplementary Fig. 11a). In addition, h_R is independent of the initial area of the resin (Supplementary Fig. 11b), and the location where h_R is measured (Supplementary Fig. 11c). We use the needed duration ( \(>_<10<<<<<\rm<\mu >>>>>>m>\) ) after which h_R decreases to 10 μm to quantify the difficulty to remove that resin. In general, the resin with lower \(_\) has shorter \(>_<10<<<<<\rm<\mu >>>>>>m>\) (Fig. 2h). However, it should be noted that \(>_<10<<<<<\rm<\mu >>>>>>m>\) of the ceramic resin ( \(_\) = 6 Pa·s) is lower than that of the resin with \(_\) = 0.7 Pa·s. This is because the ceramic resin exhibits non-Newtonian behavior, and its viscosity at 1000/s is 0.57 Pa·s (Supplementary Fig. 10a). Based on the study on the flow of a viscous liquid on a rotating disk 32 , we develop a theoretical model that predicts the relation between \(>_<10<<<<<\rm<\mu >>>>>>m>\) and ω for resin with different η by the following equation: \(t=0.75\eta ^<\omega >^(^-_^)\) , where h₀ and h are the initial and current thickness of the resin, and ρ is density. Derivation of the equation can be found in Methods. Figure 2i implies that even for a highly viscous resin (10 Pa·s), \(>_<10<<<<<\rm<\mu >>>>>>m>\) can be less than 10 s when ω = 10 5 rpm. It should be noted that in the case of printing extremely soft hydrogels (Young modulus: 4 kPa), a high angular speed may lead to severe deformation or even damage of the printed part (Supplementary Movie 6). Thus, a moderate angular speed (less than 3000 rpm) should be used for printing soft hydrogels. In addition, we also conducted experiments to investigate the effect of printed patterns on the efficiency of removing residual resin. As shown in Supplementary Fig. 12 and Supplementary Movie 7, under the same spinning speed and time, the centrifugal force can also efficiently remove the residual resin stick onto complex patterns. In conclusion, the centrifugal force avoids the direct contact to the printed parts during the process of removing residual resin so that the CM 3D printing system can print multimaterial structure with much greater area (Fig. 2j, Supplementary Table 1) and higher printing speed (Supplementary Table 2), and is compatible with a wide range of material resins whose viscosity ranging from 10 −3 to 10 1 Pa·s (Supplementary Fig. 10b, Supplementary Table 1).

CM 3D printing of digital materials

To investigate the effect of spinning speed on the transition zoom between two materials, we print grid patterns consisting of orthogonal black lines and white squares. Figure 3a presents a 130 × 70 mm grid pattern board where the width of black line is 1 mm and the distance between neighboring black lines is 2 mm. To print such a large area two-material board, the maximum spinning speed that we could apply to remove residual resin is 6000 rpm above which the printing system shakes violently due to the uneven weight distribution of the printing platform resulted from assembly error. It should be noted that the violent shaking may also be caused during printing a large volume multimaterial structure whose weight is not evenly distributed in horizontal directions. This uneven weight distribution can be balanced by printing extra counter-weight parts (Supplementary Fig. 13). The transition zoom is about 150 μm (Fig. 3a) when a 6000-rpm spinning is applied for 30 s (details on the measurement of transition zoom can be found in Methods). To print a smaller area board with two-material, the maximum spinning speed can be increased to 10,000 rpm which reduces the transition zoom to about 100 μm that is smaller than that from other multimaterial 3D printing techniques (Supplementary Fig. 1). As shown in Fig. 3c, we can print a 130 × 130 mm letter where the black characters are clearly embedded into the white board. The CM 3D printer also enables us to design and fabricate digital materials where the mechanical properties can be tuned by controlling the spatial distribution of the hard and soft voxels (Fig. 3d). By increasing the content of hard voxels from 0 to 100%, the modulus of the printed digital material raises from 0.8 MPa to 1 GPa (Fig. 3e, Supplementary Fig. 14). The capability of printing digital materials allows us to use only two base materials to design and fabricate one single part that exhibits multiple mechanical properties at different locations (Fig. 3f). We further apply this unique capability to a four-dimensional (4D) printing demonstration (Fig. 3g–j) where the palm and five fingers of a hand are formed with different digital materials, and a layer of hydrogel is printed on the top of the hand (Fig. 3g, h). After placing the hand into water for 1 h, the swelling of the hydrogel layer drives the five fingers to bend to different angles due to the different modulus (Fig. 3i). The hand finally makes a fist after being placed into water for 6 h (Fig. 3j).

CM 3D printing of soft actuator with multiple sensors

The CM 3D printing system enables direct 3D printing a soft pneumatic actuator (SPA) where the bending, pressure and temperature sensors are seamlessly integrated (Fig. 4a). The entire SPA could be fabricated in a single 3D printing with five different polymers including stretchable elastomer, hard polymer, soft polymer, conductive hydrogel, and ICE (Fig. 4b, Supplementary Fig. 15, and Supplementary Table 3). Figure 4c presents the snapshot of the printed SPA with three sensors which connect to electric leads. The SPA bends to 80° upon 8 kPa inflation pressure (Fig. 4d). In Fig. 4e, the bending process leads to an increase in resistance of the bending sensor as well as a slight increase in capacitance of the contact sensor as the bladders of the SPA compress the pressure sensor. In contrast, the resistance of the temperature sensor remains constant. When a rigid obstruction blocks the bending of SPA, the rise in inflation pressure leads to the increase in capacitance of the pressure sensor due to higher contact force applied to the SPA (Fig. 4f). The increase in temperature leads to the decrease in the resistance of the temperature sensor but the increase in the capacitance of the pressure sensor (Fig. 4g). To fully demonstrate the utility of SPA with multiple sensing capabilities, we assembled three SPAs to produce a soft robotic gripper (Fig. 4h). The bending and pressure sensors response differently when the soft robotic gripper grabs nothing, a duck, and an orange, while the resistance of the temperature sensor is constant (Fig. 4i, Supplementary Movie 8). When the robotic gripper grabs a warm or hot object (Fig. 4j), the resistance of the temperature sensor varies correspondingly (Fig. 4k) which could be used to decouple the temperature effect on the pressure sensor (Supplementary Fig. 16).

CM 3D printing of ceramic-polymer structures

The CM 3D printing system also allows us to print heterogenous 3D structures consisting of ceramic and polymer. We prepare ceramic resin by mixing ceramic particles into acrylate resin which during 3D printing process converts into solid ceramic green body that could form robust interfacial bonding with acrylate elastomer part (Fig. 5a). In a uniaxial test on a hybrid specimen composed of elastomer and ceramic green body arranged in series, the specimen breaks on the elastomer indicating that the interface is stronger than the elastomer (Fig. 5b). The results of the 90° peeling tests confirm that the above conclusion as the measured interfacial toughness is about 1200 J/mm 2 and interfacial fracture is cohesive (Fig. 5c, Supplementary Fig. 17). Utilizing the strong interfacial bonding between ceramic green body and elastomer, we can print complex ceramic structure with overhang parts. To demonstrate this unique capability, we print a ceramic spider whose body is supported by a solid elastomer part (Fig. 5d). The sintering process removes the elastomer part, and leaves a ceramic spider with an overhang body. To further demonstrate the impact of our approach to manufacture engineering parts, we design a ceramic bearing where there must be empty space between the rollers and the inner/outer ring so that the bearing can rotate freely (Fig. 5e). To support these freestanding rollers, we design and print elastomer to fill the empty space (Supplementary Fig. 18). Sintering process removes the elastomer (Fig. 5f) so that the ball bearing could rotate freely without resistance (Supplementary Movie 9). Figure 5g demonstrates a turbine where the ceramic bearing connects the metal shaft and impeller. The impeller can spin at a high speed due to the low friction of the ceramic bearing (Supplementary Movie 10). The high thermal resistance enables the ceramic bearing to work at 650 °C (Fig. 5h), and its low thermal conductivity prevents the shaft from overheating (Fig. 5i).

Discussion

We report a DLP-based multimaterial 3D printing approach that utilizes centrifugal force to realize non-contact cleaning of residual resin induced by the multimaterial switching process, and allows us to generate large-volume heterogeneous 3D objects made of materials ranging from hydrogels to functional polymers, and even ceramics. The largest area of a printed two-material structure is 180 × 130 mm, and the lowest width of the two-material transition zoom is about 100 μm. The CM 3D printing system is suitable to print various photopolymers with distinct properties and functionalities. We demonstrate that it is an ideal manufacturing tool to create multimaterial multifunctional structures and devices such as digital materials, soft robot with seamlessly integrated sensors, and ceramic structure with freestanding parts by printing ceramic and polymer together. Our method substantially enhances the multimaterial 3D printing capability for creating multifunctional heterogeneous objects.

Methods

Materials

Structures in Fig. 1a, b, Fig. 2c, d, f, and Fig. 3a–c were printed using commercial photo-curable polymer resins including Vero white (white polymer), Vero black (black polymer), Vero clear (transparent polymer), ABS plus (green polymer). Soft polymer resin and elastomer resin in Fig. 1d, f, Fig. 3d–j, Fig. 4b and Fig. 5a, d were printed using commercial photo-curable polymer Agilus. Hard polymer resins in Fig. 1d, e, Fig. 3d–j and Fig. 4b were printed using commercial photo-curable polymer Vero white and Vero clear. All commercial resins were purchased from Stratasys Ltd. (Eden Prairie, MN, USA). Hydrogels in Fig. 1c, Fig. 3g–j, Fig. 4 mainly consist of acrylamide and poly(ethylene glycol) diacrylate (PEGDA) 10 . 1 wt.% red or blue pigment was added to the hydrogel resin for Fig. 1c or Fig. 4. 5 mol/L lithium chloride was added to the blue hydrogel resin in Fig. 4 to achieve ionic conductivity 10 . SM polymer in Fig. 1e consists of 70 wt.% tert- butyl acrylate and 30 wt.% aliphatic urethane diacrylate (AUD) 16 . ICE in Fig. 1f and Fig. 4 mainly consists of butyl acrylate (BA), PEGDA, and lithium chloride (LiCl) with following weight ratios (BA:PEGDA = 98:2, BA + PEGDA: LiCl = 90:10). Stretchable elastomer in Fig. 4 was prepared by mixing ratio of AUD and epoxy aliphatic acrylate 12 . Ceramic resin in Fig. 1g and Fig. 5 was prepared by mixing 1,6-hexanediol diacrylate (Bide Pharmatech Ltd., China), PEGDA and ZrO₂ ceramic powders (6.08 g/cm 3 , d₅₀ = 0.56 μm, Shenzhen Adventuretech Co., Ltd., China) with a weight ratio of 4:1:20. Blue ceramic resin in Fig. 1g was modified by adding 1 wt.% CoAl₂O₄.

CM 3D printer

The CM 3D printing system illustrated in Fig. 2a consists of a commercial UV projector (Wintech Digital System Technology Corp, San Marcos, CA, USA), a horizontally moving stage (LTS 150, THROLABS, Newton, NJ, USA) for switching resins, a vertically moving stage (LTS 150, THROLABS, Newton, NJ, USA), and a rotating printing platform that could quickly remove residual resin through centrifugal force (Supplementary Fig. 6a). The printing platform is connected to a rotation motor through a shaft. To ensure that the printing platform can precisely return to the position before spinning, a pair of permanent magnets is equipped to the other end of the shaft, and a pair of permanent magnets with opposite directions are attached to a clamping air cylinder. As illustrated in Supplementary Fig. 6b, after rotating the printing platform to remove residual resin, the two opposite magnets quickly clamp the pair of the magnets attached to the rotating shaft so that the printing platform quickly and precisely return to its initial position. We can adjust the CM 3D printing system into three different configurations for three different printing modes. For the high-resolution-small-area mode, a Pro4710 Wintech Digital projector is used as the light engine to directly project UV patterns with maximum area of 48 × 27 mm and optical resolution of 25 μm. For the low-resolution-large-area mode, two Pro 6500 Wintech Digital projectors are combined to directly project UV patterns with maximum area of 150 × 160 mm and optical resolution of 75 μm. For the high-resolution-large-area mode, we attach a Pro4710 Wintech Digital projector which projects scrolling images to two orthogonally assembled translational stages (LTS 150, THROLABS) which can quickly move the Pro4710 projector in x and y directions. In this printing mode, the maximum area is 180 × 130 mm, and the optical resolution is 25 μm. Detailed printing process for this mode can be found in Supplementary Movie 4.

Slicing approach

We design a heterogeneous 3D structure through a commercial computer aided design software (SolidWorks), and save the design model as assembly in.STL format so that different parts are described in the same coordinate system. The STL files for the assembly were then loaded into a self-developed slicing software programmed through MATLAB (MathWorks, Natick, MA, USA). The sliced two 2D images for each layer are arranged in the order that the horizontal stage follows to deliver the resin containers for printing the corresponding parts (Supplementary Fig. 2). When we printed the digital materials in Fig. 3, we generated the bitmaps for the soft and hard parts by randomly arranging the hard pixels with a given composition and the pixels that are not occupied by the hard material are filled by the soft material (Supplementary Fig. 14).

Measurements on the thickness of the residual resin

We investigated the effects of spinning speed and duration on the thickness of the residual resin on an inversely mounted commercial spin coater (VTC-200 vacuum spin coating machine, HF-Kejing, China) which allows us to accurately control the spinning speed and duration. We chose the Indium Tin Oxide (ITO) glass as the printing substrate due to its low surface roughness, and attached the ITO glass onto the center of the spinning disc of the spinning coater. Before turning on the spinning coater, we deposited a droplet of polymer resin (diameter: ~20 mm, thickness: ~1 mm) onto the ITO glass. The spinning removes most of the residual resin but leaves a thin layer on the ITO glass. In order to measure the thickness of such thin-layer residual resin, we photo-cured it in an oxygen-free environment to eliminate the effect of oxygen inhabitation on the thickness of the cured residual resin. Finally, we measured the thickness of cured residue resin films on a surface roughness measuring instrument (SURFCOM NEX, Tokyo Seimitsu Co., Ltd., Japan).

Rheological characterization

We measured the viscosity of all polymer resins on Discovery Hybrid Rheometer (DHR2, TA instruments Inc., UK) with a steel plate geometry (diameter: 20 mm). The tests were conducted with shear rate ranging from 0.1 to 1000/s at room temperature. The plate gap was set as 200 μm.

Measurement on two-material transition zoom

The data of grayscale transition graph was obtained by customized image processing technology (written in Python 3.6, www.python.org, with OpenCV 4.3.0, https://opencv.org). Each raw image photographed by Nikon Z7 under the same lighting and camera parameters. The region of interest (ROI) was obtained through binarization and opening operation (corrosion before expansion). According to the ROI, the sampling lines are equally divided in the X or Y direction. Then the gray value of raw image was traversed and recorded according to each sample line. Finally, all the recorded values are normalized.

3D printing and characterization of soft actuator with multiple sensors

We printed the SPA with multiple sensors by following the sequence as shown in Supplementary Table 3. The SPA was printed by using five different materials, and the layer thickness was set to be 100 μm. The variations on the capacitance and resistance of the SPA in Fig. 4 were measured on a precision LCR meter (TH2838H, Changzhou Tonghui Electronic Co., Ltd., China). The testing frequency was 1 kHz.

3D printing ceramic-polymer structures

The sintering of the composite structure of ceramic and elastomer was composed of debinding process and sintering process. The debinding process was carried out in a tubular furnace in argon at 800 °C for 2 h to decompose the resin. The sintering process was carried out in a muffle furnace in air at 1450 °C for 2 h. The 90° peeling tests were performed on a MTS universal testing machine (MTS Criterion, Model 43.104 Dimensions, USA) to measure the interfacial toughness between elastomer and ceramic green body.

Theoretical modeling

We develop a theoretical model that predicts the relation between \(>_<10<<<<<\rm<\mu >>>>>>m>\) and ω for resin with different viscosities based on a previous work 32 but the thin lay of liquid is attached to the bottom surface of the disc (Supplementary Fig. 19a). For simplicity, we make following assumptions: (i) the rotating disc is infinite and horizontal; (ii) the liquid layer is radially symmetric, and extremely thin so that compared with the effect of centrifugal forces, the effect of gravity is negligible; (iii) the liquid is Newtonian so that its viscosity is independent of shear rate; (iv) the radial velocity is so small that Coriolis forces are negligible.

We create cylindrical polar coordinates (r, θ, z) with the center of the bottom surface of the disk as the origin. The disc spins with a constant angular velocity ω. The initial thickness of the liquid layer is h₀ (Supplementary Fig. 19b). Since the liquid is a Newtonian fluid, the shear force τ along the z direction can be calculated as

$$\tau=\eta \frac,$$

where \(\eta\) is viscosity, and \(v\) is velocity in the radial direction. In Supplementary Fig. 18c, the shear forces on the upper and lower surfaces of an infinitesimal element can be \(\tau dS\) and \((\tau+d\tau )dS\) where \(dS\) is the surface area of the infinitesimal element.

Based on Supplementary Fig. 19c and d, the centrifugal force acting on the infinitesimal element is:

$$_=\rho <\omega >^rdSdz.$$

The total force is balanced in the radial direction: