One definition I found that sounded pretty good:

 

Note: electric plants or power plants are not factories ( I guess they don �t really produce a "real" product (make something out of other things).

 

Cars, engines and candies can all be produced in a factory--I know too that car companies often refer to their "plant in ..."

 

I guess you kind of have to go on a case by case basis as to whether something is a plant but not a factory.

 

But there are NOT electric factories, power factories, chemical factoriesa, etc..

 

So it is not the best advise to say they are "generally interchangeable"

 

--sorry spagman, I just don �t agree

 

They are interchangeable.  I see chemical factories all the time, my husband was a scientist in one.  In AmE they are the same thing and interchangeable.  We just go with the flow here...of which sounds and flows better.  We have a car plant in our city.  But we also call it a factory.  Candy (Sweets) can be made in a plant or factory.  Nestle chocolates has a plant locally where they make candy.  We also have a chocolate factory too.  Again, they are interchangeable.  Both envelop a place where there is a final mass product, whether it goes further or not it makes no difference.  Labor organizations prefer "plant" over "factories" because it just has a better connotation.  If you really want to get technical a plant is a factory with its equipment and which of either don �t have both?

 

Just my little 2 cents.