You say this part is a piston seal? Does it look something like this?
If so I don't think scanning is needed or even appropriate. Anyone that's good with a caliper should be able to reverse engineer that into a 3D model in about 10 minutes. If you really want to be super accurate about it you could use an optical comparator instead of calipers, but even that's probably overkill.
As far as making a mold for it, there are a few ways to do it in Solidworks. Probably the most typical way is to model the seal first, model the mold block around it, then use the "combine" command to subtract the seal from the mold block. This would be the Solidworks equivalent to the boolean command in AutoCAD. But if this part is similar to the picture above, you could most likely cut the negative shape from the mold with a single revolved sketch.
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