3D printing technology is a technology that directly obtains physical objects by stacking raw materials layer by layer through a nozzle based on a 3D slicing model. This technology benefits from its unique manufacturing method, greatly increasing the geometric freedom of the model, reducing dependence on manual labor, saving raw materials and time, and is an emerging technology with low energy consumption and low emissions. Easy to achieve cross-border integration and multi scenario overlapping applications with emerging technologies such as new energy, big data, and artificial intelligence, which constitutes an important support and innovative technology for the fourth industrial revolution. The application of 3D printing technology in the construction industry began in the late 1990s, attracting research and participation from numerous enterprises and universities worldwide. Up to now, more than 40 different printing technologies have emerged. The research on 3D printing technology mainly focuses on material ratio and construction methods, with little detailed discussion and classification on 3D printing technology.
1
Classification of 3D Printing Technology and Its Application in Prefabricated Buildings
In the 3D printing technology standards jointly released by the American Society for Testing and Materials (ASTM) and the International Organization for Standardization (ISO), 3D printing is divided into seven types: stereolithography, material injection, binder injection, powder bed melting, material extrusion, directed energy deposition, and thin material stacking, There are currently four main types of applications in the domestic and international construction industry, namely binder injection, material extrusion, powder bed melting, and directed energy deposition.
Material extrusion is the process of squeezing material through a nozzle and depositing it layer by layer. This method was invented by Crump, and due to its need to extrude printing materials from high-temperature nozzles for deposition, it is also known as melt deposition molding technology, which has been commercialized and gradually widely used in the construction industry and low-cost personal 3D printers. For example, the contour process technology developed by Khoshnevis, the six axis robotic arm printing technology called XtreeE developed by Gosselin et al., and the most widely used concrete printing technology.
Adhesive spraying is the opposite of material extrusion, where the adhesive serves as the ink and selectively sprays liquid adhesive onto each layer of material to bond the powder together and deposit the powder material layer by layer. This technology is characterized by fast forming speed, color printing without support, and is often suitable for printing small and high-precision parts. It was first proposed by Pegna in 1997.
Powder bed melting is a process of selectively melting metal material powder beds using laser and electron beams as energy sources. This type of technology is mainly applied to the manufacturing of metal and glass components in buildings, which is beneficial for reducing material waste and improving production efficiency.
Directional energy deposition is the use of thermal energy to melt materials and deposit them layer by layer, mainly used for printing large-sized metal components and repairing damaged metal parts. For the thin plate lamination process, Fabriconic has developed an ultrasonic 3D printing system that can be used in the manufacturing of metal components in the construction industry. However, due to its high manufacturing cost, it has not been successfully applied in the construction industry.
Practice has proven that the 3D printing technologies that can be successfully applied in the construction industry mainly include material extrusion, adhesive spraying, and powder bed melting. Table 1 summarizes the application of 3D printing technology in prefabricated buildings in chronological order, and introduces the characteristics of different technologies. From the development path of 3D printing technology, it can be concluded that the development process of 3D printing in the construction industry has shifted from non on-site to on-site and mobile, from single materials to multiple materials and composite materials, from printing walls to printing plastic, metal, and glass components related to buildings. There are both large gantry printing systems, as well as small robots and unmanned flight printing systems.
2 Application Scenarios
2.1 Design phase
3D printing has significant advantages in non-standard components, with low production costs and high efficiency, providing a solution for personalized design of prefabricated buildings. Under traditional manufacturing technology, structural complexity is positively correlated with manufacturing costs, so product design is largely limited by manufacturing level. Architectural design often has a single form, and 3D printing is conducive to promoting the transition from "design for manufacturing" to "manufacturing for design". 3D printing technology helps to establish a data sharing cloud platform that includes 3D printing, BIM, customers, and designers, promotes communication between customers and designers, and promotes the informatization and digitization of the supply chain. The printing process of combining 3D printing with BIM is: BIM -3D model - slicing - layer merging -3D printing - physical object. BIM provides data support for 3D printing, helping to provide precise spatial positioning information for the printing of prefabricated buildings, complete high-precision printing, and improve building visualization by combining augmented reality technology, thermal imaging technology, laser technology, etc. Developed information networks and a complete and comprehensive construction engineering information library can promote multi-party collaboration, ensure building design and construction, and improve supply chain operation efficiency, Promote the transformation and upgrading of the construction industry. In addition, 3D printing helps achieve topology optimization by changing shape and structure, utilizing fewer materials to achieve more functionality, ensuring building quality, reducing building weight, and empowering the building by filling gaps with other materials, such as insulation or sound insulation.
2.2 Printing stage
The 3D printing and assembly process of architecture is shown in Figure 1, which is mainly applicable to multi-story buildings. First, print the components in the factory, complete the foundation at the construction site, transport the components to the construction site for assembly, then locally pour concrete, and finally decorate the exterior facade. This process is similar to the construction method of prefabricated buildings. 3D printing can not only print single sided walls, but also exterior finishes and interior furniture components of buildings, ultimately resulting in finished buildings through assembly. In prefabricated buildings, 3D printing can also be used for printing component templates. Formwork is an important device and facility for the production of prefabricated concrete components, and its quality directly affects the formation and quality of the components. At present, steel formwork is a type of formwork widely used by component production enterprises, with high stiffness, small deformation, long service life, but high cost investment and low recycling rate. The FreeFAB wax template developed by Laing O'Rourke can print prefabricated component templates through smaller nozzles. This wax template can be quickly created with low accuracy, and can then be cut into precise shapes through milling technology. After use, the material can be heated, recycled, and reused. Empirical research shows that FreeFAB has reduced the entire mold manufacturing time by 6.5 times. Although wax templates are easy to obtain and have low costs, further exploration and demonstration are needed in terms of technology and materials. Whether they will deform like wooden templates after being heated and wet, and whether their stiffness and printing speed can support the production of large quantities of concrete components still need further research and exploration.
2.3 Construction Phase
At the construction site, there is often a problem of significant errors in component assembly, which often leads to the need for on-site replacement or even return to the factory for reconstruction. 3D printing can scan and print the required defective parts or fillers on-site, thereby reducing the time spent in manufacturing, construction, and on-site modification processes. MIT has developed a mobile digital building platform that uses rapidly extruded foam as a template for on-site design, scanning, and printing of required components. 3D printing can also print special sleeves, hangers, parts, etc. on site, and can also detect areas that require repair, and use 3D printing to repair or directly print damaged areas. For example, Siemens repairs gas turbine burners by milling damaged areas on the surface of the department and then printing new material layers in 3D to keep them running or meet new design requirements.
2.4 Logistics stage
The production speed of prefabricated building components is relatively slow, and it is often necessary to produce components in advance. The produced components are stored in the factory or construction site for use. However, building components, due to their large volume and easy damage, require strict protection during transportation, storage, and lifting, and have high requirements for logistics and storage. 3D printing allows for flexible production and reduces inventory pressure. By utilizing the on-demand production capability of 3D printing, manufacturers can timely produce the components required by customers, reduce inventory costs, and help the supply chain join new cooperative enterprises at any time, reduce the preparation and investment of spare parts inventory, and invest relevant costs in updated design to provide feedback on demand. In addition, due to the topology optimization ability of 3D printing, the weight of building components may be lighter, so the difficulty of transportation and storage protection may be reduced.
3 Development Trends of 3D Printing Technology
3.1 Non standardized buildings
The cost of concrete printing and prefabrication production can be reduced by 25% through empirical research comparison, and the cost savings mainly come from templates and labor. When printing single sided walls, the cost of concrete printing is higher. It can be seen that the complexity of components is directly proportional to the cost-effectiveness and productivity of 3D printing.
Krimi et al. used a French construction company as an example to discuss the construction speed and breakeven of prefabricated production and concrete printing. Empirical evidence shows that the production speed of concrete printing is higher than that of prefabricated production, and much higher than that of cast-in-place production. This is because 3D printing can achieve high automation and 24-hour continuous construction, but is limited to 8 hours of construction time per day. Therefore, the efficiency of prefabricated production is higher than that of concrete printing, and the time required for subsequent surface texture processing of 3D printed components is not calculated. Finally, the breakeven point between 3D printing and traditional construction methods is calculated based on the cost. The cost of 3D printing is constant, while the cost of traditional methods is inversely proportional to the production quantity. The former is suitable for small-scale production and complex component manufacturing, while the latter is suitable for large-scale production. Therefore, saving time and costs may not necessarily be the advantages of applying 3D printing in the construction industry, but personalized customization, complex structure manufacturing, and flexible production are the key advantages.
3.2 Stable and multifunctional ink materials
Mohan et al. summarized that the main problem with concrete printing currently lies in the stability and durability of materials. Further exploration is needed to explore materials with good tensile strength and ductility, while not losing the inherent flexibility of 3D printing. Anton et al. developed an automated 3D concrete printing and prefabrication platform for customizing cylindrical components through interdisciplinary collaboration and experimentation, which is used to produce complex structural column prefabricated components. It can provide a stable curing environment and achieve concrete printing of complex structures, with the aim of promoting the application of large-scale component 3D printing in prefabricated buildings. The results indicate that the durability of prefabricated columnar components printed on concrete still needs improvement. The component developed cracks after 10 months under outdoor conditions, proving that it is not suitable for direct exposure to outdoor conditions and has low durability. Moreover, the current geometric complexity of 3D printed concrete is still limited, and it is necessary to continuously improve the printing system, search for stable and efficient ink materials and printing speed. Although it has obvious advantages in printing complex structural components, when prefabricated columns are more conventional, it does not have the strength to compete with traditional construction methods.
New ink materials can empower buildings. Winsun uses recycled waste as ink materials, such as industrial solid waste, construction solid waste, mine tailings, coal gangue, etc., and is currently developing the addition of graphene to ink. Graphene, due to its excellent performance in optics, mechanics, and electricity, can absorb electromagnetic waves and sound waves, and has good thermal conductivity, conductivity, and waterproof properties. After experiments, it has been proven that adding 0.35 g of graphene to each liter of ink can reduce the demand for cement by half, greatly improve the strength, compression resistance, and bending resistance of buildings, and reduce the large amount of greenhouse gases emitted due to the heat absorption of concrete. Adding 0.8 g of graphene to each liter of ink will increase the waterproofing of the building by 400% and enhance its durability.
3.3 Improving laws and regulations
At present, the main problem faced by 3D printing construction enterprises and similar companies is the lack of acceptance standards and specifications for 3D printing buildings, and the relevant laws and regulations are not clear. The ink materials used in 3D printing buildings are similar to traditional concrete, but have significant differences in structure. Therefore, the acceptance standards for 3D printing buildings are different from traditional buildings, and require long-term exploration and research to achieve a mature and stable state of 3D printing construction technology. This process requires cooperation with universities and research institutions. From the current situation, as rural buildings do not require complex approval and acceptance, the majority of 3D printed building applications may be in rural areas in the short term.
4 Conclusion
The digitalization and automation level of the construction industry is one of the lowest, although BIM and prefabricated production improve construction efficiency and quality, reduce engineering costs, and improve construction safety. However, the production methods of templating and mass production have led to the simplification and standardization of buildings. Although 3D printing can achieve low-cost manufacturing of complex structures and meet personalized design requirements, its large-scale application in prefabricated buildings is still in the experimental and research stage, and there is no mature and reliable technology and process yet. The stability and durability of materials still need further research and exploration. This article summarizes the current development status and future directions of 3D printing technology in prefabricated buildings. It has considerable prospects and markets, but interdisciplinary research is still needed to study the use of multiple materials and processes for printing.
