Device for granulating plastic strands

A device for granulating plastic strands includes an extruder having a nozzle plate with boreholes arranged in front of a manifold and a starting valve connected thereto, and a cutting device with a knife head arranged centrally in a receiving housing wherein the cutting knives of the knife head rotate in the region of emergence of plastic strands which are acted on radially by a coolant. The cooling is accomplished by providing a cutting plate with cooling boreholes aligned axially with the nozzle boreholes of the nozzle plate. These cooling boreholes extend up to an insulating layer except for a recess which is connected to a coolant feedline for releasing at least one flow cross section of coolant which corresponds to the cooling boreholes.

BACKGROUND OF THE INVENTION 
This invention pertains to apparatus for granulating plastic strands. 
The granulating of plastic strands by means of cutting knives immediately 
upon the emergence of the strand, referred to, as such, also as head 
granulation, requires a sensitive temperature control of the plastic 
strands within the nozzle plate all the way to the outlet for the strand. 
The temperature control must be adapted to retain the desired or 
predetermined viscosity of the individual molten strands up to their place 
of emergence, an increase in viscosity upon the emergence of the strand 
being necessary in order to obtain optimum cutting conditions. 
In known granulating devices, this temperature control is obtained 
essentially by a corresponding addition of heat within the nozzle plate or 
by special insulating measures on the nozzles, or by the direct use of a 
coolant in the region of emergence of the plastic strands. 
The processing of certain plastics which have a narrow softening range 
between the liquid and solid states, as is true, for instance, of 
polyamides and polyesters, requires a precise cooling of the plastic 
strands for the cutting process, without impairing the temperature 
conditions in the nozzle plate. 
U.S. Pat. No. 3,792,950 shows a granulating device of the type indicated in 
which nozzle inserts, formed of porous material, in the nozzle plate 
directly adjoin the conically tapering inlet channel of the nozzle bore 
and are each individually surrounded by an annular channel for the 
introduction of a coolant into the nozzle bore. 
Since in this case heat is removed by the action of the coolant on the 
inlet-channel insert, this action must be counteracted again by sufficient 
heating of each individual inlet channel insert by a suitable arrangement 
of heating channels. Thus, a large radial separation of the nozzle 
boreholes is required. Furthermore, the fine-pore nozzle inserts very 
easily become clogged, so that only carefully filtered pressurized water 
can be used, which means a high expense, in addition to which the water 
pressure thereof must be adapted to the melting pressure in the feed 
region. 
SUMMARY OF THE INVENTION 
An object of the present invention is to remedy the above-mentioned 
drawback and to provide granulating device of the aforementioned type in 
which the strand outlet nozzles are separated from the region surrounding 
the coolant feed which is necessary in order to obtain suitable cutting 
conditions, which device while spatially limited in size, after the 
emergence of the strand makes possible a precise lowering of the 
temperature of the molten strands without affecting the temperature 
thereof within the nozzle boreholes. 
This goal is achieved in accordance with the invention by means of the 
features set forth in the main claim. 
The connection of the nozzle plate with the cutting plate with the 
inclusion of the highly effective insulation and the supplementation of 
the nozzle boreholes by means of special cooling channels developed for 
cooling purposes makes possible a direct limitation of the length of the 
calibrated outlet nozzles. The cooling of the plastic strands thus takes 
place within a limited region of the cutting plate, in such a manner that 
the nozzle plate is substantially protected against heat losses by the 
coolant. The extrusion pressure of the extruder is also better utilized 
and an increase in the extrusion capacity is obtained. Furthermore, 
granulates having the smallest particle diameter can be obtained hereby 
without great expense. 
The heat insulation required for the reducing of heat losses on the 
face-side of the nozzle plate is contained completely outside the sphere 
of action of the cutting knives and permits an extensive screening of the 
nozzle plate from the influence of the coolant and the structural parts 
acted on by the latter. It serves exclusively for the purpose of heat 
insulation. The end surface of the cutting plate which is passed over by 
the cutting knives, on the other hand, is provided, without any special 
heat-insulating measures, merely with a wear-protection layer or with 
special wear inserts. The coolant used can be introduced practically 
without pressure into the cooling channels. Nor is any special preparation 
of the coolant necessary. 
With an embodiment of the granulating device in accordance with the 
features of the invention it is possible in simple manner at any time to 
control the nozzle outlet region. By displacement of the cutting plate 
together with the knife head and the collection housing, a check-up and 
cleaning of the nozzle mouth of the outlet nozzles and possibly of the 
cooling channels in the cutting plate can easily be effected at the start 
of an extrusion process as well as upon a change in the material being 
processed. The proposed development furthermore makes it possible to 
automate the placing in operation of the granulating device, this being 
achieved in accordance with another feature of the invention. 
Another feature of the invention promotes the rapid placing in operation of 
the device by a rapid and dependable connection of the cutting plate with 
the nozzle plate. The guiding of the plastic strands within the cooling 
boreholes is favored in a special manner by further features of the 
invention. 
The stream of coolant which enters directly via the annular gap into the 
coolant boreholes and as well as the stream introduced over the periphery 
of these boreholes makes possible a high velocity of flow of the coolant 
without any constrictions in or tearing apart of the plastic strands 
taking place. When water is used as a coolant, a velocity of flow which is 
up to 10 times greater than the velocity of emergence of the strand can be 
obtained in this way. The formation and growth of vapor bubbles on the 
surface of the strand is effectively prevented in this manner. 
Still further features of the invention make possible a stretch-free 
guidance of the plastic strands even in the event of a high velocity of 
flow of the coolant. 
Finally yet another feature of the invention gives assurance that upon the 
placing in operation the granulating device of the time-coordinated 
sequence of the steps to be carried out can be accurately maintained.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The granulating device 1 shown in FIG. 1 is connected with its associated 
nozzle plate 2 via a manifold member 3 and a starting valve 4 to the screw 
extruder 5. The starting valve has two passages, one passage 6 extending 
into the atmosphere and the second passage 6' producing the connection 
with the nozzle plate 2. The actuating of the starting valve 4 is effected 
via a variable electric motor 7 and a gearing 8. 
The granulating device 1 further includes a cutting plate 9 which connects 
a fully enclosed collection housing 10 to the nozzle plate 2, and a knife 
head 11 arranged centrally in the collection housing 10 and provided with 
a plurality of cutting knives 12. 
The collection housing 10 has a connection 21 to the permanent water supply 
and a connection 22 for the removal of the cut granulate particles 
together with the conveying and cooling water introduced. The knife head 
11 is connected with a drive shaft 13 which enters into the collection 
housing 10, which is sealed off by gaskets 14. The drive shaft 13 is 
guided in a bearing 40 which is located on a movable frame 15 to which the 
collection housing 10 is connected. 
By displacing the frame 15 which is movable via rollers 16 on rails 17, the 
cutting plate 9 can be moved away from the nozzle plate 2 in order to 
provide free access and brought into the position shown in FIG. 2. 
This moving of the cutting plate 9 from the nozzle plate 2 is obtained by a 
plurality of telescopic cylinders 19 (FIG. 2) which can be actuated 
hydraulically or pneumatically via a switch means 33, the cylinders being 
connected via a control valve 23 with a source of pressure fluid 24. The 
piston rods 20 of each telescopic cylinder 19, which rods are fastened to 
the outer periphery of the nozzle plate 2 and the cutting plate 9, make 
possible a uniform movement of displacement of the cutting plate 9. The 
adjusting of the nozzle plate 2 and of the cutting plate 9 is effected via 
setting pins 25. 
A limit switch 18 serves to produce a pulse for special sequential 
operations which will be explained further below. 
Upon transfer to the operating position shown in FIG. 2, a change-over 
valve is previously actuated so that the passageway 6 thereof which leads 
into the atmosphere is connected with the outside opening of the screw 
extruder 5. 
The construction of the nozzle plate 2 and of the cutting plate 9 is shown 
in further detail in FIG. 3. 
The nozzle plate 2 has a number of nozzle boreholes 26 arranged on circles 
concentric to its longitudinal axis and surrounded by heating channels 27 
in order to make up for radially discharging heat losses. The nozzle 
boreholes 26 which are connected on the inlet side to a manifold 34 and 
narrow down on the outlet side discharge directly into cooling boreholes 
28 whose diameter is greater than the diameter of the plastic strand 29 
guided therein. These cooling boreholes 28 which are arranged in the 
cutting plate 9 and are aligned with the nozzle boreholes 26 lead directly 
into the collecting housing 10. They are surrounded on the inlet side by a 
recess 30 which serves for the feeding of a coolant and is connected with 
a supply line 31 for the coolant. The feeding of the coolant is effected 
via a solenoid valve 32 (FIGS. 1 and 4) which is controlled by an electric 
switching device 33. The introduction of the stream of coolant into the 
cooling borehole 28 takes place via an annular gap 42 formed between the 
guide sleeves 38 and the surface of an insulating layer 35. 
This stream of coolant can, however, also be introduced into the cooling 
boreholes 28, exclusively or in addition, via inlet openings 39 arranged 
in the region of the recess 30 in the guide sleeves 38. 
The coolant which flows into the cooling boreholes transports and thereby 
cools the plastic strand 29 immediately upon its emergence from the 
nozzles 36. In this way, an action which centers the plastic strands is 
obtained by the passage opening 42 and the plurality of inlet openings 39. 
The cutting plate therefore has a multiple function. It not only makes 
possible a precise separation of the hot-melt region from the collection 
housing 10 which is continuously traversed by a relatively large quantity 
of cooling water, but it also assures a precise guidance of separately 
cooled plastic strands into the collection housing 10. 
Between the nozzle plate 2 and the cutting plate 9 is insulating layer 35 
which covers the end surfaces of both plates to such an extent that merely 
the cross section of passage of the outlet nozzles 36 of the nozzle 
boreholes 26 remains free. The insulating layer 35 is subjected solely to 
thermal stresses and can consist, for instance, of zirconia which is 
insensitive to wear and assures good heat insulation. Development of the 
insulating layer 35 by means of bushings in the outlet region of the 
nozzles 36 is not necessary in this case. 
The cutting plate 9 is provided in the region of the outlet side of its 
cooling boreholes 28 with a wear-resistant insert 37. Insulating measures 
are not required in this region since the insulating layer 35 is present 
between the cutting plate 9 and the nozzle plate 2. 
The outlet region of the cutting plate 9 is thus not subjected to any great 
changes in temperature, so that the insert 37 may also be formed of 
materials of low resistance to changes of temperature and of a thermal 
expansion which differs from that of the material of the cutting plate 9. 
The guiding of the plastic strands which is effected within the cooling 
boreholes 28 by means of a coolant, for instance water, makes possible a 
precise guiding of such strands up into the region of the path of the 
cutting knives 12. The cutting knives need not extend all the way up to 
the front side of the insert 37. The cutting of the plastic strands at a 
distance from these inserts thus makes possible considerable increases in 
the life of the cutting knives 12. 
FIG. 4 shows the arrangement of the cooling boreholes 28 in the region of 
the annular recess 30. The feeding of the coolant into this recess 30 and 
thus into the cooling boreholes 28 is effected via a ring conduit 41 which 
is in communication with the solenoid valve 32 and the supply line 31. The 
feeding of the water is effected with a velocity of flow which is higher 
than the velocity of emergence of the plastic strands in order to 
counteract any formation of vapor bubbles along the plastic strands. The 
granulating device is particularly suitable for granulating plastics which 
have a narrow softening range. Such plastics are, for instance, polyamides 
and polyesters, whose viscosity changes rapidly as a function of the 
temperature and which therefore customarily mean a high expense for 
control of the temperature in the region of the nozzle plates. 
The molten plastic strands emerging from the nozzle boreholes 26 pass in 
each case into a narrow stream of coolant which is adapted to the velocity 
of emergence of the strand. This stream of coolant cools the plastic 
strands uniformly and transports them into the cutting region of the 
cutting knives. The transformation of the molten strands from molten state 
to the highly viscous cuttable state therefore does not take place 
immediately upon the emergence of the strand from the nozzle boreholes but 
during the transportation through the coolant boreholes. The transport 
fluid--preferably water--surrounds the plastic strands and cools them as a 
whole only to the extent necessary in order to obtain optimum cutting 
conditions. The degree of cooling is determined here by the temperature 
and the velocity of flow of the water. These parameters, which are 
dependent on the plastic being worked, can be determined by simple 
experiment. 
When the granulating device is placed in operation, the nozzle plate 2 and 
cutting plate 9 are first of all separated from each other so that the 
nozzle boreholes 26 are initially free, as is shown in FIG. 2. 
After a pulse is produced manually via the switching device 33, there is a 
brief transfer of the starting valve 4 into the position shown in FIG. 1. 
After the nozzle boreholes 26 have been filled with plastic and the 
transfer between the nozzle plate 2 and the cutting plate 9 is freed of 
any adherent molten material, the actual starting process commences. In 
this connection the extruder is first of all turned on and brought to the 
desired through-put. The melt in this connection flows for a short time 
over the starting valve 4 onto the floor in accordance with the operating 
position shown in FIG. 2. By the connecting of the source of pressure 
fluid 24 the cutting plate 9 is then drawn by the telescopic cylinders 19 
against the nozzle plate 2. In this way, the limit switch 18 is actuated 
and the pulse formed thereby brings about, via the switching device 33, a 
movement of displacement of the starting valve 4 and an opening of the 
solenoid valve 32 so that the cooling boreholes 28 are traversed, with 
entrance of strand, by the cooling water. 
By the production of this pulse there is further effected the actuating of 
a circulating pump (not shown) which circulates the transport water which 
is introduced into the collection housing 10 via the connection 21 and 
discharged via the connection 22, this water serving for the conveying of 
the granulate. 
EXAMPLE OF OPERATION OF THE SYSTEM 
For processing there was used polypropylene which was delivered at a melt 
temperature of 280.degree. C. from the nozzle boreholes of the nozzle 
plate. 
The speed of removal of the strands is 9.25 meters/second with a diameter 
of the plastic strand of 4 mm, so that a through-put of 10 kg/hour was 
obtained. 
With a thickness of the cutting plate of 120 mm, there was obtained a time 
of stay in the cooling boreholes of 0.5 seconds. Under these conditions 
and with a water temperature of about 60.degree. C. within cooling 
boreholes of 9 mm and a velocity of flow of the water of 2.5 
meters/second, the surface of the strand in the cutting region of the 
cutting knives was cooled to about 100.degree. C. The temperature in the 
center of the strand was 200.degree. C. 
With about 10 times greater velocity of flow of the water as compared with 
the given velocity of withdrawal of the strand it was possible, with 
sufficient cooling of the strand, still to obtain a stretch-free but 
sufficiently centered guiding of the strand. Higher velocities of flow of 
the cooling water lead to undesired reductions in area and subsequently to 
the tearing of the strand. 
The centering of the plastic strands within the cooling boreholes is 
obtained by the special arrangement of the guide sleeves 38 which permit 
the entrance of the cooling water through annular slot 42 formed between 
the guide sleeves 38 and the insulating layer 35. In this case, the width 
of the annular slot can amount preferably to 0.5 to 3 mm. 
The inlet openings 39 provided in the guide sleeves also favor the 
centering of the strand and permit a flutter-free guidance of the strand 
in the cooling boreholes. 
The granulating device can be used not only in combination with a 
collecting hood which is traversed by water but also with the same success 
with the use of a so-called water-ring hood. With a water-ring hood, the 
mixture of granulate and water which is removed by the cutting knives is 
taken up, with the utilization of the existing centripetal forces, by a 
water ring rotating in the hood. If instead of using water as coolant 
agent a gaseous coolant agent is employed, for instance air, an 
interceptor hood can also be used. 
While only one embodiment of the invention has been shown and described in 
detail there will now be obvious to the skilled in the art many 
modifications and variations satisfying many or all of the objects of the 
invention without departing from the spirit thereof as defined by the 
appended claims.