Hilchos Tevilas Keilim – 20

1. A utensil that is assembled from several parts, and the way it used is in its assembled state, such as a thermos bottle or meat-grinder, should be immersed assembled. It is not necessary to dismantle the utensil to its components — even though the water will not reach the points where the components are joined. (Shu”t Har Tzvi, 108).

2. If the person nevertheless wishes to immerse this type of utensil when it is dismantled, he must make sure to completelydismantle it and immerse each piece separately that is, the utensil must either be totally dismantled or totally assembled into a single, complete unit]. (Ibid.)