The present invention relates to a method for manu¬ facturing helical screw rotors in accordance with the pre¬ amble of claim 1. The invention also relates to rotors manufactured in accordance with the method. Normally, helical screw rotors are machined from solid metal blanks. Moreover, the rotors have a complicated geometric shape, which places great demand on accuracy in manufacture. Machining of the rotors from solid rotor blanks also involves the removal of large quantities of material from the blank. All of which results in long manufacturing times and high production costs .

Consequently, there has long been a demand for a simpler method for the manufacture of helical screw rotors, without the need to observe such high precision in manufacture, such that rotors of this kind can be produced in large numbers at. reasonable cost.

Swedish Patent Applicatin No. 8603720-7 and PCT/SE87/00397 teach a number of known methods for the manufacture of helical screw rotors, in which manufacture is simplified by moulding a plastic surface layer onto a plastic or a metal rotor body, which in turn is mounted on a metal shaft. These konwn methods are based on the concept of first producing a rotor body with no particular atten¬ tion to measurement accuracies, and then moulding onto the body a surface layer formed to exact measurements. The desired result is normally achieved, when the rotor body is made of metal . Although plastic-moulded rotor bodies represent a considerable simplification in the manufacture of such bodies, subsequent shrinkage of the plastics material creates drawbacks which are not found when casting or moulding other kinds of plastic objects, where sub-

sequent shrinkage of the plastics material can be anti¬ cipated and compensated for. The particular difficulties which manifest when moulding helical screw rotors relate to the helically extending cams, which as a result of the shrinkage become distβrted in a manner which cannot readily be calculated prior to manufacture. This distortion changes the pitch of the cams in the vicinity of the ends thereof. The effect of this shrinkage-induced distortion can be eliminated in some cases, by imparting to the thin-wall lands or threads of female rotors a degree of elasticity which will allow the lands to be deformed elastically in operation. This feature will also afford certain advan¬ tages, as reported in PCT/SE86/00109.

Another solution must be found, however, in the case of male rotors and female rotors of larger land-wall thick¬ nesses.

This solution is provided by the present invention, according to which the rotor body is moulded, injection moulded, etc., from several layers of solidifying material, suitably a material composite, such as to allow especially shrinkage-induced distortion of the lands or threads to be eliminated, by application of the outer layer. The degree of distortion and measurement deviations which remain despite moulding the rotor body in several layers will not be so great as to prevent the distortion and measurement deviations from being eliminated, by moulding the outer layer of polymer-based material onto the rotor body, which layer can then be made relatively thin and consequently will have negligible shrinkage. The rotor body can be moulded in accordance with one of a number of different methods, including sintering, using many different kinds of material, as defined in claims 2 to 7. All of the layers of the rotor body will preferably be made from a polymer-based material, suitably a material composite, i.e. a material reinforced with fibres in accor¬ dance with known techniques .

The invention will now be described in more detail with reference to an exemplifying embodiment thereof illustrated in the accompanying drawing, in which Figure 1 illustrates in axial section the top half of a rotor manufactured in accordance with the invention, and Figure 2 is a sectional view taken on the line II-II in Figure 1.

The illustrated rotor, which is a male rotor, includes a shaft 1 , which may be made of steel or a reinforced plas¬ tics material of extremely high bending strength, and a rotor body 2 which has moulded thereon an outer layer 3 of polymer-based material. The illustrated rotor body 2 comprises two body layers 21, 22 of polymer-based material, of which the innermost body layer 21 is somewhat thicker than the outermost layer 22. Shrinkage of the body layer 21 shall have terminated, before moulding of the outer layer 22 is complete. Similarly, shrinkage of the outermost of said two body layers shall have terminated, prior to mould¬ ing of the thin outer layer 3 being completed, therewith to achieve the highest possible measurement accuracy and the least possible distortion of the rotor lands, in the simplest possible manner.

To ensure uniform solidification of the rotor body, the thickness of each layer will preferably be as uniform as possible. The shaft embodiment with helical lands or threads described in the aforesaid PCT/SE86/00109 can be used to this end, and when different materials are used, the thermal coefficient of expansion of the layers should be so selected in respect of the material of adjacent layers and the material of the shaft and the outer layer 3 as to ensure that the smallest possible differences in thermal expansion are obtained.

In the case of the preferred embodiment, the shaft 1 is made of steel which has a thermal coefficient of expansion of α = 12 x 10 ^~6 m/m/°K, the layer 21 is made of polyetherimide with a 30 % glass-reinforcement. ULTEM 2300V-^ (General Electric) with a density of 1.51 g/cm ³ , a bending

modulus of 9000 N/mm ² , α = 20 x 10 ^~6 m/m/°K, the layer 22 is made of a polyetherimide ULTEM 2100®with a 10 % glass- reinforcement, density 1.34, bending modulus 4500 N/mm ² , and α = 32 x 10 ^~6 . The outer layer 3 consists solely of polytetraflouroethene plastic (TeflonvE/) and varies in thickness from about 1/100 mm to 1-2 mm depending on the extent of subsequent shrinkage of the rotor body, prior to moulding of the outer layer 3 being completed.

It will be understood that the invention is not restricted to the described and illustrated embodiment thereof and that many modifications are possible within the scope of the inventive concept defined in the following claims. For instance, the various layers may include particles of metal and or ceramic material, in addition to reinforcement fibres of greater or smaller lengths. At least one. of the outer layers of the rotor body may comprise a porous plastic material, in which case the density will preferably increase in a direction towards the two faces of the layer.