Figure 1, Polyurathane resins, hardeners and the collection of parts made out of them
The materials involved in SDM fabrication of a biomimetic robot are classified into 5 categories: part material, support material, temporary material, dam material and embedded components.
The part materials are typically chosen from epoxy and polyurethane systems. The selection of the part material considers such factors as shrinkage, strength, flexible modulus, chemical stability, machinability, viscosity, gel time and curing time. The following figure shows the typical part materials we used and some sample parts made from them. Click here to see the technique information about various polyurethane materials. Click here to see the vendor list.
Figure 1, Polyurathane resins, hardeners and the collection of parts made out of them
The support material is typically chosen from different waxes. The most commonly used wax is blue wax supplied by Freeman Manufacturing & Supply Inc.. A proper candidate for the support material should have high chemical stability, good machinability, low shrinkage, and ease to remove from the part after the process. The following picture shows wax biomimetic robot fabrication. The current SDM facility is a HASS CNC machine which requires a wax palette size of 12"X12"X3". A DM2400 benchtop CNC machine is going to be used which requires a wax palette size of about 7"X7"X1.5".
Figure 2 Blue wax palettes used in polymer SDM
The temporary material is used to make internal geometry such as bearing
houses and tube connectors. It should be easily removed or dissolved after
the process in order to create the internal void in the part assembly.
The temporary material is especially useful for embedding mobile components
where the join should have a certain clearance. The typical temporary materials
are water solvable waxes, low melting point waxes. Some chemicals
such as PEG can be very easily dissolved in water. Various chemical solutions
have also been used to dissolve special types of polymers. The following
picture shows the solvable wax we are using for making internal voids.
Figure 3 Water solvable green wax used for making internal features
Dam material is used as a dam during casting to hold the liquid state
polymer material. The typical dam material we use is the clay that can
be found in a book store. The oil-based clay is chosen to avoid water contamination
to the part material. The clay and how it works is shown in the following
picture.
Figure 4 Clays used as a dam for polyurethane casting
The component material is something that is embedded into the SDMed part. The design of the embedded component and the embedding process should consider: 1) the component should not be not damaged by layer by layer machining process. 2) the component should have robust binding with the part material where the binding is required. 3) the component should keep required clearance from the part material where a free motion is required. 4)Part embedding always introduces a significant amount of bubbles. It is important to minimize the bubbling using various techiniques such as pre-vacuuming the wax palette and the component, cating under vacummn envrionment, and improving the surface finish of the embedded components. The typical embedded components include motors, valves, air cylinders, chips, and strain gage sensors. A collection of embedded components are shown below:
Figure 5 Example of embedded components such as optical fibers, sensors and actuators
While working on the multiple material system in SDM, we need to pay
a special attention to the interaction between different materials. First
of all, we have to consider if the combination of two materials will lead
to chemical reaction or contamination. Several combinations of material
systems have been tested and will be discussed in material selection section.
Second, we have to consider mechanical properties such as the strength
of the binding. This part is covered in material property test section.
Third, the choose of the material system should also consider the feasibility
and the sequence of the manufacturing process. Let us take an example of
depositing two materials: the soft polyurethane and the hard
polyurethane. Since the soft polyurethane does not machine very well, it
is necessary to range the process sequence in order to minimize the chance
for the software material to be contoured.
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Last updated: 11/22/1999