CA1323178C

CA1323178C - Method of debinding for injection molded objects

Info

Publication number: CA1323178C
Application number: CA000608577A
Authority: CA
Inventors: Masakazu Matsushita; Yoshisato Kiyota; Hiroshi Ohtsubo; Junichi Ohta
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1989-01-20
Filing date: 1989-08-17
Publication date: 1993-10-19
Anticipated expiration: 2010-10-19
Also published as: US5380476A; AU612280B2; KR900011533A; KR930002522B1; JPH02194105A; EP0379777B1; AU4006389A; DE68916778T2; EP0379777A1; JPH0647684B2; DE68916778D1

Abstract

ABSTRACT OF THE DISCLOSURE
A debinding method for use in a process for producing a sintered object, by effecting a debinding pre-treatment by heating the injection-molded object in an atmosphere of reduced pressure at a temperature at which the vapor pressure of the component of the organic binder having the highest vapor pressure does not exceed the pressure of the atmosphere, thereby removing the organic binder in the amount of at least about 18 wt%; and effecting a further debinding heat treatment in which the injection-molded object is heated to and maintained at a temperature higher than the temperature in the debinding pre-treatment.

Description

1 32~ 1 78 BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention broadly relates to a method of producing a sintered material and more particularly, to a 05 method of removing binder components from objects formed by injection molding of powder mater.ials such as metal powders, ceramics and cermets.

2. Description of the Related Art Injection molding of a sintered product containing binder components is a well-known technique suitable for use in production of objects having complicated configurations from powder materials such as metal powders, ceramics and cermets. Since organic binders are needed in the procedure, the objects that are injection molded from such bound powder material essentially require debinding, i.e., removal of binder components, and various debinding methods have been proposed for this purpose.
For instance, Japanese Patent Publication No. 61-48563 discloses a debinding method in which an injection-molded object containing binder components is placed on a binder absorbing body and is heated so that the binder components elute in the liquid phase in the order of their melting points, i.e., such that the binder component having the lowest melting point is extracted first, while an inert gas is 9upplied to the injection-molded object and the binder .

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absorbing body so as to promote evaporation of the extracted binder components in thie liquid phase.
This method basically relies upon elution of the binder components in the liquid phase, so that the obtained 05 debound object inevitably has deflects in its surface contacting the binder absorption material. When the rate of elution of the binder component is small, the strength of the injection-molded object is undesirably impaired by the molten binder component remaining in the in~ection-molded object, often resulting in a defect such as droop.
United States Patent No. 4.404,166 discloses adebinding method in which a non-saturated, chemically inert atmosphere is blown over the injection-molded object to cause the atmosphere at the surface of the object to be turbulent and unsaturated to remove a predetermined amount of binder components therefrom. Unfortunately, however, this method also fails to provide a fundamental solution to the problem of generation of defects. The generation of defects is also encountered with other known techniques.

Accordingly, an object of the present invention is to provide a debinding method for injection-molded objects, which method is capable of effecting debinding without impairi~g dimen~3ional precision of the product while 1 3231 7~

eliminating generation of defects, thereby overcomlng the problems of the prior art.
Another object of the invention is to provide a method for making injection-molded objects from a mixture 05 containing a binder by removing all or part oE the binder without significantly distorting the shape of the object.
Other objects and advantages of the invention will appear in further detail hereinafter.
SUMMARY OF THE INVENTION
The present invention is based upon an intense study conducted by the present inventors on the mechanism of generation of defects in the course of producing sintered products or objects formed from fine powder materials by injection molding.
To this end, there is provided a debinding method for use in a process for producing a sintered object. The sintered object is prepared by mixing together a material powder and an organic binder (comprising plastici2er components and binder components); subjecting the resulting 20 mixture to injection molding so as to form an injection- -molded object, removing part or all of the organic binder from the injection-molded object so as to obtain a debound object; and sintering the debound object in a heated atmosphere so as to form a sintered ob~ect.

According to this invention, the debinding method includes the steps of (a) effecti~g a debinding pre-treatment step by heating the inj~ection-molded object in a reduced pressure atmosphere at a temperature at which the 05 vapor pressure of that component of the organic binder which has the highest vapor pressure does not exceed the pressure of the surroundiny atmosphere and (b) efecting a subsequent debinding heat treatment step in which the injection-molded object is heated to and maintained at a temperature hiyher than the temperature used in the debinding pre-treatment step ~a) thereby to remove a sufficient amount of the organic binder without significantly distorting the shape of the object.

DETAILED DESCRIPTION OF THE INVENTION
The invention will be more fully understood from the following description.
The deformation, in particular the droop of the injection-molded object during debinding is related to and affected by the viscosity of the material of the injection-molded object. More specifically, a reduced deformation isattainable by increasing the viscosity of the material.
The viscosity of the material of the injection-molded object in turn varies with the temperature and the extent of removal of the binder. Namely, a higher viscosity is ~5 obtainable by lowering the temperature and by increasing the amount of removal of the binder. Thus, both a reduction of viscosity due to temperature increase and an increase of viscosity due to removal of binder take place simultaneously as the injection-molded object is heated.
05 In order to prevent deformation, in particular droop during debinding, it has been discovered to be necessary to remove a sufficient amount of binder while the temperature of the injection-molded object is still relatively low and while the viscosity of the material is still high enough so that -, ~ beir~q ~ eq/
the object after -h~ating to a higher temperature will have a viscosity which is high enough to reæist deforming forces.
The present invention effectively achieves this goal by utilizing a phenomenon wherein the evaporation temperature of the plasticizer component (consisting of a kind of wax and a plasticizer) of a binder is lowered when the pressure of the atmosphere is lowered.
According to this invention a debound object is obtained from an object ~ormed by injection molding a mixture obtained by mixing together a material powder and an organic binder, and this is accomplished by removing at least a part of the organic binder. The debinding method is characterized by the combination of a debinding pre-treatment and a subsequent debinding heat treatment.

It will be appreciated that the binder used in the rf~70~J /~5~ f~ c ~ practice of this invention may comprise one or morelbinder ~, . r~slr)S
-e0-ps~e~ts and one or more plasticizer components, and that different binder components may have vapor pressure-05 temperature relationships which differ from each other.
The debinding pre-treatment according to this invention is conducted by heating the formed object at a temperature at which the vapor pressure of that binder component which has the highest vapor pressure is less than the pressure of the treating atmosphere. If this condition is not met, the binder tends undesirably to evaporate in the formed object, so as to expand or inflate the formed object.
A pre-debound part is obtained in accordance with this invention which is resistant to deformation during subsequent debinding heat-treatment.
No practical limitation is imposed by this invention on the pressure of the atmosphere in which the debinding pre-treatment is conducted, provided that a sufficiently large reduction in the evaporation temperature of the plasticizer component of the binder is obtained as a result of the reduction of the applied pressure. In general, however, the pre-treatment atmosphere pressure is preferably not higher than lO Torr, more preferably not higher than l Torr. A phthalate ester and a paraffin wax, which are typical examples of plasticizer components of t~e binder, generally exhibit boiling temperatures ranging between about 250and 3500C and between about 350 and 4500C, respectively. The temperatures at which these plasticizer 05 materials exhibit a vapor pressure of 10 Torr are between 130 and 180C and between 180 and 2500C, respectively. A
further reduced vapor pressure of 1 Torr is obtained when the temperatures of these plasticizer materials are reduced to below about 130C and below about 180C, respectively.
Removal of the plasticizer component of the binder at a low temperature cannot be effected satisfactorily without - utilizing a reduction of the evaporation temperature caused by a reduction of the pressure of the pre-treating atmosphere.
Preferably, the debinding pre-treatment is conducted at a temperature which is 10C or more lower than the temperature at which the vapor pressure of the binder component having the highest vapor pressure equals the pressure of the pre-treatment atmosphere.
The debinding pre-treatment is executed effectively when the pre-treating temperature is not higher than about 3000C. Pre-treatment at a temperature above about 3000C
tends to cause decomposition of a thermoplastic resin which is a major binder component of the binder. Monomer components produced as a result of the decomposition of the .

734~1-3 resin are gaslEied slmultaneously wlth the decomposltion because the boilin~ temperatures of the monc,mers are lower than the decorn-posltion reaction temperature. In other ~ords, the gas pressure ls lnfluenced by the speed or rate of the decompositlon reactlon.
The speed or rate of the decomposltion reaction ln turn ls ln~lu enced by factors such as temperature or the number of bonds of a resin which remains without being perfectly decomposed. It is therefore difflcult to maintaln the gas pressure below the pres-sure of the atmosphere at a temperature above about 300C. Name-ly, the risk of generatlon of deblnding defects lncreases when thepressure of the atmosphere ls reduced at a high temperature ex-ceeding 300C. For this reason, the effectlve upper llmlt of the debinding pre-treatment temperature is preferably about 300C.
More speciflcally, the maxlmum debinding pre--treatment temperature ls preferably selected within the range of about 100 and 200C, while the debindlng holdlng time for the pre-treatment preferably ranges between about 0 and 6 hours.
Even in the case that the blnder contains two or more plasticizer components, adequate pre-treatment temperatures can ~0 easily be established based on the nature of the components.
It is preferred that about 18 wt% or more of the total amount of the blnder is removed by the novel debinding pre-treatment of this invention. When the amount of the binder removed by the pre-treatment is below about 18 wtP~, it is effect-ively impossible to obtain an appreciable rise of viscosity cause~

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by mutual contact of materlal powder particles in -the formed ob~ect. In addltlon, ln such a case the vlscoslty ls und~slrably lowered when the temperature is raised, with the re.sult that the formed part is deformed undesirably. It ls therefore eEfectivel~
necessary that about 18 wt% or more of the total amount o~ the binder is removed by the novel deblnd1ng pre-treatment.
It ls thus posslble to obtain a pre-debound object which, when sub~ected to a subsequent debindlng heat treatment, does not exhlblt any slyniflcant deformatlon, since the debindlng pre-treatment was conducted under the conditions described above.
The subsequent deblndlng heat treatment is conducted, so that the debound object is obtained wlth a high dimenslonal pre-cision, i.e., without any slgnificant deformation.
The deblnding heat treatment is preferably conducted in a non-oxldizing atmosphere, more preferably in a nltrogen gas atmosphere, or in argon gas or a mixture of nitrogen and argon.
The maxlmum debinding heat treatlng temperature ln this step pre-ferably ranges between about 450 and 650C, and the heat holding time for treatment preferably ranges .

between about 0 and 6 hours. A too rapid temperature rise of the pre-debound object may cause difficulties such as cracking or inflation. The rate of temperature rise, therefore, is preferably selected to range between 5C/h and 05 300oC/h.
The debindiny method of the present invention is used in the production of sintered products. Thus, the steps executed before and after the debinding may be those which are ordinarily employed in the production of sintered products.
The powder material used in the practice of the invention may be a metal powder, a ceramic or a cermet.
More specifically, the powder material comprises powder particles of an alloy or a metal obtained through a suitable procedure such as atomizing r reduction, a carbonyl process, pulverizing and so forth, or a ceramics or a cermet. The powder material is prepared by classifying and/or blending the desired powders. The use of the atomizing method is preferred because other methods such as the reduction method, the carbonyl process and pulveri~ing undesirably restrict the kinds of powder compositions that can be used.
The powder material suitably used in the invention has a mean particle size of about 20ym or smaller. In order to obtain superior properties of the sintered product, it is advisable that the material powder should have a mean particle size of about 7 to 14~m.
More specifically, the powder material can be from a wide selection: namely~ a metal such as a stainless steel, 05 pure iron, an Fe-Ni alloy, an Fe-Si alloy, an Fe-Co alloy and so forth, or a ceramic material such as SiC~ Si3N4, SiO2, TiC and so forth, or also a magnetic material or a cermet.
Production of an injection-molded object is conducted as follows. The powder material which may be one of the above-mentioned materials, preferably having a mean particle size not greater than about 20 ~m, is mixed and kneaded together with a binder as a forming assistant, thereby preparing an injection molding mixture.
- 15 Any known binder ~omposed mainly of a thermoplastic resin, a wax, a plasticizer or a mixture thereof may be used as the binder. A lubricant, a debinding promoter and/or another additive or additives may be included as required in the binder.
Examples of the thermoplastic resins include the acrylic resins, polyethylene resins, i~ polypropylene resins ~ and the polystyrene resins. Any of these resins may be ; used alone, or a combination of two or more of these resins may be used in the form of a resin mixture in formulating the binder.

Examples of the wax include natural waxes such as beeswax, Japan wax or montan wax, and synthetic waxes such as low-molecular weight polyethylene, microcrystalline wax and paraffin wax, for example. One of these waxes may be ~5 used alone or a combination of two or more of these waxes in the form of a wax mixture may be used.
The plasticizer may be selected in accordance with the composition of the main component of the binder. Examples of plasticizer suitably used are di-2-ethylhexyl phthalate (DOP), diethyl phthalate (DEP) and di-n-butyl phthalate (DBP). A wax also can serve as a plasticizer.
Examples of the lubricant suitably used include the higher fatty acid, fatty acid amides and fatty acid esters.
When a wax is used, the wax càn serve also as a lubricant.
The debinding promoter may be a substance capable of sublimation such as camphor, for example.
In connection with the foregoing, it is desirable that the organic binder is mainly composed of a thermoplastic resin, a wax, a plasticizer or a plasticizer mixture, wherein the total amount of wax and plasticizer is not less than about 18 wt~ of the total amount of the organic binder.
; The mix ratio of the binder to the material powder generally ranges between about 50 : 50 and 40 : 60 in terms of volume %.

The mixing of the material powder and the binder may be conducted in any suitable manner. Namely, any suitable apparatus such as a pressuri~ing Icneader, a Banbury mixer or a twin extruder may be used for mixing the material 05 powder and the binder.
The injection molding mixture thus prepared may be pelletized by means of a pelletizer or a pulverizer.
The injection molding mixture is then subjected to injection molding so as to be formed into a molded object.
The injection molding step can be conducted by use of an ordinary injection molding machine for plastics, or an injection molding machine for ceramics and metal powders, which has become available recently.
The injection molding step is preferably conducted at an injection pressure of about 400 to 2500 kgf/cm2 and at a temperature of about 100 to 180C.
The thus obtained injection-molded object, referred to also as a "formed part" is subjected to a debinding process in accordance with the present invention. The debinding 2~ is conducted, as explained herein, by a debinding pre-treatment combined with a subsequent debinding heat treatment.
In the present invention, pressure control of the atmosphere in the pre-debinding process can be conducted by utili~ing a heating furnace which is provided with a gas introduction port for introducing a small amount of a gas, as well as a gas discharging system for evacuating the furnace.
Following the said debinding heat treatment, the 05 debound ob]ect is then subjected to sintering so that a metallic or ceramic sintered part is obtained.
When the material powder is a stainless steel, the sintering of the debound object may be executed by holding the object for about 0.5 to 4 hours in a reduced pressure atmosphere of about 10-2 to 10-4 Torr at 1050 to 13000C, introducing an inert gas such as argon or nitrogen, and holding the part for about 0.5 to 2 hour at an elevated temperature of about 1200 to 1370~C. When the powder material contains only a metal having a limited tendency toward oxidation, such as Fe, Fe-~i or an Fe-Co alloy, sintering may be effected by holding the object for about 0.5 to 4 hours at a temperature of about 800 to 1300C in a reducing gas such as hydrogen gas.
In the case where a metal having a strong tendency toward oxidation is used, such as Fe-Si, sintering is conducted in the same manner as that explained in connection with a stainless steel.
After performing the series o steps as described in this invention it is possible to obtain a metallic or ceramic sintered product having superior properties.

The steps comprising the debinding pre-treatment, the debinding heat treatment and the c;intering may be conducted in independent furnaces or in a cc>mmon furnace or oven.
More specifically, these steps may be executed according to 05 any of the following methods: namely, ~1) a method in which each of these steps is carried out: in a single furnace or oven, (2) a method in which an~ two successive steps are carried out in a single furnace or oven while the other step is carried in a different furnace or oven, or (3) all the three steps are executed in a single common furnace or oven. When two consecutive steps are executed in a single furnace or oven it is not necessary to cool the object between these two steps, unless confirmation of the characteristics of the object after completion of the earlier step is required. It will be understood that omission o~ cooling of the object in such an intermediate period contributes to saving of energy.
The invention is further described with reference to the following examples of the invention which are intended to be illustrative but not to define or to limit the scope of the invention, which is defined in the appended claims.

Powder of a stainless steel SUS 316 L having a mean particle diameter of lOum was prepared as the material 1~

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powder. A binder was prepared which contained 22 wt% of phthalate ester, 18 wt% of paraffin wax and 60 wt% of methacrylate ester-vinyl acetate copolymer. The binder was mixed with the material powder in an amount of 9.8 wt%
05 to the material po~der using a pressurizing Icneader. The mixture was then pulverized so as to provide an injection molding mixture. Using this mixture, test pieces were fabricated in a rectangular parallelepiped form 20 mm long, 5 mm wide and 5 mm high.
The test pieces were placed on a stainless steel sheet in an oven such that one of the longer surfaces of each test piece contacted the stainless steel sheet. Then, the following steps (a) and (b) in accordance with the invention and the following steps (c) and (d) for comparative purposes were executed.
(a) The test piece was heated to 700C at a ~ rate of temperature increase of 150C/h in an atmosphere having a reduced pressure of 1 Torr, and was held at this temperature for 8 hours to conduct a debinding pre-treatment. The removal of the binder obtained as a resultof this pre-treatment was 22.0 wt% based upon the total weight of the test piece.
(b) The test piece was heated to 700C at a temperature increase of 150C/h in an atmosphere having a reduc~d pressure of 1 Torr, and was held at this temperature for 8 .

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hours. Then the test piece was further heated to 150C at a temperature increase of 1500C/h and so held for l hour, thereby conducting the debinding pre-treatment. The ratio of removal of the binder obtained as a result of this pre-05 treatment was 3~.4 wt~ based upon the total weight of thetest piece. Subsequently, a debinding heat-treatment was performed by heating the test piece to 650C at a temperature increase of 5C/h in a nitrogen gas atmosphere of 1 atmosphere and holding it at this temperature for 30 minutes.
(c) A debinding heat treatment was performed by heating the test piece to 650C at a temperature increase of 5C/h in a nitrogen gas atmosphere of l atmosphere and holding it at this temperature for 30 minutes.
(d) A debinding heat treatment was executed by heating the test piece to 6500C at a temperature increase of 20C/h in a nitrogen gas atmosphere of l atmosphere and holding it at this temperature for 30 minutes.
Each test piece was then subjected to a sintering process which was executed by holding the test piece for 90 minutes in an atmosphere of OoO01 Torr at a temperature o~
11500C, and~ after introducing argon gas at l atmosphere, heating each test piece to and maintaining the same at 1350C for 2 hours.

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The sintered test pieces were visually checked and changes of dimensions and sintering properties were measured, the results of which are shown in Table 1.
In this Example, phthalic acid ester and paraffin wax 05 were used as plasticizer components. The phthalic acid ester and paraffin wax exhibited boiling points of about 115~C and 165C at a total pressure of 1 Torr. Thus, the vapor pressures of these plasticizers did not exceed 1 Torr under the conditions of the debinding pre-treatment (a) mentioned above.
It will be seen that the debinding pre-treatment (a) enabled the phthalic acid ester to be removed completely, because the amount removed by this treatment equaled the amount initially added. Therefore, the subsequent heating to 150C did not cause the vapor pressures of the binder components to exceed the pressure of the treating atmosphere In Table 1, the rate of change in dimensions exceeding -0.3% suggests that droop had taken place due to flowing of the binder resin. The heightwise contraction of the sintered products according to this Example of the invention was 16.5%. This means that the height should be reduced to 4.175 mm. Taking into account a tolerance of +0.3%, samples having heights ranging between 4.162 mm and ~8 . .

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1 3~31 78 4.188 mm were and are regarded as having good or acceptable dimensional precision.

Debinding pre-treatment was executed on the same test pieces as Example 1, under the treating conditions shown in Table 2. Then, a debinding heat-treatment was conducted under the same conditions as (a) in Example 1, followed by sintering. Results of visual appearance checks of the pre-debound samples and of the debound samples, as well as results of measurements of heightwise dimensions of the samples after debinding and after sintering, are also shown in Table 2.
From the test results shown in Tables 1 and 2, it will be clearly understood that the debinding method of the present invention, which employs the recited debinding pre-treatment, enabled the sintered articles to be produced with a high dimensional precision which could not be attained by conventional methods.

Samples corresponding to runs Nos. 1-1 to 1-3 in Table 2, obtained through debinding pre-treatments according to this invention and having different rates of removal oE
binder, were subjected to debinding heat-treatments .

conducted by heating these samples from room temperature to 27QC at a temperature increase of 100C/h in a nitrogen gas atmosphere of 1 atmosphere-holding the samples in this state for 30 minutes, heating the samples up to 650OC at a ~5 temperature increase of 300C/h and holding the samples in this state for 30 minutes. These samples were cooled after the debinding pre-treatmen-t. For the purpose of comparisonr pre-debound samples were directly subjected to the debinding heat treatment without cooling, by introducing nitrogen gas at 1 atmosphere, heating the samples to 270OC at a temperature increase of 100C/h, maintaining the samples in this state for 30 minutes, heating the samples to 650OC at a temperature increase of 300C/h and holding the samples at this temperature for 30 i5 minutes. These samples were then subjected to sintering in the same manner as in Example 1. Results of visual appearance checks of the samples after debinding and of heightwise dimension measurements after debinding and after sintering are shown in Table 3.
2~ According to the invention, the binder is removed in an amount of at least about 18 wt~ by the debinding pre-treatment. It is understood that sound debound materials can be obtained irrespec~ive of any intermediate cooling after the debincling pre-treatment and regardless whether the heating is executed at a high rate of 100 to 300C/h (see samples Nos. 4-1, 4-2, 5-1 and 5-2 in Table 3) or at a relatively low rate of 5C/h (see samples Nos. 1-1 and 1-2 in Table 2).

It will also be seen that, when the amount o~ removal c~bc~
05 of the binder is belowl18 wt% after the debinding pre-treatment, the quality of the material after debinding is impaired due to inflation when the debinding heat treatment is conducted at a high rate of temperature increase as in the cases of samples Nos. 4-3 and 5-3 in Table 3. The binder removal rate below about 18 wt% also causes defects such as droop (this appears as a negative dimensional change of a large absolute value) when the debinding heat treatment is executed at a low rate of temperature rise, as in the case of sample No. 1-3 in Table 2.
From these facts, it will be seen that the debinding pre-treatment executed in accordance with the present invention provides an additional advantage that the post-treatment, i.e., the debinding heat treatment at the elevated temperature, can be accomplished in a shorter time because the heating can be performed at a higher ,temperature increase rate, regardless whether cooling is executed after the debinding pre-treatment or not.

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Table 1 De- Appell- Dimen- HeightDensity C O hem~lrks bind- rance sional change ratiocontentcontent ing Change (mm) (%) (%) (%) process (%) _ (a) Good -0.165 4.167 96.4 0.03 0.15 Sample according invention _ ___ (b) Good -0.173 4.163 96.2 0.02 0.12 Sample according to __ invention (c)Defect in -5.23 3.971 96.4 0.02 0.10 Compa-bottom rison surface _ sample (d)Inflation Not Compa-measura- rison ble . sample ., ,.~ ' '- ' ' '' o _ _ o o c~ o a~
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,~ ' ~ 32~ 1 7~3 Although this invention has been described with reference to specific powder materials and specific binding agents, either with or without a variety of additives, it will be understood that various equivalent or other 05 materials may be substituted without departing from the spirit of the pre-debindi~g and further debinding steps of the invention. Further, certain of the steps of the method used may be interposed or reversed and equivalent steps may be used, provided the powder mixture is subjected to pre-debinding followed by further debinding at a highertemperature in accordance with the spirit and scope of this invention as defined in the appended claims.

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Claims

1. A process for producing a sintered object, which comprises:
mixing a material powder and an organic binder comprising one or more plasticizer components and one or more binder components including a major proportion of a thermoplastic resin, one of the binder components having a higher vapor pressure than the others, so as to prepare a mixture;
subjecting the mixture to injection molding, so as to form an injection-molded object;
removing part or all of the organic binder from the injection-molded object, so as to obtain a wholly or partially debound object; and sintering the debound object in a heating atmosphere, so as to form the sintered object, wherein the step of removing the organic binder comprises:
(a) effecting a reduced pressure debinding pre-treatment by heating the injection-molded object which contains the thermoplastic resin as a major portion of the binder in an atmosphere at a reduced pressure and at a temperature at which (i) the vapor pressure of the said component of the binder having the highest vapor pressure does not exceed the reduced pressure of the atmosphere, (ii) the thermoplastic resin does not decompose and (iii) the most volatile component of the plasticizer has a vapor pressure lower than the reduced pressure of the atmosphere, for maintaining the viscosity of the thermoplastic resin low enough to prevent excessive droop of the object and to prevent excessive inflation of the object due to excessively rapid evaporation of the binder, so as to remove part of the organic binder and to obtain a pre-debound object; and (b) subsequently effecting a debinding heat treatment in which the pre-debound object is heated to a temperature higher than the temperature of the pre-treatment.

2. A debinding method according to claim 1, wherein the organic binder is mainly composed of the thermoplastic resin, a wax and a plasticizer, wherein the total amount of the wax and the plasticizer is equal to or greater than about 18 wt% of the total amount of the organic binder.

3. A debinding method according to claim 1, wherein at least 18 wt% of the organic binder is removed in the debinding pre-treatment (a).

4. A debinding method according to claim 2, wherein at least about 18 wt% of the organic binder is removed in the debinding pre-treatment (a).

5. A debinding method according to claim 1, wherein the debinding pre-treatment of step (a) is conducted at a temperature which is 10°C or more lower than the temperature at which the vapor pressure of the said component of the organic binder having the highest vapor pressure is equal to the pressure of the atmosphere.

5. A debinding method according to claim 1, wherein the pressure of the atmosphere in the debinding pre-treatment of step (a) is 10 Torr or lower.

7. A debinding method according to claim 5, wherein the pressure of the atmosphere in the debinding pre-treatment of step (a) is 10 Torr or lower.

8. A debinding method according to claim 7, wherein at least about 18 wt% of the organic binder is removed in the debinding pre-treatment (a).

9. A method of producing an object of precision shape from a molded object made of a mixture containing a powder and a binder which includes one or more binder components including a major portion of a thermoplastic resin, one of which components has a higher vapor pressure than the others, and wherein the evaporation temperature of the more volatile binder component decreases when the pressure of the surrounding atmosphere is decreased, which method comprises:
(a) pre-debinding the molded object by heating it containing the thermoplastic resin as a major portion of the binder in an atmosphere of predetermined reduced pressure while maintaining the atmosphere at a temperature which is at least 10°C lower than the temperature at which the vapor pressure of the more volatile binder component which has the highest vapor pressure equals the predetermined reduced pressure of the pre-debinding atmosphere;
and (b) in a separate step further debinding the object by subsequent heat treatment at a temperature higher than the temperature of step (a).

10. A method of producing an object of precision shape from a molded object made of a mixture containing a powder and a binder including a thermoplastic resin as a major portion of the binder, which object includes one or more components one of which has a higher vapor pressure than the others, and wherein the evaporation temperature of the more volatile component decreases when the pressure of the surrounding atmosphere is decreased, which method comprises:
(a) pre-debinding the shaped object by heating it containing the thermoplastic resin in an atmosphere of predetermined reduced pressure while maintaining the atmosphere at a temperature lower than the temperature at which the most vola-tile component of the plasticizer has a vapor pressure equal to the pressure of the surrounding atmosphere to prevent excessive inflation in the object due to vaporization of the binder; and (b) in a separate step further debinding the object by subsequent heat treatment at a temperature higher than the temp-erature of step (a).

11. A process for producing a sintered object, which comprises:
(1) mixing a metal, ceramic or cement material powder having a mean particle diameter of from about 7 to 20 µm and an organic binder comprising a minor portion but at least about 18% by weight (based on the organic binder) of one or more plasticizer components and a major portion of one or more thermoplastic binder resins, one of the plasticizer components having a higher vapor pressure than any other components in the organic binder, so as to prepare a mixture;
(2) subjecting the mixture to injection molding, so as to form an injection-molded object;
(3) removing part or all of the organic binder from the injection-molded object, so as to obtain a wholly or partially debound object; and (4) sintering the debound object in a heating atmosphere, so as to form the sintered object, wherein the step of removing the organic binder comprises:

(a) effecting a reduced pressure debinding pre-treatment by heating the injection-molded object which contains the thermoplastic resin as a major portion of the binder in an atmosphere at a reduced pressure and at a temperature at which (i) the vapor pressure of the said component of the binder having the highest vapor pressure does not exceed the reduced pressure of the atmosphere, (ii) the thermoplastic resin does not decompose and (iii) the most volatile component of the plasticizer has a vapor pressure lower than the reduced pressure of the atmosphere, for maintaining the viscosity of the thermoplastic resin low enough to prevent excessive droop of the object and to prevent excessive inflation of the object due to excessively rapid evaporation of the binder, so as to remove at least 18%
weight of the organic binder and to obtain a pre-debound object; and (b) subsequently effecting a debinding heat treatment in which the pre-debound object is heated to a temperature higher than the temperature of the pre-treatment, so as to substantially completely remove the binder.

12. A process according to claim 11, wherein the organic binder comprises a major portion composed of the thermoplastic resin and from about 18 to about 40% by weight (based on the total amount of the organic binder) of a mixture of a phthalate ester plasticizer and a wax, the phthalate ester plasticizer being the said component having the lowest vapor pressure.

13. A process according to claim 12, wherein the material powder and the organic binder are mixed in step (1) at a volume ratio of from about 50:50 to 60:40.

14. A process according to claim 12, wherein the debinding pre-treatment (a) is conducted at a reduced pressure of from about 0.5 to about 10 Torr at a temperature which is 10°C or more lower than the temperature at which the vapor pressure of the said plasticizer component having the highest vapor pressure is equal to the pressure of the debinding pre-treatment.

15. A process according to any one of claims 11 to 14, wherein the thermoplastic resin is a member selected from the group consisting of acrylic resins, polyethylene resins, polypropylene resins and polystyrene resins.