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process demands. Te most common foam for lost foam is expanded polystyrene (EPS) due to its low cost and widespread availability. 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 casting surface. Newly designed pattern bead


Table 1. Chemistry Limits and As-Cast Chemistry Sample for SAE 1060 Plain Carbon Steel.


Element C


Cr


Mn Mo Ni P


Si S


Required Range (Weight %) As Cast Weight % 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


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 param- eters that are developed for the process.


Case Studies


Using the 70% PMMA and 30% EPS foam and a ceramic-based refractory


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 thermodynami- cally 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


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


Fig. 3. The machined foam pattern gear teeth were 4 in. tall.


affects the mold fill time. Improper coating will result in burn-in sand, mold wall collapse, or a backpressure of gas that can cause molten metal to explode out of the sprue. Te new


coating, several grades of steel have been cast successfully via lost foam for commercial applications. Grades cast include 1060 plain carbon steel, ASTM A743 grade CF8M stain- less steel (316 SS), and ASTM A743 grade CF8 stainless steel (304 SS). As is typical in lost foam casting, the coating properties, foam material, and fluidized bed compaction details are significant parameters to con- trol. In addition, the gating system is critical to prevent mold wall collapse and control fluid flow. However, the most significant parameter is the pour temperature. Tis is controlled by hav- ing 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. Further discussions of the actual process parameters will be covered in future papers. A large gear replacement part for a steel rolling mill was made using


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.


Fig. 6. The surface of the ASTM A743 grade CF8M stainless component is shown after shot blasting.


October 2016 MODERN CASTING | 39


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