GB2127183A - Heat exchanger apparatus for extruding thermoplastic compositions - Google Patents

Abstract

A heat exchange device for controlling the temperature of a resin after leaving an extruder and prior to extrusion through a die in an extrusion process, includes a heat exchanger (10) having an inlet (14) and outlet (16) for a heat- plastified resin and an inlet (32) and an outlet (36) for a heat exchange medium (e.g. oil or water). A selective heater (20) receives the medium from the exchanger (10) and heats it to a predetermined temperature during a start-up phase. A cooler (22) receives a portion of the medium from the heater and circulates a cooling medium in heat exchanging relationship with the heat exchange medium. Control means (42) directs a portion of the heat exchange medium to said cooler (22), in response to the temperature of the resin leaving the exchanger (10), and the balance of the medium to the exchanger (10). A pump (24) for circulates the medium between the heat exchanger (10), the heater (20), the cooler (22) and the control means (42). <IMAGE>

Description

SPECIFICATION Heat exchanger apparatus for extruding thermoplastic compositions The present invention relates to an apparatus for use in the extrusion of thermoplastic compositions and more especially to an apparatus for use in the extrusion of foamed thermoplastic compositions. The apparatus disclosed herein is useful for extruding compositions comprising a major portion of at least one thermoplastic resin which is either amorphous or crystalline in nature.

In the conventional process employed in the plastics industry for extruding thermoplastic compositions, beads or pellets of at least one thermoplastic resin and various other additives are introduced into the feed zone of a screw-type extruder. In the extruder, the thermoplastic resin and additives are heated and mixed to form a substantially homogenous, continuous, flowable composition which is then forced by the screw through an extrusion die to produce a product of the desired shape and dimensions.

As the thermoplastic composition passes through the extruder, its temperature increases significantly due to the combined shear and compressive forces applied to the material by the rotating extruder screw. For a given extruder, the magnitude of the temperature increase varies according to the rotation rate of the extruder screw and the shear properties of the particular thermoplastic composition being run. While some heating is desirable and necessary for achieving satisfactory extrusion, excess heat must be removed from the material downstream of the extruder in order to retain the shape and integrity of the extruded product. Typically, this is done by passing the extrudate, in some cases at slower or lower rates, over chill rolls or through cooling vats downstream of the extrusion die.

Because the temperature of the extrudate exiting the extrusion die is proportional to the rotation rate of the extruder screw when operating under standard conditions (i.e., an increase in throughput causes a higher melt temperature), conventional extrusion lines have been limited as regards their throughput rates by the capacity of the cooling equipment downstream of the extrusion die. Even where the downstream cooling capacity is adequate, the extrudate can undergo thermal shock if its temperature is reduced too rapidly over a wide temperature differential, thereby adversely affecting its mechanical properties.

Particular problems are encountered in the extrusion of foamed thermoplastic compositions.

Extruders for foamed thermoplastic compositions are typically run at high pressures to keep the blowing agent condensed until the composition emerges from the extrusion die. If the temperature of the foamed product as it emerges from the extrusion die is significantly greater than that required to achieve satisfactory extrusion, the blowing agent will overexpand once the pressure is relieved, resulting in cell rupture and the loss of dimensional stability and compositional integrity. If the temperature is too low, expansion will be incomplete and poor density properties will result. For some polymers, such as polyethylene, the correct temperature window is only about + 2 F.

Furthermore, the problem is not only one of achieving a specific absolute temperature, but uniformity of temperature. If temperature gradients exist within the polymer mass, uneven blowing takes place, again causing ruptured cells and poor density values. At high throughputs, the existence of temperature gradients is more likely to occur.

Therefore, in connection with the extrusion of foam products, it is extremely difficult to obtain an increase in throughput for an extrusion line while at the same time not causing a deterioration in the physical properties of the resulting product, such as the size, uniformity and integrity of the cells and the density value of the foamed polymer. In addition, these problems are exacerbated when, as often desired, various additives are incorporated into the foamed product, such as, for example, a fire-retardant.

Several measures have been taken in the past to solve these problems. For example, it is common to employ two separate extruder screws connected in series. See, e.g., U.S. Patent No.

3,860,220. In this configuration the screw of the second extruder merely acts as an auger to convey the thermoplastic composition through the extruder, which is jacketed and cooled with a circulating cooling medium. However, the use of a second extruder in this capacity has proven to be a very expensive, both from an equipment and an energy standpoint, and an inefficient method for cooling a foamed material. Temperature gradients are actually produced in such a second screw, because heat is generated at the screw, while cooling is applied from the outside.

Furthermore, because of the high pressures employed in foam extrusion, problems are often encountered with the rear seals of the second extruder screw. Failure of the rear seals can result in damage to the gear box from the escaping polymer as well as undesirable leakage of the blowing agent.

Another solution is to decrease the rotational speed of the extruder screw; however, this measure is obviously antithetical to an increase in extrusion line throughput.

Other measures have included the inclusion of cooling devices in the downstream portion of the extruder (see, e.g., U.S. Patents No. 3,385,917, No. 3,151,192, No. 3,444,283 and No.

3,658,973 and British Patent No. 2,003,080) or in conjunction with the extrusion die itself (See, e.g., U.S. Patents No. 3,393,427 and No. 4,088,434 and U.S.S.R. Patent No.

592,610). These die units are very expensive to begin with and even more expensive to modify in this manner. Furthermore, they are not effective heat exchange elements, and therefore do not permit significant increases in throughput.

It is also possible to increase the amount of cooling capacity downstream of the extrusion die.

See. e.g., U.S. Patent No. 3,764,642. However, this gives rise to the problem of thermal shock, mentioned above, and moreover, the most essential cooling often is required upstream of the die orifice in order that the resin can be extruded within a certain required temperature range. This is essential in the case of foam extrusion.

Other attempts have been made to interpose some sort of a cooling device between the extruder and the extrusion die. See, e.g., U.S. Patents No. 3,310,617, No. 3,275,731, No.

3,751,377, No. 3,588,955, No. 3,827,841 and No. 3,830,901. These efforts have indeed increased the total heat exchange or cooling capacity of the extrusion line; however, they have not been successful in solving the problem of temperature uniformity, as evidenced, for example, by the need to include an additional mixing device downstream of the heat exchange or cooling device, e.g., in the U.S. Patent No. 3,588,955, Fig. 3. Furthermore, while some increase in throughput has been accomplished (See, e.g., U.S. Patent No. 3,827,841) by these prior measures, it has not been possible to achieve such increases above a certain level, while at the same time producing a foamed product having the desired physical properties.

An extrusion apparatus for thermoplastic compositions is therefore needed which will simultaneously permit increased throughput through the extruder and not result in deterioration of the physical properties of the extruded product. An apparatus is particularly needed which will permit the extrusion of foamed thermoplastic compositions at increased production rates with excellent physical properties, preferably by means of an extruder employing a single screw.

It is therefore an object of the present invention to provide an improved apparatus for extruding thermoplastic resinous materials.

It is also an object of the invention to provide a heat exchange apparatus which enables significantly increased throughputs of resinous material through the extrusion line, preferably with the use of a single screw-type extruder.

A further object of the invention resides in providing such a heat exchange apparatus which enables not only increased throughputs in material but also the production of extruded products having excellent physical properties.

Still another object of the invention is the provision of an apparatus which can control the temperature of an extruded material within narrowly determined limits.

Yet another object of the invention is to provide energy savings, preferably in excess of 30%.

In accomplishing the foregoing objects, there has been provided in accordance with the present invention a heat exchange device for controlling the temperature of a resin after leaving an extruder and prior to extrusion through a die in an extrusion process, comprising a heat exchanger having an inlet and outlet for an extruded heat-plastified resin and an inlet and an outlet for a heat exchange medium to be circulated in heat exchanging relationship with the extruded resin; a selective heater for receiving the heat exchange medium from the heat exchanger, the heater heating the heat exchange medium to a predetermined temperature during a start-up phase of the heat exchange device; a cooler adapted for receiving a portion of the heat exchange medium from the heater, the cooler including an inlet and an outlet for the heat exchange medium and an inlet and an outlet for a cooling medium to be circulated in heat exchanging relationship with the heat exchange medium; control means interposed between the heater and the cooler for selectively directing a portion of the heat exchange medium to the cooler in response to a sensed temperature condition and for directing the balance of the heat exchange medium to the heat exchanger; and a pump for circulating the heat exchange medium between the heat exchanger, the heater, the cooler and the control means.

In a preferred embodiment of the present invention, the heat exchange device comprises a self-contained cabinet having therein all requisite equipment having only the cooling medium inlet and outlet and inlets and outlet means to said extrusion die comprising exterior connections. More preferably, the self-contained cabinet is portable.

Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a simplified perspective view of an apparatus disclosed herein for extruding foamed thermoplastic compositions; Figure 2 is a perspective view of the heat exchange apparatus disclosed herein for accurate temperature control; Figure 3 is a diagrammatical schematic view of the proper arrangement and relationship between components of the present invention; Figure 4 is an enlarged longitudinal sectional elevation view of one resin heat exchanger of the cooling means of the invention, viewed along line 4-4 of Fig. 1; Figure 5 is an enlarged transverse sectional elevation view of the resin heat exchanger shown in Fig. 4, viewed along line 5-5 of Fig. 1;; Figure 6 is an enlarged elevation view of a preferred outlet nozzle means for the resin heat exchanger shown in Figs. 1 and 4-6; Figure 7 is an enlarged elevation view of a preferred inlet nozzle means for the resin heat exchanger shown in Figs. 1, 4-6; Figure 8 is a longitudinal elevation view of another embodiment of the resin heat exchanger of the cooling means according to the invention; Figure 9 is a longitudinal sectional elevation view of the tube sheet portion of the resin heat exchanger shown in Fig. 8; Figure 10 is a transverse elevation view of the inlet end of the resin heat exchanger shown in Fig. 8; Figure 11 is a transverse elevation view of the inlet end of the tube sheet portion of a resin heat exchanger of the type shown in Fig. 8; Figure 12 is a transverse elevation view of the outlet end of the resin heat exchanger shown in Fig. 8;; Figure 13 is a transverse elevation view of the outlet end of the tube sheet portion of a resin heat exchanger of the type shown in Fig. 8; Figure 14 is an enlarged sectional elevation view of the inlet valve assembly of the resin heat exchanger shown in Fig. 8; Figure 15 is an isolated transverse elevation view of an inlet plate which can be used in one embodiment of a resin heat exchanger of the type shown in Fig. 8; and Figure 16 is an end view schematic of the heat exchanger device of the present invention illustrating its preferred arrangement in a cabinet.

Referring to Fig. 1, one embodiment of the invention is a combination comprising extruder 2, a heat exchange device or more simply a cooling means 10 and extrusion die 70 as its three principal elements. The three principal elements are positioned and installed so that inlet port 14 of cooling means 10 communicates with outlet port 6 of extruder 2, and extrusion die 70 communicates with outlet port 16 of cooling means 10. In this figure, a tubular extruded product 74 is shown exiting from die orifice 72. Extruder 2 has been partially cut away to reveal barrel 4 having a single extruder screw 8 positioned therein in such a manner that the screw 8 is rotatable about its longitudinal axis. Thermoplastic material is introduced into extruder 2 through feed port 5.When producing foamed thermoplastic compositions, a blowing agent is introduced through blowing agent inlet port 7 into barrel 4 around extruder screw 8. Extruder 2 is a conventional motor-driven, single stage, screw-type extruder which is commercially available and well known to those of ordinary skill in the art.

Fig. 2 is a detailed schematic view of the heat exchange apparatus or cooling means 10 according to the invention. The cooling means 10 comprises a resin heat exchanger 12 having an inlet port 14 which is positioned downstream of and adjacent to the extruder 2 and an outlet port 16 which leads from the exchanger 12 to the extrusion die 70. The cooling means 10 further comprises a heater 20 and cooler 22 which provide temperature control for the exchange medium being cycled through the exchanger 12. While the exchange medium may be selected from any appropriate liquid having a boiling point higher than the melt point of the hot resin being introduced into the exchanger, a preferred medium is oil, e.g., Monsanto, Thermanol 55, as discussed below.The heater 20 operates to warm the oil during inital start-up in order to melt any solidified resin remaining from the previous operation and during times when the temperature of the exiting resin in outlet port 16 is indicated as being too low. The heater 20 heats the oil, which is then returned to the exchanger 12 such that the heat transfer between the cold resin and the oil is increased due to the warmer oil, and the resulting temperature of the resin is increased to the predetermined temperature. Also provided is cooler 22 which operates in the opposite manner from the heater, i.e., the cooler 22 provides for cooler oil and thus a cooler resin at outlet port 16.

In operation, there is provided an oil temperature control instrument 18, shown in Fig. 3, which monitors the temperature in the system. Fig. 3 further schematically illustrates the arrangement of cooling means 10. On initial start-up, the temperature control instrument 18 actuates the heater 20 by establishing a voltage sufficient to produce an oil temperature capable of melting any solidified resin remaining in the heat exchange device from the previous operation. The oil is pumped by pump 24 operated by motor 26 through the heater, exiting by line 28 to a T-connection 30. At the T-connection 30, the hot oil present is split, with a majority of the stream being pumped through line 32 to the resin heat exchanger 12.The minor proportion of the hot oil is pumped through line 34 to the extrusion die 70 where it will heat the extrusion die to a temperature sufficient to melt any resin residue remaining therein from the previous use. Alternatively, if higher die temperatures are required, the die may be bypassed by the hot oil, and electrical heat applied instead. The relative split of the T-connection 30 varies according to operating conditions; however in a typical operation about 90% or more of hot oil returns to the resin heat exchanger through line 32 while 10% or less is pumped to the extrusion die 70 through line 34. The passage of the minor oil stream to the extrusion die 70 is only exemplary.The minor oil stream may be blocked, and therefore, all the oil may return to the exchanger through stream 32, or the line may provide oil passage to other external needs, i.e., pressure pumps, or the line may be itself connected to the exchanger, thus providing two inlet ports to the exchanger as will be discussed in more detail below. For purposes of explanation here, however, the line will be connected to the extrusion die. As shown in Fig. 3, line 34 and the extrusion die return line 50, which will be discussed below, can be provided with separating valves 33 and 35 which allow for the on-line separation of line 34 from the extrusion die 70 should such flow no longer be necessary. By the incorporation of these values, the separation can be accomplished without having to drain the entire system prior to the separation.

The hot oil is pumped to the resin heat exchanger 12 wherein it flows in relation to the resin flow in such a manner as to establish heat transfer. Preferably, the oil flows countercurrently to the resin, thus providing greater heat transfer. As the resin begins to melt, it flows through the resin heat exchanger 12 to the extrusion die 70. The hot oil, being the source or higher temperature medium, transfers heat to the cold solidified resin. Due to this heat transfer, the hot oil leaving the exchanger 12 through line 36 has been cooled. The cooler oil finishes its cycle by returning to the heater 20 where it is again heated, and the cycle is repeated.

Once resin flow is established, a lower setting on the oil temperature control instrument 18 is possible, and when this occurs, establishment of on-line production is possible.

When on-line production is established, the cooling means 10 functions to provide for the removal of heat from the hot resin exiting the extruder 2 and to further provide for the accurate control of the resin to a predetermined temperature. Once hot resin begins to flow through the exchanger 12, temperature gauges will indicate the increase in temperature of the resin melt, and the oil temperature controller 18 will then shut off the heater. The cooling means 10 will now function to provide for cooler oil flow into the resin exchanger 12.

Referring to the oil flow cycle, a hot oil will now exit from the resin exchanger 12 through line 36 near the inlet end 14. The oil, upon countercurrent passage through the exchanger has now functioned as a heat sink to remove heat from the resin melt, the heat source, and as a result, the oil temperature is increased. The hot oil is then transported through line 36 via pump 24 to the heater 20. The passage through line 36 provides for some heat exchange between the hot oil and the ambient atmosphere. Since the oil may vary in the hundreds of degrees Centigrade compared to ambient conditions in the range of from about 10 to 40'C, the oil-to-ambient air contact provides for a major heat exchange environment, e.g., this type of heat exchange can produce up to about 90-95% of the total heat removal from the oil.This type of exchange characteristically occurs throughout the system, with the oil return line 36 being only one example.

As previously mentioned, the heater 20 has been shut off; however, passage through the heater guards the oil against thermal shock due to residual heat remaining in the heater. The oil, partially cooled due to ambient heat exchange, exits the heater 20 and proceeds to the second T-connection 38. The oil stream now splits, with a minor proportion proceeding through line 40 to the oil cooler 22 and a major proportion proceeding through line 28 to T-connection 30, wherein the oil flow is split as previously discussed, with a major portion of the stream returning to the resin exchanger. The relative proportion of the oil permitted to flow to the oil cooler 22 is determined by the solenoid valve 42 in accordance with signals from the oil temperature control unit 18.The relative proportions vary here according to the process conditions; however, the amount of oil proceeding through line 28 to the resin heat exchanger 12 generally varies from about 80-95%, with at least a 90% split being preferred. This means that only a small load is placed upon the external cooling source, for example, provided by a source of cooling water supplied to cooler 22. Adjacent to the solenoid valve 42 is a restrictor valve 41 which functions to limit the flow of the oil through line 40 caused by pressure buildup in line 28 which acts to force the oil through line 40.

The oil leaving the T-connection 38 through line 28 proceeds to the previously discussed Tconnection 30. Pressure valve 43 is provided in line 28 to further control the amount of oil returning through line 28 to the resin exchanger 12 and extrusion die 70. The proportional split at the T-connection 30 has previously been discussed; the difference between the present cooling function and the heating function described earlier is that the oil is functioning as a cooling medium during the present cycle, whereas during start-up the oil was a heating medium serving to melt the resin and establish a flow in the extrusion die and resin heat exchanger apparatus. From T-connection 30, the majority of the oil stream proceeds through line 32 to the resin heat exchanger 12. In one embodiment of the invention, as will be discussed below regarding Fig. 4, the oil is introduced into two different chambers of the exchanger 12 through line 32 and line 33. As discussed above, line 33 corresponds to line 34 of Figs. 2 and 3 which connects to the extrusion die. In the presently described embodiment, the line is removed from the extrusion die and connected directly to the exchanger to provide two individual oil flows to the exchanger. The oil stream, upon entering the resin heat exchanger, has circulated through the heating/cooling cycle, changing from a hot oil exiting the resin heat exchanger at the resin inlet port 14 to a cool oil at resin outlet port 16. The majority of cooling for the cycle has been accomplished through oil stream-to-ambient air heat exchange.

Returning to the T-connection 38 located at the exit of the heater 20, the minor proportion of the hot stream proceeds through line 40 and solenoid valve 42 to the oil cooler 22. The hot oil passes through the cooler in counter-current fashion to the cooler exchange medium. Any convenient heat exchange medium may be chosen, with water being preferred. The exchange medium enters the oil cooler 22 through line 44 at a cool temperature relative to the hot oil and will absorb the heat from the oil and subsequently be discharged via line 46 to a cooling device, e.g., cooling tower (not shown). The resulting oil, upon exiting the oil cooler, will be at a much lower temperature. The cooled oil exits the oil cooler through line 48 and unites with the oil returning from the extrusion die 70 via line 50 to form the cool oil stream 52.The oil circulated to the extrusion die 70 functions to control the temperature of the extrusion die, particularly to remove any heat buildup resulting from the extrusion process. Because the oil flow returning from the extrusion die is minor in comparison to the rest of the oil flow and the temperature of the resin melt is lower at the extrusion die in comparison to the resin melt upon passage through the resin heat exchanger, and because of ambient air-to-oil heat exchange, the introduction of the return oil via line 50 does not markedly offset the temperature of the cooled oil from line 48 exiting the oil cooler.

Stream 52 then flows to the T-connection 54 where it is united with the hot oil from line 36.

This functions to cool the hot oil of line 36. The combined stream 36 is then cycled through check valve 58 and pump 24 to the heater, whereupon the cycle is repeated.

Oil tank 60 provides for the addition of oil when required. This may be necessary either due to loss of oil through leaks or to further cool the hot oil being circulated from the exchanger.

Additionally, the pressure relief valve 62 is actuated due to pressure buildup in the system.

This typically occurs during start-up, when the melting of the cold resin results in increased oil pressure. The system typically operates at oil pressures in the range of from about 60-80 psi.

Cold start-up pressures may reach about 85 psi. These values will vary according to the type of material being extruded, as well as the cooling medium being used. The pressure of the inlet melt may range up to about 5000 psi during start-up. If excessive pressure in excess of 5000 psi is encountered, a higher temperature in the oil will be effected by means of the oil temperature control unit 18.

Once a temperature-stable process is established, the cooling means 10 operates to control the temperature of the exiting resin melt within very narrowly defined temperature ranges. For example, if a higher temperature resin melt is indicated as exiting the exchanger 12, the oil temperature control instrument 18 is actuated and more coolant oil is provided. The solenoid valve 42 is controlled by temperature control instrument 18 to allow more oil to be introduced into the oil cooler 22. The resulting oil is then returned to the resin exchanger 12 to increase the heat transfer ability of the exchanger 12 by means of a cooler oil.

In the opposite case, if the resin exiting the exchanger 12 at the outlet port 16 is cooler than required, the oil temperature control instrument 18 actuates the solenoid valve 42 to reduce the flow through the cooler 22, thereby reducing the heat transfer capacity in the exchanger and increasing the temperature of the resulting melt. The oil temperature is continually monitored at check points (not shown) by the oil temperature control unit to assure correct oil temperature.

The resin temperature control system thus provides for the establishment of a near-uniform temperature of the resin melt. This control is possible to an accuracy of about i 1 or 2 F. This control also provides for a constant temperature gradient across the resulting resin melt. This improves the dimensional stability and results in smaller cell size in the case of an expanded resin.

The amount of heat removed varies according to the process conditions; however, generally, the exchanger has demonstrated that up to 150,000 BTu/hr, or approximately 7,600 BUT/hr/sq.

ft. of exchanger surface, can be effectively removed from the resin melt. Process flow temperature reductions of from about 200-300 F can be expected as a result of the present heat exchange apparatus.

Referring now to the cooling means 10, Fig. 4 is an enlarged longitudinal sectional elevation view of one embodiment of a resin heat exchanger 12 according to the invention, taken along line 4-4 of Fig. 1. This resin heat exchanger 12 preferably comprises a three-chambered substantially cylindrical vessel adapted to transfer heat from a thermoplastic resin which is received through cooling means inlet port 14 of resin exchanger 12 from outlet port 6 of extruder 2 shown in Fig. 1. More particularly, resin exchanger 12 preferably comprises three concentrically and coaxially positioned vessels identified in Fig. 4 as outer vessel 102, middle vessel 104 and inner vessel 106. The three vessels are substantially cylindrical in shape, having annular cross sections as shown in Fig; 5, which is a transverse sectional elevation view taken along line 5-5 of Fig. 1.The lengths and diameters of the vessels are preferably designed so that inner wall 110 of outer vessel 102 and outer wall 112 of middle vessel 104 are equidistant at all points. Although none are shown in Fig. 4, it will be apparent to those of ordinary skill that positioning pins can be employed where needed to aid in maintaining the alignment of the vessels. The exact spacing in any particular design will depend on the design flow rate and the properties of the fluids involved. The interior space thus defined is identified as outer chamber 122 in Figs. 4 and 5. Similarly, inner wall 116 of middle vessel 104 and outer wall 118 of inner vessel 106 are preferably equidistant at all points, thereby defining middle chamber 124 for the flow of a heat plastified thermoplastic resinous material. Finally, inner chamber 126 is defined by inner wall 120 of inner vessel 106.

The resin exchanger 12 is adapted to receive a thermoplastic resin from extruder 12 by attaching inlet port 14 to the proximal end of middle vessel 104 in such a manner that the interior of inlet port 14 communicates with middle chamber 124. Preferably, as shown in Fig.

4, exterior wall 128 of inlet port 14 is threaded to receive inlet nozzle 130. Likewise, exterior wall 132 of outlet port 16 is preferably threaded to receive outlet nozzle 134.

Inlet nozzle 130 and outlet nozzle 134 are further described with reference to enlarged elevation views depicted in Figs. 7 and 6, respectively. Inlet nozzle 130 and outlet nozzle 134 each comprise threaded engagement members 136, 138 and elongated sleeve members 140, 142, respectively. Elongated sleeve members 140, 142 further comprise longitudinal bores 144, 146 having a diameter which is adequate to accommodate the flow of thermoplastic composition desired to pass therethrough. The length of sleeve members 140, 142 is preferably such that, when engagement member 136 of inlet nozzle 130 or engagement member 138 of outlet nozzle 134 is threaded onto cooling means inlet port 14 or cooling means outlet port 16, respectively, sleeve end faces 145, 147 will contact the end wall of inner vessel 106, as shown in Fig. 4.To permit the thermoplastic composition to flow from inlet nozzle 130 into middle chamber 124 and from middle chamber 124 into outlet nozzle 134, each nozzle further comprises a plurality of small orifices 148, 150 bored in a radial direction through that portion of sleeve members 140, 142 which is positioned inside middle chamber 124 when engagement members 136, 138 of inlet nozzle 130 or outlet nozzle 134 are threadably engaged with their respective ports and the sleeve end faces 145, 147 abut the end wall of inner vessel 106.

In accordance with the present invention, it has been discovered that the throughput of an extrusion line such as the one illustrated in Fig. 1 can surprisingly be increased by a considerable factor in comparison to similar prior art extrusion lines if measures are taken to maintain a hydraulic balance of the thermoplastic resin as it flows through the resin exchanger 12. This is achieved in the embodiment shown in Figs. 4-7 by the proper placement and sizing of orifices 148, 150 in the inlet and outlet nozzles 130, 134.

Orifices 148. 150 are distributed uniformly around the circumference of sleeve members 140, 142 in order to assure that the thermoplastic resin is metered uniformly into middle chamber 124 about its entire entrance circumference, and that it can uniformly converge again into outlet nozzle 134. This function of providing uniform distribution and collection flow can be augmented by providing a plurality of baffle or vane members 152 which are distributed about the circumference of the generally circular outer surface 11 8 of the ends of inner vessel 106 and emanate generally radially from the immediate vicinity of inlet and outlet nozzles 130, 134.

These baffle or vane members 152 can be positioned between each set of circumferentially adjacent orifices 148, 150 or between groups of circumferentially adjacent orifices 148, 150.

Baffle or vane members 152 preferably extend across the entire gap width of middle chamber 124, and they preferably extend a substantial portion of the radial distance defined by the end faces of inner vessel 106, e.g., at least half this distance and advantageously the entire distance, or even along a portion of the axially-running outer surface 1 18 of inner vessel 106.

The combined area of orifices 150 in outlet nozzle 134 is somewhat larger than that of orifices 148 of inlet nozzle 130. This slight increase in area of orifices 150 in outlet nozzle 134 is sufficient to create a uniform backpressure and hence a hydraulic balance in middle chamber 124, thereby promoting uniform mass flow of the thermoplastic composition, and thereby avoiding channeling of the thermoplastic composition through middle chamber 124 which otherwise takes place. It is not possible to give a precise quantitative relationship between the sizes of the orifices 150 and the orifices 148, since the relationship varies depending upon, e.g., the particular thermoplastic resin which is being run and the respective resin inlet and outlet temperatures. Generally, the orifices 150 in the outlet nozzle 134 should be about 15 to 50% larger in total area than the orifices 148 in the inlet nozzle 130. It is preferred to have the same number of inlet orifices 148 as outlet orifices 150, with the latter being of larger diameter.

Outlet orifices must be sized so that they will not permit channel flow within the temperature control means due to localized overheating of the resin. Once this localized overheating occurs and regions of channel flow begin, the effectiveness of the cooling means is drastically reduced.

Orifices 148, 150 in inlet nozzle 130 and outlet nozzle 134 further aid in blending the thermoplastic composition passing therethrough.

As shown in Figs. 1, 4 and 5, cooling means 10 further comprises means for receiving and circulating a temperature controlling medium on both sides of middle chamber 124 through which the thermoplastic resin flows. More particularly, first oil inlet line 32 is connected in inner vessel 106 in such a manner that it communicates with inner chamber 126 of exchanger 12.

Similarly, second cooling medium inlet line 33 shown in Figs. 1 and 4 is connected to outer vessel 102 so that it communicates with outer chamber 122. Oil inlet lines 32, 33 are preferably connected to vessels 102, 106, respectively, in such a manner that the cooling medium flowing into exchanger 12 initially contacts outer wall 112 of middle vessel 104 and inner wall 120 of inner vessel 106 near the downstream end of middle chamber 124. When constructed in this manner, the flow of cooling medium through outer chamber 122 and inner chamber 126 is substantially countercurrent to the flow of thermoplastic composition through middle chamber 124.

As previously mentioned, a satisfactory cooling medium for use with the resin exchanger 12 is an oil having a flash point higher than the temperature of the thermoplastic composition entering through inlet nozzle 130. Other suitable heat exchange fluids, such as hydraulic fluids and the like, can also be used. The cooling oil exits the resin exchanger 12 and is cycled through the coolant temperature control apparatus as previously discussed.

To promote heat transfer between the thermoplastic composition and the circulating cooling medium, it is desirable to control the flow of circulating cooling medium through outer chamber 122 and inner chamber 126 in such a manner that it maintains significant surface contact with outer wall 112 of middle vessel 104 and inner wall 120 of inner vessel 106. Although various means for accomplishing this result will be apparent to those of ordinary skill upon reading this disclosure, satisfactory results have been achieved by means of the flow control devices depicted in Figs. 4 and 5. More particularly, vertical baffles 92 have been positioned inside inner chamber 126 and affixed to inner wall 120 of inner vessel 106.Similarly, metal band 94 has been spirally wrapped around outer wall 112 of middle vessel 104 and welded thereto to direct the flow of cooling medium entering outer chamber 122 through temperature controlling medium inlet line 32 toward oil outlet ports 39, 41 in temperature controlling medium outlet line 36.

Referring again to Fig. 1, outlet port 16 is connected to and communicates with extrusion die inlet port. Upon exiting resin exchanger 12, the cooled thermoplastic composition is directed through extrusion die inlet port into and through the annular die orifice 72 in extrusion die 70.

In the case of extruding resin compositions containing a blowing agent, the extruded profile undergoes rapid expansion upon exiting the die orifice 72. Expansion of extrudate 74 occurs since the external pressure is reduced to atmospheric level upon exiting extrusion die 70, permitting the blowing agent to expand around each nucleator particle, thereby forming the individual cells. While extrudate 74 is shown in Fig. 1 as a foamed thermoplastic tube, it will be readily understood that the shape and dimensions of extrudate 74 can be varied to any desired configuration by the substitution of a different extrusion die.

Although the foregoing detailed description has been directed to an embodiment of the apparatus of the invention useful for the production of foamed thermoplastic articles, it will be apparent to those of ordinary skill in the art upon reading the specification that the disclosed apparatus is also useful for the production of thermoplastic compositions containing no blowing agent. Thus, for example, by closing blowing agent inlet port 7 by means of a valve or the like and by substituting an extrusion die comprising a horizontal slit, it is possible to extrude a thermoplastic sheet material on the same apparatus. Similarly, if there is no intention to ever produce foamed thermoplastic compositions on a particular extrusion line, there is no requirement that a blowing agent inlet port be provided in the extruder.The advantages achieved by the present invention as regards increased throughput of thermoplastic resin are also attained in the case of a non-foam extrusion process; however, the increases are often not as significant in view of the fact that non-foam processes often have less stringent temperature requirements for the resin as it enters the extrusion die.

Satisfactory thermoplastic resins for use in the subject apparatus are selected from the group consisting of both crystalline and amorphous polymers. Preferred thermoplastic resins for use in the subject apparatus include, for example, polystyrene, polyvinyl chloride, acrylonitrile-butadiene-styrene, polyethylene, polypropylene, polyalkylene terephthalates, and the like. Those of ordinary skill will appreciate that various other copolymers and terpolymers of the abovementioned crystalline and amorphous polymers can also be employed in the subject apparatus.

In addition to the thermoplastic resin, the composition provided to the feed port of the extruder can also comprise other optional ingredients, including, without limitation, lubricants, nucleators, rubbery materials, ionomer resins, coloring agents, ultraviolet light stabilizers, fireretardants and the like. Where the apparatus is employed for the production of foamed thermoplastic compositions, satisfactory blowing agents include, for example, methyl chloride, carbon dioxide, ammonia, air, n-pentane, isopentane fluorocarbons, and mixtures thereof.

Another embodiment of the resin heat exchanger according to the subject invention is described with reference to Figs. 8-15. Fig. 8 depicts a longitudinal elevation view of a resin exchanger 160 which is adapted to circulate the oil around a plurality of tubes 162 positioned inside shell 164 thereof. In addition to shell 164, resin exchanger 160 comprises inlet valve assembly 1 66, outlet thermocouple assembly 168, and tube sheet portion 170 which is normally positioned inside shell 164, but for illustrative purposes is shown in Fig. 9 isolated from the shell 164.

Shell 164 is preferably a substantially cylindrical vessel adapted for use in resin exchanger 160 of the invention by the addition of inlet shell flange 172, outlet shell flange 174, oil inlet port 176, and oil outlet port 178. Tube sheet portion 170 is adapted to be positioned inside shell 164 and bolted thereto by means of inlet tube sheet flange 180 and outlet tube sheet flange 182, or any other functionally equivalent means known to those of ordinary skill in the art. Tube sheet portion 170 comprises a plurality of tubes 162 adapted to transport a thermoplastic polymer composition from polymer inlet port 184 to polymer outlet port 186 through and generally countercurrent to the direction of the oil passing through shell 164. The flow of the oil through shell 164 can be further controlled by the addition of baffles 188 or the like to tube sheet portion 170, as shown in Fig. 9.According to one preferred embodiment depicted in Figs. 8-14, tube sheet portion 170 comprises six metal tubes which are evenly and circumferentially spaced about the longitudinal axis of resin exchanger 160.

As in the embodiments illustrated in Figs. 1, 4-7, it has been found that the throughput of a cooling device of the tube-in-shell type illustrated in Figs. 8 and 9 can also be significantly increased by maintaining a hydraulic balance across the entire cross-section of the cooling device which is devoted to the transport of the thermoplastic polymer material. As a result, a significant increase in throughput is also possible for any extrusion line in which the cooling device is employed.

A hydraulic balance is maintained across the cooling device by carefully controlling the pressure drop through the individual tubes 162. This is preferably accomplished by creating an orifice in the upstream end of each tube 162. These orifices can be either of fixed size or of variable size, as will be discussed in more detail hereinbelow. It has been found that the following relationship must be satisfied in order to enable a hydraulic balance to be maintained for heat plastified polymeric materials: Tube length .Orifice length = at least Tube diameter Orifice diameter about 25:1 This ratio is preferably within the range of about 40:1 to about 100:1, and most preferably is between about 50:1 and about 100:1. This means that the pressure drop through the orifice is at least about 25 times greater than the pressure drop through each individual tube, and most preferably about 50 times greater.

In carrying out this embodiment of the invention, it is preferred to use tubes having an internal diameter of between about 1/2 inch and about 2 inches. If the tube diameter becomes considerably smaller than 1/2 inch, the orifice must be made extremely small in order to satisfy the foregoing relationship. This in turn causes a considerable pressure build-up, which actually contributes to an increase in temperature as the polymer passes through the cooling device, due to the resulting work energy. If the tubes have a diameter of more than about 2 inches, the heat transfer coefficient between polymer located near the center of the. tube and the oil becomes too low to accomplish efficient cooling. Tubes having an internal diameter of about 1 inch have been found to offer a good compromise between pressure drop and heat transfer coefficient.

One means of providing an orifice at the inlet end of each tube 162 is to position a valve in each tube, whereby a variable size orifice is provided. This embodiment of the invention is illustrated in Figs. 8 and 10 of the drawings. In those figures, inlet valve assembly 166 comprises polymer inlet port 184, inlet flange plate 190, and a plurality of valves 192, one for each of the tubes 162. Fig. 10 depicts a transverse elevation view of inlet valve assembly 166.

Fig. 11 depicts the configuration of an inlet polymer distribution device 200, by which the flow of thermoplastic polymer composition through polymer inlet port 184 is divided and directed through passage-ways 194 to tubes 162. By including a separate valve 192 for each tube 162 to produce inlet valve assembly 166, shown in more detail in Fig. 14, it is possible to control the flow of thermoplastic composition through tubes 162 in accordance with the relationship defined above so as to maintain a hydraulic balance across the cooling device and maximize heat transfer to the oil circulating through shell 164.

Fig. 13 illustrates an outlet polymer collection device 202, by which the flow of polymer through tubes 162 is collected via passage-ways 196 and fed into polymer outlet port 186.

According to a preferred embodiment of resin exchanger 160, the temperature of the thermoplastic composition exiting each tube 162 is monitored by thermocouples 198 positioned in outlet thermocouple assembly 168 as shown in Figs. 8 and 12. For example, the thermocouples 198 may be positioned in the passage-ways 196 of the outlet polymer collection device shown in Fig. 13. When the temperature of the thermoplastic composition exiting any tube 162 becomes too great, this is indicative that that particular tube is no longer in hydraulic balance with the remaining tubes. This means that the polymer is beginning to selectively channel through the particular tube. It is then possible to place this tube back into hydraulic balance and reduce the exit temperature by reducing the flow through that tube by partially closing the corresponding valve 192 in inlet valve assembly 166.It will be apparent that this control process can be done either manually or automatically through use of conventional instrumentation linking directly the valves 192 with the respective thermocouples 198.

Generally, however, once a hydraulic balance has been achieved, it is quite stable, so that manual operation of valves 192 is entirely satisfactory.

In an alternative embodiment, an orifice can be provided at the inlet end of each tube 162 by insertion of an inlet plate 210 in place of inlet valve assembly 166. Inlet plate 210 is illustrated in side view in Fig. 15. Plate 210 is provided with a plurality of apertures 212 corresponding in number to the number of tubes 162 and positioned in plate 210 in the same spatial configuration as the tubes 162 are fixed within tube sheet 180. The size of apertures 212 must be predetermined to satisfy the above-defined pressure drop relationship for the particular tube size and polymer material which is to be processed, i.e., the latter determines the polymer viscosity which is a relevant aspect of the resulting pressure drop.With this embodiment, it is a simple matter to adapt the extrusion apparatus for processing different polymeric materials, since a number of interchangeable intermediate plates 210 having different aperture sizes can be provided, and the appropriate one can be readily inserted into the resin exchanger 160 as required.

In another important aspect of the present invention, the resin exchanger 12 is constructed so that it contains, at any one time, a volume of polymer material which is larger than the volume of heat plastified polymer contained in the extruder 2. Preferably, the volume of polymer in the cooling means is at least 2 times and more preferably at least 5 or 6 times the volume contained in the extruder. This enables the extruder to be operated at very high rates, while at the same time permitting the polymer to have a sufficiently long residence time in the cooling means so that it can be effectively cooled. By running the extruder at a higher speed, or course, the polymer exiting from the extruder will be at a higher temperature.In the case of a foamed polymer extrusion process, however, this higher temperature is advantageous while the polymer is still in the extruder, because it is easier to disperse the blowing agent uniformly therein, e.g., freon-type blowing agents are more soluble at higher temperatures.

Therefore, in accordance with the present invention, it is possible to run the extruder at a very high speed. This not only increases the throughput of the extruder line, but has the added benefit of producing an improved foamed product, i.e. a product with a more uniform cell size distribution because of improved dispersion of the blowing agent throughout the polymer.

By virtue of the increased efficiency in the resin exchanger achieved according to the invention, as a result of the hydraulic balance maintained over the resin exchanger, the polymer can be brought to an extremely uniform temperature, i.e., there are no temperature gradients within the polymer mass. This is extremely advantageous in a process for producing foamed thermoplastic products, since the resulting uniformity of expansion produces an improved product having a uniform density and cell structure.

With these foregoing features, the apparatus according to the invention is especially well suited for extruding foamed products from thermoplastic polymers which require exact temperature control and uniformity just prior to being passed through the extrusion die orifice and thereafter expanded. Temperature uniformity is assured by the above-described measure of hydraulic balance across the cooling device, and this in turn permits precise temperature control by the use of a cooling medium having a temperature which is kept at the final temperature desired for the polymer. This is possible in view of the relatively long residence times provided, and it further minimizes the possibilities for undesirable temperature gradients in the polymer.

As a result of the capability for precise temperature control, the apparatus according to the invention is particularly suited for extrusion of foamed polymers having very critical temperature control constraints, such as polyethylene. In fact, it is even possible to produce at greatly increased rates foamed polyethylene products of excellent quality which contain high percentages of additives, such as fire-retardant additives.

By employing the apparatus in this specification, it is possible to achieve throughput rates up to about five times greater than those previously experienced through use of a conventional extrusion apparatus. Furthermore, the subject apparatus permits the production of foamed thermoplastic compositions having signficantly lower densities, smaller average cell size and more uniformity of cell size than those which can be made by conventional methods.

In a further preferred embodiment of the present invention, cooling means 10 comprising resin extruder 12, heater 20, cooler 22 and the remaining apparatus components are provided in a self-contained cabinet 230 as shown in Fig. 16. Fig. 16 is a schematic illustration of cooling means 10 shown arranged in proper order in the cabinet 230. The cabinet 230, preferably mounted on casters 232, provides a space-efficient means for temperature control.

The cabinet apparatus can be attached to any of the present types of extrusion lines, requiring only a single cooling medium inlet and outlet port for attachment to, e.g., a source of water.

The cabinet apparatus provides the same accurate temperature control as previously discussed and, therefore, as a result of reduced space requirement and the ability to be easily relocated, the apparatus offers a significant and worthwhile improvement in temperature control.

While this invention has been described in relation to its preferred embodiments, it is to be understood that various modifications thereof will be apparent to those of ordinary skill in the art upon reading this specification, and it is intended to cover all such modifications as fall within the scope of the appended claims.

Claims (21)

1. A heat exchangs device for controlling the temperature of a resin after leaving an extruder or plastic pump and prior to extrusion through a die in an extrusion or other melt process, comprising: a heat exchanger having an inlet and outlet for an extruded heat-plastified resin and an inlet and an outlet for a heat exchange medium to be circulated in heat exchanging relationship with the extruded resin; a selective heater for receiving the heat exchange medium from said heat exchanger, said heater heating the heat exchange medium to a predetermined temperature during a start-up phase of the heat exchange device;; a cooler adapted for receiving a portion of the heat exchange medium from said heater, said cooler including an inlet and an outlet for the heat exchange medium and an inlet and an outlet for a cooling medium to be circulated in heat exchanging relationship with the heat exchange medium; control means interposed between said heater and said cooler for selectively directing a portion of the heat exchange medium to said cooler in response to a sensed temperature condition and for directing the balance of the heat exchange medium to said heat exchanger; and a pump for circulating the heat exchange medium between said heat exchanger, said heater, said cooler and said control means.

2. A heat exchange device as claimed in Claim 1, wherein said cooler comprises a tube-inshell heat exchanger.

3. A heat exchange device as claimed in Claim 1, further comprising conduit means for transporting the heat exchange medium in the heat exchange device, wherein said conduit means provided for ambient heat exchange between the heat exchange medium and the environment.

4. A heat exchange device as claimed in Claim 1, wherein the cooling medium comprises water.

5. A heat exchange device as claimed in Claim 1, wherein the heat exchange medium comprises an oil.

6. A heat exchange device as claimed in Claim 1, further comprising a temperature monitor for monitoring the temperature of the extended resin exiting from the heat exchange device and for providing said sensed temperature condition.

7. A heat exchange device as claimed in Claim 1, further comprising a first dividing means for proportionately dividing the circulating heat exchange medium subsequent to passage through said heater and prior to said heat exchanger, said first dividing means providing for passage of a portion of the heat exchange medium to said heat exchanger.

8. A heat exchange device as claimed in Claim 7, wherein said first dividing means further provides for passage of a portion of the heat exchange medium to an external source.

9. A heat exchange device as claimed in Claim 8, wherein said external source comprises the extrusion die.

10. A heat exchange device as claimed in Claim 7, wherein first dividing means provides for the division of the circulating heat exchange medium into two streams wherein both streams provide a passage to said heat exchanger.

11. A heat exchange device as claimed in Claim 7, wherein more than 50% of the heat exchange medium is provided to said heat exchanger.

12. A heat exchange device as claimed in Claim 7, wherein 90% of the heat exchange medium is provided to said heat exchanger.

13. A heat exchange device as claimed in Claim 7, further comprising a second dividing means for proportionately dividing the circulating heat exchange medium subsequent to passage through said heater and prior to said first dividing means, said second dividing means providing for passage of a portion of the heat exchange medium to said cooler.

14. A heat exchange device as claimed in Claim 13, wherein 80% of the heat exchange medium is provided to said first dividing means from second dividing means.

15. A heat exchange device as claimed in Claim 14, wherein 90% of the heat exchange medium is provided to said first dividing means from second dividing means.

16. A heat exchange device as claimed in Claim 1, further comprising an oil tank, said oil tank providing a reservoir for supplementing and reducing the supply of the heat exchange medium.

17. A heat exchange device as claimed in Claim 1, wherein the heat exchange device comprises a self-contained cabinet having therein all requisite equipment with only the cooling medium inlet and outlet and inlets and outlet means to said extrusion die comprising exterior connections.

18. A heat exchange device as claimed in Claim 17, wherein said self-contained cabinet is portable.

19. A heat exchange device as claimed in Claim 17, further comprising a first resin passage means disposed between said self-contained cabinet and the extruder, wherein said first resin passage means functionally engages an inlet port of said self-contained cabinet, and a second resin passage means disposed between said self-contained cabinet and the extrusion die, wherein said second resin passage means functionally engages an outlet port of said selfcontained cabinet.

20. A heat exchange device as claimed in Claim 13, further comprising inlet and outlet means for providing the heat exchange medium to the extrusion die, wherein said means provide for the application of heat to the extrusion die during start-up and and heat control during operation.

21. A heat exchange device for controlling the temperature of a resin after leaving an extruder or plastic pump and prior to extrusion through a die in an extrusion or other melt process, substantially as herein described with reference to Figs. 1 to 8 and 16, or Figs. 1 to 3 and 9 to 16 of the accompanying drawings.