manufacturing offers a rela- tively low-cost, quick method using sheets of material, such as paper, plastic or compos- ites, thermally bonded with a laser that scans the contours of each layer. Te excess material is later removed. RP equipment manu-
facturers have developed SLS equipment that prints sand molds and cores, rather than plastic patterns, offer- ing a significant opportunity to produce small-quantity casting orders without tooling. Sand printing machines can produce numer- ous small molds and cores side by side, just as plastic prototype equipment does. Tey either fuse polymer-bonded sand together or use inkjet technol- ogy to bond the sand. Te technology also allows for greater design flexibility, as the elimination of the tooling step removes some limitations from the pro- cess of achieving the desired geometry. One of the newest rapid manu-
facturing approaches is for molds and cores to be computer numerical control (CNC) machined from a block of bonded sand. It skips the pattern- making step for prototyping and short production runs, and allows designers to test a casting before creating the permanent tooling. Tis method offers a particular benefit for larger parts that cannot be produced in one piece using additive 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 fabrication and plaster molding. Te rapid manufacturing method
most often employed for diecasting 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,
Many rapid tooling methods are based on laser printing technology.
as well as for prototyping or testing. Tey also are employed as a tempo- rary substitute while the hard tooling is prepared. Rapid tooling for diecasting can
be created through the application of direct metal deposition technol- ogy, which is similar to fused deposi- tion modeling. It uses a laser to melt injected powder metal and deposit it in a precise location. For die inserts, more methods have been developed, including direct metal laser sinter- ing (DMLS), electron beam melting (EBM) and laser engineered net shap- ing (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
creates a molten pool of metal on an existing 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 com- plex designs but are limited in size and the life of the tooling, seldom exceeding 10,000 shots. CNC machining also can be used to create individual dies, as well as metal parts and tooling. And rapid solidifica- tion process tooling begins with an RP part or machined prototype, which is used
to create a ceramic negative of the die. Molten H13 or a similar steel is sprayed onto it, then the die is cut to fit and polished.
Lost Foam Inroads Rapid lost foam casting options
include cut foam patterns as well as machined and assembled alumi- num prototypes. Equipment makers have adapted
CNC machinery to cut foam patterns accurately and quickly. Tey employ special cutting tools designed for the properties of the foam material, and incorporate supports to prevent flaws and breakage. Tese patterns can be used to produce molds immediately, without any post-processing steps. Another option for lost foam casting
is to produce the tooling by machined aluminum rapid prototyping. Tis often is used during development to cast test samples of a part. Te pattern can be made in sections and glued together, and good CAD data is critical to ensur- ing the prototype is a match with the final production tooling. Te benefits to using rapid manu-
A die insert formed by the LENS process shows the complex designs possible. This insert features curved, irregularly shaped cooling channels.
facturing techniques and technology include greater design flexibility and the ability to test casting designs at a low cost, as well as quick-turn prototypes that closely resemble production product. Te possibility of using RP methods for an entire production run presents a significant savings opportunity in tooling costs as well as turnaround times. Te cast- ing supplier is a valuable resource for purchasers and designers exploring this rapidly evolving area of metal- casting technology.
January 2013 MODERN CASTING | 41
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