In German patent document 37 40 916, the application of a wear-reducing coating on the surface of a worm or screw for a plastifying device is disclosed in which molybdenum or a molybdenum-containing coating material is applied to a surface of the worm or screw body, the coating on the body is subjected to heat to effect fusion between the coating and the underlying base material, and the coated screw or worm is thereupon cooled.
As noted, the plastifying device can be an extruder, injection-molding machine or compounder and, for the purpose of this description, all such machines will be understood as comprised within the term "extruder" where that term is used to describe a machine or device embodying the invention.
An extruder can be used, for example, to produce articles of synthetic resin material, plastic or rubber and can comprise a housing and at least one worm or screw rotatable in this housing to displace the material to be processed through the housing. During the rotation of the worm or screw and the passage of the flow of the plastic or plastifiable material through the housing, the rib, thread or flight of the worm or screw may be subject to considerable and continuous wear.
In addition, there may be direct contact between the outer surface of the flight and the wall of the screw housing. To minimize mechanical deterioration of the parts in contact, good emergency running properties must be provided at the contact surfaces.
In the past this has generally been obtained by coating the outer surfaces of the flight or thread of the worm or screw with a suitable low-wear and low and reduced-friction material.
An appropriate material for this purpose is molybdenum. In Japanese patent document 61-139 682, molybdenum in pulverulent form is applied to the steel base material of the body and irradiated with laser energy. To ensure that the layer will bond firmly enough to the base material, before application of the powder, pits or depressions are generated in the surface to be coated. This pretreatment step makes the coating process complicated and expensive. Because molybdenum has a substantially higher melting point than the steel, a relatively large amount of thermal energy must be applied to the base body for effective bonding and this can give rise to deformation and cracking at the interface and surface.
Swiss patent 576,526 describes the application of a molybdenum coating to an extruder worm at the rib or thread surface by flame-spraying. According to German patent 37 18 779, this does not produce effective results because the layer with excessively high molybdenum content does not bond satisfactorily to the base body of the worm and has a tendency to spall off therefrom.
In German patent 37 18 779, an alloy is used as the material for the wear-reducing layer which contains between 40% and 70% molybdenum. This alloy can be applied to the base body by a special process, namely, plasma-powder deposition welding. The prior art process gives rise to useful results only upon the use of the expensive plasma-powder deposition welding which is of high capital cost. Furthermore, alloys are used with a molybdenum content of a maximum of 70%.
The advantageous characteristics of molybdenum, especially the good emergency running characteristics, can be utilized only with limitations. Even with this process, a relatively high amount of thermal energy must be transferred to the basic body and leading to unsatisfactory intrinsic stress characteristics with distortion and crack formation.
One could conclude that high molybdenum content with deposition welding utilizing coating materials available as powder would be obtainable by the method of German patent document 37 40 916. From the teachings of this document, the danger of crack formation in the generation of the wear-reducing coating of powder can be reduced by reduction of the molybdenum content in the coating material. In this conventional process, the protective coating is applied by thermal spraying onto the base body and by then melting the coating into the material of the base body by a laser beam remelting step. The coating material can be molybdenum or an alloy predominantly consisting of molybdenum. Even in this two-stage process, relatively high quantities of thermal energy must be applied to the base body during the thermal spraying and here as well the undesirable intrinsic stress conditions can result in distortion and crack formation.