FR2877259A1 - Moulding device for manufacturing envelope of heicoidal bellows comprises a mould and core having complementary threading to delimit moulding space and the envelope and core are unscrewed for stripping of the envelope - Google Patents

Abstract

The mould (2) threading (21) comprises a discontinuous rib (212) forming trough and crest with 5mm height/depth over the envelope and has circular section on both sides from which extend the flanks for retaining the envelope. The rib has plane surface of opposition perpendicular to the curve of the threading. Air is injected under pressure by channel between the core (1) and the envelope (3). Surface roughness of the mould is higher than core and is measured by sanding and chemical treatment. Independent claims are also included for: (1) manufacturing process by moulding; and (2) bellows envelope.

Description

Device for manufacturing by molding a bellows envelope

  comprising a screwable core and envelope retention means, and corresponding method.

  The field of the invention is that of molding techniques for hollow objects. More specifically, the invention relates to helical bellows envelope molding.

  Such bellows parts are used in various applications, these parts being able to be implemented in particular as an extensible or flexible coupling device protection element, as a flexible sheath for protecting cables (or other) or even as an envelope containing a liquid.

  Conventionally, the bellows of these envelopes comprise turns which extend radially, allowing radial, axial and angular displacements of the protected device.

  In order to improve the performance of these bellows, and in particular their service life, they are made of thermoplastic elastomer. This type of material also makes it possible to obtain low production cycle times and therefore a moderate cost.

  Bellowed envelopes are generally made by a process of extrusion blow molding or injection blow molding. These methods are expensive and do not provide a production quality that meets the current requirements, because the thickness of the walls is not controlled with sufficient precision.

  Another solution has therefore been proposed, consisting in producing bellows envelopes by injection.

  The thickness is thus guaranteed by the cavity and the core of the mold of the piece.

  However demolding requires deforming, including by inflating with compressed air to clear the turns of the core, and pulling them from the core (the mold having been previously opened).

  Such demolding mode is relatively suitable for vulcanized elastomeric bellows material that supports very large deformations and that resumes its molding form after demolding.

  However, this type of demolding generates too much stress on the thermoplastic elastomers which tend to make them yield or weaken them and to alter the life of the bellows.

  Yet another method of manufacturing gusseted envelopes having radial gussets with substantial undercuts has been proposed in the prior art.

  This process consists in molding the parts by single or multi-cavity injection, then demolding the bellows-shaped parts by the relative unscrewing of the core relative to the part.

  This new method of manufacturing bellows involves having a particular form of bellows. Indeed, the turns are helical in order to allow the unscrewing of the core or the part.

  However, helical turns induce disturbing reactions during manufacture, or even on the behavior of parts.

  Indeed, during manufacture, the relative unscrewing of the core relative to the bellows generates a problem: the bellows tends to remain stuck to the core and deforms during unscrewing.

  The bellows tightens on the core with the shrinkage and remains stuck by the adhesion / friction between the core and the bellows.

  The material is hot during demolding, the coefficient of friction is quite high, while the modulus of elasticity is lower. The bellows can therefore be deformed by pleating or twisting the turns by applying the unscrewing torque.

  If the workpiece is further cooled, the material is certainly stiffer, but shrinkage is also greater, involving a higher unscrewing torque to be applied, which would help distort the turns.

  One possible way to avoid this twist twist problem is to reduce the friction of the core to reduce the unscrewing torque.

  Several solutions exist to obtain a low coefficient of friction: - choose a bellows material with a low coefficient of friction, but this limits the choice of materials and forces the use of specific expensive materials; apply an anti-friction surface treatment to the core, but this type of treatment deteriorates with use. However, for a high-speed series production, the core undergoes a large number of moldings, which requires regular maintenance and induces corresponding costs. This solution is therefore not satisfactory for economic reasons.

  inject a fluid at the core / bellows interface to reduce friction. It is indeed possible to inflate the bellows with compressed air and easily unscrew the core.

  The implementation of the latter technique (injection) nevertheless causes a problem: it is necessary to obtain a good airtightness at the ends of the bellows in order to be able to take off the walls of the bellows from the surface 20 of the core.

  This seal is not easy to guarantee because the end of the bellows tends to open under pressure and allow air to escape without the walls of the turns of the bellows are peeled off. The result is that, in case of excessive leakage, bellows remain stuck on the core. This then causes production stops to undo the parts not demolded.

  Another solution has been proposed, which consists in molding the bellows with a bottom. The seal is thus automatically ensured at this end and the bellows comes off well.

  However, for bellows such as sheaths or guards 30 of flexible mechanisms, it is necessary to have both ends of the bellows 10 open. Thus, it is necessary to apply an additional operation of cutting the bottom of the bellows and generates a drop of material, which increases the cost of production of the part.

  Another problem arises: under the injection pressure, the core, which is cantilevered with its free end on the bottom side, flexes slightly, which can cause geometry defects with poor thickness distribution. .

  The invention particularly aims to overcome the disadvantages of the prior art.

  More specifically, the object of the invention is to propose a device for molding a helical bellows casing, with relative unscrewing of the casing and a core during demolding, which is more reliable than those of the art. prior.

  In this sense, the object of the invention is to limit, or even eliminate, the risks of crimping or twisting of the envelope during the relative unscrewing of the core.

  The invention also aims to provide such a device that does not change or little behavior characteristics of the envelopes obtained.

  The invention also aims to provide such a device that is simple in design and inexpensive to implement.

  These objectives are achieved thanks to the invention which relates to a device for the manufacture by molding of an envelope having at least one helical bellows, of the type comprising a mold and at least one core each having a helical thread complementary to one the other in order to define a molding space, said device comprising means for driving a relative unscrewing of said casing and said core to proceed with the demolding of said casing, characterized in that said threading of said mold presents means retaining said envelope during said relative unscrewing of said envelope and said core.

  In this way, the envelope is held in the mold during the unscrewing of the core, avoiding or at least limiting the risk of kinking (or crimping) of the envelope.

  According to an advantageous solution, said threading of said mold has a surface whose roughness is at least partially greater than the roughness of a surface of said thread of said core.

  This can be achieved simply inexpensively.

  In this case, said roughness of said surface of said thread of said mold is preferably obtained by one of the techniques belonging to the following group: sanding; - rolling; chemical treatment.

  According to an advantageous embodiment, said threading of said mold has at least one hollow which extends over a length less than the helical length of said thread, and preferably less than one-eighth of the circumference of a turn.

  Thus, during molding, protruding parts are formed in the bellows envelope, the recesses of the mold cooperating with the projecting portions of the envelope to retain it in the mold during the unscrewing of the core.

  According to a first variant embodiment, said retaining means comprise at least one discontinuous rib formed along said thread of said mold.

  According to a second variant embodiment, said retaining means comprise at least one hollow intended to form a pin on said envelope.

  According to a third conceivable variant, said retaining means comprise at least one hump intended to form a recess on said envelope.

  According to an advantageous solution, said retaining means are placed on said thread of said mold in correspondence with the peaks of said bellows of said envelope.

  In this way, it is possible to limit the influence of the projecting portions of the bellows on the behavior of the casing obtained, these projecting parts being able to be felt on the capacity of the bellows to deform in the event of stress exerted on the casing.

  In this case, said threading of said mold preferably has a thread base whose section has a circular portion on either side of which flanks extend, said retaining means extending within the limits of said circular portion, preferably with their width less than or equal to the corresponding circular portion of the core.

  However, according to an advantageous solution, said retaining means are placed on said thread of said mold in correspondence with the flanks and / or said recesses of said bellows of said envelope.

  In this way, the retaining means can be distributed so as to act on a larger surface of the bellows of the envelope.

  It is understood that this solution will be used more particularly for the manufacture of envelopes for which the deformation behavior of the envelope has a secondary aspect.

  Thus, sufficient relief is obtained to retain the envelope in the mold (the envelope could rip in the hollows in case of too low relief).

  Advantageously, said retaining means have a depth / height greater than or equal to about 0.015 mm and less than or equal to 5 mm and preferably less than or equal to 0.5 mm on the flanks or troughs of turns of the envelope.

  Indeed, a significant relief on the flanks or troughs of turns would promote the attachment of mud or aggregates on the envelopes, when using them, especially in an outdoor environment. However, these aggregates favor the abrasion of the turns.

  Too much relief on the surfaces outside the vertexes of turns 30 would therefore contribute to reducing the life of the bellows, which is not

desirable.

  According to a preferred solution, said hollow or said recesses have a planar surface opposed to said unscrewing, said surface being substantially perpendicular to the helical curve of said thread.

  Thus, the projecting parts made on the bellows (in correspondence with the hollow portions of the thread of the mold), have a good shear resistance that may appear under the retaining force in the mold.

  In this way, a relatively uniform support of the entire envelope is obtained, thus avoiding localized wrinkling or twisting.

  Advantageously, said retaining means are regularly distributed over the length of said thread.

  According to yet another embodiment, said retaining means have a narrower communication section with the envelope and a wider part, which retain the envelope by a tenon-mortise type connection.

  According to another approach of the invention, said core and said mold, each have a frustoconical shape, the conicity of said mold being less than the conicity of said core.

  According to another characteristic, the device comprises means for regulating the temperature of said mold to maintain it colder than said core during cooling of said bellows envelope.

  According to an advantageous solution, the device comprises a channel for injecting air under pressure between said core and the envelope.

  The invention also relates to a method of manufacturing by molding a casing having at least one helical bellows, using a device comprising at least one mold and at least one core each having a helical thread complementary to one of the other for delimiting a molding space, said device comprising means for driving a relative unscrewing of said casing and said core to demold said envelope, characterized in that said relative unscrewing is performed while exerting an action of retaining said envelope in said mold at said thread of said mold.

  The invention also relates to an envelope obtained by the method described above and having pins or retaining ribs on its outer surface.

  Other features and advantages of the invention will emerge more clearly on reading the following description of two preferred embodiments of the invention, given by way of illustrative and non-limiting examples, and the appended drawings in which: Figure 1 is a longitudinal sectional view of a device for manufacturing a bellows envelope according to the invention; Figure 2 is a cross-sectional view of a device for manufacturing a bellows envelope according to a first embodiment of the invention; Figure 3 is a partial view of a mold cavity of a manufacturing device according to a second embodiment of the invention; Figure 3b is a partial magnification of Figure 3; Figure 4 is a detailed view of a retaining means in the cavity according to a third embodiment of the invention.

  A device for manufacturing a bellows according to the invention is illustrated in FIG.

  According to the previous embodiment, such a device comprises at least two cavities 2 and a core 1. As illustrated a second core 4 molds a non-cylindrical inner shape for the large base of the bellows envelope. The geometry of the bellows envelope 3 is determined by the cavities 2 and the cores 1, 4. During demolding, the bellows envelope is maintained in the cavities 2 and the core 1 is unscrewed. It rotates inside the second core 4 and the two cores 1, 4 retreat in a support portion 7 of the tool, through the element 5, to release the bellows envelope. Then the mold is opened to eject the piece. Ejectors are used to facilitate the ejection of the envelope.

  Note that such a manufacturing device comprises means for holding each of the ends of the core 1 during molding. These holding means comprise: a bore 51 ensuring the centering of the core in the support 7 of the mold, via the element 5 ensuring the rotation and the translation of the core (screwing / unscrewing movement) (note that the element 5 and the core 1 can be formed in one piece or that the bore 51 can bear directly on the support 7 of the mold according to a design variant); a tip 6 ensuring the maintenance of the core at the end thereof opposite the support 5, via a cone 61.

  According to the principle of the invention, the thread 21 of the cavities 2 of the mold have means for retaining the casing 3 during the relative unscrewing of the casing 3 and the core 1.

  Other retaining means 8 are additionally provided at one end of the bellows. Also, a venting through the channel 62 of the nozzle 6 makes it possible not to cause depression during the unscrewing of the core 1. To properly press the casing 3 on the cavities 2 of the mold, air compressed can be injected through this channel 62.

  According to an embodiment illustrated in FIG. 2, these holding means comprise recesses 211 formed in the threading of the cavities, these recesses being of a nature to form lugs 31 on the surface of the thread of the casing 3.

  It is understood that these hollows do not extend continuously over the entire helical length of the thread (otherwise they would not be stop means). On the contrary, according to the present embodiment, the hollows form a kind of dashed, preferably regularly distributed over the length of the thread.

  In addition, it is advantageous to have the retaining means at least at both ends of the bellows and in a region towards the middle of the turns thereof.

  According to another conceivable embodiment, illustrated in Figures 3 and 3b, the recesses take the form of ribs 212 spaced from each other and formed in the threading of the mold cavities.

  It should be noted that the recess-forming recesses can be placed, as required, in the different zones of the threading of the cavities, namely: at the bottom of the impression thread, so as to form protruding parts on the crests of the bellows the envelope; at the crest of the impression thread, so as to form projecting portions at the bottom of the bellows of the envelope; on the flanks of the thread connecting the bottom to the crest.

  Preferably, the depth of the recesses (along an axis normal to the surface of the thread passing through the deepest point of the recess) is between 0.015 mm and 5 mm.

  With reference to FIG. 2, the depressions 211 have a plane surface 2111 perpendicular to the helical axis of the threading of the cavities, surface 2111 forming a free stop opposing the unscrewing movement printed by the core to the casing to better retain this one in the footprints.

  It should be noted that, according to another conceivable embodiment, the retaining means may consist of or comprise one or more bumps on the threading of the indentations.

  According to a variant of the invention shown in FIG. 4, the impression of the mold 2 comprises envelope retention means characterized by a small bead 41 having a narrower communication section 412 with the envelope and a larger portion. 411. This portion 411 further retains the casing by a tongue-and-groove connection and avoids the lugs to rip, especially in case of slight opening of the mold during the unscrewing of the core. This type of retaining means is disposed in the joint plane of the mold in order to release the pin portions in radial counterpane 411 during the opening of the mold.

  According to yet another conceivable embodiment, the roughness of the threading of the cavities is obtained by sandblasting, smoothing or by chemical treatment so that it is greater than the roughness of the threading of the core.

  In order to promote the unscrewing of the core, despite surface defects or coaxiality, it is advisable to relieve the closing force of the mold in order to reduce the pressure exerted on the material of the bellows and on the core. However, the mold remains substantially closed. A slight opening of the mold can be allowed (a few hundredths to a few tenths of a millimeter), so that the envelope of the bellows can be pushed back during the unscrewing of the core by a possible defect. In this case, it is clever to have protrusions or recesses of a height or depth greater than the opening of the mold, such that the bellows is retained further deep in the cavities. Indeed, too small hollows could let slip the protruding parts of the bellows, which by deforming, would interfere in the gap left between the bellows and fingerprints obtained by the slight opening of the mold. Advantageously, the device comprises a channel for injecting pressurized air between the core and the casing and allowing the casing to be better maintained by the retaining means, by being pressed against them.

  According to another aspect of the invention, the cooling circuit of the device is regulated so as to have mold cavities colder than the core. The desired effect is to stiffen the retaining ribs or pins, by cooling faster on their side. Another advantage is to obtain a lower coefficient of friction of the material on the core, having a stronger crystallization of the thermoplastic elastomer material on the side of the hot wall of the mold during cooling. This makes it easier to demold. Also a crystalline structure improves the grease resistance of the material, which improves the quality of the bellows for certain applications such as a transmission bellows.

Claims (1)

13 CLAIMS 5   1. Device for the manufacture by molding of an envelope (3) having at least one helical bellows, of the type comprising a mold (2) and at least one core (1) each having a helical thread complementary to one of the another for delimiting a molding space, said device comprising means for driving a relative unscrewing of said casing (3) and said core (1) to demold said casing (3), characterized in that said Thread (21) of said mold (2) has means for retaining said envelope (3) during said relative unscrewing of said envelope (3) and said core (1).   2. Device according to claim 1, characterized in that said thread (21) of said mold (2) has a surface whose roughness is at least partially greater than the roughness of a surface of said thread of said core (1).   3. Device according to claim 2, characterized in that said roughness of said surface of said thread of said mold (2) is obtained by one of the techniques belonging to the following group: sanding; knurling; chemical treatment.   4. Device according to any one of claims 1 to 3, characterized in that said thread (21) of said mold (2) has at least one recess (211) which extends over a length less than the helical length of said threading (21).   5. Device according to any one of claims 1 to 4, characterized in that said retaining means comprise at least one discontinuous rib (212) formed along said thread (21) of said mold (2).   6. Device according to any one of claims 1 to 4, characterized in that said retaining means comprise at least one recess (211) for forming a pin on said envelope.   7. Device according to any one of claims 1 to 4, characterized in that said retaining means comprise at least one bump intended to form a recess on said envelope.   8. Device according to one of claims 1 to 7, characterized in that said retaining means are placed on said thread (21) of said mold (2) in correspondence with the peaks of said turns of said casing (3).   9. Device according to claim 8, characterized in that said thread (21) of said mold (2) has a bottom of the net whose section has a circular portion on either side of which flanks extend, said means retainer extending within the boundaries of said circular portion.   10. Device according to any one of claims 1 to 7, characterized in that said retaining means are placed on said thread (21) of said mold (2) in correspondence with the sidewalls and / or said recesses of said bellows of said envelope (3).   11. Device according to one of claims 4 to 10, characterized in that said retaining means have a depth / height greater than or equal to about 0.015mm.   12. Device according to one of claims 4 to 11, characterized in that said retaining means have a depth / height less than or equal to 5 mm.   13. Device according to any one of claims 4 to 12, characterized in that said retaining means have a plane opposing surface (2111) to said unscrewing, said surface (2111) being substantially perpendicular to the helical curve of said thread ( 21).   14. Device according to any one of claims 1 to 13, characterized in that said retaining means are regularly distributed over the length of said thread (21).   15. Device according to any one of claims 4 to 14, characterized in that said retaining means have a narrower communication section (412) with the envelope (3) and a wider portion (411), which hold the casing (3) by a connection type tenonmortaise.   16. Device according to any one of claims 1 to 15, characterized in that said core (1) and said mold (2), each have a frustoconical shape, the conicity of said mold (2) being less than the conicity of said core (1).   17. Device according to any one of claims 1 to 16, characterized in that it comprises means for regulating the temperature of said mold (2) for maintaining it colder than said core (1) during cooling of said bellows envelope.   18. Device according to any one of claims 1 to 17, characterized in that it comprises a channel (62) for injecting air under pressure between said core (1) and the casing (3).   19. A method of manufacturing by molding an envelope (3) having at least one helical bellows, using a device comprising at least one mold (2) and at least one core (1) each having a helical thread. complementary to each other in order to define a molding space, said device comprising means for driving a relative unscrewing of said casing (3) and said core (1) for demolding said casing (3). ), characterized in that said relative unscrewing is performed while exerting an action for retaining said envelope (3) in said mold (2) at said thread (21) of said mold (2).   20. bellows envelope comprising helical turns made by a method according to claim 19, characterized in that said casing comprises pins or retaining ribs on its outer surface.