Mouldable composition and process

There is provided a mouldable composition for use in manufacturing a sintered metal or ceramic object, the composition comprising a metal or ceramic powder, a cross-linkable thermoplastic polymer and an amount of a cross-linking agent effective to cross-link the thermoplastic polymer at an elevated temperature. There is also provided a process for manufacturing a sintered metal or ceramic object using such a mouldable composition.

TECHNICAL FIELD 
The present invention relates to a polymeric mouldable composition which at 
lower temperature behaves as a mouldable thermoplastic material and at 
higher temperature as a thermosetting material. The present invention also 
relates to a process for producing a metal or ceramic object, using such a 
polymeric mouldable composition. 
BACKGROUND OF THE INVENTION 
The processing of particulate materials into solid bodies is a known 
technique, described, for example, in the following patent specifications: 
U.S. Pat. Nos. 4,113,480, 4,305,756, 4,544,694, 4,661,315, 4,765,950, 
5,006,164, 5,122,326, 5,240,513 and 5,337,531, European patent nos. 
0324122 and 0409646, and Japanese patent application nos. 59-121150 and 
365552. Metal powders are compounded with a binder system and shaped by 
injection moulding. The binder is then removed from the moulded green body 
prior to sintering into a solid component. The binder systems that are 
used in the prior art are progressively softenable and are removed using 
heat or partial dissolution in solvents with subsequent heating. These 
methods are always associated with the use of a thermoplastic polymer, 
having a thermoplastic behaviour throughout the heating cycle. The 
thermoplastic polymer becomes quite soft during the initial heating cycle 
which is initiated to remove wax based binder components through melting, 
and increases in softness as the temperature increases. As a result of 
this increasing softness the polymer reaches a stage where it cannot 
support the metal or powder effectively to retain the shape of the part, 
thus causing distortion, deformation defects in the parts. This also 
limits the shapes of the parts that can be processed using processes of 
the prior art. Furthermore this also limits the type, size or shapes of 
metal powders that can be used in the processes of the prior art. In order 
to reduce the effect of softening of a thermoplastic polymer binder, the 
use of fine powders, typically less than 10 .mu.m in size, and irregular 
shape have been suggested. It may also be necessary to prolong the heating 
cycle, often to uneconomical lengths. Both these have a serious impact on 
the quality, economy, usage of material and the size of the processable 
parts. Furthermore the decomposition of the binder as it moves through an 
increasingly soft phase can leave residual products that contaminate the 
metal or ceramic particulate material. Processes of the prior art also 
require the shape of the powder particles to be partially irregular or 
irregular when larger particle size (up to 10 .mu.m) powders are used, but 
spherical or near spherical when smaller sized powders are used. This is 
in order to take advantage of some packing of the particles due to 
interlocking of irregular shapes, or the high surface energy of fine 
spherical particles, to bond the particles together. This unquantifiable 
packing and energy behaviour lead to unpredictable deviations of the final 
product properties. 
Accordingly, there is a need for an improved binder composition for use in 
forming solid bodies from particulate materials. 
Furthermore in prior art processes binder compositions for metals differ 
from binder compositions for ceramics. There is therefore a need for a 
binder composition that can be used for processing metal and ceramic 
powders. 
SUMMARY OF THE INVENTION 
The present invention is directed towards addressing overcoming the 
problems of the prior art as indicated above, by providing a binder system 
that offers the green part excellent mouldability during moulding and 
provides excellent shape retention during the binder removal process. The 
binder is formulated to provide the above characteristics and can be used 
with metallic and ceramic particulate powders without having to operate 
within a narrow band of particulate material characteristics, specially 
with respect to powder particle size and shape. In the binder of the 
present invention the thermoplastic polymer is rigidised during a heating 
phase through cross-linking and is transformed into a thermoset polymer. 
The rigid and strong thermoset polymer retains the shape of the component 
during heating until neck formation between the powder particles has 
commenced. In the case of larger particle sizes, for example up to 40 
.mu.m, the neck formation takes place at higher temperatures. The rigid 
polymer retains the shape of the article until the neck formation has 
progressed sufficiently for the article to retain its shape without the 
aid of the polymeric binder. In the prior art the thermoplastic polymer 
progressively softens and fails to retain the shape until the necks are 
formed. As such it is not possible to increase the particle size beyond 
typically 10 .mu.m in prior art processes. 
The present invention further provides a binder system that during the 
binder removal process, when subjected to heat, progressively hardens 
through cross-linking, thus imparting excellent shape retention. The 
hardening progresses until the polymer begins to degrade at a higher 
temperature by gradually and progressively breaking down into gaseous 
products. The polymer therefore does not undergo a state softening. The 
degradation is completed after the metal powder particles have formed 
stable necks or links among them. 
Thus, according to a first embodiment of the invention, there is provided a 
mouldable composition for use in manufacturing a sintered metal or ceramic 
object, said composition comprising a metal or ceramic powder, a 
cross-linkable thermoplastic polymer and an amount of a cross-linking 
agent effective to cross link said thermoplastic polymer at an elevated 
temperature. 
The invention also provides a moulded green part comprising a mouldable 
composition of the first embodiment. 
According to a second embodiment of the invention, there is provided a 
process for manufacturing a sintered metal or ceramic object, the process 
comprising: 
(a) compounding forming a mouldable composition comprising a metal or 
ceramic powder, a cross-linkable thermoplastic polymer and an amount of a 
cross-linking agent effective to cross-link said thermoplastic polymer at 
an elevated temperature, said compounding occurring at a temperature less 
than said elevated temperature; 
(b) injecting said mouldable composition at a first temperature less than 
said elevated temperature into a mould to form a moulded green part; 
(c) raising the temperature of said moulded green part to a second 
temperature above said elevated temperature to cross-link said 
thermoplastic polymer, thereby forming a thermoset polymer; 
(d) raising the temperature of said moulded green part to a third 
temperature sufficient to cause neck formation between particles of said 
metal or ceramic powder; and 
(e) heating said moulded green part at a temperature sufficient to degrade 
said thermoset polymer and sinter said metal or ceramic powder, thereby 
forming said sintered metal or ceramic object. 
Suitably, in the process of the second embodiment, step (e) comprises the 
steps of (i) heating the moulded green part at a fourth temperature 
sufficient to at least partly degrade the thermoset polymer and (ii) 
raising the temperature of the moulded green part to a fifth temperature 
sufficient to sinter the metal or ceramic powder, thereby forming the 
sintered metal or ceramic object.

DETAILED DESCRIPTION OF THE INVENTION 
The mouldable composition of the first embodiment of the present invention, 
and the mouldable composition used in the process of the second 
embodiment, comprises a thermoplastic polymer in order for the composition 
to be readily injection mouldable. Typically, the thermoplastic polymer 
softens above about 120.degree. C. and is therefore of relatively low 
molecular weight. The injection moulding temperature is typically about 
140.degree. C. to 160.degree. C. 
Typically, the cross-linkable thermoplastic polymer has a molecular weight 
40,000 to 120,000, more typically 60,000-80,000. The polymers in this 
range are injection mouldable at a relatively low temperature. The 
thermoplastic polymer also includes one or more reactive groups that make 
cross-linking possible using a compatible cross-linking agent. Suitable 
cross-linkable thermoplastic polymers are any polymeric materials bearing 
reactive groups capable of reacting with a suitable cross-linking agent by 
an addition or condensation mechanism. For example, the thermoplastic 
polymer may bear reactive hydroxyl groups, for cross-linking using a 
cross-linking agent having reactive carboxyl, ester, anhydride, aldehyde, 
isocyanate, epoxy or hydroxyl groups; or the thermoplastic polymer may 
bear reactive carboxyl groups for cross-linking using a cross-linking 
agent having reactive hydroxyl or amino groups; or the thermoplastic 
polymer may bear reactive amino groups for cross-linking using a 
cross-linking agent having reactive carboxyl, ester, anhydride, aldehyde, 
isocyanate or epoxy groups; or the thermoplastic polymer may bear reactive 
epoxy groups for cross-linking using a cross-linking agent having reactive 
hydroxyl or amino groups; or the thermoplastic polymer may bear reactive 
olefinic groups for cross-linking using a cross-linking agent also having 
reactive olefinic groups. Other examples of cross-linkable thermoplastic 
polymers and suitable cross-linking agents for those polymers will be 
readily apparent to persons of ordinary skill in the relevant art. It will 
be appreciated that the cross-linkable thermoplastic polymer may include 
more than one kind of reactive group. 
In a preferred embodiment of the invention, the cross-linkable 
thermoplastic polymer is a poly(vinyl acetal). More preferably, the 
cross-linkable thermoplastic polymer is poly(vinyl butyral) or poly(vinyl 
formal). Typically, the butyral or formal content is in the range 75% to 
85% expressed as a percentage of reactive hydroxy groups. Even more 
preferably, the cross-linkable thermoplastic polymer is a poly(vinyl 
butyral). 
The primary function of the cross-linking agent is to cross-link the 
molecules of the thermoplastic polymer to form a three dimensional 
thermoset polymer network with high rigidity. When the cross-linkable 
thermoplastic polymer is a poly(vinyl acetal) the cross-linking agent may 
preferably be a melamine, diisocyanate, dialdehyde, phenolic, epoxy or 
urea. It is more preferred to use melamines or diisocyanates as 
cross-linking agents for producing the above characteristics. A 
particularly preferred cross-linking agent is hexamethoxymethyl melamine 
when the cross-linkable thermoplastic polymer is a poly(vinyl butyral). 
Using these cross-linking agents, cross-linking is markedly accelerated in 
the presence of nitrogen gas and heat. 
Premature cross-linking of the cross-linkable thermoplastic polymer at the 
injection moulding temperature, and at the temperature at which the 
mouldable composition of the first embodiment is compounded, is prevented 
through compounding and injection moulding the mouldable composition under 
a nitrogen-free inert atmosphere and/or at a temperature below a 
temperature above which cross-linking of the thermoplastic polymer 
commences. Typically, cross-linking of the thermoplastic polymer commences 
at a temperature of at least about 140.degree. C., more typically at least 
about 170.degree. C., even more typically at least about 200.degree. C. 
The rate of cross-linking is controlled by the quantity of the 
cross-linking agent in the mouldable composition, the temperature during 
removal of any plasticisers or other additives after injection moulding, 
and/or the application of a nitrogen atmosphere during the removal of 
additives. Typically, the cross-linking agent is included in the 
composition in an amount of from about 2.0% to 10.0%, more typically from 
about 6% to about 9% by weight, based on the weight of the cross-linkable 
thermoplastic polymer. 
The mouldable composition of the first embodiment and the mouldable 
composition used in the process of the second embodiment may include one 
or more processing aids such as plasticisers, lubricants and surfactants. 
It will be appreciated that where the cross-linkable thermoplastic polymer 
is cross-linkable by a free-radical cross-linking reaction, the mouldable 
composition further comprises a free-radical polymerisation initiator, and 
optionally an activator, selected to initiate the cross-linking reaction 
at above the preselected temperature. Suitable initiators include dialkyl 
peroxides, such as di-t-butyl peroxide, and alkyl hydroperoxides, such as 
t-butyl hydroperoxide. Suitable amounts of free-radical polymerisation 
initiator depend on the chemical nature of the free-radical polymerisation 
and the desired polymerisation (cross-linking) temperature, and may be 
readily determined by persons of ordinary skill in the art with no more 
than routine trial and error. 
The plasticiser, lubricant and/or surfactant, when used, typically melt at 
a temperature below a temperature at which the thermoplastic polymer cross 
links, or are soluble in water or an appropriate non-aqueous solvent, so 
that they can be melted or dissolved in water or solvent and removed after 
the mouldable composition has been injection moulded. Suitable 
plasticisers, lubricants and/or surfactants include waxes such as 
partially saponified montan ester wax, montanic acid wax or polyethylene 
wax, polytetrafluoroethylene (PTFE) and PTFE compounded with micronised 
wax and optionally Amide, modified polyfluo-carbons, stearic acid, oleic 
acid, glycols such as di- and tri-ethylene glycol, dipropylene glycol and 
alkoxylated glycols or related derivatives having surface-active 
properties, such as octyl- or nonyl-phenol ethoxylates or octyl- or 
nonyl-phenoxy ethanol. A surfactant, when used, activates or conditions 
the surface of the metal or ceramic particles to facilitate the coating of 
the particles with the thermoplastic polymer. A lubricant, when used, 
facilitates the flow of powder within the polymer. Typically, a lubricant 
is not miscible in the polymer. 
The quantity of plasticiser used in a mouldable composition of the 
invention may be varied over a wide range. Typically, the quantity of 
plasticiser is in the range of from about 30% to about 200% by weight of 
the weight of the cross-linkable polymer, more typically from about 50% to 
about 140% by weight. The quantity of surfactant or lubricant is typically 
in the range of from 0% to about 20% by weight of the cross-linkable 
polymer, more typically from 8% to about 12%. Even more typically, the 
quantity of plasticiser is in the range of about 20% to about 45% by 
volume of the binder, the quantity of the surfactant or lubricant is in 
the range 3% to 5% by volume of the binder, the quantity of the 
crosslinkable polymer is in the range 40% to 70% by volume of the binder 
and the crosslinking agent is in the range 2% to 10% by volume of the 
crosslinkable polymer. 
In contrast to prior art processes, in the process of the present invention 
processing aids such as plasticisers and surfactants may be selected so as 
to be removed from the moulded composition independently of the primary 
binder material, namely the cross-linkable thermoplastic polymer, 
conferring greater processing flexibility on the process of the present 
invention. 
The mouldable composition of the present invention may include metal or 
ceramic powder, The particle size of the metal or ceramic particles in the 
composition of the invention is typically in the range from about 0.3 
.mu.m to about 40 .mu.m. The shape of the particulate material can be 
variable and is not an important factor in the process of the present 
invention. The relative amount of metal or ceramic particles to binder may 
vary over a wide range. Typically the ratio is between 35 parts by volume 
of particles to about 65 parts by volume of binder, to about 80 parts by 
volume of particles to about 20 parts by volume of binder. 
A composition of the present invention is typically compounded by being 
granulated before being injection moulded to the required shape. The 
compounding is preferably carried out under a nitrogen free inert 
atmosphere. During the injection moulding, the material is protected from 
nitrogen inside the barrel of the injection moulding machine. Thus, 
cross-linking of the thermoplastic polymer is avoided during the 
compounding and injection moulding stages. The temperature of compounding 
and injection moulding is preferably controlled in the region of 140 to 
170.degree. C. The material produced below 170.degree. C. can be recycled 
for injection moulding without any loss of its properties. The composition 
of the invention typically shows excellent injection mouldability between 
140 and 160.degree. C. The temperature above which cross-linking of the 
thermoplastic polymer occurs, may be adjusted by the amount of 
cross-linking agent included in the mouldable composition. 
The cross linking transforms the thermoplastic polymer to a rigid thermoset 
polymer through the temperature-related activation of the cross-linking 
reaction with the cross-linking agent. Upon cross-linking, the transformed 
polymer, which then behaves like a thermoset, demonstrates no softening 
with the increasing of the temperature. The rigidity of the polymer 
increases progressively with temperature above the cross-linking 
temperature conferring excellent shape retention on the moulded 
composition while any low molecular weight processing aids such as 
plasticisers, lubricants or surfactants are removed from the composition. 
The rigid transformed polymer continues to retain its shape as the 
temperature is raised, while the particles of the metal or ceramic powder 
begin to form necks between each other providing absolute shape stability. 
During this stage, the thermoset polymer which has formed typically begins 
a degradation process into gaseous products and eventually decomposes 
entirely into gaseous products, as the temperature is raised further, 
without undergoing a phase of melting. Thus, the moulded part is assured 
of excellent and absolute shape retention throughout the binder removal 
process. The decomposition of the polymer leaves no residual products that 
can contaminate the moulded object. 
Best Mode and Other Modes of Carrying Out the Invention 
A typical mouldable composition in accordance with the first embodiment of 
the invention comprises 35-80% by volume of a metal or ceramic powder and 
65-20% by volume of binder consisting of 70-40 parts by volume of 
cross-linkable thermoplastic polymer, 2-10 parts by volume of 
cross-linking agent, 20-45 parts by volume of plasticiser and 3-5 parts by 
volume of a surfactant. 
A more typical mouldable composition in accordance with the first 
embodiment of the invention comprises 40-60% by volume of a metal or 
ceramic powder and 60-40% by volume of binder consisting of poly(vinyl 
butyral), from 0.06 to 0.09 parts by weight of hexamethoxymethyl melamine, 
based on the weight of poly(vinyl butyral), from 0.5 to 1.4 parts by 
weight of partially saponified montanic ester, based on the weight of 
poly(vinyl butyral), and from 0.08 to 0.12 parts by weight of stearic 
acid, based on the weight of poly(vinyl butyral). 
In a typical process in accordance with the second embodiment of the 
invention, mouldable composition according to the invention comprising 
35-80% by volume of a metal or ceramic powder and 65-20% by volume of 
binder consisting of 70-40 parts by volume of cross-linkable thermoplastic 
polymer, 2-10 parts by volume of cross-linking agent, 20-45 parts by 
volume of plasticiser and 3-5 parts by volume of a surfactant/lubricant 
are compounded at a temperature in the range of 140 to 170.degree. C. in a 
nitrogen-free atmosphere, typically argon. 
The mouldable composition is then injection moulded at the first 
temperature in the range of 140 to 170.degree. C. to form a moulded green 
part. In the injection moulding step, in which the mouldable composition 
of the invention is typically not in contact with nitrogen gas, the 
temperature is insufficient to initiate substantial cross-linking of the 
thermoplastic polymer. The moulded green part is typically subjected to 
binder removal as follows. 
The plasticiser and surfactant are melted away from the part through slow 
and progressive increase of temperature in the range of 40.degree. C. to 
280.degree. C., typically under a nitrogen gas atmosphere. 
The cross-linking agent in the mouldable composition initiates 
cross-linking of the thermoplastic polymer during the temperature increase 
in the range of 140.degree. C. to 280.degree. C. catalyzed and accelerated 
by the nitrogen gas environment above 140.degree. C. That is, in the 
presence of nitrogen gas, the cross-linking typically occurs at a 
temperature lower than in the absence of nitrogen gas. 
During this activation period, the relatively soft thermoplastic polymer 
changes its characteristic through cross-linking to that of a thermoset 
polymer thus increasing in rigidity. The increase in rigidity due to this 
transformation results in excellent shape retention of the polymer, and 
thus of the moulded part. This is of extreme importance in preventing any 
collapsing of the metal or ceramic powder or deformation of the part. The 
absolute shape retention provided through the backbone polymer that is now 
a thermoset is of importance to provide extreme end product accuracy in 
dimensions and contours. 
After removal of any plasticiser(s) and/or surfactant(s), at a third 
temperature in the range of about 380 to 480.degree. C., neck formation 
between the powder particles begins and the shape retention is now 
progressively provided more by a network of inter particle necks that are 
formed, rather than by the strength of the rigidified polymer. At this 
stage the function of the rigidified polymer is complete. 
Also in the temperature range of from 380.degree. C. to 480.degree. C., the 
polymer gradually begins to degrade, without softening or melting, into 
lower molecular weight gaseous products which are purged away from the 
moulded part. Elevating the temperature of the part further, to the range 
of 480.degree. C. to 600.degree. C., completes degradation of the polymer 
into gases which are purged away from the binder-free body. At this point, 
all of the polymer is removed from the part without leaving residual 
products. The temperature is further elevated to the sintering temperature 
of the metal or ceramic material, typically above 1100.degree. C., where 
the product is fully sintered into a solid body of density above 98% of 
the theoretical density. 
FIG. 1 illustrates in graphical format a typical temperature-time heating 
profile used in a process of the present invention. Total time taken for 
the complete removal of the binder materials is reduced to 12 to 14 hours. 
This may be compared to prior art processes in which soft thermoplastic 
polymer deforms under the pressure of melting and evaporating. Plasticiser 
and surfactant require very slow heating to reduce such pressures, and 
binder removal times in excess of 16 to 20 hours or more are typically 
reported for prior art processes. 
The present invention provides a number of advantages in comparison to the 
prior art. Firstly, since the softening character of the polymer is 
eliminated in the mouldable composition of the present invention, and the 
degradation of the polymer occurs at a higher temperature where neck 
formation can take place between particles of the metal or ceramic powder, 
thus assuring absolute shape retention, the use of much more economical 
coarser powders, typically up to 40 .mu.m, is facilitated by the process 
of the present invention in contrast to the more expensive (less than 
about 10 .mu.m) powders of the prior art processes. Since the shape 
retention is completely established without being influenced by the shape 
of the powder particle the present invention makes the system independent 
of the shape of the powder particles and expands the size from sub-micron 
(typically 0.3 .mu.m) sizes to 40 .mu.m. 
Further, the present invention provides a binder composition which may be 
used with both metal and ceramic particles. 
In addition, the present invention provides a more flexible process for 
forming moulded green parts for production of sintered metal or ceramic 
objects, by providing a greater choice of metal or ceramic particle 
loadings and processing temperatures, and for providing for the removal of 
relatively low molecular weight processing aids such as plasticisers, 
lubricants and surfactants, independently of removal of the polymeric 
binder material. For example, in prior art processes the volume ratio of 
metal or ceramic powder to total volume of the composition is typically in 
the range of by 50-65% by volume, whereas in the present process may be in 
the range of 35 to 80% by volume. 
EXAMPLES 
The following examples are provided to illustrate the invention, but are 
not to be construed as in any way limiting the invention. 
Example 1 
5000 g of 316L Stainless Steel powder of average particle size 30 .mu.m was 
compounded with 262 g of poly(vinyl butyral), 17 g of hexamethoxymethyl 
melamine (cross-linker), 362 g of polyethylene glycol, and 25 g of stearic 
acid to a homogeneous feedstock in a planetary mixer. The green parts were 
moulded using the feedstock in an injection moulding machine. The glycol 
in the parts was then dissolved in water at 80.degree. C. The dried parts 
were then heated in a nitrogen carrier gas to a temperature above 
280.degree. C. activating the cross-linking agent and transforming the 
poly(vinyl butyral) to a rigid polymer offering absolute shape retention. 
The nitrogen gas catalyzed the transformation of the thermoplastic 
poly(vinyl butyral) into a rigid thermoset polymer assuring the shape 
retention of the part. The temperature was further increased to 
380.degree. C. in which temperature range, the gradual degradation of the 
polymer and neck formation between the metal powder particles began. The 
temperature was further increased to 480.degree. C. at which temperature 
the thermoset polymer was completely decomposed into gaseous products 
which were removed by the carrier gas nitrogen. The temperature was 
further elevated to 1360.degree. C. to produce a solid stainless steel 
body of density 98.9% of the theoretical. 
Example 2 
4500 g of Alumina powder and 135 g of Magnesium Oxide powder of average 
particle size 1.6 .mu.m was blended and compounded with 248 g of 
poly(vinyl butyral), 21 g of hexamethoxymethyl melamine (cross-linker), 
260 g of partially saponified montanic ester (plasticiser), 20 g of 
octylphenoxyethanol (surfactant) into a homogeneous feedstock. 
Green bodies were moulded using the feedstock and were subjected to 
progressive increase in temperature under a nitrogen atmosphere to melt 
away the plasticiser while activating the cross-linking of the 
thermoplastic poly(vinyl butyral) polymer into a rigid thermoset plastic 
polymer in the temperature range of 120.degree. C. to 280.degree. C. The 
temperature was progressively increased above 380.degree. C. At these 
temperatures the degradation of the rigid polymer was completed and the 
neck formation between the powder particles began. The temperature was 
further increased to 480.degree. C. at which temperature the polymer was 
completely degraded into gaseous products which were entrained and removed 
with the nitrogen carrier gas. The parts were subsequently sintered at 
1490.degree. C. to form solid alumina ceramic bodies. 
Example 3 
5000 g of 316L stainless steel powder of average particle size 30 .mu.m was 
compounded in a planetary mixer with 230 g of poly(vinyl butyral), 18 g of 
hexamethoxymethyl melamine (cross-linker), 207 g of partially saponified 
montanic ester (plasticiser) and 24 g of stearic acid into a homogeneous 
feed stock. The feedstock was injection moulded to form the green bodies 
using an injection moulding machine. The temperature of the green bodies 
was increased progressively from 120.degree. C. to 180.degree. C. under 
nitrogen gas to melt away the plasticiser and to initiate the 
cross-linking of the polymer. The temperature was further increased to 
280.degree. C. to complete the cross-linking. The temperature was further 
increased to 380.degree. C. and then to 480.degree. C. At this temperature 
the rigid polymer was completely degraded into gaseous products and necks 
were formed between the powder particles. The parts were sintered at 
1360.degree. C. to a density of 99% of the theoretical. 
Example 4 
6000 g (769 cc) of 316L stainless steel powder of average particle size 32 
mm was compounded with 1270 g (1154 cc) of binder, amounting to a volume 
loading of 40% metal powder, under argon into a homogeneous feedstock. The 
binder consisted of 762 g of poly (vinyl butyral), 381 g of partially 
saponified montanic ester (plasticiser), 63 g of octylphenoxyethanol 
(surfactant) and 54 g of hexamethoxymethyl melamine (cross-linker). 
The feedstock was injection moulded to green objects using a plastic 
injection moulding machine. The temperature of the green bodies were 
increased progressively from 120.degree. C. to 180.degree. C. under 
nitrogen gas during which cross-linking is initiated and then to 
280.degree. C. to complete cross-linking. When this temperature has been 
reached the plasticiser and the surfactant have left the green body. The 
temperature is further increased to 380.degree. C. and then to 480.degree. 
C. and subsequently to the sintering temperature of 1350.degree. C. The 
sintered body showed excellent shape retention and a shrinkage of 22% from 
the green stage to the sintered stage. The sintered density of the body 
was 98.7% of the theoretical. 
Example 5 
FIG. 1 illustrates in graphical format a typical time-temperature heating 
profile used in a process of the present invention. Referring to FIG. 1, 
the temperature of a moulded green part of the invention is raised at (1) 
at a rate of 100.degree. C. per hour to a temperature of 120.degree. C., 
where it is held at (2) for two hours. During this time, plasticiser and 
any other low-melting additive separates from the moulded green part and 
is removed from it. At (3) the temperature of the moulded green part is 
raised at 35-40.degree. C. per hour to 180.degree. C. and held there for 
one hour at (4). During this time, removal of plasticiser and other 
low-melting additive(s) is completed. At (5) the temperature is raised at 
45-50.degree. C. per hour to 280.degree. C., at which it is held for 30 
minutes at (6). During heating step (5) and holding step (6) cross-linking 
of the cross-linkable thermoplastic polymer occurs to form a thermoset 
polymer which confers rigidity on the moulded green part. At the end of 
the hold time at 280.degree. C., the moulded green part is heated at (7) 
at a rate of 65-70.degree. C. per hour to 380.degree. C., and is held at 
that temperature for 30 minutes at (8). During the heating step (7) and 
the holding step (8) neck formation between particles of metal or ceramic 
in the mouldable composition commences. Subsequently, the temperature is 
further raised at (9) at a rate of 80-90.degree. C. per hour to 
480.degree. C., during which the thermoset polymer begins to degrade with 
the formation of gaseous degradation products. The temperature of the 
moulded part is held at (10) for 30 minutes at 480.degree. C. At the end 
of this time, the temperature is raised at 100-120.degree. C. per hour 
(11) to a final temperature (not shown) in the range of 1300-1500.degree. 
C. to sinter the metal or ceramic particles present in the moulded part 
and to complete degradation of the thermoset polymer into gaseous product. 
The foregoing examples clearly demonstrate that the present invention 
provides a unique method to manufacture high precision metal components 
eliminating the problems and the defects associated with the prior art. 
The defects and problems in the prior art which are due to the softening 
of the thermoplastic backbone polymers with increase in temperature for 
binder removal is eliminated through the use of the cross-linker which 
transforms the thermoplastic polymer into a rigid thermoset polymer during 
the binder removal.