Table 1. Chemistry Limits and As-Cast Chemistry Sample for SAE 1060 Plain Carbon Steel.
Te foam used in lost
foam casting requires a special bead, and the type of foam bead needed for specifically steel had ad- ditional requirements due to the process demands. Te most common foam for lost foam is expanded polysty- rene (EPS) due to its low cost and widespread avail- ability. It is usually molded at a density of 1.2-1.5 lb./cu.ft (0.02-0.03 g/
cu.cm). More recently, the use of expanded polymethyl methacrylate (PMMA) beads have led to reduced carbon pickup. It also reduces carbon defects such as lustrous carbon, which occurs when liquefied polymer is trapped between the metal and coating, causing a carbon film on the cast- ing surface. Newly designed pattern bead
materials specially made for irons and steel are now available. One example of this is a blend of 30% EPS and 70% PMMA, formulated to minimize the carbon available for pickup. Tis is accomplished by creating foam polymers containing atoms that, upon vaporization, are thermodynamically predisposed to joining with the carbon. Tus, the carbon is no longer available for pickup into the molten metal itself. Te compounds that form are non- degrading to the cast steel or cast iron alloy.
Coatings for lost foam patterns
Element Required Range (Weight %) As Cast Weight % C
Cr
Mn Mo Ni P
Si S
0.55-0.65 Nil
0.60-0.90 Nil Nil
0.02
0.15-0.30 0.05 Max
0.62 0.04 0.68 0.01 0.03 0.02 0.27
0.002
sure of gas that can cause molten metal to explode out of the sprue. Te new coatings on the market are designed for the higher pour temperature of steel and iron as compared to aluminum. It is important to have strict adherence to the process control parameters that are devel- oped for the process.
Case Studies Using the 70% PMMA and 30%
Fig. 3. The machined foam pattern for the gear teeth was 4 in. tall.
are critical and have been studied in depth. Te coating controls the removal of the vaporized foam gas from the mold. It has been deter- mined that the coating thickness affects the mold fill time. Improper coating will result in burn-in sand, mold wall collapse, or a backpres-
EPS foam and a ceramic-based refractory coating, several grades of steel have been cast successfully via lost foam for commercial applica- tions. Grades cast include 1060 plain carbon steel, ASTM A743 grade CF8M stainless steel (316 SS), and ASTM A743 grade CF8 stainless steel (304 SS). As is typical in lost foam casting, the coating properties, foam mate- rial, and fluidized bed compaction details are significant parameters to control. In addition, the gating system is critical to prevent mold wall collapse and control fluid flow. However, the most significant parameter is the pour tempera- ture. Tis is controlled by having a higher than normal superheat of at least 2,900F (1,590C) and pouring speeds of at least 80 lbs. (36.3 kg) per second, and preferably 100 lbs. (45.4 kg) per second. A large gear replacement part for
Fig. 4. The surface of the as-cast SAE 1060 steel gear’s teeth shows the surface finish matches that of the foam.
Fig. 5. The bllown foam pattern used for the ASTM A743 grade CF8M stailness steel component features metal cage pieces that are about a quarter-inch thick.
42 | METAL CASTING DESIGN & PURCHASING | Nov/Dec 2016
Fig. 6. The surface of the ASTM A743 grade CF8M stainless component is shown after shot blasting.
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