This invention relates to apparatus for accurately and uniformly cooling a preformed polymeric sheet. It further relates to apparatus for extruding and cooling a preformed polymeric sheet wherein the gap between adjacent rolls is measured and controlled for accurately and gently cooling the polymeric sheet with minimum stress.
Polymeric extruded or coextruded sheets are usually extruded out of a slit die of appropriate width, as indicated in the U.S. Pat. Nos. 3,918,865, 3,940,221, 3,959,431, 4,533,510 and 5,466,403, to Frank R. Nissel 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 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. The rolls typically apply embossing to provide a high quality surface impression on the sheet.
Traditionally,the sheet die exit is horizontal and the sheet runs through a cooling roll stack at any angle, either upwardly or downwardly. Vertical, horizontal or various angled roll stacks may be used.
For various purposes, the art has made efforts from time to time to measure the gap distance between cooling rolls. Gauging the gap distance has been achieved mechanically through the use of feeler gauges. Unfortunately, feeler gauges lack the precision needed for maintaining optimal cooling and surface impression. They provide only a go/no go type reading without giving accurate quantitative measurements. They further tend to scratch the roll surfaces. Feeler gauges also pose the danger of being accidentally drawn into the rolls and permanently damaging them.
To avoid these dangers, methods have been introduced where indirect measurement of the gap is taken. This includes measuring the sheet material which passes between the rolls, measuring the distance between the mounting blocks used to support the rolls, and measuring reduced diameter barrel portions. These methods, however, do not and cannot account for actual differences in the diameter of the roll, caused by changes in temperature and other factors. An accurate way to measure the gap between the rolls would be to measure the gap directly, but the reported attempts have not succeeded.
Several devices which monitor a sheet are used to influence the thickness of the sheet product. For example, U.S. Pat. No. 3,809,907 which issued to Schuller et al on May 7, 1974, discloses a device for controlling a bank of viscous material that is formed on, and builds up in front of, the nip of a pair of spaced rolls. Specifically, the rolls are first maintained in close proximity such that the molten material applied to an entry side of the rolls forms a bank of hot material against and before passing through the rolls. The amount of molten steel accumulation is measured by a pair of laser emitters and detectors, located at or near the ends of the rolls. But the laser measures the amount of molten material that is banked up before it flows between the rolls, not the actual roll gap.
Turning now to the field of plastic sheets, extrusion dies with 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 cooling of a preformed polymeric sheet without excessively stressing the polymeric sheet product.
There is a need to provide an apparatus for extruding polymeric sheet materials with an extrusion die capable of pre-forming polymeric sheets of accurately predetermined thickness, and a cooling device having cooling rolls defining a gap of precisely known dimension through which the polymeric sheet passes for the purpose of cooling or embossing, wherein the rolls rapidly and uniformly cool the polymer while exerting only minimal pressure on the extruded sheet.
This invention includes an extruder and cooling rolls that have an adjustable gap through which a preformed polymeric sheet passes. The gap between the rolls is accurately measured and controlled to allow the rolls to exert only a minimal pressure on the polymeric sheet and to impart the best surface finish to the sheet while subjecting it to the lowest possible stress level.