Controlling Refractory Coatings CAST TIP—Technical Information and Practice
M
any metalcasting facili- ties use refractory coat- ings in the core room and on the molding line to improve casting
surface finish. The Baume test can be used on the coatings at each work station as part of a control program. But utilizing the test alone may not provide sufficient information to fully control coating parameters. Refractory coatings have two inter-
active parameters that affect casting finish: the refractory components and the rheological characteristics of the coating. The coating performance depends on a consistent deposit of the refractory components on the core or mold, and the rheological characteristics (viscosity/flowability) determine the consistency of the ap- plied coating. Baume measurement and hydrometers are strongly influ- enced by both refractory concentra- tion (i.e. density) and viscosity, so the contribution of the two elements to specific gravity cannot be segregated. This can result in undetected changes to the coating deposit on the core or mold, which can lead to inconsistent layering and random casting surface finish defects. When the Baume test is used in combination with the Specific Gravity Gravimetric Method, the combined results can be a more useful diagnostic tool. Many metalcasting facilities also include a viscosity test in their refrac- tory coating control test procedures.
Technical Information and Practice The specific gravity and refrac-
tory component in a coating can be measured by using a combination of the following two test procedures rec- ommended in the American Foundry Society (AFS) Mold and Core Test Handbook: • AFS 4409-00-S, Specific Gravity, Baume Hydrometer Method
• AFS 4410-00-S, Specific Gravity 76
Calculating Specific Gravity Specific Gravity =(Wf
-We Example: Wf Vt
=final weight of filled container=927.32g We
=weight of empty container=51.12g
=volume of graduated container=500mL (927.32g–51.12g)/500mL Specific Gravity=1.7524 g/mL
Gravimetric Method (density or weight per gallon)
Baume Method The refractory component of a
coating is usually heavier than water, so coatings have a lower specific grav- ity when they are diluted with water from a concentrated to application level. Baume is a simple control test to help measure dilution consistency. However, there is potential for operator variability, and test parameters must be carefully controlled. Operator consis- tency in placing the hydrometer into the coating and length of test time are critical. The AFS test procedure recom- mends the following controls: • use a clean and dry hydrometer; • stir the coating sample gently so that no air is introduced, and let it settle one minute after stirring;
• place the hydrometer gently into the sample at least 0.5 in. away from the container wall and in such a manner that it does not rise back up in the liquid;
• allow the hydrometer to settle for one minute before reading the gage value;
• record the temperature of the sample.
Gravimetric Method Specific gravity is a quick test that
allows inferences to be drawn about the
total solids and refractory components present in a coating, independent of the rheological characteristics. If the dilution factor of the coating is constant (i.e. the same amount of water is added each time), then the principle variable in the test is the amount and type of refractory present. The gravimetric method requires a graduated container with a capacity of 100-500 mL, a 1,000-gram or greater balance capacity with a sensitivity of 0.01 grams, and an immersion ther- mometer with a range of 50-100F. The AFS test procedure recommends the following steps: • use a clean and dry graduated container;
• measure and record the temperature of the coating sample;
• zero the balance prior to testing, place the empty container on the balance and note the weight (des- ignated as We
);
• fill the container to the top mark with coating, which should be fully mixed prior to testing; stir the sample gently so that no air is introduced;
• calculate the test results by subtract- ing the weight of the empty container from the final weight of the full con- tainer, and divide this difference by the volume of the container.
• place the full container on the scale and record the weight (designated as Wf
); MC MODERN CASTING / March 2010 )/Vt
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92