PROCESS FOR THE PREPARATION OF AN ARTICLE BY POWDER INJECTION MOLDING

Abstract

A process for the preparation of an article by powder injection molding, the process including the steps of providing a feedstock containing a powder of a ceramic, a metal or a metal alloy dispersed in a binder, heating the feedstock, and injecting the heated feedstock into a mold cavity of a mold, where it cools and hardens to the configuration of the mold cavity. The process is characterized in that step c) is carried out under the application of vibrational energy onto the feedstock.

Claims

1. A process for the preparation of an article by powder injection molding, the process comprising the steps of a) providing a feedstock containing a powder of a ceramic, a metal or a metal alloy dispersed in a binder, b) heating the feedstock, and c) injecting the heated feedstock into a mold cavity of a mold, where it cools and hardens to the configuration of the mold cavity, wherein step c) is carried out under the application of vibrational energy onto the feedstock.

2. The process according to claim 1, wherein the mold is subjected to vibration at a frequency corresponding to the resonant frequency of the mold.

3. The process according to claim 1, wherein the mold is subjected to vibration at a frequency above 10 kHz.

4. The process according to claim 1, wherein the mold is subjected to ultrasonic vibration.

5. The process according to claim 1, wherein heating of the feedstock according to step b) is at least partially carried out in a barrel, from which the heated feedstock is transferred through a transfer channel to the mold cavity of the mold, the barrel and/or transfer channel being subjected to vibration at a frequency corresponding to the resonant frequency of the barrel and/or the transfer channel.

6. The process according to claim 1, wherein the mold is kept at a temperature in a range from room temperature to 60° C.

7. The process according to claim 1, wherein the feedstock comprises zirconia powder.

8. The process according to claim 1, comprising the further step of d) subjecting the article obtained to a machining process.

9. The process according to claim 1, wherein the article is a dental implant or a dental implant abutment.

10. The process according to claim 8, wherein the article is a dental implant comprising an internal thread and an external thread, the internal thread being formed in step c) and the external thread being formed in step d).

11. A powder injection molding apparatus for carrying out the process according to claim 1 comprising A) an injection unit containing a heatable barrel, a nozzle and conveying means arranged in the barrel for conveying feedstock in direction from a feed zone of the barrel towards the nozzle, and B) a mold enclosing a mold cavity fluidically connected with the injection unit via a transfer channel, wherein the apparatus further comprises C) a vibrational energy generator and a transducer for transducing the vibrational energy received from the vibrational energy generator to the mold.

12. The powder injection molding apparatus according to claim 11, wherein between the transducer and the mold the apparatus further comprises an amplitude transformer for transforming the wave amplitude of the vibrational energy received from the transducer and transferring it to the mold.

13. The powder injection molding apparatus according to claim 12, wherein the transducer or the amplitude transformer is in direct contact with the mold.

14. A process comprising preparing a dental article using the powder injection molding apparatus according to claim 11.

Description

EXAMPLES

[0093] The present invention is further illustrated by way of the following examples together with the attached figures, of which

[0094] FIG. 1 shows schematically a powder injection molding apparatus where the mold has a bi-bloc configuration according to the present invention;

[0095] FIG. 2 shows an article obtainable by the process of the present invention using an apparatus as shown in FIG. 1;

[0096] FIG. 3 shows schematically a subassembly of a further embodiment of the powder injection molding apparatus, which subassembly comprises the transducer, the clamping unit and the mold in the separated state; and

[0097] FIG. 4 shows a cross section along the line IV-IV of the subassembly of FIG. 3 in the assembled state of the mold.

[0098] As shown in FIG. 1, the powder injection molding apparatus comprises a mold 10 containing two mold parts, namely a first mold part 11a and a second mold part lib, clamped together by a clamping unit 12 and enclosing a mold cavity 14.

[0099] The first mold part 11a has a first contact surface facing the second mold part in the assembled state and the second mold part has a second contact surface facing the first mold part in the assembled state. The first and the second contact surface form in the assembled state of the mold a parting surface 13 between the first mold part 11a and the second mold part 11b.

[0100] The clamping unit 12 designed to retain the first mold part 11a and the second mold part 11b in contact with each other under the effect of a clamping force. The clamping unit 12 comprises a fixed part 12b and a movable part 12b, wherein the movable part 12a can be moved relative to the fixed part 12b along the longitudinal axis L.

[0101] The powder injection molding apparatus also comprises a guiding element not shown in FIG. 1, which is designed to bring the first mold part 11a and the second mold part 11b in contact with each other or to separate them from each other. The guiding element drives the first mold part 11a and the second mold part 11b in a direction perpendicular to the parting surface, which direction defines a longitudinal axis L of the mold.

[0102] The apparatus further comprises an injection unit 16 containing a barrel 18 extending along the longitudinal axis L and arranged on the side of the fixed part 12b opposed to the mold, a hopper 20 for feeding the feedstock into a feed zone 22 of the barrel, a nozzle 24 and a screw (not shown) arranged axially in the barrel and designed for conveying the feedstock in direction from the feed zone 22 towards the nozzle 24, thereby passing a compression zone and a metering zone (not shown) of the barrel. For heating the feedstock on its path towards the nozzle 24, heating elements 26 encasing the barrel 18 are provided.

[0103] The nozzle 24 opens into a transfer channel 28 arranged in the mold 10 and leading into the mold cavity 14.

[0104] The apparatus further comprises a vibrational energy generator 30 in the form of an ultrasound-generator 300 as well as a transducer 32 for transducing the vibrational energy received from the ultrasound-generator 300 to the mold 10. Between the transducer 32 and the mold 10, an amplitude transformer (or “booster”) 34 is arranged, designed for amplifying the wave amplitude of the vibrational energy received from the transducer 32. In the embodiment shown in FIG. 1 the transducer 32 is arranged to transmit the vibrational energy in a direction essentially parallel to the parting surface.

[0105] As indicated in FIG. 1 with dashed lines, the mold 10 acts as a resonator (or sonotrode) 36, vibrating with standing waves at its resonant frequency and thereby applying vibrational energy to the feedstock. Hence, no additional sonotrode is required within the structure of the apparatus shown in FIG. 1.

[0106] The subassembly, including the transducer 32, the clamping unit 12 and the mold 10, shown in FIG. 3 and FIG. 4 is part of a further embodiment of the powder injection molding apparatus, wherein the mold 10 has a bi-bloc configuration and the transducer 32 is aligned with the longitudinal axis of the mold 10.

[0107] The same reference numbers are used in FIG. 3 and FIG. 4 for parts with the same effect as for the embodiment of FIG. 1. Further, since the embodiment of FIG. 3 and FIG. 4 is structured similarly to the embodiment of FIG. 1, only the differences are described below. It is noted that the subassembly of FIG. 3 is represented in separated state of the mold 10, whereas FIG. 4 represents the same subassembly in the assembled state of the mold 10.

[0108] In the present embodiment, the transducer 32 is connected along the longitudinal axis to the first mold part 11a by way of a spring wave 33 and is arranged to transmit the vibrational energy in a direction essentially perpendicular to the parting surface 13.

[0109] The first mold part 11a and the second mold part 11b are each made of massive, essentially cylindrical bloc extending along the longitudinal axis L and form a planar parting surface 13. The mold cavity 14 is formed at the parting surface 13 and is fluidically connected to the hopper 20 (not shown) for feeding the feedstock by way of the transfer channel 28 extending radially with respect to the longitudinal axis L.

[0110] The first mold part 11a and the second mold part 11b comprise four cylindrical first retaining holes in the form of blind holes 42 and four cylindrical second retaining holes in the form of blind holes 44, respectively, each extending essentially perpendicularly to the parting surface 13 and arranged opposite to each other respective to the parting surface 13 as illustrated in FIG. 4. The axis of the four first blind holes 42 are arranged at the corner of a square centered on the longitudinal axis. The four second blind holes 44 are arranged similarly to the four first blind holes 42.

[0111] Further, the clamping unit 12 comprises four cylindrical first pins 46 fixed by way of screws 54 to the movable part 12a and four cylindrical second pins 48 fixed by way of screws 54 to the fixed part 12b. The four first pins 46 and the four second pins 48 extend essentially perpendicularly to the parting surface 13. Further, the axis of the four first pins 46 are also arranged at the corner of a square centered on the longitudinal axis and the four second pins 48 are arranged similarly to the four first pins 46.

[0112] The diameter and the length of the four first pins 46 and of the four second pins 48 is chosen such that they can be inserted in the four first blind holes 42 and in the four second blind holes 44, respectively, and cooperate in the assembled state of the mold with a retaining portion in the form of a bottom 50 of the first blind holes and with a retaining portion in the form of a bottom 52 of the second blind holes to retain the first mold part 11a and the second mold part 11b in contact with each other.

[0113] Using a CIM apparatus as schematically illustrated in FIG. 1, a ceramic part made of yttria-stabilized zirconia was prepared as follows:

[0114] A feedstock of yttria-stabilized zirconia powder dispersed in a binder (Tosoh PXA 233PH) was provided and filled into the mold cavity of the injection molding apparatus.

[0115] Prior to injection, the mold had been subjected to a pre-drying at a drying temperature in a range from 40° to 60° C. for about 6 hours. For injection, the temperature of the mold was set to about 30° C.

[0116] The temperature of the feedstock increased from its path from the feed zone to the nozzle, according to Table 1 below:

TABLE-US-00001 TABLE 1 Temperature profile of feedstock Feed zone 110° C. Compression zone 150° C. Metering zone 170°-190° C. Nozzle 180°-200° C.

[0117] Injection was carried out at an injection speed of 10 ccm/s and an injection pressure of 800 bar, followed by a holding period of 20 seconds at 800 bar. During the injection and the holding period, vibrational energy was applied to the mold, the frequency of the vibration being 20.98 kHz at a starting phase and 21.94 kHz at a subsequent working phase.

[0118] The resulting green body was then debinded in air, according to the temperature scheme shown in Table 2:

TABLE-US-00002 TABLE 2 Temperature scheme for debinding Temperature rise from room temperature to 150° C.: 1.5 hours Keeping temperature at 150° C. 0.5 hour Temperature rise from 150° C. to 400° C.: 25 hours Temperature rise from 400° C. to 450° C. 1 hour Keeping temperature at 450° C. 2 hours Temperature decrease from 450° C. to room temperature Natural cooling

[0119] Finally, the debinded brown body was then sintered according to the temperature scheme given in Table 3:

TABLE-US-00003 TABLE 3 Temperature scheme for sintering Temperature rise from room temperature to 800° C.: 8 hours Keeping temperature at 800° C. 1 hour Temperature rise from 800° C. to 1000° C.: 2 hours Temperature rise from 1000° C. to 1450° C. 9 hours Keeping temperature at 1450° C. 2 hours Temperature decrease from 1450° C. to room temperature Natural cooling

[0120] In a first series of tests, the effect of subjecting the mold to ultrasonic vibration on the bend strength of a cylindrical molded article was examined. To this end, cylindrical articles were produced by injecting different commercial feedstocks (Catamold TZP-A, PXA 211PH, PXA 233PH, TCP 0036) into a radial bi-bloc mold, but under different process parameters as given in Table 4:

TABLE-US-00004 TABLE 4 Process parameters of first test series Use of ultrasonic vibration Sample during Holding Bend No. Feedstock injection parameters strength 1.1 Catamold yes 800 bar for 997 ± 72 MPa TZP-A 30 seconds 1.2 Catamold no 800 bar for 794 ± 152 MPa TZP-A 30 seconds 1.3 PXA yes 1000 bar for 1199 ± 92 MPa 211PH 30 seconds 1.4 PXA no 1000 bar for 943 ± 147 MPa 211PH 30 seconds 1.5 PXA yes 600 bar for 1102 ± 118 MPa 233PH 20 seconds 1.6 PXA no 600 bar for 918 ± 111 MPa 233PH 20 seconds 1.7 TCP yes 1000 bar for 958 ± 117 MPa 0036 30 seconds 1.8 TCP no 1000 bar for 726 ± 137 MPa 0036 30 seconds

[0121] Thus, for all feedstocks given in Table 4, a bend strength increase was determined. Specifically, the highest increase (of 24%) has been determined for feedstock TCP 0036, whereas the highest absolute bend strength was determined for PXA 211PH.

[0122] In a further series of tests, the impact of the holding pressure on the molded cylindrical samples based on feedstock TCP 0036 has been analysed, leading to the results shown in Table 5.

TABLE-US-00005 TABLE 5 Analysis of impact of holding pressure Use of ultrasonic Sample vibration during Holding No. injection parameters Bend strength 2.1 yes, for a duration 1000 bar for 958 ± 117 MPa of 30 seconds 30 seconds 2.2 no 1000 bar for 726 ± 137 MPa 30 seconds 2.3 yes, for a duration 1200 bar for 1004 ± 162 MPa of 30 seconds 30 seconds 2.4 no 1200 bar for 795 ± 71 MPa 30 seconds

[0123] In a still further series of tests, a molded article as shown in FIG. 4 has been prepared. To this end, feedstock PXA 233PH has been injected into a radial mono-bloc mold under different injection and vibration parameters, as given in Table 6.

TABLE-US-00006 TABLE 6 Process parameters for molding article of more complex geometry Use of ultrasonic Sample vibration during Holding No. injection parameters Bend strength 3.1 yes, for a duration 400 bar for 776 ± 127 MPa of 10 seconds 10 seconds 3.2 no 400 bar for 586 ± 146 MPa 10 seconds 3.3 yes, for a duration 800 bar for 769 ± 142 MPa of 10 seconds 10 seconds 3.4 no 800 bar for 568 ± 133 MPa 10 seconds

[0124] By the process described above, the molded article shown in FIG. 2 was prepared using an apparatus as schematically depicted in FIG. 1 featuring a bi-bloc mold of a respective inner geometry. The specific molded article 38 had the shape of a dental article comprising an inner thread 40 (for connecting an abutment), but which in the portion to be inserted into bone did not exhibit an outer thread for a primary fixation of the implant, said portion being thus cylindrical.

[0125] Analysis of the bend strength of these samples confirmed the results given above. Specifically, a bend strength improvement of 32% was determined for the samples subjected to a holding pressure of 400 bar and of 35% for the samples subjected to a holding pressure of 800 bar.

[0126] The examples thus show that also for complex geometries such as the one shown in FIG. 4 having an inner thread, an improved bend strength is achieved, proving that the present invention can be applied to the preparation of a dental implant.

[0127] A still further bend strength improvement has been achieved by the additional use of an amplitude transformer (“booster”), by which the wave amplitude of the vibrational energy received from the transducer was amplified. The respective results are shown in Table 7 below.

TABLE-US-00007 TABLE 7 Impact of amplitude transformation on bend strength improvement Use of ultrasonic Sample vibration during Holding No. injection parameters Bend strength 4.1 no 800 bar for 640 ± 64 MPa 10 seconds 4.2 10 seconds 800 bar for 856 ± 79 MPa 10 seconds 4.3 10 seconds by 800 bar for 940 ± 107 MPa use of a booster 10 seconds

[0128] As shown in Table 7, a bend strength improvement of about 34% was achieved by applying ultrasonic vibration onto the mold, and the improvement was further increased to 47% by the additional use of a booster.

LIST OF REFERENCE NUMERALS

[0129] 10 mold [0130] 11a, b first and second mold parts, respectively [0131] 12 clamping unit [0132] 12a, b movable and fixed parts, respectively [0133] 13 parting surface [0134] 14 mold cavity [0135] 16 injection unit [0136] 18 barrel [0137] 20 hopper [0138] 22 feed zone [0139] 24 nozzle [0140] 26 heating elements [0141] 28 transfer channel [0142] 30; 300 vibrational energy generator; ultrasound-generator [0143] 32 transducer [0144] 33 spring wave [0145] 34 amplitude transformer [0146] 36 resonator [0147] 38 molded article [0148] 40 inner thread [0149] 42 first blind holes, first retaining holes [0150] 44 second blind holes, second retaining holes [0151] 46 first pin [0152] 48 second pin [0153] 50 bottom of the first blind holes, retaining portion of the first retaining holes [0154] 52 bottom of the second blind holes, retaining portion of the second retaining holes [0155] 54 screw [0156] L longitudinal axis