Abstract:
Accurately and uniformly cooling a preformed plastic sheet that is formed at elevated temperature in an elongated plastic sheet die, using upper, middle and lower vertically arranged cylindrical cooling rolls closely spaced to one another to engage the sheet between them, and providing one or more auxiliary cooling rolls connected to provide additional cooling for the sheet.

Description:
This invention relates to apparatus and method for accurately and uniformly cooling a hot, extruded polymeric sheet, and for providing auxiliary cooling for the sheet under mechanically stable, self-supporting conditions and without sagging or distortion. 
     Polymeric extruded or coextruded sheets are usually extruded out of a slit die of appropriate width, as indicated in the U.S. Patents to Frank R. Nissel U.S. Pat. Nos. 3,883,704, 3,918,865, 3,940,221, 3,959,431 and 4,533,510, for example. The hot sheet is then cooled by passing it through a pair or a series of temperature-controlled rolls. Although various roll numbers and arrangements may be used, three rolls are often used, sometimes in vertical planar alignment with each other. The gaps between the rolls are desired to be precisely adjusted according to the desired final sheet thickness. This precision is necessary for a variety of reasons, including elimination of air entrapment between the rolls, which causes adverse or uneven heat transfer or cooling. 
     Extruded plastic sheet is usually cooled with a vertically arranged three-roll stack wherein the middle roll is fixed in position but rotatable and the two upper and lower rolls are movably arranged to press gently against the sheet passing around the middle roll. This pressure assures air free contact with the rolls contacting the sheet for improved cooling. 
     In the field of plastic sheets, extrusion dies having variable lip portions have been used to form polymeric sheets of varying widths and thickness. Once extruded through an adjustable lip die, the polymeric sheet is substantially at its desired thickness. However, there is a demand for rapid cooling of a preformed polymeric sheet without excessively stressing the polymeric sheet product. 
     With increasing extrusion capacity, these cooling rolls have to be increased in diameter to provide sufficient cooling of the sheet. This diameter cannot always be increased sufficiently to assure adequate cooling of the sheet before leaving the last roll under mechanically stable, self-supporting conditions because of the increase of the die discharge distance from the first roll entrance. This excessive distance sometimes can cause sheet sagging and distortion prior to cooling, among other problems, because of the fluidity of the molten polymer. Fluid polymers like polyester are especially difficult to cool; they come out of the die with a consistency like liquid tar and are not very self supporting. 
     It has now been discovered that there is great advantage in adding a further roll (or set of rolls) to the system to assure sufficient cooling of the sheet. Such a cooling roll (or rolls) follow the third roll of the roll stack, but are not pressed against it because the gap would interact with the position of the third roll with respect to the second. It is, however, desirable to maintain a minimum gap between the bottom roll and each such auxiliary roll to minimize distortion of the still soft sheet between rolls. On the other hand, a substantial gap is required to thread the sheet through the roll train safely during start-up. 
     It is an object of this invention to create an improved apparatus and method of achieving a wide roll gap for threading with a closely controlled roll gap for accurate processing while using one or more auxiliary cooling rolls. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a conventional three-roll sheet take-off stack with equal diameter rolls and an extruder sheet die positioned at its upper entrance. 
     FIG. 2 shows a similar arrangement, but with larger rolls, also of equal diameter, for achieving larger cooling capacities, as is often required. 
     FIG. 3 shows a cooling roll arrangement with a smaller diameter upper roll and an inclined die position. 
     FIG. 4 shows a system according to this invention with two auxiliary cooling rolls positioned at an acceptable distance from the primary roll stack for threading. It is typically a distance of approximately 4″ or more. 
     FIG. 5 shows another embodiment of this invention wherein the auxiliary cooling rolls are movably arranged to permit their positioning toward and away from to the primary roll stack. These rolls can be moved either as a group or individually, so that a close or wide gap can be obtained between the first and the second auxiliary and other cooling rolls. 
     FIG. 6 shows two auxiliary cooling rolls in accordance with this invention wherein both are individually movable so that the roll gap between them can also be minimized or maximized for better sheet equipment handling and improved cooling. 
    
    
     SUMMARY OF THE INVENTION 
     Various relative auxiliary roll arrangements are shown in FIGS. 3 to  6  of the drawings as specific examples for illustrative purposes, but are not intended as limitations on the scope of the invention. The rolls may be differently arranged with respect to each other, the purpose in all cases being to provide a better cooling path for the sheet, adjustable to provide a minimum of unsupported sheet while providing significant additional cooling. The closer proximity of the auxiliary rolls also increases the overall contact length with the sheet and therefore accelerates and improves cooling. 
     Turning to the drawings, FIGS. 1 and 2 are drawings of conventional arrangements and are not intended to be within the scope of this invention. The number  10  represents a die, receiving polymer from an extruder; the number  11  designates three cooling rolls arranged in a vertical stack. The hot polymer is ejected from the die  10  and flows between the uppermost two rolls  11 ,  11 , is wound part way a round the middle roll  11  and party way around the lower roll  11 , and then transported as a sheet S to the nip between two takeup rolls  12 ,  12 . 
     FIG. 2 is the same as FIG. 1 except that the cooling rolls are larger to provide more cooling for the sheet. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 is like FIG. 2 except that the upper roll  13  is smaller than the lower rolls  11 ,  11 , and the die  10  is tilted, allowing it to deposit a sheet of hot polymer upon an upper surface of the middle roll  11 , with closer spacing between the die  10  and the upper surface of the middle cooling roll  11 . This has advantages for feeding low viscosity polymers such as polyethylene terephthalate (PET). 
     Turning to FIG. 4, two additional substantially horizontally arranged auxiliary cooling rolls  14 ,  14  are provided. They are spaced apart from each other to provide further cooling to the sheet S prior to guiding it between takeup rolls  12 ,  12 . The rolls  14 ,  14  need not be of the same diameter with any of the preceding cooling rolls. 
     In FIG. 5 the arrangement is the same as FIG. 4 except that the two auxiliary cooling rolls  14 ,  14  are mounted upon a horizontally-oriented movable support  15 , so that they and their support can be moved back and forth in a generally horizontal direction, between the position shown in solid lines and the alternative position shown in dotted lines in FIG.  5 . 
     FIG. 6 differs from FIG. 5 in that each of the auxiliary cooling rolls  14 ,  14  and their supports  16 ,  16  are independently capable of moving back and forth in a generally horizontal direction, toward and away from the lowermost cooling roll  11 . Again, the solid lines indicate one available relative position, and the dotted lines indicate another available relative position of the substantially horizontally oriented rolls  14 ,  14 . Various other positions may be substituted. 
     The substantially horizontal movability of auxiliary cooling rolls  14 ,  14  is important because, whenever the apparatus is to be started up, the original starting polymeric material from the die  10  can be quite soft, or even simply a molten glob, as in the case of polyester, for example. As the cooling process settles down after start-up, it takes on the configuration shown in FIG. 5, making it a necessity to move the horizontally oriented auxiliary cooling rolls  14 ,  14  back and forth with respect to each other after start-up. FIG. 6 shows that the individual horizontally oriented cooling rolls can be independently moved back and forth, and that the operator can shift them independently of the other horizontally oriented auxiliary cooling rolls, however many there might be. 
     An advantage of the small roll  13  at the top of the vertical stack is to allow the operator to position the sheet die  10  as close as possible to the middle cooling roll. Otherwise, there is a danger that air will enter underneath the soft fluid polymeric sheet that is being introduced, thereby developing air bubbles which undesirably penetrate into the sheet as it is being cooled. Often this is not necessary to do, especially in processing high molecular weight polymers. 
     In all cases in accordance with this invention in FIGS. 3-6, the letter “E” is intended to indicate an extruder for blending an extrudable plastic material and delivering it through the die  10 . Further, the die  10  is a sheet die having an elongated transverse lip capable of delivering a polymeric material in sheet form. The entire sheet is preformed in the sheet die  10  and the rolls  13 ,  11 ,  11 ,  14 ,  14  perform primarily a cooling function but not a sheet-forming function. 
     Further, the rolls  13 ,  11  and  14  are driven at compatible peripheral velocities without substantially varying the tension on the sheet S as it passes from roll to roll, thereby applying uniform tension as sharply distinguished from a varying tension such as the tension used to stretch a sheet. 
     In accordance with this invention, a substantially steady tension is applied at all times to the sheet as it is drawn through the cooling rolls by the draw rolls  12 ,  12 , and there is no effort to thin or stretch the sheet. Indeed, the sheet is cooled substantially independently of any significant stretching or thinning. Further, in accordance with this invention, there is no attempt to emboss either surface of the sheet S; its surface is kept smooth as it is drawn over and under the cooling rolls. 
     The die  10  is an elongated die, having a length equivalent to the width of the sheet that is being formed, and can be angled at an angle to the horizontal in a manner to permit the exit portion of the die lip to approach very closely to and above the middle cooling roll  11 , as shown in FIGS. 3-6. This is advantageous in the case of a very soft polymer such as PET which is quite readily flowable; the soft polymer is supported by the middle roll  11  almost immediately after it has left the die  10 . Otherwise there is a likelihood of sagging developing, especially when the extruded soft polymer sheet is at a high temperature. 
     It is in many cases acceptable to operate the apparatus without using an angled die as shown in FIGS. 3-6, especially in the processing of a relatively high viscosity polymer such as polypropylene (PP) and polystyrene (PS). 
     It is not important that the top roll diameter be any different than other rolls in the system. 
     It is also important to mention that the auxiliary cooling rolls  14 , 14  need not be the same diameter as any of the other preceding rolls. In some cases, we may need to build a machine with 3×12″ diameter vertically arranged rolls to quickly cool both surfaces of the polymer, and then use two or more larger diameter cooling rolls to complete the cooling process. 
     Alternatively, it is possible, for example, to use 3×30″ diameter vertically arranged rolls to accomplish most of the cooling and add a set of one or more smaller auxiliary cooling rolls to complete the cooling process. This might be preferred to a system using 3×36″ or 3×42″ diameter vertically arranged rolls where cost, size and shipping can all become important. 
     The maximum number of auxiliary cooling rolls  14 , 14  that might be used is indefinite. On lines where they have been used, 2 or 3 such rolls  14  are normal. However, one such roll  14  could be sufficient for some applications and as we try to achieve higher capacities, four or more may be necessary. 
     Although this invention has been disclosed in regard to specific embodiments thereof, which embodiments are provided as examples, the invention is not limited to the specific details disclosed; its scope is defined in the appended claims.