VAROTTO MARCO (IT)
SALPIETRO FRANCESCO (IT)
VAROTTO MARCO (IT)
| JP2001232688A | 2001-08-28 | |||
| US20030029876A1 | 2003-02-13 | |||
| GB1511847A | 1978-05-24 | |||
| US6506044B1 | 2003-01-14 |
CLAIMS
1. Injection press for the manufacture of pieces moulded from polymer material, which said pieces may consist of parts of two different colours, or a single colour, of the type comprising: • a lower base (2);
• a plurality of first columns (3a, 3b, 3c, 3d) or (30a, 30b, 30c, 3Od) mounted on said base (2), which act as support for an upper fixed crosspiece (4);
• a mobile crosspiece (5), which slides on said first columns (3a, 3b, 3c, 3d) or (30a, 30b, 30c, 3Od), first means (6a) designed to lock said mobile crosspiece (5) in relation to said upper fixed crosspiece (4) being provided;
• a first and second mould Si and S 2 mounted between said lower base (2) and said mobile crosspiece (5); • second means designed to exert the force required to close said moulds
S 1 and S 2 , counteracting the pressure exerted by a molten polymer injected by a pair of injectors (11); characterised in that:
• the lower parts or mould halves Sn and S 2i of said first and second moulds S 1 and S 2 are mounted side by side on said lower base (2);
• the upper parts or mould halves S 1s and S 2s of said first and second moulds S 1 and S 2 are mounted side by side on said mobile crosspiece
(5);
• a cavity designed to be filled with molten polymer is present in said first upper mould half Si s and said second lower mould half S 2 , or, alternatively, in said second upper mould half S 2s and said first lower mould half S-π; third means (10) and fourth means (10a) being designed to move said first and second upper mould halves S 1s and S 2s parallel to themselves, so as to bring said upper mould half S 1s or S 2s , which contains said impression, in correspondence with said second or first lower mould half S 2 i or S 1 J respectively, which contains said impression to be filled with molten polymer.
2. Injection press as claimed in claim 1 , characterised in that said injectors (11) are both positioned at the same level. 3. Injection press as claimed in claims 1 and 2, characterised in that said third means (10) are designed to move one of said first or second upper mould halves Si s or S 2s parallel to itself, so as to bring it outside said mobile crosspiece (5).
4. Injection press as claimed in claims 1 to 3, characterised in that said third means (10), designed to move one of said first or second upper mould halves S 1s or S 2s parallel to itself, so as to bring it outside said mobile crosspiece (5), are also designed to reinsert it, after performing a lateral movement due to the sliding effect of a slide (20), in correspondence with said second or first lower mould half Sa or S 1 , respectively.
5. Injection press as claimed in claims 1 and 2, characterised in that said fourth means (10a) are designed to move one of said first or second upper mould halves Si s or S 25 parallel to itself, so as to bring it in correspondence with second or first lower mould half S 2 j or Sn respectively.
6. Injection press as claimed in claims 1, 2, 3 and 5, characterised in that it includes: • a first movement of said first or second upper mould half S-| S or S 23 , effected by said third means (10), so as to bring it outside said mobile crosspiece (5);
• a second movement of said second or first upper mould half S 23 or Si Sj effected by said fourth means (10a), in order to move it in correspondence with said first or second lower mould half Sn or S 2i respectively; said first and second movements causing coupling to be effected between the two mould halves containing said cavity designed to be filled with the molten polymer. 7. Injection press as claimed in claims 1 to 5, characterised in that it includes
• a first movement of said first or second upper mould half Si 3 or S 23 , effected by said third means (10), so as to bring it outside said mobile crosspiece (5);
• a second movement of said second or first upper mould half S 23 or Si 3 , effected by said fourth means (10a), designed to move it in correspondence with said first or second lower mould half Sn or S 2 j respectively;
• a third movement of said first or second upper mould half Si 3 or S 2s , effected by said third means (10), designed to move it in correspondence with said second or first lower mould half S 2 i or Sn respectively, after performing a lateral movement via said slide (20); said first, second and third movements causing coupling to be effected between the two mould halves containing said cavity designed to be filled with molten polymer, by means of a mutual exchange of position between said first and second upper mould halves Si s and S2s. 8. Injection press as claimed in claims 1, 2, 3, 5 and 6, characterised in that said first columns (3a, 3b, 3c, 3d) have a stiffness such that (as regards their position in relation to the action line of the resultant of clamping forces F 3 , which occurs when there is only one mould in press (1), in an asymmetrical position, and said mobile crosspiece (5) is locked in relation to said fixed upper crosspiece (4)) said clamping force F 3 causes uniform lengthening of said first columns (3a, 3b, 3c, 3d), so as to maintain said mobile crosspiece (5) parallel to said lower base (2).
9. Injection press as claimed in claim 8, characterised in that it includes second columns (9a, 9b) mounted on said base (2), which said mobile crosspiece (5) slides in relation to said second columns (9, 9a), fifth means being installed which are designed to lock and unlock said second columns (9a, 9b) in relation to said mobile crosspiece (5), which said second columns (9a, 9b) have a stiffness such that (as regards their position in relation to said first columns (3a, 3b, 3c, 3d) and the action line of the resultant of clamping forces Fi and F 2 , which occur when said two moulds Si and S 2 are present in press
(1) and said mobile crosspiece (5) is locked in relation to said fixed upper crosspiece (4)) said clamping forces Fi and F 2 cause uniform lengthening of said first columns (3a, 3b, 3c, 3d) and said second columns (9a, 9b), so as to maintain said mobile crosspiece (5) parallel to said lower base (2), said second columns (9a, 9b) being locked in relation to said mobile crosspiece
(5).
10. Injection press as claimed in any of claims 1 to 9, characterised in that said second means designed to exert the force required to clamp said moulds Si and S 2 , counteracting the pressure exerted by said molten polymer injected by said pair of injectors (11 ), are designed to act separately on each of said two moulds Si and S 2 .
11. Method for optimisation of the movements of the moulds in an injection press (1), as claimed in claims 1 , 2, 3 and 5, characterised in that it includes:
• a first movement of said first or second upper mould half S-ι s or S 25 , effected by said third means (10), so as to bring it outside said mobile crosspiece (5); • a second movement of said second or first upper mould half S 2S or S-| S , effected by said fourth means (10a), in order to move it in correspondence with said first or second lower mould half Sn or S 2 j respectively; said first and second movements causing coupling to be effected between the two mould halves containing said cavity designed to be filled with the molten polymer. 12. Method for optimisation of the movements of the moulds in an injection press
(1a), as claimed in claims 1 to 5, characterised in that it includes: • a first movement of said first or second upper mould half Si s or S 2s , effected by said third means (10), so as to bring it outside said mobile crosspiece (5);
• a second movement of said second or first upper mould half S 2s or S-i s , effected by said fourth means (10a), in order to move it in correspondence with said first or second lower mould half S 1 , or S 2 j respectively;
• a third movement of said first or second upper mould half Si s or S 2s , effected by said third means (10), designed to move it in correspondence with said second or first lower mould half S 2 j or Si 1 respectively; said first, second and third movements causing coupling to be effected between the two mould halves containing said cavity designed to be filled with molten polymer, by means of a mutual exchange of position between said first and second upper mould halves Si s and S 2s . |
INJECTION PRESS FOR THE MANUFACTURE OF ONE- AND TWO-COLOUR PIECES
This invention relates to an injection press for moulding pieces from polymer material, which said pieces may be constituted by two identical materials with different colours.
Said injection press can also produce pieces of a single colour, but with higher productivity than the injection presses according to the prior art.
For the manufacture of two-colour pieces moulded from polymer material, such as crosslinked EVA foam, with separation of said two colours taking place along a surface, the prior art uses two mould halves separated by a third element which acts as clamping system for both said mould halves. In practice, there is a lower mould containing a first impression. Said lower mould half is closed at the top by the lower part of said third element, the upper part of which closes an upper mould half containing a second impression.
Three elements are consequently superimposed, namely the lower mould half, the intermediate element and the upper mould half, which forms two impressions into which the molten polymer can be injected, in the two chosen colours.
After injection, the two mould halves are distanced from the intermediate element, which is moved laterally so that the two mould halves can be joined. The clamping of the two mould halves places the polymers in the two impressions in contact with one another; said polymers are still partly molten, as crosslinking has only just begun, and has produced the initial consolidation required to ensure that there is no leakage from the impression when the moulds are opened. The two parts therefore bond, while the crosslinking reaction continues, leading to the formation of a single two-colour piece. After clamping, the foaming agent begins to react, generating a great increase in pressure in the partly molten mass, and promoting bonding between the upper and lower parts. The actual expansion obviously takes place on final opening of the moulds, namely when crosslinking has been completed, allowing the piece to reach the required size and density. If the piece is to be made in a single colour, the unit described can meet this requirement simply by eliminating the intermediate element and placing the two mould halves in contact with one another. The result is a single cavity into which the molten polymer is injected in the required colour, according to the ordinary procedures. This procedure is performed when a production run involves pieces of the same shape and size, whether two-colour or one-colour, because it allows the use of the same equipment, with no need for a new mould.
The machines according to the prior art, of the type described, require the use of an intermediate element of constant height. This is because the extruders or injectors used to inject the molten polymer are at a fixed height, so that if the height of the intermediate element is varied, the upper extruder is unable to operate.
In theory, a mobile supporting structure could always be made for the upper injector, so that it can be moved vertically in order to adapt to intermediate elements of various heights. However, the manufacture of such a structure is very expensive, and introduces excessive complications. This drawback does not constitute a limitation in the case of manufacture of two- colour pieces, because the intermediate element is made with such a height that the injection points of the two mould cavities coincide with the centre line of the two extruders. However, as said machine according to the prior art has two extruders, it would be able to inject simultaneously into two moulds, suitable to make two one-colour pieces, if the injection points were at the same level as the two extruders. In general, this situation does not arise, because the heights of the moulds are based on the shape of the pieces. If the moulds had a height such that the injection point of the upper mould were below the level of the upper extruder, it would always be possible to insert spacers, whereas in the opposite case the problem clearly could not be solved.
The drawback of the machines according to the prior art designed to make two- colour pieces is consequently their limited versatility because, in general, if used to make one-colour pieces of any shape, they cannot perform two injections simultaneously, although they have two extruders. Moreover, if the material used to fill the mould cavities is a polymer foam, such as crosslinked EVA, it is known that after the crosslinking stage of the injected products, the moulds must be opened as quickly as possible, to allow correct expansion of the products. In the specific case of one-colour products, both the superimposed moulds should open quickly and simultaneously, to prevent deformation of the moulded products. If it is assumed that both the intermediate plate and the upper mould half move along a vertical axis, the latter should move at a very high speed, namely twice the speed of the intermediate plate, and consequently twice the already high speed used in current machines which inject crosslinked EVA foam, and this would make its manufacture very complicated, apart from the high cost of the machines.
Finally, in the case of two superimposed one-colour moulds, the operations involved in demoulding the injected products and frequent cleaning of the injection channels are difficult, as are the inspection and cleaning of the cavities of the two moulds. The purpose of this invention is to offer an injection press, as claimed in claim 1 , designed to manufacture both two-colour and one-colour pieces of any shape, equipped with a pair of extruders or injectors able to perform two mouldings at the same time, with considerable versatility as regards the height of the moulds, which depends on the height of the pieces to be made, which said injection press can also make two-colour pieces.
This result is achieved by a press in which two moulds are housed, next to one another, on the lower bed, which said two moulds are fed by a pair of injectors installed at the same height. In this configuration, the machine according to the invention is an injection press with two workstations, of known type, which is highly versatile as regards the height of the moulds. If the possibility of moving the mould halves laterally is added, said injection press also becomes suitable to make two-colour pieces.
The invention also relates to a method of optimising said movements, as claimed in claims 11 and 12. Unlike the injection presses having two workstations according to the prior art, the injection press according to the invention can also make two-colour pieces. Unlike the injection presses for the manufacture of two-colour pieces according to the prior art, the injection press according to the invention can simultaneously perform two mouldings of one-colour pieces of any shape, with much greater versatility.
The invention will now be described according to two preferred embodiments, by way of example but not of limitation, with reference to the annexed figures, wherein:
• figures 1 and 2 show a press according to a first embodiment of the invention, in two perspective views and four orthogonal views;
• figures 3 (a, b, c) show the procedure for moulding ordinary one-colour pieces with the press according to said first embodiment of the invention;
• figures 4 (a, b, c, d) show the procedure for moulding two-colour pieces with the press according to said first embodiment of the invention;. • figures 5 and 6 show a press according to a second embodiment of the invention, in two perspective views and five orthogonal views;
• figures 7 (a, b, c) show the procedure for moulding ordinary one-colour pieces with the press according to said second embodiment of the invention;
• figures 8 (a, b, c, d, e, f) show the procedure for moulding two-colour pieces with the press according to said second embodiment of the invention; In figs. 1 and 2, numeral (1) indicates an injection press according to a first preferred embodiment of the invention.
Said press (1) comprises a base (2) into which four first columns (3a, 3b, 3c, 3d) are inserted.
Said four first columns (3a, 3b, 3c, 3d) act as support for an upper fixed crosspiece (4).
A pair of second columns (9a, 9b), fitted with means of known type designed to make said second columns (9a, 9b) integral with (or release them from) upper crosspiece (4) at various times are also mounted on lower base (2). The function of said second columns (9, 9a) will be explained below. An intermediate crosspiece (5) slides on said first columns (3a, 3b, 3c, 3d), the movements of which said intermediate crosspiece are controlled by a cylinder (6) integral with upper fixed crosspiece (4). Locking means (6a) of known type, designed to secure intermediate crosspiece (5) in relation to upper fixed crosspiece (4), are also provided. Said cylinder (6) and said locking means (6a) are preferably of the fast-acting type, to allow mould opening which is fast enough for the demoulding of the expanded pieces.
A number of cylinders (7), for example four, designed to exert mould clamping strength, are mounted on base (2). A first and second plate (8, 8a), each of which is suitable to receive a lower mould half, are mounted on said cylinders (7), the corresponding upper mould half being fixed to said intermediate mobile crosspiece (5).
Press (1) according to the invention also includes a first mechanism (10) for the lateral removal of one of the two upper mould halves, and a second mechanism (10a) to traverse the other upper mould half from a first position in which it is in correspondence with one of the lower mould halves, to a second position in which it is in correspondence with the other said lower mould half. As said mechanisms
(10, 10a) are known in themselves, they will not be described further, whereas their function will be clarified below. The moulds for manufacture of the mouldings are mounted between plates (8, 8a) and intermediate mobile crosspiece (5). In particular, the upper mould halves are
mounted below intermediate crosspiece (5), whereas the lower mould halves are mounted on plates (8, 8a), with one mould half on each of the two plates (8, 8a). For the manufacture of ordinary one-colour pieces, the procedure involves moving intermediate crosspiece (5) with the aid of cylinder (6) until the moulds are clamped, and locking the intermediate crosspiece in said mould-clamping position with said locking means (6a).
At this point lower cylinders (7), designed to exert the clamping force required to counteract the pressure of the injected polymer, perform their action, and said locking means (6a) are counteracted. The fact that the lower mould halves are mounted on separate plates (8, 8a) and that cylinders (7) exert their thrust independently of one another readily compensates for the inevitable differences of height between the moulds due to imprecise manufacture. The operation of press (1) according to the invention for the moulding of ordinary one-colour pieces is illustrated schematically in fig. 3 which, for the sake of simplicity, only illustrates base (2), mobile crosspiece (5) and columns (3a, 3b, 3c, 3d).
Fig. 3a shows a first and second mould S 1 and S 2 positioned between base (2) and intermediate mobile crosspiece (5), said first mould Si consisting of a first lower mould half S-π and a first upper mould half Si s and said second mould S 2 consisting of a second lower mould half S 2 j and a second upper mould half S 2s . The lower mould halves have a height Hi, suitable to bring the injection point to the height of a pair of injectors, not illustrated in fig. 3. The total height H of the moulds is obviously the sum of said height Hi of the lower mould halves and height H 2 of the upper mould halves. If the heights H of the two moulds are different, it is sufficient to insert spacers to obtain both the same total height and the height of the injection points of both moulds at the level of the corresponding injectors. In fig. 3a, moulds Si and S 2 have just been clamped by mobile crosspiece (5), due to the effect of cylinder (6) which is integral with upper fixed crosspiece (4). In fig. 3b, the lower mould halves are pressed against the upper mould halves by cylinders (7), mobile crosspiece (5) being maintained in position by locking means (6a). At this stage the moulds are filled by the injectors. This is followed by consolidation of the pieces, with the moulds clamped. In fig. 3c, mobile crosspiece (5) is raised, the moulds open and the consolidated pieces are demoulded.
The procedure described is an ordinary process for the manufacture of any kind of one-colour pieces. It will be shown below how injection press (1 ) according to the invention is suitable to make two-colour pieces, according to the sequence shown in fig. 4. Fig. 4a shows a first and second mould Si and S 2 mounted on press (1). The first upper mould half Si s contains an impression which corresponds to the upper part of the piece to be made, while the first lower mould half Su is a simple smooth lid. The second lower mould half S 2i contains an impression which corresponds to the lower part of the piece to be made, while the second upper mould half S 2s is a simple smooth lid.
In the configuration shown in fig. 4a, with the moulds clamped, injection of the molten polymer in two colours is performed, for each of the two moulds Si and S 2 , by a pair of injectors (11 ). After a certain time, in which a first partial crosslinking produces a first partial consolidation, upper crosspiece (5) is raised (fig. 4b), opening the moulds. At this point mechanisms (10) and (10a) act on the upper mould halves, traversing them laterally. In particular, first mechanism (10) traverses second upper mould half S 2s to bring it outside crosspiece (5), while second mechanism (10a) traverses first upper mould half Si s until it is in correspondence with second lower mould half Sa.
At the next stage, illustrated in fig. 4c, the mould formed by second lower mould S 2 J and first upper mould Si s is clamped, thus placing the polymers contained in the two impressions in contact with one another, so that they bond together due to the crosslinking reaction, leading to the formation of a single two-colour piece. The foaming agent contained in the polymer exerts its pressure during the crosslinking stage, and at that stage the mould, consisting of parts S 2 j and Si s , is constantly kept perfectly clamped by the action of cylinders (7) which develop force F 3 . The actual expansion obviously takes place on final opening of the moulds, namely when crosslinking has been completed, allowing the piece to reach the required size and density.
The opening of the mould (fig. 4d) must be very fast to prevent deformation of the piece which is still hot, and the opening space must preferably be large to allow easy demoulding of the piece, together with easy cleaning of the injection channel and everything else necessary to prepare the mould for the next manufacturing cycle.
During the injection stage, intermediate mobile crosspiece (5) is held closed by locking means (6a) which counteract clamping forces F-i and F 2 , transmitting them to upper fixed crosspiece (4) which, in turn, transmits them to first columns (3a, 3b, 3c, 3d) and second columns (9a, 9b) which, in this case, are rendered integral with crosspiece (4).
The sum of the areas of the right-hand columns (3a, 3b) (fig. 4) is equal to the sum of the areas of all the left-hand columns (3c, 3d) and (9a, 9b). When there are two moulds on the press, as in fig. 4a, and clamping forces Fi and F2 exert their action symmetrically, the two right-hand columns (3a, 3b) undergo the same lengthening as all the other four left-hand columns (3c, 3d and 9a, 9b); in this way upper crosspiece (4), and intermediate crosspiece (5) with it, remain parallel to lower base (2), thus allowing correct mould clamping.
However, as shown in fig. 4c, when there is only one mould in the press, clamping force F 3 is exerted asymmetrically, with the result that the action on the right-hand columns is greater than the action exerted on the left-hand columns. Consequently, if all the columns (3a, 3b, 3c, 3d, 9a, 9b) remained engaged, there would be different lengthenings between the right- and left-hand columns, thus causing inclination of crosspieces (4) and (5) and incorrect mould clamping. To eliminate said drawback, this invention provides for said second columns (9a, 9b) to be released so that they are no longer integral between base (2) and upper crosspiece (4).
In this way, the action of force F 3 is only exerted on the remaining columns (3a, 3b, 3c, 3d), whose cross-sectional areas are such that the lengthening of the right-hand columns (3a, 3b) is exactly equal to the lengthening of the left-hand columns (3c, 3d).
As all the columns are made of the same material, in particular steel, their stiffness will be proportional to the cross-sectional area of the columns The pre-requisite will therefore be that the moments on upper fixed crosspiece (4) are balanced, lengthening between the various columns being equal. Thus if the total area of right-hand columns (3a, 3b) is called AD, and the total area of left- hand columns (3c, 3d) is called As, the pre-requisite must be:
A 0 * b = As * a
Moreover, the total area of all the left-hand columns (3c, 3d, 9a, 9b) is equal to total area A D SO as to comply with the condition of equal lengthening, between the left-hand and right-hand columns, when forces Fi and F 2 exert their symmetrical action.
This result is achieved by including means of known type designed to make the columns in question integral with lower base (2) and/or upper fixed crosspiece (4) at various times.
In other words, it can be said that when forces Fi and F 2 exert their symmetrical action, left-hand columns (3c, 3d, 9a, 9b) and right-hand columns (3a, 3b) operate, whereas when force F 3 exerts its predominant asymmetrical action on the right, the same right-hand columns (3a, 3b) also operate, but only columns (3a, 3b) operate on the left, as the remaining columns (9a, 9b) are disengaged. In the preferred case described, second columns (9a, 9b) are hooked onto or released from upper fixed crosspiece (4). However, one skilled in the art would have no difficulty in finding other clamping or release combinations of first columns (3a, 3b, 3c, 3d) or second columns (9a, 9b) which fulfil the condition of balance of the moments on upper fixed crosspiece (4), and consequently on mobile crosspiece (5). In conclusion, it can be said that said first columns (3a, 3b, 3c, 3d) and said second columns (9a, 9b) have a stiffness which (as regards their position in relation to the action line of the resultant of clamping forces F-i and F 2 , which occur when said two moulds Si and S 2 are present in press (1), or of clamping force F 3 , which occurs when there is only one mould in press (1 ), in an asymmetrical position) is such as to cancel out the moments on said mobile crosspiece (5), depending on whether or not said second columns (9a, 9b) are locked in relation to said mobile crosspiece (5).
In accordance with a second embodiment of the invention, the moulds are moved in such a way as to prevent said imbalance from occurring. In such case the presence of six columns (3a, 3b, 3c, 3d, 9a, 9b) with differentiated cross-sectional areas is unnecessary; four equal columns (30a, 30b, 30c, 3Od) can be used, as shown in figs. 5 and 6, which illustrate an injection press (1a) in accordance with said second embodiment of the invention. Press (1a) in accordance with said second embodiment of the invention also provides that said first mechanism (10) for the lateral removal of the upper mould halves is mounted on a slide (20) that traverses said mechanism (10) parallel to itself.
The manufacture of ordinary one-colour pieces, illustrated in fig. 7, is performed with press (1a) in exactly the same way as with press (1), and consequently will not be further described.
It will be shown below how injection press (1a), in accordance with said second
embodiment of the invention, is suitable for the manufacture of two-colour pieces, according to the sequence shown in figs. 8 (a, b, c, d, e, f), without causing asymmetries of the forces applied to the structure of press (1a). Fig. 8a shows a first and second mould Si and S 2 mounted on press (1). The first upper mould half Si s contains an impression which corresponds to the upper part of the piece to be made, while the first lower mould half Sn is a simple smooth lid. The second lower mould half S 2 i contains an impression which corresponds to the lower part of the piece to be made, while the second upper mould half S 2 s is a simple smooth lid. In the configuration shown in fig. 8a, with the moulds closed, injection of the molten polymer in two colours is performed, for each of the two moulds Si and S 2 , by a pair of injectors (11 ).
After a certain time, in which a first partial crosslinking produces a first partial consolidation, upper crosspiece (5) is raised, opening the moulds (fig. 8b). At this point (fig. 8c) mechanisms (10) and (10a) act on the upper mould halves, exchanging their mutual positions, by traversing on their plane. In particular, first mechanism (10) traverses second upper mould half S2 S to bring it outside the contour of second lower mould half S 21 , while second mechanism (10a) traverses first upper mould half Si s to move it in correspondence with second lower mould half S2 J . Then, with a lateral movement of first mechanism (10) controlled by slide (20), and a return movement of first mechanism (10), second upper mould half S 2s is brought in correspondence with first lower mould half Su. In practice, the two upper mould halves Si s and S2 S exchange positions. This configuration is shown in fig. 8d. The next stage, illustrated in fig. 8e, involves clamping of the mould formed by second lower mould S 2 i and first upper mould S 1s , thus placing the polymers present in the two impressions in contact with one another, so that they bond together due to the crosslinking reaction not still completed an the pressure generated by foaming agent, leading to the formation of a single two-colour piece. Simultaneously the two lids, namely the two mould halves without impressions S-π and S 2S , are placed in contact.
The foaming agent contained in the polymer exerts its pressure during the crosslinking stage, and at this stage the mould, consisting of the two mould halves S 2 i and Si s , is constantly maintained perfectly clamped by the action of cylinders (7). The actual expansion obviously takes place on final opening of the moulds, namely when crosslinking has been completed, allowing the piece to reach the
required size and density.
During the injection stage, intermediate mobile crosspiece (5) is held closed by locking means (6a) which counteract the clamping force, transmitting it to upper fixed crosspiece (4). In view of the simultaneous presence of moulds Si and S 2 between lower base (2) and mobile crosspiece (5), there are no imbalances in the injection stage and consequently, the four columns (30a, 30b, 30c, 3Od) are equal. As will clearly appear from the above description, the injection press according to the invention, in both the embodiments described, is able to perform two simultaneous mouldings of one-colour pieces with considerable versatility as regards the height of the moulds, or to mould two-colour pieces. In practice, these machines are designed to produce two-colour pieces, but are also able to perform two simultaneous mouldings of one-colour pieces. Conversely, the machines designed according to the prior art for moulding two-colour pieces can only perform two simultaneous mouldings of one-colour pieces if the height of the two moulds is such as to make the centre line of the injection points coincide with the centre line of the injectors, but even in that case, expensive mechanisms are required to effect fast, simultaneous opening of the two superimposed moulds. The invention has been described according to two preferred embodiments, by way of example but not of limitation. One skilled in the art could devise numerous other embodiments, all of which fall into the ambit of protection of the following claims.