How to get the best from your impregnation plant
Checking Out The System – Gel Time
Gel Time
Gel Time of the sealant is all important. Unless treating virgin components, the porosity is likely to be contaminated with a cocktail of cutting oils, water, emulsifiers etc., some of which contain anti oxidants. The latter can poison the sealant, especially if the sealant is low in catalyst thus making it impossible for the sealant to cure within the porosity. It also depends on the quality of sealant you have chosen as to the type and percentage of catalyst you can use, so please check with your supplier if you feel it necessary to exceed the recommended level of catalyst.
All too often, catalyst strength is kept to a minimum because of the fear of the sealant curing in the autoclave. A cure in the test tube will only tell you that the sealant will cure in the test tube, but this is quite different to the cure in the porosity in both size and the risk of contamination in the latter.
Test tube cured sealant also benefits from exothermic reaction during curing. In comparison, it is very unlikely that any exothermic action would occur within the porosity due to the minute amount present. It follows therefore that non cure within the porosity is the greater risk. It is also important to note that once a sealant has been subjected to a cure cycle and remains uncured, it is unlikely to cure even if reheated. There is a greater problem here in that the uncured sealant within the porosity becomes a contaminant for further impregnations, rendering the casting scrap.
Therefore it is important always to have the maximum allowable catalyst present in the sealant. The illustrations above show that with little catalyst in the sealant, the slug in the test tube will appear stress free see Fig 1. On the other hand, sealant with the correct amount of catalyst may be found to be crazed and fractured – See Fig 2/3. This should be ignored, as this condition does not relate to the sealant deposit within the porosity. Note how the crazing in the test tube diminishes as the test tube gets smaller as with no crazing as with Fig 4.
Sealant Storage
When working with higher levels of catalyst in the sealant, it is important to ensure that the temperature control system fitted to the sealant storage tank is working correctly to maintain permissible storage temperature to supplier's recommendations. Bear in mind that a chilled sealant chills the casting resulting in a greater heat rise to the ultimate curing temperature. Also, to over cool the sealant will encourage condensation of moisture from the atmosphere which will be be detrimental to sealant performance.