POWDER INJECTION MOLDING. The Powder Injection Molding (PIM) process is said to be a combination of conventional powder metallurgy and plastic injection.

POWDER INJECTION MOLDING

POWDER INJECTION MOLDING• The Powder Injection Molding (PIM) process is said to be a

combination of conventional powder metallurgy and plastic injection molding technology because it brings together the diversity of materials of conventional powder metallurgy and the geometric freedom of part design associated with thermoplastic injection molding.

• The combination of these technologies allows for complex shapes to be produced to near full-density for high performance applications.

• Some of the advantages of PIM over conventional powder metallurgy are that it allows more design flexibility, closer tolerances and gives more uniform shrinkage during sintering.

• Historically, the concept of injection molding originated from the metal die casting industry wherein the technology of injecting molten metal into closed dies to obtain desired shapes evolved.

• Later, this molding concept was successfully exploited in the plastics industry.

• Powder injection molding is an extension of the plastic injection molding process.

PROCESS OUTLINE:• The PIM process begins by mixing selected

powders with a suitable combination of binders, lubricants and plasticizers. The mixture, usually termed the ‘Feedstock’, may be either granulated or may be used in the form of a thick slurry for injection molding into the desired shape, employing conventional plastic injection molding practices.

• The shaped parts removed from the die must have an adequate green strength and stiffness to be handled. Subsequently, the binder and other unwanted additives are removed from the compact by a process known as ‘debinding’.

• Then, the parts are sintered to densify the resulting porous compacts to yield near-net shape and high performance components. The parts shrink considerably during sintering and the final part is a reduced version of the as-molded green shape.

• The product can be used without further processing or it may be further densified, heat-treated, machined or surface treated.

As noted in the figure, the basic stages involved in forming a component by PIM include the following.

1) selecting and tailoring a powder for the process;

2) mixing the powder with a suitable binder for producing either a homogeneous slurry or granular pellets of mixed powder and binder (i.e. feedstock);

3) injection molding of the feedstock to obtain the required shape;

4) removing the binder from the molded parts (debinding);

5) densifying the compact by high-temperature sintering;

6) post-sintering processing, as appropriate, including machining, heat treatment, surface finishing or further densification.

Schematic Diagram of MIM

MATERIALS FOR PIMThe feedstock for powder injection molding consists of two components:

a) the particulate materials and

b) the binder and other additives.

PARTICULATE MATERIALS:• metal, alloy, ceramic or cermets powders.• There are some apparent conflicts in the powder

characteristics needed. • For example, an irregularly shaped or ligmental

powder will raise the viscosity of feedstock mixture and yield a lower packing density during injection molding. This requires more binder (i.e. low powder loading) and results in more sintering shrinkage; however, due to particle interlocking, the shapes produced will have a high green strength, increased compact strength after binder removal and better shape retention during sintering.

• On the contrary, a spherical powder results in high packing density and offers comparatively less resistance to flow during injection molding, thereby minimizing the amount of binder that must be added as well as reducing the sintering shrinkage. However, because there is no mechanical interlocking between spherical particles, the green part will have a comparatively lower strength, especially after removal of the binder.

• Fine powder • Fine powders have the advantage of better sinterability and

ease of molding and they expose the injection molding machine screw to less risk of damage.

• In addition to faster sintering kinetics, fine powders also result in more homogeneous microstructures since diffusion lengths and times for migration of alloying constituents will decrease with reduced particle size.

• Finer powders also allow for more intricate geometries, thinner walls, sharper edges and better surface finish in parts.

• Additionally, the smaller particles, because of comparatively higher inter-particle friction, exhibit a desirable increase in the compact strength during debinding that reduces the possibility of distortion or slumping in processing.

• However, very fine particles cause difficulty in attaining a high packing density because of agglomeration.

• On the other hand, coarser powders generally give lower sintered densities and larger residual pores and also pose difficulties in molding.

• To obtain a high sintered density in the final product, a high packing density is required in the green compacts.

• In general, the lower the initial packing density, the greater the sintering shrinkage needed to attain a high final density.

BINDERS AND OTHER ADDITIVES:• The binder is not only a necessary aid for promoting

viscous flow in the feedstock during injection molding, but it should also maintain the shape of the green part after removal from the mould.

• After molding, the binder has to be removed carefully without impairing the integrity of the molded part. Even though the binder is just an intermediate processing aid it has considerable influence on the success of the PIM process.

• A binder system that may produce excellent flow characteristics during molding, but presents difficulties during its subsequent removal, is not suitable for PIM purposes.

• Generally, a binder system consists of the following components:

1) Major binder component:• This is responsible for holding the compact in shape during

debinding until the commencement of sintering and determines to a large extent the final binder properties. It is normally a high molecular weight thermo-plastic; such as, polystyrene, poly- propylene and paraffin wax.

2) Minor binder component:• This is generally added to alter the viscosity of the binder

system and is removed early in the heat treatment (debinding). If this component, which is usually a thermo-plastic or an oil, forms a continuous, separate phase, then its removal would create pore channels throughout the molded product to facilitate the removal of the evolving gas(es). Examples are paraffin wax and beeswax.

3) Plasticisers:

• These minor additives are included to enhance the moldability of the feedstock mixture. Examples are carnauba wax and glycerin.

4) Processing aids:

• Some minor additives (surfactants) are included to improve the wetting between the binder and powder. Other minor components may be used to facilitate release of the product from the mould.

• German has classified binders into five different categories, as given below:

1) thermoplastic compounds,

2) thermosetting compounds,

3) water-based systems,

4) gellation systems, and

5) inorganics.

PREPARATION OF FEEDSTOCK:• The first processing step in the manufacture of a PIM part

is to produce an appropriate feedstock. • The selected powders are mixed in precise proportions

with suitable thermo-plastic binders and other additives. • These polymeric additions often comprise as much as 30-

40 volume percent of the feedstock. • To obtain high density green compacts and reproducible

results, it is essential to optimize the composition of the powder-binder mixture.

• In general, it is considered best to use the minimum amount of binder that gives good flow behavior since the binder must eventually be removed from the powder. However, too little binder results in a highly viscous mixture causing mould filling difficulties and voids formation. During debinding, these voids can cause cracking due to internal vapour build-up, as a result of degradation of polymers.

• On the other hand, an excess of binder is wasteful and requires comparatively longer debinding times and also results in greater dimensional shrinkage during sintering. During molding, excess binder can separate from the powder, leading to inhomogeneities in the molded compact and possible dimensional control problems.

• Moreover, excessive binder loading levels usually cause deformation, sagging and blistering during subsequent binder removal stage.

• The ideal corresponds to the case where particles are coated with a uniform and very thin layer of binder and with no voids in the binder.

• Thus, it is necessary that the binder fills all of the void space between the particles while maintaining a reasonably low viscosity.

MIXING• Once a powder and binder have been selected, the next

concern is to mix these ingredients to prepare a homogeneous feedstock on both large and small scales of size.

• The homogeneity of the feedstock composition is crucial, since inhomogenieties cannot be removed by subsequent processing.

• Compositional variation on a large scale of size, i.e. on a scale of size that is a moderate fraction of the molding, will lead to non-uniform shrinkage on a similar scale of size and the distortion of the molding during sintering.

• A very homogeneous mixture on a small scale of size, e.g. 100 particle scale, is important as it determines the homogeneity of porosity on the same scale of size after the binder is removed and this determines how the material shrinks during sintering on this small scale.

• Variations in porosity from place to place on a small scale lead to the formation of enlarged pores.

• An ideal mixing operation should result in a uniform distribution of powder in a matrix of binder on all appropriate scales of size with the binder filling the holes between the particles and coating each powder particle. This will ensure a thin liquid film between the particles at the injection molding temperature, thus promoting good rheological properties, and will result in maximum packing of powder in the molding cavity.

• There is a possibility of segregation when the mixture has particles of different sizes, shapes, or densities. As the particle size decreases there is greater inter-particle adhesion and friction, making the problem of agglomeration more acute but reducing segregation.

• An important requirement in mixing is to break up agglomerates to attain uniform particle packing and porosity on a small scale of size. Failure to do this can affect the final microstructure causing both residual porosity and non-uniform grain size.

• Agglomerates can also decrease the packing efficiency, which may increase the viscosity of the feedstock mixture.

• The agglomeration of the powder can also be reduced by the addition of appropriate dispersing and coupling agents within the binder system.

• The mixing variables that affect the homogeneity of a feedstock can be broadly classified into four categories, namely those associated with the powder, binder, the mixer and the way of ingredients are added to the mixer.

TYPES OF MIXERS• Different types of mixers

• These include mixers incorporating plug extrusion, double planetary, twin-screw extrusion, sigma-blade, Z-blade, milling, and impeller concepts.

• Consequently, there can be a wide variation in the quality of the mixtures. Several problems in the molding, debinding and sintering stages can be traced to improper mixing procedures.

MOLDING:• In its simplest form, molding consists of heating the

feedstock pellets/granules to a sufficiently high temperature such that the binder is melted and making the mixture fluid, then forcing this mixture into a cavity where it cools and forms the compact shape.

• The objective is to attain the desired shape free of voids or other defects and with a homogeneous distribution of powder.

• The feedstock must therefore have sufficiently low viscosity at the molding temperature to flow freely into the mould and subsequently to leave minimal residual stresses.

Feedstock pellets and worms for molding

• PIM processors have adopted the principles and equipment used in plastics injection molding.

• Since the material properties and molding requirements of PIM parts are substantially different from those of plastics, the injection molding machines and techniques for PIM parts require modification.

• Avoid premature freezing.

• Some differences in mould design, and especially in the gating system, are needed to accommodate the highly viscous PIM feedstock.

• To obtain reproducible results, molding is often carried out under strictly controlled conditions of temperature of feedstock and of pressure and flow rate which are influenced by the configuration of the barrel, runners, gates and moulds.

• Control of these parameters within the required levels can now be achieved with the advent of closed-loop computer-based, control systems applied to an injection molding machine.

• In general, the minimum barrel temperature should be above the melting point of the highest melting component of the binder.

• The maximum mould temperature should be below the lowest recrystallisation temperature among the binder components.

• A high temperature difference between barrel and mould results in a large thermal shrinkage of molded parts thus requiring a higher packing pressure or longer packing time to offset the thermal shrinkage. However, the powder mixture quickly freezes in the mold, rendering a longer packing time ineffective.

• Defects occurring during the molding stage include voids, short shots, jetting, poor ejection, parting line flash, surface blistering, cracking, formation of weld lines and sink marks, cold flow patterns, warpage, non uniform densities and poor dimensional control. Some of the defects, such as sink marks, exterior cracks and voids are apparent by visual observation.

• Unfortunately, most of the defects only become apparent after debinding or sintering. These defects may be eliminated through adjustments in the time, temperature, and pressure parameters of the molding cycle.

• However, it requires extensive trial-and-error experiments to determine the proper molding conditions.

• An injection pressure which is too low leads to weld lines and cold flow patterns on the surface, whereas too high a pressure can result in residual stresses or even cracking after mold release.

• Moreover, a pressure release directly after mould filling would cause a back flow from the molding into the plastification unit and should be avoided.

Overview of a horizontal injection molding machine and key components

DEBINDING:• The binders and additives used in the molding steps

are removed by a processing which has come to be known as “debinding”.

• Thus, a major difference between polymer injection molding and powder injection molding occurs after molding, when the binder is removed from the compact prior to sintering.

• German has identified six debinding techniques which may be used in PIM, broadly categorized as either solvent or thermal processing, as shown in Figure.

• The debinding process may be a combination of more than one of these processes.

• Commercially, total thermal debinding and solvent extraction followed by thermal debinding are the two popular debinding methods.

• Recently, another technique known as ‘catalytic debinding’ has been reported whereby acidic vapors are used to remove the binder from green compacts produced from BASF feedstocks.

• Solvent debinding generally offers better shape retention as well as a shorter cycle time compared to thermal debinding.

• The binders used in the common solvent debinding approach have an inherent disadvantage in moldability, making it difficult to mould highly complex components.

• In addition, there is an environmental pollution problem with the commonly used debinding solvents.

• However, the recent trend is to use water-soluble binders, thus water leaching is employed to remove a major proportion of the binder, which is quite a safe practice.

• Thermal debinding uses very low heating rates and thus requires long times.

• High heating rates can result in distortion, slumping or even failure of the component.

• For thermal debinding, concurrent use of wicking enhances the shape retention. However, the removal of wicking powder after debinding is a time-consuming practice; therefore, the use of wicking may not be practicable in some commercial production.

• Several methods have been developed in order to overcome the problems associated with classical thermal debinding.

• They are mainly based on the principle that a major fraction of the binder is removed chemically followed by a short thermal debinding treatment.

• With the solvent extraction method, the green component is immersed in a suitable organic fluid, which dissolves the binder partially.

• This results in the formation of open pore channels. Thus, the remaining binder fraction can be removed relatively easily and in a shorter time through the open pore structure by thermal means.

• In a recent development a highly concentrated acid is added to the gas atmosphere in order to remove a binder based on polyoxymethylene by catalytic phase erosion, known as catalytic debinding.

• During debinding this binder develops formaldehyde.

• One problem with these methods is that they have to use special devices and/or chemical substances, which are particularly hazardous for health and the environment.

• The debinding rate depends on several factors including compact size, packing density, particle size, porosity, binder chemistry, debinding mechanism, heating rate, solvent or atmospheric composition and flow rate and placement in the debinding apparatus.

SINTERING• The ISO definition of the term ‘sintering’ reads:-

‘The thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of particles’.

• The definition by Thummler from the point of view of physical chemistry is:

‘Sintering is a thermally activated mass transport process which leads to strengthening of particle contacts and/or a change in porosity and pore geometry accompanied by a reduction of the free energy. A liquid phase can take part in the process.’