Microwave technology brings LOI testing up to speed.
Loss on ignition (LOI) has for several years been a recognized control property for such molding materials as raw, resin-coated or reclaimed sands, and a variety of the additives used with them. The main method for determining LOI involves heating samples to temperatures at which organic materials decompose and certain inorganic components volatilize. Any resultant loss in weight from the sample is the LOI measurement.
LOI measurement indicates the amount of non-active combustibles in raw sand. For green molding sand, these combustibles consume water added to activate the clay. In chemically-bonded sand, they absorb binder and reduce its effectiveness. Thus, LOI measurements can tell a good deal about the overall quality of a foundry's sand system.
LOI testing can be an effective quality check of binder additions to sand mixes, and has often been discussed as a control tool for the sand reclamation process. But unlike tests for compactibility or compressive strength, which take little time and can be done on-line for immediate control, conventional LOI testing produces results too slowly to be used as an effective quality control tool. For it to be of maximum value to foundrymen, it must provide the ability to detect changes quickly and make the proper adjustments.
Standard AFS test methods for LOI on new and reclaimed sands, as well as clay combustibles in clay bonded sands, require a 2-hr ignition in a muffle furnace at 1800F (982C). With time needed for sample weighing and cooling in a desiccator, the total testing process approaches 3 hr, rendering LOI useless as an on-line control tool.
While there have been attempts to cut testing time for chemically bonded and resin-coated sand by using smaller samples and shorter furnace times, accuracy may be lost through less than representative sample size and incomplete combustion.
A new LOI method has been developed that uses a microwave muffle furnace and crucibles to obtain combustion. This method can provide results in 20 min without sacrificing accuracy.
This modified LOI furnace looks like a household microwave oven. Inside, however, is a ceramic fiber-insulated chamber serving as the furnace, and a silicon carbide microwave absorbent that acts as the heating element [ILLUSTRATION FOR FIGURE 1 OMITTED]. As this heating element uses microwave energy to maintain the interior temperature of the chamber, the ceramic material provides very effective insulation, so that the outside remains near-ambient despite the 1800F inside. The furnace design promotes a large volume of air flow around the heated chamber, providing enough oxygen for rapid, complete combustion of organic materials in the sample.
Though the microwave method accepts porcelain, graphite and platinum crucibles, it is maximized using lightweight quartz fiber crucibles [ILLUSTRATION FOR FIGURE 2 OMITTED]. The quartz variety is lighter and holds less sample, allowing for faster heating and cooling. Another benefit to the new crucible is its porous nature, which allows oxygen to get to the sample for rapid oxidation. This plays a role in cutting test time, compared to the relatively static conditions found in the conventional muffle furnace/porcelain crucible combination.
In performing the microwave furnace LOI test, the following procedure is followed:
1. Preheat microwave furnace to 1700F (926C).
2. Fire crucibles to assure dryness and lack of organic contamination.
3. Place fired crucibles in a desiccator and allow to cool.
4. Record the tare weight of the crucible.
5. Weigh 8 g of sample into crucible using an analytical balance.
6. Place crucible and sample inside microwave and hold for 12 min.
7. Remove samples from furnace and cool to room temperature in desiccator.
8. Reweigh crucibles and calculate percent weight loss as LOI.
Typical Test Time: 20 min.
Considerable work has been done to correlate the results between the above testing procedure and the conventional muffle furnace method. A plot of microwave LOI and muffle furnace LOI is shown in Fig. 3. The data represents 75 resin-coated sand samples ranging in LOI from about 2-5%. Regression analysis proves good correlation of the testing methods and the high [R.sup.2] value assures the method is usable with existing specifications.
The chart in Fig. 4 shows similar results across a broad range of foundry materials tested. Here, the [R.sup.2] value is even higher than that of the resin-coated sand.
Table 1. Gage R&R Study of LOI Results for Various Foundry Materials
Materials LOI Muffle LOI Microwave Furnace Furnace
Washed and Dried Silica Sand 0.058 0.056 Ester Cured Phenolic Reclaimed Sand 0.729 0.734 Urethane Nobake Reclaimed Sand 1 1.07 1.09 Lake Sand 0.251 0.253 Resin Coated Sand 3.25 3.22 Clay 13.99 13.95 Bank Sand 0.253 0.261 Chromite -0.346 -0.351 Urethane Nobake Reclaimed 2 4.03 4.07 Furan Nobake Reclaimed 2.08 2.06
A second correlation was run as a Gage R&R study. The samples were selected to represent a broader range of materials used in foundry core and mold processes, including silica sands with LOI of 0.1-0.6%, reclaimed sands with LOI of 1-3%, and clay with LOI of 14%. Table 1 shows the close proximity of the measurements.
These findings bear out good correlation between the muffle furnace method and microwave LOI testing, and existing AFS specifications will likely be applicable to either method. The microwave method is also able to handle the various types of foundry materials to which LOI is an applicable process control indicator. In providing test results up to 10 times faster than the conventional method, microwave testing allows technicians to get an early handle on quality problems that might just be creeping into the sand system. It also saves on energy costs and the considerable expense of replacing conventional muffle tubes, which are subjected to 2 hr of punishment every time the traditional test is run.
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|Title Annotation:||loss on ignition|
|Author:||Cavanaugh, James E.|
|Date:||Feb 1, 1996|
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