BIANCHI, Giancarlo (Gusbi Officina Meccanica S.p.a, Via Alessandria 39, pv Vigevano, I-27029, IT)
GUSBI OFFICINA MECCANICA S.P.A. (Via Alessandria 35, pv Vigevano, I-27029, IT)
MIOTTO, Alfonso (Wintech S.r.l, Via Pò 39, ve Mellaredo Di Pianiga, I-30030, IT)
BIANCHI, Giancarlo (Gusbi Officina Meccanica S.p.a, Via Alessandria 39, pv Vigevano, I-27029, IT)
| CLAIMS 1. A plant for producing footwear, in particular boots, of the type comprising: • one or more mold-holders (7, 7a, 7b, 7c); • one or more injectors for reactive mixtures (306, 307); characterized in that • each of said one or more mold-holders (7, 7a, 7b, 7c) is adapted to receive one or more molds (1a,s, 1ad); • each of said molds (1as, 1ad) comprises two or more half-molds (3as, 4as, 3ad, 4ad), said half-molds mutually interfacing at surfaces P, to form a closed cavity, and said joining surfaces P forming gaps which, when said mold (1as, 1 ad) is in the operating position, are not present in the top part of said cavity. 2. The plant according to claim 1 , characterized in that it also comprises one or more presses (302, 304), each of said presses being provided with an injector for thermoplastic polymers (303, 305). 3. The plant according to claims 1 and 2, characterized in that said one or more mold-holders (7, 7a, 7b, 7c) open in vertical direction. 4. The plant according to claim 3, characterized in that said vertical opening of said one or more mold-holders (7, 7a, 7b, 7c) takes place by means of an upward rotational movement of the cover (9, 9b) of said mold-holder (7, 7a, 7b, 7c) about a point (10) of the base (8, 8a, 8b, 8c) of said mold-holder (7, 7a, 7b, 7c). 5. The plant according to claim 4, characterized in that the cover (9, 9b) of said mold-holder (7, 7a, 7b, 7c) performs an upward rotation through around 80°. 6. The plant according to one or more of claims 1 to 5, characterized in that it is provided with rotary table (301 ) comprising a plurality of rotating supports (13) adapted to support said mold-holders (7, 7a, 7b, 7c), rotation of said rotary table (301 ) carrying said mold-holders (7, 7a, 7b, 7c) to said injectors for thermoplastic polymers (303, 305) and said injectors for reactive mixtures (306, 307). 7. The plant according to one or more of claims 1 to 6, characterized in that said mold-holder (7a, 7b, 7c) is hinged in a point (12) of a support (13) and provided with means (14), adapted to tilt said mold-holder (7a, 7b, 7c), which act between said mold-holder (7a, 7b, 7c) and said support (13). 8. The plant according to claim 7, characterized in that said tilt is of 15÷20°. 9. The plant according to one or more of claims 1 to 8, characterized in that said mold (1 as, 1ad) is mounted in two or more units on the same mold-holder (7a, 7b, 7c). 10. The plant according to one or more of claims 1 to 9, characterized in that the final density of the expanded polymer produced by the reactive mixtures is greater than 100 kg/m3. |
PLANT FOR THE PRODUCTION OF SHOES PRODUCED BY MOULDING THERMOPLASTIC POLYMERS AND REACTIVE MIXTURES
DESCRIPTION
The present invention relates to a plant for producing footwear, in particular boots, molded with reactive mixtures, such as two-component polyurethane, with thermoplastic polymers, and with both the materials to produce the same piece. In the case of molded pieces in thermoplastic material, multiple molds are usually produced, comprising a plurality of cavities, for each of which one piece will be produced in each injection cycle. When designing said molds care must be taken that the opening and closing maneuvers are possible in relation to the press on which they must be mounted, while the problem of venting of air to allow injection of the molten material is not relevant. This is due to the fact that the injection pressures are so high (hundreds of bar) that the air present in the mold is vented completely through the join lines of the half-molds which, although precise, are not airtight.
In the case of pieces molded in two-component polyurethane (PU) having a certain degree of expansion, the injected material is not a molten polymer, but a reactive mixture of two components (isocyanate and polyol), and related additives, including a blowing agent, said reactive mixture being susceptible to react chemically to form, at the end of the reaction, the material that forms the final product. As a rule, the two-component polyurethanes are used with a certain degree of expansion, which implies that a predetermined quantity of said mixture, which does not fill the cavity of the mould completely, is injected, or even poured, into this mold. Said cavity will be filled completely when, with the mixture still in liquid state, the blowing agent has completed its action. Subsequently, the chemical reaction between isocyanate and polyol is completed, with formation of the final material.
Therefore, filling of the mold is not obtained as a result of a very high pressure imparted by the injector, but by the much lower pressure (a few bar, according to the formula) produced by the blowing agent.
In the situation described, venting of the air, and of the gases produced by the reaction between components, is not so easy as in the case of injection of thermoplastic materials and requires particular attention in positioning the join lines of the half-molds, so that there is always a join line in the top part of the mold, when this is in operating position.
It should be observed that it is not possible to produce a small hole in the top part of the mold, as expansion takes place with the reaction mixture still in fluid state, and this would flow copiously through said hole.
The critical nature of said problem is such that the molds used for injection of polyurethane reactive mixtures are often mounted on mold-holders that allow tilting of these molds in such a manner as to allow the reactive mixture to fill the lower part of the mold and expand upward. In this manner the air present in the cavity is vented progressively through the join lines of the half-molds. Venting of air is facilitated by the fact that the expansion speed of the mixture is not particularly high, and therefore the air is able to exit completely even if the pressure exerted by the expanding mixture is relatively low.
In the specific case of producing boots, prior art provides that molds are held, during production of pieces, in such a manner that the presence of a join line between the half-molds, through which air and reaction gases are vented, is always guaranteed in the upper part of the cavity.
The mold must therefore be arranged in such a manner that the boot is in vertical position, or in horizontal position but with the tip facing downward.
In the case of molding thermoplastic polymers, as due to the high injection pressures there is no problem of venting the gases present in the mold, it is possible to position a pair of molds, left-right for the simultaneous production of two boots, left-right.
In fact, by positioning the molds with the separation surface between half-molds in substantially horizontal position, the mold can open with a movement in vertical direction.
In the case of molding with two-component polyurethane, the separation surface between the half-molds is placed in substantially vertical position. By placing two molds in the same mold-holder, this should carry out lateral movements that would make it practically incompatible with the rotary table plants normally used to produce two-component polyurethane pieces, both due to the excessive horizontal dimensions of the mechanisms required and in relation to the position of the injectors.
A first drawback is therefore a direct consequence of the fact that the mold-holders for molding thermoplastic polymers are very different from the mold-holders for molding with reactive mixtures. Consequently, a plant equipped for productions with reactive mixtures, is not able to house molds for thermoplastics, and vice versa. This means that if both productions are to be carried out, it is necessary to be equipped with both types of plant, resulting in noteworthy economic expenditure, especially if the production does not saturate the productivity of both. The patent DE 3731518 describes a plant equipped for productions both with reactive mixtures and with thermoplastics, suitable for producing footwear comprising parts in both said materials. With said plant molds for reactive mixtures and molds for thermoplastics can be used simultaneously, but the molds to produce the uppers must be held in vertical position and the molds themselves open with movements in horizontal direction, and consequently it is practically impossible to mount two molds on the same mold-holder.
It must be noted that the two-component polyurethanes are used in numerous cases with high and very high degree of expansion, in order to obtain a final density often below 50 kg/m 3 , so as to obtain flexible foams for paddings or rigid foams for fillings. In these situations, the problem of venting air and reaction gases from the mold is effective. In fact, if the necessary measures are not taken, defects caused by the occlusion of air and reaction gases, often also visible on the outer surface of the molded piece, can occur, these defects having the appearance of open concavities and therefore lack of material or closed concavities with air and reaction gases trapped inside. This fact has convinced experts in the sector that said problem represents a criticality in the production of any piece in two- component polyurethane material.
Therefore, the aforesaid drawbacks are all consequences of the fact that the problems of venting gases make it necessary to mount the molds for two- component polyurethanes in a different manner to the molds for thermoplastics. It follows that if it were possible for the venting of gases to be made independent from the specific mounting of the mold on the mold-holder, the problems set forth above would be solved automatically.
Therefore, a test campaign was carried out on a mold for producing boots in thermoplastic polymers, with supply channels modified to receive a two- component polyurethane reactive mixture, to verify if, with appropriate modifications, it were possible to obtain pieces without defects, even if there are no gaps for venting air and reaction gases in the top part of the cavity of this mold. The experimentation, conducted with reactive mixtures with a low degree of expansion, typical of the production of boots and footwear in general, surprisingly showed that, although the joining plane of the half-molds is in substantially horizontal position, and therefore there are no gaps due to the join between the two half-molds in the top part of the closed mold, the air and reaction gases are nonetheless vented completely, in the same manner as occurs in the case of injection of thermoplastic materials.
Therefore, it was ascertained that the conviction of the problem of venting air and reaction gases from the mold, as indicated above, is a mere technical prejudice, at least in the case of reactive mixtures with low degree of expansion, i.e. with final densities in the order of 100 kg/m 3 and over.
The object of the present invention is to overcome said technical prejudice, proposing a plant in accordance with claim 1 , for producing footwear, in particular boots, molded with reactive mixtures, such as two-component polyurethane, with thermoplastic polymers, and with both the materials to produce the same piece. Said plant comprises:
• one or more mold-holders;
• one or more injectors for reactive mixtures;
and is characterized in that
• each of said one or more mold-holders is adapted to receive one or more molds;
• each of said molds comprise two or more half-molds, said half-molds mutually interfacing at surfaces P, to form a closed cavity, and said joining surfaces P forming gaps which, when said mold is in the operating position, are not present in the top part of said mold.
According to a preferred embodiment, said plant also comprises one or more presses, each of said presses being provided with an injector for thermoplastic polymers, in such a manner as to produce, on the same plant, footwear made of both said materials.
Said molds are suitable to be used with reactive mixtures, such as two-component polyurethane, with thermoplastic polymers, and with both the materials to produce the same piece.
The use of plants according to the invention therefore allows the following advantages to be achieved:
• strong increase in productivity of the plants for two-component polyurethane items, in particular boots and footwear in general, through the simultaneous production of two or more pieces, this result being obtained through simultaneously mounting at least two molds on the same mold-holder;
• possibility of producing pieces in thermoplastic material and in two-component polyurethane on the same plants;
• possibility of producing, in the same plant, pieces comprising parts in thermoplastic material and in two-component polyurethane.
Said molds, suitable to produce pieces in thermoplastic material, in two- component polyurethane, and pieces in which thermoplastic materials and two- component polyurethanes are present simultaneously, form the subject matter of a simultaneous patent application by the same applicant.
The invention shall now be described, purely by way of non-limiting example, according to a preferred embodiment and with reference to the accompanying figures, wherein:
• Figs. 1 (a, b) show a typical mold for producing a boot;
• Figs. 2 and 3 (a, b) show, in closed position and in open position, a pair of molds "left" and "right", positioned on a single mold-holder;
• Fig. 4 shows a typical rotary table plant suitable to produce boots in thermoplastic material;
• Fig. 5 shows a thermoplastic material production cycle of a pair of boots;
• Fig. 6 shows a typical mold for two-component polyurethanes;
• Fig. 7 shows a typical rotary table plant suitable to produce boots in two- component polyurethane material;
• Fig. 8 shows a tilting mold-holder for mounting molds for polyurethanes, according to the invention;
• Fig. 9 shows a pair of molds, according to the invention, into which reactive mixture has just been injected;
• Fig. 10 shows a plant according to the invention, suitable to produce boots in thermoplastic material and in two-component polyurethane and a combination of these;
• Fig. 11 shows a thermoplastic material production cycle of a pair of boots using the plant according to the invention;
• Fig. 12 shows a two-component polyurethane production cycle of a pair of boots using the plant according to the invention;
• Figs. 3 (a, b) compare two different designs of sole;
• Fig. 14 shows the possible interference between the sole/plates and the injection pipes;
• Figs. 15 (a, b) show a process for producing soles;
• Fig. 16 shows a mold-holder provided with means suitable for the maneuver on the sole/plates;
• Fig. 17 shows another process for producing soles;
• Fig. 18 shows injection of a reactive mixture for forming the uppers, subsequent to forming the soles with two-component polyurethane;
• Fig. 19 shows the mold-holder for producing soles in thermoplastic material; • Fig. 20 shows injection of a reactive mixture for forming the uppers, subsequent to forming the soles with thermoplastic polyurethane.
Figs. 1 to 7 and the relevant description relate to prior art. The purpose for which they are indicated herein is to clarify the context in which the technical prejudice overcome by the invention was formed, showing how an apparently small modification of the molds is able to introduce important variations in the production cycles, with noteworthy economic consequences.
Each boot produced with the molding plant is commonly produced, according to prior art, in two materials. The top part of the boot, i.e. the upper, is produced with a first softer material, while the bottom part, i.e. the sole, is produced with a second material, more resistant to abrasion. According to prior art, with equipment of the type shown in Figs. 1 to 5, only boots in thermoplastic material can be produced.
Figs. 1 (a, b) show a typical mold (1 ) for producing a boot as described above. Said mold (1 ) comprises a last (2), which forms the inner surface of the boot, a pair of half-molds (3, 4) which form the outer surface of the same boot, a first sole/plate (5), which serves to close the half-molds (3, 4) at the bottom during injection of the first material into the mold to produce the upper, and a second sole/plate (6), in which the pattern of the sole of the boot is etched. The process involves fitting a liner onto the last (2), closing the half-molds (3, 4) and injecting a first soft thermoplastic material into the cavity between said half-molds (3, 4) and said last (2), in such a manner as to form the upper of the boot. The purpose of the liner fitted previously onto the last (2) is to provide an internal finish of the boot that is comfortable to wear. After injection of the first material, the second sole/plate (6) replaces the first sole/plate (5) and the second material is injected into the new cavity thus obtained, thereby producing the sole of the boot. Fig. 2 shows, according to a plan view, a pair of molds, "left" (1s) and "right" (1d), for producing simultaneously a left boot and a right boot, positioned on a single mold-holder (7) and with the plane P of separation of the half-molds, visible in the subsequent Figs. 3a and 8a in substantially horizontal position.
Besides the plane P of separation between the half-molds, an oval surface (not referenced) is also present at the top edge of the upper, due to separation between the half-molds and the last which forms the inner surface of the boot. There is also a second surface (again not referenced) of separation between the half-molds (3, 4) and the sole plates (5, 6).
Figs. 3 (a, b) show a side view of said pair of molds (1s, 1d) mounted on said mold-holder (7), in closed position (Fig. 3a) and in open position (Fig. 3b).
Said "left" mold (1s) comprises a last (2s), which forms the inner surface of the left boot, a pair of half-molds (3s, 4s), which form the outer surface of the same boot, a first "left" sole/plate (5s), a second "left" sole/plate (6s).
Said "right" mold (1d) comprises a last (2d), which forms the inner surface of the right boot, a pair of half-molds (3d, 4d), which form the outer surface of the same boot, a first "right" sole/plate (5d), a second "right" sole/plate (6d).
The mold-holder (7) comprises a base (8) and a cover (9) hinged in a point (10) of said base (8). As a result of a hydraulic cylinder ( 1), which acts between the base (8) and the cover (9) of the mold-holder (7), this can open, performing an upward rotation to allow opening of the molds (1s/1d).
The mold-holder (7) also comprises a rotating plate (12) on which the sole/plates (5s, 5d) and (6s, 6d) are mounted. Said rotating plate (12) is mounted slidingly and rotatingly on said base (8) in such a manner that, with an appropriate combination of translating and rotating movements, indicated by the arrows, it is capable of carrying the sole/plates (5s, 5d) and (6s, 6d) alternatively to close the molds (1s, 1d) at the bottom.
Fig. 4 shows a typical plant (100) suitable to produce boots in thermoplastic material, using said pair of molds (1s, 1d) mounted on said mold-holder (7).
Said plant (100) comprises a rotary table (101 ) with a plurality of mold-holders (7) with respective pairs of molds (1s, 1d). In the example of Fig. 4 there are 12 stations (from a to n), on each of which one mold-holder (7) can be positioned. The plant (100) also comprises a first press (102) and a first injector (103) to inject said first thermoplastic material into said mold at high pressure to produce the upper.
The plant (100) also comprises a second press (104) and a second injector (105) to inject said second thermoplastic material into said mold at high pressure to produce the sole.
In accordance with the above, the first material, to produce the upper, will be softer, while the second material, to produce the sole, will be more resistant to wear.
A plant (100) thus equipped allows a pair of boots to be produced simultaneously and is therefore capable of achieving a high hourly production.
Fig. 5 shows a thermoplastic material production cycle of a pair of boots using said rotary table plant (100). The rotary table (101) rotates in the direction indicated by the arrow carrying each mold-holder (7) to occupy in sequence the stations from a to n, where the following operations are carried out:
• in the position (a), after having fitted the liners onto the respective lasts (2s, 2d), the first thermoplastic material is injected to produce the uppers, the molds (1s, 1d) being closed at the bottom by the first sole/plates (5s, 5d);
· in the position (m) the first sole/plates (5s, 5d) are automatically replaced by the second sole/plates (6s, 6d); • in the position (I) the second thermoplastic material is injected to produce the soles, the molds (1s, 1d) being closed at the bottom by the second sole/plates (6s, 6d);
• the mold remains closed until the position (f), to allow solidification of the injected materials;
• in the position (e) the mold-holder opens to allow, also in the subsequent positions (d) and (c), the operations to remove the injected products and to prepare the molds for the subsequent injections;
• in the position (b) the mold-holder closes once again automatically, to start a new production cycle.
Total venting of the air and gases described above is guaranteed by the high injection pressures of the thermoplastic materials, which are thus well compressed onto the surfaces of the molds to produce excellent products faithfully reproducing the patterns etched in these molds.
The process for filling the molds with the two-component polyurethane material instead takes place with low pressure. For this reason, to obtain a product that correctly reproduces the patterns etched on the molds, maximum care must be take to expel the air contained therein and the gases coming from the chemical reaction.
For the reasons described above, in machines suitable to produce boots in polyurethane material, specific positions of the molds which allow a better production process are currently adopted. However, with the aforesaid positions, in practice each mold-holder can only contain a single mold therein, instead of the two molds that can be contained in the case of thermoplastic material. Therefore, to produce a pair of boots two mold-holders are necessary, resulting in a noteworthy reduction in production capacity with respect to the production of boots in thermoplastic material.
For the production of boots in two-component polyurethane material the position of the mold-holder must guarantee easy filling of the mold contained therein, avoiding high counter pressures which would have a negative effect on correct filling of the mold cavity. In practice, the mold is arranged in such a manner that the reactive mixture can be injected into the lower part thereof, said mixture expanding upward, following the chemical reaction, and filling the cavity completely. Due to the this arrangement and to the fact, already mentioned, that the join between the half- molds is such that, in operating position, there is a gap in the top part thereof through which air and reaction gases are vented, it is possible to obtain high quality pieces, without defects such as gas inclusions, etc.
In a mold position that is commonly adopted, the longitudinal axis of the last is in vertical position, as shown in Fig. 6, in which a typical mold (30) for two- component polyurethanes, mounted on a mold-holder (31 ), is visible. Said mold (30) comprises the two half-molds (33) and (34) which close over the last (32), the cavity being closed in the bottom part by the sole/plates (35) and (36).
In another position used (not shown) the longitudinal axis of the boot is horizontal, while that of the sole is vertical and with the tip of the sole facing downward. In this way, the rear join line between the two half-molds is facing upward.
Fig. 7 shows a typical plant (200) suitable to produce boots in two-component polyurethane material. Said plant (200) comprises a rotary table (201 ) with a plurality of molds (30) mounted on the respective mold-holders(31 ).
The example of Fig. 7 shows 12 stations (from a to n), on each of which one mold- holder (31 ) can be positioned.
The plant (200) also comprises a first injector (203) for injecting said first two- component polyurethane material into the mold, for producing the upper, and a second injector (202) for injecting said second two-component polyurethane material into the mold, for producing the sole.
As can be seen from Figs. 6 and 7, a mold for two-component polyurethanes opens with a fan-like movement, engaging the space at the side of this mold. For reasons of overall dimensions, it is evidently impossible to use two or more horizontally adjacent molds, as occurs with thermoplastic materials. However, by placing the molds vertically, injectors would have to be arranged at two different heights, either making them slide vertically or duplicating them. Both these solutions are too costly and complex to be implemented. In practice, it is less penalizing to accept the limited productivity of these types of plants.
It is evident that if it were possible to have molds suitable for molding pieces in two-component polyurethane that can be mounted in at least two units on a same mold-holder, obviously without the drawbacks mentioned above, a drastic increase in productivity could be achieved, with consequent noteworthy economic benefits. If, moreover, the mold-holder were of the same type both for molding thermoplastic polymers and two-component polyurethane materials, the same plants would become suitable to produce pieces with both types of materials and also pieces comprising both types of materials.
Therefore, a test campaign was carried out, with two-component polyurethane, on a pair of said molds for thermoplastics (1s, 1d), mounted in a pair on said mold- holder (7), as shown in Figs. 2 and 3, to verify whether, with appropriate modifications, it would be possible to obtain pieces without defects, even if, in the top part of the cavity of this mold, there are no gaps through which to vent the air and reaction gases, obviously apart from the oval shaped vent at the top edge of the upper, due to the separation between the half-molds and the last which forms the inner surface of the boot, said gap being considered insufficient to vent the gas alone, due to said technical prejudice.
In actual fact, a second gap also exists in the bottom part of the boot, at the contact of the sole plates (5as, 5ad, 6as, 6ad) with the half-molds (3as, 3ad, 4as, 4ad). Said second gap is however ineffective for the purpose of venting gases. In fact, as the reactive mixture is injected into the bottom part of the boot, it is this that closes the second gap, preventing the passage of gases.
The modifications made involved the feed channels, which were modified to be suitable to inject reactive mixtures, in particular two-component polyurethanes, and the mold-holder which was modified to be able to tilt.
The resulting molds (1as, 1ad) and the mold-holder (7a) are shown in the subsequent Figs. 8 and 9).
Each of said molds (1as, 1ad), modified to receive reactive mixtures, comprises a last (2as, 2ad), which forms the inner surface of the boot, a pair of half-molds (3as, 4as, 3ad, 4ad), which form the outer surface of the same boot, a first sole/plate (5as, 5ad), a second sole/plate (6as, 6ad).
The experimentation has surprisingly shown that, although the joining plane P of the half-molds (3) and (4), is in substantially horizontal position, and therefore in the top part of the closed mold there are no gaps due to the join between the half- molds (3as, 4as, 3ad, 4ad), the air and reaction gases are in any case completely vented, in the same manner as occurs in the case of injection of thermoplastic materials.
However, it is preferable, although not essential, to tilt the mold-holder during the injection and expansion step of the reactive mixture. Said tilting of the mold-holder also involves tilting of the plane P, however said tilting is not able to guarantee that there will be gaps in the top part of the closed mold due to the join between the half-molds (3as, 4as, 3ad, 4ad), for venting of air and reaction gases, both because the mold-holder, as can be seen in Fig. 8b, is rotated about an axis perpendicular to the longitudinal axis of the boot, and because said tilt is in any case limited to 15÷20°.
Another modification involved the rotation carried out by the cover (9) of the mold- holder (7a) which was increased from 40÷45° to around 80°, to facilitate application of release agent.
Figs. 8 (a, b) show a mold-holder (7a), the base (8a) of which is hinged in a point (12) of a rotating support (13) which is part of the rotary table production plant on which said mold-holder (7a) is mounted.
A hydraulic cylinder (14), which acts between the base (8a) of the mold-holder (7a) and the rotating support (13), allows the tilt of the mold-holder (7a) to be varied, taking it from the substantially horizontal position (Fig. 8a) to the tilted position (Fig. 8b).
In practice, the experimentation has shown that the greater the final density of the material is, the less the mold-holder requires to be tilted during the reaction. With the densities normally required to produce boots, tilting is not essential, but only preferable. The suitability of whether or not to tilt the mold-holder (7a) is also related to the filling speed of the mold. In practice, for filling speeds below 100 grams of reactive mixture per second, the mold holder can remain in horizontal position, while for higher filling speeds it can advantageously be tilted by 15÷20°. Fig. 9 shows a pair of molds (1as, 1ad) into which polyurethane reactive mixture has just been injected through the injection pipes (15s, 15d), for the left mold and for the right mold respectively. The quantity of material injected is predefined, for each mold, with an electronic volume control system of known type. The polyurethane material is very fluid during injection and the quantity of material injected will fill only a part of the molds up to a level (16s, 16d), respectively for the left mold and for the right mold, corresponding to the volumes predetermined for each mold, in the manner described above.
As soon as the material injection step has terminated, the injection pipes (15s, 15d) are closed by the shutters (17s, 17d), to prevent the injected material from exiting from the molds.
The chemical reaction of the material, which starts immediately after injection, creates an increase in volume of this material, filling the cavity completely.
During the expansion step, the polyurethane material thrusts the air contained in the molds and the gases released by the chemical reaction forward. These gases, thrust forward in this manner, exit from the mold through the joining surface P of the half-molds. In the top part of the mold, where there are no escape routes, a bubble could form, but this is vented following the increase in pressure, due to the blowing agent, in the final step of the reaction.
The tilt of the mold-holder (7a), as shown in Fig. 8b, facilitates accumulation of the reactive mixture in the bottom part of the mold and subsequent expansion in the direction of the length of the boot. The tilt of the mold-holder has as positive effect a reduction in the dimensions of the gas bubble that forms in the top part of the mold and therefore creates more favorable conditions for venting thereof.
The mold-holder (7a) then returns to horizontal position in all the other stations. Fig. 10 shows a plant (300) suitable for producing boots in thermoplastic material and in two-component polyurethane, and a combination thereof, which uses said pair of molds (1as, 1ad) mounted on said mold-holder (7a).
Said plant (300) comprises a rotary table (301) with a plurality of mold-holders (7a) and the respective pairs of molds (1as, 1ad). In the example of Fig. 10 there are 12 stations (from a to n), on each of which one mold-holder (7a) can be positioned. The plant (300) also comprises a first hydraulic press (302) and a first injector (303) for injecting said first thermoplastic material into the mold at high pressure, to produce the upper.
The plant (300) also comprises a second press (304) and a second injector (305) for injecting said second thermoplastic material into the mold at high pressure, to produce the sole.
The plant (300) also comprises a third (306) and fourth (307) injector, respectively for injecting said first two-component polyurethane material, to produce the upper, and said second two-component polyurethane material, to produce the sole, into the mold.
Fig. 11 shows a production cycle of a pair of boots, in thermoplastic material, using said rotary table plant (300). The rotary table (301) rotates in the direction indicated by the arrow carrying each mold-holder (7a) to occupy in sequence the stations from a to n, where the following operations take place:
· in the position (a), after having fitted the liners onto the respective lasts (2as, 2ad), the first thermoplastic material is injected, by means of the injector (303), to produce the uppers, the molds (1as, 1ad) being closed at the bottom by the first sole/plates (5as, 5ad);
• in the position (m) the first sole/plates (5as, 5ad) are automatically replaced by the second sole/plates (6as, 6ad);
• in the position (I) the second thermoplastic material is injected, by means of the injector (305), to produce the soles, the molds (1as, 1ad) being closed at the bottom by the second sole/plates (6as, 6ad);
• the mold remains closed until position (f), to allow solidification of the injected materials;
• in the position (e) the mold-holder opens to allow, also in the subsequent positions (d) and (c), the operations to remove the injected products and to prepare the molds for the subsequent injections;
• in the position (b) the mold-holder closes again automatically, to start a new production cycle.
Fig. 12 shows a two-component polyurethane production cycle of a pair of boots using said rotary table plant (300). The rotary table (301 ) rotates in the direction indicated by the arrow carrying each mold-holder (7a) to occupy in sequence the stations from a to n, where the following operations take place:
• in the position (b), after having fitted the liners onto the respective lasts (2as, 2ad), the first polyurethane material is injected, by means of the injector (306), to produce the uppers, the molds (1as, 1ad) being closed at the bottom by the first sole/plates (5as, 5ad), while the mold-holder (7a) is preferably in tilted position;
• in the position (m) the first sole/plates (5as, 5ad) are automatically replaced by the second sole/plates (6as, 6ad);
• in the position (I), the second polyurethane material is injected, by means of the injector (57), to produce the soles, the molds (1as, 1ad) being closed at the bottom by the second sole/plates (6as, 6ad);
• the mold remains closed until position (f), to allow solidification of the injected materials;
• in the position (e) the mold-holder opens to allow, also in the subsequent positions (d) and (c), the operations to remove the injected products and to prepare the molds for the subsequent injections;
• in the position (b) the mold-holder closes again automatically, to start a new production cycle.
In Figs. 13 (a, b) two different designs of sole are compared. In one case (Fig. 13a) it can be noted that the sole remains completely below the profile of the last, while in the other case (Fig. 13b) the sole rises onto the edges of the last.
In this second case (Fig. 14), if the injection pipes were once again the pipes (15s, 15d) of Fig. 9, when the sole/plates (6as, 6ad) come into contact with the half- molds (3s, 4s, 3d, 4d), the polyurethane material would be obliged to pass through a narrow space (18s, 18d). The polyurethane material, injected through said holes (15s, 15d) would in this case encounter great resistance to filling the mold and often this is unacceptable, as it might be unable to enter the mold.
The production process according to the invention is illustrated in the subsequent Figs. 15 (a, b).
The second sole/plates (6as, 6ad) are mounted on a plate (19) operated by hydraulic cylinders (20) adapted to move said second sole/plates (6as, 6ad) from a first position (Fig. 15a) to a second position (Fig. 15b).
When the sole/plates (6s, 6d) are in the first position, they produce a closed cavity together with the half-molds (3as, 4as, 3ad, 4ad) and leave the injection pipes (21 as, 21 ad) open. When the sole/plates (6as, 6ad) are in the second position, they are in contact with the bottom edge of the half-molds (3as, 4as, 3ad, 4ad), reducing the volume of said cavity and closing the injection pipes (21 as, 21 ad). The polyurethane reactive mixture is injected when the sole/plates (6as, 6ad) are in the first position. The reactive mixture, injected in a predetermined quantity, collects in proximity of the outlet of the injection pipes (21 as, 21 ad), and then, following a movement caused by the hydraulic cylinders (20) which carries the sole/plates (6as, 6ad) into contact with the bottom edge of the half-molds (3as, 4as, 3ad, 4ad), the reactive mixture expands into the cavity, while return of the mixture upstream is prevented by the fact that the plates (6as, 6ad) close the injection pipes (21 as, 21 ad). The subsequent expansion of the mixture will allow complete filling of the cavity.
Fig. 16 shows a mold-holder (7a) provided with the front plate (19) and hydraulic cylinders (20) for the maneuver on the sole/plates (6as, 6ad).
In a different process to produce boots, the soles and formed first, followed by the uppers.
The process to form the sole is shown in Fig. 17.
The mold-holder (7b) is equipped with two mechanisms. A first mechanism (41 ), anchored to the base (8b) of the mold-holder (7b), and a second mechanism (42) anchored to the cover (9b) of said mold-holder (7b).
Said first mechanism (41 ) comprises a support (43), hinged in a point (44) of the base (8b) and operated by a hydraulic cylinder (45), tilting about a point (46) integral with said base (8b). The sole/plates (6bs, 6bd) bearing the pattern of the left sole and of the right sole respectively are mounted on said support (43).
Said second mechanism (42) comprises a hydraulic cylinder (48), integral with the cover (9b) of the mold-holder (7b), adapted to move the covers (49s, 49d) which mate with said sole/plates (6bs, 6bd) to form the right and left soles.
An injector (50) is positioned in such a manner that it can pour the two-component polyurethane material onto the sole/plates (6bs, 6bd).
After pouring of the polyurethane material onto the sole/plates (6bs, 6bd) has been completed, the cylinder (48) thrusts the sole/covers (49s, 49d) downward, which thus close the sole/plates (6bs, 6bd) to form, in this manner, the left and right soles. The soles thus produced can have complex patterns and thin thicknesses, as the polyurethane material is injected with the "mold open" and therefore without any counter-pressure to obstruct forming of these soles.
After the step to form the sole and after the sole/covers (49s, 49d) have been moved away from the sole/plates (6bs, 6bd), these latter are overturned on the half-molds (3as, 4as, 3ad, 4ad), as shown in Fig. 17b, as a result of the thrust of the tilting cylinder (45) on the support (43) of said sole/plates (6bs, 6bd).
Fig. 18 shows injection of polyurethane material, suitable to form the top part of the boots, in the molds (1 as, 1ad). The reactive mixture is injected into the molds through the injection pipes (15s, 15d); after injection, the injection pipes (15a, 15d) are closed by the shutters (17s, 17d).
At the end of the process to inject the material of the upper, the shutters (17s, 17d) are operated to respectively close the injection pipes (15s, 15d).
A further process for forming the sole is shown in Figs. 19 e 20; In said process the soles are formed with thermoplastic material, such as TPU (thermoplastic polyurethane).
Fig. 19 shows the mold-holder (7c) which comprises a base (8c) configured according to an arrangement indicated with the reference (53), while the remaining components are very similar to those of the mold-holder (7a).
Two pairs of sole/plates (54s, 54d) are mounted on the front plate (12).
In this method the soles are formed first using first sole plates (54s, 54d) mounted on the front plate (12) which close over the respective sole/covers (56s, 56d). The thermoplastic material is injected through the injection pipe (57).
The sole/plates (54's, 54'd), shown in Fig. 20 in the top part of the front plate (12), are identical to the sole/plates (54s, 54d); in fact, the front plate (12) rotates through 180°, in accordance with the production sequence of the machine, and therefore the pair of sole/plates (54s, 54d) and the pair of sole/plates (54's, 54'd) cyclically assume the bottom and top position.
In practice, the soles are produced as described above in the bottom area of the front plate and, simultaneously in the top area, in the sole/plates (54's, 54'd) are located the soles already produced when, in the immediately preceding cycle, the same sole/plates were located in the bottom area of the front plate (12).
After producing the soles in the manner described, the two-component polyurethane material in injected between the half-molds (3s, 4s, 3d, 4d) and the sole/plates (54's, 54'd) (Fig. 21 ) through the injection pipes (15s, 15d) in a similar manner to that already described in Figs. 9 and 18; but with the soles (58s, 58d) that were already produced previously. In this case, therefore, the boots are already completely finished in the position b of Fig. 12.
The plant described is also suitable to produce boots in which the leg is relatively rigid in the rear part and softer and more flexible in the top part. This type of production (not represented) is carried out by fitting a liner, of suitable consistency, onto the last (2as, 2ad) and using the half-molds (3as, 3ad, 4as, 4ad) which produce a cavity into which to inject the material, which only covers a part of said liner. The plastic material thus formed will cover the leg of the boot only in the bottom part, giving it relative rigidity, while the upper part of the leg, being formed only by the uncovered liner, will be much more soft and flexible.
When said liner is covered by injecting a reactive mixture, the filling and gas venting process is identical to the one described above in the case in which the liner is covered completely with the plastic material.
As will be apparent from the description above, all the production limits in the case of use of reactive mixtures, in particular polyurethane, were due to the persistence in time of a technical prejudice, which had in fact prevented research from carrying out experimentation on the use of reactive mixtures in molds for thermoplastic polymers.
In the example of embodiment described, reference was made to a plant suitable both for the use of thermoplastic polymers and of reactive mixtures. However, with the teachings contained in the description it is also possible to produce a plant equipped only for the use of reactive mixtures.
The invention has been described purely by way of non-limiting example according to a preferred embodiment. Those skilled in the art may find numerous other embodiments, all falling within the scope of protection of the claims below.
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