There are a few names we can use to describe a system where a stronger country acquires colonies in order to get raw materials. The term most commonly used is called imperialism. An imperialistic country wants to acquire colonies for many reasons. One of those reasons is to get raw materials cheaply. It usually is cheaper for a country to get raw materials from its colonies than by buying them from other countries. Plus, the imperial power can sell the finished products back to the colonies and make a lot of money.
Another term that can be used is mercantilism. Mercantilism is a system where the mother country (country owning the colony) is supposed to benefit by having the colony. The colony provides the raw materials, and the mother country provides the finished product. The mother country benefits by getting raw materials cheaply and by selling the finished products back to the colony. The system of mercantilism is designed to benefit the mother country.