Trainer midsole
The part that fails in a washing machine, and the part nobody was trying to clean.
What not to do
Machine washing is unsafe on Trainer midsole
Unsafe
Do not do this. The risk is established, and it is described below.
The part the prohibition is actually about. No authoritative source gives any cleaning method for a midsole at all.
Evidence
Strongly supported
Sources
Can I Put Shoes in the Washing Machine?
Altra Running (VF Corporation) · Manufacturer documentation
Relied on for: That machine washing breaks down the glue and foam forming the shoe's internal structure, causing loss of structural integrity. (states this directly)
In detail
The foam layer between the upper and the ground, bonded to both with adhesive. It matters to cleaning for one reason: it is what the machine destroys. A manufacturer's stated objection to washing trainers is that the cycle breaks down the glue and the foam that form the shoe's internal structure, and a hot wash adds heat at around 130°F or hotter to the agitation. The consequence is loss of structural integrity rather than a mark on the surface, which is why the damage is invisible until the shoe stops behaving as it did. No source gives a cleaning method for a midsole — the maker's instruction is to hand-clean the shoe without soaking it, and to keep direct heat away while it dries. The midsole is the reason for a prohibition rather than a part with a procedure of its own.
Sources for this page
Sources
Can I Put Shoes in the Washing Machine?
Altra Running (VF Corporation) · Manufacturer documentation
Relied on for: That machine washing breaks down the glue and foam forming the internal structure, that a hot wash reaches around 130°F or hotter, that the consequence is loss of structural integrity, and that the shoe must not be soaked or dried with direct heat. (states this directly)