Field of the invention

The invention relates to a method for manufacturing a thermoplastic element. Also, the invention relates to a resin mixture for use in said method for manufacturing a thermoplastic element. Furthermore, the invention relates to a device for manufacturing a thermoplastic element.

Background Recently, high-performance thermoplastics are emerging as a promising substituent for metals and thermoset materials in many industries with a desirable combination of properties, including high stiffness to weight ratio, damage tolerance, and recyclability.

The adaption of such high grade thermoplastics, for additive manufacturing (AM) processes is an extremely challenging task, considering the high processing temperatures and complications associated with the high melt viscosity.

By the same token, the limited success in the production of durable and satisfactory thermoplastic AM-based end products is correlated with the insufficient bond strength between printed layers, thermal degradation of thermoplastics at the elevated temperatures, and high void content of the parts. The high melt viscosity of engineering thermoplastics prevents sufficient bond formation between printed layers and accounts for major failures in many AM-based parts.

One method to enhance the bonding quality in such thermoplastic AM-based end products is to increase the processing temperature. However, application of high temperatures creates a critical processing trade-off in which higher bonding quality at elevated temperatures can stimulate higher locked-in residual thermal stress, and intensify the thermal degradation which are highly detrimental to mechanical performance and geometric accuracy of the produced parts. In severe cases such as large-scale structures, warpage and thermal cracking are expected phenomena. Alternatively, AM methods with the capability to manufacture parts at room temperature such as solvent-cast 3D printing or the so-called liquid deposition modeling (LDM) are considered to overcome some of the limits imposed by the thermally-driven processes. LDM processes, still in their infancy, are only applicable to a very narrow group of polymers which are soluble in specific solvents. Moreover, solvent-polymer phase separation during a drying process leads to high porosity in the structure which is very detrimental to mechanical performance of the produced parts. The inherent weakness of unreinforced polymers is another parameter hindering the mechanical performance of the thermoplastic AM-based products even when engineering thermoplastics are used.

Therefore, in recent years, polymeric composite materials and their implementation into AM processes have attracted tremendous attention. Various reinforcement types such as carbon nanotube, graphene, and short or long fibres are considered to effectively increase the mechanical performance of components. Among them, continues fibre reinforcement appears to be a most effective filler to improve the mechanical properties and extend the applicability of polymeric AM parts for aerospace and other high-performance structures. However, AM of such hybrid systems is not commonplace due to process complications and unidentified microstructure evolution and their relation to mechanical performance.

Most of the reports in this area are focused on replacement of thermoplastic filaments with a pre-impregnated filament including time-consuming and expensive pre-process steps and yet not achieving the mechanical properties of conventionally produced composites.

It is an object of the present invention to mitigate or overcome the disadvantages from the prior art.

Summary of the invention The object is achieved by a method for manufacturing a thermoplastic element, which method comprises the steps of: a) providing a resin mixture in which said resin mixture is substantially liquid at room temperature, and comprises at least one photo-initiator species; b) curing said resin mixture at substantially room temperature using UV-light. Accordingly, the method establishes a new technology named reactive liquid deposition modeling (RLDM) which substitutes the melt processing of thermoplastic material in conventional AM methods by an in-situ polymerization of thermoplastic resin at room temperature. The method of the invention offers enhanced bonding quality while having high process flexibility to achieve high-quality products. The method overcomes the current AM fabrication process enigmas such as thermal degradation, residual thermal stress, and energy waste by eliminating the temperature/energy/time required for melt processing of polymers.

The method requires a resin fulfilling some requirements as being cured at room temperature, proper viscosity and also resulting in thermoplastic structure after curing.

According to an aspect there is provided a resin mixture in accordance with claim 3. Since such a resin mixture is in the liquid state at room temperature, it allows curing at or around this temperature with a minimal thermal effect to create warpage or delamination of the AM structure produced from the resin mixture. It is observed that the temperature is not significantly changing during polymerisation. No additional activation by heating the resin is needed. According to a further aspect there is provided a resin mixture as described above, wherein the photo-initiator is selected from a group of UV activated peroxide photoinitiators.

According to a further aspect, such an UV activated peroxide photo-initiator is chosen from the group comprising Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) and 2,2-Dimethoxy-2- phenylacetophenone (acetophenone) or a mixture thereof. Such species are suitable for UV curing while being stable at room temperature. According to a further aspect, there is provided a resin mixture as described above, wherein the resin mixture comprises between 1 and 3 wt.% and preferably approximately 2 wt.% of said photo-initiator species. Advantageously, by restricting the amount of photo-initiator species, the effect of the polymerisation reaction on the temperature of the created structure is kept at low values. It is observed that temperature during AM remain below about 40°C for said amount of photo-initiator species.

According to a further aspect, there is provided a resin mixture as described above, wherein the resin mixture comprises a resin that is selected from a group of liquid reactive thermoplastic resins.

According to a further aspect, the group of liquid reactive thermoplastic resins comprises methyl methacrylates, acrylonitrile-butadiene-styrene, polyacrylates, polyamides, polyesters, polycarbonates, and polyurethanes. According to a further aspect, there is provided a resin mixture as described above, wherein the resin mixture comprises between 97 and 99 wt.% and preferably approximately 98 wt.% of said reactive liquid thermoplastic resin.

According to an aspect, there is provided a device for manufacturing a thermoplastic element, wherein the device comprises a, preferably moveable, nozzle for extruding a resin mixture as described above, and a UV-light source arranged at or near an outlet of the nozzle for curing said extruded resin mixture.

The device allows to print a structure from the liquid resin mixture by supplying the liquid resin mixture on a substrate through the nozzle. The curing of the resin mixture is done simultaneously by exposing the resin mixture at the outlet of the nozzle to UV radiation generated by the UV source.

According to a further aspect, there is provided a device as described above wherein the nozzle comprises a throughflow channel for the resin mixture and an resin mixture inlet in fluid communication with the throughflow channel for feeding the resin mixture to the throughflow channel.

According to a further aspect, there is provided a device as described above, wherein the nozzle further comprises a fibre inlet for feeding at least one fibre to the throughflow channel, wherein the fibre inlet is either the resin mixture inlet or an additional inlet, debouching into the throughflow channel. The provision of both a fibre inlet and a resin mixture inlet allows that AM production of structures of resin mixture based thermoplastic composites in accordance with the method as described above is achieved.

According to a further aspect, there is provided a device as described above wherein the fibre inlet connects to the throughflow channel at a first position and the resin mixture inlet connects to the throughflow channel at a second position, wherein the first position is arranged upstream with respect to the second position as seen in a throughflow direction of the at least one fibre and the resin mixture. Advantageously, such arrangement of the fibre inlet and the resin mixture inlet provides that efficient wetting of fibre(s) with the resin mixture can be achieved. Advantageous embodiments are further defined by the dependent claims.

Brief description of drawings

Embodiments of the present invention will be described hereinafter, by way of example only, with reference to the accompanying drawings which are schematic in nature and therefore not necessarily drawn to scale. In the drawings, identical or similar elements are indicated by the same reference sign.

The scope of the invention is only limited by the definitions presented in the appended claims. Figure 1 A-1 D schematically show an additive manufacturing process according to an embodiment of the invention;

Figures 2A, 2B schematically show a cross-section of a respective tool for AM according to an embodiment;

Figures 3A, 3B schematically show a cross-section of nozzle for 3D printing according to an embodiment;

Figure 4 schematically shows a cross-sectional view of a nozzle for 3D printing according to an embodiment, and

Figure 5 shows a perspective view of a tool for AM according to an embodiment. Detailed description

The method according to the invention to create thermoplastic structures by additive manufacturing is based on the use of a resin mixture of a resin and a photo-initiator. The resin mixture is configured to have the property that polymerization is triggered by UV activation of the photoinitiator and takes place at or around room temperature without the need for external input of thermal energy. This allows printing of thermoplastic products from the liquid state by extrusion of the resin mixture in combination with a substantially direct exposure to UV radiation of the extruded resin mixture.

The RLDM produced parts are benefiting from a novel interphase formation mechanism by the liquid phase printing principles and in-situ polymerization of monomers. In this regard, a newly deposited layer containing highly mobile monomers with the capability to act as a solvent for the already solidified layers can partially dissolve solidified layers into the newly deposited part as shown step by step in Figure 1A- 1D. In an embodiment, the resin mixture comprises between 1 and 3 wt.% and preferably approximately 2 wt.% of photo-initiator species, and as balance liquid reactive thermoplastic resin.

As shown in Figure 1A a first step of printing a liquid layer 52 of the resin mixture on an already polymerized sublayer 51 is shown. The contact at the interface 53 between the liquid monomers in the liquid layer 52 and the polymer 51 is observed to facilitate interdiffusion of the liquid phase in layer 52 and the solid state in layer 51 (Figure 1 B). Monomers are schematically indicated by short strings 56, polymer strings schematically by longer strings 57. Subsequently, in-situ polymerization and interphase formation takes place while the liquid phase is exposed to UV radiation R (Figure 1C). During polymerization a strong bonding between the already present polymerized sublayer 51 and the freshly created polymerized sublayer 55 is obtained (Figure 1 D).

Furthermore, by using a relatively small amount of photoinitiator in comparison with the amount of resin, the production of polymerization heat is kept at a minimum which reduces thermal effects such as thermal stress between different printed sublayers. In an embodiment, the resin mixture comprises between 1 and 3 wt.% and preferably approximately 2 wt.% of photo-initiator species, and as balance liquid reactive thermoplastic resin. Accordingly, warpage and/or delamination between sublayers 51 , 55 are prevented.

In a further embodiment, the photo-initiator species is chosen from a group UV activated peroxide based photo-initiators.

In a further embodiment, the photo-initiator is selected from a group comprising Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) and 2,2-Dimethoxy-2- phenylacetophenone (acetophenone) or a mixture thereof.

In an embodiment, the liquid reactive thermoplastic resin is a methyl methacrylate- based liquid resin. Alternatively, the liquid reactive thermoplastic resin may be selected from a group comprising acrylonitrile-butadiene-styrene-based resins, polyacrylate-based resins, polyamide-based resins, polyester-based resins, polycarbonate-based resins, and polyurethane-based resins.

The resin mixture is obtained by providing the photo-initiator species and the thermoplastic liquid resin, which components are then mixed. Optionally, the resin mixture is degassed which may improve the formation of a bubble-free thermoplastic polymer. Figures 2A, 2B schematically show a cross-section of a respective tool for AM according to an embodiment.

In Figure 2Aa tool for additive manufacturing of thermoplastic structures is depicted. The tool 100 comprises a nozzle 110, a substrate 120 and a UV radiation source 130. The nozzle is provided with inlet 112 for coupling to a container (not shown) of the resin mixture which is to supply the resin mixture 52 into the nozzle. According to an embodiment, the nozzle comprises a throughflow channel 114 for the resin mixture and a resin mixture inlet in fluid communication with the throughflow channel for feeding the resin mixture to the throughflow channel. Within the tool, the throughflow channel is arranged to output 116 the resin mixture onto the substrate.

To allow printing patterns, the nozzle 110 is moveable relative to the substrate 120 in a manner that the resin mixture is deposited on the substrate 120. Preferably, the nozzle and substrate are moveable relative to each other in three orthogonal directions indicated by arrows X, Y, Z.

The substrate 120 can be an auxiliary surface to support the deposited resin mixture. Alternatively, the substrate 120 may be a previously deposited polymerized layer 51 on which the liquid resin mixture is deposited.

The UV radiation source 130 is coupled to the nozzle 110 to enable that the liquid resin mixture is exposed to UV radiation after the deposition on the substrate 120.

The UV radiation source 130 is mounted adjacent to the nozzle 110 at some distance from the substrate and arranged with the radiation source facing towards the substrate 120.

In a further embodiment, the radiation source is positioned to illuminate the substrate closely to the outlet 116 of the nozzle to initiate polymerization directly after release from the nozzle and cure the resin mixture into a thermoplastic polymer 55. Between the outlet 116 and the cured thermoplastic the resin mixture is polymerizing in a resin gelation portion 58 of the extruded resin mixture.

Figure 2B shows a tool 150 for additive manufacturing in a further embodiment. Similar as the tool as described above with reference to Figure 2A, the tool 150 comprises a nozzle 151 , a substrate 160 and a UV radiation source 170.

The nozzle comprises an inlet 152 for coupling to a resin mixture container and a second inlet 154 for input of short or long fibres 60. The nozzle 151 is adapted to wet or impregnate the fibre(s) 60 with the resin mixture 52 before the combination of liquid resin mixture and wetted/impregnated fibre(s) is deposited through the outlet 156 on the substrate 160.

Advantageously, the tool 150 according to the embodiment provides to create fibre enforced composite thermoplastic structures by means of AM / 3D printing. The UV radiation source 170 is positioned adjacent the outlet 156 of the nozzle in a similar manner as described above with reference to Figure 2A and is arranged to cure the liquid resin mixture 52 and wetted/impregnated fibre(s) 60.

According to an embodiment, the nozzle thus further comprises a fibre inlet for feeding at least one fibre to the throughflow channel, wherein the fibre inlet an additional inlet, debouching into the throughflow channel.

Figures 3A, 3B schematically show a cross-section of a nozzle 200; 220 for 3D printing according to an embodiment.

In Figures 3A, 3B exemplary arrangements are shown of the fibre inlet 204; 224 and the resin mixture inlet 202; 222 on the nozzle.

The nozzle is configured in a manner that the fibre inlet 204; 224 connects to the throughflow channel 206; 226 at a first position P1 and the resin mixture inlet 202; 222 connects to the throughflow channel 206; 226 at a second position P2, wherein the first position P1 is arranged upstream with respect to the second position P2 as seen in a throughflow direction indicated by arrow V of the at least one fibre and the resin mixture.

Accordingly, the resin mixture is added to the fibre(s) that already entered the throughflow channel 206; 226 which enhances the wetting/impregnation of the fibre(s). The resin mixture inlet can be positioned to enter the nozzle substantially perpendicular to the transport direction of the fibres through the throughflow channel, i.e., perpendicular to the longitudinal axis of the throughflow channel as shown in Figure 3A. Alternatively, the resin mixture inlet can be slanted relative to the transport direction, as shown in Figure 3B. Figure 4 schematically shows a nozzle 400 for 3D printing according to an embodiment in a cross-sectional view.

In an embodiment, the nozzle 400 is provided with a first inlet 402 for fibre(s), a second inlet 404 for resin mixture and a throughflow channel 406 which is provided with an outlet 408 for extruding the resin mixture mixed with the wetted/impregnated fibre(s). The second inlet enters the throughflow channel at entry 405 relatively remote from the outlet 408, such that the throughflow channel extends over a length 407 from the level of the entry 405 to the outlet 408, to allow sufficient wetting/impregnating of the fibre(s) by the resin mixture. The throughflow channel 406 is embodied as a tapered chamber in which a cross- section of the outlet 408 is smaller than a cross-section of the fibre inlet 402.

The channel forming the resin mixture inlet 404 has a constant cross-section along its length, but could be tapered as well towards the entry 405 thereof into the throughflow channel 406.

Figure 5 shows a perspective view of a tool 500 for AM according to an embodiment. The tool 500 comprises a nozzle 502, one or more brackets 504 and a fibre container 506 or fibre tube 506. The nozzle 502 has been described with reference to Figure 4. An outlet indicated by arrow 508 of the fibre container 506 is coupled to the fibre inlet of the nozzle. The resin mixture inlet 507 is positioned downstream of the fibre inlet and connects to the throughflow channel of the nozzle. The one or more brackets 504 are also connected with a proximal end 504A thereof to the nozzle at or close to the level where the fibre inlet is positioned. On the distal end 504b of the one or more brackets a mount 510 is provided for the UV radiation source. The mount is configured to allow in use direct illumination of the UV radiation source towards the outlet 514 of the nozzle. In an embodiment the bracket provides that a mount area 512 for the UV radiation source is placed under an angle a to direct the output of the radiation source towards the area adjacent the outlet.

In an embodiment the UV radiation source comprises at least one UV emitting LED. The at least one LED may be DC driven by a corresponding power source.

As shown in Figure 5, the tool 500 may be provided with a plurality of brackets 504 to provide distributed illumination in an area surrounding the outlet 514 of the nozzle.

For example, three brackets 504 positioned at intermediate angles of about 90°, may be provided to provide illumination of the outlet from three sides during use. In an embodiment, the at least one LED is configured to emit radiation with at least wavelength in a range in the UV part of the spectrum which extends between about 350 and about 410 nm.

The at least one LED may have an output power of at least 250 mW upto about 2W Alternatively, the UV radiation source may comprise a laser capable of emitting UV radiation.

The tool 500 is configured to be positioned over a substrate (not shown) and moveable relative to the substrate.

The invention has been described with reference to some embodiments. Obvious modifications and alterations will occur to those of skill in the art upon reading and understanding the preceding detailed description. In addition, modifications may be made to adapt a particular material to the teachings of the invention without departing from the essential scope thereof. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims.