Investment Casting Patterns
The AM pattern will be used during manufacturing, in the case of investment casting. The residue left in the mold when the pattern is melted or burned out is in this case undesirable. Any sealant used to flatten the surface during pattern preparation should be carefully selected so as not to unintentionally produce unnecessary residues.
The AM parts can be printed on the casting tree or added manually to the casting tree after AM. Figure 9.П displays rings made from a material jetting device.
FIGURE 9.11 Rings for investment casting, made using a ProJet© CPX 3D printer.
In the first picture, a collection of rings is shown on the construction platform; each ring is supported by a secondary support material in white. A close-up of the ring pattern is shown in the second picture. The third picture shows the metal rings still attached to the cast tree. In this instance, after AM, but before casting, the rings were added to the tree. When using Quickcast style stereolithography, hollow, truss- filled shell patterns must be drained of liquid prior to investment. The hole(s) used for drainage shall be covered to prevent investment from reaching the interior of the pattern. Since thermosets are photopolymer materials, they must be burned rather than melted out of the investment.
The resulting component is brittle and fragile when using powdered materials as investment casting patterns such as polystyrene from a polymer laser sintering process, or starch from a binder jetting process. To seal and reinforce the part for the investment process, the part is pre-investment infiltrated with an investment casting wax.
Sand Casting Patterns
Binder jetting and PBF processes can be used to directly create sand mold cores and cavities using a thermoset binder to bind sand in the desired shape. One benefit of these direct approaches is that complex geometry cores can be produced that would be very difficult to produce by any other method, as illustrated in Figure 9.12. To prepare AM sand casting patterns for casting, loose powder is removed, and the pattern is heated to complete the thermoset binder cross-linking and eliminate by-products of moisture and steam. For certain cases, additional binders are applied before heat to the pattern to increase handling resistance. When the pattern is heat treated, the corresponding core(s) and/or cavity are assembled and hot metal is poured into the mold. Sand patterns are collected after refrigeration using tools and bead blasting.
In addition to directly creating sand casting cores and cavities, AM can be used to create parts that are used instead of traditional wood or metal patterns from which sand casting molds are made. In this case, the AM part is built as one or more portions of the component to be cast, separated along the parting line. The part is put in a jar, the sand mixed with the binder is poured around the part, and the sand is
FIGURE 9.12 Sand casting pattern for a cylinder head of a V6, 24-valve car engine (left) during loose powder removal and (right) pattern prepared for casting alongside a finished casting.
compressed (pounded) so that the binder keeps the sand together. The box is disassembled, the sand mold is removed from the box, and the pattern is removed from the mold. The mold is then reassembled with its complementary mold half and core(s) and the molten metal is then poured into the pipe.
