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Development in Powder Co-Injection Moulding
Development in Powder Co-Injection Moulding D. J. Stephenson School of Industrial and Manufacturing Science, Cranfield University, Cranfield, U.K. Submitted by P. A. McKeown (11, School of Industrial and Manufacturing Science, Cranfield University, Cranfield, U.K.
Abstract Powder co-injection moulding (PCIM), is a novel technique for the surface engineering of PIM components. The process uses two moulding feedstocks which are injected into a mould sequentially, so that one feedstock forms the surface layer of the component and the other forms the core. Work to date has shown that the injection moulding time and the viscosity ratio of the two feedstock materials are critical in determining the surface layer profile. This has been confirmed through finite element modelling predictionsof the skin layer thickness distributionwhich correlate well with experimental observations. Keywords:
Powder Injection Moulding, Coating, Modelling
1 INTRODUCTION In recent years developments in materials processing have continued to focus on methods for the reduction of manufacturing costs whilst enhancing component performance. Major factors that contribute to a reduction in manufacturing costs include the ability to manufacture to 'net-shape' and more efficient processing through a reduction in the 'process chain'. Enhanced performance can be realised through better control of processing parameters, improved microstructural control and the application of surface engineering principles. Powder metallurgy based processes can satisfy many of these requirements. In particular, the application of powder injection moulding (PIM) is now more common for the high volume production of small, intricately shaped components. PIM is an extension of conventional polymer injection moulding. Polymer based binders are mixed with metal or ceramic powders to produce a granulated feedstock which is then injection moulded to form a green component. The component is then debound to remove the polymer, before sintering to the required density. Shrinkage during sintering should be uniform so that the component retains the shape of the mould. Providing the magnitude of the shrinkage can be predicted, tolerances of 0.2 to 0.3% of nominal dimensions can be achieved [I]. However, the ability to manufacture components that are also free from defects requires the use of appropriate processing parameters during the moulding, debinding and sintering cycles [2]. Surface engineering has developed considerably over the last few decades such that surface coatings are now a well accepted part of component design. For engineering applications surface treatments and coatings are most often used to enhance corrosion and wear resistance. The substrate is chosen to ensure optimum structural efficiency and the coating system used to tailor the surface properties and provide the required level of environmental resistance.
Annals of the ClRP Vol. 49/1/2000
Many of the conventional methods of applying surface coatings to components result in a significant increase in cost due to both the additional processing stage and the high capital cost of equipment. There would clearly be advantage in developing methods of surface engineering which can be undertaken as part of the primary manufacturing process, thus ensuring cost-effective manufacture, particularly when large numbers of complex shaped components are to be produced. Current research at Cranfield is addressing this requirement by combining conventional PIM with powder metallurgy surface engineering [3,4].
2 POWDER CO-INJECTION MOULDING Powder co-injection moulding (PCIM) combines the well established polymer sandwich moulding process [5] with conventional PIM in order to produce small complex components which are surface engineered in-situ. Sandwich moulding is normally used to manufacture laminated polymer structures by injection moulding. Two thermoplastics are injected simultaneously or sequentially, so that one forms a skin layer at the surface surrounding the second material that forms the core. Applications include the production of high quality surface finishes on foamed or low cost cores and the selective fibre reinforcement of the skin layer. In PCIM, a twin barrel injection moulding machine is used, with the two polymer feedstock materials replaced by two PIM feedstock materials. The Cranfield Dassett DM30 twin-barrel injection moulding machine enables sequential co-injection moulding to be undertaken by using a second barrel mounted at right angles to the first. Switch over between barrels is accomplished by a pneumatic valve positioned just behind the heated injection nozzle. The three stage moulding cycle is illustrated in Figure 1. In stage 1, material from barrel A is injected into the mould to form the skin layer. In stage 2, the switch over valve is repositioned to inject core
191
1 2;
.. ...
..
Figure 1: The three stages of the PCIM cycle material from barrel B. As the core material enters the mould it forces the skin material to the outer regions of the mould caw. In stage 3 injection reverts to barrel A and a small volume of skin material is applied to complete the surface layer at the injection point. The moutd is then packed under pressure. 2.1 Pmceumg . conditions and examptea of PCIM A typicat feedstock composition used when moulding a staintess steel alumina - carbonyl iron system is given in Table 1 [3]. In this example both camuba wax and paraffin wax are added to the polypropylene in order to modify the feedstock rheotogy and ease debinding. A two stage debinding process is used, in which the wax binder constituents are removed by solvent debinding in heptane at W C ,followed by thermal debinding to bum off the thermoplastic. The solvent debinding cycle generates interconnecting capdlary channels which aid the removal of polymer during the subsequent thermal debinding cycle. Binder, wt-% pdypropylene camuba wax Paraffinwax stearic Powder loading, vol.-% Skin 316L ParticulatealuminaCore OM carbonyl iron
60.0 7.5 31.5 1.o
60.0 5.0 47.5
TabIe 1: Typical composition of feedstocks used for PClM The potential of the PClM process has been demonstrated through the manufacture of components such as the gear wheels discussed below. Several material combinations have been studied including metals, ceramics and composites. Figure 2 itlustrates a gear wheel moutded from two thermoplastics to demonstrate the skidcore effect. The core consists of
192
Figure 2: Thermoplastic gear wheel showing skidcore
Figure 3: Section through stainless steellcarbonyl iron gear wheel following PCIM.
the black themoptastic and the skin, the white thermoplastic. A section through a similar gear wheel, moulded with two metal feedstocks is shown in Figure 3. In this example the core is carbonyl iron and the skin is a 31st stainless steet. The skin thickness varies from a minimum at the sprue to a maximum (complete skin layer) in the outer tooth region. Figure 4 illustrates a section thrwgh a-oomposite gear wheel following PCIM, debindmg and sintering. In this example the stainless steel coating contains 15 vol% A 1 2 0 3 particulate (13vm) to provide additional wear resistance at the component surface. Figure 4 demonstrates the high sintered density that can be achieved and excellent quality of the interface region. Similar success has been achieved with PCIM of ceramics. An alumina gear wheel is shown in Figure 5, after moulding and then following debinding and sintering. In this example the surface skin layer contains 20% ZrOz to provide a zirconia toughened alumina
conductivity) result in a more uniform skin thickness and greater core penetration towards the component edge. Experimentally, PClM has been undertaken successfully for a wide range of skin-core ratios, between 0.2:l and 8:l. This provides the process with considerable flexibility in terms of controlling the skin thickness distribution. Core: corbonyl iron
1 mm
Figure 4: Section through fully debound and sintered gear wheel showing skinlcore interface.
The sintering characteristics of the skin and core materials must be reasonably compatible if fully dense components and adherent coatings are to be produced. A difference in sintering rate between the core and skin materials may result in high levels of retained porosity and interfacial stresses, leading to prior coating adhesion. Thus, a critical part of the PClM process is to ensure compatible sintering rates between the skin and core materials, even when large differences in composition may exist. The densification rate can be controlled in a number of ways: material composition particle size distribution rn
Figure 5: Alumina gear wheel with ZTA surface a) Before and after sintering b) Skidcore interface (SEM image).
surface region. The interface region is well defined and sintered densities approaching the theoretical can be achieved. 2.2
Processing considerations
To ensure the successful exploitation of the PClM process, two major processing concerns must be addressed. These are: rn
The control of skin layer thickness and its variation across a component surface.
rn
Differential sintering rates between the skin layer and core materials.
Recent research [6] has shown that the skin layer thickness distribution depends on the skin-core volume ratio, skin-core viscosity ratio and the injection moulding time. The injection moulding time has a major effect on the frozen skin layer thickness developed during the early part of the injection moulding process. For short injection times, say less than 1 s,more of the skin layer remains in the molten state. This material can be pushed ahead of the core material via the 'Fountain effect' when injecting from the second barrel. Short injection times produce a less uniform skin thickness distribution, with more skin material at the component outer edge. For longer injection times, only a small volume of molten skin material may be ahead of the core and therefore there will be a higher probability of core breakthrough at the surface. Longer injection times also increase the skin thickness along the major dimension of the component but provide a more uniform thickness distribution. The relative viscosity also affects the skin thickness distribution, with a low skin-core viscosity ratio producing a distribution similar to the short injection time. Higher skin-core viscosity ratios (for a constant thermal
particle loading (volume fraction in feedstock)
Generally the material composition is fixed although additives may be used to modify sintering rate. Examples include the use of a transient liquid phase to accelerate sintering rates or the addition of ceramic particulate to metals (e.9. A1203 additions to carbonyl iron) to retard the rate of densification. Particle loading can also be varied over a wide range. For example, a reduction in the carbonyl iron loading from 60 vol% to 47.5 VOW has been used to reduce the sintering rate until it matches a more coarse stainless steel powder moulded at a 60 vol% loading. Another approach used to overcome differences in particle size is to use a master batch powder. For example, to produce a stainless steel a coarse master batch stainless steel powder (20 pm) can be blended with a fine carbonyl iron powder (5 pm) in the appropriate proportions. The fine carbonyl iron powder accelerates the sintering rate whilst a homogeneous stainless steel is produced following the sintering cycle. Figure 6 illustrates a PClM section where the skin layer consists of master batch 316L stainless steel and fine carbonyl iron and the core material is only carbonyl iron. Figure 6a shows the part sintered structure highlighting the coarse and fine powders in the outer skin layer. Figure 6b shows the more homogeneous structure produced following the complete sintering cycle.
Figure 6: PClM sections from 316L ss master batch skin and carbonyl iron core. a) part sintered b) fully sintered.
193
When modifying the feedstock characteristics in order to match sintering rates, the situation is further complicated by associated changes that may occur in the heological properties of the feedstock materials. This includes changes to viscosity and thermal conductivtty which both influence the mould filling process and final skin layer thickness distribution.
to predict the position of the skin-core interface at specific locations is also required. The through thickness structure of the skin layer often has an asymmetric distribution and currently the software only predicts core/skin thickness ratio at specific points across the section.
Suitable conditions for successful sintering can be determined experimentally and the heological properties and thermal characteristics of the feedstock materials quantified. Using these data it is necessary to idenm the range of injection moulding conditions which will produce a given skin thickness distribution and this can be achieved using finite element modelling.
4 CONCLUSIONS
3 MODELLING OF THE PClM PROCESS C-MOLD, a commercial finite element modelling programme, has been used to simulate the PClM process and predict skin thickness distribution. The software, designed for polymer injection moulding, includes a simulation of the polymer sandwich moulding process and this has been adapted to model the PClM process. A finite element mesh representation of the gear wheels shown in Figures 2 to 6 was made and the heological characteristics of feedstock materials measured using capillary rheometry. Heat capacity and thermal conductivity were also been determined. The simulation predicts the relative skin and core fractions at any point across the component. Figure 7 shows the predictedand measured skin thickness fraction distribution for a stainless steeVcarbony1 iron system following PCIM. As can be observed from Figure 7 the model is able to predict the trend in data successfully.
C
0
5 10 15 20 25 30 Radial Position (mm)
Figure 7: Predicted and measured skin thickness values for stainless steeVcarbonyl iron PClM gear wheel.
However further improvements are required. Currently residual stress in co-injection moulded components cannot be modelled. This is a key aspect of improving dimensional tolerance on PClM components. The ability
194
1. PClM is a novel process for the manufacture of surface engineered components produced from metal, ceramic and composite powders. 2. The surface layer thickness distribution depends on the injection moulding parameters, for example injection time, and the heological properties of the feedstock materials. Thermal conductivity is of particular importance as this Influences the feedstock viscosity during moulding and hence the frozen skin thickness.
3. Successful processing by PClM requires that the surface and core powders exhibit compatible sintering characteristics. In particular, sintering rates must be controlled to minimise interfacial stress that could result in delamination of the skin from the core. 4. Successful methods for the control of differential sintering rate include varying the powder loading, modlfying material composition and the use of master batch powders to vary particle size.
5. Modelling using C-MOLD has demonstrated the ability to predict the skidcore thickness ratio as long as the heological and thermal characteristics of the feedstocks are quantified. 5 ACKNOWLEDGEMENTS The author would like to thank the Engineering and Physical Sciences Research Council for supporting this research and Dr J R Alcock for his significant contribution to this work
6 REFERENCES [l] German, R. M., 1990. Powder Injection Moulding, Metal Powder Industries Federation, Princeton, New Jersey. (21 Gelin, J.C.. Bamere,T. Dutilly,M.,l999, Experiments and Computational Modeling of Metal Injection Molded Parts, Annals of the CIRP, 4811: 179-182 [S] Stephenson, D. J., 1994, Surface Modification Technologies, vol VII, The lnst .of Materiak43-53. [4] Alcock, J. R., Logan, P. M., Stephenson, D. J., 1998, Surface Engineering by &injection Moulding, Surface and Coating Technology, 105: 6571. [5] Meyer. W., 1989. Plastics Technology: 109-112 [6] Alcock, J. R., Hanson, S. M. J., Stephenson, D. J., 1998, Surface Layer Thickness Control in Metal Injection Moulding, PM 98, vol3: 27-31
Abstract Powder co-injection moulding (PCIM), is a novel technique for the surface engineering of PIM components. The process uses two moulding feedstocks which are injected into a mould sequentially, so that one feedstock forms the surface layer of the component and the other forms the core. Work to date has shown that the injection moulding time and the viscosity ratio of the two feedstock materials are critical in determining the surface layer profile. This has been confirmed through finite element modelling predictionsof the skin layer thickness distributionwhich correlate well with experimental observations. Keywords:
Powder Injection Moulding, Coating, Modelling
1 INTRODUCTION In recent years developments in materials processing have continued to focus on methods for the reduction of manufacturing costs whilst enhancing component performance. Major factors that contribute to a reduction in manufacturing costs include the ability to manufacture to 'net-shape' and more efficient processing through a reduction in the 'process chain'. Enhanced performance can be realised through better control of processing parameters, improved microstructural control and the application of surface engineering principles. Powder metallurgy based processes can satisfy many of these requirements. In particular, the application of powder injection moulding (PIM) is now more common for the high volume production of small, intricately shaped components. PIM is an extension of conventional polymer injection moulding. Polymer based binders are mixed with metal or ceramic powders to produce a granulated feedstock which is then injection moulded to form a green component. The component is then debound to remove the polymer, before sintering to the required density. Shrinkage during sintering should be uniform so that the component retains the shape of the mould. Providing the magnitude of the shrinkage can be predicted, tolerances of 0.2 to 0.3% of nominal dimensions can be achieved [I]. However, the ability to manufacture components that are also free from defects requires the use of appropriate processing parameters during the moulding, debinding and sintering cycles [2]. Surface engineering has developed considerably over the last few decades such that surface coatings are now a well accepted part of component design. For engineering applications surface treatments and coatings are most often used to enhance corrosion and wear resistance. The substrate is chosen to ensure optimum structural efficiency and the coating system used to tailor the surface properties and provide the required level of environmental resistance.
Annals of the ClRP Vol. 49/1/2000
Many of the conventional methods of applying surface coatings to components result in a significant increase in cost due to both the additional processing stage and the high capital cost of equipment. There would clearly be advantage in developing methods of surface engineering which can be undertaken as part of the primary manufacturing process, thus ensuring cost-effective manufacture, particularly when large numbers of complex shaped components are to be produced. Current research at Cranfield is addressing this requirement by combining conventional PIM with powder metallurgy surface engineering [3,4].
2 POWDER CO-INJECTION MOULDING Powder co-injection moulding (PCIM) combines the well established polymer sandwich moulding process [5] with conventional PIM in order to produce small complex components which are surface engineered in-situ. Sandwich moulding is normally used to manufacture laminated polymer structures by injection moulding. Two thermoplastics are injected simultaneously or sequentially, so that one forms a skin layer at the surface surrounding the second material that forms the core. Applications include the production of high quality surface finishes on foamed or low cost cores and the selective fibre reinforcement of the skin layer. In PCIM, a twin barrel injection moulding machine is used, with the two polymer feedstock materials replaced by two PIM feedstock materials. The Cranfield Dassett DM30 twin-barrel injection moulding machine enables sequential co-injection moulding to be undertaken by using a second barrel mounted at right angles to the first. Switch over between barrels is accomplished by a pneumatic valve positioned just behind the heated injection nozzle. The three stage moulding cycle is illustrated in Figure 1. In stage 1, material from barrel A is injected into the mould to form the skin layer. In stage 2, the switch over valve is repositioned to inject core
191
1 2;
.. ...
..
Figure 1: The three stages of the PCIM cycle material from barrel B. As the core material enters the mould it forces the skin material to the outer regions of the mould caw. In stage 3 injection reverts to barrel A and a small volume of skin material is applied to complete the surface layer at the injection point. The moutd is then packed under pressure. 2.1 Pmceumg . conditions and examptea of PCIM A typicat feedstock composition used when moulding a staintess steel alumina - carbonyl iron system is given in Table 1 [3]. In this example both camuba wax and paraffin wax are added to the polypropylene in order to modify the feedstock rheotogy and ease debinding. A two stage debinding process is used, in which the wax binder constituents are removed by solvent debinding in heptane at W C ,followed by thermal debinding to bum off the thermoplastic. The solvent debinding cycle generates interconnecting capdlary channels which aid the removal of polymer during the subsequent thermal debinding cycle. Binder, wt-% pdypropylene camuba wax Paraffinwax stearic Powder loading, vol.-% Skin 316L ParticulatealuminaCore OM carbonyl iron
60.0 7.5 31.5 1.o
60.0 5.0 47.5
TabIe 1: Typical composition of feedstocks used for PClM The potential of the PClM process has been demonstrated through the manufacture of components such as the gear wheels discussed below. Several material combinations have been studied including metals, ceramics and composites. Figure 2 itlustrates a gear wheel moutded from two thermoplastics to demonstrate the skidcore effect. The core consists of
192
Figure 2: Thermoplastic gear wheel showing skidcore
Figure 3: Section through stainless steellcarbonyl iron gear wheel following PCIM.
the black themoptastic and the skin, the white thermoplastic. A section through a similar gear wheel, moulded with two metal feedstocks is shown in Figure 3. In this example the core is carbonyl iron and the skin is a 31st stainless steet. The skin thickness varies from a minimum at the sprue to a maximum (complete skin layer) in the outer tooth region. Figure 4 illustrates a section thrwgh a-oomposite gear wheel following PCIM, debindmg and sintering. In this example the stainless steel coating contains 15 vol% A 1 2 0 3 particulate (13vm) to provide additional wear resistance at the component surface. Figure 4 demonstrates the high sintered density that can be achieved and excellent quality of the interface region. Similar success has been achieved with PCIM of ceramics. An alumina gear wheel is shown in Figure 5, after moulding and then following debinding and sintering. In this example the surface skin layer contains 20% ZrOz to provide a zirconia toughened alumina
conductivity) result in a more uniform skin thickness and greater core penetration towards the component edge. Experimentally, PClM has been undertaken successfully for a wide range of skin-core ratios, between 0.2:l and 8:l. This provides the process with considerable flexibility in terms of controlling the skin thickness distribution. Core: corbonyl iron
1 mm
Figure 4: Section through fully debound and sintered gear wheel showing skinlcore interface.
The sintering characteristics of the skin and core materials must be reasonably compatible if fully dense components and adherent coatings are to be produced. A difference in sintering rate between the core and skin materials may result in high levels of retained porosity and interfacial stresses, leading to prior coating adhesion. Thus, a critical part of the PClM process is to ensure compatible sintering rates between the skin and core materials, even when large differences in composition may exist. The densification rate can be controlled in a number of ways: material composition particle size distribution rn
Figure 5: Alumina gear wheel with ZTA surface a) Before and after sintering b) Skidcore interface (SEM image).
surface region. The interface region is well defined and sintered densities approaching the theoretical can be achieved. 2.2
Processing considerations
To ensure the successful exploitation of the PClM process, two major processing concerns must be addressed. These are: rn
The control of skin layer thickness and its variation across a component surface.
rn
Differential sintering rates between the skin layer and core materials.
Recent research [6] has shown that the skin layer thickness distribution depends on the skin-core volume ratio, skin-core viscosity ratio and the injection moulding time. The injection moulding time has a major effect on the frozen skin layer thickness developed during the early part of the injection moulding process. For short injection times, say less than 1 s,more of the skin layer remains in the molten state. This material can be pushed ahead of the core material via the 'Fountain effect' when injecting from the second barrel. Short injection times produce a less uniform skin thickness distribution, with more skin material at the component outer edge. For longer injection times, only a small volume of molten skin material may be ahead of the core and therefore there will be a higher probability of core breakthrough at the surface. Longer injection times also increase the skin thickness along the major dimension of the component but provide a more uniform thickness distribution. The relative viscosity also affects the skin thickness distribution, with a low skin-core viscosity ratio producing a distribution similar to the short injection time. Higher skin-core viscosity ratios (for a constant thermal
particle loading (volume fraction in feedstock)
Generally the material composition is fixed although additives may be used to modify sintering rate. Examples include the use of a transient liquid phase to accelerate sintering rates or the addition of ceramic particulate to metals (e.9. A1203 additions to carbonyl iron) to retard the rate of densification. Particle loading can also be varied over a wide range. For example, a reduction in the carbonyl iron loading from 60 vol% to 47.5 VOW has been used to reduce the sintering rate until it matches a more coarse stainless steel powder moulded at a 60 vol% loading. Another approach used to overcome differences in particle size is to use a master batch powder. For example, to produce a stainless steel a coarse master batch stainless steel powder (20 pm) can be blended with a fine carbonyl iron powder (5 pm) in the appropriate proportions. The fine carbonyl iron powder accelerates the sintering rate whilst a homogeneous stainless steel is produced following the sintering cycle. Figure 6 illustrates a PClM section where the skin layer consists of master batch 316L stainless steel and fine carbonyl iron and the core material is only carbonyl iron. Figure 6a shows the part sintered structure highlighting the coarse and fine powders in the outer skin layer. Figure 6b shows the more homogeneous structure produced following the complete sintering cycle.
Figure 6: PClM sections from 316L ss master batch skin and carbonyl iron core. a) part sintered b) fully sintered.
193
When modifying the feedstock characteristics in order to match sintering rates, the situation is further complicated by associated changes that may occur in the heological properties of the feedstock materials. This includes changes to viscosity and thermal conductivtty which both influence the mould filling process and final skin layer thickness distribution.
to predict the position of the skin-core interface at specific locations is also required. The through thickness structure of the skin layer often has an asymmetric distribution and currently the software only predicts core/skin thickness ratio at specific points across the section.
Suitable conditions for successful sintering can be determined experimentally and the heological properties and thermal characteristics of the feedstock materials quantified. Using these data it is necessary to idenm the range of injection moulding conditions which will produce a given skin thickness distribution and this can be achieved using finite element modelling.
4 CONCLUSIONS
3 MODELLING OF THE PClM PROCESS C-MOLD, a commercial finite element modelling programme, has been used to simulate the PClM process and predict skin thickness distribution. The software, designed for polymer injection moulding, includes a simulation of the polymer sandwich moulding process and this has been adapted to model the PClM process. A finite element mesh representation of the gear wheels shown in Figures 2 to 6 was made and the heological characteristics of feedstock materials measured using capillary rheometry. Heat capacity and thermal conductivity were also been determined. The simulation predicts the relative skin and core fractions at any point across the component. Figure 7 shows the predictedand measured skin thickness fraction distribution for a stainless steeVcarbony1 iron system following PCIM. As can be observed from Figure 7 the model is able to predict the trend in data successfully.
C
0
5 10 15 20 25 30 Radial Position (mm)
Figure 7: Predicted and measured skin thickness values for stainless steeVcarbonyl iron PClM gear wheel.
However further improvements are required. Currently residual stress in co-injection moulded components cannot be modelled. This is a key aspect of improving dimensional tolerance on PClM components. The ability
194
1. PClM is a novel process for the manufacture of surface engineered components produced from metal, ceramic and composite powders. 2. The surface layer thickness distribution depends on the injection moulding parameters, for example injection time, and the heological properties of the feedstock materials. Thermal conductivity is of particular importance as this Influences the feedstock viscosity during moulding and hence the frozen skin thickness.
3. Successful processing by PClM requires that the surface and core powders exhibit compatible sintering characteristics. In particular, sintering rates must be controlled to minimise interfacial stress that could result in delamination of the skin from the core. 4. Successful methods for the control of differential sintering rate include varying the powder loading, modlfying material composition and the use of master batch powders to vary particle size.
5. Modelling using C-MOLD has demonstrated the ability to predict the skidcore thickness ratio as long as the heological and thermal characteristics of the feedstocks are quantified. 5 ACKNOWLEDGEMENTS The author would like to thank the Engineering and Physical Sciences Research Council for supporting this research and Dr J R Alcock for his significant contribution to this work
6 REFERENCES [l] German, R. M., 1990. Powder Injection Moulding, Metal Powder Industries Federation, Princeton, New Jersey. (21 Gelin, J.C.. Bamere,T. Dutilly,M.,l999, Experiments and Computational Modeling of Metal Injection Molded Parts, Annals of the CIRP, 4811: 179-182 [S] Stephenson, D. J., 1994, Surface Modification Technologies, vol VII, The lnst .of Materiak43-53. [4] Alcock, J. R., Logan, P. M., Stephenson, D. J., 1998, Surface Engineering by &injection Moulding, Surface and Coating Technology, 105: 6571. [5] Meyer. W., 1989. Plastics Technology: 109-112 [6] Alcock, J. R., Hanson, S. M. J., Stephenson, D. J., 1998, Surface Layer Thickness Control in Metal Injection Moulding, PM 98, vol3: 27-31