Views: 15 Author: Site Editor Publish Time: 2021-10-25 Origin: Site
Rapid manufacturing china has developed rapidly. Many small and medium-sized enterprises have long relied on the production of simple castings with low added value, but are afraid of complex castings with higher technical difficulties. Because for complex castings, especially for thin-walled castings with complex shapes and produced in small batches, casting has always been a problem.
What's more, customers now have high requirements for short cycle and low cost, and traditional craftsmanship is difficult to handle. The emergence and development of rapid manufacturing provides a promising way to solve the above problems, so we should make good use of this advanced technology to serve the foundry industry.
Rapid manufacturing technology is an emerging technology that integrates computer-aided design, precision machinery, CNC laser technology and materials science. It uses the principles of software discretization and material accumulation to convert the CAD model of the designed object into a physical sample. Because RP technology uses the principle of transforming three-dimensional shapes into two-dimensional planes and layered manufacturing, it can handle arbitrarily complex components, and the more complex the components, the more it can reflect its superiority.
At present, a variety of rapid manufacturing methods have been studied at home and abroad. The more mature and practical ones are mainly LOM, SLA, SLS, FDM, and 3DP. Each of these technologies has advantages and disadvantages. The specific principles and characteristics are as follows:
1. Stereo Lithography (SLA):
SLA is formed by using ultraviolet laser to scan photosensitive resin, and the photosensitive resin is cured layer by layer under the action of laser. The SLA prototype can replace wooden molds in casting and wax molds in investment casting.
SLA technology has fast forming speed and high precision. Since the resin used in SLA is a thermosetting photosensitive resin, it cannot be lost by heating but can only be lost by burning. Therefore, the firing process is relatively high during investment casting, and the prototype is best made into a hollow structure. The SLA prototype is more sensitive to water, and it is easy to absorb moisture and deform in a humid environment for a long time, so pay attention to moisture. SLA's equipment and operating costs are relatively high, and it is suitable for manufacturing thin-wall fine-profile precision casting models.
2. Layered Entity Manufacturing (LOM):
The LOM process is a three-dimensional entity that is stacked layer by layer by laser cutting thin materials according to the contour information obtained by layering the parts. The main features of LOM are lower equipment and material prices, and lower operating costs. After the prototype is processed, it has good strength, high precision and no deformation. It can replace complex wooden molds in casting, and is especially suitable for single-piece or small batch production in small and medium-sized foundries.
3. Selective Laser Sintering (SLS):
The principle is: the laser beam scans the powder under the control of the computer, and scans the entity formed by the melting and bonding of part of the powder. The biggest feature of SLS technology is a wide range of forming materials. Plastics, ceramics, metals, wax powder, peritoneal sand and their composite materials can all be used as forming materials for SLS to prepare corresponding functional parts, metal parts, wax molds and sand molds (cores) , So the applicability is strong. SLS is currently the most widely used method in China to make wax molds for investment casting. It not only has relatively low cost, but also has high precision, good surface quality and no support. It can produce wax molds with extremely complex shapes to produce complex castings.
4. Fuse Deposition (FDM):
Fuse deposition liquefies the material and deposits it layer by layer through a nozzle to form parts. The forming materials include plastic filaments, wax filaments, etc., so plastic prototypes and wax molds are prepared. There is no expensive laser in this process, so the equipment is low in price, and because the material is completely melted, the strength of the part is high. However, the forming speed is slow, the precision of the parts is poor, the surface quality is poor, and the nozzle of the spinneret is prone to casting and clogging.
5. Three-dimensional printing (3DP):
The working principle of 3D printing is similar to that of inkjet printers. The materials of this technology include ceramics, metal/organic composite materials and so on. This technology has low cost, fast forming speed and high precision, but the surface of the part is rough, the strength is not high, and the print nozzle is easy to cause blockage. In casting, it is suitable for the verification and production of prototypes or the manufacture of plaster molds.
1. Investment casting:
The wax model obtained by SLS rapid prototyping can be directly used for investment casting to obtain metal parts. This kind of rapid prototyping process does not need to produce profiling, greatly shortens the production cycle of the blank, and can realize small batch production.
SLS not only makes wax molds fast, relatively cheap, and has high surface quality, but also the properties of the wax used are similar to those of high temperature wax or medium temperature wax in general casting. It can be realized by slightly changing the current baking process in investment casting. Roasting of SLS wax material is released. SLS wax molds are widely used in investment casting. They can be used to cast small precision castings as well as large complex castings. In particular, they can be welded with conventional low-melting wax modules to form composite wax molds, that is, they can be molded with ordinary wax materials. The simple part of the casting is made, and the complicated part is made with SLS, which greatly improves the production efficiency and reduces the cost of wax mold production.
Resin parts made by SLA method can also be used for investment casting. SLA resin parts have high strength and good surface quality, so they can be used to cast very fine small castings. However, due to the complex firing process and the requirement to use SLA special resin that is easy to burn, the manufacturing cost of prototype parts is also high, so it is not as widely used in China as the SLS method. FDM can also extrude wax, but the forming speed of FDM is slow and the surface quality is not high, so there are not many applications in this area.
2. Gypsum type and ceramic type precision casting:
Gypsum and ceramic castings have the advantages of high precision and low surface roughness values. There are many ways to use rapid prototyping to obtain cast ceramic or plaster molds:
(1) Use the prototype made by LOM, SLA, SLS, and 3DP as a master mold, and directly turn it into a ceramic or plaster mold. For parts with complex shapes and difficult to draft, you can first turn it into a silicone rubber mold. This method is characterized by low cost and can be produced in small batches, but this method is not suitable for parts with complex shapes and cannot be drawn;
(2) Similar to the method of investment casting, use SLS, SLA, 3DP or FDM to make wax molds, and then melt or burn out.
3. Sand casting:
Traditional sand casting generally requires the manufacture of wooden molds first, and the manual method of making wooden molds is often difficult to fully meet the requirements in terms of technology, production cycle, precision and material. The LOM model obtained by rapid prototyping can be directly used in casting production instead of wooden molds. Due to the special treatment, the paper mold is as hard as hardwood, and the surface is smooth, which is good for sand casting. It works below 200°C and can reproduce 50 to 200 sand molds.
Its advantages are low cost, fast manufacturing speed, high-precision patterns can be produced in a short time without high-level wood molders and corresponding woodworking machinery, and its advantages are particularly prominent for large and medium-sized castings with complex shapes. Such as engine block, cylinder head, intake (exhaust) air pipe and chassis transmission clutch housing, etc. can be manufactured by this method.
The replacement of wood molds by paper molds or resin molds greatly reduces the manufacturing cycle and cost of metal mold blanks, and the improvement of precision can reduce machining allowances and processing costs. SLS sintered coated sand can be used to directly form sand molds (cores) for casting. This method is economical and fast, especially for castings with particularly complex inner cavity runners, which have advantages that other methods cannot match.
