ADDITIVE MANUFACTURING

The SLS process, also known as selective laser sintering, uses powdered materials for forming. Powder material is spread across the surface of the already formed part and leveled; a high-power CO₂ laser scans the cross-section of the part on the freshly laid layer; the powder material is sintered together under the high-intensity laser irradiation to form the cross-section of the part, which bonds with the underlying formed section; after one layer's cross-section is sintered, a new layer of powder material is spread, and the next cross-section is selectively sintered.

The greatest advantage of the SLS process lies in its wide range of material options, such as nylon, wax, ABS, resin-coated sand (shell sand), polycarbonates, metal and ceramic powders-all of which can be used as sintering materials. The unsintered powder on the powder bed acts as a support structure for the sintered portions, thus eliminating the need for a support system (both hardware and software). The SLS process is closely related to casting technology; for example, sintered ceramic molds can be used as shell molds and cores, wax patterns can be used as investment casting patterns, and models sintered from thermoplastic materials can be used as lost foam patterns. SLS can also be employed to manufacture metal or ceramic parts.

SLM (Selective Laser Melting) is a primary technological approach in metal additive manufacturing. This technology uses a laser as the energy source, scanning layer by layer across a metal powder bed following paths defined in 3D CAD slice models. The scanned metal powder is melted and solidified to achieve metallurgical bonding, ultimately producing the metal part designed in the model.

The main difference between SLM and SLS technology is that SLM directly applies thermal action to the metal powder via the laser, without relying on binder powders. The metal powder is melted and solidified to achieve metallurgical bonding, resulting in metal parts of the designed structure. To achieve better melting of the metal, SLM technology requires a laser beam with high absorptivity by the metal. The advantages are that SLM uses pure metal powder, enabling formed metal parts to achieve a density close to 100%; mechanical properties such as tensile strength are superior to castings and can even reach levels comparable to forgings. The density, mechanical performance, and forming accuracy are all better than SLS. However, during the metal powder melting process in metal 3D printing, because the parts are often complex, support materials are needed. After the part is completed, the supports must be removed, and the surface of the part must be post-processed.

The 3DP (Three-Dimensional Printing) technology is similar to the SLS process, using ceramic or gypsum powders for forming. The difference lies in the fact that the powder material is not bonded by laser sintering of solid particles but by a binder printhead that jets transparent or colored binder along the cross-section path of the part, solidifying the powder. The powder in other areas serves as support, and then another layer of powder is spread, repeating the process until printing is complete.

3DP offers advantages such as fast forming speed, high production efficiency, and low material costs. Additionally, by adding pigments to the binder, colored prototypes can be produced. The forming process does not require supports, and removing excess powder is relatively easy, making it particularly suitable for prototyping complex internal cavities.
 

The working principle involves heating and melting filamentous thermoplastic material through a nozzle. The nozzle tip has a fine orifice (typically 0.2–0.6 mm in diameter). Under computer control, the nozzle moves along the X-axis while the build platform moves along the Y-axis, positioning at specified coordinates according to the 3D model data. The molten liquid material is extruded and eventually solidifies. After one layer is deposited, the build platform descends along the Z-axis by a predetermined layer thickness, and the material is extruded and deposited onto the previously cured layer, building the final product through successive layer accumulation.

FDM features low cost, fast speed, ease of use, simple maintenance, compact size, and no pollution. It significantly shortens product development cycles and reduces costs, enabling rapid response to market changes and customized customer needs. It is widely applied in fields such as industrial manufacturing, medical, construction, education, and consumer products.

Stereolithography (SLA) primarily uses photosensitive resin as the raw material, leveraging the rapid curing property of liquid photosensitive resin under ultraviolet (UV) laser beam irradiation. The photosensitive resin is generally liquid and undergoes polymerization and solidification immediately upon exposure to UV light at specific wavelengths (250 nm–400 nm). SLA focuses a UV laser of a particular wavelength and intensity onto the surface of the photocurable material, solidifying it in a point-by-point, line-by-line, and layer-by-layer sequence, thus completing the drawing of one cross-sectional layer. These layers are then stacked to complete a 3D solid print.

The working principle: A vat is filled with liquid photosensitive resin, which cures rapidly when exposed to the UV laser beam emitted by the laser. At the start of forming, the movable build platform is positioned just below the liquid surface by a distance equal to one layer thickness. The focused laser beam, following the equipment's instructions, scans the cross-sectional contour along the liquid surface; the resin in the scanned area cures quickly, completing one cross-sectional layer and forming a thin plastic sheet. The platform then descends by one layer thickness, and the next cross-section is cured, building up the layers to form a 3D solid object.