The invention relates to the field of 3D printing of metal objects, also known as “additive manufacturing” or “rapid prototyping”. In 3D printing, a 3D object is created by building it layer by layer. The name “3D printer” in this disclosure should be widely interpreted as any system that generates a 3D object from computer data. There were several attempts to 3D print ceramic molds or shells and cast metal into them to create metal objects. Prior art approaches used jetting of a binder onto a powder bed to generate a ceramic mold, however molds were rough and weak and needed many steps to strengthen them. The traditional way of casting into a ceramic shell is known as investment casting, or lost wax casting. This process was used for thousands of years with excellent results; however, it is a labor intensive and slow process comprising of many steps. The current invention overcomes these problems by extruding premixed ceramic paste using a 3D printer to build a casting shell in a single step. One of the difficulties in direct extrusion is the “shear thinning” behavior of ceramic pastes. Another problem when using ceramic materials is shrinkage and distortion during drying. A third problem is the abrasive nature of most common ceramics such as alumina, silica, zirconia etc. Most ceramics are high hardness materials and even when ground to a very fine powder act as abrasives. The invention solves all these problems in an economical way and allows the fabrication of ceramic objects of arbitrary shape at high speeds and high dimensional accuracy. The invention is particularly useful for the creation of metal casting ceramic shells, similar to shell casting and investment casting. Shell casting uses sand with a phenolic binder to create a thin shell into which the molten metal is cast. It is limited to parts without complex internal structures. Investment casting, also known as lost wax castings, allows the creation of complex objects however the current process has many steps and the time to make a casting shell is long (5-10 days) because of the need to dry multiple layers of ceramic slurry.
A 3D printer creates a ceramic casting shell of high accuracy. Casting molten metal into this shell creates an accurate metal object. The ceramic shell is formed from a paste made from a low hardness ceramic, dried by freeze drying. To overcome the shear thinning behaviour of ceramic pastes a positive displacement pump is in close proximity to the nozzle.
Referring to
In order to avoid shrinkage and distortion during drying of object 7, freeze drying is used. This is a commercial process combining fast freezing at a very low temperature (about −50 degrees C.) followed by applying a vacuum to sublimate the water and slowly heating the drying part to accelerate the drying. Commercial freeze dryers suitable for current invention are readily available, their typically used to dry food. Well known suppliers are Labconco Corp (USA) and Harvest Right (USA).
When freeze drying is used, the process can be accelerated by keeping the area above build table 8 at a low temperature, preferably sub zero, by blowing chilled air at the nozzle or other means, e.g. placing the complete machine in a refrigerated chamber. It is possible to generate different resolutions from a single nozzle. The reason it is possible to generate a different deposited line width from the same nozzle size is that the deposited line diameter is a function of two parameters: flow rate and relative motion speed between nozzle and object. A slow motion and a high flow rate will create a thick line, as a large amount of material is deposited per unit length. A low flow and high motion speed will generate a thin deposited line, as needed for high resolution. The flow rate is controlled by the metering pump 5. While
Sometimes novel properties can be achieved by co-deposition of materials. For example, interleaved deposition of two reactive materials, such as a ceramic paste and an activator fluid. This can be handled by multiple nozzles. Also, several nozzles allow deposition of several types of ceramic pastes without needing to clean the nozzle.
The details of the nozzle and pump are shown in
The preferred embodiment uses a vane pump but many other types of positive displacement pumps can be used, such as a swash-plate pump, a progressive cavity pump, a peristaltic pump or any other type of small flow high pressure positive displacement pump.
When the shell 7 has horizontal surfaces a support structure 26 is usually needed. The preferred method uses a separate feeder, pump and nozzle for the support material. The art of automatically designing the supports is well known in 3D printing. It is desired that the ceramic materials used for the shells should have the following properties:
It was found that to meet these requirements it is best to use material with a Moh scale hardness below 6, and preferably below 3. It was also found out that for best results the ceramic paste should comprise a base material, water, a binder and a lubricant. The binder can be organic, inorganic or a mixture of both. It was also found out that adding water increases be porosity of the shell after drying, which is highly desirable for a casting shell as the hot gasses created in the casting process can escape. The best base materials were found to be Talc (Moh hardness of 1), Kaolin (Moh hardness of about 2.5) and Magnesium oxide (Moh hardness of about 5.8). The best inorganic binders were Sodium Silicate, Potassium Silicate, Aluminum Oxide Hydroxide and Vee Gum (Magnesium Aluminum Silicate). The best organic binders were Arabic Gum, Poly Vinyl Acetate, Poly Vinyl Alcohol and water based Phenolics. The lubricant is added is small quantities to prolong pump life. Out of the large number of possible ceramic materials the following three gave the best results:
For Non-Ferrous Castings:
This ceramic is best under 1000 deg C. but for small castings that cool rapidly it can be used to 1200 deg C.
For Ferrous and Other High Temperature Castings:
For a Support Structure than can be Easily Removed after Drying:
Talc powder with water added to form a thick paste (no binders used), optionally add 1-5% motor oil as lubricant.
The above formulations can be used as casting shells right after drying or can be baked to increase strength. Baking is typically done at temperatures of 200 to 500 degrees C. Higher temperature baking can be used to further increase strength but some dimensional changes may occur.
When the system is used to make permanent ceramic parts harder ceramic materials such as alumina or Zirconia can be used, mixed with water to form a paste and about 1% to 10% of binder such as Poly Vinyl Alcohol. Such parts are sintered at high temperatures (1500 to 2000 degrees C.) after drying to achieve full strength. This process is well known.
The shells made had a wall thickness from 1 to 30 mm, however walls are build up similar to corrugated cardboard, with significant air spaces incorporated inside wall.
The internal inserts in the mold (to create cavities) can be built up with large voids in order to speed up clean-out and reduce cost as well as reduce drying times.
The 3D printing process allows adding periodic pinholes in the shell, which helps gasses escape. The surface tension of the metal normally prevents the molten metal from filling these pinholes. The molten metal entering the pinholes solidifies because of the low heat capacity of the minute amount of metal. The pinholes are typically from 0.1 mm to 1 mm. The shell can be made as one piece, similar to an investment casting shell, from several pieces similar to shell moulding or from several pieces plus inserted cores. The different pieces can be held together during the casting process or bonded together, after drying, with the same ceramic paste used to make them or a specialized bonding paste having a higher concentration of binders. Other ceramic adhesives can be use as well for bonding shells parts.
A complete fabrication system may comprise a 3D printer 10 generating a casting shell 7. Shell 7 is moved (manually or automatically) to a freeze dryer having a vacuum tight chamber. A fan can be used to blow chilled air from this chamber to 3D printer 10 before the vacuum is applied. After drying the shell can be moved (manually or automatically) to casting station. Filling shell 7 from bottom instead of top offers some advantages, as known in the art of foundry work.
Number | Date | Country | |
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62578488 | Oct 2017 | US |