DESCRIPTION The invention relates to a protection element for a stair tread, comprising at least one element portion intended for placement on at least part of the tread portion of the stair tread. The invention further relates to a method for manufacturing a protection element according to the invention for a stair tread. Protection elements as referred to in the above introduction are known for protecting the treads of a staircase. Usually they consist of an element portion made of a metal, which is fixed to the stair tread in a known manner, for example by screwing, glueing and the like. The object of the invention is to provide an improved protection element which on the one hand is of a light but strong construction and on the other hand possesses good antiskid properties, and which is furthermore easy to fit. According to the invention, the protection element is to that end built up of at least one first sublayer of a plastic material obtained by means of an injection moulding process. More specifically, in one embodiment of the invention the protection element is. built up of at least two sublayers made of different materials, at least one of which sublayers has been formed on the other sublayer by means of an injection moulding process. This makes it possible to form the protection elements of lightweight materials, wherein the various sublayers can be designed to have mutually different properties. Thus, materials that provide the required strength can be used for the lower(most) sublayer, which is to be placed directly on the tread, whilst a material that in particular has good antiskid properties can be used for the other sublayer, which functions as the tread portion. More specifically, in a special embodiment all the sublayers of the protection element have been formed by means of an injection moulding process. The layered structure of the protection element makes it possible to use sublayers having mutually different properties. Thus, materials providing the required strength can be used for the lower(most) sublayer, which is to be placed directly on the tread, whilst a material having in particular good antiskid properties can be used for the other sublayer, which functions as the tread. In a special embodiment, the sublayer that can be placed into contact with at least part of the tread is made of a thermoplastic material in order to obtain said antiskid properties. The sublayer most remote from the tread may be made of a thermoplastic elastomer, for example TPE-E, TPE-U, EPDM, NR, SBR or LSR. More in particular, at least one of the sublayers may be made of a metal, wood or a textile. In special embodiments the thickness of the sublayer that can be placed into contact with at least part of the tread may range between 0.5 and 3.5 mm, whilst the thickness of the sublayer most remote from the tread may range between 0.5 and 3.5 mm. In yet another functional embodiment, the protection element comprises a second element portion, which is connected to the first element portion at an angle. Said angle may be 90°. The method according to the invention is characterized at least by the steps of: A) placing a first sublayer into the moulding cavity of an injection moulding device; B) forming at least one further sublayer of a further plastic material on the first sublayer by means of an injection moulding process. This makes it possible to form protection elements consisting of layers of different materials, with the various sublayers having mutually different properties. In a special embodiment of the method, step A comprises the step of Al) forming the first sublayer of a first plastic material by means of an injection moulding process. Another step comprises the successive forming of the various sublayers by means of an injection moulding process. In a special embodiment of successive formation of the various sublayers by means of an injection moulding process, the method according to the invention may further comprises the steps of C) allowing the first plastic material of the first sublayer formed by means of an injection moulding process in step Al to cool down; D) enlarging the moulding cavity of the injection moulding device; E) forming a next sublayer of a plastic material in the space thus formed in the moulding cavity, using an injection moulding process. The method according to the invention may furthermore comprise the step of F) repeating steps D and E one or more times so as to create a multilayer protection element by injection moulding, wherein each layer can be designed to have its own specific material property. The method as well as embodiments of a protection element according to the invention will be explained with reference to a drawing in the detailed description below. In the drawing: Figure 1 shows a first embodiment of a protection element according to the invention; Figures 2a-2c are various views of the embodiment of a protection element according to the invention that is shown in figure 1. For a better understanding of the invention, like parts are indicated by identical numerals in the description below. Figure 1 shows an embodiment of a protection element according to the invention for a stair tread 1. The protection element 10 is to be placed on the tread portion Ia of the stair tread 1 and is built up of a first element portion 11 and a second element portion 12, which extend at an angle, preferably an angle of 90°, to each other. Thus the protection element 10 can easily be supported on the edge Ic of the stair tread 1, with the first element portion 11 being placed on the tread portion Ia and the second element portion 12 abutting against the face edge Ib of the stair tread 1. According to the invention, the protection element 10 has a layered structure comprising at least two sublayers 13, 14. Said at least two sublayers 13, 14 are made of different plastic materials, and at least one of the layers 14, 13 has been injection-moulded on the other sublayer 13, 14. It is preferable, however, to form the protection element 10 as a whole by means of an injection moulding process, in which case all the sublayers 13, 14 are sequentially injection-moulded. This makes it possible to form the protection element 10 of lightweight materials, and the various sublayers 13, 14, etc, can be designed to have mutually different properties. Thus, a material that primarily provides strength and stiffness may be used for the lower(most) sublayer 13, which is placed on top of the tread portion IA of the stair tread 1, whilst materials having good antiskid properties, for example, may be used for the upper(most) sublayer 12. According to the invention, the sublayer 14 may furthermore be provided with cuts 15, which on the one hand have an antiskid effect but which, in addition, are also capable of discharging any sand and dirt. Said recesses furthermore prevent undesirable warping of the protection element. In another embodiment, the recesses 15 can be filled with the material 13 of the first element portion 11, which enhances the stiffness of the protection element. Preferably, the first element portion 11 and the second element portion 12 extend at an angle of 90° with respect to each other. Figures 2a, 2b and 2c show larger-scale views of an embodiment of the protection element according to the invention. As in particular figure 2c clearly shows, the layered structure of the protection element 10 (both element portions 11 and 12) is depicted. As already noted before, at least one sublayer 13, 14 may have been formed on the other sublayer 14, 13 by means of an injection moulding technique. It is preferable, however, to form both sublayers sequentially by means of an injection moulding process. Furthermore it is possible to use more than two sublayers 13-14, etc. The layered structure of the protection element not only provides an aesthetically attractive appearance, but more particularly it also has a functional advantage, because it makes it possible to design the various sublayers 13-14 to have properties that the other sublayer(s) do not necessarily have. This enables an efficient use of the right materials at the required locations in the protection element 10 according to the invention. As figure 2 shows, the protection element, and in particular the second element portion 12, has an upright edge 20 that functions as an abutment edge for the second and further sublayers 40, etc. The upright edge 20 furthermore prevents the second sublayer 14 from being undesirably detached or stripped from the first sublayer 13. As a result, an adequate adhesion of the second and further sublayers to the first sublayer 13 is obtained, which adhesion can be improved by forming one or more openings 21 in the first sublayer 13, in which openings a part 14a of the material of the second sublayer 14 can be received. Since the production technique of the protection element according to the invention makes it possible use different materials having different physical properties for the various sublayers 13, 14, the first sublayer 13, which is placed on the tread portion Ia of the stair tread 1, is made of a thermoplastic or a mixture of thermoplastics. A filler may be used, if desired. Examples of suitable materials for the first sublayer 13 are: ABS, ABS/PC, ASA, CA, EVA, PA 6, PA6 (mod. +25% GF), PAT6.6, PA6.6 (mod. +25% GF), PA12, PA12 (mod. +25% GF), PBT, PC, PC/PBT, PE, PET, PMMA, POM, PP, PPO mod., PEE (mod.), PS, PSU, PVC hard, SAN. Optionally, the sublayer 13 or 14 may be made of a base material other than a plastic. Think in this connection of steel, textile or wood, which is embedded (whether or not entirely) in a further sublayer 14 or 13 of plastic material, using an injection moulding technique. According to the invention, the second sublayer 14 is preferably made of a thermoplastic elastomer such as TPE-E, TPE-U. Furthermore, the second sublayer 14 may also be made of an elastomer, such as EPDM, NR, SPR or LSR. The method according to the invention for forming a protection element as described above consists of an injection moulding process, and in principle comprises the steps of: A) placing a first sublayer 13 (or 14) into the moulding cavity of an injection moulding device; and B) forming at least one further sublayer 14 (or 13) of a specific plastic material on the first sublayer 13 (or 14) by means of an injection moulding process. On the one hand, the sublayer 13 (or 14) can be placed into the injection moulding device without being injection-moulded therein, in particular if use is made of a sublayer 14 (or 13) consisting of a metal, wood or a textile. In that case the preformed element that forms the sublayer 13 is placed into the injection moulding device and subsequently provided with a second (and possibly third and fourth, etc) sublayer 14 by means of an injection moulding process. According to another aspect of the method according to the invention, both the first sublayer and the further sublayers are injection moulded in the injection moulding device. In connection with the forming of the next sublayers on the first sublayer it is preferable for the materials used for the various sublayers to have a shrink percentage of the same order, more in particular substantially the same shrink percentage. When a next sublayer is being formed on the first sublayer, it is preferably to clamp down the first sublayer in the moulding cavity so as to prevent shrinkage of the first sublayer. After one or more next sublayers have been formed, the protection element thus formed can be removed from the moulding cavity, after which the various sublayers (consisting of materials having substantially the same shrink percentage) will shrink in a uniform and even manner, so that they will not warp. The building up of the protection element according to the invention in layers can also take place by allowing the injection moulding device to cool down after the formation of the first sublayer 13 (or 14) therein so as to obtain a more easily manageable temperature, and subsequently enlarging the moulding cavity of the injection moulding device, after which the further sublayer 14 (or 13) is formed in the space thus created, using an injection moulding technique. This manufacturing step can be repeated a number of times, so that a multilayer structure is obtained, from which the protection element 1 according to the invention is obtained. It will be apparent that the invention as described herein provides a protection element which on the one hand is light in weight and has a long life and which furthermore can be produced in a simple manner in large numbers, but which above all possesses an improved functionality, because the various sublayers can be designed to have mutually different physical properties. More specifically it has become apparent that a protection element according to the invention will not warp during manufacture when the properties of the various sublayers are optimally geared to each other. Think in this connection of the thickness as well as the mechanical properties of each sublayer. Preferably, the materials used for the various sublayers have a shrink percentage of the same order, more in particular substantially the same shrink percentage. The combination of standard PA6 (polyamide) and TPE-U (thermoplastic elastomer polyurethane) has appeared to be very suitable. Experiments have shown that a maximum (and sufficient) adhesion between a sublayer PA6 and a sublayer TPE-U is obtained when the time that passes before the second material (second sublayer) is formed on the first material (first sublayer) is kept as short as possible (a few seconds). When the time that passes before the second material is applied after the first material has solidified is about 1 minute, the adhesion will be significantly less strong and no longer acceptable.