Patent Application
20160288405
The invention concerns an apparatus and a method for stamping a thermoplastic matrix composite blank. The invention is more particularly but not exclusively adapted to the transformation of a composite blank formed of a lamination of plies, including continuous reinforcement fibers such as carbon, glass or aramid fibers in the form of fabrics or non-crimped layers, in a matrix made of a thermoplastic polymer such as polyether ether ketone or PEEK, polysulfone or PPS or polyetherimide or PEI, without these application examples being exhaustive. The invention is more particularly adapted to the stamping of a consolidated blank or a partially consolidated blank.
Stamping is an operation whose purpose is to deform a flat raw part, known as a blank, between a die and an effector comprising a rigid punch or a bladder, in order to urge the blank against the die so that it fits its shape, and to conform said blank to a tridimensional shape. In the case of stamping a blank made of a composite material with a fibrous reinforcement, in the form of continuous fibers, this shape modification implies inter-laminar slipping between the plies and, locally, modifications of the distance between the fibers in controlled proportions, for passing from the plane configuration to the shaped configuration, the fibers not being able to deform plastically. These inter and intra-laminar movements of the reinforcements are made possible by the matrix viscosity, the matrix being brought to its melting temperature for performing the stamping of a blank with a thermoplastic matrix.
Thus, the typical steps for the stamping a thermoplastic matrix composite blank comprise:
This method of shaping is satisfactory, but presents some drawbacks related notably to the use of the polyimide film. Actually, being stamped with the blank, said film is not reusable, although being of a high price when the polymer constituting the composite matrix exhibits a high melting point, for example PEEK. In addition, as the film is not plastically deformable at the stamping temperature, it tends to wrinkle during this operation, the wrinkles being printed in the piece and causing fibers undulations.
Document FR 2 922 276 describes a method and a device for stamping a machined composite blank with a consolidated thermoplastic matrix; said device uses a transfer frame, that holds the blank on its periphery, without a support film, both during the heating and the transfer of the blank on the die. This same device of the prior art uses a locator traversing a bore made in the blank, which allows to position the blank in relation to the frame. This device of the prior art proves to be poorly reliable in the framework of an industrial production. In fact, the loss of the mechanical properties of the blank during heating frequently leads to its falling from the frame and even when the blank is further held by the locator, the bore tends to deform around said locator, leading to defects such as resin squeezing, pinching or undulations of the fibers; these defects, considering the continuous nature of the fibers, are likely to propagate at remote distances from the locator. During stamping, if the blank does not succeed in escaping from the locator, the latter causes tearing and hinders the interlaminar slipping, thus leading to defects.
The document FR 2 354 870 describes a device for stamping of a thin sheet of a thermoplastic material serving as coating to a container. Such a thin sheet, non-reinforced by fibers, tends to shrink during heating. For avoiding this phenomenon, the sheet is hooked on needles on its periphery. For avoiding the sheet tearing during stamping, a blank holder is used to wedge the edges of the sheet between the needles and the formed part. Such a method cannot be used in the case of a continuous fibers reinforced composite blank, the blank holder preventing the interlaminar slipping that is required for the forming. The term “machined blank” designates a blank which underwent a preparation, especially by material removal usually by high pressure water jet cutting or by routing with a cutting tool, in order to bring it to an adapted contour, or for cutting out particular shapes, such as a bore or a recess.
The invention aims at solving the drawbacks of the prior art, and concerns to this end an apparatus for stamping a thermoplastic matrix composite blank, said apparatus comprising for heating and stamping the blank:
Thus, the heat sink is able to locally keep the blank temperature below the melting temperature of the polymer constituting the matrix, during the heating and the transfer of the blank, so that, in the vicinity of the heat sink, the blank is not brought to its melting temperature, is not subject to bulking and keeps some mechanical properties. Consequently, the locator ensures an efficient backing of the blank, without deformation of the blank around this locator.
This invention is advantageously implemented in accordance to the embodiments described hereunder which are to be considered individually or according to any operating technical combination.
Advantageously, the heat sink consists in the cleat holding the locator. This embodiment is particularly simple and economical to implement.
Advantageously, the heat sink cleat is made of a material whose thermal diffusivity is higher or equal to 10 times the thermal diffusivity of the polymer constituting the matrix of the blank. Thus, the cleat provides an efficient cooling of the blank face that touches it.
Advantageously, the cleat comprises a surface treatment to limit its absorption of infrared radiation. This embodiment, compatible with the previous, is advantageous when the blank heating, prior to its stamping, is performed by radiant panels. Thus, the cleat does not undergo, or only to a small extent, the heating by the heat source.
Advantageously, the blank comprises at its periphery:
Thus, this guiding hole is able to orient the blank with regard to the die after the blank was transferred on the stamping tool.
Advantageously, the apparatus of the invention comprises:
Thus, the blank is advantageously transferred on the die means of the frame, said frame being able to retract from the die for not impeding the stamping.
Advantageously, the stamping die comprises:
Thus, the guiding and positioning means of the die are replacing the frame positioning means once said frame clears out.
Advantageously, the guiding hole of the blank is a slotted hole. Thus, said hole enables the positioning of the blank in relation to the die while not impeding its sliding in the stamping direction.
The invention also concerns a method for stamping a composite blank including a continuous fibrous reinforcement in a thermoplastic matrix, using an apparatus in accordance with the invention; said method comprises the steps consisting in:
Thus, the method of the invention allows to heat and to transfer the blank on the tooling, without being hindered by any blank support during the stamping. Thus, the mechanical quality of the parts obtained by this method is improved with regard to the stamping method of the prior art.
Advantageously, the method of the invention comprises before step i) a step consisting in:
Thus, the blank is perfectly positioned in relation to the die, but no obstacle impedes the interlaminar slipping of plies during the stamping operation.
The invention is described hereunder according to its preferred embodiments, in no way limiting, and in reference to
E=√{square root over (λρ.C)}
where λ is the thermal conductivity coefficient of the material, ρ its density and C its specific heat. The thermal diffusivity of a material quantifies its capacity to exchange heat with its environment. Thus the thermal diffusivity of PEEK is 330 J·K−1·m−2·s−1/2, the thermal diffusivity of copper is 36 000 J·K−1·m−2·s−1/2, that of steel is 14 000 J·K−1·m−2·s−1/2. When a first material with thermal diffusivity E1 is brought to the temperature T1, then put in contact with a second material with thermal diffusivity E2 and temperature T2, the instant temperature T at the interface of two materials is given by the relation (neglecting the contact resistance):
Thus, in the exemplary embodiment where the polymer constituting the matrix of the blank is a PEEK, the cleat (220) is, for example, made of steel and its thermal diffusivity is 40 times higher than the one of the PEEK. Thus, the cleat absorbs the heat of the blank by conduction, next to the locator, so that the blank area (240), sensibly in contact with the cleat (220), is not brought to the melting temperature of the matrix upon the heating prior to the stamping of the blank. Thus, the stratification layers remain stuck to each other, in the vicinity of the locator (221) and of the hole (121) receiving said locator, and do not deform when the rest of the blank is brought to melting temperature the matrix. The blank (100) remains consequently well backed and well positioned in the frame with the help of the cleats and the locators (221), which enable to make the transfer of the blank from the heating station to the stamping tool without any risk of separation of the blank from the frame, and, on the other hand, to preserve the positioning of the blank and master this positioning in relation to the stamping tooling.
Returning to
In
After being positioned in relation to the die (400), the blank (100) is immediately stamped by a rigid punch or a bladder so as to conform it to the stamping die shape. During this stamping operation, the material constituting the blank undergoes slipping (551, 552) with variable direction and amplitude, on the blank surface, depending on the shape of the die.
Returning to
The aforementioned description and the embodiments indicate that the invention reaches the targeted objectives, and more specifically it enables to heat a blank on a transfer frame in view of the hot stamping of said blank, ensuring an efficient backing of the blank on the frame during the transfer, an accurate position of the blank in relation to the forming die, without impeding the stamping operation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13 60992 | Nov 2013 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/073617 | 11/4/2014 | WO | 00 |
