Actually I think it's more expensive to make plastic/aluminum rather than
all aluminum. I think the reason they do plastic tubes is that aluminum
isn't resiliant enough for the tubes and when you clamped down on the
tubes with the heater hose clamps they would probably fracture at that
point after a few years, unless you made the tubes really thick (like as
as A/C evaporator core tubes)
I suspect they went to aluminum heater cores because soldered copper/brass
will rot out if the antifreeze isn't kept changed. (like if someone runs
instead of antifreeze for a long time, or they use old antifreeze that has
all the corrosion inhibitors used up) Of course if they don't design the
to aluminum sealing area right, it will leak there, and if they use the
materials for the plastic, the plastic will get hard and crumble. By
soldered copper brass is a bit easier to get right in the manufacturing.
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