Three-dimensional printing

This molding method is a rapid prototyping method with the deepest research in the world, the most mature technology, and the most widely used. In 1984, the three-dimensional printing and forming technology was still in the laboratory research stage. In 1988, the first operational manufacturing system was commercialized. In 1989, American Chryster first applied this technique in engineering practice, but it did not receive enough attention until 1992. The trailblazer of this technology is the United States 3D System Co., Ltd., which is the world's largest and first RP manufacturer. The market share of the product is about 30%. The SLA series of products produced by the company are the first to dominate and form a monopoly market.

1) Process principle

Stereolithography (SLA - Stereolithography Apparatus), also known as stereolithography, photocuring and so on. The basic process principle (shown in Figure 4) is to use CAD to perform 3D geometric modeling of the required prototype, generate data files and process the surface model. The internal and external surfaces of the model are discretized with small triangular planes, and the STL (Stereolitho-graphy) file format, which is generally adopted by the rapid prototyping manufacturing system and is the default industry standard, is obtained. The model is cut by equidistant or unequal distance treatment methods to form a series of horizontal cross-section sheets parallel to each other from the bottom to the top, ie, the computerized model is cut into a series of cross-sections. The scanning line algorithm is used to generate the best path for each section piece including both the section profile path and the inner scan path. At the same time, the model is positioned on the forming system and the support structure is designed.

The slice information and the generated route information are used as a command file (CLI file) for controlling the molding machine, and a numerical control command is given to a molding machine at each level. The thinner the stratification, the higher the precision of the generated parts, and the purpose of using unequal thickness stratification is to accelerate the molding speed.

The laser beam in the laser molding machine is scanned according to numerical control instructions, and the liquid photosensitive resin contained in the container is cured and bonded together layer by layer. The curing process begins with the first layer of liquid on the working platform. After the first layer is cured, the working platform is lowered along the Z-axis by a certain distance (ie, the layer thickness, and considering material and process factors), so that a new layer of liquid resin is covered Above the solidified layer, a second layer is cured. This process is repeated until the final layer is cured and a three-dimensional prototype entity is created. The liquid photosensitive resin contained in the liquid storage tank will be solidified in a certain area to form a solidification point under the irradiation of a certain wavelength (eg, 325 nm) and an intensity ultraviolet laser. When the forming starts, the working platform is at a certain depth below the liquid surface, such as 0.05-0.2 mm. The focused laser spot is cured point by point according to computer instructions on the liquid surface. The resin that was not exposed to laser light after the completion of one layer of scanning was still liquid. Then the lifting platform drives the platform down one more layer. The highly just-formed layer is covered with a layer of resin and a second layer is scanned to form a new processing layer and is firmly connected to the solidified part.

For a molding machine that scans using a laser deflection mirror, since the laser beam is deflected, the focal length and the liquid spot size change when the laser beam is shot, which directly affects the curing of the thin layer. In order to compensate for changes in focal length and spot size, the speed of laser beam scanning must also be adjusted in real time. In addition, the scanning speed must also be adjusted in accordance with the thickness change of the material layer to be processed (layer thickness change) when making each thin layer.

2) System composition

The three-dimensional printing and forming system is usually composed of a laser, an XY motion device or a laser deflection scanner, a photosensitive liquid polymer, a polymer container, control software, and a lifting table.