Impregnation of porous parts is a common technique employed in a variety of industries for a variety of reasons. Stone, brick, ceramic, wood, polymer, aggregate, cermet., and porous metal parts, for example, are commonly impregnated. Commonly, a sealant is impregnated into the part because the porosity is undesirable in the intended end use of the part. In some applications, it is only necessary to seal the pores on the surface of the part. In other applications, thorough impregnation of the part is necessary. Further, in certain applications, it may be possible to over-impregnate a part, so the level of impregnation should be carefully controlled.
For example, separator plates are a component of fuel cells, including solid polymer electrolyte fuel cells. Separator plates are electrically conductive and substantially impermeable to the fluid reactants and/or coolants used in the fuel cell or fuel cell stack. They are commonly made from graphitized carbon, carbon-resin composites, or graphite. The plates are typically impregnated with a resin that assists in imparting the necessary impermeability and mechanical stability. The plates should be thoroughly impregnated with resin.
Expanded graphite sheets, such as the material available from UCAR Carbon Technology Corp. (Danbury, Conn., U.S.A.) under the tradename GRAFOIL, may be used to form separator plates for fuel cells. Expanded graphite sheets are useful in this regard because they are relatively light, flexible and amenable to lowcost manufacturing methods, such as embossing. Separator plates made from expanded graphite sheet may be impregnated with a suitable resin in order to achieve the desired impermeability and mechanical stability discussed above. It is important that such plates be sufficiently impregnated to meet performance requirements. At the same time, it is possible to over-impregnate the plates, resulting in degradation or loss of desired structural and/or functional properties. Accordingly, impregnation process control is an important aspect of separator plate manufacture.
Conventional impregnation process control methods typically rely on a consistent time to sufficiently impregnate a part. Based on such methods, an optimum time can be selected to ensure adequate impregnation without much wasted time or expenditure. However, where relatively subtle process and/or material changes can drastically affect the proper impregnation time necessary to achieve the desired impregnation level, such methods are unsatisfactory. For example, the variability of different grades, lots and batches of expanded graphite sheet, as well as variations in separator plate processing or design, has made it virtually impossible to determine an appropriate impregnation time beforehand for a given lot of separator plates.
Current methods use the impregnation time from the previous batch of plates as the initial time estimate for impregnation of the next batch, taking into account other factors such as plate thickness and density. Since the level of impregnation can only be assessed after the impregnation process is complete, entire batches of parts may have to be scrapped due to incorrect estimates of the impregnation time. This approach is costly in terms of time and materials, and is poorly suited to high-volume production methods.
Accordingly, a method of controlling the impregnation of porous parts that allows the extent of impregnation to be predictably controlled is desirable.
The present process and apparatus address one or more of the problems associated with the prior art impregnation control processes. Specifically, the present process and apparatus allow the extent of impregnation of a porous part to be controlled during the impregnation process.