GB2163780A - Method of moulding powder materials - Google Patents

Abstract

Green mouldings of metal powder and/or ceramic powder are obtained by filling a cooled mould with a mixture of the metal and/or ceramic powder containing 30 to 55% of tertiary butanol as binder. The mixture is at a temperature above the melting point of tertiary butanol, and is cooled in the mould to a temperature below the melting point of methanol. The use of tertiary butanol as binder leads to easy mould release, and since it can be removed from the green moulding at low pressure and relatively low temperature, without decomposition, it is reusable, avoiding the problems of the known decomposable organic binders, and the corrosion resulting from aqueous binders.

Description

SPECIFICATION Method of moulding powder materials.

The present invention relates to a method of moulding metal powder or ceramic powder.

It is well known in the artthat injection moulding is an effective method for producing mouldings of complicated shapes from metal powders, such as powders of 2% N-1-98% Fe, SUS 316, stellite hard metals, and from ceramic powders, such as powdered alumina, silicon carbide, silicon nitride orzirconia. For instance, the production of small machine components from 2% Ni-98% Fe or stainless steel, and the production of components of 161 cm2 cross-sectiona I area and 5cm thickness, through the injection moulding of mixtures of metal powders and organic binders have been disclosed in Industrial Heating, May, 1984, p14-17.

Also, the application of such methods to the field of ceramics, e.g., the manufacture of turbine blades from silicon carbide, has been described in Journal of Engineering for Power: July1982, Vol. 104, p601-606, while examples of the application of such methods to the manufacture of high-density components from silicon nitride have been described in Industrial Hearing, January 1984, p3942.

These injection moulding methods generally com prise the following five steps: (1) a mixing step, involving mixing a starting mate rial powder with an organic binder to form a thermo- plastic mixture; (2) an injection step, involving softening the mixture in a heating cylinder and then injecting the mixture into a mould under pressure; (3) a mould release step, involving opening the mould and removing the green moulding; (4) a dewaxing step, involving removing the organic binderfrom the green moulding; and (5) a sintering step, involving sintering the moulding to produce a high-density product.

Thesuccessorfailureoftheinjection moulding process is dependent upon the type of organic binder used, and the results of these steps are influenced by the organic binder.

An organic binder is added to a metal or ceramic powderto provide the desired mouldabiiity. If the mouldability ofthe powder material containing the organic binder is not good, such defects as knit lines, weld lines or sink marks are produced in the resulting moulding.

Whiie a large amount of an organic binder must be employed to improvethe mouldability of a powder material, increased amounts ofthe binder tend to cause such defects as cracking, foaming distortion, etc., of the moulding in the course of the dewaxing step for removing the binder. Thus, a binder having such properties asto minimize its amount, and prevent the occurrence of defects during the dewaxing step has heretofore been sought.

Previously-used, organic binders include generally those prepared by adding a small amount of thermo plastic resin, oil orthe liketo low-molecularweight polyethylene, polystyrene, paraffin or microcrystalline wax. Other known binders include polypropylene, polyvinyl alcohol, polyvinyl butyral, polyethylene glycol, methyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, atactic polyethyl cellulose, and hydroxyethyl cellulose.

It is also known to add a small amount of stearic acid to such binders for the purpose offacilitating the mould release.

On the other hand,the conventional techniques used for molding metal or ceramic powders having the following disadvantages (1)to (4).

(1) In the conventional methods,thedewaxing step usually involves heating a moulding from room temperature up to 400 to 500 C, thereby decomposing the binderand removing it by vaporization. In this case, it is essential that the moulding is heated slowly up to the maximum temperature, so that the rate of generation of the vapour produced by decomposition of the binder does not exceed the rate at which the vapour is discharged to the outside through the voids within the moulding. If this is not done, the vapour pressure within the moulding is increased and cracking, foaming, distortion or the like results. Consequently, the dewaxing step requires a long period (e.g.

70 to 100 hours), which destroys the inherent high productivity of injection moulding methods; and this is the most serious disadvantage of the injection moulding methods of powder materials.

(2) The second disadvantage is the waste of heat energy due to the extended heating of mouldings at 400 to 500"C. Since the temperature of the waste gas is low, it is difficult to make use of it economically as effective heat energy.

(3) Also, while the dewaxing is dependent on the thermal decomposition of the binder, as mentioned previously, itisdimcultto ensure perfectdewaxing and usually small amounts of carbon and oil remain in the moulding. This caused deterioration in the properties of mouldings after sintering.

(4) The oil recovered during the dewaxing step is a decomposition product of the binder, and therefore cannot be reused as a binder. Thus, the oil is usually discarded and this increases the costs of moulded products.

Afreeze casting process is known as a method for overcoming the foregoing deficiencies.

W.D.Jonesand E.M. Gralapublishedtheirresearch on this process in 19/8 and 1961, respectively, ("Powder Metallurgical Techniques Course: Moulding of Metal Powders, 1964, Auk.25, Nikkan Kogyo Shimbun-sha). In the examples described by E.M.

Grala, a high viscosity slurry is first prepared by mixing titanium carbide powder with a 40% solution of rubber or 25% aqueous solution of paste, and the slurry is poured into an injection unit. While vibrating the injection unit to prevent solidification of the slurry, the slurry is rapidly exposed to a vacuum, thereby removing the aircontained in the slurry. Then, a mouldisfittedtotheinjection unitandtheslurryis injected into the mou Id, while continuously vibrating the injection unit.After the mould and slurry have been removed from the injection unit, they are immersed in a mixture of petroleum and dry ice to cool at -40;C for5 to 10 minutes, or cooled in a petroleum tank at -3 for 45 to 60 minutes, thus completely freezing theslurry. Afterthefrozen moulding has been removed from the mould, it is vacuum dried for2 hours in a vacuum container, or it is covered in China clay and subjected to air-drying for48 hours. Then, the moulding is vacuum sintered at 1250for 1 hour, and a sintered productoftitanium carbide is produced.

Also, in 1984, Nakagawa etal, described a freeze injection moulding process ("Freeze Injection Molding Process", Nikkan Kogyo Shimbun-sha, 1984, June, 15). While this process is similarto the previously mentioned process, in that moulding is performed using liquid water, and mould release is effected by using ice to bindtheformed green moulding, the process is employed for injection moulding.

For example, alumina powder of 1 ,am or less is mixed with about40 volume % of water give plasticity to the powder material at room temperature, injected into a mould cooled to a temperature between -5 and -10 C, provided with the desired strength by freezing the water, and the moulding is removed from the mould by opening it. In this case, the freezing has the effect of facilitating mould release.

Then, the moulding is dehydrated. Since the satu rated vapour pressure of water is 3333 Pa (25mmHg) at 25"C, which is very high compared with that ofthe conventional organic binders, a greater part of the water content can be removed by vaporization at room temperature, and natural drying is also possible.

Also, vacuum drying is effective for increasing the rate of dehydration, and the dehydration time can further be reduced by applying additional heating.

Thus, this process overcomes the above-mentioned disadvantage (1), that is, the excessively long dewaxing time required by the conventional injection moulding methods. Also, this process is advan tageous from the standpoint of thermal energy, since no high temperature heating is required, and moreoverwater is inexpensive as compared with any organic binder. Also, it can be recovered and reused if desired.

In the case ofthe ordinary injection moulding method useng an organic binder, since the deformation propertiesofa plasticized material comprising a binder and a powder material aretemperaturesensi- tive, mixture heating-and plasticizing-mechanisms must be incorporated in an injection moulding machine and the injection temperature must be strictly controlled. The deformation properties of a plasticized material employingwaterasa binder, however, are stable at around room temperature, so that plasticization and moulding can be effected by separate machines. For example, a combination of a kneader and a die forging machine can be used, so that the kneading is effected at room temperature and the plasticized material is loaded into the die ofthe die forging machine.

While the freeze injection moulding processes have a numberof advatages, as mentioned above, they have the disadvantage that only a limited number of powder materials can be moulded using water. In other words, most of the metal powders tend to oxidize when they are in contact with water, and the resulting oxides impede sintering ofthe metal powders, producing problems with respectto the strength and toughness of molding. Also, as in the case of ceramic powders, the micronization of particles not only increases the amount of adsorbed water, but also particularly results in a rapid increase inthe amount of OH- ionsadsorbed on the resulting active surface portions having large bond energies.

As a result, the finerthe micronization of the particles, the greater will be the amount of OH ions, which require high temperatures for heir removal.

The particles including strongly adsorbed OHions generally impede sintering. In the case of magnesia powder it is well known that the adsorbed OH- ions cause the growth of abnormal particles during sintering. It is also known thatsilicon nitride reacts with adsorbed water and releases ammonia, being converted into silica. Also, tungsten carbide reacts with adsorbed waterattemperatures in the region of 1200"C, releasing hydrogen and carbon monoxide.

Thus, moulding by freeze casting techniques cannot be employed with powder materials which will be modified bytheadsorption of water.

The present invention has been made to overcome the foregoing deficiencies of conventional moulding methods using freeze moulding techniques, such as freeze casting, freeze injection moulding,freeze forg- ing and the like, and it is the primary object ofthe invention to provide a moulding method for powder materials in which a metal or ceramic powder is not contaminated by a binder, and removal ofthe binder after moulding is easy.

The present invention provides a method of moulding powder materials which comprises filling a cooled mould with a mixture containing 30 to 55% by volume oftertiarybutanol,the balance comprising a metal powderandlora ceramic powder; the mixture having a first temperature above the melting point oftertiary butanol, lowering the surface temperature ofthe resulting moulding to a second temperature below the melting point oftertiary butanol whereby to solidify the tertiary butanol, and removing the resulting green moulding from the mould.

With a view to overcoming the foregoing deficiencies in the prior art, the present invention features the use as a binderoftertiary butanol, (CH3)3 COH, (2-methyl-2-propanol) in place of the water in the freeze-casting moulding process.

As a binderforfreeze moulding purposes, the tertiary butanol shows properties which are superior to those of water. The density of tertiary butanol at 25"C is 0.78 glcm3, which is only 78% ofthat of water, and its molecularweight is 74.12, which is 4.1 times that of water. Therefor, the number of mols per unit volume ofthetertiary butanol is 1.05 x 10-2 g mol/cm3, which is about one fifth that of water (5.56 x 10-2 mol/cm3). This means that if the voids between the particles ofthe starting powder are filled, the amount of gas produced during the removal of the binder is only one fifth ofthat in the case of water, and this is extremely advantageous in reducing the binderremoving time.

Also, since the melting point of tertiary butanol is 25.66"C, and close to room temperature, the freezing can be effected by the circulation of cold water, and it is not necessary to use any special freezing equipment, such as is required in the case of water. Also, while the heat of solidification of water is 79.4 cal/cm3, that oftertiary butanol is 17.1 cal/cm3, whose ratio to theformeris only about 1:4.6, and this makes the use oftertiary butanol as a binder more advantageous from the standpoint of energy consumption.

Moreover, the vapour pressure of tertiary butanol at 25"C is 5733 Pa (43mmHg) and this is higherthan that of water, 3333Pa (25mmHg). This also makes tertiary butanol advantageous over water in consideration of the ease of removal of binder, while the heat of vaporization at 250C ortertiary butanol is 118 cal/cm3 which is about one fifth that of water (583 cal/cm3).

This means that the energy required for the removal of binder is very small as compared with the case of water.

After the removal of the binder at around room temperature, a part of the tertiary butanol is adsorbed to the surface of the powder material. In this case, while water is adsorbed as OH- ions, tertiary butanol is adsorbed as alkyl groups. In contrastto OH- ions, the desorption of alkyl groups is relatively easy, and they have no detrimental effect on the sintering ofthe moulding.

Moreover, tertiary butanol is a stable single-compo nentsubstance, and there is practically no danger of it decomposing or deteriorating on vaporization, condensation and solidification, thus making it possible to recover and reuse it. This makes tertiary butanol even more advantageous than water compared with conventional organic binders.

This tertiary butanol binder is added in an amount which is mainly dependent on the particle size distribution ofthe powder material used. The binder must completely fill the voids among the particles of the powder material, and empirically it is necessary furtherto increase the amount by several volume percent.

If the amount of the binder is below 30 volume %, it is impossible to obtain suitableflow properties for moulding. If the amount exceeds 55 volume %, not only is a considerabletime required forthe removal of the binder, but also there is the danger of causing defects. Thus, the binder must be added in an amount between 30 and 55% by volume.

On the other hand, thefirsttemperature (i.e. the heating temperature of the mixture in the heating cylinder) must generally be between 26 and 40 C. The reason is that, sincethe melting point of the tertiary butanol is 25.6"C, the heating temperature must be higherthanthis,inorderto maintainthetertiary butanol in a molten condition, while heating the mixture above 400C is disadvantageous in that the vapour pressure of the tertiary butanol is increased, and the dissipation loss is excessively increased.

As regards the cooling ofthe moulding in the mould (the second temperature), the moulding is generally maintained at25 C or less, since it is necessary that at least the surface of the moulding must be maintained below the melting point, so asto solidify the moulding. Cooling the moulding to an excessively low temperature is not desirable, however, since it results in a decrease in the rate of removal of bender in the following step. Also, the simple process of watercooling is not then sufficient, thus requiring a special thermal source and therefore the mould must be cooled to adjust at least the surface temperature of th ulding to the second temperature between 5 and 25"C.

While the mould release of the moulding frozen by the tertiary butanol is good, even better mould release can be ensured by preliminarily mixing a mould release agent, for example stearic acid, as in the case of conventional powder moulding methods.

From the foregoing description it will be seen that the method of the invention employing tertiary butanol as a binderforthefreeze moulding of metal powders or ceramic powders, is an economical powder moulding method, in that the binder can be removed in a short period of time, that moulding can be effected at around room temperaturewith the resulting energy saving, that there is no contamination of the starting powder material, as in the case of freeze moulding methods using water, and thatthe tertiary butanol can be rocovered and reused.

With the present invention, the scope of application to metal and ceramic powders covers ceramic powders such as silicon nitride, silicon carbide, alumina, zirconia and 2-titanium boride, and metal powders such as Ni-Fe alloy, stainless steel, Stellite (TRADE MARK) and hard metals.

The following Examples describe in greater detail the powder moulding method in accordance with the invention.

EXAMPLE 1 Astarting powderwas prepared by mixing 92 weight % of silicon nitride powder having an average particle diameter of 0.75,am, with 6 weight % of Y203 and 2 weight % of Al203 as sintering aids, and then a mixture of 60 volume % ofthe starting powder and 40 volume % oftertiary butanol powderwas prepared.

The mixture was kneaded in a pressure-assisted kneader under a nitrogen atmosphere. The inner wall of the kneader mixing ta u was controlled at30 C by an electric heater so that the tertiary butanol was melted and the kneading was advanced. At the expiration of 12 hours afterthe start of the kneading, the heater power supply was cut off and the kneading was continued for30 minuteswhile cooling the mixture by circulating cooling water at 20 Cthrough the mixing tank and the jacket attached to the blades. During this period,thetertiary butanol was frozen so that it became the binder ofthe starting powder and the mixture was formed into small pellets. The pellets were taken out and loaded into the hopper of a screw in-line type injection moulding machine.

The outlet temperature ofthe heating cylinder ofthe injection moulding machine was set at 30"C and also cooling water at 2Otwas run around the mould.

Then, a sequence of plasticizing operations including die closing, cylinder advance, injection, dwell pressure application, die opening, moulding ejection, cylinder withdrawing, screw withdrawing and screw turning were performed.

The binder in the vicinity ofthe moulding surface was solidified and hardened, and mould release was effected easily. The moulding was loaded into a vacuum dryer, and after setting the temperature at 25the dryer was evacuated to attain the maximum degree of vacuum of 1.33Pa (10-2 Torr). At 3 hours, vacuum drying was stopped and-the moulding was removed.Then,the moulding was loaded into a vacuum pressure sintering furnace which was then maintained at 1.33 Pa (10~ 2Torr)/1200^Cfor3 hours, thereby removing the adsorbed substances. The sintering furnace was subsequently maintained at 1800 C in a nitrogen atmosphere of9.8 Kg/cm2G for3 hours and then the pressure was reduced and the moulding left to cool.By using this procedure, mouldings were produced, employing two sizes of rectangular cavity, respectively of 43.8 mmx 14.8 mm x19.1 mmand43.8mmx7.4mmx19.1 mm.

As a result, no defects due to the removal ofthe binderwere seen in eitherthe thinner mouldings of 7.4 mm thickness orthe thicker mouldings of 14.8 mm thickness, and in either instance, the resulting sintered moulding showed a theoretical density ratio of 98% and uniform contraction.

EXAMPLE2 An example ofthe freeze forging method using tertiary butanol as binderwill now be described.

Astarting powder was prepared by mixing 92 weight % of silicon nitride powder having an average particle diameter of 0.75,am,with 6 weight % of Y203 and 2 weight % of Al203 as sintering aids, and then a mixtureof60volume % ofthestarting powderand40 volume % oftertiary butanol powderwas kneaded in a pressure-assisted kneaderundera nitrogen atmosphere. The inner walk ofthe kneader was maintained at 30"C by an electric heater. As a result, the tertiary butanol was melted and the kneading was advanced.

Simultaneously with the kneading, pressure was intermittently applied to the material to be kneaded by the press cover of the kneader, effecting deaeration and compaction. After 12 hours,theplasticized kneaded material was taken out, formed in a rectangularshape, loaded into a forging mould cooled by circulating water at 20"C, subjected to pressing and moulding with delay, and then held for about3 minutes. In the moulding, the tertiary butanol in the vicinity ofthe surface was solidified and mould release was easily effected. The moulding was then loaded into a vacuum dryersothataftersettingthe temperature at 25"C, the dryer was evacuated to the maximum degree of vacuum of 1.33 Pa (1 0-2 Torr) and the tertiary butanol was removed by evaporation.

After3 hours, drying was stopped and the moulding was taken out.

The moulding was then loaded into a vacuum pressure sintering furnace, held in a vacuum of 1.33 Pa (10-2Torr) at 1000 Cfor3 hours to remove the adsorbed substances, held in a nitrogen atmosphere of 9.8 Kg/cm2G at 1800for3 hours and, after restoring the vacuum to normal pressure, the moulding was cooled by standing.After cooling, the moulding was taken out and any burrs were removed by diamond grinding.

By using the above-mentioned procedure, mouldings were produced employing two sizes or rectangu larcavity, of43.8 mmx 14.8 mmx 19.1 mm and 43.8 mmx 7.4 mm x 19.1 mm, respectively. No defects due to the removal of the binderwerefound either in the thinner or in the thicker mouldings, and in both instances the resulting sintered mouldings had a theoretical density ratio of 98% and uniform contraction.

EXAMPLE3 An example of metal powderfreezeforging using tertiarybutanol as binderwill now be described.

Amixture of 60 volume % of SUS 316 powder having a particle diameter of S to 20,am and 40 volume % oftertiary butanol was kneaded by the technique described in Example 2, in a pressure-assisted kneader under a nitrogen atmosphere at 30"C. After 12 hours, the plasticized kneaded material was taken out, formed into a rectangular shape, loaded into a forging die cooled by the circulation of cooling water at 20"C, readily subjected to pressing and moulding and held for3 hours. In the resulting moulding, the tertiary butanol in the vicinity ofthe surface was solidified and hardened and mould release was easy.This moulding was loaded into avacuum dryersothataftersetting the temperature to 250C, the dryerwas exhausted to a maximum vacuum of 1.33 Pa (10-2Torr) and the tertiary butanol was removed by evaporation.

After3 hours, the drying was stopped and the moulding ws taken out. The moulding was loaded into a vacuum sintering furnace, held in a vacuum of0.133 Pa (10-3Torr) at800 Cfor6 hours thus removing the adsorbed substances, and raised to 11 00"C and, after restoring normal pressure, the moulding was cooled by standing. After cooling, the moulding was taken out and any burrs were removed bya grinder.

By the above-described procedure, mouldings were produced in two sizes of cavity of 43.8 mm x 14.8 mmx 19.1 mm and 43.8 mm x7.4mm x 19.1 mm, respectively.

No defects due to the removal ofthe binder were seen in eitherthe thinner or in the thicker mouldings.

In both instances, the resultinsintered mouldings showed a theoretical density of 99% and uniform contraction.

EXAMPLE4 An example ofthefreexeforging of a mixed powder of metal and ceramic, using tertiary butanol as a binderwill now be described. 90 weight % oftungsten carbide having an average particle diameter of 1.5 m and 10 weight % of cobalt powder having an average particle diameter of 1.3 clam were mixed and ground in acetone in a wet ball mill (stainless steel pot loaded with WC-Co alloy balls) for48 hours and dried. A mixture containing 60 volume % ofthe resulting starting powder and 40 volume % of tertiary butanol powder was kneaded for 12 hours at 30 C under nitrogen, as in the technique of Examples 2 and 3.

Afterforming into a rectangularshape,the material was loaded into a forging die cooled by the circulation of coolingwaterof 20 C, subjectedto pressing and moulding without delay and maintained for about 3 minutes. In the resulting green moulding, the tertiary butanol in the vicinity ofthe surface was solidified and hardened and mould release was effected easily. The moulding was loaded into a vacuum dryer and dried at 25"C, under a pressure of 1.33Pa (10-2 Torr) to remove tertiary butanol by evaporation. After3 hours, the drying was stopped and the moulding was taken out, and loaded into a vacuum sintering furnace. After removing the adsorbed substances from the moulding by holding it at 6.66 Pa (5 x 10-2Torr) and 1200"C for3 hours, the moulding was maintained at 141 so and after restoring to normal pressure with nitrogen the moulding was cooled by standing. After cooling, the moulding was taken out and any burrs were removed by diamond grinding.

By using the above-mentioned procedure, rec tangular mouldings of the same two sizes as in the previous Examples were prepared. No defects due to the removal of binder were seen in either the thinner or in the thicker mouldings, and in both instances, the resulting sintered mouldings showed a theoretical density ratio of 99.5% and uniform contraction.

Claims (9)

1. A method of moulding powder materials which comprises filling a cooled mould with a mixture containing 30 to 55% by volume oftertiary butanol, the balance comprising a metal powder and/or a ceramic powder, the mixture having a first tempera- ture above the melting point oftertiary butanol, lowering the surface temperature of the resulting moulding to a second temperature below the melting point oftertiary butanol whereby to solidify the tertiary butanol, and removing the resulting green moulding from the mould.

2. A method according to claim 1, wherein the first temperature is from 26 to 40"C.

3. A method according to claim 1 or 2 wherein the second temperature is from 5 to 25"C.

4. A method according to any preceding claim, wherein said metal powder is Ni-Fe alloy, stainless steel, Stellite or a hard metal.

5. A method according to any of claims 1 to 3, wherein said ceramic powder is silicon nitride, silicon carbide, alumina, zirconia or 2-titanium boride.

6. A method according to claim 5, wherein said ceramic powder is mixed with a sintering aid.

7. A method according to any preceding claim, wherein the green moulding is held under vacuum to removethetertiary butanol and sintered at elevated temperature.

8. A method according to claim 1 and substantially as hereinbefore described with reference to any of the Examples.

9. Mouldings where obtained by a method accord ingtoanyofthepreceding claims.