WO2004037902A1

WO2004037902A1 - Method of manufacturing 3d articles and articles made by such methods

Info

Publication number: WO2004037902A1
Application number: PCT/GB2003/004564
Authority: WO
Inventors: Jacek Paul Obuchowicz; Ranjana Chhaganbhai Patel; Richard John Peace
Original assignee: Huntsman Advanced Materials (Switzerland) Gmbh
Priority date: 2002-10-23
Filing date: 2003-10-22
Publication date: 2004-05-06
Also published as: GB0224716D0; AU2003274360A1

Abstract

A method for producing a 3D article comprising the steps of: forming a precursor in sequential cross-sectional layers in accordance with a model of the precursor by defining a layer of a powder material, consolidating a portion of the powder in a pattern corresponding to the respective cross-sectional layer of the model and repeating these steps to form successive layers. Within the precursor during its formation, an internal zone bounded by the consolidated material is defined. A curable material is established within the internal zone, which includes the unconsolidated powder, and the curable material is cured to form the 3D article in the form of a final composite solid body.

Description

Method of Manufacturing 3D Articles and Articles made by Such Methods

Technical Field

The present invention relates to a method of manufacturing 3D articles and articles made by such a method. The formed articles may be composites of rapidly manufactured, internally structured porous parts, which may take the form of shells or skeletons, having an outside skin covering the external surfaces, and includes secondary cured materials.

These parts may be made from various programmed machines that produce with suitable materials, three dimensional objects.

Background Art

In the past, a number of techniques and materials have been proposed to produce three dimensional articles from programmed data. These include stereolithography, selective laser sintering, fusion deposition and ink jet methods, as described in US-A- 4,575,330, US-A-5,059,266 and US-A-5, 140,937.

There is increasing need to produce such products as rapidly as possible and with ever increasing performance attributes. Speed of producing the prototype is necessary to cut the overall time to production. This is especially needed for short volume product runs, with a fast turn-round from initial design to actual production.

Additionally there is a need to produce from rapid prototyping techniques, 3D objects having actual functionality for technical testing and use. Laser address systems, such as stereolithography and laser sintering, although of high accuracy, are very- demanding with respect to the time taken to build the 3D article. They are also limiting with regard to the range of materials that can be used and therefore cannot easily produce high performance final cured products. High mechanical strength, impact toughness, and thermal tolerance under load are some of the desired attributes that are being sought.

Certain systems, e.g. ink jet/powder based systems, can yield prototypes in a short time of build, however the parts produced are fragile. Surface infiltration with curable resins has been advocated in the past to produce some final functionality, but surface infiltration techniques are difficult to implement and even then smooth infiltration via the surface of the formed part into the interior part is poor. This lowers the final performance achievable.

US-5,824,260 and US-6,364,986 disclose a method of making an object by first forming a shell using stereolithography; the shell contains internal voids from which uncured stereolithography liquid resin is removed. The voids are then filled with strengthening materials, e.g. a mixed epoxy resin and hardener, to form a high strength part. The removal of the uncured stereolithography resin from the shell and re-filling it with other strengthening materials is time consuming.

WO02/064354 discloses a method of making a composite article by buildmg it up in successive layers, each layer being produced by forming a layer of powder and depositing a liquid, e.g. using an inkjet printer, on areas of the powder layer that are to form part of the article. The powder is removed in those areas where no liquid is deposited and the areas of the powder layer to which liquid has been applied are cured. The powder and liquid may together form a curable resin.

Disclosure of Invention It is an object of the present invention to provide a straightforward method for producing a 3D article using layerwise buildmg techniques, which results in an article of increased strength.

It is a further object to provide a method capable of producing rapid prototypes and usable products, at least in small production runs, starting with layerwise building techniques.

According to the invention, there is provided a method for producing a 3D article comprising: (a) forming a precursor in sequential cross-sectional layers in accordance with a model of the precursor, each layer being formed by:

(i) defining a layer of a powder material and (ii) applying a liquid, preferably a reactive liquid, to the powder material to consolidate a portion of the powder material in a pattern corresponding to the respective cross-sectional layer of the model and repeating these steps (i) and (ii) to form successive layers;

(b) establishing within the precursor during its layerwise formation an internal zone bounded by the consolidated material, the zone including at least a portion of unconsolidated powder material;

(c) establishing within the internal zone a quantity of curable material; and

(d) curing the curable material within the internal zone to form the 3D article in the form of a final composite solid body.

The curable material mentioned in step (c) may comprise the unconsolidated powder referred to in step (b), in which case the powder is reactive. Alternatively, the unconsolidated powder may be inert and simply form a filler for curable material introduced into the internal zone.

Preferably substantially all of the powder in the internal zones that remains unconsolidated after the precursor is formed is retained within the internal zone, which avoids the time-consuming step of removing it.

The consolidation of -the precursor preferably occurs by the powder and the liquid forming a curable composition that at least partly cures shortly after the liquid is applied.

The consolidated portion, i.e. the precursor, will include the outside surface layer and may also include internal features to provide support within the internal zone.

Preferably each formed layer has a ratio of consolidated to not consolidated powder material ranging from 1:10 to 1:1. At the 1:10 ratio, there maybe no or little internal structure, resulting in retention of original untreated build material inside the internal zone, with only the edge surface being consolidated. At the 1:1 ratio, there will be a significant proportion of the internal zone consolidated. This consolidation could be highly structured, and could take a geometric pattern e.g. honeycombed or ladder like, or could be more random such as sponge textured, criss-crossed or wholly random and variable. At its simplest, the precursor could be formed as an outside skin of the article.

The precursor may include some internal structure, e.g. internal walls. The internal structure could be smooth and even throughout the internal body, or maybe variable. In some areas, such as around curves and corners, a finely dimensioned structure may be formed for additional definition and strength, while in straight areas, larger dimensioned geometric designs can be tolerated. The internal structure may have dimensions measured in mm to microns. If, within the final article, the cured material within the zone(s) is stronger than the outside shell originally formed as the precursor during step (a), it may be desirable to make the precursor shell walls relatively thin.

The powder layer may readily be formed, e.g. using a doctor blade. Any method that accurately deposits the liquid onto the powder in step (a)ii) can be used but deposition using an ink jet printing head as described in WO02/064354 is preferred. Suitable equipment for forming powder layers and applying liquids thereto by ink jet printing equipment is available commercially, e.g. from Z-Corporation of 20 North Ave. Burlington, MA 01803, U.S.A. The precursor may be formed in several ways.

Firstly, the powder can comprise a reactive component that reacts with the liquid to form a composite that solidifies or cures up in the shape of the precursor. This includes the case where the powder recrystalises on application of a liquid to form a solid precursor structure, e.g. calcium sulphate, which recrystalises on application of an aqueous solution.

Secondly, the powder may be inert, e.g. aluminium oxide, and the liquid is intrinsically reactive and solidifies or cures when jetted. In this connection, the liquid may be a binder that binds the powder; in the case of a binder, it is usually necessary later on to infiltrate the bound powder with a curable material in order to provide the precursor with substantial strength. This is important since the prcursor will generally form the outside of the final 3D article. Thirdly, the powder material may itself be intrinsically curable without any additional additives, e.g. it may be a mixture of materials that will react together when heated, in which case the consolidated material forming the precursor can be cured directly by heating. A suitable intrinsically curable powder material is for example described in O00/157110 Al, which discloses a composition comprising (A) an epoxide containing at least two epoxy groups per molecule, (B) a polyol containing at least two hydroxy groups per molecule or a primary or secondary amine containing at least two amino-hydrogen atoms per molecule, and (C) a solid compound containing at least two isocyanate groups; such a powder exhibits high storage stability. In this case, the liquid may be curable or a binder.

As discussed below, if the precursor includes gaps between the powder grains in the consolidated material, the precursor can be infiltrated with a suitable curable material that can be cured up in the final composite solid body to increase the strength of the precursor; in this case, the infiltrating material could be such as to react with the liquid and/or the powder of the consolidated precursor to form a curable mixture or the infiltrating liquid could in itself be a curable composition It has also been found that the fabrication techniques of the present invention help to reduce the time of build, while ultimately producing final articles of increased strength.

WO02/064354 discloses 3D articles built using powders and ink jetted fluids to bind the powder according to programmed data, and reference should be made to that specification for further details of forming the powder layer and apply the liquid in a desired pattern in step (a) and also details of suitable reactive powder/liquid combinations.

The powder within the internal zone may be intrinsically curable, e.g. it will be cured on heating or when subject to radiation, e.g. US6437045, 6433084, 6169158, 6165558. Alternatively, in the same way as the precursor may be infiltrated with a curable material through the surface, so the material could also infiltrate into the internal zone and the powder within it. When infiltration is used, the infiltrating liquid is preferably either intrinsically curable or is curable with the powder in the internal zone. Ultimately, the resultant cured composite has surprisingly greater strength than a similar parts where no internal zones are formed.

As will be apparent, when infiltrating liquid into the precursor or into the internal zone, it is necessary for the outer wall of the precursor to be porous to the infiltrating liquid. Such porosity will generally be provided by gaps between particles in the precursor, which can be brought about by applying an appropriate amount of liquid in step a(ii) that gaps remain between the powder particles.

The powder material used and retained within the internal zone may be a finely divided expansible reactive material which, on curing, e.g. by heating or on exposure to microwaves, expands to fill the internal zone. The expansion of the powder will be beneficial within the internal zone since it will fill the whole internal zone. On the other hand, it is usually undesirable that the walls of the precursor should expand through the use of foaming powders because the dimensions of the precursor will generally be set by the consolidation of the precursor in step (a) above and the degree of expansion by foaming is hard to control. However, the same powder is used to form the precursor walls as is used in the internal zone, and so a powder that expands on curing can only be used when either expansion of the precursor can be tolerated or when expansion of the precursor is somehow avoided when curing the powder in the internal zone. Avoidance of the expansion of the powder in the precursor can be achieved by solidifying the precursor to such an extent before curing step (d) that any expanding/foaming agent in the powder will not operate to expand the precursor. Such a result can also be achieved, for example, by using a reactive powder that contains a foaming agent that only operates at elevated temperature; in this case, the powder forming the precursor could be cured at room temperature on application of the liquid in step (a) to form a rigid structure so that, when the composite is heated to cure the powder within the internal zones, the foaming agent will foam the powder in the internal zone but will not expand the rigid precursor. Foamable compositions that melt and then foam up giving a foam having a high compression strength are described in WOOO/24559.

When material is^' infiltrated through the precursor, it may be introduced through the precursor into the unconsolidated region by vacuum and/or under pressure. The curing may be achieved by heat or microwave radiation, or any other suitable curing mechanism.

Suitable liquid curable resins that may be infiltrated range from addition polymerisation types to photocuring types. This flexibility of using a secondary resin for infiltration enables a variety of properties to be attained. These include high heat deflection temperatures, high water resistance, high tensile strengths and modulii, better flame resistance (necessary in aero, auto and rail applications), and impact toughness. The infiltrating liquid and/or the powder and/or the liquid applied in step a) may be mixtures which contain latent toughening agents, e.g. butadienes or siloxanes which phase separate out on curing, or toughening elastomeric particles (nano or micro sized particles), particularly surface reactive particles for example as disclosed in

International Application PCT/EP03/04231.

Pigments and fillers including inorganic and organic materials such as metal powder, e.g. aluminium particles or whiskers, silica and polymeric core shell particles may also be present as matrix strengthening ingredients; solid particles will generally be incorporated into the powder.

It is especially preferred to incorporate nano scale particles into the powder, the applied liquid or (when used) the infiltrating liquid; nano scale particles are particles having a size of not greater than 50nm along their longest dimensions; nano scale particles give the 3D article increased strength and water resistance. They may be:

• nano-scale metal particles, e.g. copper, aluminium, nickel, zinc, silver etc, which are available commercially from e.g. Argonide Corporation.

• nano-scale platelet minerals, e.g. nanoclays (also known as "planomers"), which are commercially available under the trade name Claytone^® or Cloisette,

• flat nano-scale carbon sheets, which are commercially available under the trade name Quasam; see also US-6080470,

• rounded nano-scale particle, e.g. made of silica or barium sulphate, which are also commercially available; nano-silica particles are available under the trade name Highlink OG ^® from Clariant International Ltd of Muttenz , Switzerland;

• nanocarbon materials, e.g. buckminsterfullerenes,

• nano-tubes, e.g. carbon nano tubes, which are commercially available from Nanoledge S A, or Nanocarbon Inc,

• nano-scale salt and ceramic particles: e.g. nano titanium dioxide from Chengyin Technology (China) Ltd, nano barium sulfate (from Solvay), or nano-glass;

• nano-polymers, e.g. nano elastomeric particles available from from Hanse Chemie, or as described in EP0938026A1;

• nano-organic metals, e.g. nano-polyanilines, which are conducting polymeric particles from Ormecron GmbH;

• nano-light emitting organic pigments and polymeric/oligomeric materials, or

• fibres, e.g. alumina fibres, such as nano alumina fibres from Argonide Corporation.

The nano-scale particles may be porous on an Angstrom scale.

A single form of nano-particles may be used or a mixture of compatible nano particles could be used either of the same or of a different chemical type.

The fillers, especially the nano-scale fillers, maybe suitably surface treated, e.g. with organosilanes and surfactants, to make them compatible with the material that they are dispersed in and may be ionised, e.g. by plasma treatment, so that they do not clump together and are more readily dispersed..

Especially preferred are those compositions, which can cure at low temperatures, e.g. less than 100°C, preferably less than 40°C.

Preferably, the curing of the powder in the zone in step d) and the curing of the consolidated powder are such that they do not result in substantial volume shrinkage of the precursor, preferably a shrinkage of less than 1%. Good wetting is important with respect to the internal surfaces of the article.

Powder/ Applied Liquid System

The powder and the liquid applied in step (a) may be chosen freely so long as they forms a stable precursor that can be subject to the curing step (d). Preferred powder/liquid curing resin compositions for forming the consolidated precursor of the curable composition within the internal zones include epoxy/hardener systems, isocyanate/polyol systems, anhydride/epoxy systems maleimide/epoxy systems and acrylic/epoxy/amine mixes. Some preferred powders are epoxy based powders, e.g. as disclosed in US6437045, 6433084, 6169158, 6165558. As discussed above, the powder may be a foamable reactive powder which on heating will foam up to fill the cavity of the fornied article and adhesively bond any internal supports, so that a fully strengthened part is obtained. Especially preferred are the powders which foam, adhere and deliver good compressive strengths.

In one embodiment, the liquid applied in step (a) is aqueous based and the powder includes a crosslinkable agent that crosslinks when the aqueous liquid is applied. A powder/liquid system that may be used in the present invention is as follows, by way of example:

Powder

The "crosslinkable agent" in the powder is a monomer, oligomer, polymer, or polymer mixture that has functional groups capable of forming covalent bonds (crosslinks), either with itself or with the functional groups of other crosslinkable agents.

Preferably, the crosslinkable agent is selected from the group consisting of amino resins, phenol resins, and mixed amino/phenol resins. Amino resins, phenol resins, and mixed amino/phenol resins are derived from the reaction of formaldehyde and an amine, a phenol, or a mixture of an amine and a phenol, respectively. Most preferably, crosslinkable agents are selected from the group consisting of melamine-formaldehyde resins, urea-formaldehyde resins, melamine-urea-formaldehyde resins, melamine- phenol-formaldehyde resins, benzoguanamine-formaldehyde resins, glycoluril- formaldehyde resins, and acetoguanamine-formaldehyde resins. Additionally, the crosslinkable agent may be a glyoxal resin or a methylol carbamate. The powder may comprise from about 1% to about 60% by weight of crosslinkable agents in powder form. The size of the particles should be less than the thickness of the layers to be printed. The shape of the particles may be regular or irregular.

When a standard aqueous binder is applied to a powder system comprising a crosslinkable agent, the crosslinkable agent dissolves and may begin to polymerize and crosslink with itself and other water soluble functional resins, thereby contributing structure and strength to the printed article. The crosslinking occurs within the layer being formed, as well as with previously formed layers. In order to achieve these results, the crosslinkable agent should be soluble in water at room temperature. Preferably, the agent will a) be crosslinkable under ambient conditions; b) have the ability to catalyze in moderately acidic solutions; and c) have a relatively rapid cure rate.

Thin films of the crosslinkable agent may be coated onto a filler material such as glass spheres, flakes or fibers, and used in this form in the powder systems described herein. In this embodiment, lower volumes of binder fluid are required to solubilize the crosslinkable agent. The time required for the solubilized agent to fully penetrate the agent on the other surface treated particles is reduced, and consequently, green strength development and final strength development are enhanced. This embodiment also helps to reduce the spread of fluid into non-image areas.

Acid catalysts may be used to accelerate crosslinking of the crosslinkable agent, and these include, e.g., lewis acids (such as ammonium chloride, tin (II) chloride, magnesium chloride, cobalt, sulfate, or iron III chloride), which may be incorporated into the powder (but tend to reduce storage stability). Acid catalysts may also include, e.g., blocked acid catalysts such as Nacure (King Industries, Norwalk, Connecticut), which release acid upon heating and can be incorporated into the powder, binder, or redox systems herein described.

The powder may also contain a "strengthening component" that melts and flows upon heating, then resolidifies on cooling, or preferably thermal cures. The powder to which the strengthening component is added can be any powder suitable for 3D printing, such as a standard starch/cellulose or plaster powder, and the crosslinkable agent- containing powder systems described above. Preferably, the strengthening component is substantially inert when contacted with an aqueous binder at ambient temperature. In other words, when the 3D article is initially printed, the strengthening component does not contribute substantially to the article's strength. However, when the article is heated, e.g., in an oven, the strengthening component melts and flows within the article, filling gaps and pores and engulfing other components of the powder, and then it resolidifies or cures, which adds substantially to the strength of the 3D article.

Preferably, the strengthening component reacts to produce a thermoset polymer when it is heated. By way of example, the strengthening component may comprise a blend of an epoxy resin and a carboxyl group-containing polyester (such as those used in powder coating applications, e.g. U.S. 6,117,952), which reacts when heated to produce a thermoset polymer.

The powder may contain other ingredients such as adhesives, fillers, cohesive aids, thickeners, and polyols, which improve the material system's performance in 3D articles, and provide desired mechanical properties in the printed article.

Adhesives suitable for the material systems of this invention should be water soluble at room temperature, so that the adhesive is activated when contacted by the aqueous binder fluid. Examples include polyvinyl alcohol and poly(ethyloxazoline). In general, the adhesive should be milled; preferably to less than 100 microns, and more preferably in the range of 20-40 microns. In any event, the adhesive powder should be fine enough to enhance dissolution in the aqueous binder, without being so fine as to cause "caking", an undesirable phenomenon wherein unactivated powder adheres to the printed article, resulting in poor resolution. The powder may contain 0%.to about 90% by weight of one or more adhesives. Fillers may be included in the powder and should be insoluble, or only slightly soluble, in the aqueous liquid, should be readily wettable, should be capable of adhesively bonding with the adhesive components of the powder system; may be coated (e.g., with aminosilanes) or uncoated; and should not render the powder system unspreadable. Examples of suitable fillers, which may be used alone or in combination, include glass spheres, flakes or fibers; inorganic mineral fillers (such as wollastonite or mica); clay fillers (such as Kaolin); starches (such as maltodextrin); plaster; polymeric fibers (such as cellulose fiber); ceramic fiber; graphite fiber; limestone; gypsum; aluminum oxide; aluminum silicate; potassium aluminum silicate; calcium silicate; calcium hydroxide; calcium aluminate; sodium silicate; metals; metal oxides (such as zinc oxide, titanium dioxide and magnetite); carbides (such as silicon carbide); borides (such as titanium diboride); and inert polymers such as polymethylmethacrylate, polysterene, polyamide and polyvinyl chloride.

The filler component may include a variety of particle sizes, ranging from about 5 microns up to about 200 microns. Generally speaking, the mean size of the particulate material cannot be larger than the layer thickness. Large particle sizes may improve the quality of the printed article by forming large pores in the powder through which the binder fluid can easily migrate. Smaller particle sizes may serve to reinforce article strength. A distribution of particle sizes may be particularly desireable, as it may increase the packing density of the particulate material, which in turn may increase both article strength and dimensional control. Ideally, the powder systems of this invention may contain from about 0% to about 60% by weight of a main filler (such as glass spheres), and from about 0% to about 30% by weight of one or more other fillers (such as inorganic mineral fillers and/or clay fillers). Solid glass spheres are preferred as the main fillers, because they are readily wetted by liquid components of the composition (e.g. surfactants and wetting agents).

Cohesive aids provide light adhesion between the powder grains, thereby reducing dust formation and promoting even spreading of the powder. Examples include polyethylene glycol, sorbitan trioleate, citronellol, ethylene glycol octanoiate, ethylene glycol decanoiate, ethoxylated derivatives of 2,4,7,9-tetramthyl-5-decyn-4,7-diol, sorbitan mono-oleate, sorbitan mono-laurate, polyoxyethylene sorbitan mono-oleate, soybean oil, mineral oil, propylene glycol, fluroalkyl polyoxyethylene polymers, glycerol triacetate, oleyl alcohol, and oleic acid. The powder systems of this invention may contain 0% to about 10% by weight of one or more cohesive aids.

Thickeners work to increase the viscosity of the fluid binder, thus minimizing the diffusion of the binder into the surrounding powder. It is believed that thixotropic agents such as CARBOPOL® EZ-2 (polyacrylic acid from Noveon, Inc., Cleveland,

OH), when added to the powder system, will mediate the settling of fine, denser filler materials during the initial stage of binder migration. Furthermore, the swelling of the EZ-2 polymer may counteract to some degree the natural tendency of the amino resins to shrink upon curing. Finally, the acidic character of EZ-2 is believed to catalytically assist the polymerization of the crosslinkable agent. The material systems of this invention may contain about 0% to about 15% by weight of one or more thickeners.

Surfactants increase the solubility in the aqueous liquid of lipophilic powder components. Surfactants may be present in the binder system and/or in the powder system. For example, the powder system may contain up to about 6% of one or more surfactants. Examples of surfactants include perfluoroalkyl polyethers.

The powder system may comprise a polyol to increase the extent of crosslinking of the crosslinkable agent. Examples of polyols include tetramethylol methane, glycerol, sorbitol, erythritol, polyvinyl alcohol and trimethylolpropane. The powder systems of this invention may contain from 0% to about 90% by weight of one or more polyols.

The Aqueous Liquid Applied to the Powder The aqueous liquid is preferably a binder system is selected to provide the degree of solubility required for the various powder components described above. Powder systems of this invention are compatible with standard aqueous binders, such as ZB-7 (Z Corporation). The binder system of this invention comprises mainly water, but may comprise other additives known to those of skill in the art, such as surfactants, humectants, water absorbing moieties, and dyes.

Infiltrating Liquids

Liquids that may be used for infiltration in accordance with the present invention include epoxy/hardener systems preferably comprise low viscosity (<2500cps, highly preferred being <500cps) diglycidyl epoxies, such as bisphenol A or F di glycidyl epoxides, or perhydrogenated bisphenol A diglycidyl epoxide. Diluent epoxies include bisglycidyl butane-diol types. Also, cresol and phenol novalac epoxies can be used, as can the newer types of epoxies derived from siloxane or butadiene sub-units. Polyepoxides and pre-polymers containing epoxide functionality may be used to provide further reactivity. Isocyanates/polyols systems will produce polyurethanes and as with the epoxides, polyisocyanate compounds maybe advantageously utilised. The anhydrides/epoxy systems yield polyesters, and the maleimide/epoxy systems yield polyamides. The acrylic/epoxy/amine hybrid mixes can used to obtain controlled reactions: pure epoxy/amine systems can exotherm out of control resulting in warpage and shrinkage problems. Addition (e.g. approx 10% by weight) of acrylic compounds to the epoxy/amine mix can control exotherming effects: the mechanism is believed to involve a fast Michael addition of amine to the acrylic to produce a gel, within which epoxy reacts with the residual amine giving the required additional strength. The invention also extends to a composite part made by the methods described, having tensile strength greater than 10 MPa, preferably greater than 20MPa and more preferably greater than 30MPa.

The method of the present invention can be applied to make rapid prototypes or manufactured products especially those required to be durable during use. The strong, tough composite articles made in accordance with the invention can find uses in aerospace, automobile and rail applications, in leisure goods, in electrical casings and in other areas.

It may especially be applied for making dental implants, e.g. tooth crowns, and other prosthetic devices, for example substantially two-dimensional prosthetics, e.g. a bone plate, such as an insert in a long bone or a cranial plate, or a substantially three- dimensional prosthetic. As typical examples of three-dimensional prosthetic devices may be mentioned inter alia joints, e.g. femural head replacement; complex fracture repairs; and bone restructuring in reconstructive surgery, e.g. jaw-bone, hip-joints, shoulders, elbows, knuckles and more complicated arrangements, e.g. a knee which is believed to combine a hinge action with a gliding and rolling action.

For example, a dental crown may be rapidly formed in a dental surgery or laboratory by taking a scan of the tooth to obtain the shape of the tooth surface onto which the crown is to fit. The tooth shape is analysed from the scan in terms of thin cross sections. A crown having a surface to match with the tooth can then be made by depositing successive layers of a wear resistant powder material such as aluminium oxide and depositing a binder or curable resin on the powder layer in a shape corresponding to the successive cross sections of the tooth and the desired crown surfaces using an inkjet print head to form a precursor of the desired shape of the crown. The amount of liquid binder or resin deposited is sufficient to bind the aluminium oxide particles while at the same time leaving voids between the particles. The precursor is then placed in a bath of a tough curable epoxy resin within a vacuum chamber; the epoxy resin is drawn by the vacuum into the precursor and infiltrates it. The resin is then cured to form an extremely tough crown in a shorter time than is generally possible presently.

The invention also extends to the products made by those methods.

Brief Description of the Drawing

The invention may be carried into practice in various ways and some embodiments will now be described in the following non-limiting Examples and by reference to Figure 1 , which shows the test bars with a variety of internal structures.

Examples Formation of Partially Consolidated Test Bars using Powders and Ink Jetted

Fluid Binder. Machine: Z-Corporation Z4063D Printer

Test bars with external dimensions 100mm x 10mm x 4mm, having internal structures as shown in Figure 1 , were built using commercially available calcium sulphate di- hydrate based powder ZP102 and aqueous binder ZP50 (Z-Corporation). ZP102 powder recrystalises when contacted by an aqueous solution to form a porous consolidated precursor.

Each layer thickness was 0.1 mm (0.004 ins). Figure 1 illustrates four structures in schematic cross-section.

The formed parts were allowed to remain in the bed for 1 hour after printing before drying at 40°C in air overnight. Preparation and use of Composition 1

Bisphenol A/F Epoxy resin Resin LY113 (100 parts by weight) available from Nantico was mixed with amine hardener HY97 ( 32 parts by weight) also available fromNantico Ltd. The two components were mixed thoroughly for five minutes by hand prior to use.

Determination of Flexural Strength

The test bars were treated with Composition 1 according to the described examples and cured as described.

This procedure produced composite test bars made up of the original rapid manufacture built material and the secondary curable Composition 1.

Flexural strengths of the final composite test bars and comparison test bars were determined according to ISO 178 using a crosshead speed of 2mm / min.

Examples 1 - 5 : Parts Prepared Using Powder Technique

No Removal of the residual original build material

Test bars having internal structures as shown in Figure 1 were prepared on a Z406 3D Printer as described above.

The residual unbound powder filling the internal cavities of the part was not removed. Comparative examples are test bars which are completely solid having no internal structure. Example 0 is base test bar which is 100% consolidated and not treated with Composition 1. Example 1 is same as Example 0, but treated with Composition 1. The Examples 2-5, test bars were totally immersed in Composition 1 in a shallow dish. The dish was placed in a vacuum oven and the system evacuated for five minutes to eliminate residual air in the article. The test bars were then removed from the vacuum oven, the excess resin removed and the test bars were cured at room temperature for four hours on some release paper.

The composite test bars produced were post-cured in an oven at 200°C for 2 hours to give the final fully cured composite test bars.

Example Break Bar Flexural strength Structure MPa

Comparative Examples

Solid

0 No treatment 7.1

1 Solid 31.1

Examples

2 A 37.7

3 B 33.7

4 C 36.9

5 D 42.2

Surprisingly, the parts having internal structure consisting of unconsolidated powder and cured Composition 1 (Examples 2 — 5) were found to have a greater strength than the solid bar (Example 1). Particularly, Example 5, where there was no consolidation of the internal powder, gave the highest strength, indicating that penetration of Composition 1 into such a structure with unconsolidated internal machine can be achieved very effectively.

Claims

1. A method for producing a 3D article comprising the steps of:

(a) forming a precursor in sequential cross-sectional layers in accordance with a model of the precursor, each layer being formed by:

(i) defining a layer of a powder material and (ii) applying a liquid to the powder material to consolidate a portion of the powder material in a pattern corresponding to the respective cross-sectional layer of the model and repeating these steps (1) and (ii) to form successive layers;

(c) establishing within the internal zone a quantity of curable material; and (d) curing the curable material within the internal zone to form the 3D article in the form of a final composite solid body.

2. A method as claimed in Claim 1, wherein the liquid applied in step a(ii) is reactive with the powder material or is curable.

3. A method as claimed in Claim 2, in which the powder material includes a reactive component, and wherein, in the consolidation step, the reactive liquid reacts with the reactive component of the powder.

4. A method as claimed in Claim 3, in which the reactive component in the powder comprises a hydroxy, amine, acrylic, epoxy, isocyanate carboxylic or ester group.

5. A method as claimed in any preceding Claim, in which the unconsolidated material retained in the internal zone specified in step (c) constitutes at least part of the said quantity of curable material specified in step (d).

6. A method as claimed in any preceding Claim, in which curable liquid material is introduced into the internal zone after the precursor has been formed by infiltrating said liquid through the consolidated precursor.

7. A method as claimed in Claim 6, in which the infiltrating curable material reacts with the portion of the powder material which remains unconsolidated and which is retained in the internal zone.

8. A method as claimed in Claim 6 or Claim 7, in which the infiltrating curable material is introduced into the internal zone by vacuum and/or under pressure.

9. A method as claimed in any preceding Claim, in which the curable material in the zone that is subject to curing in step (d) comprises epoxy/amine, acrylic/epoxy/amine, acrylic/epoxy, isocyanate/polyol, epoxy/isocyanate/polyol, epoxy/anhydride, epoxy/polyester mixtures and mixtures thereof, optionally containing latent phase separating toughening additives, pigments and/or fillers.

10. A method as claimed in any preceding Claim, wherein a nano-scale material is incorporated:

(a) in the powder,

(b) in the applied liquid or,

(c) when used, in the curable liquid material introduced into the internal zone after the precursor has been formed by infiltrating said liquid through the consolidated precursor.

11. A method as claimed in any preceding Claim, in which, during formation of the precursor, an internal support structure is defined within the internal zone.

12. A method as claimed in any preceding Claim, in which the curing in step (d) is achieved by heat or microwave radiation.

13. A method as claimed in any preceding Claim, in which the powder material comprises organic or organometahc polymers, oligomers or monomers and the liquid applied in step a(ii) comprises aqueous or non aqueous jettable curable resin.

14. A method as claimed in any preceding Claim, in which the curing comprises heating at a temperature in the range 60 to 120°C for a period of 0.5 hr to 4 hrs.

15. A method as claimed in any preceding Claim, in which substantially all of the powder in the internal zone following step (a) is present in the internal zone during step (d).

16. A 3D article made by the method of any preceding Claim.