The invention relates generally to rotary screw extruders and methods of extruding hygroscopic synthetic resinous materials. More particularly, the invention is concerned with a rotary screw extruder and a method which devolatilize and dry hygroscopic synthetic resinous materials while the materials are extruded into a finished product thereby eliminating a preliminary dehumidification of the material.
In the past, it has been common to extrude hygroscopic, synthetic resins, such as ABS (acrylonitrile-butadiene-styrene resins), into finished products. However, such extrusion has in the past been preceded by a dehumidifying procedure in which the hygroscopic resins are pre-dried for several hours. When the dehumidifyed resins are extruded through a single vented extruder, a finished product substantially free of pock marks, dimples, streaks, surface roughness, and poor gloss results. A good dehumidification system is not only expensive but also requires a substantial electrical power supply. In addition, the systems occupy valuable floor area in manufacturing operations.
When the hygroscopic resinous material is dehumidified in a hopper dryer, the temperature must be carefully controlled to prevent overheating. Even with careful temperature control, experience has shown an occasional batch of material will become overheated and fuse together causing both loss of time and waste of material.
Another frequent problem with hopper dryers is the difficulty in maintaining uniformly dehumidified material for subsequent use in an extruder. This problem results from the non-homogeneity of moisture content in hygroscopic particulate material.
Previous combinations of a dryer and an extruder have inadequately accommodated the need for rapid color changes often encountered in production operations. The inadequacy results directly from the lead time required to dry a differently colored stock of hygroscopic material for extrusion.
Some rotary screw extruders in the past have employed reduced pressure vent sections to remove volatiles from the extrudate. Typically, however, these vented sections are pressure sealed from one another such as the vented sections of U.S. Pat. No. 2,992,679 which issued to W. W. Twaddle. But, such sealing between sections reduces the effective devolatilization length and is ineffective for use to extrude hygroscopic synthetic resinous materials.
Other extruders have employed continuous devolatilization sections in the extruder screw such as disclosed by U.S. Pat. No. 3,524,222 to Gregory et al. This type of extruder results in nonuniform flow rates which are unsuitable for the extrusion of high quality finished products. Nonuniform flow rates are aggravated by pressure gradients which often occur in long vent sections. Moreover, the extrudate in such extruders frequently develops thermal nonuniformity which is induced by volatile substances undergoing a liquid-vapor phase change. Since viscosity is highly dependent on temperature, such thermal non-uniformity also has an adverse effect on extruded products.
To overcome problems similar to the above, the extruder of the present invention incorporates a varying depth screw in which pumping sections have increasingly greater flow capacities. Such screw design has been used in the past in applications where material is added to the extrudate during extrusion such as illustrated by U.S. Pat. No. 3,287,477 to P. E. Vesilind. Such known screws, however, are used to provide a constant flow rate of extrudate through the extruder and not to provide an efficient means of drying and devolatilizing hygroscopic materials.
It would therefore be desirable to provide a rotary screw extruder which would eliminate problems of the type noted in the extrusion of hygroscopic synthetic resinous materials.