U.S. patent application number 16/452809 was filed with the patent office on 2019-10-24 for method for manufacturing a dental prosthesis.
This patent application is currently assigned to DEGUDENT GMBH. The applicant listed for this patent is DEGUDENT GMBH. Invention is credited to Stefan FECHER, Jorg HACHENBERG, Elmar HOCK, Rudi STEINKE, Lothar VOLKL, Markus VOLLMANN, Irmgard WISSEL, Gerhard ZELLMANN.
Application Number | 20190321147 16/452809 |
Document ID | / |
Family ID | 43823088 |
Filed Date | 2019-10-24 |
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United States Patent
Application |
20190321147 |
Kind Code |
A1 |
HACHENBERG; Jorg ; et
al. |
October 24, 2019 |
METHOD FOR MANUFACTURING A DENTAL PROSTHESIS
Abstract
A method for manufacturing a shaped body, comprising creating a
mixture of a metal powder and binding agent, compacting the mixture
to form a green compact, heating the green compact to a debinding
start temperature T.sub.1, debinding the green compact by
controlled heating of the green compact from start temperature
T.sub.1 to end temperature T.sub.2 at a heat-up rate R.sub.1,
presintering the debindered green compact to the presinter end
temperature T.sub.VS at a heat-up rate R.sub.HVS, cooling the green
compact from the presinter end temperature T.sub.VS at a cool-down
rate R.sub.KVS, whereby at least the heat-up rate R.sub.HVS, the
presinter end temperature T.sub.VS, and the cool-down rate
R.sub.KVS are tuned relative to each other in such a way that the
presintered green compact forming a blank has a surface porosity of
16% to 22% after presintering, and machining and sintering of the
blank to form the shaped body.
Inventors: |
HACHENBERG; Jorg;
(Aschaffenburg, DE) ; STEINKE; Rudi; (Hanau,
DE) ; VOLLMANN; Markus; (Gelnhausen, DE) ;
WISSEL; Irmgard; (Freigericht, DE) ; ZELLMANN;
Gerhard; (Linsengericht, DE) ; HOCK; Elmar;
(Mombris, DE) ; FECHER; Stefan; (Johannesberg,
DE) ; VOLKL; Lothar; (Goldbach, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DEGUDENT GMBH |
Hanau |
|
DE |
|
|
Assignee: |
DEGUDENT GMBH
Hanau
DE
|
Family ID: |
43823088 |
Appl. No.: |
16/452809 |
Filed: |
June 26, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15178697 |
Jun 10, 2016 |
|
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|
16452809 |
|
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13292132 |
Nov 9, 2011 |
9393088 |
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15178697 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22F 3/24 20130101; C22C
19/05 20130101; B22F 2003/248 20130101; C22C 19/053 20130101; B22F
3/11 20130101; B22F 3/02 20130101; A61C 13/0022 20130101; A61K 6/84
20200101; B22F 2003/247 20130101; B22F 3/1021 20130101; B22F
2301/15 20130101; C22C 19/055 20130101; B22F 3/16 20130101; C22C
19/07 20130101 |
International
Class: |
A61C 13/00 20060101
A61C013/00; C22C 19/07 20060101 C22C019/07; C22C 19/05 20060101
C22C019/05; B22F 3/24 20060101 B22F003/24; B22F 3/10 20060101
B22F003/10; A61K 6/04 20060101 A61K006/04; B22F 3/16 20060101
B22F003/16; B22F 3/11 20060101 B22F003/11; B22F 3/02 20060101
B22F003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 9, 2010 |
EP |
10 19 0512.3 |
Claims
1. A method of manufacturing a dental prosthesis, or a part
thereof, the method comprising: preparing a green compact from a
cobalt-chromium alloy consisting of: cobalt: 50 to 70% by weight;
chromium: 20 to 35% by weight; molybdenum: 0 to 10% by weight;
tungsten: 0 to 20% by weight; other elements: less than 10% by
weight; wherein a sum total of the elements adds up to 100% by
weight; heating the green compact; debinding the heated green
compact; presintering the debindered green compact to form a blank
having a surface porosity of from 16% to 22%; processing the blank;
and sintering the processed blank to a final density to form the
dental prosthesis, or the part thereof.
2. The method according to claim 1, wherein the blank is processed
by machining.
3. The method according to claim 1, wherein the blank is processed
using computer-aided manufacturing technology.
Description
[0001] This application is a continuation of U.S. application Ser.
No. 15/178,697, filed Jun. 10, 2016, which is a divisional
application of U.S. application Ser. No. 13/292,132, filed Nov. 9,
2011, now U.S. Pat. No. 9,393,088, which claims priority to EP
application number 10 19 0512.3, filed Nov. 9, 2010, the contents
of which are incorporated herein by reference.
[0002] The invention relates to a method for the manufacture of a
shaped body, in particular of a dental prosthesis or part thereof,
by way of mixing of a metal powder.
[0003] The invention also relates to a green compact for the
manufacture of a dental prosthesis or of a part thereof.
[0004] In recent times CAD/CAM technology (Computer Aided Design,
Computer Aided Manufacturing) is more frequently being used in the
manufacture of dental prostheses such as crowns or bridges, whereby
CAD/CAM is applied in particular in the area of ceramics. EP-B-1
067 880 should be referenced as an example for this.
[0005] DE-C-199 38 144 describes a method for the manufacture of
dental prostheses, whereby a ceramic-based presintered moulded
blank is machined using a milling process and subsequently is
sintered to full density.
[0006] WO-2009/120749 discloses the use of a CAD/CAM milling
process in the manufacture of a dental prosthesis. For this
purpose, at first a metal powder is mixed with a binding agent,
whereupon a moulded blank is produced by way of metal powder
injection moulding. A milling process is used to create a shaped
body from this, which corresponds to the dental prosthesis to be
produced taking into account the contraction occurring during
sintering.
[0007] In accordance with EP-A-1 764 062, a shaped body produced
from a dental alloy consists of a dental alloy powder that is
sintered to full density by hot-isostatic pressing.
[0008] In accordance with DE-A-103 42 231 it is known to
manufacture a shaped body using powder-metallurgical processes,
whereby during the machining the body is open-pored and has not
been sintered to full density. Only after the final shape has been
created, the open pores of the shaped body are filled with a second
alloy in a further processing step by means of an infiltration
process. The use of two alloys is a disadvantage.
[0009] US-A-2005/0023717 discloses a method for the manufacture of
dental restorations using a free-forming process, in particular a
rapid-prototyping process. Favoured materials to be used are
powders of non-oxidizing metals. Preferred use is given to noble
metals.
[0010] The reference Rodrigues et al.: "Powder metallurgical
Processing of Co-28% Cr-6% Mo for dental implants: Physical,
cheanical and electrochemical properties" Powder Technology, 2006
(2011), 233-238, describes a method for the manufacture of
restoration elements. In this, a biocompatible
cobalt-chromium-molybdenum alloy is mixed with a moulding agent, is
heated to a temperature suitable for burning out the moulding
agent, and is subsequently sintered to full density.
[0011] U.S. Pat. No. 4,996,022 discloses a method for the
manufacture of a sintered body. As starting material one uses a
powdered metal such as iron or nickel.
[0012] An iron-powder mixture that contains up to 1% organic
binding agent is used in the manufacture of a sintered moulded part
in accordance with AT-A-505 698. A final sintering is performed
after presintering and cooling.
[0013] The objective of the present invention is to further develop
a method and a presintered green body of the above-mentioned type
so as to provide a shaped body, in particular a dental prosthesis
or part thereof, that can be manufactured with very low tolerances
and allows problem-free wet or dry machining, whereby in particular
it should be possible to use a ceramic material for veneering
purposes. Disadvantages known in the art should be avoided.
[0014] Another objective is to provide a green compact that can be
machined with high accuracy in a simple manner, in order to
subsequently be able to use it to manufacture a highly precise
shaped part, in particular a dental prosthesis or a part
thereof.
[0015] In accordance with the invention, this objective is met
chiefly though a method for the manufacture of a shaped body, in
particular a dental prosthesis or a part thereof, that is
characterized by the following process steps: [0016] Producing a
mixture of a metal powder and a binding agent, [0017] Compacting
the mixture to form a green compact, [0018] Heating the green
compact from room temperature to a debinding start temperature
T.sub.1, [0019] Debinding the green compact by controlled heating
of the green compact from the debinding start temperature T.sub.1
to a debinding end temperature T.sub.2 at a heat-up rate R.sub.1 in
a manner that rules out damage to the green compact, [0020]
Presintering the debindered green compact, whereby the green
compact is heated to a presinter end temperature T.sub.VS at a
heat-up rate R.sub.HVS, [0021] Cooling the green compact from the
presinter end temperature T.sub.VS at a cool-down rate R.sub.KVS,
whereby at least the heat-up rate R.sub.HVS, the presinter end
temperature T.sub.VS, and the cool-down rate R.sub.KVS are tuned
relative to each other in such a way that the presintered green
compact forming a blank possesses a surface porosity between 16%
and 22% after presintering, [0022] Material-removing machining of
the blank, and [0023] Sintering the machined blank to final density
to form the shaped body.
[0024] Surface porosity here denotes the fraction of the surface
that is not filled with material if viewed in a metallographic
section.
[0025] In particular it is intended to use as metal powder a dental
metal alloy in form of a cobalt-chromium or nickel-chromium
alloy.
[0026] For a cobalt-chromium alloy the composition should be chosen
as follows:
Cobalt: 50% to 70% by weight Chromium: 20% to 35% by weight
Molybdenum: 0% to 10% by weight Tungsten: 0% to 20% by weight Other
elements: less than 10% by weight, whereby the sum total is 100% by
weight.
[0027] Also an option is the use of a nickel-chromium alloy of the
following composition:
Nickel: 50% to 70% by weight Chromium: 20% to 35% by weight
Molybdenum: 0% to 10% by weight Tungsten: 0% to 20% by weight Other
elements: less than 10% by weight, with a sum total of 100% by
weight.
[0028] Other elements that may be considered are in particular
manganese, silicon, and nickel in case of the cobalt-chromium
alloy, cobalt in case of the nickel-chromium alloy, and beryllium,
cadmium, lead, iron, aluminum, titanium, carbon, nitrogen, oxygen,
sulphur and other elements with a weight fraction of less than
1%.
[0029] In particular it is intended that the mixture compacted into
the green compact possess a surface porosity, corresponding to the
volume porosity, of between 16% and 27%, preferably between 18% and
22%. This porosity is created by the areas of the green compact
that are filled with air or binding agent in between the metal
powder particles.
[0030] It is also intended that after heating of the green compact
to the presinter end temperature T.sub.VS, the green compact be
held at the presinter end temperature T.sub.VS for the duration of
a holding time t.sub.VS and subsequently be cooled at a cool-down
rate R.sub.KVS.
[0031] In particular it is suggested that the green compact be
cooled at the cool-down rate R.sub.KVS to a temperature T.sub.3,
whereby in particular T.sub.3.ltoreq.T.sub.2, 450.degree.
C..ltoreq.T.sub.3.ltoreq.650.degree. C., and preferably T.sub.3 is
approximately 600.degree. C.
[0032] In this, debinding and presintering should be performed in
the absence of oxygen, in particular under an inert gas atmosphere,
particularly preferred under an argon atmosphere. Other options are
a reducing atmosphere or vacuum.
[0033] For the debinding, the green compact preferably is heated to
a debinding start temperature T.sub.1 with 350.degree.
C..ltoreq.T.sub.1.ltoreq.550.degree. C. After reaching a
temperature T.sub.1, in particular reaching the temperature
T.sub.1.apprxeq.450.degree. C., a slow heating takes place, whereby
the heat-up rate during the debinding process should not exceed 20
K/min. A preferred range for the heat-up rate is 1 K/min to 5
K/min. In particular it is intended that in the region above
500.degree. C., in particular from above 550.degree. C. to the
debinding end temperature T.sub.2, with 550.degree.
C..ltoreq.T.sub.2.ltoreq.650.degree. C., in particular
T.sub.2.apprxeq.600.degree. C., one choose a heat-up rate between 1
K/min and 5 K/min. After reaching the debinding end temperature
T.sub.2 the green compact should be held at this temperature for a
duration t.sub.2 with 1 min.ltoreq.t.sub.2.ltoreq.20 min. However,
this is not obligatory and mainly dependent on the chosen heat-up
rate.
[0034] Irrespective of the preferred parameters, which were
provided as examples and shall not limit the scope of protection of
the invention, the heating must be performed in such a manner that
the debinding takes place in a controlled manner, so that the green
compact is not damaged and rendered unserviceable. This controlled
heating, which is essential to prevent damage to the green compact,
can be performed by an average expert without any problems after
carrying out several simple trials.
[0035] After debinding, heating to the presinter end temperature
T.sub.VS takes place, whereby in principle the heating rate
R.sub.HVS may be chosen freely.
[0036] In order to obtain the desired surface porosity of the
presintered green compact of between 16% and 22%, in particular
between 18% and 20%, the invention intends that the presinter end
temperature T.sub.VS, the heat-up rate R.sub.HVS, possibly the
holding time t.sub.VS at the presinter end temperature T.sub.VS,
and the cool-down rate R.sub.KVS be tuned relative to each other.
In case of a very slow heating to the presinter end temperature
T.sub.VS, e.g. using a heat-up rate between 1 K/min and 10 K/min,
it is not required that the debindered green compact be held at the
presinter end temperature for a time period t.sub.VS.
[0037] The cool-down rate may also be used to influence the holding
time t.sub.VS, in particular to the extreme degree that cool-down
commences immediately upon reaching the presinter end temperature
T.sub.VS.
[0038] The relative tuning of the parameters for the purpose of
obtaining the desired surface or volume porosity of between 16% and
22%, in particular between 18% and 20%, can be performed taking
into account the details provided as examples in the following.
[0039] If for example the presinter end temperature T.sub.VS is in
a range between 650.degree. C. and 750.degree. C., the heat-up rate
R.sub.HVS and/or the cool-down rate R.sub.KVS should be between 1
K/min and 200 K/min, preferably between 1 K/min and 50 K/min, and
particularly preferred between 1 K/min and 20 K/min, whereby after
reaching the presinter end temperature T.sub.VS, a holding time
t.sub.VS of between 10 min and 200 min, in particular between 30
min and 100 min, particularly preferred between 50 min and 80 min,
should be adhered to.
[0040] If the presinter end temperature T.sub.VS is between
750.degree. C. and 850.degree. C., the heat-up rate R.sub.HVS
and/or the cool-down rate R.sub.KVS should be between 5 K/min and
200 K/min, in particular between 5 K/min and 20 K/min. After
reaching the presinter end temperature T.sub.VS, one should
preferably choose a holding time t.sub.VS between 5 min and 60 min,
in particular between 10 min and 30 min.
[0041] However, it is also possible to set the presinter end
temperature T.sub.VS in the range between 850.degree. C. and
950.degree. C., for example. In this case, the heat-up rate
R.sub.HVS and the cool-down rate R.sub.KVS should be in the range
between 15 K/min and 200 K/min, preferably between 15 K/min and 50
K/min. Preferred holding times t.sub.VS at this presinter end
temperature T.sub.VS are in the range between 5 min and 30 min, in
particular between 10 min and 20 min.
[0042] In order to achieve the desired surface porosity or volume
porosity of the presintered blank, the presinter end temperature
may also be in the range between 950.degree. C. and 1100.degree. C.
In this case, the heat-up rate R.sub.HVS and the cool-down rate
R.sub.KVS should be between 30 K/min and 200 K/min, preferably
between 30 K/min and 100 K/min. For the above parameters one
preferably sets a holding time t.sub.VS 5
min.ltoreq.t.sub.VS.ltoreq.20 min.
[0043] For a cobalt-chromium alloy of the above-described
composition one preferably chooses a presinter end temperature
between 650.degree. C. and 750.degree. C. and a holding time
between 50 min and 70 min at the presinter end temperature
T.sub.VS, whereby the heat-up rates are in the range between 10
K/min and 30 K/min.
[0044] In other words: various heat-up rates, presinter
temperatures, holding times, and cool-down times may be chosen,
which must be tuned relative to each other in such a way that they
yield a surface porosity of the presintered green compact, which
may also be referred to as blank, of between 16% and 22%.
[0045] In particular, the parameters should be tuned in such a way
that they yield a surface porosity between 18% and 20%.
[0046] In particular it is intended that prior to debinding the
green compact possess a porosity that is not more than 5% higher
than the surface porosity after the presintering. In particular,
this difference should not exceed 2%.
[0047] Consequently, the invention is also characterized by the
fact that the compacted green compact that is used in the
manufacture of the shaped body possesses a porosity between 16% and
27%, in particular between 18% and 22%.
[0048] If the porosity of the compacted green compact prior to
debinding is always above the porosity of the presintered green
compact, i.e. the blank, then the porosity may also be equal
without leaving the scope of the invention.
[0049] A correlation exists between the heat-up rate, the presinter
end temperature, the holding time, and the cool-down rate. For
example for lower heat-up and cool-down rates one should select
shorter holding times. The reverse also applies. All rates and
holding times are decisively determined by the choice of presinter
end temperature. Holding times shorter than 5 min are less
suitable, since in particular for blanks of larger sizes a
homogeneous heat penetration and presintering can not be ensured
for shorter holding times. Holding times in excess of 60 min are
also detrimental, since a longer dwell time favours the undesired
formation of an oxidation layer.
[0050] Shaped bodies with a corresponding surface porosity possess
excellent machining characteristics to allow production of in
particular a dental prosthesis or part thereof. Highly precise
machining can be performed with low tool wear.
[0051] After cooling the blank to room temperature,
material-removing machining is performed to create the shaped body,
whereby the processes of milling and grinding shall be named.
[0052] The final step performed is the one of sintering to full
density.
[0053] It is in particular intended to use as metal powder a
nickel-chromium-based or cobalt-chromium-based metal powder, in
particular a dental alloy powder in the form of a cobalt-chromium
alloy, preferably a cobalt-chromium-molybdenum alloy.
[0054] Preferred binding agents are wax- and/or cellulose-based
binding agents.
[0055] In particular it is intended that for the purpose of
achieving a surface porosity between 16% and 22%, in particular
between 18% and 20%, the green compact be held at the presinter end
temperature T.sub.VS for a time period t.sub.VS in accordance with
the relation
t/2<t.sub.VS<2t (1).
t is computed using the equation:
t = t 0 ln ( c 0 c ) exp ( T 0 T VS ) ( 2 ) ##EQU00001## [0056]
with [0057] c=desired surface porosity fraction of the green
compact after presintering, [0058] c.sub.0=surface porosity
fraction of the green compact after debinding, [0059]
t.sub.0=material constant in min, [0060] T.sub.0=material constant
in Kelvin, [0061] T.sub.vs=presinter end temperature at the holding
time t.sub.vs with 650.degree.
C..ltoreq.T.sub.vs.ltoreq.1100.degree. C.
[0062] c.sub.0, i.e. the surface porosity fraction of the green
compact after debinding, can be determined by interpolation of
measurement results, whereby on principle the following relation
applies: c.sub.0-c.ltoreq.5%, in particular c.sub.0-c<2%.
Preferably, one should specify as additional condition:
c<c.sub.0.
[0063] The material constant t.sub.0 can also be determined by
interpolation of measurement results. When using
cobalt-chromium-based metal powder or equivalent materials one
finds t.sub.0=0.0125 min.
[0064] When using cobalt-chromium-based or equivalent metal powder
the corresponding material constant will be T.sub.0=11000 K.
[0065] It should be noted that T.sub.VS is to be entered in the
above equation in Kelvin rather than in degree Celsius.
[0066] If the presintering in accordance with this relation is
performed by holding the presinter end temperature for the duration
of a holding time t.sub.VS, then cool-down and heat-up rates should
be chosen to be so short that the bulk of the presintering will
take place during the holding time. In particular, during heating
with a nearly constant heat-up rate R.sub.HVS and cooling with a
nearly constant cool-down rate R.sub.KVS, the heat-up period
between 650.degree. C. and the presinter end temperature T.sub.VS
and the cooling period between the presinter end temperature
T.sub.VS and 650.degree. C. should satisfy the following
relation:
T VS - 650 .degree. C . R HVS + T VS - 650 .degree. C . R KVS <
2 t , ( 3 ) ##EQU00002##
whereby temperatures should be specified in degree Celsius. An
additional condition that must be satisfied is that the presinter
end temperature T.sub.VS and the difference between c.sub.0 and c
are tuned relative to each other so that negative heat-up and
cool-down rates are ruled out. Moreover, the maximum presinter end
temperature T.sub.VS should not exceed 1100.degree. C.
[0067] Consequently, relation (3) represents a condition that must
be satisfied by the specified parameters in order to be able to use
a cobalt-chromium-based or equivalent metal powder to produce a
presintered blank that possesses a surface porosity between 16% and
22%.
[0068] Surprisingly it has been realized that a presintered
blank--irrespective of the existing surface porosity--not only can
be machined with the desired accuracy, but that in addition after
the final sintering an absolutely void-free veneering is possible
irrespective of the residual surface porosity. The reason for this
most likely is that the presinter steps and specified parameters
according to the invention result in a residual surface porosity
that after complete sintering does not form a connected system but
exists in isolated occurrences. This does not only provide the
option of a non-porous ceramic veneering, as already mentioned, but
also ensures the necessary corrosion resistance. It could further
be determined that the necessary dimensional accuracy can be
achieved after machining and the subsequent dense-sintering, i.e.
that the contraction is uniform and shape-preserving.
[0069] As a further development of the invention, it is intended
that the mixture of alloy powder and binding agent be subjected to
axial or isostatic pressing at a pressure p with 100
MPa.ltoreq.p.ltoreq.1,000 MPa, in particular with 200
MPa.ltoreq.p.ltoreq.600 Mpa.
[0070] It is further preferred and intended that the green compact
or the debindered green compact and the presintered green compact
be heated under an inert-gas or forming-gas atmosphere or in
vacuum. These measures ensure that only a very thin oxide layer
accumulates on the surface, which can be easily removed, e.g. by
polishing, without the polishing after the complete-sintering being
affected by the residual surface porosity.
[0071] The blank produced in this manner can subsequently be
machined using in particular wet- or dry-working tools, whereby in
particular with CAM technology the blank can be used to created any
desired number of shaped bodies with corresponding dimensions, in
particular of dental prostheses such as crowns or bridges, in
particular by milling or grinding. Profiling also represents a
viable option.
[0072] The invention is further characterized by a green compact
intended for the manufacture of a dental prosthesis or part
thereof, whereby the green compact is a presintered green compact
made from a dental metal alloy and possesses a surface porosity of
between 16% and 22%. It is particularly intended that the dental
metal alloy be a nickel-chromium or cobalt-chromium alloy.
[0073] In particular it is intended that as dental alloy metal
powder one use a mixture of 50% to 70% by weight of cobalt, 20% to
35% by weight of chromium, 0% to 10% of weight by molybdenum, 0% to
20% by weight of tungsten, less than 10% by weight of one or
several other elements, in particular one or several elements from
the group comprising manganese, silicon, nickel, beryllium,
cadmium, lead, iron, aluminum, titanium, oxygen, nitrogen, and
sulphur, with possible use of other elements with a weight fraction
of less than 1% by weight, whereby the sum total adds up to
100%.
[0074] The invention is further characterized by the fact that one
uses as dental alloy metal powder a mixture of 50% to 70% by weight
of nickel, 20% to 35% by weight of chromium, 0% to 10% by weight of
molybdenum, 0% to 20% by weight of tungsten, and less than 10% by
weight of one or several other elements, in particular one or
several elements from the group manganese, silicon, cobalt,
beryllium, cadmium, lead, iron, aluminum, titanium, oxygen,
nitrogen, sulphur, and possible other elements with a weight
fraction of less than 1% by weight, whereby the sum total is 100%
by weight.
[0075] Further details, advantages, and features are not only found
in the claims and the characteristic features specified therein,
but also in the following description of preferred embodiment
examples.
[0076] In the manufacture of a dental prosthesis we used a metal
alloy with the composition [0077] 26 to 30% by weight of Cr, [0078]
5 to 7% by weight of Mo, [0079] in total between 0.01 and 1.5% by
weight of at least one of [0080] the elements Mn, Si, Fe, C, Ni,
[0081] remainder Co (61.5% to 68.99% by weight) whereby the sum
total is 100% by weight. To produce the powder, we at first
produced, melted, and atomized a metal alloy. The mean grain size
was in the region between 5 .mu.m and 50 .mu.m. Subsequently a
wax-based binding agent was added, specifically approximately 2% by
weight of the metal powder. The mixture produced in this manner was
subjected to axial pressing to produce green compacts with a
disk-shaped geometry. The diameter was approximately 10 cm and the
thickness approximately 1 cm. Different dimensions are
feasible.
[0082] This was followed by debinding. For this, the green compacts
were at first heated to 450.degree. C. using any desired heat-up
rate. Heat-up above 450.degree. C. occurred slowly, whereby we
chose 3 K/min as preferred heat-up rate. After reaching the
temperature T.sub.2, which was approximately 600.degree. C., the
green compacts were held there for a time t.sub.1 of approximately
10 min. These parameters are in principle sufficient to ensure
elimination of the binding agent.
[0083] A green compact that had been subjected to debinding was
subsequently presintered to create a CoCrMo blank. For this
purpose, the green compact--in accordance with an alternative
method--was rapidly heated to a temperature in the region of
approximately 800.degree. C. (heat-up rate in the region of 90
K/min) and was held at this temperature for a time period of
approximately 20 min. This was followed by cooling, which initially
took place at a constant rate and then at a lower rate.
[0084] This method satisfied the relations and conditions of
equations (1), (2), and (3):
t = t 0 ln ( c 0 c ) exp ( T 0 T VS ) = 0.0125 min ln ( 0.20 0.19 )
exp ( 11000 K 1073 K ) .apprxeq. 18 min ##EQU00003##
and thus t/2<t.sub.VS<2t, in this case 9
min<t.sub.VS<36 min and
T VS - 650 .degree. C . R HVS + T VS - 650 .degree. C . R KVS <
2 t ##EQU00004## here : ##EQU00004.2## 800 .degree. C . - 650
.degree. C . 90 K / min + 800 .degree. C . - 650 .degree. C . 90 K
/ min .apprxeq. 3.3 min < 36 min . ##EQU00004.3##
[0085] Micrographs of blanks produced in this manner showed an open
surface porosity in a range between 16% and 22%, with a large
number between 18% and 20%. These blanks were easy to work with,
without any risk of high tool wear, which is known to have a
detrimental effect on the precision of the machining.
[0086] The surface porosity allowed an uncomplicated processing
using a CAM machine. For this, the blank was mounted in the CAM
machine using a holding device. This was followed by
material-removing machining, whereby regions of the blank were
machined using a wet system and a dry system. When milling dry, the
dust generated during the machining was removed by means of a
class-H vacuum cleaner. The wet processes used were grinding
processes. In particular when using the wet machining, no
disadvantages were encountered.
[0087] The bodies machined from the blank possessed dimensions that
took into account the contraction occurring during sintering to
final density. After contraction we determined that the contraction
took place uniformly and in a shape-preserving manner subsequently,
the surface of the shaped body was polished or a ceramic veneer was
attached in the usual manner, which could be achieved in an
absolutely void-free manner, without the existing residual surface
porosity causing any problems.
[0088] Even though the invention was explained using the example of
dental prostheses, this shall not place any limitations on the
invention.
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