Field of the Invention

This invention relates to a metal or ceramic powder injection moulding or extrusion composition including a reactive carrier; to a method of injection moulding or extruding an article which involves the use of the composition; and to an article made by that method.

Background to the Invention

Injection moulding is a well known technique for the manufacture of products, particularly components having a complex profile. A flowable material, often a plastics material, is injected into a mould and there allowed to harden to give a product of the desired shape.

Whilst injection moulding and extrusion are techniques well suited for the manufacture of plastics products, they are less useful in the manufacture of ceramic or metal products which might sometimes be preferred to plastics products because of their enhanced chemical and physical properties. Originally, metal and ceramic components were made by pressing a metal or ceramic powder into a mould. This technique was, however, only useful for very simple shapes of component. Accordingly, injection moulding techniques were sought which would enable the rapid production of more complex shapes from metal and ceramic materials.

The injection moulding of metal or ceramic materials involves the use of a carrier, in which the metal or ceramic material in powdered form is dispersed to give a flowable mixture for injection into a mould. Typical carriers, also known as "binders", used in such techniques are thermoplastics polymers and waxes. Following injection of the carrier/powder mixture into the mould, the carrier must then be debound to form a near-net-shape green compact. In known techniques, the carrier is burned off by the application of high temperatures, typically in a furnace.

Removal of the carrier raises a number of problems, the main one being that it can often take several days for all of the carrier to be removed, particularly since the carrier/powder mixtures typically contain up to 40% of the carrier. Although the debinding process is referred to as "burning", this is somewhat of a misnomer. With the thermoplastic or paraffinic wax polymers commonly used as carriers, the heating process initiates "cracking" of the polymer chains, thereby releasing volatile fractions. This is necessarily a time-consuming process, partly because the polymer "cracking" is inherently slow and also because one must ensure that there is no significant build-up of volatile fractions within the green compact, especially those of relatively high molecular weight, as this will detrimentally affect the integrity of the finished product. Thus, one of the major advantages of the injection moulding technique, i.e. its speed, is immediately negated by the time-consuming carrier removal step.

A further disadvantage in the use of such techniques is that the carrier is lost as volatile fractions, over a relatively long period of time, and cannot easily be collected for subsequent reuse. Yet another disadvantage is that if too much heat is applied, the green body may suffer from carbon deposits.

US patent no. 4906424, herein incorporated by reference, discloses the application of reaction injection moulding techniques to ceramic green bodies. A homogeneous mixture of finely divided ceramic or metallic material and a polymerisable monomeric or oligomeric binder is injected into a mould and polymerized therein to form the green body. Preferred monomeric binders are di- and tri-acrylate or - methacrylate esters of polyols. The low viscosity of the monomeric binder allows an injection moulding composition including greater than 50 vol.% ceramic powder to be produced. However, the preferred debinding process still involves "burning off" of the polymerised binder.

There is therefore a need for an injection moulding technique which combines the usual advantages of injection processes - i.e. speed of manufacture and the ability to produce complex product shapes - and yet suffers from none of the above described disadvantages and which can be used to make products from metal or ceramic materials having all the desirable chemical and physical properties of those materials.

Statements of the Invention According to a first aspect of the present invention there is provided a flowable composition, for use in an injection moulding or extrusion process, including a carrier comprising a monomeric or oligomeric species in which a ceramic and/or metallic powder is dispersed, the monomeric or oligomeric species being reversibly polymerisable so as to yield a solid polymer, which in turn is capable of undergoing a thermally activated depolymerisation reaction so as to yield the monomeric or oligomeric species in fluid form.

The use of a composition in accordance with the present invention, in an injection moulding or extrusion process, allows carrier removal times, after the carrier/powder mixture has been injected into a mould or extruded from a die, to be greatly reduced relative to those in conventional techniques. The solid polymer form of the carrier is removed from the green compact by initiating a thermally activated chemical depolymerisation reaction; the monomer or oligomer is thus released extremely quickly on heating. Accordingly, the use of a carrier in accordance with the present invention allows an injection moulding or extrusion process to proceed much more quickly than conventional processes, but with no loss in effectiveness or in quality of the final product.

Moreover, because the carrier reverts to its original monomeric or oligomeric form during the depolymerisation reaction, the released carrier may be collected and recycled in subsequent injection moulding or extrusion processes. Additionally, the monomeric or oligomeric molecules are sufficiently small to enable their escape from the green

compact almost immediately upon depolymerisation. There is little or no build-up of volatile carrier in the green compact and thus no adverse effect on the integrity of the finished product.

All that is needed in order to initiate the depolymerisation of the carrier is the application of heat to an appropriate activation temperature. The application of heat at the carrier removal stage is a step already carried out in known injection moulding processes. Thus, use of a carrier in accordance with the present invention need not involve any undue modification of existing techniques or apparatus.

The binder is preferably one which is depolymerisable from its solid to its fluid form by means of a chain reaction or "unzipping" . The presently preferred monomeric species is a cyanoacrylate, such as ethyl-alpha-cyanoacrylate. Such compounds, once polymerised, undergo a rapid thermally activated depolymerisation reaction - known as "unzipping" - which can be completed within a matter of seconds, after depolymerisation, virtually no undesirable residues are left.

Following initiation of the cyanoacrylate polymerisation reaction with an alkaline initiator, the carrier solidifies rapidly to form a hard solid polymer - of importance following injection of the binder/powder mixture into a mould or its extrusion through a die. The polymerised green body must be susceptible to being handled during transfer from the mould or die to the depolymerising furnace.

The reaction conditions under which the monomeric species undergoes polymerisation to the solid polymer may vary from one carrier in accordance with the invention to another. In the case of σyanoacrylates, for instance, the reaction conditions concerned would be alkaline conditions. Thus, initially, the carrier would need to be kept at an acid pH, to ensure that it remained in its monomeric or oligomeric form and that the carrier/powder mixture remained flowable. Alkaline conditions in the mould itself would then be used to

cause solidification - i.e. polymerisation - of the carrier.

On thermally activated depolymerisation, the carrier may be converted to a liquid form of its monomer or oligomer, since a liquid is more easy to control and collect than a gas. The liquid monomer can be collected relatively easily during the depolymerisation reaction, and reused as a carrier in further injection moulding processes. This is of particular advantage where a cyanoacrylate is used as the carrier, since cyanoacrylates tend in general to be expensive materials. Alternatively, the carrier may escape in gaseous form and be collected by condensation.

Whether depolymerisation yields a liquid or a gaseous monomer product will depend to an extent on the temperatures used during carrier removal in an injection moulding process. Thus, a carrier in accordance with the invention is chosen according to whether it depolymerises to an acceptable form under the temperature conditions desired to be employed in any particular process.

The carrier of the present invention is preferably not unduly toxic, and preferably polymerises rapidly under the appropriate reaction conditions. Polymerisation in a matter of minutes or even seconds is attainable with cyanoacrylate carriers.

The composition preferably comprises up to around 75% of the ceramic and/or metal powder, more preferably up to around 60%. The ratio of powder to carrier will be chosen according to the particular ceramic/metal, the conditions under which it is to be used and the article which is to be made by the injection moulding or extrusion process. The concentration must be chosen so that the overall composition is still flowable. At higher concentrations of the metal/ceramic material, problems of unacceptably high viscosity may arise. It has been found, however, that in a composition in accordance with the invention, such problems can be overcome by incorporating the

ceramic and/or metal material in the form of a powder comprising a mixture of powder sizes.

The composition may include a polymerisation inhibitor such as p-toluene-sulphonic acid.

According to a second aspect of the present invention there is provided a method of injection moulding an article, comprising dispersing a ceramic and/or metallic powder in a carrier to form a flowable composition, the carrier comprising a monomeric or oligomeric species, the monomeric or oligomeric species being reversibly polymerisable so as to yield a solid polymer, which in turn is capable of undergoing a thermally activated depolymerisation reaction so as to yield the monomeric or oligomeric species in fluid form; injecting the composition into a mould; and causing the carrier to polymerise in the mould.

Preferably, conditions in the mould are such as to promote polymerisation of the carrier. For example, the mould may have a polymerisation initiator coated on its surfaces. Where the carrier is a cyanoacrylate, the initiator is preferably pyridine or tertiary butyl amine.

According to a third aspect of the present invention there is provided a method of extruding an article, comprising dispersing a ceramic and/or metallic powder in a carrier to form a flowable composition, the carrier comprising a monomeric or oligomeric species, the monomeric or oligomeric species being reversibly polymerisable so as to yield a solid polymer, which in turn is capable of undergoing a thermally activated depolymerisation reaction so as to yield the monomeric or oligomeric species in fluid form; and passing the composition through an extrusion die while simultaneously causing the carrier to polymerise in the die before its emergence as an extrusion.

Either method may include mixing the composition with a polymerisation initiator prior to its injection into the mould

or its being passed into the die. This mixing may be achieved by means of a mixing head. For cyanoacrylates, the initiator may be alkaline. Preferred polymerisation initiators are pyridine and tertiary butyl amine.

Once the polymerised article has been removed from the mould or extruded from the die, either methods may include subsequently heating the composition so as to cause depolymerisation of the carrier and hence its release from the composition.

In either method, the released carrier may be collected and removed, preferably for re-use in the method but alternatively to waste. The temperature to which the composition is heated so as to cause depolymerisation of the carrier may be such that the carrier is released in the form of a liquid or a gas. The carrier may be collected by distillation if in gaseous form.

Either method may then include sintering the resulting article and optionally finishing the article by machining or polishing etc.

Either method may include mixing a polymerization inhibitor with the carrier prior to dispersion therein of the powder. For cyanoacrylate carriers, the preferred inhibitor is acidic, for example p-toluene-sulphonic acid. Preferably, the mixture of cyanoacrylate and p-toluene-sulphonic acid contains at least 0.5% acid.

According to a fourth aspect of the present invention there is provided an injection moulded article, made by a method in accordance with the second or third aspect of the present invention.

The present invention will now be described by way of example only.

Detailed Description

The flowable injection moulding or extrusion composition of the present invention is achieved by dispersing ceramic or metal powder in a reactive binder. Any ceramic or metal which can be finely divided may be used. In the following examples, a ceramic powder of irregularly shaped silicon nitride particles with a particle size of less than 350 micrometres is used; similarly, a metal powder of spherical gas atomised 316 L stainless steel particles with a particle size of less than 25 micrometres is used. The composition of the stainless steel includes 16-19% Cr, 10-12% Ni and 2-3% Mo Rem Fe.

A carrier for use in the composition is chosen so as ideally to perform the following functions: -

1. To create a flowable and mouldable composition on mixing with a ceramic/metal powder, using the lowest possible concentration of carrier.

2. To produce distortion free mouldings or extrusions having adequate mechanical integrity.

3. To allow rapid and uniform carrier release, "debinding", with minimal slumping and leaving as little as possible carrier residue.

The present invention provides a particular form of carrier, namely a reactive carrier. A consideration in selecting a suitable reactive carrier is the mechanism and kinetics of the debinding process. Many known reactive systems have undesirable thermal breakdown behaviour; these must be avoided in carrying out the present invention.

The reactive binder may be any reversibly polymerisable monomeric or oligomeric species having the desired properties. Preferred binders are cyanoacrylates and in the following examples, ethyl-alpha-cyanoacrylate supplied by Loctite is used. The composition of the product supplied is 99.9 wt.%

ethyl-alpha-cyanoacrylate, 10 wt.% polyalkyl methacrylate thickener and 0.2 wt.% organic stabilisers. The specific gravity of the composition is 1.05 to 1.10 gem ^"3 .

The present invention offers the possibility of using low viscosity pre-polymer systems as carriers in injection moulding processes. Such carriers, in particular cyanoacrylates, are monomers which polymerise rapidly so as to cause a correspondingly rapid in-mould solidification. Furthermore, such carriers can easily be thermally degraded, leaving no residue. Because the degradation process involves the quantitative conversion of the polymer to the original monomer, recovery of the carrier material and possible recycling is also possible.

Cyanoacrylate carriers offer a number of advantages over the conventional carriers such as waxes and thermoplastics. These include:

1. Low viscosity of the carrier.

2. High polarity of the carrier, giving good wetting and binding.

3. Rapid solidification, even at room temperature. Reaction times to provide a number average molecular weight in excess of 10 ⁶ can be less than 1 second.

4. Rapid debinding on application of heat. The polymer degrades by an unzipping mechanism having a low - less than 23 Kcal mole ^"1 - activation energy.

5. Relatively high maintenance of mechanical strength during the unzipping process.

6. The possibility of recovery and recycling of the product of the thermal breakdown.

7. The possibility of using relatively coarse grain metal/ceramic powders in injection moulding compositions. This would represent a significant economic advantage over current methods, which necessitate the use of expensive fine powders.

Cyanoacrylates possess the desirable property that both the polymerisation and depolymerisation processes proceed very quickly. The polymerisation process for ethyl-alpha- cyanoacrylate and cyanoacrylates in general has an initiation, propagation and termination stage, and both inter- and intra¬ molecular transfer may be possible. Depending on the nucleophile and the ester, the initiation stage can involve up to three monomer additions before the anion or Zwitterion is capable of rapid polymerisation.

A kinetic scheme for polymerisation using a strong base is as follows:

initiation: Nu ^" + M > NuM ^" NuM ^" + M > NuM ₂

propagation: Z" + M > Z^

ZIx + M > z; ₂

termination: Z ₂ + H ^* > Z _t2

Owing to the stability of the carbanion, transfer reactions are not of major importance. Termination is generally via impurities or may be capped by the addition of strong acids.

A kinetic scheme for polymerisation by Zwitterion formation is as follows:

initiation: P _y + M ===== ^* Z" ^* Z" + M > ^* z

propagation: ^* Zj + M > ^* Zj _tl

Termination may be by end group protonation or by inter- or intra-molecular transfer.

The overall degradation reaction is believed to be a free radical unzipping of the chain with an effectively infinite zip length. Evidence for this is firstly that the molecular

weight distribution is not markedly changed after 50% weight loss and secondly that the quaternary carbon confers the radical stability normally required for an unzipping process, so transfer reactions are not likely owing to the absence of alpha-hydrogens.

In addition to the powder and binder, the composition may also include additives such as polymerisation inhibitors as discussed in the following examples. A suitable inhibitor is p-toluene-sulphonic acid.

Once the composition is formed and homogeneously mixed, it can be injected into a mould or an extrusion die by known techniques. Where the composition is injected into a mould, an injection pressure of up to 10,000 to 15,000 psi may be used. Polymerisation of the binder in the mould may be catalysed or initiated by means of a suitable initiator which can, for example, be coated on the surfaces of the mould or mixed with the composition during injection by means of a mixing head. Where the composition is injected into an extrusion die, a catalyst or initiator may similarly be present, but would usually be mixed with the composition just prior to or simultaneously with its injection into the die.

Conditions in the mould or die are maintained so as to promote polymerisation of the binder within a relatively short time, preferably minutes or less than 1 minute. Since the productivity of the process depends upon the dwell time within the mould or the extrusion die, it is preferred that polymerisation should proceed as quickly as possible. In the case of a mould having an initiator coated on its surfaces, the green body can be demoulded so long as the binder at its surfaces has polymerised, even though the binder towards the centre of the green body may not have polymerised fully at that stage.

Polymerisation of the binder results in a small but significant reduction in the volume which it occupies. This causes the particles of ceramic or metal to be pressed tightly

together and results in a more handleable product once the binder has been removed and prior to sintering. When the polymerisation of the binder is complete throughout the green body, the binder can be removed by the application of heat, for example in a furnace. A temperature of 200°C is sufficient for the de-binding step and initiates the "unzipping" depolymerisation described above. Within a few seconds, the depolymerisation process can be complete, with the monomeric species being given off in gaseous form. The gaseous cyanoacrylate can be recovered and recycled in known manner by means of a distillation column.

With the binder thus removed, the green body can proceed to the sintering furnace, where it will be sintered at elevated temperatures in accordance with standard practice. The sintered product may finally be machined or polished as required.

Illustrative examples of compositions according to the present invention and their properties will now be discussed as follows:

Example 1

A ceramic powder of irregularly shaped silicon nitride particles with a particle size of less than 350 micrometres was mixed in the ratios 1:1, 3:2, 1:2 and 1:3 with Loctite ethyl-alpha-cyanoacrylate. Initiation and polymerisation occurred within five seconds in all cases, regardless of mixing ratios. It was found, however, that the powder should be added to the monomer and not vice versa.

Example 2

A metal powder of spherical gas atomised 316 L stainless steel particles with a particle size of less than 25 micrometres was mixed in the ratios 1:2, 1:1, 2:1 and 4:1 with Loctite ethyl- alpha-cyanoacrylate. Initiation was instantaneous on addition of stainless steel powder to the monomer.

The above examples demonstrate the extreme reactivity of cyanoacrylate monomer when mixed with untreated powder samples. Although the processing technology involved in producing the silicon nitride and 316 L stainless steel powder results in very pure products, it appears that there is sufficient surface contamination to cause initiation. In the case of silicon nitride, there is the possibility of trace amounts of ammonia being present. In both cases, atmospheric moisture may have been absorbed and there is also the possibility of Bronsted base sites being present on the powder surface.

Example .3

The silicon nitride ceramic powder of example 1 was mixed in the ratios 1:1 and 2:1 with Loctite ethyl-alpha-cyanoacrylate but with pretreatment of all glassware and powders. The glassware was pretreated by washing in 10% nitric acid, drying in a circulating warm air oven. The ceramic powder was either dried or acid washed and dried and the results were as follows:

Table 1

Mixing ratio Retardation time Retardation time

Si ₃ N _« : (sees) powder dried powder acid washed

E.C.A. and dried

1:1 90 100

1:1 110 90

1:1 80 mean time=93 sec 85 mean time=92 sec

2:1 75 70

2:1 95 94

2:1 100 mean time=96 sec 110 mean time=98 sec

These results indicate that acid washing of the ceramic powder had little or no effect on delayed initiation.

Example 4

Example 2 was reproduced but with pretreatment of the metal powder and glassware as described in example 3. No inhibition of retardation was experienced.

As illustrated by example 3 and table 1 it was found that predrying or acid washing and predrying the silicon nitride powder resulted in a retardation period of approximately 90 seconds being obtained. This indicates that some of the surface contaminants had been removed, but not in sufficient amounts to delay initiation to any significant extent. No retardation was seen with stainless steel powders, as is evinced by example 4. This may be due to the surface area of the stainless steel powder being extremely high, thus exposing a greater contamination area.

Example 5

In this example, the monomer solution was pretreated by heating to 25, 35 and 45 ^β C. Both pretreated and untreated ceramic and stainless steel powders were added to the monomer at each of these temperatures and the results were very similar to those obtained in examples 1 to 4 above. This indicates that preheating the monomer had little or no effect on delaying initiation.

Example 6

In this example, the monomer solution was pretreated by the addition or p-toluene-sulphonic acid, a strong acid inhibitor. Solutions of 0.01%, 0.1%, 0.5%, 1%, 2%, 3% and 4% were evaluated. The standard mixing procedure involved making an inhibited monomer solution, containing 0.5% inhibitor for use with stainless steel powder and 1% inhibitor for ceramic powder. The solution was allowed sufficient time for mixing and dissolution of the inhibitor. The monomer was weighed into a clean dry glass container using a top hand balance. The powder was then added in discrete amounts under constant stirring to ensure

homogenous mixing. Compacts were then moulded by finger rolling.

It was found that after the addition of greater than 0.5% inhibitor the inhibitor became less soluble in the monomer and required up to one hour to dissolve completely. After 48 hours the inhibitor monomer solution become quite discoloured. In addition, a slight viscosity increase was noticed.

Figure 1 is a plot of inhibition time versus volume mixing ratio of ceramic to ethyl-alpha-cyanoacrylate for a range of inhibitor concentrations. Neither powder nor glassware were pretreated. At less than 0.5% inhibitor, initiation was not sufficiently delayed, but it can nevertheless be seen that there is a dependency between the mixing ratios and inhibition periods for specific levels of inhibitor.

Figure 2 is a plot of inhibition time versus volume mixing ratio of stainless steel to ethyl-alpha-cyanoacrylate for a range of inhibitor concentrations. Neither powder nor glassware was pretreated. At an inhibitor concentration of 0.01%, inhibition of up to 30 minutes was obtainable for level weight mixing ratios 2:1 and 4:1. Again, there is a dependency between mixing ratios and inhibition periods for specific levels of inhibitor.

The results from this example demonstrate that the inhibition period is dependent on the concentration of inhibitor and on the mixing ratio of the powder at specific levels of inhibitor. They also demonstrate that the addition of high concentrations of inhibitor increases the premould binding capacity of the monomer solution. Inhibition is considered to be the very rapid termination by acid fully initiated chains. It follows therefore that, for successful inhibition, the concentration of acid must be greater than the concentration of basic species which can act as an initiator. The concentration of acid must be high enough to overcome initiation effects of impurities

present on the powders and glassware. By increasing the amount of powder in the system, the concentration of impurities is increasing and thus decreasing the maximum inhibition period.

Thus, polymerisation can readily be controlled, provided that the acid is added to the monomer and not employed as a powder surfactant. An ability to prevent the monomer from polymerising prior to injection of the carrier/powder mixture into a mould or an extrusion die is a mandatory requirement.

Example 7

This example involved the evaluation of polymerisation initiators. Under evaluation where pyridine/pyridine vapour, tertiary butyl amine and its vapour and caffeine 1%, 2%, 3% and 4% made up in an aqueous solution with D.I. water. The liquid initiators were applied to a compact by wetting a clock glass with approximately 1ml of the initiator. The compact surface was then exposed to the initiator by dapping the compacts by use of tweezers. The compacts were exposed to the vaporised initiators by insertion of a wire through a sample and placement of the sample above the vapour stream in a round bottom flask. The flask was placed in a heating mantle and temperatures of 25°C to 35°C were found to be sufficiently high to vaporise the amines. This approach was only used for ceramic compacts.

Stainless steel mixes of 10:1 and 15:1 using 0.5% inhibited monomer solution were evaluated. It was found for three weights, 150 mg, 300mg and 400mg, that initiation regardless of sample weight was instantaneous on application of the initiators pyridine and tertiary butyl amine. It was found that none of the caffeine solutions initiated a 300mg sample of the 15:1 or 10:1 stainless steel mix.

Ceramic mixes of 2:1 and 3:1 bound using 1% inhibited monomer were evaluated. It was found for three weights, 150 mg, 300mg and 450mg, that initiation was not instantaneous and generally took between 30 and 45 minutes before cure. It was found that the 2:1 mixes generally cured at the later time. Sample size did not appear to be a significant feature in onset of the cure. Nevertheless, it was found that the reapplication of initiator after 15 minutes resulted in a rapid cure in all of the above cases. Both pyridine and tertiary butyl amine were evaluated and initiator type had little significance.

None of the caffeine solutions was successful in initiating a 300mg 2:1 or 3:1 mix sample.

These tests establish that either pyridine or tertiary butyl amine would satisfy the need for a polymerisation initiator to ensure in-mould or in-die solidification of the cyanoacrylate monomer to form green compacts.

Rheological Analysis

A Haake viscometer in pk mode was used to examine ceramic 1:1, 2:1 and 3:1 mix samples and stainless steel 5:1, 10:1 and 15:1 mix samples under shear rates of 128, 256 and 512 sec" ¹ . A pk I, 0.3° cone was used at a temperature of 19 ^β C throughout.

With the metallic powder, the viscosities of the mixtures are dependent both on powder loading and shear rate. These two parameters become very significant at high mixing ratios. At a sheer rate of 128s ^"1 the viscosity increase from 150 to 800 mPas on increasing the mixing ratio from 10:1 to 15; 1. The result is a significant reduction in the fluid type characteristics of the mix. At viscosities of 800 mPas, a mix of this nature would be unsuitable for conventional low pressure injection moulding systems. When the sheer rate is increased from 128 to 512s ^"1 , the viscosity drops from 800 to 250 mPas, indicating the mix

has very strong pseudo-plastic characteristics. It would appear that at this mixing ratio the capacity of the binder has been exceeded leading to breakup of the powder and the binder system under high sheer rates, thus leaving it unsuitable for injection moulding. From extrusion testing using a simple syringe extruder and injector system it was found that the 15:1 mix sample broke up when injected. This behaviour could easily lead to serious problems in mould filling.

The lower mixing ratio shows some pseudo-plastic tendency, but the change in the viscosity is relatively low and is unlikely to cause significant problems. From extrusion testing is appears that mould filling would not be a problem as viscosities are sufficiently low for injection moulding.

In the case of ceramic mix samples, a similar trait was noticed. It was found that the highest mixing ratio 3:1 showed the greatest pseudo-plastic tendencies. At higher viscosity, the tendency of powder/binder system to break up under high sheer rates would make it unsuitable for injection moulding. In the case of stainless steel, it would appear lower mixing ratios are more suitable for injection mouldings.

However, additional data obtained using a bi-modal (two size) distribution of water atomised, stainless steel powders, with sizes in the range 30 micrometres to 100 micrometers, has suggested that around 70% powder mixes could readily be injection moulded or extruded. Similar considerations are expected to apply to bi-modal ceramic powders.

Density Evaluation

The green density of moulded compacts was determined using the volume swell tester EBO 5. For each green density calculation, three samples were weighed and the mean value

calculated. The density of moulded silicon nitride compacts was between 92% and 99% theoretical green density. For stainless steel compacts, densities of 98% to 99% theoretical green density were attained employing a bi- modal distribution of particles sizes.

Debinding Characteristics

The debinding characteristics of both ceramic and stainless steel compacts were investigated using dynamic and isothermal thermogravimetric analysis. In both cases, samples heated dynamically at a rate of 20 ^C C per minute remained stable and in the case of stainless steel samples, stability was retained at a heating rate of 100"C per minute. Rapid debinding of the cyanoacrylate binder was observed in samples heated to temperatures of 200 ^β C and

300°C. Debinding to a residual organic binding content of less than 6% was achieved in two to ten minutes, depending on the debinding temperature employed.