Molded bodies, made from foam plastics have an advantage over molded bodies made from dense plastic, because they can be fabricated on the one hand independently of wall thickness and on the other hand they have considerably lower weight. This results in a considerable saving of material without a sacrifice in the strength of the molded body. The disadvantage of molded bodies made from plastic which contains a blowing agent is that it produces a rough surface. If this rough surface is not desirable, only an additional working step can make the molded body of the required smoothness. To later improve the surface of a molded body made from foamed plastic, a number of procedures are known, all of which require a high expenditure which usually cannot be justified. German Pat. No. 1,778,457 describes a procedure for fabrication of plural-layer molded bodies with a foamed core and a non-foaming thermoplastic outer skin; first a plug is pushed into the mold and filled with an uncompleted charge of non-foaming thermoplastic material; thereafter, before solidification of the inside of this plug to the first charge, a second charge is injected which contains a blowing agent where the material of the second charge presses the material of the first charge to all sides into the mold so that it is completely filled. Devices for the execution of this procedure are described in the German Pat. No. 1,779,280, the German Pat. No. 1,814,343, and the German Pat. No. 2,007,238; the second charge is introduced only when the introduction of the first charge has been interrupted or completely ended. However, this procedure often leads to undesirable markings on the surfaces of the finished molded bodies. Furthermore, it is possible, especially on molded bodies with complicated molds, that the first injected charge is not pressed uniformly against the walls of the mold by the following foaming charge, but is rather driven so far apart that it tears. In this case, the smooth surface of the outer skin is interrupted and the molded body is not usable. To prevent this from happening, the amount of the non-foaming thermoplastic materials is often increased by a certain amount which amount is not necessary for the fabrication of a molded part with a non-foaming thermoplastic outer skin. This naturally leads again to an additional weight increase.
Another procedure has been suggested (which does not belong to this art), especially for injection molding of plastic parts with thick walls and smooth surface and porous core; wherein, first, the material forming the smooth surface and, thereafter, the inner plastic is injected into the mold which contains the blowing agent. First, a part of the material which forms the smooth surface is injected and, thereafter, the plastic containing the blowing agent with more material forming the smooth surface is injected at the same time. The device for execution of this procedure contains an injection head which is connected with two injection cylinders. Within the injection cylinder, which takes in the material forming the smooth surface, is arranged a piston-cylinder operating as an injection piston which takes in the material containing the blowing agent. This piston and cylinder carries on its front end a displacement jet and its rear end is equipped with channels; it is capable of being pushed into the inside of the piston-cylinder carrying the material containing the blowing agent. Furthermore, the injection jet is equipped with a gate-valve movable transversely of the flow channel. By this design there is the condition that the outlet nozzle arranged on the mold body extends relatively far into the closed injection jet, so that the mold ejection procedure is greatly handicapped. In addition, this device is complicated and expensive, because of the shape of the injection jet. After the hardening of the molded body, the outlet nozzle has to be removed by cutting or breaking off, which requires an additional working procedure which should not be overlooked.
In the German Pat. No. 1,154,264 a device is described for a continuous extrusion press for endless mold bodies or of plate-shaped elements containing a foamed material core and an outside skin made from thermoplastic material, in which the jet is arranged centrally inside the injection head of the extrusion press for the exit of the foam plastic and is surrounded by a jet for the exit of the foam plastic material, from which the thermoplastic material exits along with the foamed plastic. Based on the working procedure of this device, these mold bodies of plate-shaped design elements have front faces which are not provided with a layer of thermoplastic material; this means that formed bodies or plate-shaped design elements may not be fabricated where its foam material core is completely surrounded by a cover of dense thermoplastic material.
The present invention provides apparatus for the discontinuous fabrication of mold bodies made with several layers of thermoplastic material, which has a simple design by which the different materials can be injected separately or in common. Furthermore, by means of this device, it is possible for the thickness of the individual layers of the finished mold bodies to be changeable. To solve this task, according to this invention, it is suggested that the ring jet be limited in its movement through the injection head and that an axially-displaceable ring jet guided within the injection head be provided in the form of a closing sleeve whose bore forms the central jet equipped with a displaceable closure needle.
According to a further characteristic of this invention, the front faces of the sleeve and the closure needle are closed in their injection position in the plane of the front face of the injection head. By this method, it is assured that, in the area of the injection head, no outlet nozzle is created, so that the molded bodies may be removed without any difficulty by opening the mold.
The front face of the sleeve, closure needle, and injection head can form a part of the inside wall of the mold. This design would be used in the case where no outlet nozzle on the molded body should be created. Therefore, the expenditure of work to remove the outlet nozzle is eliminated. In addition, a molded body made by this device contains a covering made from dense thermoplastic material which completely surrounds the foam material core. Connected to the sleeve and the closure needle are separate or common displacement drives, where the use of a common displacement drive can bring the sleeve against the force of a spring through the closure needle into its arrested position. According to a further characteristic of this invention, the displacement drives are controllable for the sleeve and the closure needle in accordance with the position of the piston within the injection cylinders, so that the opening and closing of the central jet and the ring jet may be selectively chosen.