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perfecting materials

Glossary



Abradable coatings

Sealing within a turbine or compressor which the rotating blades are touching during operation. Used to minimize gaps and by this to increase efficiency.

Active brazing

A brazing process using active brazing fillers which are capable of reacting with ceramics or other poorly wettable materials.

APS

Atmospheric plasma spraying where process is operating under atmospheric conditions.

Binder

The polymer mixture that provides lubrication to the powders. It is critical to the fluidity during moulding and to green strength of the compact. Binder formulation goes hand in hand with debinding process selection.

Bond coat

Intermediate coating between component and functional coating; responsible for good adhesion to bulk material, can reduce stresses by equalizing different thermal expansion properties of coating and bulk material.

Braze alloy or filler metal

Additive which is used to achieve metallurgical bonding between parent materials.

Brazing

Metallurgical joining technique where the parent materials stay solid and only braze alloy melts.

Burnoff

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

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, titanium), while others require a precise final carbon level (carbon steel, tool steel, cemented carbide).

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.

Ceramic injection moulding (CIM)

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

Coarse grain annealing

Coarse grain annealing results in a microstructure which enables machining with higher cutting speed. In most cases coarse grain annealing is followed by a normalizing to get tougher materials properties.

Coining

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

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.

Debinding

A step between moulding and sintering where the majority of the binder used in moulding is extracted by heat, solvent, catalysis, or other techniques.

Delamination

The cracking of a moulded 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 or sometimes given as a ratio to theoretical (100%) density

Dimensional control

The repeatability of final dimensions in a PIM operation as measured by scatter. Usually quantified by the standard deviation (or a multiple of standard deviations) observed in a dimension as normalized by that dimension, expressed as a percent, for example 0.5%.

Feedstock

The mixture of powder and binder used in injection moulding.

Finishing operations

The steps applied to a P/M part 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.

Flux

Additive which removes oxides from component surface and enables wetting and spreading of the braze alloy.

Flux free

Brazing process where no additives are used. Oxide removal is secured by reducing gas atmosphere or decomposition in vacuum.

Gate

The constricted opening into the flow path at the entry to the die cavity in the injection moulding tool set.

Green strength

The strength of the as-moulded (“green”) component.

High temperature brazing

Brazing process where the braze alloy melts above 900°C.

Hot isostatic pressing

A process combining temperature and high pressure gas to fully densify a sintered PIM structure. Usually 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.

Infiltration

The process of filling the pores of a compact with a lower melting temperature metal or alloy.

Interface

Contact area between bulk material and coating

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.

Metal injection moulding (MIM)

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

Near net shape process

A technology capable to produce a compact close to the final dimensions with only minor need for secondary operations.

Net shape process

A technology capable to produce a compact to the final dimensions without the need for secondary operations.

Nitriding

Thermo-chemical process where nitrogen is enriched in the part surface. This results in increased wear and corrosion resistance and fatigue strength. In practice gas nitriding and plasma nitriding are applied.

Nitrocarburizing

Thermo-chemical process where nitrogen and carbon is enriched in the part surface. Target is an increase in wear resistance, frequently in combination with improvement of corrosion resistance (see post oxidation). Nitrocarburizing can be performed in gas atmosphere, salt bath or plasma.

Normalizing

Material will be transferred back in its original condition. Especially coarse grain or as formed microstructure can be eliminated. Depending on initial state normalizing results in a fine grained microstructure with round grains and almost equal size.

Parting line

The linear mark on a compact where two separate tool or die pieces mated during moulding.

Porosity

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

Post oxidation

Process to establish a thin, dense oxide layer on nitrided parts to increase corrosion resistance. Dark grey optical appearance is another reason for use.

Powder injection moulding (PIM)

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

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.

Protective gas atmosphere brazing

Flux free brazing process performed in reducing or inert gas atmosphere, i.e. hydrogen or argon. No oxidation and discoloration of parts.

Runner

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

Secondary operations

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

Setter

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

Shrinkage

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

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 moulding pressure.

Sintering

The thermal process which bonds and densifies the moulded powders. Most of the mechanical, magnetic or other properties of a PIM part are developed in the sintering cycle

Solution annealing

Heat treatment process which is frequently used with austenitic, stainless steels, nickel based alloys. Parts are treated in the range between 1000 and 1250°C for 15 minutes up to few hours (depending on component weight and wall thickness) and then quenched.

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.

Sprue

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

Stress relieving

Heat treatment in order to reduce residual stresses being introduced by previous manufacturing processes (i.e. forming, machining, welding, hardening, etc.).Stress relieving of steels is performed between 550 and 650°C.

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.

Supersolidus sintering

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

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.

Thermosetting

Binder or polymer hardening due to heating to a temperature where cross-links form between the polymer chains.

Vacuum annealing

Heat treatment performed in vacuum furnaces to prevent parts from oxidation.

Vacuum brazing

Flux free brazing process performed in vacuum atmosphere in the range of 10-2 to 10-6mbar. No oxidation and discoloration of parts.

Vacuum debinding

The extraction of binder by progressive heating in a vacuum. Usually vacuum debinding is performed with a gas inlet to drag escaping binder to the pumps.

Vacuum infiltration

Mounting of spray coating samples at reduced pressure (vacuum) in order to fill the voids of open porosity structures with imbedding compound to improve the quality of metallographic analysis.

Water-based binder

Mostly gelation binders that become rigid after moulding by the formation of a long-range molecule with water in the structure. Other forms rely on water saturated with silicates that form glasses on freezing.

Water soluble binder

A class of binders that rely on polymers that are water soluble so solvent debinding takes place in water. Most common are polyethylene oxide, polyvinyl alcohol, and polyethylene glycol.

Wax-polymer

A general term for polymers used as binders that consist of low molecular weight polymers and wax fillers. The polymers are usually simple molecules such as polyethylene, polypropylene, or co-polymers such as ethylene vinyl acetate. These are the classic binders used in PIM processing.

Weld line

A linear defect or mark on a compact surface where two portions were bonded, either in moulding or subsequently, such as in sinter bonding.

Wetting

Spreading of braze alloy on component surface

Wicking debinding

A popular means of binder extraction, especially for wax-based binders, where the compact is packed in wicking powder and heated to a temperature where the wax melts. The molten wax flows by capillary action into the surrounding packing powder.

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