The invention relates to a new and improved method of fabricating hollow cylinders of machines which are subjected to a high working pressure and pronounced erosion effects. Such machine parts are subjected to extremely great wear by virtue of resulting internal pressures in the order of magnitude of 1,000 to 2,000 bar, the frequently high corrosion loads, and last but not least by virtue of the fillers used in processing plastics. It has been found that certain fillers such as, for instance, glass fibers can reduce the service life of such parts to as much as 1/10 of the original value. Since in injection molding machines approximately 0.25 mm constitutes the maximum permissible clearance between worm and cylinder, the significance of the wear factor can be appreciated and, accordingly, on the part of the machine manufacturer considerable efforts have already been made to solve this technological problem.
The solution presently most employed is the use of nitrided steels, which render possible the application of a wear-inhibiting nitrided layer to the inner cylinder surface by means of the methods used therefor (bath nitriding, gas nitriding, ion nitriding). However, there results the drawback that during the treatment a pronounced distortion or, respectively, a change in dimensions frequently occurs and the thin nitrided layer is non-uniformly taken off already during the mechanical finishing of the cylinder. Since the cylinder material has relatively low strength, the wear following the machining of such weak locations progresses extremely rapidly. For the same reason the use of case hardened layers, also, has not been sufficiently satisfactory. Attempts have also already been made to provide hot-work steels with hard surface layers, for instance by treating with boron, flame spraying or by means of the so-called CVD-methods (chemical vapor-phase deposition). Therein, however, problems have occurred with respect to dimensional changes due to the required high coating temperatures and to chipping-off of the hard layers during operation.
An interesting although expensive solution is represented by the manufacture of so-called "bimetallic cylinders". These are understood to be primarily used for machines having larger worm diameters. Specially developed coating alloys are applied to a pre-fabricated part thereof to form an approximately 1.5 to 4 mm thick layer thereon in a hermetically sealed furnace at high rotational speeds needed until cooling (see, for example "Industrieanzeiger" Nr. 60, dated July 20, 1973, pages 1403/4). The respective centrifugal casting, however, it not easy to control; also, the brittleness of the bimetallic layers can lead to problems.