Selective Laser Sintering

Leander F. Pease III, Powder-Tech Associates, Inc.

Selective laser sintering (SLS), developed by DTM of Austin, Texas (Ref 3), uses laser energy to transform metal powders into useful tooling and parts. Figure 10 is a diagram of the first part in the process, wherein a loose, polymer coated metal powder is laser fused into the required shape. A CAD model of the part is numerically transformed into thin slices, 75 to 250 /Jm (0.003 to 0.010 in.). In the machine shown in Fig. 10, a very thin layer of heat-fusible powder is deposited on top of the build cylinder (step 1). Then, the powder layer is fused by the laser, as it traces out the computergenerated slice (step 2). The layer so formed is held together by the strength of the fused plastic coating on the 55 /'m (0.002 in.) particles. The work stations lower this fused layer by 75 to 250 /Jm (0.003 to 0.010 in.), and a new layer of powder is rolled into place (step 3) atop the fused layer. Laser fusing of this second layer bonds the particles to each other and to the layer below. The support platform moves the part downward a layer at a time (step 4), and the process repeats itself until the part is fully formed, that is, built up layer by layer. The chamber size is 38 by 33 by 41 cm (15 by 13 by 16 in.); parts and molds 15 by 25 cm (6 by 10 in.) thick have been produced.

C02 laser beam

Fig. 10 Process for producing a selective laser-sintered tool. Numbered steps in the figure are described in the text. Source: Ref 3

After the fusing step, the part has a green strength of about 2.7 MPa (400 psi) and requires care in handling. The unfused powder falls away from the fused part and is recycled. The fused green part is impregnated with acrylic resin containing a cross-linking polymer and then dried at 50 °C (122 °F). The dried part is then placed on an alumina plate in a graphite crucible. Blocks of copper are also placed in the crucible to serve as an infiltrant for the porous laser-formed part. The assembly is heated in 70%N2-30%H2 at 0.5 °C/min (1 °F/min) to 1120 °C (2048 °F). At 300 °C (572 °F) the polymer burns out, but the cross-linked polymer remains to strengthen the part and prevent slumping. At 700 °C (1292 °F) the iron particles begin to sinter and bond. At 1083 °C (1981 °F) the copper melts and runs laterally into the porous steel preform. It infiltrates from the sides and then runs up into the part by capillary action. There is enough copper present to fill all the pores, but the copper does not adhere to the alumina plate on which the infiltrated part now rests. The furnace then returns to room temperature, and the assembly is removed.

The infiltrated part contains about 60% Fe, 39% Cu, and 0.8% C. Its properties are:

Ultimate tensile strength, MPa (ksi)

475 (69)

Yield strength, MPa (ksi)

255 (37)

Elongation in 25 mm (1 in.), %

15

Hardness, HRB

75

Elastic modulus, GPa (106 psi)

207 (30)

Coefficient of thermal expansion, mm/mm • °C (106 in./in. • °F)

14.4 (7.99)

The thermal conductivity is important for injection molding tools and is 185 W/m • K (107 Btu/ft • h • °F) or about six times that of P20 steel. During infiltration, the part shrinks 2.5% linearly, and this is compensated at the initial part design by making it over size. The infiltrated parts have a tolerance of ±0.25 mm (±0.010 in.) over distances of 13 cm (5 in.). Walls can be as thin as 1 mm (0.040 in.). All parting lines and shutoffs for molding tools must be machined and therefore have 0.5 mm (0.02 in.) added at laser forming for that purpose. The as-infiltrated surface roughness is 0.25 /'m (10 ^in.) Ra and may be polished to 2.5 to 25 nm (0.1 to 1 /'in.). Some polishing may also be done in the green state.

In all, the process takes about 5 days and results in a mold that is ready for plastic injection molding or MIM. Such a mold can produce 50,000 plastic parts in its useful lifetime. The main use for SLS via the DTM process is making tooling for the injection molding of plastics.

The infiltrated material produced by SLS does not match properties of pressed-and-sintered materials (such as in MPIF Standard 35), and so could not be used in functional prototype parts. In principle, other alloy steels could be selective laser sintered to form 60% dense preforms that would then be sintered and shrunk to the correct density to serve as P/M prototype parts. This would involve linear shrinkages of 7 to 10% with difficulties in maintaining tolerances. In general, parts sintered to such a degree would not have the same properties as the standardized P/M grades. (The prolonged sintering would result in erroneously high elongation and toughness.)

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