Due mainly to the sometimes very low bulk density of the ma- terial fed, it is in the first(rear) part of the screws where the channel volume has to be maximized. As we do not want too large a pitch, for such a pitch would decrease residence and therefore heating time, it is the channel depth that has to be increased. Because the screws are intermeshing and conjugated, and their interaxis is fixed, the larger the channel depth h, the larger the screws external diameter D. This results in screws with larger diameter in the rear than in the front: for cleaning, the screws cannot be extracted through the front of the barrel, but it is the barrel that is to be pulled away toward the front(tips) of the screws.