Definitions

• the present invention refers to a method and an apparatus for moulding articles in polyurethane material, as well as the articles obtained by the claimed moulding method.
• the invention further concerns a die mould construction or apparatus for the production of articles in polyurethane material according to the method referred to above, in particular for the production of shoe soles and portions thereof, articles for car industry and the like, as well as technical articles.
• a prescribed amount of liquid material consisting of a mixture of at least a polyole and an isocyanate with reactans, is charged by casting, pouring or by injection into the cavity of a mould.
• the mould must be in closed condition when the material is fed into the cavity by injection methods, or the same mould is open during charging with casting or pouring methods and than the mould must be immediately closed after the charging of the liquid polyurethane, to avoid overflow from the open mould.
• the mould remains closed untill the polymerization process is sufficiently advanced to open the mould for demoulding the shaped article.
• the liquid material will tend to foam or to partially expand into a closed space; it is therefore necessary to provide means for venting the air or gases from the mould, particularly the air which has been trapped within the polyurethane material and the internal walls of the mould, in order to prevent the formation of air bubbles or air cavities on the outer surface of the moulded article.
• the venting of air or gases which tends to adher to inner surface of the mould, is critical. To such end vents holes or small air exit channels are usually provided in suitable positions of the mould cavity.
• an object of this invention is to provide a moulding method for manufacturing articles of polyurethane material by which it is possible to eliminate or substantially reduce defects caused by air-bubbles on the outer surface of moulded articles.
• a further object of the invention is to provide moulded articles in polyurethane material, having an higher finishing degree of the outer surfaces.
• a further object of this invention is to provide a method for manufacturing articles moulded in compact anhydrous, or semi-compact polyurethane containing low percentages of water in their formulation, by using moulds having an extremely simple construction through which the air-bubble problem can be obviated and which allows to apply high pressures without producing excessive overflows to maintain at the minimum the wasted material.
• a method for moulding articles in polyurethane material having surfaces free from air bubbles or air cavities in which a liquid polyurethane material is fed into the cavity of a mould, characterized by the steps of: allowing said liquid polyurethane to start an initial phase of the polymerization process in which the polyurethane state changes into an intermediate polymeric or semi-solid condition between said liquid and solid states, maintaining the polyurethane in an unrestained an flowing condition; applying a pressure to the polyurethane at a time during said intermediate polymeric condition urging and compacting the same polyurethane towards the cavity of the mould; and maintaining the pressure on the polyurethane for at least part of the remaining time portion of the polymerization process.
• intermediate polymeric condition or semi-solid state means a condition of the polyurethane during the polymerizing process, which is successive to the liquid state at which the polyurethane material has been fed into the mould, and comprised within a time interval during which the passage of the polyurethane material from liquid to solid state takes place when the polyurethane material has not yet fully hardened or polymerized and remains in a state capable of subjecting to plastic deformations.
• the exact instant for applying the pressure to material in the mould may varies according to the formulation and characteristics of the polyurethane or according to other specific requirements of the moulded article; therefore on the ground of the general principles of this invention according to which the compression phase of polyurethane takes place into the mould during the polymerization process when the polyurethane material has begun but has not yet completed its transition from liquid to solid state, in practice the starting moment for applying pressure can be exactly determined by specific trials.
• the value of the pressure or compression applied to the polyurethane material will vary according to specific working conditions or characteristics of the material, from the shape and dimensions of the cavity in the mould or the moulded articles.
• the compression or the pressure exerted on the polyurethane material in the mould must be greater than the specific reaction exerted by semi-solid polyurethane to be compressed and compacted into the mould, and must be maintained for a period of time sufficient to allow compacted the polyurethane to assume a stable and definitive solid state.
• the pressure which must be applied to the polyurethane material may vary from few kg/sq.cm, for example from two kg/sq.ca up to and beyond one hundred kg/sq.cm.
• a particular aspect of this invention in so far as it relates to compact, anhydrous polyurethane materials is that the starting of compression is best chosen immediately after the material begins to gel (gel time) and before the moment at which a filament is formed (filament time).
• the starting of compression takes place after cream has begun to form (cream time), preferably after the beginning of the expansion phase and before the moment of filament formation (filament time).
• the moment for starting the compression phase of polyurethane material within the mould may be chosen between 10% and 70% of the time interval comprised between the gel time and the filament time, for compact anhydrous polyurethane materials, and likewise between the cream time and the filament time for semi-compact polyurethane materials containing a percentage of water.
• polyurethane Under certain conditions or in order to produce specific articles it is preferable to cause the polyurethane to flow during compression and to work with excess quantities of polyurethane material, also called “extra-charge", which may vary from 5% to 40% of the weight of the moulded article, further providing the mould with a compression chamber in which the cavity of the mould opens.
• Figures 1 to 4 show an open mould for the production of shoe soles in polyurethane material, obtained by pouring into the cavity of the mould a liquid polyurethane mixture produced in a suitable mixing head 9, from metered quantities of a polyole and an isocyanate for compact polyurethane.
• the mould 10 consists of a lower mould section 11 and an upper mould section or lid 12 to close the lower section 11.
• the lower section 11 of the mould comprises a shaped cavity 13 which reproduces the shape of the article to be produced, while the upper section or mould lid 12 presents a downwardly protruding male or plug member 14, capable of penetrating into the cavity 13 of the lower section of the mould.
• the plug member 14 may consist of a single piece or may by fixed in an interchangeable way to the closing lid 12 directly, or, as in figures 1 and 2, through an intermediate ram or piston member 15 which adapts into a compression chamber or cavity 16 at the lower mould section provided over and around the mould cavity 13 such as to constitute a widening extension of mould cavity capable of being totally or partially filled by an extra-charge of the polyurethane mixture.
• the lower section 11 of the mould may be fixed in an usual way to a support member or located on a turning table together with a respective lid or upper mould section 12, for example hinged at 18 to the turning table structure.
• Closing means 19 are provided for closing the lid 12 and locking the same against the lower part of the mould 11 said closing means being of any type, whether mechanical, hydraulic or pneumatic or their combination.
• Reference C in figure 1 indicates a settable time control device operatively connected to actuating means 19 to close the mould or to apply a pressure for compacting the semi-solid polyurethane material after a pre-fixed time delay from the feeding of the polyurethane into the mould.
• the polyurethane mixture provided by a mixing device 9 in liquid form is firstly poured or fed in the prescribed quantity into the mould cavity 13, with the mould in the open condition of figure 1; than the polyurethane mixture is let to change his physical state in an unrestrained condition to assume an intermediate polymeric or semi-solid state in which the polyurethane material may flow and may be compacted to assume and maintain the shape of the mould cavity.
• the mould When the polyurethane has reached the required semi-solid condition, well before the filament time, the mould is closed, and a pressure is applied to the material in the cavity 13 of the mould and in the compression chamber 16 by ram member 15, causing the same polyurethane to flow inside the mould cavity and to be compacted against the shaped surface of the mould cavity, thus removing or preventing the formations of air-bubbles or air cavities on the outer surface of the moulded article.
• the pressure on the polyurethane material is maintained for a length of time necessary the polyurethane to polymerize and to harden, than the moulded article may by demoulded.
• the intermediate ram member 15 of the upper lid 12 carries out a double function acting as both a compressing member for the polyurethane mixture in the mould cavity, and as a sealing member along the opposite, nearby edges of both sections 11 and 12 of the mould.
• the smaller length and width dimensions of the ram member 15 with respect to the lid 12 also define peripheral steps 20 and 21, respectively 22 and 23 on the opposite edges of both sections of the mould which, when the mould is closed, create a labyrinth seal to allow a strong pressure to be applied on the polyurethane mixture. It is nevertheless clear that both the mould and the sealing system can be differently obtained and changed from the above embodiment.
• Figure 4 of the attached drawings shows an enlarged view of the lower mould section 11 and of the polyurethane mixture 24, while figure 7 shows the bottom face of a moulded sole 30.
• the bottom 25 of the cavity 13 of the mould in figure 4 include cavities or grooves 26 which determine protrusions 29 at the bottom surface of the sole facing the ground. Small air-bubbles 27 may be trapped in these cavities or grooves 26 of the bottom surface of the mould cavity during the feeding of liquid polyurethane mixture which, due to the strong surface tension of the polyurethane which prevent the liquid to penetrate and fill said grooves.
• air-bubbles 27 remain trapped between the surface 25 of the bottom of the mould cavity and the polyurethane material 24, causing the formation of imperfections or corresponding air-cavities 28 on the surface of the moulded article 30.
• the moment at which the compression of the polyurethane material in the mould should occur, the period for which the required pressure should be applied and the value of the pressure may vary from material to material and may also depend on the shape and dimensions of the articles to be moulded.
• compression parameters In order to obtain good results, such compression parameters must be determined through preliminary trials, however it must occur within a time interval in the polymerization process of the polyurethane, which follows the end of the pouring or feeding step of the polyurethane into the mould cavity and which precedes the instant in which the filament forms, normally referred to as "filament time". It is in this time interval, specifically after the gelification phase or the formation of cream, in which one must act by compressing the polyurethane material into the mould in order to obtain the desired results. Better results may be obtain causing the polyurethane material to flow into the mould cavity during the closure of the mould as well as during the initial steps of the compression.
• the polyurethane material may be of a compact or anhydrous type, or alternatively of semi-compact type containing a certain quantity of water through the base formulation or through atmospheric humidity.
• the polyurethane material is obtained from a polyole for compact polyurethane, of polyether or polyester base, and from an isocyanate such as, for example, a phenyl-methane - 4, 4′ - deisocyanate prepolymer (MDI) which are mixed in the necessary quantities in order to obtain a required liquid mixture of polyurethane.
• MDI phenyl-methane - 4, 4′ - deisocyanate prepolymer
• the graph in figure 5 shows the characteristic periods of a compact anhydrous polyurethane, expressed in seconds.
• T0 represents the starting point for pouring or feeding the polyurethane mixture into the mould
• T1 represents the end of pouring or feeding phase.
• the time interval T1-T0 will vary from case to case and in general will depend on the quantity of the mixture to be fed into the mould 10 and on the delivery of the mixing apparatus.
• the polyurethane material case referred to the first characteristic time interval of the polymerization process is the period necessary for gelification, commonly called "gel time", indicated by T2 which represents the starting moment of the gelification phase of the polyurethane mixture.
• filament time The time necessary for filament formation, commonly called “filament time” starts at T2, and is defined by moment T3 in the diagram of figure 5 and, as known, the time before T3 constitutes an intermediary critical phase in the polymerization or hardening process of the polyurethane material. Further characteristic time intervals of this process are the so-called “tack-free time” indicated by T4, and the “pinch time” shown by T5, after which the moulded article can be removed from the mould.
• the starting point TX for applying the pressure or the compacting compression of the polyurethane material in the mould cavity may vary, depending on specific parameters relating to the type of polyurethane, as well as the characteristics of the article to be produced, in general terms it can be stated that the moment for closing of the mould in figure 1, being the starting moment for the compression of the polyurethane material, will occur between 10% and 70% of the time interval T2-T3 which runs from "gel time” T2 and "filament time" T3.
• the graph in figure 6, as in figure 5, indicates the characteristic moments in the polymerization process for a semi-compact polyurethane material which has absorbed or contains a percentage of water.
• times T0, T1, T3, T4, and T5 have the same meaning as the corresponding times indicated in figure 5; since the presence of water in the polyurethane composition brings about the formation of a cream, and given such cream to grow, or to expand by increasing its volume, the previously referred to time T2 has been substituted by time T6 being the instant cream begins to form, commonly called “cream time”, to which time T7 must be added immediately thereafter, called “expansion time”, from which the expansion phase of the mixture occurs in the mould; the expansion terminates at time T8 following time T5 referred above after which the moulded article may be removed from the mould.
• the time TX for exerting a pressure, or a compacting compression on the polyurethane material in the mould follows the time T1 at the end of the pouring or feeding step of the mixture into the mould, and preferably falls between the "cream time” T6 and the "filament time” T3, or, more precisely, after the expansion time T7 of the polyurethane material.
• the starting point TX for the compression of the polyurethane material or for the closing of the mould in an open mould pouring system will depend on the polyurethane material and on the characteristics of the article to be moulded. Such moment TX shall tend to occur, as a general guide, during a period again within 10% and 70% of the time interval running from the "cream time" T6 to the "filament time” T3.
• the amount of pressure to apply to the material in the mould as well as the manner of applying such pressure may be important.
• pressures exceeding 2kg / sq. cm In practice it is worthwhile to operate under much higher pressures, in the order of a few tens of kg sq. cm., and in certain cases even in the order of a few hundred kg /sq. cm. or beyond, according to the requirements of the article to be moulded.
• the same pressure or the same compressing action on the polyurethane material in the conditions specified above may take place in a single phase, gradually increasing the pressure, or in several successive steps, acting contemporaneously with increasing pressure levels applied to the entire free surface of the material in the mould or beginning from one or more distinct points and then extending the pressure to the entire free surface of the material within the mould cavity in such a way as to produce a partial flow of the material from the cavity inside the mould, already in polymerization stage, causing in this way a plastic flow within the entire mass of material which assists in adhering to the deepest and smallest cavities of the mould, eliminating any air bubbles to be trapped and preventing the formation of air bubbles on the surface of the moulded article.
• FIG. 1 and 2 illustrate by way of example the starting of the polymerization process and the change of state occurring between the initial liquid state of the mixture and the semi-solid state when, the material swells slightly.
• the polyurethane material in the mould is in an evolving polymeric state and is capable of being subjected to non-elastic or plastic deformations, maintaining a shape which ensures the perfect adhesion to the mould impression or design of the surface of the internal cavity of the mould.
• the invention further concerns a particular apparatus and mould structure conceived for manufacturing shoe soles of polyurethane material though not limited to these, in which the mould has been provided with e compression chamber and in which a plug member of the mould is supported by the lid through an intermediate ram portion having a ram action which applies final pressure to the material in the mould and to a greater surfaces area than that of the article to mould, thus allowing strong pressure to be applied and a better seal to be performed during the compression phase.

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