Is it possible to kosher a blender for Pesah, and, if so, how is this done?
A blender consists of three parts the grinder, the jar, and the base in which the motor is situated. The base just needs to be thoroughly cleaned to ensure that all residue of food is removed. If the jar is made of glass, then it, too, needs simply to be cleaned, as glass does not absorb Hametz.
If, however, the glass is made from plastic (as most blenders are nowadays), then it might require koshering, depending on the circumstances. If it had been used with only cold Hametz, then it needs only to be thoroughly cleaned, since cold food is not absorbed by the walls of a utensil. If, however, the blender was used with hot Hametz, such as to prepare hot cereal, then the jar must be koshered with hot water. The precise method of koshering depends on how the blender had been used. If one had poured hot water in the blender and then added the cereal, then the blender absorbed Hametz as a Keli Sheni ("second utensil," as opposed to the original utensil in which the water was heated). Therefore, it suffices to pour boiling water from a Keli Sheni into the utensil. If one had first placed the cereal and then poured the boiling water, then the blender absorbed Hametz as a result of hot water poured from its original utensil. Therefore, one should kosher it by pouring on it boiling water directly from the kettle. However, since most people do not always remember how the blender was used, it is preferable to perform standard "Hagala" by immersing it in boiling water. This is the ruling of Hacham Ovadia Yosef. According to some Poskim, the immersion should be done three times, because plastic must be treated like earthenware (which must be immersed three times).
The grinder must be thoroughly cleaned and then immersed in boiling water. Special care must be taken to remove all pieces of food, as oftentimes some small particles can get stuck inside grinder.
Summary: To kosher a blender for Pesah, one must thoroughly clean the entire apparatus the base, the grinder and the jar and then immerse the grinder in boiling water. If the jar is made from plastic, then it, too, should be immersed in boiling water, preferably three times. If it is made from glass, it does not need immersion in boiling water, and it suffices to ensure that is perfectly clean. If one knows that the blender was never used with hot Hametz, then even if the jar is made from plastic it does not require koshering.