How to get the best from your impregnation plant
Checking Out The System – Condition of the component prior to impregnation
Condition of the component prior to impregnation
It is important to know what processes, if any, the component is going to be subjected to ahead of the point of carrying out the impregnation process. As mentioned earlier the cocktail of chemicals trapped within the porosity can have a dramatic effect on whether the sealant cures or not. See the diagram to the right.
Stress relieving from the foundry can be important. Some heat treatments call for oil quench, some call for hot water. Needless to say the latter is preferred. Avoid oil at all cost – porosity saturated with oil is a non-starter. Once the oil has penetrated the porosity, it cannot be removed. This means that if oil testing is required, it should be carried out after impregnation.
Preparatory Drying
Oven drying has always been the traditional method of preparing the component by, cooking the components at around 150ºC for one to two hours. Often this procedure is the first thing to be dropped when operations or operators are under time constraints. Again, there is no way of telling whether a component needs drying or not. Making light of decision making – if it doesn't look as if it needs it why do it?
For some time we have been aware of the virtues of vacuum drying and its benefits over oven baking. The presence of both temperature and vacuum allows significantly greater scope in the removal of highly volatile matter such as that used in cutting fluids. The dynamic effect of differential pressure across the cross section within which the defect is sited also has the effect of forcing out liquids from the cavity that would not otherwise budge by the oven method.
By far the better and recommended method for drying is by vacuum. As an example:
To vacuum dry a batch of components preheated to say 80ºC and vacuumed down to 20mb equates to 340ºC in an oven at atmospheric pressure. This is not to say that desirable results cannot be obtained at lower vacuumed temperatures. With aluminum and plastics playing an even greater part in our technical advancements such an oven temperature as this would cause havoc and be unacceptable to component requirements. Needless to say that there are instances where the volume of water within a cavity exceeds the latent heat stored in the surrounding metal or composite material would have the effect of freezing of the entrapped moisture. In such cases, oven drying would be more appropriate. This should not necessarily be a problem with powdered parts as liquids can and should be avoided with effective production planning.