Hot molding method

Abstract

A hot molding process is described for a powder material comprising expanded particles and unexpanded particles wherein the powder material is coated with an epoxy resin while it remains stirred to thicken the material.

Description

[0001] The present invention relates to a hot molding method. In particular, the method facilitates molding a solid composite object having an inner core and an outer shell made of carbon.

[0002] As is known from EP 3 372 366, the material of EP 2 697 028 is not without disadvantages and has the problem of high volatility. EP 3 372 366 solves this problem by spraying the material with water, oil, phthalate, glycol or high boiling hydrocarbon. However, experience has shown that objects made by moulding the material treated in this way are prone to cracking or splitting, which is undesirable, especially when the moulded part is the inner core of a valuable carbon coated object.

[0003] The main object of the present invention is to propose a different method of molding with the above-mentioned material.

[0004] Another object of the present invention is to propose a molding method using the above-mentioned material which eliminates or at least alleviates the problems encountered with the method of EP 3 15 372 366.

[0005] A method is then proposed for hot molding of a powder material comprising or consisting of expanded particles and non-expanded particles, said particles being made of a plastic material, being closed in shape, being hollow and filled with a gas, said method comprising a step of coating the particles with an epoxy resin by depositing a micron layer of the resin on the particles before molding.

[0006] This coating step has various embodiments, for example, it is carried out as follows

[0007] - spraying epoxy resin on the material, and / or

[0008] - atomizing the epoxy resin onto the material (for example, known foggers or atomizers can be used as atomizers), and / or

[0009] - kneading the material with the epoxy resin by mechanical action, more preferably by placing the powder material and the epoxy resin in a container and deforming the container to mix and knead its contents until a uniform wet paste is obtained. Preferably, the container is a deformable container, such as a silicone bag, and kneading is performed by squeezing the container.

[0010] The coating step is preferably performed before and / or during the conveying of the material in the mold, more preferably while the material is kept stirred; or it can also be performed while the material is applied to or in the mold.

[0011] The spraying of the epoxy resin has the effect of agglomerating particles of the powder material thereby preventing them from floating in the environment. The wet material forms a wet, sticky paste which is easily spread within the mould, particularly on top of the fibre reinforcement layer above which it is desired to transfer the mould cavity shape.

[0012] Preferably, in the deposition step, the epoxy resin is added to the powder material in an amount of 1 to 300% by weight of the powder material, in particular 3 to 250%, more preferably 5 to 200%. These values ​​experimentally give the best results.

[0013] Gas-filled plastic microspheres can be used as particles.

[0014] Specifically, the powder material to be molded is preferably composed of 10-70% by weight of expanded microspheres and 90-30% by weight of non-expanded microspheres, which are made of plastic material, are closed in shape, hollow and filled with (combustible) gas. These values ​​ensure the excellent results of favorable performance and weight suitable for application, especially impact absorption and lightness. Expanded microspheres are indispensable for the present invention and act as adhesives or fillers for other non-expanded particles. In fact, expanded microspheres are filling ingredients (fillers) and act as adhesives by preventing other heavier expandable (not yet expanded) microspheres from settling to the bottom of the mold by gravity and thickening. Instead, expanded microspheres keep the microspheres in expansion suspended and evenly distributed in the material in this way. This is why the presence of expanded and unexpanded microspheres ensures the uniformity of density of the whole piece, thereby ensuring the consistency of mechanical properties.

[0015] The microspheres are typically spherical and very small (10-40 μm in diameter). Note, however, that this size is not required.

[0016] We use the term "core" here to define an object obtained by molding only powder material.

[0017] The advantage of the material is that it provides shape memory for the core. When the core undergoes deformation, the expanded and unexpanded microspheres will compress or expand within their mass. Due to the elasticity of the microspheres, when the stress ceases, each microsphere returns to its original state, so the material returns to its original shape.

[0018] It is also noted that the core can react to the second deformation in the same way as to the first deformation, with obvious advantages of safety and repeatability of the response to bumps.

[0019] The core is preferably used as an expanding core or nucleus in the molding of the fiber reinforcement. The core expands in the mold and pushes the fiber reinforcement toward the mold wall to fill virtually all cavities due to particle expansion, compacting the material layers and enabling the fiber reinforcement to replicate the mold excellently.

[0020] For example, in this method

[0021] Baking the epoxy-coated powder material in a mold to form a solid core,

[0022] placing the solid core in the cavity of a mould and applying a layer of fibre reinforcement material between the mould cavity surface and the core,

[0023] - Baking the assembly of core and fiber reinforcement in a mold until the fiber reinforcement is cured to form a composite object having a rigid outer shell made of cured reinforcement and said solid core as an inner core.

[0024] For example, in this method

[0025] Applying a layer of fiber reinforcement material to the mold cavity,

[0026] · Place the densified material on the fiber reinforcement,

[0027] Close the mold cavity and heat the two materials inside the cavity.

[0028] In a variant, the fiber reinforcement layer is applied directly to the core, or also to the mold and the core together.

[0029] Or in this method

[0030] Filling the mold cavity with the powder material added with epoxy resin,

[0031] closing the mold cavity and heating the material within the cavity to bake and cure the material to form a solid piece,

[0032] Applying a layer of fiber reinforcement material to the cavity surface of the mold,

[0033] Placing the solid part into a mold cavity,

[0034] Close the mold cavity,

[0035] - Heating the solid piece and the layers within the cavity to solidify the layers and obtain a composite piece.

[0036] The advantage of spraying the powder material with epoxy resin is that it provides greater rigidity to the core without giving it criticality to fracture. This is true not only in the process of molding the core alone or in the process of molding the core with the fiber reinforcement, but also during the life of the final composite. In fact, although epoxy resin itself is a glassy material that is easy to break, in a small amount, when it forms a film on the microsphere, it can surprisingly bend without breaking. Spraying epoxy resin on microspheres takes advantage of the behavior of this solid resin at the micrometrical level, which is no longer an ordinary thin layer that will break, but an epidermis that can be elastically bent. Therefore, the film gives the core flexibility and resistance.

[0037] The resin layer deposited on each microsphere has an (average) thickness of from a few micrometers (= millionths of a meter) to a few hundredths of a millimeter, for example from 1 μm to 300 hundredths of mm. Statistically, this thickness may also vary from particle to particle, taking into account the stochastic nature of the deposition technique.

[0038] Spraying epoxy resin on the powder material activates and enhances the change in behavior of the hardened resin under stress (from brittle to elastic) because the resin finds a surface area on which to spread with micron thickness. As a result, the molded core becomes elastic and has shape memory.

[0039] Another surprising effect is that the epoxy resin film has the ability to return to its original shape after the stress is removed when it is subjected to stress.

[0040] Epoxy also solves the problem of gases released from the core into the composite object when subjected to high temperatures after molding (imagine an object operating in the Sahara Desert), which causes deformation and structural stress. Epoxy strives to encapsulate gases that escape locally from the material where they are, without the gas being able to aggregate with other nearby gas dispersions to form bubbles. Epoxy is not gas permeable and forms a crystalline skin on the microspheres on which it is deposited that entraps gas.

[0041] Epoxy resins also solve the problem of undesirable re-expansion of the core when returning to high temperatures, such as 90-100°C (imagine an object operating in the Sahara Desert). The core tends to deform as much as 0.5-5% of the linear dimension, because the unexpanded microspheres always tend to expand. As mentioned above, epoxy resins are elastic, but their elasticity is limited and actually forms an internal constraint to uncontrolled or excessive expansion, thereby ensuring the dimensional quality of the object.

[0042] In theory, epoxy is a thermoset: after it reacts, it no longer changes shape. It would seem pointless to use epoxy for an inner core when molding a core or carbon object where it is desired that the core still expand during the molding process to replicate the mold cavity itself or press (i.e. expand) against the carbon so that the carbon replicates the mold cavity. However, surprisingly, during the molding of the expanding microspheres under pressure, the epoxy yields (also benefiting from its own softening caused by the heat inside the mold), deforms, allows the core to expand, and then retains the acquired shape as the core cools.

[0043] Finally, another advantage of epoxy resin is that a smaller amount of microspheres can be used when the microspheres are wetted with it. A lower material density for the same volume in the finished core is sufficient (up to 1 / 3 times the material weight for the same volume). The particles wetted by the resin are arranged relatively randomly, even separated and imperfectly, when first contacted, rather than flowing and fitting together perfectly like sand. The larger spaces between them give the wet powder material a lower density.

[0044] Another aspect of the present invention relates to the powder material having particles coated with epoxy resin, which is considered as a precursor and intermediate, stand-alone product for hot molding.

[0045] Another aspect of the invention relates to a solid expandable core that can be inserted into a mold to mold a composite object, for example comprising carbon, which comprises a powder material having epoxy resin-coated particles according to any of the variations defined above, or consists entirely of said powder material, wherein said powder material has been hot molded in a mold to produce said core.

[0046] Another aspect of the invention relates to a composite object produced by hot moulding in a mould, which is formed of the following parts

[0047] The internal expansion core and

[0048] A shell covering the above core and made of a fiber reinforcement that solidifies after baking in a mold.

[0049] Advantageous examples of fiber-reinforced layers used in the method and composite object are layers of carbon or glass or aramid fibers.

Claims

1. A method for hot molding a powder material comprising or consisting of expanded particles and unexpanded particles, said particles being made of a plastic material, being closed in shape, being hollow and being filled with a gas, Therein a micron layer of epoxy resin is deposited on the particles before molding.

2. The method of claim 1, wherein the epoxy resin is sprayed onto the powder material.

3. A method according to any one of the preceding claims, wherein the powder material is kneaded with an epoxy resin in a container.

4. The method according to any one of the preceding claims, wherein an epoxy resin is added to the powder material in an amount of 1% to 300% by weight of the powder material.

5. A method according to any one of the preceding claims, wherein The epoxy-coated powder material is baked in a mold to form a solid core, placing the solid core in a mold cavity and applying a fiber reinforcement layer between the mold cavity surface and the core, The assembly of the core and the fiber reinforcement is baked in a mold until the fiber reinforcement is cured, thereby forming a composite object having a rigid outer shell made of the cured reinforcement and the solid core as an inner core.

6. A powder material for hot molding, comprising or consisting of expanded particles and unexpanded particles, said particles being made of plastic material, being closed in shape, being hollow and being filled with a gas, It comprises a micronized layer of epoxy resin deposited on the particles.

7. A method or material according to any preceding claim, wherein the powder material consists of 10-70 wt% expanded particles and 90-30 wt% unexpanded particles.

8. A method or material according to any preceding claim, wherein the layer has a thickness of 1 μm to 300 hundredths of a millimeter.

9. An expanded core insertable into a mould for moulding a composite object, for example comprising carbon, comprising or consisting entirely of a material according to any of the preceding claims.

10. A composite body produced by hot moulding in a mould, which is formed by the following parts: an internal expanded core according to claim 9, and A shell covers the core and is made of carbon fiber layers that are cured after being baked in a mold.