Preparation of sintered zirconia body.

The invention relates to a method for producing a high-density sintered body made of zirconium dioxide by (rod) non-sprue injection molding of a zirconium dioxide mixture, followed by binder removal and firing.

Injection molding is one of the common techniques for producing sintered zirconia; see USSN 07/588 180, filed September 26, 1990, or EP 0 420 284 A2, published April 3, 1991. Sintered zirconia is generally made by mixing zirconia powder with an organic binder to form a mold composition and injecting the composition into a mold to form a green body of predetermined shape, followed by binder removal and firing.

The mold used for the injection molding of ceramic materials, including zirconia, generally has a cavity corresponding to the desired product and a sprue system, including a mold gate, a trough and a gate here the cavity. The mixture is melted in a cylinder of the injection molding machine. The molten mixture is introduced into the bar gate of the mold from a nozzle at the distal end of the cylinder and then fills the trough until it reaches the gate. After pouring, the mixture fills the cavity where it is cooled and solidified. Finally, the mold is opened and the molded mixture or green body is removed therefrom. Since the green body has a beard corresponding to the bar sprue and the trough at this time, the beard must be cut off before the desired green body can be obtained.

In the prior art, molds for use in the injection molding of ceramic mixtures are designed to have a bar gate with a large draw or taper, a thick runner and a gate with a large cross-sectional area because the ceramic mixtures have a considerably reduced fluidity reduced ability to fill very narrow areas and a considerably reduced green strength or Punch resistance as plastics.

In particular, the ingot and pouring trough, which do not form part of the final molding, were necessary and indispensable in the prior art injection molding of ceramic materials. From the standpoint of the efficient use of a source mix, the bar sprue and pouring troughs (so-called "cold pouring trough") parts have been recovered for reuse as the source mix. However, this recycling process had the disadvantage of increased contamination and poorer flow of the mixture. Furthermore, the process required additional steps of cutting the molding from the trough part at the sprue and removing the remaining sprue from the molded article.

Included in the injection molding of plastics is a special molding technique that uses a mold that does not have a sprue and a runner, and is known as "molding without a sprue and runner" or "hot runner molds"; see Japanese Patent Application Kokai (JP-A-) No. 30143/1985 and 206613/1985. Although the molding process without the sprue and the trough provides for the efficient use of a source mixture, its use is limited to those plastic mixtures which show good flow behavior. No example is found where the technique is successfully applied to ceramic or other blends that have increased filler loading.

We have already found out that a zirconium dioxide powder is suitable for injection molding if it fulfills an optimal balance between particle shape, average particle size and specific surface area (USSN 07/588 180 or EP 0 420 284 A2). Continuing research, we have found that sprueless injection molding, which was limited to free flowing plastic compounds, can be applied to a zirconia mixture under certain conditions.

It is therefore an object of the invention to use a sprueless injection molding technique in the manufacture of a sintered zirconia body. Another goal is to produce a sintered zirconia body with high density in a relatively simple manner and in high yields.

According to the first aspect of the present invention, there is provided a method of injection molding a zirconia mixture in a mold defining a cavity and having a launder and a sprue located at the end of the launder in fluid communication with the cavity. The zirconia mixture consists of an organic binder and a zirconia powder which contains 2 to 10 mol% of Y & sub2; O & sub3; contains and has a specific surface area measured by the BET method of up to 12 m² / g and a product of average particle size times the specific surface area of ​​up to 3 μm m² / g. In a first or injection step, the molten zirconia mixture is passed through the trough and injected through the gate into the cavity while the trough is heated sufficiently to hold the zirconia mixture in the molten state. In this case the gate can also be heated if necessary. In a subsequent solidification step in which the launder is kept hot enough to keep the zirconia mixture melted, the gate is cooled sufficiently to solidify the zirconia mixture and the cavity is cold enough to allow the zirconia mixture to solidify. Then the zirconia mixture solidifies in the cavity and the gate, while the zirconia mixture remains melted in the launder. Finally, the solidified part at the gate and the solidified part in the cavity are separated by rupture, whereby the solidified part in the cavity is obtained as a sprueless molded article.

According to a second aspect, the present invention is directed to a method for producing a sintered zirconia body, comprising the steps of (1) injection molding a zirconia mixture into a mold according to the molding method of the first aspect, whereby a sprueless green body is obtained, (2) removing the Binder from the green body and (3) burning the green body to a sintered body.

The single figure is a schematic cross-sectional area of ​​a four cavity mold used in the injection molding process according to the present invention.

In general, the flow of mixtures loaded with ceramic particles improves as the particle shape approaches the true spherical shape. On the other hand, particles get caught and interfere with one another to a greater extent while reducing the fluidity if they have more angular elevations and are more agglomerated. Provided that zirconia particles are actually spherical, their product of average particle size times specific surface area is about 1 µm m² / g. In other words, if the product of average particle size times specific surface area approximates 1 m 2 / g, the particles approximate the true spherical shape more closely, show less agglomeration and thus show a more uniform flow behavior. It should be noted that the average particle size is determined by a centrifugation sedimentation method.

The zirconia powder used herein should have an average particle size times its specific surface area of ​​up to 3 µm m² / g, preferably 1.0 to 2.7 µm m² / g. The powder should have a specific surface area of ​​up to 12 m² / g measured according to the BET method. A zirconia powder with a specific surface area of ​​more than 12 m² / g requires an undesirably large amount of organic binder, resulting in a mixture which undergoes considerable deformation due to shrinkage after firing. The preferred specific BET surface area is 5 to 10 m 2 / g. Furthermore, a smaller average particle size of, say, up to 0.4 µm is preferred. Because the sprue of the sprue-less injection mold has a considerably smaller cross-sectional area than conventional mold gates, larger particles can cause blockage near the sprue, making the flow of the mixture unstable, thereby reducing molding yield or reducing the wear on the sprue and in it "Chip" heater is reduced. However, there is a certain lower limit to the average particle size because as the average particle size decreases, the BET specific surface area increases and the amount of binder added increases. For this reason, a particle size of 0.2 to 0.35 µm is more preferred.

The zirconia powder used herein should contain 2 to 10 mole%, preferably 2 to 8 mole%, of yttria (Y 2 O 3) in mixed crystal form. If the mixed crystal yttria content is less than 2 mol%, problems arise including reduction in strength and uneven thermal expansion. When the yttria in solid solution form is more than 10 mol%, problems arise including reduced strength and a decrease in solid electrolyte performance associated with a decrease in electrical conductivity at high temperatures.

These zirconia powders can be prepared, for example, by the methods of JP-B 43286/1986, JP-A 185821/1988 and JP-A 97134/1986 or US Pat. No. 4,873,064.

The organic binders used herein can be selected from those that are commonly used in the injection molding of ceramic materials.

Examples include the following:

(a) hydrocarbon waxes such as paraffin wax;

(b) plasticizers including phthalates such as dioctyl phthalate, dibutyl phthalate, diethyl phthalate etc. and adipates such as dioctyl adipate, dibutyl adipate etc .;

(c) Deflocculants or liquefying agents, such as aliphatic amines:

(d) dispersants such as cationic, anionic and nonionic surfactants;

(e) oils such as mineral oil, tung oil and coconut oil;

(f) low molecular weight compounds such as fatty acids, fatty acid esters and alcohols; and

(g) thermoplastic resins such as polyacrylic resin, polystyrene, polypropylene, polyethylene, polyamide, and ethylene-vinyl acetate copolymers.

Among them, polyacrylic binders, polyamide binders, wax binders, polyolefin binders, polyvinyl acetate binders and mixtures thereof are preferred.

The organic binder is preferably added to the zirconium dioxide powder in such a way that the binder fills the mixture about 45 to 57% by volume, in particular about 47 to 55% by volume. When the amount of the organic binder exceeds 57% by volume, there are disadvantages in terms of the long time for removing the binder, the severe deformation due to shrinkage after firing, and a low density of the sintered body. If the amount of the organic binder added is less than 47% by volume, the mixture flows less smoothly, causing clogging in the vicinity of a sprueless bar gate having a limited cross-sectional area, which prevents the mold cavity from being fully loaded with stable flow. In addition, the sprue wears out, which leads to a shorter service life of the mold.

The zirconia powder and the organic binder can be kneaded by conventional grinding methods, preferably a pressure kneader at 90 to 180 ° C for a period of 30 minutes to 5 hours. If desired, the zirconia mixture can contain any other additives, e.g. Contain silane coupling agents, sintering aids such as clay, silica, alumina and titanium dioxide, and colorants such as transition metal compounds and rare earth metal compounds.

For injection molding purposes, the zirconia mixture should flow evenly. A convenient index for the flow of a zirconia mixture is the apparent melt viscosity of the mixture at a particular shear rate, as determined by a capillary rheometer or the like. The preferred zirconia mixture should have an apparent melt viscosity of up to 1,000 Pa s (10,000 poise), more preferably up to 500 Pa s (5,000 poise), particularly up to 340 Pa (3,400 poise), at a shear rate of 1,000 / s and 160 ° C have. The lower limit of the viscosity is not particularly limited, although it is usually 50 Pa s (500 poise). Although the apparent melt viscosity decreases with temperature, the temperature itself is limited by the heat resistance of the organic binder used. Then the applicable temperature range of the mixture is generally between 100 and 200 ° C when current molding conditions are taken into account. Likewise, the apparent melt viscosity decreases as the amount of binder added increases. The amount of binder added should preferably be between 45 and 57% by volume of the mixture, since more binder, as already mentioned, could lead to quality problems.

The zirconia mixture is pelletized in a conventional manner and injection molded using a castless mold for molding. A variety of molds with a special gate shape and a unique heating mechanism are commercially available for molding plastics, and any of these can be used herein.

The sprue is most often a point sprue. The gate diameter is generally determined according to the void volume, although it may also be limited by the desired product configuration. The ratio of the sprue diameter (mm) to the cavity volume (cm 3) is preferably between 0.05 and 15, more preferably between 0.1 and 10, in particular between 0.2 and 5. If this ratio is less than 0.05, the cavity would not be filled completely or only over a longer period of time. A ratio greater than 15 means that the gate is larger with respect to the product (or cavity), which places some restrictions on the location of the gate or requires machining to cut gates. The gate diameter is usually up to 3 mm, and preferably up to at least 0.5 mm, since too small a cross-sectional area of ​​the gate would limit the filling of the cavity. It is recommended that the sprue diameter is between 0.5 and 3 mm, in particular between 0.6 and 2 mm.

Referring to the single figure, a mold 10 is explained in combination with a nozzle 1 that extends from a melting cylinder (not shown). The mold 10 includes a distributor block 2, a first cavity plate 3, a second cavity plate 4 and a core plate 5 in a side-by-side position. Manifold block 2 defines a manifold that is connected to the nozzle pass and branches into four passes at 15. The cavity plates 3 and 4 define the pouring channels or pouring channels 12 located therein, which communicate with the distributor lines by means of the blocking fragments 11 and end in the point gates 9 with a reduced cross-sectional area. The second cavity plate 4 and the core plate 5 nestle against one another and define the cavities 13 between them. Each pouring channel 12 is in liquid communication with the cavity 13 through the point gate 9. A heater 6 is mounted in the manifold block 2 for heating the manifold. A jacket heater 7 with a “chip” heater 8 at the downstream end is used to heat the launder in launder 12. Although the gutter 12 is branched through the manifold and connected to four cavities 13 in the illustrated embodiment, the manifold is not always necessary and is either a combination of a single nozzle, a single gutter and a single cavity, or a combination of a single nozzle , more than two troughs and more than two cavities acceptable.

In the injection step, a molten zirconia mixture 14 is introduced from the nozzle 1 into the manifold 2 and then into the trough 12 and then injected into the cavity 13 through the gate 9. At this stage, the runner 12 and the gate 9 are heated so hot that the zirconia mixture is kept molten by the actuation of the heaters 6, 7 and 8. At this stage the cavity 13 is usually cold. The injection conditions can be selected appropriately, although it is preferred to have an injection pressure of 9.8 to 98.0 MPa (100 to 1,000 kg / cm²), more preferably 9.8 to 83.3 MPa (100 to 850 kg / cm²) ), use z u. The spraying speed can be 2 to 20 cm / s, although the spraying speed can vary depending on the product shape.

Next, the fragment heater 8 is turned off so that the gate is sufficiently cooled to allow the zirconia mixture to solidify. The cavity is cold enough to allow the zirconia mixture to solidify. When heaters 2 and 7 are on, trough 12 is kept hot enough to keep the zirconia mixture melted. Then the zirconia mixture solidifies in the cavity 13 and the sprue 9, while the zirconia mixture remains melted in the trough 12.

After the solidification step, the core plate 5 is dismantled, and the solidified part at the point gate and the solidified part in the cavity are mechanically separated by rupture, whereby the solidified part in the cavity is obtained as a sprueless green body. Particularly when the green body is removed, tensile and shear stresses are applied to the point gate in order to break the solidified gate part there. In the case of plastic resin mixtures, cutting off or cutting off the sprue is quite difficult, and there often occurs the phenomenon that part of the remaining part is stretched in a thread-pulling manner. Then the product must subsequently be machined on the cutting edge side, which represents an additional step. However, in the case of a zirconia mixture, it can be easily squeezed off at the point gate, resulting in a smooth, flat cutting surface and thereby eliminating the additional machining step, which contributes to a substantial improvement in manufacturing efficiency. Strictly speaking, the cut area on the gate contains fine irregularities as a result of breakage, and if such irregularities are undesirable, the gate or fracture limes should be removed outside the final contours or on an unimportant surface.

The green body obtained by injection molding in a mold in a sprueless manner does not require an additional step in the green step. The elimination of an additional processing step means that the process does not have the risk of damaging the molded article by breaking and tearing during processing or being destroyed by inadvertent dropping. This is an advantage over conventional injection molding to produce a molded article with a sprue and gutter part present at the same time.

The green body is then removed from the binder and finally fired. Binder removal is typically carried out by heating the green body from ambient temperature to about 500 ° C at a rate of 5 to 150 ° C / h to remove the organic binder by pyrolysis. The atmosphere in which the green body is heated to burn off the binder is usually air, although an inert atmosphere such as a helium and argon as well as nitrogen atmosphere can also be used, both at atmospheric pressure and under pressure. The burn-off time, depending on the type of binder, is generally in the range of 5 to 100 hours, preferably 10 to 50 hours.

Finally, the body is fired into a zirconia sintered body, preferably at a temperature of 1,300 to 1,600 ° C, more preferably 1,350 to 1,500 ° C, 0.2 to 10 hours, more preferably 1 to 4 hours. The combustion atmosphere can be air, although an inert atmosphere, such as an He, Ar or nitrogen atmosphere or the like, can also be used.

The resulting sintered body made of zirconium dioxide has a high sintered density, corresponding to a relative density of 99% or higher, in particular 99.5% or higher, although the exact density depends on the firing temperature. The density is determined by Archimedes' method. Sintered zirconia bodies containing yttrium oxide in mixed crystal form vary in their theoretical density in accordance with the amount of the yttrium oxide mixed crystal and the crystalline phase. Table 1 shows the amount of yttria mixed crystal, the theoretical density and a density corresponding to a specific gravity of 99%. Table 1 Yttria mixed crystal and theoretical density Crystalline phase Theoretical density * (g / cm³) Relative density 99% (g / cm³) tetragonal cubic * Density of 2 wt% HfO₂ containing ZrO₂.

If a zirconia mixture is found to be less flowable during injection molding so that the cavity could be filled unevenly with zirconia particles, deformation can often occur during firing, resulting in extremely low manufacturing yields. The use of the zirconia mixture of the present invention, which flows well, eliminates the deformation during firing, which ensures a very high manufacturing yield.

The sintered body can be used for the end product without further processing, although processing or finishing could be carried out if desired.

The zirconia mixture injection molding method and the zirconia body sintered body manufacturing method according to the present invention have the following advantages. Injection molding in a sprue-free manner eliminates the waste-generating consumption of zirconia mixture, the recycling of such waste and the additional processing of the green body at the gate, all of which contribute to a leap in manufacturing efficiency. The zirconia mixture used offers the advantages of a reduced time for binder removal and a high dimensional accuracy due to the minimal shrinkage after firing, since it has an excellent fluidity and a relatively low binder content. The resulting sintered zirconia body has features of low contamination, a high sintered density corresponding to at least 99% of theory, white transparency, surface smoothness and gloss.

EXAMPLES

example 1

A zirconia powder with an average particle size of 0.25 µm determined by the centrifugation sedimentation method, a BET specific surface area of ​​9.1 m² / g and a product of average particle size times specific surface area of ​​2.28 µm m² / g which contained 3 mol% of ytrium oxide in mixed crystal form (produced by the process described in JP-A 185821/1988), An organic binder in the form of Seramo IB-27 polyacrylic binder (trade name of a mixture of acrylic resin, ethylene-vinyl acetate copolymer, paraffin and dibutyl phthalate, available from Daiichi Kogyo Seiyakti KK) was added so that the binder made up 52.5% of the volume , The mixture was ground in a pressure kneader at 150 ° C for 1 hour and pelletized by a pelletizer, whereby an injection molding mixture was obtained. The mixture was measured for flow using an Instron capillary rheometer (Model 3211 from Instron Co.) under the following conditions: capillary diameter: 1.27 mm (0.05 inches), length: 12.7 mm (0.5 inches) and 160 ° C. It had an apparent melt viscosity of 200 Pa s (2,000 poise) at a shear rate of 1,000 / s.

The mold used is a sprue-free injection mold with four box-shaped cavities, with the dimensions 25 x 22 mm, 20 mm depth and 1 mm thickness (volume 1.9 cm³), and a point sprue of 0.8 mm in diameter (trade name Spear System, available from Seiki KK). The ratio of sprue diameter to cavity volume was 0.42 (mm / cm³). Using this mold, the mixture was injection molded into a green body under the following conditions.

molding conditions

Cylinder temperature: 160ºC

Mold temperature: 30ºC

Heating temperature: 160ºC

Heating cycle: 10 s on / 27 s cycle

Injection pressure: 76.44 MPa (780 kg / cm²)

Spray speed: 8 cm / s

When the green body was removed from the mold, it was essentially flat at the gate and did not require any surface treatment. The green body was removed from the binder and fired under the following conditions.

Binder removal conditions

Temperature range: 20 to 500ºC

Heating rate: 10 to 20 ° C / h

Binder removal time: 24 h

firing conditions

Air: 1,450 ° C, 2h

The resulting box-shaped sintered body had no cracks and deformations and looked white, shiny and highly transparent. It had a linear shrinkage of 21.9% and a high density of 6.08 g / cm³ or a relative density of 99.6%. The molding yield was 100%.

Example 2

A mixture was obtained as in Example 1, except that the binder was 50.7% by volume. The mixture was measured for flow as in Example 1 to give an apparent melt viscosity of 300 Pa s (3,000 poise) at a shear rate of 1,000 / s. Using a similar mold, the mixture was injection molded under the following conditions.

molding conditions

Cylinder temperature: 170ºC

Mold temperature: 35ºC

Heating temperature: 160ºC

Heating cycle: 10 s on / 47 s cycle

Injection pressure: 76.44 MPa (780 kg / cm²)

Spray speed: 12 cm / s

The resulting green body was as good as in Example 1. It was removed from the binder and fired as in Example 1.

The sintered body had no cracks and deformations and appeared white, shiny and highly transparent. It had a linear shrinkage of 21.0% and a high density of 6.08 g / cm³ or a relative density of 99.6%. The molding yield was 100%.

Example 3

The following binder was added to 100 parts by weight of zirconia powder as in Example 1. Ingredients parts by weight of acrylic resin (BR 105 from Mitsubishi Rayon KK) ethylene-vinyl acetate copolymer (Ultracene 633 from Toso KK) paraffin wax (pure analytical) dibutyl phthalate (pure analytical)

The mixture in which the binder was 52.5% by volume was kneaded in a pressure kneader to form a mixture as in Example 1. The mixture was measured for flow as in Example 1, whereby an apparent melt viscosity of 200 Pa s (2,000 poise) was found at a shear rate of 1,000 / s. Thereafter, the mixture was injection molded using a similar mold under the following conditions.

molding conditions

Cylinder temperature: 160ºC

Mold temperature: 30ºC

Heating temperature: 160ºC

Heating cycle: 10 s on / 35 s cycle

Injection pressure: 76.44 MPa (780 kg / cm²)

Spray speed: 8 cm / s

The resulting green body was as good as in Example 1. It was removed from the binder and fired as in Example 1.

The sintered body had no cracks and deformations and appeared white, shiny and highly transparent. It had a linear shrinkage of 22.0% and a high density of 6.09 g / cm³ or a relative density of 99.8%. The molding yield was 100%.

Example 4

The following binder was added to 100 parts by weight of the same zirconia powder used in Example 1. Components parts by weight acrylic resin (BR 105 from Mitsubishi Rayon KK) ethylene-vinyl acetate copolymer (Ultracene 633 from Toso KK) paraffin wax (analytically pure) dibutyl phthalate stearic acid

The mixture in which the binder was 45.9% by volume was kneaded in a pressure kneader at 140 ° C for 45 minutes to form a mixture. The mixture was measured for flow as in Example 1, whereby an apparent melt viscosity of 150 Pa s (1,500 poise) was found at a shear rate of 1,000 / s. Thereafter, the mixture was injection molded using a mold as in Example 1 under the following conditions.

molding conditions

Cylinder temperature: 140ºC

Mold temperature: 40ºC

Heating temperature: 150ºC

Heating cycle: 10 s on / 47 s cycle

Spray pressure: 20.58 MPa (210 kg / cm²)

Spray speed: 12 cm / s

The resulting green body was as good as in Example 1. It was removed from the binder and fired as in Example 1.

The sintered body had no cracks and deformations and appeared white, shiny and highly transparent. It had a linear shrinkage of 18.6% and a high density of 6.08 g / cm³ or a relative density of 99.6%. The molding yield was 100%.

Example 5

A zirconia powder with an average particle size of 0.34 µm determined by the centrifugation sedimentation method, a BET specific surface area of ​​7.2 m² / g and a product of average particle size times specific surface area of ​​2.45 µm m² / g containing 3 mol% of ytrrium oxide in mixed crystal form was mixed with an acrylic binder as in Example 1 so that the binder was 50.7% by volume. The mixture was kneaded in a pressure kneader to form a mixture as in Example 1. The mixture was measured for flow as in Example 1, which found an apparent melt viscosity of 250 Pa s (2500 poise) at a shear rate of 1000 / s. Thereafter, the mixture was injection molded using a similar mold under the following conditions.

molding conditions

Cylinder temperature: 160ºC

Mold temperature: 30ºC

Heating temperature: 160ºC

Heating cycle: 10 s on / 35 s cycle

Injection pressure: 76.44 MPa (780 kg / cm²)

Spray speed: 8 cm / s

The resulting green body was as good as in Example 1. It was removed from the binder and fired as in Example 1.

The sintered body had no cracks and deformations and appeared white, shiny and highly transparent. It had a linear shrinkage of 20.8% and a high density of 6.08 g / cm³ or a relative density of 99.6%. The molding yield was 100%.

Example 6

A zirconia powder with an average particle size of 0.52 µm determined by the centrifugation sedimentation method, a BET specific surface area of ​​5.7 m² / g and a product of average particle size times specific surface area of ​​2.96 µm m² / g containing 3 mol% of ytrrium oxide in mixed crystal form was mixed with an acrylic binder as in Example 1 so that the binder was 48.8% by volume. The mixture was kneaded in a pressure kneader to form a mixture as in Example 1. The mixture was measured for flow as in Example 1, which found an apparent melt viscosity of 400 Pa s (4,000 poise) at a shear rate of 1,000 / s. Subsequently, the mixture was injection molded using a similar mold under the following conditions.

The resulting green body was as good as in Example 1. It was removed from the binder and fired as in Example 1.

The sintered body had no cracks and deformations and appeared white, shiny and highly transparent. It had a linear shrinkage of 19.8% and a high density of 6.06 g / cm³ or a relative density of 99.3%. The molding yield was 95%.

Comparative Example 1

A zirconia powder with an average particle size of 0.24 µm determined by the centrifugation sedimentation method, a BET specific surface area of ​​17.4 m² / g and a product of average particle size times specific surface area of ​​4.18 µm m² / g containing 3 mol% of ytrrium oxide in mixed crystal form was mixed with an acrylic binder as in Example 1 so that the binder made up 57.8% by volume. The mixture was kneaded in a pressure kneader to form a mixture as in Example 1. The mixture was measured for flow as in Example 1, which found an apparent melt viscosity of 350 Pa s (3,500 poise) at a shear rate of 1,000 / s. Thereafter, the mixture was injection molded using a similar mold under the following conditions.

molding conditions

Cylinder temperature: 170ºC

Mold temperature: 35ºC

Heating temperature: 170ºC

Heating cycle: 10 s on / 77 s cycle

Injection pressure: 102.9 MPa (1050 kg / cm²)

Spray speed: 12 cm / s

In comparison to Example 1, the injection pressure was increased and the mold was worn. The resulting green body was as good as in Example 1. It was removed from the binder and fired as in Example 1.

The sintered body was deformed. It had a linear shrinkage of 24.9% and a high density of 6.04 g / cm³ or a relative density of 99.0%. The molding yield was 70%, which was lower than in all examples.

Comparative Example 2

A zirconia powder as in Comparative Example 1 was mixed with an acrylic binder as in Example 1 so that the binder was 56.1% by volume. The mixture was kneaded in a pressure kneader to form a mixture as in Example 1. The mixture was measured for flow as in Example 1 to find an apparent melt viscosity of 450 Pa s (4,500 poise) at a shear rate of 1,000 / s. Thereafter, the mixture was injection molded using a similar mold under the following conditions.

molding conditions

Cylinder temperature: 170ºC

Mold temperature: 35ºC

Heating temperature: 170ºC

Heating cycle: 10 s on / 67 s cycle

Spray pressure: 127.4 MPa (1300 kg / cm²)

Spray speed: 12 cm / s

In comparison to Example 1, the injection pressure was increased and the mold was worn. The resulting green body was removed from the binder and fired as in Example 1.

The sintered body was deformed. It had a linear shrinkage of 24.3% and a high density of 6.04 g / cm³ or a relative density of 99.0%. The molding yield was 40%, which was significantly lower than in all examples.

Comparative Example 3

A zirconia powder with an average particle size of 0.55 µm, determined by the centrifugation sedimentation method, a BET specific surface area of ​​6.0 m² / g and a product of average particle size times specific surface area of ​​3.30 µm m² / g containing 3 mol% of ytrrium oxide in mixed crystal form was mixed with an acrylic binder as in Example 1 so that the binder was 52.5% by volume. The mixture was kneaded in a pressure kneader to form a mixture as in Example 1. The blend was measured for flow as in Example 1, which found an apparent melt viscosity of 200 Pa s (2,000 poise) at a shear rate of 1,000 / s. Thereafter, the mixture was injection molded using a similar mold under the following conditions.

molding conditions

Cylinder temperature: 160ºC

Mold temperature: 30ºC

Heating temperature: 160ºC

Heating cycle: 10 s on / 55 s cycle

Injection pressure: 50.96 MPa (520 kg / cm²)

Spray speed: 12 cm / s

In comparison to Example 1, the shape was badly worn. The resulting green body was removed from the binder and fired as in Example 1.

The sintered body was badly deformed.

It had a linear shrinkage of 21.6% and a high density of 6.00 g / cm³ or a relative density of 98.3%. The molding yield was 80%, which was lower than in all examples.

Comparative Example 4

A zirconia powder as in Comparative Example 1 was mixed with an acrylic binder as in Example 1 so that the binder was 50.7% by volume. The mixture was kneaded in a pressure kneader to form a mixture as in Example 1. The mixture was measured for flow as in Example 1, whereby an apparent melt viscosity of 250 Pa s (2500 poise) was found at a shear rate of 1000 / s. Thereafter, the mixture was injection molded using a similar mold under the following conditions.

molding conditions

Cylinder temperature: 160ºC

Mold temperature: 30ºC

Heating temperature: 160ºC

Heating cycle: 10 s on / 65 s cycle

Injection pressure: 76.44 MPa (780 kg / cm²)

Spray speed: 12 cm / s

In comparison to Example 1, the shape was badly worn. The resulting green body was removed from the binder and fired as in Example 1.

The sintered body was deformed. It had a linear shrinkage of 20.7% and a high density of 5.99 g / cm³ or a specific gravity of 98.2%. The molding yield was 50%, which was significantly lower than in all examples.

The parameters and results of these examples and comparative examples are listed in Table 2. Table 2 Example Comparative Example Zirconia Mixture Zirconia Powder Average Particle Size (µm) Spec. BET surface area (m² / g) product (um.m² / g) yttrium dioxide content (mol%) binder type quantity (vol%) apparent viscosity [Pa.s (poise) at shear rate 1000 / s] descriptions of the shape void volume ( cm³) Point gate diameter (mm) Point gate diameter / cavity volume (mm / cm³) Forming conditions Spray pressure [MPa (kg / cm²)] Spray speed (cm / s) Results Burning shrinkage (%) Density (g / cm³) Relative density (%) Yield ( %)