What are the geometric processing problems in 3D printing?What are the geometric processing problems in 3D printing?

[Foreword] In recent years, the term 3D printing has gradually moved from strange to familiar to people’s lives and work. However, there are still many students who do not know enough about 3D printing. The author has written this article to briefly explain the 3D printing technology to our computer graphics students. The purpose of this article is not to explain the concepts and applications of 3D printing in detail. Many articles can be found online. Our focus is to explain some of the problems and opportunities that 3D printing brings to researchers engaged in computer graphics and geometric modeling and processing. Of course, the author’s understanding of some contents of this article is also very limited, and it is worth further study and communication. There is something wrong, and readers are requested to understand the correction.

I. Background: Traditional manufacturing process

How are the items used in our lives made? There are many methods and processes for manufacturing articles, and traditional manufacturing methods can be summarized as the following two:

1. Material manufacturing process. For example, casting is a metal thermal processing process. Liquid metal (such as copper, iron, aluminum, tin, lead, etc.) is cast into a cavity (called a mold) that is compatible with the shape of the part. The material can be sand or metal. Even ceramics), after it is cooled and solidified, the method of obtaining parts or blanks. Humans mastered this manufacturing process thousands of years ago. For example, the bronze vessels unearthed during the Spring and Autumn Period and the Warring States Period were manufactured by casting.

Another example is forging, which is a manufacturing process in which forging presses are used to apply pressure to metal blanks to cause plastic deformation to obtain forgings with certain mechanical properties, shapes and sizes. Mankind also mastered this manufacturing process thousands of years ago, which is commonly known as “iron-casting”. Generally, because forging can eliminate defects such as looseness in the as-cast state generated by the metal during the smelting process and optimize the microstructure, the mechanical properties of the forging are generally better than those of the same material.

The other is stamping, which is a forming process that applies external force to the plates, strips, pipes, and profiles by means of a press and mold to plastically deform or separate them, so as to obtain a workpiece (stamped part) of the desired shape and size. . Many items in life, such as the body of a car, the shell of a container, instruments, household appliances, office machinery, living utensils, etc., are stamped parts. Stamping and forging are both plastic processing (or pressure processing), collectively called forging.

Since these processing techniques change the material from one form to another in the process of processing an article, the material has not increased or decreased, so it is called an equal material manufacturing process.

2. Subtractive manufacturing process. Generally refers to the process method of processing parts on CNC machine tools. Turning, milling, and planing are four basic processing methods, including turning, milling, planing, and grinding. Different parts require different processing methods. Some parts require multiple methods to complete the processing of the parts. Because this processing technology removes excess material from the workpiece, the removed material is wasted (called waste), so it is called a subtractive manufacturing process.

Second, 3D printing is not mysterious: it is just a new type of manufacturing and processing technology

3D printing technology appeared in the late 1980s to early 1990s (also known as rapid prototyping technology), and it has been less than 30 years. The principle is simple: a technology that uses 3D digital model files as input, uses powder-like metals or plastics and other adhesive materials to print objects layer by layer to construct objects.

In terms of images, ordinary printers output 2D images or graphic digital files to paper through ink; 3D printers output solid raw materials (such as metal, ceramic, plastic, sand, etc.) as a thin layer (physical With a certain thickness), and then iteratively repeats layer by layer, and finally becomes a physical object in space. Therefore, when 3D printing outputs a certain layer, the process is similar to inkjet printing. Just like building a house, it is accumulated by brick, and 3D printed items are accumulated by the raw materials.

3D printing is also called additive manufacturing because materials are stacked layer by layer. 3D printing is not a mystery. Compared with a millennium-level material manufacturing process and a century-old subtractive manufacturing process, it is only a new process for manufacturing and molding, with a history of less than 30 years.

Comparison: What are the advantages and disadvantages of 3D printing compared to other manufacturing processes?

Compared with iso material manufacturing process and subtractive manufacturing process, 3D printing has many advantages, and many articles have been analyzed and explained in detail. The author believes that, compared with the traditional manufacturing process, 3D printing has the following three main advantages:

1. Design space is unlimited. For articles with complex geometric structures (such as articles with very complicated topological structures or cavity structures inside), traditional manufacturing processes cannot be processed, and the articles need to be disassembled and processed separately for reassembly. And 3D printing decomposes objects into layers of 2D areas, so there is no problem in processing any complex object, and the processing accuracy depends only on the smallest material particles that can be output by the printer. This is the biggest advantage that 3D printing brings us, allowing designers to design arbitrary complex geometric shapes with unlimited design space. It is this advantage that gives us a lot of problems to be solved in terms of geometric design and optimization, which will be detailed later.

2. Zero skill manufacturing. Traditional manufacturing process equipment is large and expensive and requires high skills to operate. While 3D printers (such as FDM 3D printers) are small and cheap, some have already entered the home and are simple and convenient to use; compared to expensive molds, 3D printing requires only a digital file to be molded. Therefore, through 3D printing, the personalized design and customization of products can be easily realized, which greatly shortens the product development time. This advantage has given our non-mechanical research workers the ability to carry out related geometric, structural, and material research, greatly deepening and expanding related research problems in manufacturing.

3. Unlimited combinations of materials. Multi-nozzle 3D printers can print multiple materials in combination. Through the stacking and combination of materials, printed items have physical and mechanical properties that are different from those of a single material. Therefore, through the combination of different materials, “new materials” with different properties can be produced. This advantage provides us with the control of the distribution of materials to control the physical, mechanical and structural characteristics of the article, which can produce a variety of articles and increase product flexibility.

In summary, the three most important advantages of 3D printing technology are speeding up the product development process, providing personalized and customized products, and increasing production flexibility. From the perspective of the molding process, 3D printing breaks through the traditional molding method. There is no need to first make molds and mechanical processing. Through the combination of rapid automatic molding hardware system and CAD software model, various complex shapes can be manufactured, which makes the product design and production cycle. It is greatly shortened, and the production cost is greatly reduced.

Of course, as a young molding process, 3D printing still has many shortcomings, such as slow molding time, low accuracy, few types of materials, and unable to mass-produce. At this stage, the actual use of 3D printing still belongs to the field of rapid prototyping, that is, the production of prototypes for enterprises before production of formal products, also known as the prototype in the industry. Therefore, the 3D printing molding process currently exists as a method complementary to the traditional manufacturing process, and it will take time to become a mainstream manufacturing technology. However, we must believe that the human pursuit of technology is infinite. With the continuous development of 3D printing equipment and printing materials, 3D printing technology will be more and more widely used.

From another perspective, 3D printing technology has moved manufacturing from the factory to the home, spawned a large number of individual designers (ie, makers), and inspired infinite possibilities for creative design. This is the author’s opinion that 3D printing technology can bring us the most significant meaning, which will be explained in detail later.


Fourth, positioning: related fields of 3D printing

3D printing is a new type of rapid prototyping technology, which integrates cutting-edge technologies in many fields such as digital modeling technology, electromechanical control technology, information technology, materials science and chemistry, etc., involving many fields.

The author believes that 3D printing, as a manufacturing process, mainly involves three aspects:

1. Material. That is, the materials used in 3D printing include resin, metal, ceramic, plastic or natural materials, etc. These materials are stacked to form real functional products.

2. Machine. 3D printing equipment uses materials (design data and production data) to achieve product molding. There are many types of existing 3D printing equipment, which will be introduced later.

3. Modeling. This is the software part of the 3D printing process. Including slicing, model construction and optimization, forming process control, etc. This is the software part of the 3D printing process.

In the first course on 3D printing done at the 2014 Siggraph Asia International Conference Course (link), the author put forward the 3M concept of 3D printing, namely material, machine, modeling, which is like three table legs. The desktop that supports manufacturing and its development (3M + 1M) are indispensable.

If you compare 3D printing to a dish, the material is the raw material of the dish, the equipment is the pot, and the modeling is the recipe and the production method. Modeling, as the “brain” of 3D printing, plays a vital role in the forming process. This is exactly the research work of our researchers who are engaged in computer graphics and geometric modeling. Therefore, computer graphics is an indispensable research area in 3D printing. In computer graphics, a large number of research papers on geometry, structural design and optimization have appeared in recent years. The author has also done a series of research work in this area, which will be described in detail later.


V. Introduction to commonly used materials for 3D printing

“Smart women are hard to cook without rice.” Materials are the material basis of 3D printing, and it is one of the bottlenecks restricting the development of 3D printing. The materials used in 3D printing mainly include engineering plastics, rubber, photosensitive resins, gypsum, metals, and ceramics. In the field of biological applications, there are artificial bone meal and cell biological materials. These materials are developed for 3D printing equipment and processes, and have different forms, such as powder, filament, lamellar, and liquid. For example, the particles of powdery 3D printing materials have a spherical shape with a radius of less than 100 microns.

The following is a brief introduction to some commonly used 3D printing materials (mainly from Medtec). More detailed information can be obtained online.

Engineering plastics refer to industrial plastics used as industrial parts or housing materials. They are plastics with excellent strength, impact resistance, heat resistance, hardness and aging resistance. Engineering plastics are currently the most widely used type of 3D printing materials. Commonly used are ABS materials, PC materials, and nylon materials. ABS material is a thermoplastic engineering plastic commonly used in FDM (fused deposition modeling) rapid prototyping process. It has the advantages of high strength, good toughness, and impact resistance.

The photosensitive resin is UV resin, which is composed of polymer monomer and prepolymer, and a photo (ultraviolet light) initiator (or photosensitizer) is added to it. Under a certain wavelength of ultraviolet light (250 ~ 300nm), it can immediately cause the polymerization reaction to complete curing. Photosensitive resins are generally liquid and can be used to make high-strength, high-temperature-resistant, waterproof materials. At present, the research of 3D printing technology of photosensitive materials mainly includes 3Dsystem Corporation and Israel Object Corporation. Common photosensitive resins are somos NEXT material, resin somos11122 material, somos19120 material and epoxy resin.

Rubber materials have the characteristics of various grades of elastic materials. The hardness, elongation at break, tear strength and tensile strength of these materials make them very suitable for applications that require non-slip or soft surfaces. 3D printed rubber products mainly include consumer electronics, medical equipment, automotive interiors, tires, and gaskets.

Metal materials: In the field of national defense, developed countries in Europe and the United States attach great importance to the development of 3D printing technology, and have invested heavily in research. 3D printed metal parts have always been the focus of research and application. The metal powder used for 3D printing generally requires high purity, good sphericity, narrow particle size distribution, and low oxygen content. At present, the metal powder materials used in 3D printing mainly include titanium alloy, cobalt chromium alloy, stainless steel and aluminum alloy materials.

Ceramic materials have high strength, high hardness, high temperature resistance, low density, good chemical stability, corrosion resistance and other excellent characteristics, and have a wide range of applications in the aerospace, automotive, biological and other industries. However, the hard and brittle nature of ceramic materials makes them particularly difficult to form.


Introduction to 3D printing equipment technology

There are many existing 3D printing equipment, and the equipment is designed in cooperation with materials. Here is just a brief list of some common 3D printing equipment (mainly from Medtec), more detailed information can be obtained from the Internet.

1. Layered solid molding process (LOM): This is the oldest 3D printing molding technology. LOM technology uses paper, PVC film and other materials, which are cheap and have high molding accuracy. The laser cutter will cut the film along the contour line of the workpiece. The liftable table can support the molded workpiece and reduce the thickness of a material after each layer is formed to feed a new layer of material to be bonded and cut. In the end, the hot-pressed parts will glue the film in the molding area layer by layer.

2. Three-dimensional light curing molding process (SLA): With photosensitive resin as the material, under the control of the system, the UV laser will scan the liquid photosensitive resin to make it solidify layer by layer. The liquid tank will be filled with liquid photosensitive resin first, and the ultraviolet laser beam emitted by the helium-cadmium laser or argon ion laser will be scanned line by point on the surface of the liquid photosensitive resin according to the layered cross-section data of the workpiece under the control of the computer. This causes a thin layer of resin in the scan area to polymerize and cure from a thin layer that forms the workpiece.

3. Selective laser sintering process (SLS): The SLS process uses powdery materials. The laser is used to scan and irradiate the powder under the control of a computer to achieve sintering and bonding of the materials. In this way, the materials are layered and layered to achieve molding. Firstly, a layer of powder is flattened onto the upper surface of the formed workpiece by using a pressure roller. The numerical control system controls the laser beam to scan and irradiate the powder layer according to the cross-sectional profile of the layer to raise the temperature of the powder to the melting point, thereby sintering and The formed parts below are bonded.

4. Three-dimensional printing process (3DP): The working principle is similar to the inkjet printer, and it has the same function as the SLS process.

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