SHAPE DEPOSITION MANUFACTURING
What is SDM?
Advantages of process
Disadvantages of process
WHAT IS SDM?
Shape deposition manufacturing (SDM) is a solid freeform fabrication (SFF) process which systematically combines material depostion with material removal processes. It is a layering process where prototypes are relatively identical with the final product.
This process is being updated at Carnegie Mellon University and Stanford University, where their mission is to enable the rapid manufacture of high-quality, complex designs which could not be practically fabricated with conventional manufacturing processes and also to take advantage of the vast, existing CNC milling machine infrastructure throughout the world by creating SFF processes which can be implemented by simply adding depostion apparatus to a CNC machine.
How design information is transferred to the machine/building system:
The design is created using a 3 dimentional CAD process where it is broken down into triangular facets. It is sent to the building device after a series of post-processing functions has been performed (ie. supports are created).
The design is created as a series of 2 dimentional "slices"; there is no standard format for which these slices are generally created. After post-processing (adding supports), the design is sent to the machining process.
Part layers can be classified into three different categories.
Layers without undercuts are the easiest to make using SDM. The primary part material is deposited and machined. Then the support material is deposited and the surface is planed. Then the whole layer is shot peened to compensate for tensile forces.
Layers with undercuts are a little more complex. The difference is that support material must be deposited and machined first. Then the primary material can be deposited. The cavity created by the machined support material makes up the shape of the layer of the part. Once the primary material is deposited and machined, the layer is shot peened.
Layers with and without undercuts simply utilize both processes.
In the figure below, the process is shown very clearly. For layer (a), it is all undercut and therefore the support material is placed first and then the part material is deposited. In layer (b), there are no undercuts. The part material is placed and machined. Then the support material is placed and planed. In layer (c), there are places that have undercuts and places that do not have undercuts. The support material to the left must go down first and be machined. Then the part material to the right (non-undercut) is deposited and machined. More support material is placed around that part material, and then the last of the part material is placed (undercut). Once the surface is planed and shot peened, that layer is completed. The layer are almost fused together. When a layer is added it melts onto the previous layer and bonds with it.
Note: Objects can also be embedded into the part material. This is done during the process of depositing the part material. Objects such as thermocouples or electronics can be deposited within the structure of a part. Also, heterogeneous parts can be made. Parts can be made easily with a multiple number of materials.
For the whole part, layer after layer) is completed until the full geometry is acheived. Then the support material is removed. This is usually done by melting the support material.
MATERIALS USED IN SDM PROCESS
Metals: These are generally created from steel, using copper as the supporting and sacrificial structures. Because of the high temperature gradients involved in the cooling process of metal components, shot peening is necessary to force the metal together as it solidifies.
Ceramics: Much of the research centering around the use of ceramic materials has been of using molten sand with a metal support/sacrificial structure. Ceramic parts created using SDM are used in high temperature conditions such as engine components.
Plastics: SDM techniques involving plastics include such materials as Photosensitive resins and such processes as Selective Laser Sintering.
The per part costs for the SDM process are not availiable. SDM is not a commercial system, but an experimental one, so that changes and revitions are constantly taken place.
ADVANTAGES OF SDM
DISADVANTAGES OF SDM