Comparison of several common rapid prototyping processes

FDM

The rapid prototyping process of filamentous material selective cladding (Fused Deposition Modeling) is a method of heating and melting various wire materials (such as engineering plastics ABS, polycarbonate PC, etc.) without relying on laser as a molding energy Referred to as FDM.

The principle of selective cladding of filamentary materials is as follows: under the control of the computer, the heating nozzle performs XY plane movement according to the cross-sectional profile information of the product parts, and the thermoplastic filamentary material is sent to the hot melt nozzle by the filament supply mechanism and is It is heated and melted into a semi-liquid state, and then squeezed out. It is selectively coated on the worktable and quickly cooled to form a thin layer outline of about 0.127 mm thick. After the formation of one layer of cross-section is completed, the workbench is lowered to a certain height, and then the next layer of cladding is carried out, as if the cross-section of the layer is "drawn", and so on, finally forming a three-dimensional product part.

This process also has a variety of materials to choose from, such as engineering plastics ABS, polycarbonate PC, engineering plastics PPSF, and mixtures of ABS and PC. This process is clean, easy to operate, does not generate garbage, and can be safely used in the office environment without the risk of toxic gas and chemical pollution. Suitable for conceptual modeling of product design and shape and function testing of products. The specially developed medical-oriented material ABS-i, because of its good chemical stability, can be sterilized by gamma rays and other medical methods, and is particularly suitable for medical use.

FDM-fused deposition molding schematic

Figure 1 Schematic diagram of FDM technology nozzle

The advantages of FDM rapid prototyping technology are:

1. The manufacturing system can be used in an office environment without pollution by toxic gases or chemical substances;

2. One-time molding, easy to operate and no garbage;

3. The unique water-soluble support technology makes it easy to remove the support structure, and can quickly build bottle-shaped or hollow parts and one-time assembly structural parts;

4. The raw materials are provided in the form of material rolls, which are easy to handle and quickly replace.

5. A variety of materials can be used, such as various colors of engineering plastics ABS, PC, PPSF and medical ABS.

The disadvantages of FDM rapid prototyping technology are:

1. The molding precision is lower than the advanced SLA process abroad, and the highest precision is 0.127mm

2. The surface finish of the molding is not as good as the advanced SLA process abroad;

3. The molding speed is relatively slow

SLA

Selective curing of photosensitive resin is a process using the principle of Stereolithography, or SLA for short, which is the earliest rapid prototyping technology.

The resin tank is filled with liquid photosensitive resin, which will quickly cure under the irradiation of the ultraviolet laser beam. At the beginning of the molding process, the liftable table is at the height of the next layer thickness of the liquid surface. The focused laser beam is scanned along the liquid surface under the control of the computer according to the requirements of the cross-sectional profile, so that the scanned area The resin is cured to obtain a resin sheet of this cross-sectional profile. Then, the table is lowered by the height of a thin layer, and the cured resin sheet is covered with a new layer of liquid resin for the second layer of laser scanning and curing. The newly cured layer is firmly bonded to the previous layer. This is repeated until the entire product is finished. Finally, the lifting table lifts out the surface of the liquid resin, removes the workpiece, and performs cleaning, removing support, secondary curing, and surface finish treatment.

The photosensitive resin selective curing rapid prototyping technology is suitable for the production of small and medium-sized workpieces, and can directly obtain resin or similar engineering plastic products. Mainly used for prototyping conceptual models, or for simple assembly inspection and process planning.

SLA－Photocuring schematic diagram

The advantages of SLA rapid prototyping technology are:

1. The surface quality is good;

2. The molding accuracy is high, the accuracy is 0.1mm (the domestic SLA accuracy is between 0.1-0.3mm, and there is a lot of fluctuation);

3. The system resolution is high.

Technical disadvantages of SLA rapid prototyping:

1. A special laboratory environment is required, and post-processing of molded parts is required, such as secondary curing, moisture-proof treatment and other processes.

2. Poor dimensional stability. Over time, the resin will absorb moisture in the air, causing warpage and deformation of the soft and thin part, which will greatly affect the overall dimensional accuracy of the molded part;

3. The life of the helium-cadmium laser tube is only 3000 hours, and the price is relatively expensive. Because the entire cross section needs to be scanned and cured, the molding time is longer, so the production cost is relatively high.

4. The type of materials that can be selected is limited and must be photosensitive resin. Workpieces made of such resins cannot be tested for durability and thermal performance in most cases, and photosensitive resins pollute the environment and cause skin allergies.

5. The support structure of the workpiece needs to be designed to ensure that each structural part made during the molding process can be reliably positioned. The support structure needs to be manually removed before it is completely cured, which is easy to damage the molded parts.

SLS

Selective laser sintering of powder materials (Selected Laser Sintering) is a rapid prototyping process, referred to as SLS.

Selective sintering of powder materials A carbon dioxide laser is used to selectively sinter powder materials (mixed powders such as plastic powder and binders), which is a rapid prototyping method that integrates three-dimensional solids from discrete points.

The selective sintering of powder materials uses a carbon dioxide laser to selectively sinter powder materials (mixed powder of plastic powder, ceramic and binder, mixed powder of metal and binder, etc.), which is a layer-by-layer integration of discrete points Three-dimensional solid process.

Before starting processing, first warm the working chamber filled with nitrogen and keep it at the melting point of the powder. During molding, the feed cylinder rises and the powder spreading roller moves. First, a layer of powder material is laid on the working platform, and then the laser beam sinters the solid part of the powder according to the cross-sectional profile under computer control, so that the powder melts and forms a layer of solid profile. After the sintering of the first layer is completed, the table is lowered by the height of a cross-section layer, and a layer of powder is laid, and the next layer of sintering is carried out, so that a three-dimensional prototype part is formed. Finally, after 5-10 hours of cooling, the parts can be removed from the powder tank. Unsintered powder can support the workpiece being sintered. When the sintering process is completed, remove the parts. The selective sintering process of powder materials is suitable for forming small and medium parts, and can directly reach plastic, ceramic or metal parts. The warpage of parts is smaller than that of liquid photosensitive resin selective curing process. However, this process still needs to scan and sinter the entire section, plus the studio needs to be heated and cooled, and the molding time is longer. In addition, due to the limitations of powder particle size and laser spot, the surface of the part is generally porous. After sintering the mixed powder of ceramics, metal and binder and obtaining the prototype part, it must be placed in a heating furnace to burn off the binder and infiltrate the filler into the pores, and the post-processing is complicated.

The rapid prototyping process of powder material selective sintering is suitable for the visual performance of product design and the production of functional test parts. Since it can be sintered with various metal powders for post-treatment such as copper infiltration, the products it produces can have mechanical properties similar to metal parts, but due to the rough molding surface, copper infiltration and other processes are complicated. So it needs to be further improved.

SLS-Schematic diagram of selective laser sintering

The advantages of SLS rapid prototyping technology are:

1. Compared with other processes, it can produce harder molds.

2. A variety of raw materials can be used, including engineering plastics, waxes, metals, ceramics, etc.

3. The construction time of the parts is short, and can reach 1in / h height.

4. No need for design and construction support.

The disadvantages of SLS rapid prototyping technology are:

1. There is laser loss, and a special laboratory environment is required, which is expensive to use and maintain.

2. Preheating and cooling are required, and post-processing is troublesome;

3. The molding surface is rough and porous, and is limited by the powder particle size and laser spot.

4. The processing chamber needs to be continuously filled with nitrogen to ensure the safety of the sintering process, and the processing cost is high.

5. Forming