POWDER%20INJECTION%20MOLDING

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POWDER INJECTION MOLDING
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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.

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• 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.

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• 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.

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• 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.

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• 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.

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Schematic Diagram of MIM
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• MATERIALS FOR PIM
• The feedstock for powder injection molding
  consists of two components
• a) the particulate materials and
• b) the binder and other additives.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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Feedstock pellets and worms for molding
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• 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.

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• 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.

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• 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.

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• 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.

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Overview of a horizontal injection molding
machine and key components
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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.