FOAM EXTRUSION APPARATUS AND METHOD

Abstract: A foam extrusion system and method employs a horizontal vacuum chamber (10) with a bulkhead (11) and extrusion die (16) at one end of the chamber, the extrusion die being inside the chamber when the bulkhead is closed. A liquid baffle, preferably water, is provided at the opposite end of the chamber through which the extrudate passes from the chamber to atmosphere. The baffle comprises a pond (30) of liquid having a higher level in the chamber and a lower level (31) outside the chamber. The higher level of the pond is formed between a dam plate (20) and a seal plate (12), the latter forming the opposite end of the chamber. The seal plate includes a size adjustable window (34) through which the extrudate passes, the extrudate being guided by a conveyor (43) to extend inclined downwardly into the pond and to exit through the seal plate and pond with a large radius upwardly curved section. The level of liquid in the chamber is controlled by circulating liquid (50, 51) from the higher level of the pond within the chamber to the lower level of the pond outside the chamber with the amount of liquid circulated being inversely proportional to the absolute pressure in the vacuum chamber. Optionally the circulated liquid may pass through a cooler (61) to control the liquid temperature. The size adjustment of the window or exit orifice in the seal plate may be used to control the flow of liquid from the lower level pond portion outside the chamber to the higher level pond portion inside the chamber, particularly during start up.

Description: This invention relates generally to a foam extrusion apparatus and method, and more particularly one such method and apparatus utilizing a large horizontal vacuum chamber. It has been shown that extrusion of foamable plastic into a vacuum chamber will produce a high quality low density foam. Such products are produced commercially by extruding the plastic melt into an inclined barometric leg which is in the form of a large diameter pipe which forms a vacuum chamber. The lower end of the pipe which is of substantial length projects into a pond of liquid, preferably water. When the chamber is under vacuum the water is drawn from the pond at least partially to fill the chamber. The water serves to seal the chamber and cool the extrudate. The inclined configuration permits the extrudate to be extracted from the chamber on a continuous basis with the aid of a belt conveyor. Examples of such installations may be seen in US-A-3,704,083; 4,044,084; 4,199,310; 4,234,529; 4,247,276; and 4,271,107. One of the principal problems of the inclined barometric leg installations such as shown in such publications is the cost of construction. Because of the length of the leg or chamber and its inclination, the die end where the extruders are located must be at a substantial elevation above the exit or pond end. Such difference in elevation not only creates construction problems but also material handling problems during operation. Attempts have been made to employ a horizontally extending vacuum chamber, and reference may be had to US-A-4,487,731 and 4,486,369 for an illustration of horizontal chamber vacuum extrusion systems. As seen, such systems avoid a liquid pond or baffle but instead use an exit chamber which must be cycled from atmospheric to vacuum. Also, the extrudate must be cut and/or stacked within the chamber which makes service and control of such equipment difficult. Also, spray cooling of the product within the chamber may be inadequate for a large cross sectional size extrudate. Other types of vacuum extrusion apparatus may be seen in US-A-3,822,331; 1,990,434; and 2,987,768, but such are obviously not suitable for continuous foam extrusion, particularly extrudates of large cross sectional size. It is accordingly desirable to employ a horizontal chamber which nonetheless utilizes a water baffle or seal in which the extrudate is immersed as it exits the vacuum chamber. In the production of extruded foam material it is known (EP-A-0 260 949) to extrude a foamable plastic melt into a vacuum chamber and passing the foamed extrudate through a body of liquid which is partially within and partial outside of the vacuum chamber to exit the chamber through such body of liquid. However, in this known foam extrusion apparatus the foamed extrudate is immersed in the body of liquid only outside of the vacuum chamber while in the vacuum chamber the liquid seeping through the exit of the vacuum chamber into the vacuum chamber is circulated into the liquid outside the vacuum chamber thus that the higher level of the liquid is outside the vacuum chamber. Furthermore, it is in the production of extruded thermoplastic foam bodies known (EP-A-0 260 948) to provide an extrusion die on a bulkhead plate at one end within a vacuum chamber. In this known apparatus the foamed extrudate is cooled within the vacuum chamber by spraying liquid onto the surface of the endless extrudate so that it is not in contact with the body of fluid within said vacuum chamber. Also in this known apparatus the liquid level in the vacuum chamber is lower than the liquid level outside the vacuum chamber. In both cases the water baffle or seal of the apparatus is only outside the vacuum chamber so that the extrudate is immersed in the liquid only after it has left the vacuum chamber. A foam extrusion system and method employs a horizontal vacuum chamber with a bulkhead and extrusion die at one end of the chamber, the extrusion die being inside the chamber when the bulkhead is closed. A liquid baffle, preferably water, is provided at the opposite end of the chamber through which the extrudate passes from the chamber to atmosphere. The baffle comprises a pond of liquid having a higher level in the chamber and a lower level outside the chamber. The higher level of the pond is formed between a dam plate and a seal plate, the latter forming the opposite end of the chamber. The seal plate includes a size adjustable window through which the extrudate passes, the extrudate being guided by a conveyor to extend inclined downwardly into the pond and to exit through the seal plate and pond with a large radius upwardly curved section. The level of liquid in the chamber is controlled by circulating liquid from the higher level of the pond within the chamber to the lower level of the pond outside the chamber with the amount of liquid circulated being inversely proportional to the absolute pressure in the vacuum chamber. Optionally, the circulated liquid may pass through a cooler to control the liquid temperature. The size adjustment of the window or exit orifice in the seal plate may be used to control the flow of liquid from the lower level pond portion outside the chamber to higher level pond portion inside the chamber, particularly during start up. To the accomplishment of the foregoing and related ends the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed. In the annexed drawings: Figure 1 is a side elevation of a vacuum foam extrusion system in accordance with the present invention with the liquid circulation and level controls shown schematically; Figure 2 is a front view of the seal plate showing the variable gate through which the submerged extrudate passes; and Figure 3 is a somewhat enlarged side elevation of the system at the seal plate separating the two levels of the liquid pond. Referring first to Figure 1 there is illustrated a large elongated horizontally extending vacuum chamber 10 which includes a bulkhead 11 at one end and a seal plate 12 at the opposite end. The chamber is substantially elongated and is horizontal throughout its length and is in the form of a large diameter pipe. The chamber may be formed of interconnected and sealed large diameter concrete pipe sections such as shown in US-A-4,199,310. The bulkhead 11 may be mounted for movement on a carriage 14 for movement toward and away from the end of the vacuum chamber 10. On the exterior of the bulkhead 11 there is provided one or more extruders 15 while an extrusion die 16 is on the interior of the bulkhead. Also mounted on the interior of the bulkhead are shaping rolls to control the configuration and shape of the extrudate exiting the die. For more detail as to the configuration of the shaping rolls and the manner of mounting both the die and shaping rolls on the bulkhead, reference may be had to US-A-4,234,529; 4,247,276 and 4,469,652. In any event, the vacuum chamber may be opened and closed by moving the bulkhead 11 horizontally toward and away from the end of the chamber. Positioned in the chamber fairly close to the bulkhead 11 is a dam plate 20, the upper edge 21 of which extends somewhat above a horizontal centerline of the chamber. Positioned outside the chamber beyond the seal plate 12 is a containment structure 24 which includes side walls 25 and 26 and an end wall 27 remote from the chamber 10. The opposite end wall 28 of the containment structure 24 includes the seal plate 12. The containment structure 24 together with the vacuum chamber 10 forms a containment for a pond 30 of liquid, preferably water, extending from the dam plate 20 to the containment structure wall 27. As indicated, a portion 31 of the pond is outside the vacuum chamber 10 while a portion 32 is inside the vacuum chamber. Such portions are on opposite sides of the seal plate 12 and communication between such portions is obtained by a window 34 in seal plate 12. A vertically movable gate 36 mounted on the seal plate and driven vertically by a motor 37 controls the size of the opening of window 34. The foam extrudate 40 leaves the die 16 to expand and be shaped and then passes under the end 42 or tail pulley of a conveyor belt 43 which is inclined downwardly to bring the extrudate below the level of the pond 30 in the vacuum chamber 10. The foam extrudate will float up against the conveyor belt, and the speed of the conveyor belt is closely controlled to draw the foam extrudate from the vacuum chamber. The conveyor belt 43 passes through the window 34 and then curves upwardly as indicated at 45 terminating in head pulley 46 driven from drive 47. The curvature of the conveyor belt need not necessarily be uniform but in no case should the curvature be on a radius less than 50 m. Reference may be had to US-A-4,044,084 and 4,199,310 for disclosures of such conveyors used in inclined barometric legs or vacuum chambers. After the extrudate 40 exits the portion 31 of the pond 30 outside of the vacuum chamber 10 it is exposed to atmosphere, and the extrudate proceeds in the direction of the arrow 48 for subsequent cutting and processing. When the gate 36 is in its most closed position it provides fairly close clearance for the conveyor 43 and extrudate 40 and acts as a gross flow control valve for water moving from the portion 31 of the pond 30 outside of the chamber to the portion 32 inside the chamber, particularly during start-up. In order to control the level of the water within the vacuum chamber 10 there are provided two water circulation pumps 50 and 51 which draw water from the bottom of the vacuum chamber as indicated at 52 and 53, respectively, just inside the dam plate 20. The outlet of the pumps passes through check valves 54 and 55, respectively, and through power operated valves 56 and 57, also respectively. Such power operated valves are in pump outlet lines 59 and 60, respectively, which extend to cooling tower 61 through respective valves 62 and 63. The cooling tower may be bypassed by closing the valves 62 and 63 and causing the water to flow through lines 64 and 65 leading directly into the pond 31 outside of the vacuum chamber. Such lines may also include respective valves 66 and 67 which are opened and closed alternately with valves 62 and 63. Water from the cooling tower 61 returns to the pond 31 through line 69, again outside of the vacuum chamber. Also, in addition to the outlet check valves 54 and 55 each pump is provided with inlet and outlet shut off valves 72 and 73 as well as a pressure gauge 74. The pumps may be utilized concurrently or separately. Vacuum is established in the vacuum chamber 10 by one or more vacuum pumps 77. The vacuum level is controlled by vacuum controller 78 through an air bleed valve 79. The vacuum controller 78 is also electronically connected to both level control 80 and motor 37, the latter controlling the position of gate 36 on the seal plate. The level controller connection is by a set point control connection 81. The level controller 80 is connected to both the vacuum and water sides of the vacuum chamber through valves 83 and 84. The level controller in turn controls level control valves 56 and 57 in the pump discharge lines 59 and 60, respectively. The vacuum chamber 10 just to the inside of the dam plate 20 is provided with a sump and drain 86 in the event water splashes over the top 21 of the dam plate. During operation of the system the pumps 50 and 51 function to draw water from the portion 32 of the pond within the vacuum chamber and deliver that water either directly or through the cooling tower to the portion 31 of the pond outside the vacuum chamber or exposed to atmosphere. The water thus flows from one end of the pond to the other and circulates back through the adjustable window 34 in the seal plate 12. Before vacuum is established in the vacuum chamber 10 the water level on both sides of the seal plate 12 will be at a common level. At this point the pumps 50 and 51 are turned on but the level control valves 56 and 57 are closed. When the vacuum chamber is closed, by moving the bulkhead 11 against the end of the chamber, the vacuum controller 78, being set to a desired level, turns on the vacuum pumps 77. As the vacuum level goes up (absolute pressure goes down) water will be drawn from the exterior of the chamber into the interior of the chamber causing the water level on the interior of the chamber to rise. During this process, the level controller begins to open the level control valves 56 and 57 and the vacuum controller begins to close gate 36 through motor 37. The control of the gate 36 reducing the size of the orifice acts as a gross flow control valve for water moving into the vacuum chamber. Both the vacuum controller 78 and the level controller 80 have optimum adjustable set points. When the vacuum controller 78 reaches its set point, it begins to maintain the desired vacuum level by actuating the bleed valve 79. When the level controller 80 reaches its set point it will begin to modulate the level control valves 56 and 57 maintaining the water level within the vacuum chamber within a given narrow range simply by increasing or decreasing the flow or circulation of water from the vacuum chamber to the atmospheric portion of the pond 31. As the vacuum level increases, the water level inside the vacuum chamber increases and in order to maintain the water level at an acceptable level the water flow through the valves 56 and 57 also increases. During operation of the system the vacuum controller resets the level controller 80 on a continuing basis which in turn modulates the level control valves 56 and 57. In other words, the vacuum controller is the master and the level controller is the slave. By resetting the level controller constantly, the vacuum controller then can anticipate changes in the water level thus maintaining a precise water level inside the vacuum chamber in response to the vacuum level. However, the gate 36, once in its final position, will normally stay in that position and is used as such gross flow control valve only during start up or shut down of the system. In any event the full system permits the water level inside the vacuum chamber to be controlled in response to the vacuum level within the chamber thus obviating the necessity for a long and inclined barometric leg. The system may also operate with less total volume of water. With the present invention there is provided a foam extrusion line which includes an elongated horizontally extending chamber with the chamber being horizontal throughout its length. The chamber is provided with an extrusion die at one end forming a foam extrudate within the chamber and a water baffle at the opposite end to enable the chamber to be maintained under vacuum. The foam extrudate is guided through the baffle to atmosphere for subsequent cutting and processing. In order to maintain the liquid level within the vacuum the liquid is circulated from the higher level of liquid within the vacuum chamber to the lower level of liquid outside the chamber during extrusion. The amount of liquid circulated is inversely proportional to the absolute pressure in the vacuum chamber.

Claims: Foam extrusion apparatus, comprising a vacuum chamber (10), a bulkhead plate (11) and an extrusion die (16) at one end of said chamber, a liquid baffle at the opposite end of said chamber operative to seal the chamber and through which the extrudate (40) passes from the chamber to atmosphere, said liquid baffle comprising a pond of liquid (30) having a higher level (32) in the chamber and a lower level (31) outside the chamber, means (50, 51) to circulate liquid from the higher level to the lower level during extrusion to maintain such higher level in the chamber, and means (43) to guide said extrudate into the higher level pond within the chamber and then through said liquid baffle to exit to atmosphere from the lower level pond outside the chamber. Foam extrusion apparatus as set forth in claim 1, characterized in that the amount of liquid circulated is inversely proportional to the absolute pressure in the vacuum chamber (10). Foam extrusion apparatus as set forth in claim 1 or 2, characterized in that said chamber (10) is horizontal and that the extrudate (40) is guided downwardly into the higher level pond (32) within the chamber. Foam extrusion apparatus as set forth in anyone of claims 1 to 3, characterized in that a dam plate (20) is provided within said chamber (10) and a seal plate (12) at the end of said chamber opposite said die (16) to form the pond of higher level (32) in the chamber. Foam extrusion apparatus as set forth in claim 4, characterized in that an exit orifice (34) for the extrudate (40) is provided in said seal plate (12) below the pond level, means (37) controlling the size of said exit orifice. Foam extrusion apparatus as set forth in claim 5, characterized in that said means (37) controlling the size of said exit orifice (34) comprises a motor (37) driven gate (36) on said seal plate (12). Method of preparing foam plastic, comprising the steps of extruding a foamable plastic melt into a vacuum chamber (10), passing the foamed extrudate (40) into a body of liquid (30) which is partially within and partially outside of such chamber (10) to exit the chamber through such body of liquid, the body of liquid having a higher level in the chamber (10) and a lower level outside said chamber, the level of that portion (32) of the body of liquid (30) within such chamber (10) into which the extrudate (40) passes being controlled by circulating liquid from that portion (32) within the chamber to that portion (31) outside the chamber (10) from which the extrudate exits to atmosphere. Method as set forth in claim 7, characterized in that the amount of liquid circulated is inversely proportional to the absolute pressure within such vacuum chamber (10). Method as set forth in claim 7 or 8, characterized in that the step of controlling comprises the step of controlling a level control valve (56, 57) in the outlet line (59, 60) of a circulating pump (50, 51). Method as set forth in claim 9, characterized in that the level control valve (56, 57) is in turn controlled by a level controller (80).

Inventors: PAGAN, RAFAEL
Applicants: U.C. INDUSTRIES, INC.
Application year: 1988
Main classification: B29C67
IPC codes: B29C44, B29C47