EP0318608A1 - Method and apparatus for the moulding of articles in polyurethane - Google Patents

Abstract

A method for manufacturing moulded articles in polyurethane; according to the invention, a liquid polyurethane (24) is fed into the cavity (13) of a moulded(10) and maintained in an unrestrained condition to assume a flowing polymeric condition intermediate between liquid and solid states; a pressure is applayed to the polyurethane material (24) at said polymeric condition urging the polyurethane (24) against the mould cavity (12) and removing air-bubbles (27).

Description

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.

According to Prior Art method of manufacturing polyurethane articles, particularly polyurethane shoe soles or portions thereof in compact and semy-compact polyurethane, 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.

In the Prior Art methods for manufacturing polyurethane articles 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. However in the Prior Art production of polyurethane articles having protrusions or particularly complex markings and designs, for example in moulding of the ground-most surfaces of shoe soles, or whenever the articles require an extremely high quality or finishing of the outer surface, the presence of air bubbles or air cavities of even of small dimensions, can not be prevented.

The above problem is prevalently due to the fact that the internal pressure developed by gases which causes the expansion or the foaming of the polyurethane material against the internal surfaces of the mould, is rather low and little to more than one kg/sq.cm.; therefore no possibility exist to increase said pressure and to influence positively it from outside in order to obtain more precise markings or designs reproduction on the moulded article. In any event, in the Prior Art method of manufacturing polyurethane articles the pressure cannot be increased to any significant extent.

Furthermore to apply a pressure to a liquid polyurethane mixture into a closed cavity of a mould in order to achieve higher compression inevitably requires particularly complex construction of the moulds to prevent the overflowing of the mixture and in practice does not adequately solve the problem of eliminating air bubbles because the bubbles trapped between the internal walls of the mould and the mixture are merely compressed by the polyurethane against the inside walls of the mould.

From tests and experiments it has been now devised that the problem can be positively solved by applying a pressure for compacting the polyurethane mixture in the mould when the same mixture is in a particular condition during the polymerization process, in which the polyurethane material is capable of assuming and maintaining the shape of the internal impressions or design of the cavity of the mould.

Therefore 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.

According to the invention, a method for moulding articles in polyurethane material having surfaces free from air bubbles or air cavities has been provided 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.

For the purpose of present invention, "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. In general term 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. In any event, 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). In the case of semi-compact polyurethane materials containing a certain percentage of water, 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). In general terms, 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.

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.

The method and apparatus according to this invention will be further illustrated here below with reference to the manufacturing of shoe soles, it being understood, nevertheless, that the invention may be applied for manufacturing articles requiring a perfect and faithful reproduction of the design of the shaping cavity of a mould. In the drawings:

Fig. 1 shows a longitudinal section of an open mould for the manufacture of a shoe sole according to the invention, in which the polyurethane material has been completely fed into the mould cavity;

Fig. 2 is a cross sectional view along the line 2-2 of figure 1;

Fig. 3 is a view similar to that of figure 1 with mould in the closed condition;

Fig. 4 is an enlarged view showing a portion of figure 1;

Fig. 5 is a graph illustrating the succession in time of key moments in the process for a compact, anhydrous polyurethane;

Fig. 6 is a graph illustrating the succession in time of key moments for a semi-compact polyurethane containing a small percentage of water.

Fig. 7 is a bottom view of a sole moulded according to the invention.

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. In the illustrated example, 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 working of the apparatus is now described: 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. 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 use of an appropriate pressure for flowing and compacting the polyurethane into the mould, applied at a appropriate time as referred above, in which the pressure value will depend from properties and formulation of polyurethane materials, as well as the flowing of the same material at the above mentioned physical conditions inside the mould, prevent the formulation of air bubbles and air cavities on the outher surface of the moulded article, and a better reproduction of the design; furthermore, applying a pressure on the polyurethane material as referred above, will improve the properties of moulded articles, such as the wear stand allowing to produce articles, in particular shoe soles or portion thereof, of reduced weight.

In the shown example, 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. In the present case for the manufacturing of shoe soles, the use of a mould in which the polyurethane mixture is poured in an open mould has been shown, however the invention is also applicable to injection moulding systems providing the mould with ram members and actuating means suitable for applying a pressure on the polyurethane mixture 24, as described above.

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. Therefore unless said air-bubbles are removed before the polymerization and hardening of the polyurethane, 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.

As a result of trials and experiments undertaken with compact or semi-compact polyurethane materials, it has been discovered that the cause of the above negative phenomena is due to an insufficiency of pressure usually created by the expansion process of the polyurethane material in a closed mould and by the fact that the full pressure mainly acts when the critical points of the polymerization process has already occurred.

In the polymerization and hardening process of the polyurethane material, a short period of time exists in which the mass of material changes from liquid to solid state. This change from liquid to solid state occurs through modification of the molecular structures of the polyurethane material, allowing its assume a semi-solid state as previously referred to.

According to the invention it has thus been discovered that by applying a pressure or a compression for compacting the polyurethane material after the feeding into the mould has been stopped, with a delay which can be determinated in advance within a specific time interval during the polymerization process when the material has reached the referred to semi-solid condition and is still in a flowable state, it is possible to positively influence the moulded articles, preventing the formation of surface air- cavities and thereby obtaining a perfect reproduction of the design of the mould cavity.

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. 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 above is explained in greater detail with reference to the graph of figures 5 and 6. As described above, 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. Generally 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.

In detail, 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, while 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. In 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. 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.

According to this invention, and in the case of an anhydrous polyurethane material, it has been found worthwhile to apply the pressure to the material, for example through the same ram action exercised by the closure of the lid 12 in figure 1, or in other suitable way, at the moment TX following the time T1 at the completion the feeding of the mixture into the mould, preferably between time T2 or gelification time and time T3 or filament time. Since 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.

In the example of figure 6, 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.

In figure 6 as well, 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. As in the prior case, 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.

Trials have been carried out in different working conditions for both types of material using materials of the Italian firm Routinol, sold commercially under the trade mark "Routinol SP/A 65-90" and "Routinol SP/C 65-00B" respectively. Moulds for manufacturing shoe-soles having fine markings on the bottom were used. In both cases optimal results that is optimal reproductions of the impression or design of the moulds, free from air bubbles, were obtained closing the moulds or, more precisely, starting the compression of the polyurethane material, after approximately 15% ÷ 25 seconds from the time T1, keeping the mould closed for approximately 80 ÷ 130 seconds with an approximately 25 ÷ 35 kg / sq. cm. pressure value. In such conditions an extra-charges has been provided, being an excess weight of approximately 20 ÷ 25% of polyurethane material fed into the mould, even if in general terms it is possible to operate with extra-charges or excess weights greater or less, the limits being between 5% and 40% in weight with respect to the moulded article.

Depending on the desired aims as a function of the article to be moulded, the amount of pressure to apply to the material in the mould as well as the manner of applying such pressure may be important. In general term it is necessary to operate under pressures greater than that which would be generated spontaneously in a closed mould merely through self-expansion of the mixture to compact the polyurethane material, for example under 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. The dotted lines in figures 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. In this phase of the polymerization process 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. It is stressed, nevertheless, that the above description and illustrations through the attached drawings are purely intended as example of this invention, which in substance consists of the application of pressure and the compressing of polyurethane material in the mould during its transitional phase between liquid and solid states, maintaining such compression at least until such time as the polymerization process has yet to go beyond a critical phase although not yet definitively ended.

As referred to, 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.

Claims (18)

1. Method for moulding articles in polyurethane material in which a liquid polyurethane (24) is fed into the cavity (13) of a mould (10) letting the same polyurethane (24) to polymerize to a solid state and to assume the shape of the cavity (13) of the mould (10) characterized by the steps of: allowing said liquid polyurethane (24) to start an initial phase (11-13) of the polymerization process in which the state of the polyurethane (24) changes into an intermediate polymeric condition between said liquid and solid states; maintaining the polyurethane (24) in an unrestrained and flowing condition; applying a pressure to compact the polyurethane (24) at a time (TX) during said intermediate polymeric condition urging the same polyurethane towards the cavity (13) of the mould (10); and maintaining the pressure on the polyurethane for at least part of the remaining time portion of the polymerization process.

2. Method according to claim 1, particularly for compact anhydrous polyurethane materials, in which, after the feeding of the polyurethane material into the mould, the polymerization process comprises a gel time (T2) from which the gelification of the polyurethane begins as well as a filament time (T3) in which the formation of a filament may take place characterized by compressing the polyurethane material (24) in the mould (10) starting from a time (TX) between the gel-time (T2) and the filament time (T3).

3. Method according to claim 2, characterized by the fact that the time (TX) for starting the compression of the polyurethane material (24) occurs within the 10% and 70% of the time interval between the gel-time (T2) and the filament time (T3).

4. Method according to claim 1, in particular for semi-compact polyurethane materials in which, after the time (T1) at which the feeding of the polyurethane (24) into the mould (10) is stopped, the polymerization process comprises a cream time (T6) from which the formation of a cream begins, as well as a filament time (T3) in which the formation of a filament of polyurethane may take place characterized by compressing the polyurethane material (24) in the mould (10) from a time (TX) between the cream time (T6) and the filament time (T3).

5. Method according to claim 4 in which, from an intermediate expansion time (T7) during the formation of the cream the polyurethane material is subjected to an expansion of volume, characterized by compressing the polyurethane (24) in the mould (10) from a time (TX) following said expansion time (T7).

6. Method according to claim 4, characterized by the fact that the time (TX) at which the pressure is applied to the polyurethane material (24) in the mould (10) occurs within 10% and 70% of the time interval between the cream time (T6) and the filament time (T3).

7. Method according to claim 1, characterized by compressing the polyurethane material by continuously increasing the pressure during a single compression phase.

8. Method according to claim 1, and one or more of claims 2 to 8, characterized by applying the pressure to the polyurethane material (24) in the mould (10) acting contemporaneously on the whole surface area of the material (24) in the mould cavity (13).

9. Method according to claim 1, and one or more of claims 2 to 8, characterized by applying the pressure to the polyurethane material (24) in the mould starting from at least one intermediate point on the upper surface area of the material (24) in the mould cavity (13), causing a flow of polyurethane within said mould cavity (13).

10. Method according to claim 1, characterized by compressing the polyurethane material (24) in the mould (10) at pressure value greater than 2 kg / sq. cm.

11. Method according to claim 1, characterized by compressing the polyurethane material (24) in the mould (10) at pressure value greater than 10 kg / sq. cm.

12. Method according to claim 1, characterized by compressing the polyurethane material (24) in the mould (10) at a pressure value greater than 100 kg / sq. cm.

13. Method according to any one of preceding claims from 1 to 14, characterized by feeding an extra-charge of polyurethane material (24) into the mould cavity (13) with respect to the weight of the moulded article.

14. Method according to claim 13, characterized by the fact that the extra-charge of the polyurethane material is between 5% and 40% of the weight of the moulded article.

15. Method according to claim 1, particularly for moulding a shoe sole or portion thereof in polyurethane materials, free from air bubbles, in a mould comprising a lower mould section (11) having a shaped cavity (13) having a bottom surface defining the design of the lower portion of the sole and an upper mould section (12) having a plug member (14) which penetrates the shaped cavity (13) of the lower mould section (11), characterized by providing said lower section (11) with a compression chamber (16) in which said shaped cavity (13) of the lower mould section (11) opens, feeding said mould cavity (13) and compression chamber (16) with an polyurethane material (24) until said mould cavity (13) and at least a portion of said compression chamber (16) are filled with liquid polyurethane; allowing said polyurethane to change from liquid state into said intermediate polymeric condition; closing the mould to penetrate said plug member (14) in the polyurethane within said compression chamber (16) and said mould cavity (13) causing a flow of the polyurethane material between opposite sealing surface of said upper and lower mould sections (11,12) and applying a pressure on the polyurethane material (24) within said compression chamber (16) and said mould cavity (13).

16. Apparatus for moulding articles in polyurethane material, according to the method of preceding claims, said apparatus comprising first and second mould sections (11,12) defining a mould cavity (13) for the article to be moulded; actuating means (19) for opening and closing said mould sections (11,12) and means (9) for feeding a polyurethane mixture into said mould cavity (13), characterized in that said upper mould section includes ram means for compressing the polyurethane material into the mould cavity, and settable time control means operatively connected to said actuating means (19).

17. Apparatus according to claim 16 for moulding polyurethane articles particularly but not exclusively for moulding shoe soles, in which the mould comprises a lower mould section (11) defining a shaped cavity (13) for the article to be moulded, and an upper mould section (12) having a plug member (14) penetrating into said cavity (13) of the lower mould section (11), characterized in that said lower mould section (11) further includes an enlarged cavity (16) defining a compression chamber for receiving an extra charge of polyurethane material said enlarged cavity (16) opening into and being positioned above said mould cavity (13), and in that said plug member (14) is attached to the upper moulded section (12) by intermediate ram section (15).

18. Apparatus according to claim 17, characterized in that the peripheral edges (22,23) of the enlarged cavity (16) of the lower mould section (11) and corresponding peripheral edges (20,21) of said upper mould section (11) an said intermediate ram section (15) comprise matching stepped surfaces means around said enlarged cavity (16).