Simple Solutions That Work! Issue 14

28 units, automatically record the actual vacuum of each test making both the internal and external quality teams happy. 2. Vacuum System Leakage Any leakage within the vacuum system will influence the RPT sample yielding an inaccurate test result. The vacuum should not only reach the desired vacuum setting, but it should reach the setting within 30 seconds. Common causes for vacuum leakage are damaged O-rings on the vacuum dome or a small piece of foreign material on the vacuum plate preventing a proper seal. When a system leak prevents the dome from reaching the proper vacuum, RPT sample analysis will indicate a good melt, when in fact it is not. Advanced automated RPT testing equipment will have built-in leak detection to ensure the vacuum setting is reached in the correct amount of time aborting the test at the first indication of any vacuum problem. 3. Atmospheric Pressure Changes Swings in atmospheric pressure (low pressure day vs high pressure day) can influence the RPT test results. To combat this when using a manual RPT vacuum, many foundries perform a daily calibration of the mechanical vacuum gauge using a manometer. Advanced Automated RPT systems will compensate for the ambient pressure and always pull the same amount of vacuum no matter the weather conditions. 4. Sample Solidification Time Sample solidification timing is another common source of operator influenced variation in the RPT process. A minimum solidification time under vacuum must be met and strictly adhered to. The typical solidification time for a standard 100g sample is 7 minutes. Leaving the sample under vacuum for a longer time has no effect on the result, however early sample removal greatly effects the integrity of the RPT test. If using a manual RPT vacuum, a timer should be located near the unit and used for every test. Advanced RPT systems contain an integrated vacuum timer which ensures the vacuum runs for the set parameter and then releases the vacuum at the end of the cycle. 5. Sample Analysis The greatest source of variation, by far, lies within how most aluminum foundries analyze their RPT samples. The old way to measure the RPT sample was by cutting it in half on a saw, sanding the surface, and comparing to a chart. This method is time consuming, operator dependent, and fraught with variation. The location of the saw cut and how the sample is prepared can often change the result and once prepared, the grading of the sample is a judgement call. Three people can compare the same sample to the same chart and get three different results. Advanced RPT systems on the market Contact: BRAD HOHENSTEIN [email protected] completely eliminate this source of variation by measuring the specific gravity of the RPT sample. Note that on most RPT sample charts there is a specific gravity number associated with each porosity level. The automated systems simply measure the specific gravity directly. No more cutting and polishing. No more judgement calls. Just set the sample on the RPT system scale, weigh in air and then weigh in water. Within a few seconds the specific gravity measurement is complete. Some of the new automated RPT systems will even determine the percent porosity along with the specific gravity number. Implementing one of the advanced RPT systems is simple. The foundry will set a target number based on their alloy and rather than relying on an operator to make a judgement call, the actual specific gravity or percent porosity of the sample is compared against the target. The old method was to compare the cut sample to a chart. No need to cut and prepare the RPT sample.