A common method for forming parts from plastic involves the extrusion of material from an extruder. The extruded plastic, or extrudate, however, remains in the molten state immediately upon exiting the extruder. As a result, the extrudate is subject to undesirable deformation until it cools sufficiently to become solid.
Over the years, efforts have been made in the plastics industry to control the size and shape of extruded plastic profiles and extruded plastic tubing as they cool and harden. One widely used method of cooling extruded profiles consists of passing the profile onto an air rack after it exits the extrusion die. Compressed air is directed or blown onto the extrudate to facilitate cooling while the air rack maintains the size and shape of the profile as it solidifies. Another method of maintaining the size and shape of the extruded plastic involves the spraying or cascading of water onto the extrudate after it exits the extruder, or immersing the extrudate in a water bath as it exits the extruder. Spraying and water tanks of this type have been commonly used in the cooling of extruded plastic profiles. However, tanks of this type are often required to be of substantial length to insure that the extrudate is sufficiently cooled and hardened before exiting the tank. As a result, commercially available spraying and water tanks are typically of substantial length, twelve feet or longer, to ensure sufficient cooling.
Typically, alternative methods include passing the extrudate through a sizing and cooling tool or die, which is immersed and contained within a water tank, to help maintain the size and shape of the extrudate as it cools. Effective sizing tools or dies are often quite complex and require precision machining. Often, a sizing tool or die is comprised of a combination of precision machined tapered components which impart the desired size and shape to the extrudate. Various types of sizing or calibrating tools appear in Kessler, U.S. Pat. No. 4,181,487 and Gauchel, U.S. Pat. No. 4,411,613. Due to the precision machining required, effective sizing tools or dies are often quite expensive. Furthermore, sizing tools or dies are often of substantial length, typically six to eight inches or longer.
It is has been found that water in the spraying or water tanks exerts a pressure on the extrudate which can cause unwanted deformation of the extrudate as it cools. Extruded tubing and other hollow extruded articles are particularly susceptible to deformation during the cooling process. To help eliminate the deformation caused by the water in the cooling tanks, it has been found advantageous to place the water under a negative pressure. This has been accomplished through the use of a vacuum pump to create a less than atmospheric pressure in the water of the cooling tank. The vacuum within the cooling tank coupled with the higher pressure within the hollow portion of the extrudate helps to maintain the shape of the extrudate during the cooling process. Sizing tools have also been adapted with vacuum lines to enable vacuum pressure to be applied to the extrudate as it passes through the sizing tool.
Vacuum sizing tanks with complex vacuum pressure control systems have been developed, such as ones disclosed in DeCoursey, U.S. Pat. No. 5,008,051 and Preiato, U.S. Pat. No. 5,340,295, the disclosure of which are herein incorporated by reference. The use of elaborate vacuum sizing tanks with carefully controlled vacuum pressures are particularly useful where precision tubing is involved. However, commercially available vacuum sizing tanks of this type are expensive and of substantial length to accommodate a variety of applications. Furthermore, carefully controlled vacuum pressure is often unnecessary for profile parts and low precision hollow extruded parts.
The above-described water tanks typically consist of an enclosed trough or close-walled tank holding water the entire length of the tank, rendering further modification or manipulation of the extrudate impractical, after it passes through the sizing tool or exits the tank. Vacuum sizing tanks are structurally enclosed to hold the vacuum pressure, which also renders further manipulation or modification of the extrudate after passing through the sizing tool impractical.
Therefore, it is an object to the present invention to provide a versatile vacuum sizing system wherein the vacuum sizing tank is substantially reduced in length, and wherein said apparatus is useful for a variety of applications. It is a further object of the invention to reduce the high cost of lengthy precision machined sizing tools by substantially decreasing the length of the sizing tool or die. It is a further object of the invention to provide a vacuum sizing system wherein the shape of the extrudate can be further manipulated or modified in an ambient air environment after passage through a vacuum sizing tank.