EP0853995A1 - Injection moulding composition containing metal oxide for making metal shapes - Google Patents

Abstract

Moulding material containing 20-50 vol.% of a powder of one or more metal oxides, optionally together with metal carbides and/or nitrides which are not reduced by hydrogen, in a fluid binder, ≥ 65 vol.% of the powder having NOTGREATER 0.5 mu m particle size, with the remainder having NOTGREATER 1 mu m particle size, and ≥ 90 vol.% of the powder consisting of metal oxides, which are reduced by hydrogen. Preferably, the fluid binder is an organic polymer. Also claimed is the use of hydrogen-reducible metal oxides of particle size NOTGREATER 0.5 mu m for producing injection moulding materials. Further claimed is a method of producing metal mouldings by injection moulding the above moulding material, removing the binder and then reducing and sintering the resulting moulding in the presence of hydrogen.

Description

The invention relates to molding compositions containing metal oxides, in particular Injection molding compositions which are suitable for the production of metal moldings, and processes for the production of metal moldings.

In the production of small complex metal moldings after the Powder injection molding processes are metal powders with powder diameters of 2 to 40 µm mixed with a flowable binder and this Mixture, as is common in the processing of plastics, using injection molding machines injected into a mold under pressures up to 2000 bar. In the Mold solidifies the injection molding compound, usually because the mold is lower Surface temperature than the injected mass and that Binder in the mold to a temperature below the glass temperature or melting temperature is cooled.

Then the mold is opened and the molded part is removed. The binder is then removed from the molding thus formed, and the molding should not be deformed. The binder can be removed in various ways. It is possible to thermally decompose the mostly organic binder by carefully increasing the temperature over a longer period of time and thus to remove it. The binder can also be constructed so that it is partially soluble in a solvent and this portion can be extracted with the solvent. The further binder fraction is then thermally decomposed, which can be done more quickly than in the first variant, because after the extraction of the soluble binder fraction an open-pored body is already present and the thermal decomposition does not build up any internal pressure that could destroy the molding. The binder is most elegantly removed using a catalytic process, the binder used being, for example, a polyacetal which is directly depolymerized below its melting temperature under the influence of gaseous acids without the formation of a liquid phase to form gaseous formaldehyde. This process runs from the outside to the inside of the molding walls, which means that the entire gas exchange can also only take place in the already porous parts by volume, and likewise no disadvantageous internal pressure can be built up. This process has the further advantage that the debinding process takes place below the melting point of the binder, and the molding does not disadvantageously change its dimensions. Moldings which are very true to dimension are thus obtained. The deviation of the linear dimensions from the target dimension is a maximum of +/- 0.3%, often less. However, the roughness depths of the molded parts are essentially determined by the powder size used, so that roughness depths R _Z of 1 μm are not undershot. Metal powder with a diameter smaller than 2 µm would be required to produce parts with a smaller surface roughness. However, the production of such metal powders is extremely expensive or there are considerable difficulties in handling such fine metal powders. The ratio of surface to volume increases with decreasing particle size, which makes the metal powders more and more chemically reactive. Base metals, such as iron, cobalt, zinc or nickel, become pyrophoric and cannot be processed in air.

In addition, in the production of metal powders by spraying Metal melts barely fell below particle sizes of 5 µm. Often leave the metal powders do not shred even further by grinding, because they're too ductile.

However, there is a demand for finer molding compounds for production of metal moldings, ever since using newer techniques to produce finer mold inserts for the injection molding process. With the LIGA processes are used, for example, to produce tool inserts which parts are manufactured by injection molding, the dimensions in µm range and roughness in the nanometer range.

In the LIGA process, a light-sensitive polymer layer is placed on a base plate, a so-called photoresist, applied and through a mask, which contains the structures to be produced in cross section. The portions of the polymer layer exposed through the mask become soluble and can therefore be washed out. The trenches that were created electroplated through a metal layer, after which the remaining one Photoresist is dissolved. The metal structure thus obtained can be used as a mold can be used for an injection mold.

The object of the invention is to provide molding compositions or injection molding compositions for the production of metal moldings that have a property profile have their use in very fine mold inserts, for example allowed from the LIGA process. The shaped bodies obtained in this way are said to Fineness and surface quality of those produced using the LIGA process Conform shapes.

The object is achieved by molding compounds containing a flowable Binder 20 to 50 vol .-%, based on the total volume of the molding compound, a powder of one or more metal oxides and optionally metal carbides and / or metal nitrides that cannot be reduced with hydrogen, wherein at least 65% by volume of the powder has a particle size of maximum 0.5 µm and the rest of the powder a maximum particle size Have 1 micron, and at least 90 vol .-% of the powder with hydrogen reducible metal oxides exist.

According to the invention, it was found that instead of the large-grain, poorly accessible and difficult to handle metal powder, metal oxide powder with particle sizes below 1 µm for the production of the molding compositions can use. The molding compound or injection molding compound is deformed to a shaped body, debinds the shaped body and sinters it under Reduction of metal oxides in a hydrogen-containing, reducing atmosphere.

A powder is used which has a particle size of at least 65% by volume of maximum 0.5 µm, the rest of the powder having a Has particle size of at most 1 micron. Point particularly preferably at least 80% by volume of the powder has a particle size of at most 0.5 µm on. At least 90 vol .-% of the powder consist of hydrogen reducible Metal oxides, the remaining portion of the powder not being made with hydrogen reducible metal oxides, metal carbides and / or metal nitrides consists.

Suitable metal oxides are those which can be reduced and sintered with hydrogen, so that metal moldings can be produced from the inside by heating under a hydrogen atmosphere or in the presence of hydrogen. Examples of metals whose oxides can be used can be found in groups VIB, VIII, IB, IIB, IVA of the periodic table. Examples of suitable metal oxides are Fe ₂ O ₃ , FeO, Fe ₃ O ₄ , NiO, CoO, Co ₃ O ₄ , CuO, Cu ₂ O, Ag ₂ O, WO ₃ , MoO ₃ , SnO, SnO ₂ , CdO, PbO, Pb ₃ O ₄ , PbO ₂ , Cr ₂ O ₃ . The lower oxides are preferably used, such as Cu ₂ O instead of CuO and PbO instead of PbO ₂ , since the higher oxides are oxidizing agents which, under certain conditions, can react, for example, with organic binders. The oxides can be used individually or as mixtures. For example, pure iron moldings or pure copper moldings can be obtained. When using mixtures of the oxides, alloys and doped metals are accessible, for example. For example, steel parts are produced from iron oxide / nickel oxide / molybdenum oxide mixtures and bronzes from copper oxide / tin oxide mixtures which may also contain zinc, nickel or lead oxide. Particularly preferred metal oxides are iron oxide, nickel oxide and / or molybdenum oxide.

The metal oxides used according to the invention with a particle size of A maximum of 1 µm, preferably a maximum of 0.5 µm, can be divided into different Process, preferably by chemical reactions. Out Solutions of metal salts can include the hydroxides, oxide hydrates, Carbonates or oxalates are precipitated, the particles optionally accumulate very finely in the presence of dispersants. The rainfall are separated and washed to the highest possible purity brought. The precipitated particles are dried and heated by elevated temperatures to the metal oxides.

It is also possible to get very finely divided metal oxides directly in one step. For example, by burning iron pentacarbonyl with oxygen, extremely fine, spherical iron oxide particles with specific surfaces of up to 200 m ² / g are obtained.

The metal oxides or at least 65% by volume of the powder used according to the invention preferably have a BET surface area of at least 5, preferably at least 7 m ² / g.

In addition to the metal oxides that can be reduced with hydrogen, other metal compounds that cannot be reduced during sintering, such as metal oxides, metal carbides or metal nitrides that cannot be reduced with hydrogen, can also be present. Examples of oxides are ZrO ₂ , Al ₂ O ₃ and TiO ₂ . Examples of carbides are SiC, WC or TiC. An example of a nitride is TiN.

Preferably, that used in the molding compositions according to the invention Powder at least 90% by volume, particularly preferably at least 95% by volume, based on the powder, on hydrogen-reducible metal oxides. If metal oxides, metal carbides and / or not reducible with hydrogen Metal nitrides are used, they are preferably in amounts of 1 to 10, particularly preferably 2 to 5% by volume, based on the powder, in front.

The powder used according to the invention is present in the molding compositions in quantities from 20 to 50% by volume, preferably 25 to 45% by volume, particularly preferably 30 to 40 vol .-%, based on the total volume of the molding compound.

The powder used in the molding compositions according to the invention is distributed in a flowable binder. If necessary, additional a dispersant can be used. According to a preferred embodiment The invention consists of the molding composition from that described above Powder, a flowable binder and optionally a dispersant.

According to a further embodiment of the invention, the molding compound has In addition to these components, other components as described below are described.

The total volume of all ingredients of the molding compound results in each Fall 100 vol%.

All binders which are suitable for use in the powder injection molding process can be used as flowable binders. They are preferably flowable at the processing temperature so that they can be injection molded into molds. For example, the binders described above in the prior art can be used. It is therefore possible to use binders which are thermally decomposed and thus removed, binder mixtures, some of which can be extracted with solvents and the other part can be thermally decomposed, or binders which are used, for example, in the form of a polyacetal which is below its melting temperature below the influence of gaseous acids can be depolymerized directly to gaseous products without the formation of a liquid phase. Suitable binders are known to the person skilled in the art. The flowable binder preferably contains an organic polymer. A polyoxymethylene copolymer is preferably used, as described, for example, in EP-A-0 444 475, EP-A-0 446 708 and EP-A-0 444 475. It is preferably a polyoxymethylene copolymer which contains 0.5 to 10, preferably 1 to 5 mol% of butanediol formal as comonomer. Polybutanediol formal can be used as an additional binder.
A mixture of 75 to 89% by weight of polyoxymethylene copolymer which contains 2 mol% of butanediol formal as a comonomer and has a melt index of about 45 g / 10 min at 190 ° C. and 2.16 kg coating weight is particularly preferred, and 11 to 25 wt .-% polybutanediol formal with a molecular weight M _n of about 20,000 used.

All dispersants used for Dispersion of metal oxide particles of the specified particle size in Binders are suitable. A suitable class of substances for the dispersants are alkoxylated fatty alcohols or alkoxylated fatty acid amides.

Other suitable ingredients of the molding compositions are those during processing processing stabilizers used by polyoxymethylene.

The molding compositions according to the invention are for injection molding usable from metal moldings. It is used to manufacture the Molding the organic and inorganic components in suitable Mixers mixed. This is preferably done in a kneading device while melting the flowable binder. After solidification the molding compositions are preferably granulated. You can be injection molded by known methods, preferably at melt temperatures from 170 to 200 ° C. The shape used preferably has a temperature of 120 to 140 ° C.

The binder is then removed from the moldings thus obtained. Depending on the binder used, this can be done by slowly heating, Treat with a solvent and then heat or Treat with an acid and heat up. This is preferably done Debinder simultaneously with heating to reduce and sinter the Shaped. The molding in the presence of hydrogen is preferred under hydrogen atmosphere, at a rate of 1 to 20 ° C / min, preferably 2 to 10 ° C / min to the material-specific sintering temperature heated, 1 to 20, preferably 2 to 10 hours at Leave the sintering temperature and then cool down. During the slow The binder is removed by heating. The one used for reduction Hydrogen preferably has a maximum dew point of -10 ° C, particularly preferably from less than -40 ° C. The dew point will be chosen so that a reduction under the Reaction conditions is possible.

To reduce Cr ₂ O ₃ , for example, an extremely dry hydrogen with a dew point of less than -40 ° C is required. The reduction is carried out at temperatures above 1500 ° C., particularly preferably above 1600 ° C. When sintering chromium-containing alloys, the alloy components often sinter at 1200 to 1300 ° C, while when using Cr ₂ O ₃ this can still remain in the molding unreduced. In the production of, for example, stainless steels with a chromium content of approximately 13 to 20% by weight, the chromium content is therefore preferably used as ferrochrome with a particle size of the particles of at most 1 μm. The volume fraction of the ferrochrome is preferably less than 35% by volume. It is thus possible to produce stainless steels alloyed with chromium and, if appropriate, nickel and molybdenum, without fear that non-reduced Cr ₂ O ₃ remains in the otherwise already sintered molding.

The invention also relates to a method for producing molded metal bodies by injection molding a molding compound as described above is, in a form, removal of the binder from the thus obtained Molding and reducing and sintering the debindered molding into one Shaped metal body in the presence of hydrogen. The removal takes place of the binder preferably thermally in one step with the reducing and sintering by heating the molding to the sintering temperature in Presence of hydrogen.

With reducing sintering, the moldings shrink up to 5 times, based on volume or up to half based on linear Dimensions. This high shrinkage is just for making very Small structures are an advantage because the injection mold is about the Factor 2 can be made larger in every dimension and therefore very large fine details can be formed. The dimensional tolerances of the sintered Moldings are preferred despite the high absolute shrinkage maximum +/- 0.3%, particularly preferably +/- 0.15%.

The surface roughness R _Z is preferably less than 1 μm, R _a less than 0.2 μm, measured in accordance with DIN 4768 or DIN 4768/1.

The invention is explained in more detail below with the aid of examples.

Examples

The injection molding compounds listed in the examples below were Manufactured according to a uniform procedure, thermally debindered and when the material is adequate Sintering reducing temperatures under hydrogen.

A thermoplastic polyoxymethylene copolymer which contained 2 mol% of butanediol formal as a comonomer and had a melt index of about 45 g / 10 min at 190 ° C. and 2.16 kg coating weight was used as the flowable binder. Polybutanediol formal with a molecular weight M _n of approximately 20,000 was used as an additional binder. Solsperse® 17000 from ICI was used as the dispersant for dispersing the inorganic powders. The amounts are given in the table below.

The organic and inorganic components of the molding compound were in a paddle mixer of 1 l useful content melted at 190 ° C and for Kneaded for 90 min. The blade mixer was then cooled and the mass solidified and granulated in the rotating kneader. The so obtained Injection molding compounds were at 180 ° C melt temperature in a to 130 ° C tempered mold for a bending rod with the dimensions 1.5 x 6 x 50 mm injected.

The bending rods produced in this way were in a tube furnace under a hydrogen atmosphere (Hydrogen with a dew point around -10 ° C) with a Speed from 2 ° C / min to the specified material-specific Sintering temperature heated and for 2 hours at the sintering temperature leave. The oven was then cooled. During the slow heating up depolymerized the polyoxymethylene and the polybutanediol formal in the temperature range from 220 to 300 ° C without formation of cracks in the thin-walled bending rod. The bending bars were made on a powder bed Aluminum oxide powder with about 5 µm grain size stored to prevent shrinking to facilitate.

All of the molding compositions listed in the examples resulted in flawless, crack-free moldings, although the volume shrinkage is sometimes 80% scam.

The surface roughness values obtained with a polished injection mold were in any case less than 1 µm for R _Z and less than 0.2 µm for R _a . (Composition in grams)
Example No. Oxides used 1 2nd 3rd 4th 5 6 7 8th 9 10th Fe ₂ O ₃ 9m ² / g 2257 Fe ₂ O ₃ 20m ² / g 1890 2000 197 Fe ₂ O ₃ 40m ² / g 1050 NiO 7 m ² / g 155 2264 679 Cu ₂ O 9m ² / g 2700 2112 1974 MoO ₃ 11m ² / g 1890 WO ₃ 10m ² / g 2721 SnO ₂ 13m ² / g 423 968 Organic components Polyoxymethylene 653 681 848 567 625 584 560 592 507 684 Polybutanediol formal 53 85 106 106 53 85 101 85 106 90 Solsperse 17,000 51 71 92 92 51 82 87 82 92 77 Sintering temp. in ° C, more linear 700 700 600 850 980 1450 1450 820 1090 1170 % Shrinkage 41.3 44.3 54.2 42.3 42.5 49.8 49.3 42.2 32.9 41.4

Claims (10)

Molding composition containing 20 to 50 in a flowable binder Vol .-%, based on the total volume of the molding compound, of a powder from one or more metal oxides and possibly not with Hydrogen reducible metal carbides and / or metal nitrides, wherein at least 65% by volume of the powder has a particle size of at most 0.5 µm and the rest of the powder has a maximum particle size of 1 µm have, and at least 90 vol .-% of the powder with hydrogen reducible metal oxides exist. Molding composition according to claim 1, characterized in that at least 65% by volume of the powder has a BET surface area of at least 5 m ² / g. Molding composition according to claim 1 or 2, characterized in that the flowable binder contains an organic polymer. Molding composition according to one of claims 1 to 3, characterized in that that it contains a dispersant for the powder. Molding composition according to one of claims 1 to 4, characterized in that as metal oxides which can be reduced with hydrogen, Fe ₂ O ₃ , FeO, Fe ₃ O ₄ , NiO, CoO, Co ₃ O ₄ , CuO, Cu ₂ O, Ag ₂ O, Bi ₂ O ₃ , WO ₃ , MoO ₃ , SnO, SnO ₂ , CdO, PbO, Pb ₃ O ₄ , PbO ₂ , Cr ₂ O ₃ or mixtures thereof. Molding composition according to one of claims 1 to 5, characterized in that that the powder 1 to 10 vol .-% not reducible with hydrogen Metal oxides, metal carbides, metal nitrides or mixtures thereof with one Contains particle size of maximum 0.5 microns. Use of molding compositions according to one of claims 1 to 6 as Injection molding compounds for the production of metal moldings. Use of hydrogen reducible metal oxides with a Particle size of maximum 0.5 µm for the production of injection molding compounds. Process for the production of metal moldings by injection molding a molding composition according to any one of claims 1 to 6 in a mold, Removal of the binder from the molding and Reduce and sinter the debindered molding to a metal molding in the presence of hydrogen. A method according to claim 9, characterized in that the removal of the binder in one step with reducing and Sintering by heating the blank to the sintering temperature in Hydrogen is present.