Kelim used for kneading – the surface on which the kneading takes place, the rolling pins, and the parts of mixing and kneading machines that come into direct contact with dough – must be kashered by means of hagala. Even though the dough is cold, since the acidity of the ḥametz in the dough can intensify the absorption of the taste of ḥametz, they must be kashered via hagala (Ri, Raavyah, Rosh, SA 451:17). Some are more stringent, maintaining that hagala is ineffective for such kelim (Rashi, Rabbeinu Tam), because it is hard to clean off the dough that was absorbed in crevices. However, they can be kashered by means of light libun. If they have no crevices, they can by kashered via hagala (Rema 451:16-17; MB 94).
Mixers have holes whose purpose is to ventilate the motor, so that it does not overheat. Flour and pieces of dough splatter into these holes, and there is concern that, when used with Pesaḥ foods, pieces of ḥametz will fall into the food. Since it is hard to clean, it is better not to kasher it, but when there is great need for a mixer, one must open the motor compartment and clean it thoroughly.