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Injection molding metals & ceramics glossary

acicular powder: Needle shaped particles, generally good for debinding strength but detrimental to forming process.

admixed powder: A small, discrete powder mixed with another powder for lubrication, bonding, or alloying. One common means of forming PIM alloys or composites is via admixed powders.

agglomeration: A tendency for small PIM particles to stick together and appear as larger particles. It is a common problem with the ceramics.

air classification: The most common means for separation of specific size classes of PIM powders by differential settling in a high velocity air stream.

alloy powder: A powder in which each particle is composed of the same alloy of two or more constituents, especially common in gas or water atomized powders.

alumina: The compound of aluminum and oxygen Al2O3 that is the most common material used in ceramic PIM.

angle of repose: A measure of the interparticle friction and ease of shape retention during debinding. It is the angle from the horizontal plane that a pile of loose powder will assume when freely poured through an orifice.

apparent density: The mass of a unit volume of powder in the loose condition, usually expressed in g/cm3.

aspect ratio: The ratio of the maximum particle dimension to the minimum dimension.

atomization: The dispersion of molten metal into droplets by a rapidly moving stream of gas or liquid (usually water), or by centrifugal force.

atomized powder: Alloy or metal powder produced by the disintegration and subsequent solidification of a molten metal stream.

attrition: A mechanical milling or grinding process that typically employs a stirred or tumbled container filled with small balls that collide against particles mixed with the balls. The process is widely used for deagglomeration, partial alloying (W-Cu, WC-Co), and particle size reduction in ceramics.

barrel: The heated portion of the feedstock flow path before the nozzle. The barrel holds the feedstock under pressure while providing heat to melt the binders.

BET surface area: The specific surface area as measured by gas adsorption according to the Brunauer, Emmett and Teller theory. Widely used in characterization of ceramic PIM powders and expressed as square meters per gram, m2/g.

bimodal: A particle size distribution exhibiting two mode sizes. Although such a powder can result from mixing two different particle size distributions, some powder production techniques naturally produce this distribution. Its main advantage is in producing a higher solids loading in the PIM feedstock. It is common to use trimodal mixtures in ceramic PIM.

binder: The polymer mixture that provides lubrication to the powders. It is critical to the fluidity during molding and to green strength of the compact (which is expelled during sintering). Binder formulation is the major proprietary secret in the field and binder formulation goes hand in hand with debinding process selection.

Bingham flow: Viscous flow of a feedstock with an initial yield strength, meaning the stress must exceed the yield strength prior to initiation of flow.

blending: The thorough intermingling of powders of the same nominal composition, for example the combination of water atomized and gas atomized stainless steels.

burnoff: The removal of the polymer binder via preheating prior to sintering.

capillary rheometer: A device for measuring viscosity by applying a pressure on molten feedstock and determining the flow rate dependence on applied stress as flow occurs through a small capillary tube.

carbo-nitride: A heat treatment in a nitrogen and methane atmosphere that causes both carbon and nitrogen to diffuse into the steel surface for strengthening and hardening.

carbon control: A measure of the ability to remove the PIM polymer without contamination of the powder. Carbon control is quantified by the final carbon level and the uniformity of that level between parts, days, and feedstock batches. For some materials the desire is no final carbon (austenitic stainless steel, alumina, titanium), while others require a precise final carbon level, and in the extreme cemented carbides require very high carbon levels.

carbonyl powder: Predominantly an iron powder, but may also be nickel. These powders are prepared by the thermal decomposition of a metal carbonyl molecule. The resulting particle size is typically in the size range from 1 to 10 µm.

carburization-decarburization: Two events critically related to carbon control, since the addition of carbon to a material occurs by carburization from binder or atmosphere sources, while decarburization occurs by reacting carbon in the material with hydrogen, oxygen, or carbon monoxide.

case carburize: A post-sintering heat treatment aimed at diffusion of carbon into the surface of a ferrous PIM compact. The carbon increases strength and hardness.

catalytic debinding: The partial removal of the filler polymer via heating in an atmosphere containing an agent that induces depolymerization. The classic process involves extraction of polyacetal using a nitrogen atmosphere doped with nitric acid.

cemented carbide: A solid composite consisting of a metal carbide and a binder phase, usually cobalt or nickel aluminide. The composite is formed by liquid phase sintering a mixture of the carbide and binder metal powders. They are produced by PIM and used for tooling.

centrifugal atomization: The formation of spherical particles by combining a melt with a centrifugal force such that the melt is thrown off into droplets which spheroidize prior to solidification.

cermet: A body consisting of ceramic particles bonded with a metal.

clamping force: One of several measures of the capabilities of a molding machine. In this case the available force for holding the mold together while pressurized feedstock is filling the cavity. If the applied pressure times the projected part area exceeds the clamping force, then the cavity will open or flash during molding.

classification: Separation of a powder into fractions according to particle size.

closed-loop feedback control: A molding concept for precise dimensional control where the pressure inside the molding cavity is monitored during filling and used to adjust the molding machine operation to ensure repeatable filling, weight, and final dimensions. The dimensional scatter in final parts is greatly reduced by using this control logic as compared to open-loop or adaptive controls.

closed pore: An isolated pore not linked to the external surface.

coarsening: The progressive enlargement of the grain size or pore size during sintering due to diffusion, coalescence, or solution-reprecipitation processes. With respect to property control, microstructure coarsening is very important.

coining: The final pressing of a sintered compact to obtain a definite surface configuration, flatness, or surface finish.

comolding: Also known as two color molding because of the use in plastics. Two materials are shot into the same cavity from different injector units to form layers, interconnections, or other differences in materials with position in the cavity.

composite: A mixture of two or more powders to form a multiple phase structure, typically designed to deliver properties which are a hybrid of the constituent properties.

compression: The removal of air from melting feedstock by applying heat and pressure. In a reciprocating screw molding operation this is achieved by tapering the screw to reduce the space for the feedstock during metering toward the screw tip.

critical loading: The maximum volume fraction of solid particles which can be incorporated in a polymer binder without forming pores while still allowing flow in normal injection molding situations.

cross linking: The formation of bonds between polymer chains to give rigidity and strength to the polymer. Thermosetting polymers that harden on first heating are examples of cross linked polymers.

cycle time: A critical measure of molding equipment productivity, it is the time for completion of one molding cycle. It can be measured from the time to start filling to the start of the next fill. Cycle times from a few seconds to five minutes are encountered in PIM production.

debinding: A step between molding and sintering where the majority of the binder used in molding is extracted by heat, solvent, catalysis, or other techniques. The debinding techniques are highly variable between production sites. Thermal debinding often leads to lower precision and is slower. Hence, newer PIM operations rely on solvent, solvent vapor, vacuum, wicking, or catalytic debinding, and in many cases through a combination of methods.

delamination: The cracking of a molded compact, often leading to a hairline crack that is hidden until debinding or sintering.

densification: The change in porosity divided by the initial porosity due to pressing or sintering.

density: The mass divided by the volume, usually expressed in g/cm3 (equivalent to Mg/m3) or sometimes given as a ratio to theoretical density.

dew point: A measure of atmosphere purity, it is the temperature where moisture condenses out of a process atmosphere.

differential scanning calorimetry: Determination of the heat flow into or out of a PIM sample. Usually it is applied to lower temperatures where the polymer melts, crystallizes or evaporates.

differential thermal analysis: The careful measure of temperature and temperature difference during heating for the PIM material and a reference. Whenever the polymer melts or a phase transformation occurs the sample will lag behind the reference, while if there is a reaction the sample will heat faster than the reference.

dilatant flow: Viscosity that changes with flow conditions where the mixture actually dilates under stress.

dilatometry: Measurement of dimensional change during thermal processing to determine the sources of sintering densification, phase transformation, or other causes of dimensional control problems. Usually dilatometry is conducted with a pushrod that makes contact with the specimen in a furnace.

dimensional control: The repeatability of final dimensions in a PIM operation as measured by part to part, day to day, and batch to batch scatter. Usually quantified by the standard deviation (or a multiple of standard deviations, perhaps as large as six) observed in a dimension as normalized by that dimension, expressed as a percent, for example 0.1%.

ductility: A measure of the permanent stretching or deformation a material can take prior to failure. Ceramics have no ductility, while materials such as stainless steel and aluminum exhibit large ductilities, often measured at 60% stretch or elongation prior to failure.

ejection: The final stage of molding where the powder-binder compacts is forced out of the die.

elastic modulus: The material property linking stress and strain. In PIM feedstock it also is important because it determines residual stress and the stress relaxation time in cooling.

elasticity: The spring back of feedstock after ejection from a tool set. Formally, the elastic modulus is the material parameter that links stress to strain, but in PIM it is largely related to die sticking and dimensional control problems since ejection and debinding induce stresses that might cause warpage or loss of component precision.

elemental powder: Powder of a single chemical species like iron, nickel, titanium, copper or cobalt, with no alloying ingredients.

elongation to fracture: A measure of ductility, since this is the amount of permanent plastic stretch a material undergoes prior to failure in a tension test. The most common measure of ductility.

equiaxed powder: Particles with approximately the same size in all three (perpendicular) dimensions. A sphere formed by gas atomization is the classic example of this desired particle shape.

feedstock: The mixture of powder and binder used in injection molding. It involves decisions on the powder composition, particle characteristics, binder formulation, mixing practice, and ratio of powder to binder. Feedstock generation is a business in itself.

filling: The first phase of the molding cycle where feedstock is flowing into the mold cavity under pressure from the screw.

finishing operations: The steps applied to a P/M material after sintering to tailor the dimensions, properties, or attributes to the application; examples include machining, polishing, heat treatment, straightening, and electroplating steps.

flashing: A lip of extruded feedstock that penetrates along the parting line of a die cavity due to excessive pressure, poor tool tolerances, or binder separation from the powder.

flow analysis: Computer simulated molding to assess the location of the gate, runner, cooling passages and other important aspects of tool design and molding to minimize errors in production.

flow time: The time required for a powder sample to flow through an orifice in a standardized test. The flow time gives a measure of the interparticle friction. Most powders used in PIM are not free flowing.

fracture toughness: A measure of the resistance to crack propagation in a material, formally related to the applied stress and crack size.

freeze firing: A concept developed in the 1960's using water as the binder and freezing in the tool cavity. The frozen compact is ejected and subjected to sublimation to extract the binder.

gas atomized powder: A rounded or spherical powder formed by the disintegration of a melt stream by a gas expansion nozzle. The particles solidify during free flight after atomization.

gate: The constricted opening into the flow path at the entry to the die cavity in the injection molding tool set. It should be the first portion of the flow path to solidify after filling the mold.

gelation: A binder setting process where a macromolecule grows in a binder solution to form a highly interlinked structure with most of the binder trapped in cells formed by the long-range molecule.

granulation: A term describing the agglomeration of powder or the breaking apart of lumps, runners, sprues, or parts that are reformed into pellets for reloading into the molding machine.

green strength: The strength of the as-molded component.

hard metal: Another name for the cemented carbides which reflects their high hardness after sintering.

hardness: A formal test of resistance to penetration by a pointed or rounded indentor under a given load. There are many useful hardness tests where the depth or width of the penetration is measured. Often this relates to other material properties such as strength.

heavy alloy: A range of high density alloys based on tungsten with small concentrations of alloying additions such as nickel, iron, or copper. These alloys are liquid phase sintered from mixed elemental powders to create a composite material. Most of the applications are for thermal management heat sinks or military projectiles.

hot isostatic pressing: A process combining temperature and high pressure gas to fully densify a sintered PIM structure. Used only for very high performance structures.

hot runner : A tool cavity where the flow path is kept hot between shots to eliminate recycle of runners and sprues. The built-in heaters and valves are coordinated with the molding machine to ensure no freezing in the flow path.

infiltration: The process of filling the pores of a compact with a lower melting temperature metal or alloy. It is one means of forming low cost tooling or making dense structures via molding a porous preform and filling the pores with liquid metal, such as for SiC-Al or W-Cu.

injection molding: A hydrostatic forming technique for shaping powders using plastic binders and relatively low temperatures and pressures.

interparticle friction: The friction between powders which limits sliding, packing, and densification.

invar: Low thermal expansion alloys of iron-nickel-cobalt where a martensitic phase transformation is balanced against the thermal expansion coefficient to give a near zero thermal expansion coefficient over a range of temperatures.

irregular powder: A powder which lacks shape symmetry in the individual particles.

jetting: A condition that arises with the rapid filling of an injection mold where the feedstock shoots across the mold and fills back toward the gate. Generally this is unacceptable for quality components.

knit line: A linear defect occurring where feedstock streams merge in the cavity because of two gates or flow around a core or other solid portion of the die.

kovar: A glass to metal sealing alloy used for microelectronic packaging and other situations where a matched or graded structure is required.

low pressure molding: The use of lower pressures and low viscosity binders (largely water-based or wax-based) to fill out a complex tool cavity without packing the shape. This route is successful for components where internal flaws are not a concern, such as nozzles, spray tips, or other geometries where external geometry is the key concern.

lamination: A layered structure or cracking in the pressed compact resulting from ejection stresses exceeding the green strength.

liquid phase sintering: Sintering at a temperature where a liquid and solid coexist due to chemical reactions, partial melting, or eutectic liquid formation. It is most useful for stainless steels, and alloys containing phosphorous, boron, copper, or silicon.

lubricant: An organic additive which is mixed into the feedstock to minimize die wear and aid in ejection after compaction.

mean size: The average value from the particle size distribution.

mechanical alloying: The formation of an alloy powder by milling elemental powders for a prolonged time; frequently used to create amorphous or dispersion strengthened alloy powders via attritor milling.

median size: The centroid of the particle size distribution, where half of the particles are larger and half are smaller; not necessarily the mean, but easily identified as the 50% value.

melt index: A measure of flow at low shear strain rates, where a capillary tube is used to extrude molten feedstock under a dead load. The melt index depends on the capillary tube diameter and applied load, but is always reported as the grams of feedstock collected from the tube in ten minutes.

metering: Controlled forward extrusion of molten feedstock past the screw tip and check ring to ensure the proper shot volume is ready for the next filling event.

microstructure: The detailed information on the microscopic phases, pores, grains, defects, heterogeneities, and other property controlling features.

mixing: The thorough intermingling of powders of two or more different compositions.

mold flow simulation: Several computer simulations exist for simulation of the flow, packing, sizing, cooling, and other events during PIM.

mold release: A spray or coating that reduces component sticking to the die cavity, aiding ejection without defects.

moldability: A relative measure of the ease of filling out a tool cavity during injection molding. It can be determined by the length of filling for a long, narrow passage. The current best test of plastic moldability is the spiral, which has been altered to a zig-zag to induce powder-binder separation along the flow path.

multimodal: A powder which exhibits several modes, possibly generated by blending several monosized powders.

nanoscale: Powders or microstructures with sizes that can be measured in nanometers. Typically the powders are less than 100 nm in size, or less than 0.1 µm.

net shape: A compact manufactured to the final density and dimensions without the need for machining. PIM is naturally a net shape process.

Newtonian flow: An idealized viscosity situation applicable to a few fluids where there is no sensitivity to the shear strain rate, only temperature.

nodular powder: Irregular particles with knotted, rounded shapes. This is characteristic of water atomized powders.

open pore: A pore completely through a compact from one surface to another. During thermal debinding the pores must be open to allow the escape of evolving vapors.

oxidation-reduction: A combination of atmosphere-powder reactions that can extract oxygen or deposit oxygen in a powder compact during heating, especially during debinding and sintering. Reduction conditions are usually required for sintered metallic materials.

packing pressure: The peak pressure encountered in the molding operation once the die is filled, prior to freezing of the gate. It is precisely controlled for optimized final dimensional control.

particle size: The controlling linear dimension of an individual particle, as determined by analysis with screens or other instruments.

particle size analyzer: An automated device for determination of the particle size distribution. These are widely used in the quality control function as part of PIM to ensure repeatable powders for the process.

parting line: The linear mark on a compact where two separate tool or die pieces mated during shaping. In injection molding it is where the two halves of the die joined together.

pelleting or pelletizing: The formation of discrete chunks of feedstock with repeated sizes and shapes that allow easy flow and filling of the molding machine.

planetary mixer: Usually these have two mixing blades that rotate around individual shafts and the two blades further rotate around a center axis. The net effect is intermixing and stirring and shear to produce PIM feedstock, usually in a heated vessel.

plunger molding: A hydraulic plunger is used to push molten PIM feedstock into a die cavity, where motion and pressure are controlled by the applied hydraulic pressure.

pneumatic molding: Use of an air pressure head over molten feedstock to push it into a die cavity. This is the lowest cost and least precise form of PIM, yet is widely employed in the fabrication of large ceramic structures.

polydisperse: Implies a broad powder size distribution, covering a wide range of particle sizes with no clear mode size.

pore size: The size of the holes or voids between powder particles, often measured by mercury porosimetry on open pores.

porosimeter: A device for measuring the pore size of the open pores using high pressures and mercury intrusion techniques.

porosity: The amount of void space in a powder compact. Most PIM materials have less than 5% porosity after sintering.

powder: Particles of matter characterized by a small size, less than 1 mm in size. Most PIM powders are below 20 micrometer in maximum size.

prealloyed powder: Each particle contains an intimate mixture of two or more elements in a prescribed ratio to form an alloy; examples include brass, bronze, steel, and stainless steel.

presintering: The heating of a compact to a temperature lower than the normal sintering temperature to gain strength for subsequent handling, including machining. This is often performed by heating above the highest temperature required to burnoff the polymer.

pressure-assisted sintering: Sintering with the application of an external pressure. It is often performed by initially sintering in vacuum and subsequently pressurizing the furnace to densify any remaining closed pores. Best applied to high performance alloys or difficult to sinter alloys.

pressure control: The final phase of molding where the gate is not frozen and the quantity of feedstock in the die cavity is controlled by the pressure, thereby ensuring uniform weight and dimensions in the final PIM part. Proper control is achieved by monitoring pressure sensors in the die cavity wall.

pseudoplastic flow: A form of viscous flow where there is a strain rate sensitivity to the viscosity and generally it is the opposite from dilatant flow, since the viscosity decreases with higher strain rates.

pycnometer: A device for measuring the theoretical density of a loose powder or preform.

reciprocating screw: Injection molding with a screw located in a heated barrel. During compression, metering, melting, and forward advance of the feedstock, the screw is turning, while during mold filling it becomes a plunger to quickly fill the cavity.

reduced powder: Metal powder produced by the chemical reduction of a compound, most typically a metal oxide.

refractory: A metal or ceramic having a high melting temperature, usually over 1700 C. Example refractory metals are tungsten, molybdenum, rhenium, and zirconium, while example refractory ceramics are alumina, zirconia, yttria, and chromia.

rheology: The study of elastic, plastic, and viscous flow of polymers and feedstocks used in PIM.

runner: A portion of the feed path for filling an injection molding die; the runner is between the sprue and gate, the latter being the inlet to the actual die cavity.

screw: The key portion of an injection molding machine for metering and filling the die cavity. It has a taper along the length to compress the feedstock as it is metered for removal of trapped air and a check ring at the tip to allow forward plunging motion during mold filling.

screw molding: A reciprocating screw is used to mix, pressurize, deair, and convey molten feedstock to finally be injected into a tool cavity. This is the most widely employed means of injection molding precise components.

secondary operations: Those activities performed to adjust dimensions or properties of a compact after sintering.

segregation: Nonuniform distribution of ingredients, such as powder separation by size, shape, or density, or chemical separation in the microstructure of a solidified material.

setter: The tray or shaped substrate for support of PIM compacts during debinding and sintering.

shear rate: A measure of the rate feedstock is deformed and forced to flow into a die cavity. It has units of inverse seconds. Actually it is the shear strain rate, representing the sheared change in length divided by the original length in unit time. For PIM shear rates of several thousand per second are normal, meaning the slug of feedstock is quickly deformed to fill out the cavity.

shot size: A historical basis for sizing molding machines. This is usually given in terms of the amount of polystyrene that can be moved through the nozzle in a single forward motion of the screw or plunger.

shrinkage: A decrease in dimensions of a compact which occurs during sintering.

sigma mixer: A closed mixer consisting of two blades that look like the Green capital letter sigma. These blades lift and separate feedstock to form a mixture of moderate homogeneity. Major advantage for PIM is the small batch size.

sink mark: A shallow surface cavity that forms during cooling. It is usually indicative of under packing prior to gate freezing and is corrected by a higher molding pressure.

sintering: The thermal process which bonds and densifies the molded powders. It increases the compact strength via diffusion or related atomic level events. Most of the mechanical, magnetic or other properties of a PIM part are developed in the sintering cycle.

sintering diagram: A process map showing the interaction of the key variables with respect to the densification of a powder. The variables include particle size, grain size, temperature, time, and applied pressure.

solids loading: The relative powder volume in feedstock designed for binder-assisted shaping. A common injection molding feedstock will have a solids loading of 0.6 or 60 vol.% powder.

solvent debinding: The extraction of the binder or some portion of the binder by leaching into a solvent. This is performed by immersion or exposure to solvent vapors. The solvent might be flammable, toxic, or simply water or ethanol.

spherical powder: Powder with a uniform spherical shape and a size which can be characterized by a diameter. Gas atomized powders are often spherical.

sprue: The initial inlet into the die set for injection molding feedstock. The sprue is usually filled by the nozzle and is tapered for easy extraction after mold filling. It feeds the runner system, which in turn feeds the gate.

stainless steel: A wide range of alloys based on iron and chromium that give corrosion resistance in most common corrosive environments. The most popular PIM alloys are 300 series that contain high nickel levels, 400 series that have little nickel, and precipitation hardened alloys such as 17-4 PH.

stress relaxation: The removal of residual stresses in a PIM component by controlled cooling or reheating to relax the polymer entanglement or other sources of stress.

superalloy: The highest performance alloys used for the most demanding applications. Typically these are formulated with a wide range of components, based on nickel, with tailored properties that allow use in moving components in jet turbines.

supercritical extraction: The use of high pressure and moderately high temperatures to heat a solvent over the critical point for binder removal. The most common supercritical fluid used in PIM is carbon dioxide. Over the critical point the gas is compressed to a density equal to that of the liquid, so there is no volume change on binder removal.

supersolidus sintering: A liquid phase sintering process applied to prealloyed powders where sintering occurs over the solidus temperature, thereby nucleating liquid within the particles.

surfactant: A surface active agent added to the binder system in PIM feedstock to induce binder wetting of the powder surface. These are the major trade secrets, but many powders prove responsive to simple soap compositions.

tap density: The density of a powder obtained when it is vibrated for a prolonged period. The tap density represents the highest packing density possible for a powder without the application of pressure.

tensile strength: The maximum strength attainable prior to failure during a uniaxial tension test.

thermal debinding: Extraction of the polymer phase in debinding by the application of heat. The classic processes relied on slow heating in air to evaporate the binder over a period of days.

thermogravimetric analysis: A fancy term that means measuring the sample weight change versus temperature while heating. It is used for determination of polymer evaporation temperatures by performing constant heating rate experiments and recording the sample weight versus temperature.

thermoplastic: Binders or polymers that soften on heating, but stiffen on cooling, and can repeat the heating and cooling process without undergoing any long-range chemical bonding changes.

thermosetting: Binder or polymer hardening due to heating to a temperature where cross-links form between the polymer chains. Unlike a thermoplastic that can be softened by reheating, a thermosetting binder becomes hard on the first heating cycle and can not be subsequently softened.

thixomolding: A direct injection

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