MiM secondary operations

MiM parts can be subject to post sintering operations



As all metal parts, MiM & MiM-Like parts can be subject to secondary operations such as machining, heat treatments and surface finishing, etc.
Despite one of the aims of making a part by MiM process is to achieve net shape, some parts can require adding final details. Moreover, the secondary operations costs are often very significant, and can achieve in some cases 50% of part total cost.

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Pollen AM MiM secondary operations Pellet extruder

Machining operations


In many cases MiM & MiM-Like are a near net-shape technology, with final details added via secondary operations.
Machining operations are usually carried out after sintering, also it can be done on the green part, but due to the low strength of these green parts it is made on rare occasion such as machining hard and abrasive materials once sintered (tungsten, titanium, etc.).
It is also possible to machine the parts after a presintering operation at a low temperature (temperatures will depend on the alloys).
The parts are then sintered and eventually sized.

MiM & MiM-Like parts can accept a wide range of machining finishing treatment for instance, broaching, sizing, burnishing, grinding, deforming, deburring, abrading, etc.
The machinability of a mechanical component, independently of being wrought or derived from powder metallurgy, is mainly influenced by, the workpiece itself, the tool and the cutting conditions.
The primary factors influencing the machinability of sintered materials, being also responsible for their properties, can be divided into the following groups:
- Manufacturing and processing technique, (forming, debinding and sintering conditions);
- MiM feedstock, (powders specifications, binder/powder ration, etc.);
- Alloy chemistry, resulting microstructure, mechanical properties, and porosity (shape, size, distribution of pores and of non-metallic inclusions).
In some cases, to improve the machinability of MiM & MiM-Like parts they are impregnated with oils by submerging parts in a heated oil bath for several hours.




Pollen AM MiM machining operations Pellet extruder


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Pollen Am MiM heat treatment secondary operations - Pellet extruder

Heat treatment


As cast or wrought metal parts, MiM & MiM-Like parts can be heat treated to increase strength, hardness and wear resistance with similar results.
The degree of hardening is determined by the percentages of carbon, alloying elements, and residual porosity in the material.
The most common heating operations are, resintering, through hardening, case hardening, tempering, etc.
The part thus produced is a preform, called "green part". The level of binder contained in the green parts varies according to its chemical nature and the powders used; the percentage of binder is generally between 35 and 50% by volume.

Hot isostatic pressing (HIP) is a secondary process used to increase the density of MIM components after sintering, to increase the ductility and fatigue resistance of high-end materials. Under typical pressures between 400 and 2,070 bar and temperatures up to 2 000 °C materials can reach 100 % of its maximum theoretical density.

Resintering is usually made at higher temperature and under conditions similar to those of the first sintering operation. It relieves stress or removes the mechanical constraints imparted during coining or repressing and for further densification.

Hardenability for a wrought material can be defined as the capacity of the microstructure of a metal to be transformed from austenite to martensite at a given depth when cooled rapidly. In wrought materials hardenability is mainly controlled by the chemistry and the grain size, whereas on sintered materials is it linked the porosity.

Carburizing is a heat treatment in which the carbon content of the surface of a low-carbon steel is increased by exposure to an appropriate atmosphere at a temperature in the austenite phase fields. Nitriding is a surface hardening heat treatment that introduces nitrogen into the surface of steel while it is in the ferritic conditions.

Carbonitriding is a surface hardening heat treatment that introduces carbon and nitrogen into the austenitic steel.



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Surface treatment


Surface treatments are very common for MiM & MiM-Like parts, its aim is to improve functions and aesthetics. All common surface treatments are compatible with MiM & MiM-Like parts such as steam oxidizing, coloring, plating, impregnating, dipping, or spraying.

The first step in selecting a surface modification is to determine the surface and substrate engineering property requirements, such as: - abrasion wear resistance under conditions of low or high compression loading:
- resistance to scuffing and seizure;
- bending or torsional fatigue strength;
- rolling contact fatigue;
- resistance to case cracking (surface collapse);
- resistance to corrosion.

The thickness of the engineered surface can vary from several millimeters for weld overlays to a few micrometers of physical or chemical vapor deposited coatings, while the depth of surface modification induced by ion implantation is < 0.1 mm.



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Pollen Am MiM surface treatment - Pellet extruder


Pollen Am MiM why and when using it - Pellet extruder

MiM & MiM-Like

Why and when using it?


It exists a wide range of metalworking technologies, all are not adapted to each part complexity and volume requirements. MiM & MiM-Like are generally used to produce complex shaped, high density, high-performance metal parts.

MiM is mainly used in the automotive, medical and dental instruments, Computer and electrical applications, aerospace and defence markets, etc.
MiM-Like answers a large range of market applications from unique parts to medium series, where traditional MiM part are not competing due to mould cost.

MiM process is best applied to small and medium size parts that are often complex in nature with high annual part volumes. Thanks to pellet MiM-like additive technology such as Pam Series MC, MiM is now economically viable from 1-unit part and with no tooling investment.

MiM & pellet MiM-like processes are compatible with almost all type alloys such as ferrous and non-ferrous alloys, titanium, copper, nickel, etc.

Benefit from enhanced properties: MiM & MiM-Like parts are typically 96% to 99% dense, approaching wrought material properties and can achieve higher mechanical strength, corrosion resistance, magnetic properties than conventional powder metallurgy processes and traditional casting.

Reduce design constraints: MiM & MiM-Like offer design flexibility as traditional plastic injection moulding process. For example, some complex parts produced with conventional powder metal processes will require secondary operations, using MiM processes allow producing net shape parts.

Avoid subassemblies: MiM & MiM-Like processes can be used to combine different shapes into one, to avoid assemblies and reduce labour and additional operation costs.

Nevertheless, since every process comes with its own constraints, it is necessary to take into account some MiM conception guidelines during the design phases.


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MiM & MiM-Like economical aspects

Why and when using it?


The MiM process is an indirect metal application involving different steps. It is important to take this into account from the design phase to achieve the best production cost possible via MiM & MiM-Like processes.

MiM is particularly useful to produce small, medium and complex parts in large quantities. Pellet MiM-like additive manufacturing process allows making MiM economically efficient from the first parts.

Like all production processes, MiM & MiM-Like costs arise along the different steps of the process (man power, equipment, materials, etc.), and sum up with indirect costs to obtain final cost.

Pollen Am MiM part cost structure representation - Pellet extruder

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Raw materials
MiM feedstock is obtained by mixing thermoplastic resins and metal powders. The metal powder is the first source of cost is the raw materials. Powders are usually very fine (below 20 μm), obtained by gas atomization and then sort. This powder is more expensive than bulk wrought material.

As the raw materials are expensive, the parts should be as light as possible.
The use of coring and the reduction of thickness, though it cannot be extreme to avoid moulding difficulties, and the use of coring, are both good ways to reduce part weight and eventually part cost.

Mixing
Mixing is the process of feedstock preparation (addition of binder to the powders), it as a rather stable value when applied to well-established MIM feedstocks, just a few euro per kilogram, i.e. an additional cost to the powder cost.

Should a special feedstock be developed for a new application, it will suffer from added costs coming from some development activity:
- these costs will eventually be diluted over the quantity produced, but, of course, if a part requires a special alloy and only in a small quantity, that is the worst case in terms of cost.

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Pollen Am Pim feedstock fabrication - Pellet extruder
Mould cost is an important item. Any error in the mould will be copied forever into the sintered parts.
Mould cost depends on:
- mould size;
- mould material;
- mould complexity: number of cavities (very important); tolerance of dimensions; number of slides; presence of additional movements (unscrewing, side movements, etc.) special heating/cooling devices;
- mould finish.

One of the interests to use Pam technology to 3D print MiM feedstock is to avoid mould cost and lead time.

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Pollen Am Mould costs are important in the PiM part production - Pellet extruder
Debinding is a step that depends strongly on the actual feedstock used.
The specific debinding strategy (thermal, solvent, chemical degradation, batch or continuous) will imply different equipment costs.

Equipment cost for a single part will depend on the actual number of parts that can be processed in a single cycle and the cycle time (for batch debinding) or in a given time (continuous debinding).

Cycle time, or the speed of the continuous debinding, depend on the component.

In general, thick components will debind slower than thin components but actual debinding times vary with the feedstock and the debinding strategy so no figures can be given here. Debinding can vary from a few hours to several days.

Each strategy will imply some fluid consumption (solvents, chemical agents, gases) and some energy, roughly proportional to cycle time and to the number of parts processed.
Personnel cost will be due to pick & place activities, quality checks (defects, weight loss), and equipment operation.

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Pollen Am Debinding costs are important in the PiM part production - Pellet extruder
Sintering, like moulding, is common to all MIM variants.
It may be batch or continuous, in graphite lined or metal-lined furnaces, under hydrogen or inert gas or vacuum, slower or faster because of the furnace capabilities, but after all the same material (with the same starting powders) will have to reach a typical temperature to be sintered.

Equipment cost for a single part will depend on the actual number of parts that can be processed in a single cycle and the cycle time (for batch sintering) or in a given time (continuous sintering).
Cycle time, or the speed of the continuous sintering, depend on the component.
In general, thick components will have to be sintered slower than thin components. The variation in cycle time is much less than for debinding, roughly from a few (6-8) hours to 24 hours.
Each furnace use energy and gases, and these costs will refer to the number of parts processed.

An additional source of cost is the necessity of sintering supports. These can be disposable MIM feedstock supports that sinter together with the part and are removed later,or ceramic supports that do not change during sintering and are used in many cycles until they eventually get damaged and have to be replaced.

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Pollen Am Sintering costs are important in the PiM part production - Pellet extruder
These costs are often very significant:
- tumble deburring / finishing;
- thermal treatment (quench & temper, ageing, etc.);
- plastic deformation (coining, sizing, straightening, etc.) ;
- machining (taps, threads, milling, turning, grinding, etc.).

- Sometimes even 50% of part total cost;
- Sometimes not carried out at the MiM producer, increase cost by adding transportation (and introduce external variables in production times); - The golden rule is to try to avoid these costs in the first step, by optimising the design (e.g. avoiding “over tolerancing”);
- Dealing with small parts, often an initial cost for the setup of the operation (small tools for handling, etc.) adds to the cost per part;
- These costs do not reduce with lot size as much as MiM costs (less dependent on quantity).

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Pollen Am Secondary operations costs are important in the PiM part production - Pellet extruder
1 - Mould production: a good balance between mould complexity and subsequent secondary operations must be sought. The larger the lot size, the higher complexity in the mould to avoid expensive secondary operations on a large production.

2 - Use common alloys, reduce the thickness, and use cores to minimise materials cost.

3 - Useless overspecification, especially on tolerances, increases part cost without actual value in the application.

4 - Reduce sintering cost, that is usually a significant fraction of the cost as the equipment is expensive, sintering supports should be used only when it is impossible to modify the design of the part.

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Pollen Am Mim economic aspect general guidelines  - Pellet extruder
For the sake of simplicity, all costs are normalised to the cost of this MiM part, so that, in arbitrary units, it is equal to 1.
Most of the cost is due to the feedstock (36%), followed by the secondary operations (23%) and the sintering (20%).

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Pollen Am MiM costs repartition - Pellet extruder