METHOD FOR MOLDING THIN PARTS MADE OF SINGLE-COMPONENT POLYURETHANE PLASTICS AND APPARATUS FOR PERFORMING THE METHOD

Abstract: A method for molding thin parts made of single-component polyurethane plastics consisting in depositing on the lower part (118) of the mold (116), while the mold (116) is at least partially open, the fluid plastic material in the various regions in an amount which is proportional to the width of the impression, and subsequently, closing the mold (116), compressing the fluid material, and allowing the system to cool.

Description: METHOD FOR MOLDING THIN PARTS MADE OF SINGLE-COMPONENT POLYURETHANE PLASTICS AND APPARATUS FOR PERFORMING THE METHOD TECHNICAL FIELD The present invention relates to a method for molding thin parts made of single-component polyurethane plastics and to the apparatus for performing the method. BACKGROUND ART It is known that plastics in general and single- component polyurethane plastics in particular are melted, for molding, in plasticizing units at temperatures above 160°C and are then injected into closed molds which are cooled or temperature-controlled to temperatures below 50°C. When the plastics in fluid condition is pushed into the mold, as it makes contact with the cold surfaces it begins to harden, consequently reducing the feed passage thicknesses. Feeding is allowed to continue by the hot central part of each section of the passing fluid. This effect becomes more relevant when the part to be obtained is thin with respect to the length along which the material must flow in order to fill the mold and the impression formed in the mold therefore also has a thin cross-section. The term "thin" is used to designate a length-to- thickness ratio of at least 100. This problem is currently dealt with in various manners which generally tend to increase one or more process parameters. In a first case, high injection speeds are used, but such speeds require greater mold closing forces (in order to contrast the resulting high pressures) and consequently entail strengthening the structures of the machine and the closure means. By way of example, closure forces in excess of 100 tons for each impression are required in the case of molds for shoes. The current trend is also to obtain, at the end of the molding process, parts which have no burr in order to avoid subsequent finishing treatments. In a second case, the temperatures of the injected material are set higher than the normal molding temperatures of said material, or multiple injections in heated ducts are performed, but in such cases the constructive complexity of the mold increases. Another case in which the structure of the mold becomes more complicated relates to the provision of surface coatings which facilitate the flow of the material. DISCLOSURE OF THE INVENTION The aim of the present invention is to provide a method for molding thin parts made of single-component polyurethane plastics which uses smaller mold closure forces than those currently believed necessary to obtain the same products with conventional injection methods. A primary object is to provide a method which allows to simplify the molding apparatus, with a consequent cost reduction. Another important object is to provide a method which can be performed in the field of shoe manufacturing by direct molding onto an upper and for manufacturing soles to be joined to the upper at a later time. Another important object is to provide a method which can be performed, in the shoe manufacturing field, in all kinds of molding machine, whether static or rotary. This aim, these objects and others which will become apparent hereinafter are achieved by a method for molding thin parts made of single-component polyurethane plastics consisting in: — depositing on the lower part of the mold, while said mold is at least partially open, the fluid material in the various regions in an amount which is proportional to the width of the impression; — closing the mold, compressing the fluid or pasty material and allowing the system to cool. Advantageously, the dispensing point is simultaneously placed within the impression during deposition. BRIEF DESCRIPTION OF THE DRAWINGS Further characteristics and advantages of the invention will become apparent from the following detailed description of the steps for the execution of two embodiments of the method, illustrated only by way of non- limitative example in the accompanying drawings, wherein: Figure 1 is a schematic view of a molding apparatus adapted for a first way of performing the method; Figure 2 is a partial schematic view of the molding apparatus adapted for a second way of performing the method. WAYS OF CARRYING OUT THE INVENTION With reference to Figure 1, a mold for manufacturing shoes comprises, according to conventional configurations, a supporting frame, generally designated by the reference numeral 10, whereto a rotary last holder 11 is rigidly coupled in an upward region and can move vertically by being mounted on a slider 12 which is slidable on said frame 10, actuated by a pneumatic cylinder 13. The rotary last holder 11 supports a cover 14 and an upper-holding last 15 which are fixed thereto and are conveniently moved, in successive steps, so as to close in an upward region the mold, generally designated by the reference numeral 16, which is fixed to the lower part of the frame 10. The mold 16, in a per se known manner, comprises upper rings 17 and a bottom 18. The bottom 18, mounted on a lower pneumatic cylinder 19, can move from a lowered position, termed open-mold position, to an elevated position, termed closed-mold position, in which its perimetric edges 20 abut against perimetric edges 21 of the cover 14 when said cover is enclosed between the rings 17. The impression of the tread of the shoe is formed in this position, when the cover 14 is closed onto the mold 16, while a connecting mid-sole is formed between the already-injected tread and the bottom of the upper fitted on the last 15 when the upper-holding last 15 replaces the cover 14. An injector 22, also known per se, is combined with the mold and in this case is surrounded by heating resistors 23 and is provided with temperature probes 24 which allow to control its temperature. The injection head 25 has injection nozzles 26 which in this case are in the lower part, so that the material exits downward from above in the form of one or more ribbons or strings. In the method according to the invention, which is adapted for molding single-component polyurethane plastics with a length-to-thickness ratio of at least 100, the injector 22, which is arranged so as to have a horizontal axis, can move so that it can arrange itself, with the head 25, above the bottom 18 of the mold 16 when said mold is open. The plastics in fluid condition (with a temperature on the order of 200°C) that arrives from the injector 22 is deposited by means of the head 25 on the bottom 18 of the mold 16, which is temperature-controlled to a temperature below 50°C. The plastics in fluid condition is deposited in the various regions in amounts which are proportional to the width of the impression and the head 25 of the injector 22 can be moved simultaneously in order to place the dispensing point within the impression. The injection speed can be variable and the transfer speed of the plastics dispensing point can also be variable. In other cases, the injector can be fixed and the mold 16 can be moved appropriately. After depositing the plastic material on the bottom 18, the mold 16 is closed, compressing said material, and the system is allowed to cool. The mold 16 is then opened and the cover 14 is replaced by the upper-holding last 15 to mold a connecting mid-sole; this operation can be performed with the same above-described operating steps for the tread. At this point it should be observed that the deposition of the fluid plastic material while the mold is open and the subsequent closure allow to reduce the required closure forces by 70% and more with respect to the forces currently provided in machines for molding the same material . Low closure forces allow burr to form, but the subsequent overmolding of the completion mid-sole already entails the need for a final trimming operation, which is accordingly used to eliminate the burr as well. in practice it has been observed that the intended aim and objects of the present invention have been achieved. The above-described method in fact allows to drastically reduce closure forces and this considerably simplifies the structure of the machine and of the mold. The foregoing has a positive effect on costs. The method can of course be carried out in single-mold machines as well as in multiple-mold machines, whether static or rotary. With reference to Figure 2, an apparatus for molding soles for shoes has a mold 116 which is composed of a cover 114 and of a bottom 118. The mold or die 116 includes an injector 122 which is of the same type as the previously described injector 22 and is likewise provided with an injection head designated by the reference numeral 125. The mold 116 is mounted on a machine which is not shown for the sake of simplicity. The method is performed in the same manner as the previously described one adapted for direct injection on an upper, which in particular provides for the deposition, while the mold 116 is open, of the plastics in fluid condition through the injection head 125 onto the various regions of the impression of the bottom 118 in an amount which is proportional to the width of the impression. The subsequent step provides for the closure of the mold 116, producing compression of the material, followed by cooling and extraction. The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. All the details may further be replaced with other technically equivalent elements. In practice, the single-component polyurethane plastic materials used, so long as they are compatible with the contingent use, as well as the dimensions, may be any according to requirements. The disclosures in Italian Patent Application No. PD97A000280 from which this application claims priority are incorporated herein by reference.

Claims: CLAIMS 1. A method for molding thin parts made of single- component polyurethane plastics, characterized in that it consists in: — depositing on the lower part (18, 118) of the mold (16, 116), while said mold is at least partially open, the plastic material in fluid condition in the various regions in an amount which is proportional to the width of the impression; — closing the mold (16, 116), compressing the fluid or pasty material and allowing the system to cool. 2. The method according to claim 1, characterized in that it consists in moving the dispensing point (25, 125) inside the impression simultaneously with the deposition. 3. The method according to one or more of the preceding claims, characterized in that it consists in depositing the plastic material in fluid condition with a variable injection speed. 4. The method according to one or more of the preceding claims, characterized in that it consists in moving the plastic material dispensing point (25, 125) with a variable speed. 5. The method according to one or more of the preceding claims, characterized in that it consists in depositing said fluid plastic material by directing the material flow downwards . 6. An apparatus for performing the method according to one or more of the preceding claims, characterized in that it comprises at least one injector (22, 122) which can move so as to protrude, with its injection head (25, 125), between the mold parts (14, 114, 18, 118) while said mold (16, 116) is at least partially open. 7. The apparatus for performing the method according to one or more of claims 1 to 5 , characterized in that it comprises a fixed injector (22, 122) and a mold (16, 116) which can move so as to place the injection head (25, 125), while the mold (16, 116) is at least partially open, between its components.

Inventors: FEBBRARI ITALO, FEBBRARI, ITALO
Applicants: FEBBRARI ITALO, MAIN GROUP SPA, FEBBRARI, ITALO, MAIN GROUP S.P.A.
Application year: 1998
Main classification: B29D
IPC codes: B29C31, B29C43, B29D31