Additive Fabrication, Applications, Technology

Metal Castings – Investment Castings

Additive technologies involve the use of injection molds which can produce components faster and at lower costs than the traditional use of subtractive technology. Additive technologies can be utilized as investment casting patterns. Casting methods are one of the first industrial processes developed by humans and have been utilized for thousands of years. The results can yield detailed and intricate results. One of the first materials used for the casting process was bees wax. This process is so adaptable that the forms of the bees have been used as patterns for producing detailed and stunning gold jewelry.

One of the modern applications for additive casting patterns is creating environmentally friendly and socially conscious jewelry. On the other end of the spectrum, applications for casting patterns have produced products that contain a variety of metals and can weigh several hundred pounds.

Additive casting patterns involve a thick coating or investing, which is a pattern that melts or burns out quickly as opposed to a material like ceramic, which doesn’t. A gate can be built into the pattern for allowing liquid materials to be poured into the mold. Passageways can be created to allow for hot air and melted and burned pattern materials to escape. Invested patterns can be placed into a furnace to be fired. This allows burn out or the pattern to melt and fuse the ceramic into a solid, hollow mold. At this point, molten metal can be poured into the ceramic mold and after the liquid metal cools and becomes solid, the mold can be broken, revealing the desired object. Excess material needs to be removed and the object will usually require substantial cleaning.

Additively-generated Patterns

Other types of casting can be created from additively-generated patterns. They can be created from thermoplastic extrusion using wax, plastics, 3D printing and inkjet technology that utilize wax-like plastics. These types of materials require being melted or burned very cleanly from the investment. Patterns created from these processes can be any size and range from tiny to several meters. The highest resolution produced for these products is from inkjet technology or 3DP. It can be used for creating large envelopes for industrial sized castings.

Another type of production for patterns for investment casting is stereolithography. The drawback of stereolithography is that the photopolymer materials used in this process are more difficult to burn out than materials in the other types of processes. They also have a habit of expanding and cracking the mold. In order to counteract these issues, 3D Systems has created a special build style called QuickCast. The improvements to the system included creating a photopolymer pattern built into thin, hollow sections which crumple during the burnout process. The lack of expansion results in less material to remove after the completion of the process.

Sand Castings

Sand casting is a process that begins by compacting fine and moist sand around a box-like framework constructed out of wood. At the end of the process, the pattern is removed from the sand which leaves behind a cavity that can be filled with molten metal. The metal cools and then hardens and is removed from the sand. The sand is then able to be recycled. This process also required the extra material to be removed from the finished product and clean-up performed.

Sand casting holds the option to skip the step of building a pattern mold. This can be advantageous if very few castings are required or if the patterns being produced are very expensive. It’s most beneficial during the early stages of a project before the final dimensions, as well as other parameters, have already been determined.

A system that does have size limitations for the molds it can produce is laser sintering. This system fuses polymer coated sand one layer at a time to form sand casting molds. This method has been coined DirectCroning by EOS GmbH.

A process that removes the use of additive fabrication altogether is offered by Clinkenbeard & Associates. In this process, large blocks of sand are created using a polymer binder. The blocks are then machined utilizing CNC techniques and diamond tools to create a mold for metal.

Large parts can be accommodated by ExOne and the German company Voxeljet Technologies. Their process involves machines that utilize a wide area inkjet which bonds layers of sand into core patterns and sand castings in a build chamber, weighing several tons. This allows large volumes of several cubic meters to be produced, similar to 3D printing developed by MIT.

Sand castings can also be produced by Laminated Object Manufacturing (LOM). The main US producer of this technology shut down a decade ago limiting the growth of this application. LOM is available from some service companies and can produce large parts similar to wood patterns.