* Express means clearly stated or explicitly declared.
* Expressed is the past participle of the verb "to express," and it refers to something that has been communicated.
Therefore, "express purpose" is the correct term to use when referring to a purpose that is clearly stated or explicitly declared.
Here are some examples:
* The express purpose of the meeting was to discuss the budget.
* The contract stated the express purpose of the agreement.
"Expressed purpose" is not grammatically incorrect, but it's not the preferred term in this context. It might be used in a situation where someone is describing the purpose that was expressed, but it's less common than "express purpose."