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Refractory powder - Al2O3
Al2O3 refractory powder is a material used in the sintering process of printed parts with Filamet™ metal filaments (The Virtual Foundry). These metallic filaments are formed by a very high percentage of metal (80 % - 95 %) and the rest by PLA. This high metal content does not increase the complexity of printing, since all Filamet™ filaments are used with the same printing configuration as the PLA, being ideal materials for any type of FDM 3D printer (industrial or desktop).
Al2O3 or alumina (aluminum oxide) is a white powder of great hardness that is commonly used to improve the resistance of clays and enamels. In the sintering process, it is normally used as a refractory powder due to its high melting point (2072 °C), much higher than the melting point of the main metals used by the industry (stainless steel, copper, bronze, high carbon steel, aluminum, etc.)
Once a piece is printed with one of the Filamet™ metal filaments, it is necessary to carry out the sintering process, in an open environment or in a vacuum or inert environment, to remove the binder polymer (PLA). For sintering the piece is placed in the crucible completely surrounded by the Al2O3 refractory powder, ensuring that the entire surface of the piece is completely covered and with some separation with the walls of the crucible. The product obtained after sintering is a totally metallic piece, with the real properties of the metal such as electrical conductivity, post-processing by sanding and polishing, or even welding union; but with a certain porosity and a reduction in volume due to the loss of PLA.
Depending on the metal that needs to be sintered and the type of oven used, different materials are needed. In an open oven, only Filamet ™ copper and bronze can be sintered in combination with Al2O3 refractory powder. In the case of using a vacuum or inert oven, any metal can be sintered using as Al2O3 refractory powder. The following table shows the necessary refractory powder for each metal and furnace, data verified by The Virtual Foundry to date.
To know more about the entire sintered process you should visit the "Tips for Use" section.
Sintered in open environment
Sintered in vacuum or inert environment
After printing, the pieces need to be sintered to remove the PLA that is part of Filamet ™. Sintering can be in an open environment (only for Filamet ™ of copper and bronze) or in a vacuum or inert environment (Filamet ™ of copper, bronze,316L stainless steel and others).
For sintering in an open environment a refractory vessel (crucible) and Al2O3 refractory powder is required. The process begins by sanding the rough edges of the piece for best results. Before inserting the piece into the crucible, a 1.25 cm layer of Al2O3 should be placed at the bottom of the crucible, then position the piece on the Al2O3 and in the center, without touching the walls. In the case of sintering several pieces at once, there should be a separation of 1.25 cm between them. Subsequently, the piece is covered with Al2O3, leaving a 1.25 cm layer on top, it is struck in a crucible so that the powder is distributed around the entire piece and is crushed so that it is completely compacted. The refractory powder is covered with coconut shell charcoal and crushed again. Finally, the crucible is covered with a stainless steel lid that covers at least 80 % of the last layer of coconut shell coal. Once the previous process is completed, the crucible is introduced into the oven and it is time to program the oven for sintering. From room temperature it rises to 205 ºC, from there to 815 ºC for bronze or 1010 ºC for copper, the temperature is increased 100 ºC / h. From these temperatures up to 870 ºC for bronze or 1060 ºC for copper, the temperature is increased 25 ºC / h. Once these temperatures have been reached, they should be maintained between 1.5 hours up to 4 hours depending on the size of the piece (pieces from 5 g to 10 g - 1.5 to 2.0 hours; pieces from 10 g to 25 g - 2.0 to 3.0 hours; pieces from 25 g to 40 g - 3.0 to 4.0 hours)
The user must take into account that these times and temperatures are indicative and may vary depending on many aspects, such as the oven model used for example. At the end of the sintering cycle, the oven door should be opened slightly (approximately 3 cm) until the oven temperature drops below 535 °C, then it must be fully opened and allowed to cool until the crucible can be operated in a safe way.
For sintering in a vacuum or inert environment, a crucible (cooking vessel) and refractory powder are needed: Al2O3 for copper, bronze and stainless steel. The piece is prepared for sintering by placing it inside the crucible and covering it with refractory dust, taking into account that there must be at least 10 mm of dust between the surfaces of the piece and the crucible. The crucible is then placed in the oven at room temperature. The temperature is increased to 205 °C in a time of 200 minutes. Then, in a 180 minute interval the temperature is raised to 400 °C. In a 180 minute stretch and depending on the material, the configuration of the next step will be to increase the temperature and maintain it for a certain time (see values for each material in the table). Finally, let the oven cool down to room temperature.
Both with sintering in an open environment and in a vacuum or inert environment the mechanical properties of the final product are directly related to the time that the printed part is maintained at the sintering temperature. If the final product is powdery and brittle, the sintering time has not been sufficient. If the print shows a surface similar to wrinkled skin, it is over sintered. Finally and as a general rule, during the sintering process the volume of the piece is reduced by 7 %.
Once sintered the piece can be sanded and polished in the same way as a metal but following a series of indications. With water sandpaper it is possible to eliminate the printing lines and other small deformations because the loose particles during sanding adhere to the gaps due to the heat of friction. In case of using 3M sandpaper or radial disc it is recommended to start sanding with a grain of 120 (80 for 3M radial disc), being careful not to deform the most delicate areas, such as corners. Once the entire surface has been sanded, a sandpaper of the next grain should be used and so on until it increases 6 or 7 (4 times for the 3M radial disc). Before moving on to the final polishing it is recommended to use a 3000 grit sandpaper, with which a certain gloss is achieved. Finally and once the piece is cleaned with a flannel cloth, the piece can be polished. TVF recommends using a rotating tool with a polishing disc and polishing wax to make polishing faster and more efficient. Simply apply a little polishing wax on the polishing disc and polish with constant movements throughout the piece so as not to generate excess heat, which can deform the piece. In addition to sanding and polishing parts manufactured with Filamet™, they can be carved, melted, welded and smoothed with heat application.
Refractory powder for sintering printed parts with Filamet™ metal filaments.