I haven't worked with HDPE, but I have found that several polymer mechanical properties are sensitive to premold drying... If not done properly, you literally can lose as much as 50% of some mechanical properties. The polymer manufacturer should be able to tell you if HDPE is sensitive to drying...
Pay special attention to the pre-mold drying process... I've seen inattention to this detail result in nearly 50% reduction in some mechanical properties.
I would look at Mil-Std-810...There's a wealth of information in there, solar exposure is just one of the topics (section 505.4). Perhaps you can extract what you need.
While I am certainly not experienced in this field, just a couple of observations...
When looking at the expansion properties of the rail, you
cannot do that exclusively unless the rail is completely isolated, which it is not! You must then consider the SYSTEM, which then requires that you...
