There is known for instance from German Pat. No. 1,037,116 a below-water granulating device with water feeding means within the range of the nozzle plate of an extruder. According to this device a flow of cooling water either through separate nozzle bores or through a hollow drive shaft for the cutting tool head of the device is used to flush granules cut adjacent to the nozzle plate toward and into an outlet. However, it has been found that the inherent propelling action of the cutting tools impedes complete removal of the granulate pellets out of the range of the cutting tool head as is required.
An under-water granulation is particularly suitable for thermoplastic degraded synthetic plastics materials which pass through the nozzle plate of an extruder at temperatures within the range of 190.degree. to 260.degree. C.
At higher temperatures as they particularly occur at processing of degraded thermoplastic synthetic plastics materials, for instance degraded polypropylene in an extruder the melt particles which are cut when in hot condition rigidify much slower than normal. As a result, part of the hot cut granulate tends to form lumps or adheres within the range of the middle of the nozzle plate and on the cutting tool head.
These disadvantages are particularly pronounced with thermoplastics which are processed at relatively low viscosities, for instance polyamides and polyesters.
According to a granulating device disclosed in German published application Ser. No. 2,455,757 the synthetic plastics to be processed when the melted strands emerge from the nozzle plate are granulated in an air stream and to effect cooling are fed to a ring of water spiral-like circulating in the dome or cap of the device. There are provided in the cutting tool head bores for feeding a gaseous medium into the range between the cutting tool head and the nozzle plate. The feeding of such flow of a gaseous medium which is directed toward the middle of the nozzle plate serves to obtain a pressure balance between the space defined by the cutting tool head and the nozzle plate and the remaining space in the device. A flushing action with respect to the cutting tools is not obtained as the feed of the gaseous medium does not extend into the range of the plane in which the cutting occurs. Moreover, to obtain an adequate flow of the gaseous medium an additional source of power is required. The referred to dome is designed to granulate the synthetic material in dry condition and, hence, it not suitable for processing synthetic materials requiring the afore-mentioned higher temperatures.