does. Tey either fuse polymer-bonded sand together or use inkjet technology to bond the sand. Te technology also allows for greater design flexibility, as the elimination of the tooling step re- moves some limitations from the process of achieving the desired geometry. Production-wise, sand printed cores and molds are available in a variety of molding materials and binders, includ- ing organic and inorganic, and can be used with most metals, including aluminum, magnesium, copper-base, iron and steel. Suppliers of sand print- ing technology continue to research new materials and binders to use. Build volumes range from 2.6 x 1.6 x 1.3 ft. to 13.1 x 3.3 x 2.3 ft. For producing patterns, sand casting
is one area where SLA does not shine as brightly as the other additive methods, due to the patterns’ lack of rigidity. Tis has been improved in recent years, how- ever. SLS parts offer a surface finish that interlocks with sand grains, but these pat- terns also have limited durability. FDM parts withstand repeated use, but defects are a concern due to their porosity. Additional additive methods used
to produce patterns for molds and cores include solid ground curing and laminated object manufacturing. Solid ground curing involves building incre- mental layers of liquid photopolymer that are covered by a photomask and cured with a 2kW ultraviolet lamp. No support structure or post curing is necessary. Laminated object manufacturing offers a relatively low-cost, quick method using sheets of material, such as paper, plastic or composites, thermally bonded with a laser that scans the contours of each layer. Te excess material is later removed. Another rapid manufacturing ap-
proach for molds and cores is to computer numerical control (CNC) machined them from a block of bonded sand. It skips the patternmaking step for prototyping and short production runs, and allows design- ers to test a casting before creating the permanent tooling. Tis method offers a particular benefit for larger parts that can- not be produced in one piece using addi- tive RP equipment. In addition, robotically automated production lines can produce machined molds quickly.
A Diecasting Difference For diecasting, the options for
rapid manufacturing are machined tooling, laser-based die insert fabrica- tion and plaster molding. The rapid manufacturing method most often employed for diecasting
2014 CASTING SOURCE DIRECTORY
A die insert formed by the LENS process shows the complex designs possible. This insert features curved, irregularly shaped cooling channels.
Many rapid tooling methods are based on laser printing technology.
production is plaster molding. Depending on the required surface finish and accuracy of a diecast part, an RP-generated pattern can be used to create a rubber mold, which is then filled with plaster to form a mold the metal is poured into to produce a casting. Plaster castings often are used to eliminate hard tooling costs for parts with tolerances suited to this method, as well as for prototyping or testing. Tey
also are employed as a temporary substi- tute while the hard tooling is prepared. Rapid tooling for diecasting can be
created through the application of direct metal deposition technology, which is similar to fused deposition modeling. It uses a laser to melt injected powder metal and deposit it in a precise loca- tion. For die inserts, more methods have been developed, including direct metal laser sintering (DMLS), electron beam melting (EBM) and laser engineered net shaping (LENS). DMLS machines create a die insert
by sintering thin layers of powdered metal. EBM is similar to SLS or DMLS,
except it uses an electron beam to melt the powder. It can only be used on iron. With LENS technology, a laser cre- ates a molten pool of metal on an exist- ing metal substrate. Ten, metal powder is added as the work piece is moved through a programmed path, building layers to create the final piece. Laser-based RP methods for die
inserts allow for more complex designs but are limited in size and the life of the tooling, seldom exceeding 10,000 shots. CNC machining also can be used to
METAL CASTING DESIGN & PURCHASING 13
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