1. Field of the Invention
This invention relates to an apparatus and method for granulating plastic strands that emerge from nozzles in molten form and are supplied to a following, downward inclined runoff channel in which the plastic strands are transported by a liquid stream, in particular water stream, the apparatus further having a dewatering zone for the plastic strands and a granulator disposed after said dewatering zone for granulating the solidified plastic strands with the aid of at least one knife assembly.
2. Description of the Related Art
U.S. Pat. No. 5,182,115 discloses an apparatus for cooling, drying and granulating strands emerging from nozzles in molten form having a runoff channel disposed with its receiving-side end below the nozzles, and further having a device for producing a coolant stream on the runoff channel, while the runoff channel is followed by a granulator and furthermore a dewatering zone is disposed in the runoff channel before the discharge-side end, the runoff channel being provided in said dewatering zone with openings for free passage of the coolant.
In addition the runoff channel in this known apparatus is provided in its bottom in the area after the dewatering zone with admission nozzles for a gas or air stream over such a length and so close together that the strands are guided for removal of residual water along the runoff channel in largely frictionless fashion with respect to the bottom into the granulator disposed at its discharge-side end with a degree of dryness permitting immediate processing. This granulator has essentially two draw-in rolls for the plastic strands one above the other within a corresponding housing, further a knife roll cooperating with a corresponding counterknife. This knife roll and the counterknife granulate the plastic strands supplied by the draw-in rolls, and the granules thus produced by the granulator fall out through a discharge shaft for further processing.
In addition U.S. Pat. No. 4,180,539 discloses an apparatus of the above-defined type wherein the runoff channel is followed by an upward inclined dewatering and drying zone for the plastic strands that consists essentially of a circulating, endless and latticed conveyer belt on which the plastic strands leaving the runoff channel and thereby entraining at least part of the liquid stream are transported further toward the granulator. Above this circulating latticed conveyer belt a number of air nozzles are disposed for directing relatively sharp air jets ("air knives") onto the surface of the plastic strands. These air knives serve to dry the strands on their surfaces during transportation, whereby they blow away the water sticking thereto and remove it downward through the lattice belt. The latticed formation of the transport belt is necessary in the known apparatus of U.S. Pat. No. 4,180,539 in particular because relatively sharp air jets are directed onto the strands from above so that the air must likewise be removed downward through the lattice belt since the strands would otherwise not have the right contact pressure on the surface of the lattice belt; the strands subjected to the air knives would instead "fly up" and "swirl around" as it were. In addition, the known dewatering and drying zone ensures only a dewatering and drying process with respect to the strand surfaces while the interior of the strands still has an elevated temperature compared to the surface, so that the plastic strands entering the granulator directly after the dewatering and drying zone are not yet equalized in their temperature profile.
It is fundamentally necessary, however, that the plastic strands to be processed into granules be neither too soft nor too brittle when they are supplied to the knife roll of the granulator. In view of this requirement and also because of the extremely different properties of the plastics marketed in granular form, these properties ranging from "very elastic" to "extremely brittle", the cooling devices used between an extruder and a granulator in known apparatuses are already adapted by the manufacturer to the type of plastic later to be processed in order to ensure that the newly extruded plastic strands are cooled with such an intensity that they have the desired strength properties when they reach the knife roll of the granulator. When processing certain plastics, in particular glass fiber-reinforced plastics, one requires that the granulating temperature be over about 100.degree. C., whereby the temperature profile should be as homogeneous as possible across the strand cross section. The same requirement also holds quite generally for example in the processing of plastics that are fundamentally relatively poor heat conductors.
In the apparatus known from U.S. Pat. No. 4,180,539 the dewatering and drying process does not permit the plastic strands to be given a homogeneous temperature distribution across the entire strand cross section before they enter the granulator, because the dewatering and drying process concentrates in particular on the strand surfaces.