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Microextrusion is a microforming extrusion process performed at the submillimeter range. Like
extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex c ...
, material is pushed through a die orifice, but the resulting product's
cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...
can fit through a 1mm square. Several microextrusion processes have been developed since microforming was envisioned in 1990. Forward (ram and
billet A billet is a living-quarters to which a soldier is assigned to sleep. Historically, a billet was a private dwelling that was required to accept the soldier. Soldiers are generally billeted in barracks or garrisons when not on combat duty, alth ...
move in the same direction) and backward (ram and billet move in the opposite direction) microextrusion were first introduced, with forward rod-backward cup and double cup extrusion methods developing later. Regardless of method, one of the greatest challenges of creating a successful microextrusion machine is the manufacture of the die and ram. "The small size of the die and ram, along with the stringent accuracy requirement, needs suitable manufacturing processes." Additionally, as Fu and Chan pointed out in a 2013 state-of-the-art technology review, several issues must still be resolved before microextrusion and other microforming technologies can be implemented more widely, including deformation load and defects, forming system stability, mechanical properties, and other size-related effects on the
crystallite A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...
(grain) structure and boundaries.


Development and use

Microextrusion is an outgrowth of microforming, a science that was in its infancy in the early 1990s. In 2002, Engel ''et al.'' expressed that up to that point, only a few research experiments involving micro-deep
drawing Drawing is a form of visual art in which an artist uses instruments to mark paper or other two-dimensional surface. Drawing instruments include graphite pencils, pen and ink, various kinds of paints, inked brushes, colored pencils, crayons, ...
and extruding processes had been attempted, citing limitations in
shearing Sheep shearing is the process by which the woollen fleece of a sheep is cut off. The person who removes the sheep's wool is called a '' shearer''. Typically each adult sheep is shorn once each year (a sheep may be said to have been "shorn" or ...
on billets and difficulties in tool manufacturing and handling. By the mid- to late 2000s, researchers were working on issues such as billet flow, interfacial friction, extrusion force, and size effects, "the deviations from the expected results that occur when the dimension of a workpiece or sample is reduced." Most recently, research into using ultrafine-grained material at higher formation temperatures and applying ultrasonic vibration to the process has pushed the science further. However, before bulk production of microparts such as pins, screws, fasteners, connectors, and sockets using microforming and microextrusion techniques can occur, more research into billet production, transportation, positioning, and ejection are required. Microextrusion techniques have also been applied to
bioceramic Bioceramics and bioglasses are ceramic materials that are biocompatible. Bioceramics are an important subset of biomaterials.J. F. Shackelford (editor)(1999) ''MSF bioceramics applications of ceramic and glass materials in medicine'' Bioceramics r ...
and
plastic extrusion Plastics extrusion is a high-volume manufacturing process in which raw plastic is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weatherstripping, fencing, deck railings, window frames, plastic film ...
and the manufacture of components for resorbable and implantable
medical device A medical device is any device intended to be used for medical purposes. Significant potential for hazards are inherent when using a device for medical purposes and thus medical devices must be proved safe and effective with reasonable assura ...
s, from
bioresorbable stents In medicine, a stent is any device which is inserted into a blood vessel or other internal duct to expand it to prevent or alleviate a blockage. Traditionally, such devices are fabricated from metal mesh and remain in the body permanently or unti ...
to controlled drug release systems.


Microextrusion processes

Like normal macro-level extrusion, several similar microextrusion processes have been described over the years. The most basic processes were forward (direct) and backward (indirect) microextrusion. The ram (which propels the billet forward) and billet both move in the same direction with forward microextrusion, while in backward microextrusion has the ram and billet moving in opposite directions. These in turn have been applied to specialized applications such as the manufacture of microbillet, brass micropins, microgear shafts, and microcondensers. However, other processes have been applied to microextrusion, including forward rod–backward cup extrusion and double cup (one forward, one backward) extrusion.


Strengths and limitations

Strengths of microextrusion over other manufacturing processes include its ability to create very complex cross-sections, preserve chemical properties, condition physical properties, and process materials which are delicate or dependent on physical or chemical properties. However, microextrusion has some limitations, though primarily related to the need for improvement of the relatively young process. Dixit and Das described it thus in 2012:
With the scaling down of dimensions and increasing geometric complexity of objects, currently available technologies and systems may not be able to meet the development needs. New measuring devices, principles and instrumentation, tolerance rules, and procedures have to be developed. Materials
database In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases sp ...
s with detailed information on various
materials Material is a substance or mixture of substances that constitutes an object. Materials can be pure or impure, living or non-living matter. Materials can be classified on the basis of their physical and chemical properties, or on their geologic ...
and their properties/interface properties including microstructures and size effect would be very useful for product innovation and process design. More studies are necessary on micro/nanowear and damages/failures of the micromanufacturing tools. The forming limits for different types of materials at the microlevel must be prescribed. More specific considerations must be incorporated into the design of machines that are scaled down for microforming to meet engineering applications and requirements.


Further reading

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References

{{reflist Forming processes Medical technology Drug manufacturing