WO2016012002A1

WO2016012002A1 - Method for producing multi-component workpieces by way of 3d printing

Info

Publication number: WO2016012002A1
Application number: PCT/DE2015/100289
Authority: WO
Inventors: Fabian SCHÜTT; Jörg BAHR; Jürgen CARSTENSEN; Rainer Adelung; Victor KAIDAS
Original assignee: Christian-Albrechts-Universität Zu Kiel
Priority date: 2014-07-25
Filing date: 2015-07-09
Publication date: 2016-01-28
Also published as: DE102014110505A1

Abstract

The invention relates to a method for producing, by 3D printing, a workpiece that is made up of a plurality of polymer compositions. During each printing interval one polymer composition, which is flowable, at a time is applied to predetermined portions of the unfinished workpiece, and prior to the last printing interval nano- to micro-scale particles are sprinkled during at least one sprinkle interval onto a predetermined portion of at least one of the previously applied polymer compositions.

Description

DESCRIPTION

Method for producing multi-component workpieces by means of 3D

print

The invention relates to an additive manufacturing method for three-dimensional (3D) workpieces, in particular a so-called 3D printing method.

A 3D printer enables layered, three-dimensional buildup of a workpiece from one or more source materials. The starting materials are thereby changed during the construction process at least in their state of matter and usually also chemically. One of the earliest 3D printing techniques is stereolithography. In this case, in a trough which contains a liquid photopolymer precursor, a lowerable platform is arranged just below the liquid surface. By means of a light source, which emits usual ultraviolet (UV) light, the precursor is then polymerized and cured at predetermined locations on the platform. After curing a layer, the platform is lowered and washed over again by precursor, whereupon the next layer is hardened by means of targeted irradiation, and so on. Finally, a hardened body of the desired shape can be lifted out of the trough and possibly fed to a complete curing. One of the known disadvantages of stereolithography is that the chemical polymerization process is often accompanied by shrinkage of the material, which can lead to distortions and torsions in the final product. On the other hand, protrusions in the end product can only be produced with the aid of additional support structures formed from the same polymer, which can be removed mechanically, usually by hand, in a post-processing. The methods of selective laser or electron beam melting or sintering are commonly applied to metallic materials in powder form, in addition to models and prototypes and functional metal parts - eg rare spare parts - manufacture. The functional principle is similar to stereolithography, but the powder grains are not chemically bonded here, but thermally fused at predetermined locations. The method is also applicable to ceramics and glass and in extreme cases can even be carried out with focused sunlight.

An alternative powder layer printing method envisages spraying a liquid binder-here briefly referred to as adhesive-onto the powder layer at predetermined locations in order to bond the powder grains. The adhesive may be a photocuring polymer and is often an organic material. If the workpiece is intended for mechanical stresses, then it is lifted out of the powder after the 3D printing as a green compact and sintered in a furnace to burn out the organics and, if necessary, its pore space is infiltrated. For this purpose, reference is made, for example, to the publications DE 600 08 778 T2 and DE 10 2004 012 682 A1, which deal with powder printing processes, wherein provision is made of a layer of particulate material, that is to say a powder. Since the workpiece in the above-described powder printing process remains in the surrounding powder until the end of printing, no support structures are required for projections.

The aforementioned 3D printing methods are characterized in that the workpieces produced ultimately consist of only one material, the starting material being presented in a completely disordered manner. The shape and / or structure of the workpiece is impressed by selective - ie limited to predetermined locations - energy input. In this case, the selectively sprayed on adhesive can also be understood as an aid to use the per se omnipräsente heating power of the furnace only in the selected areas for sintering. The method according to the invention described below is not a powder layer printing method.

There are still 3D printers that build the workpiece by selectively placing one or more source materials. In this case, a starting material is always applied in a flowable form and then cured within a short time at the place of its placement. It is common to print thermoplastics from heatable extruder nozzles, wherein the nozzles are electronically controllably moved in a predetermined area and are caused to flow out of a reservoir during predetermined pressure time intervals for extruding a polymer composition. Known printable polymers are for example acrylonitrile-butadiene-styrene - short: ABS - with a melting point of 220-250 ° C and polylactides - short: PLA - with a melting point of 150-160 ° C.

When so-called melt layers, engl. Fused Deposition Modeling, FDM, the polymers are applied layer by layer and can cure sufficiently even during the printing of a single layer, either by cooling and solidification, or also by the action of a polymerizing energy supply, e.g. UV lighting that they can form the basis for the next printing plane. Different layers formed from the same polymer usually adhere well to each other and further fuse together by fusing and / or polymerizing as the printing process progresses.

In melt layers, overhangs again require support structures that support the weight of the extruded polymer until it is sufficiently solidified in itself. It is now common to use 3D printers with a plurality of extruder nozzles that can apply different polymer compositions, such as PLA and ABS, to the same workpiece. This is referred to as multi- or polyjet modeling, MJM or PJM for short. An important advantage of the MJM or PJM is the ability to create workpieces in a single printing operation, which are an assembly of multiple materials and hereafter referred to as multi-component workpieces.

This allows, inter alia, to provide the necessary support structures during printing from a material that can be chemically removed after the print product has fully cured, e.g. by immersing the printed product in a solvent that dissolves the material of the support structures and does not attack the material of the workpiece.

In particular, multicomponent workpieces can also be functional devices. Since, in principle, all thermoplastics are suitable for 3D printing, functional polymers such as the piezoelectric polyvinylidene fluoride, in short: PVDF, can be embedded anywhere in a printed workpiece. In addition, almost all thermoplastics can be additized to further functionalization, for example, metallic particles can be mixed in a thermoplastic to the electrical or optical properties of the polymer composition - the term should here and below also any admixtures of particles, so-called "filier" to the polymer It is thus conceivable to print a plastic object from a plurality of different polymer compositions which, for example, is able to show a defined color change on its surface under mechanical load, be it pressure or bending, or the like.

In addition, by means of MJM or PJM, it is advantageously possible to reduce the manufacturing costs of 3D printed workpieces by printing inside areas of the work piece with less expensive materials. For example, a body of ABS may be externally provided with a PVDF layer to provide better chemical resistance. So it does not have to consist entirely of the more expensive PVDF. One of the main problems of the MJM or PJM today, however, is that in practice it is limited to relatively few starting materials that can be durable printed together. In fact, many combinations of polymer compositions show only very moderate or no tendency to adhere to one another or even more to remain together under mechanical stress.

From the article by Xin Jin et al., "Joining the Un-Joinable: Adhesion Between Low Surface Energy Polymers Using Tetrapodal ZnO Linkers", Adv. Mat., 24, 42, pp. 5676-5680, 2012, it is apparent that zinc oxide particles in Tetrapod form can be added to the polymers prior to curing in order - similar to microscale staples - to hold together non-adherent polymer layers of polytetrafluoroethylene, short: PTFE, and a polysiloxane, also known as silicone, It is reported that the peel strength, engl "Peel strength", the compound thus achieved is 200 N / m and thus is comparable to the removal of an adhesive strip of glass.

The object of the invention is now to propose a 3D printing method which is particularly advantageous for the production of multi-component workpieces because, inter alia, it also results in improved adhesion of the various polymer compositions to one another after completion. The object is achieved by a method for 3D printing of a workpiece formed from a plurality of polymer compositions, wherein during each printing time interval one of the polymer compositions is applied fluently to predetermined areas of the unfinished workpiece, characterized in that before the last printing time interval nano- to microscale particles are scattered for at least one scattering time interval to a predetermined area of at least one of the previously applied polymer compositions.

The dependent claims indicate advantageous embodiments. The invention is based on the basic idea of a mechanical anchoring of various, poorly to not at all adherent polymer compositions from the work of Xin Jin et al. (2012) and applies it to the knowledge of the inventor for the first time on 3D-printable thermoplastics. It should be noted that the idea of powdery solid particles in very small amount alone as a primer - and not significantly contributing to the material structure - to sprinkle on previously extruded polymer pastes, apparently for the SD pressure has not yet been considered.

Preferably, the particles to be scattered have diameters from the interval 10 nanometers to 100 micrometers. Larger particles, in particular particle agglomerates, may also be suitable in some cases.

The comments from Xin Jin et al. (2012), it is essential for adhesion improvement that nano- to micro-scale particles be mechanically bonded to the surface of a first polymer composition, and that structures protruding from the surface be present to allow these structures to be enclosed by a second polymer composition. By contrast, particles present off the interface between the polymer compositions do not play an important role, i. it is not expedient to add the said particles to one of the polymers before the actual printing process.

Rather, the particles should, if possible, individually or as agglomerates on the surface of a flowable polymer, there - mechanically anchored, for example by sinking, so that after the solidification of the polymer firmly bonded to this, protruding from the surface structures in average predictable density available. When applying the next polymer layer in the 3D printing process, these structures are enclosed and thereafter act cohesively in the area between the polymer layers. This is particularly advantageous according to the invention, when the printing process provides two different polymer compositions to stack each other. It is within the scope of the invention to reserve the sprinkling of the nano- to micro-scale particles only those inner boundary surfaces of the workpiece, on which a material change is provided. However, the invention is not to be construed as limited thereto. However, the spreading should not take place continuously during the entire 3D printing of a workpiece, but only during predetermined scattering time intervals. These are preferably in a predetermined relationship to the printing time intervals in which each one of the polymer compositions is extruded. In a preferred embodiment of the invention, at least one printing time interval and at least one scattering time interval may overlap, ie the extrusion of the polymer composition and the scattering of the particles take place at times simultaneously. It is intended to sprinkle the particles onto that still flowable paste which has just left the heated extruder die.

This can be achieved by placing a controllable scattering head in the immediate vicinity of the extruder die, i. the spreading head is moved together with the nozzle. A scattering head can be a per se known, miniaturized design of a screw conveyor, the powder transported from a entrained reservoir with controllable speed to a scattering outlet. Numerous other possible embodiments of a suitable scattering head are suggested by the prior art.

It should be remembered that the scattering must be done on the immediately previously extruded polymer, as long as it still has a flowable surface. The scattering time interval thus begins only after the printing time interval, and the scattering head has to be moved over the deposited polymer after a predetermined time. Therefore, the arrangement of the spreading head can temporarily change the permissible direction of movement of the extruder die during printing restrict. Alternatively, however, several scattering heads, for example four, can be arranged around the extruder nozzle so that the nozzle can be moved freely in four directions and the scattering is always possible.

The method according to the invention can also be carried out with 3D printers which are available today and which have a plurality of extruder nozzles but no co-moving scattering head. Then there can be no overlapping of print time and scatter intervals, because the extruder nozzles are moved away from the workpiece after depositing a polymer layer so as not to obscure it, and the scattering of the particles is carried out by a separate device. In the simplest case, this can be a sieve that is manually guided over the workpiece. For the purpose according to the invention, of arranging the particles at inner boundary surfaces of the workpiece, it is preferable to provide a plurality of scattering time intervals and intermediate printing time intervals. In other words, the process steps of polymer extrusion, ie conventional 3D printing, and scattering will alternate in a predetermined manner.

Although the sequence of process steps is initially similar to the procedure in the known powder layer printing, but it must be remembered that here the liquid applied polymer is the workpiece material and the scattered solid particles serve as adhesion promoters, which make no significant contribution to material. In fact, they do not readily adhere to the polymer. Rather, it requires an additional process step, which causes an increase in the flowability in a predetermined range of the previously applied polymer composition, so that the scattered particles form a mechanical anchoring in this area with the flowable polymer, for example, partially sink. After re-solidification of the polymer, the anchored particles provide those structures on the polymer surface which are to be enclosed by the subsequently imprinted polymer layer to improve adhesion. Advantageously, it has been found that this additional process step for 3D printable thermoplastics is very easy to perform with conventional 3D printers. Preferably, one of the heated extruder dies may be provided with a sufficiently high temperature and caused to sweep that portion of the printed polymer surface where short term thermal softening of the polymer is desired. During this "thermal sweep", the scattered particles are already on the polymer and mechanical anchoring occurs as softening proceeds, and it is advantageous if the extruder die used as a heating element does not extrude any polymer during the thermal sweep Thus, even a wire or pin intended solely as a heating element can be moved over the workpiece in a controlled manner.Of course, commercially available 3D printers do not contain such an element today, and such retrofitting would in most cases be uncomplicated.

As an alternative to thermal softening of the polymer, chemical softening by spraying a solvent of the polymer composition to be softened is also possible. It should be noted that the solvent may be applied only in small quantities and should be slightly volatile under the atmospheric conditions of 3D printing or at least in a simple and fast manner should be removable. Otherwise, it would hinder, delay or even fail the progress of printing.

With reference to the figures described below, which are electron micrographs of an embodiment, the invention will be illustrated. Showing:

Fig. 1 is a plan view of a printed from PLA polymer surface, sprinkled with zinc oxide tetrapods and by passing a Heating element was thermally softened in predetermined areas;

FIG. 2 shows a detail enlargement of the surface of FIG. 1 within the previously softened area; FIG. Fig. 3 is a surface section of FIG. 1 after the partial printing of a layer of ABS.

The plan view of a PLA printed polymer surface can be seen in FIG. After depositing in a complete layer, 3D printable thermoplastics are usually already sufficiently solidified to print the next layer resting thereon. After printing such a layer, one sprinkles nano- to microscale particles, here: zinc oxide tetrapods based on Xin Jin et al. (2012), these are still lying, but can be removed by a puff of air or by suction. They do not add anything to the liability improvement. The lines shown in Fig. 1 (white) surround a right angle bent strip of slightly more than 1 millimeter width on the surface. This strip is the predetermined area of the polymer in which an increase in fluidity is desired. In the embodiment, one of the extruder dies with a temperature above 200 ° C is moved close to the polymer after the tetrapods have been scattered. As the polymer softens, the tetrapods partially sink into the surface and remain mechanically anchored when the polymer solidifies again.

FIG. 2 shows a detail enlargement of the resulting polymer surface with anchored tetrapods. FIG.

On the PLA layer with tetrapods, an ABS layer is printed in such a way that a recess remains, which allows the free view of the PLA layer. 3 shows a section of the PLA tetrapod surface with partial coverage by the ABS layer. Particularly in the right-hand area of the image, it can be seen that the flowable ABS encloses the structures protruding from the PLA layer. After the solidification of the ABS, the polymers are thus mechanically anchored along their common interface. As in the work of Xin Jin et al. The tetrapods act as staples here.

With the invention, even polymers could be printed together that normally show no tendency for adhesion to each other and for which there are no common adhesives. The nano- to micro-scale particles not only take over the role of a universal, mechanically acting adhesive by sprinkling according to the invention, but the not adhering to the workpiece - anchored - particles are to be removed with simple means after the end of a scattering interval again from the workpiece. Preferably, such particles can be blown by a blast of air from the workpiece or sucked by vacuum. They are only effective as adhesion promoters, where they can just rest and anchor when softening a polymer.

With regard to the choice of material for the particles is basically limited. If one chooses a thermal softening of the polymers, the particles should only be correspondingly temperature-resistant. Preferably, one can use metallic or ceramic particles, wherein electrical properties of the material rather not important, for example, zinc oxide is a semiconductor. More powerful is the morphology of the particles, because, of course, spherical particles, of course, do not provide any structures that could be well-enclosed by two different polymer layers at the same time. Tetrapod particles, also known as whiskers, appear particularly well suited for use in 3D printing. Tetrapods of zinc oxide are advantageously inexpensive to produce. However, many nanoparticles, including spherical ones, tend to form agglomerates that are designed as open-pore microparticles. Such agglomerates can absorb a flowable polymer by capillary forces in their pore space and then adhere firmly to the polymer after solidification of the polymer. Further details can be found in the not yet published application DE 10 2013 107 833.8. In this respect, the use of open-pored particles, in particular particle agglomerates, a preferred embodiment of the invention, since the cost of such powders are almost marginal.

Claims

PATENT APPLICATIONS

A method of 3D printing one of a plurality of

In each case, one of the polymer compositions is flowable onto predetermined areas of the unfinished workpiece during each pressure time interval, characterized in that nano- to microscale particles are applied to at least one of the previously applied polymer compositions during at least one scattering time interval before at least one scattering time interval be scattered.

2. The method according to claim 1, characterized in that the scattered particles have diameters from the interval 10 nanometers to 100 micrometers.

3. The method according to any one of the preceding claims, characterized

in that at least one scatter time interval and at least one print time interval overlap.

4. The method according to any one of claims 1 or 2, characterized in that a plurality of scattering time intervals and intermediate printing time intervals are provided.

5. The method according to any one of the preceding claims, characterized

characterized in that before the start of a scattering interval, a step of increasing the flowability of a predetermined area of the previously applied polymer composition is carried out.

6. The method according to claim 5, characterized in that a heating element is guided along the applied polymer composition and this softens thermally.

7. The method according to claim 5, characterized in that a film of a solvent of the polymer composition is sprayed on.

8. The method according to any one of the preceding claims, characterized

characterized in that metallic or ceramic particles are scattered.

9. The method according to any one of the preceding claims, characterized

characterized in that open-pore particles, in particular agglomerates, are scattered.

10. The method according to any one of claims 8 or 9, characterized in that the particles consist of metallic zinc or zinc oxide, in particular of tetrapodic zinc oxide.

1 1. A method according to any one of the preceding claims, characterized

in that the particles not adhering to the workpiece are removed from the workpiece after the end of the at least one scattering time interval.

12. The method according to claim 11, characterized in that the particles are blown by a puff of air from the workpiece.

13. The method according to claim 11, characterized in that the particles are absorbed by means of negative pressure.