Detailed 3D printing technology classification, understanding 3D printing process

               
Update:21.01.2019
               
Summary:

The professional name of 3D printing is “additive manuf […]

The professional name of 3D printing is “additive manufacturing”: Additive refers to the manufacturing method of 3D printing by depositing or bonding raw materials into material layers to form a three-dimensional entity. Therefore, Lipson and Kuman combine 3D printing into two categories according to material combination: (1) selective deposition method; (2) selective bonding method.

In addition, there is another classification method in the industry. According to the material form and process realization method, it can be subdivided into the following five categories: (1) high energy beam sintering or melt forming of powder or filament material, such as laser selection. Selective Laser Sintering (SLS), Selective Laser Melting (SLM), Laser Engineering Net Shaping (LENS), etc.; (2) Wire extrusion hot melt forming, such as fused deposition molding ( Fused Deposition Modeling, FDM), etc.; (3) liquid resin photocuring molding, such as Stereo Lithography Appearance (SLA), Digital Light Processing (DLP), etc.; (4) Molded body printing, printing three-dimensional (Three Dimensional Printing, 3DP) and the like; (5) sheet / plate / block material forming an adhesive or welding, such as laminated object manufacturing (Laminated Object Manufacturing, LOM) and the like.

The following is a brief introduction to the representative process.

1. Laser selective sintering SLS

SLS is also called selective laser sintering. The principle of the process is to pre-lay a layer of powder material (metal powder or non-metal powder) on the workbench. Under the control of the computer, the laser sinters the solid part powder according to the interface contour information. Then continue to circulate, layering and forming. The biggest advantage of the SLS process is that it can be used as a sintering target for a wide range of materials such as nylon, wax, ABS, polycarbonate, metal and ceramic powder. The unsintered portion of the powder bed becomes a support structure for the sintered portion, so that the support system need not be considered.

The disadvantages of SLS are: the prototype structure is loose, porous, and has internal stress, making variability; post-processing of ceramics and metal parts is difficult; preheating and cooling are required; the surface is rough and porous, and is affected by the size of the powder particles. Laser spot limitation; the molding process produces toxic gases and dust, which pollute the environment.

2. Fused deposition molding FDM

FDM is to melt and melt the filamentous hot-melt material, and the three-dimensional nozzle is selectively coated on the table according to the sectional profile information under the control of the computer, and rapidly cooled to form a section. After one layer is formed, the machine table is lowered by one height (ie layer thickness) and the next layer is formed until the entire solid shape is formed. There are many kinds of molding materials, generally thermoplastic materials, such as wax, ABS, PC, nylon, etc., which are fed in the form of filaments. The molded part has high strength and high precision, and is mainly suitable for forming small plastic parts.

Each layer of the FDM process is stacked on the upper layer, and the upper layer plays a role in positioning and supporting the current layer. When the shape changes greatly, the upper contour can not provide sufficient positioning and support for the current layer. This requires designing some auxiliary structures - "support" to provide positioning and support for the subsequent layers to ensure smooth forming process. achieve.

The FDM process does not use lasers, and it is easy to use and maintain, and the cost is low. Prototypes made with ABS are widely used in product design, testing and evaluation due to their high strength. The most obvious limitation of FDM technology is surface accuracy. Due to the semi-molten plastic extrusion, the surface accuracy is rougher than SLA, and comparable to SLS. FDM prototypes can be used for milling, drilling, grinding, lathe machining, etc. In order to compensate for the lack of surface accuracy and enhance the details of the features, when there are special quality requirements, the user usually performs secondary processing to enhance the details of the prototype.

3, light curing molding SLA

SLA is the earliest practical rapid prototyping process. It uses liquid photosensitive resin as the raw material. The principle of the process is to focus the laser to the surface of the photocurable material with a specific wavelength and intensity of laser, so that it can be solidified from point to line and from line to surface. Level drawing work, then the lifting table moves the height of one layer in the vertical direction, and then solidifies another layer. This layer superposition constitutes a three-dimensional entity. The advantage of the SLA process is that it has high precision and good surface effect. Therefore, it is mainly used to manufacture a variety of molds, models, etc. After the parts are finished and polished, the accumulation marks of the layers can be removed. SLA molded parts generally have a layer thickness of 0.1mm to 0.15mm. Years of research have improved the cross-section scanning method and resin molding properties, making the process the highest

The accuracy has been able to reach 16μm (0.016mm).

The limitations of SLA mainly include the need for support, resin shrinkage leading to decreased precision, and photocurable resin with certain toxicity. In addition, the photocuring process has high operating costs, low strength and inelastic parts, and cannot be assembled. The raw materials of the light curing process equipment are very expensive and there are not many types. After the parts of the light curing equipment are completed, they need to be cured twice in the curing chamber of the ultraviolet light to ensure the strength of the parts.

4, three-dimensional printing 3DP

The 3DP process was invented by Paul Smith, a student at the Massachusetts Institute of Technology, and his instructor, Professor Elesacs. This process is similar to the SLS process and is formed from powder materials such as ceramic powder and metal powder. The difference is that in the 3DP process, the material powder is not joined by sintering, but the cross section of the part is "printed" on the material powder by means of a nozzle with an adhesive such as silica gel.

3DP is a method using droplet ejection technology. The specific process is as follows: under the control of computer, the nozzle is selectively sprayed and bonded on a layer of powder material previously laid according to the information of the current layered section. The agent is used to bond a part of the powder to form a thin layer of cross section; after the upper layer is formed, the working platform is lowered in thickness, the lower layer is laid, and then the selected area is sprayed with a binder to form a thin layer and bond with the formed part. This cycle continues until the part is finished. The place that is not sprayed and bonded is dry powder, which plays a supporting role in the forming process, and is easier to remove after forming.

The advantage of 3DP technology is that it has a fast forming speed, no support structure, and can output color printing products. It can output up to 390,000 colors, and the color is very rich. It is also the 3D printing technology that is closest to the finished product in terms of color appearance. 3DP technology also has shortcomings. Firstly, the direct strength of powder bonding is not high, it can only be used as a test prototype. Secondly, due to the working principle of powder bonding, the finished surface is not as smooth as SLA, and the fineness is also inferior. Therefore, in order to produce sufficient strength. The product also requires a series of follow-up processes. In addition, the technology for manufacturing related material powders is relatively complicated and costly.

5, digital light processing molding DLP

The DLP process is a 3D object in which a photosensitive resin is solidified layer by layer by visible light using a light curing and projector DLP technique, and a 3D object is created by stacking layers from top to bottom.

It is very similar to the previous SLA light curing. The only difference is that the light of the SLA is concentrated to move on the surface, while the DLP is to hit the light on one surface, a bit like a projector, light curing layer by layer, so the speed Faster than the same type of SLA stereolithography.

At the same time, it uses a high-resolution digital light processing (DLP) projector to cure the liquid photosensitive resin, and each layer is cured by slide-like sheet curing. Based on this technology, an ultra-thin 3D printing layer can be realized with high molding precision, and it can match the durable plastic parts of injection molding in terms of material properties, details and surface finish.

6, layered entity manufacturing LOM

The LOM process is based on the contour of each section of the 3D CAD model. Under computer control, the command to control the laser cutting system is issued to move the cutting head in the X and Y directions. The feeding mechanism sends the hot-melt-coated foil (such as coated paper, coated ceramic foil, metal foil, plastic foil) to the top of the workbench in sections. The laser cutting system cuts the paper on the workbench along the contour line with a carbon dioxide laser beam according to the cross-sectional profile extracted by the computer, and cuts the uncontoured area of ​​the paper into small pieces. Then, a layer of paper is pressed and bonded together by a hot pressing mechanism. The lifting table supports the workpiece being formed and, after each layer is formed, reduces the thickness of the paper to feed, bond and cut a new layer of paper. Finally, a three-dimensional prototype part surrounded by many small scraps is formed. Then take it out and remove the excess scrap pieces to get a three-dimensional product.

LOM is suitable for large and medium-sized prototypes, with small warpage deformation, short molding time, long laser life, good mechanical properties, no need to design and manufacture support structure, suitable for conceptual modeling and function of product design. Sex test parts. Its advantages are: faster forming speed. Since it is only necessary to use the laser beam to cut along the contour of the object, it is not necessary to scan the entire section, so the molding speed is fast, and thus it is often used to process large parts with simple internal structures. The biggest disadvantage of LOM is the low material type, low performance of the parts, and its performance is close to the wood mold.

0