Al-Silicate -200#


Filler load Viscosity


Slurry Density wt.-%


sec [Z5] g/ccm


68 11


1.90


TecoSil Hybrid


69 11


1.85


TecoSil Hybrid+


72 11


1.95


Figure 3: Plate weight curves and slurry properties. Both are a function of specific gravity of the according material and of particle size distribution. Despite lower specific gravity compared to Al-Silicates, the slurries made with Hybrid flours reach comparable levels of slurry density. The very broad PSD is responsible for the high plate weight. During shell build, the high plate weight translates into higher layer thickness, and hence into faster shell build-up.


dimensional control/accuracy. Obviously, the “hot” properties of materials (used in PIC shells) directly reflect the dimensional stability/control of parts being cast. Most aluminosilicates used in the PIC industry contain a fairly significant amount of cristobalite excluding Imerys’ Molochite and Mulgrain 60 which contains little to no appreciable cristobalite). Because of the alpha to Beta-cristobalite phase transition that takes place, upon heating the shell, some loss of dimensional control is inherent to back-up systems based on that material. Of course, this might not matter for larger or less intricate parts. But, for smaller, quite intricate parts, requiring very tight dimensional control (like turbocharger wheels, for example), this can be a real problem. Obviously, fused silica, has a very low coefficient of thermal expansion, and, upon firing, its devitrification to B-cristobalite is quite seamless, in terms of comparable thermal expansion. TecoSil Hybrid


contains an appreciable amount of fused silica, and ®


the ability of the product of this range to help solve problems related to this issue is without question.


Again, one of the main features of using a large percentage of fused silica in a shell back-up system is the fact that it can be used to produce castings with excellent dimensional control. Many turbocharger wheel producers are using fused silica-based back-up slurry, for that very reason. The US PIC market has, for years, recognized/greatly utilized the benefits of fused silica in PIC shell systems. Many of Imery’s customers are using an almost 100% fused silica shell, while others have developed shell systems that contain other types of material but still a very large percentage of fused silica. Obviously, there are some “issues” inherent to using a large percentage of fused silica in PIC shells. Generally speaking, fused silica- based back-up systems don’t have the green strength of aluminosilicate shells, and can develop problems (cracking, etc.)


during de-waxing. Also, the creep/hot deformation resistance of shells based on fused silica can sometimes be lower than required. Obviously, there are some very successful, broad distribution, fused silica-based, backup systems available in the market today- that is without question. Imerys has two such systems- Teco-Sil B1 and B1+. But, one of the reasons that Imerys developed Teco-Sil Hybrid range is to help combat problems that can be encountered when using a higher percentage of fused silica in these systems. The aluminosilicate present in these products helps with green strength and creep resistance. Because most aluminosilicates tend to have a higher coefficient of thermal expansion than fused silica, one would assume that using a blend of the two materials would adversely affect the properties obtained when only using a high percentage of fused silica in back-up systems. But, that is not the case, at all. The thermal expansion properties of Teco-Sil Hybrid is quite acceptable, when compared to fused silica-based back-up systems (Figure 2). In fact, the stucco has a much bigger impact on thermal expansion than the flour.


Faster Shell Build-Up Throughput through the shell-room is a factor for foundries, no matter where (in the world) they are located. Broad distributions-based back-up systems help build shell thickness with a reduced number of coats, thus saving time and improving throughput. Teco-Sil Hybrid products are efficiently blended, and very


particle


tightly-controlled, size


in distribution. terms The of shell


building properties of Teco-Sil Hybrid are outstanding. But, they are even further enhanced in Hybrid+. Hybrid+ contains a unique/extremely compatible mineral additive that enhances rheology, viscosity/plasticity, and particle packing. At the same comparable viscosity, the filler load of this product is outstanding, thus increasing plate weight and building shell thickness (versus standard products) at a very high rate (Figure 3). This occurs without adverse shell permeability problems. Costs savings, through efficiency, can definitely be achieved by using these blended products, but


April 2017 ❘ 23


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