In many cases the volume injected is increased to permit a reliable production system using a scrap system over dimensioned in relation to the piece.
 

As for the control system, it is important to bear in mind that the hydraulic system must be highly dynamic to enable injection of extremely low volumes with times of injection under 0.1 s.
 

7.2.2 Microinjection mold

In the micromolding process, as well as injection machine with especial characteristic could be necessary special auxiliary equipment.


7.2.2.1 Design

The recommendation is a small sprue that involves high precision in the control of the material to inject, in fact, it is less than 1mm3 of plastics

When small machines are used in the process of microinjection it is necessary a mold with an overdimensioned scrap in relation to the injected volume. This permits to avoid a precise control of the micro volume injected, as the scrap will offset that volume.

The use of a larger runner, in addition to distributing the polymer from the entrance to the mold cavity or cavities, aims to prevent degradation of the material in the plasticizer due to an excessive time of residence in the barrel. Therefore, the runner volume must be calculated to ensure the constant renewal of the polymer and the transformation in each injection of at least a volume equal or greater than the screw diameter.

The main disadvantage in an injection system with a large runner volume is the need of a greater total cycle time, due to the cooling time needed for the scarp, much higher than the needed to cool the micro pieces.


Mols-temperature-control-unit:cooling and heating

Due to the high aspect ratio of micropieces, plastic solidifies instantly when it comes into contact with the walls of the mold. This phenomenon may be only partially offset by modifying some machine parameters as the injection speed or the bulk temperature of the polymer.

A high-speed injection can automatically raise the temperature of the plastic because of the shear produced. In fact, depending on the complexity of the piece geometry it is possible that an increase of the injection speed is not enough to reproduce the form of the mold completely. Therefore, it will require an increase in mold temperature, which can become considerable: it could even be necessary to heat the mold at a temperature between 40 and 50 ° C above Tg (glass transition temperature) of the thermoplastic material.

If the reproduction problem still persists, a final option is to develop a vacuum injection, ie, causing a depression in some areas of the mold before injecting. For that, it is necessary to install an O-ring perimeter, usually fixed on the side of the mold, a valve in the mouthpiece of machine and a small vacuum pump, which will close the mold.

In micro-injection it is recommended that the cooling of the cavity is performed by Conformal Cooling Channels, which has a more efficient control of the mold temperature, improving the quality of the injected pieces and reduces the cycle time.
It is also possible to use the Variotherm system, which involves heating the mold cavity by induction and equip the mold with a cooling circuit (with liquid) such that during injection the liquid temperature is greater than during the cooling. This system allows the mold of being thermally insulated.

When it is necessary to reach mold temperatures between 100 and 200 ºC, another alternative is to implement small heat heating resistances, commanded by thermocouples.

As it happens in high dimension molds, it is also possible to use classical thermal channels through which steadily circulates a fluid previously tempered. (Case study in the micro-gear mold)


7.2.2.2 Material and recoveries

The steel moulds for microinjection have to be highly resistant to corrosion and wear. The area of the cavity is usually coated.


Cavity centre systems

The best guides focused on the process of microinjection are those of square or rectangular section. In addition, we a system of floating cavities is need.

With square or rectangular section guides it is possible to get a better centred cavity because the adjustment takes place between two parallel sides and also ensures that tensions (in the moment of closing the mold) are lower than with other conic geometry guides.

The system of floating cavity (moving part of the mold) is to encircle the cavity through O-rings so that the focusing guides are outside the floating system. This system allows offset the efforts that appear during the centre process and get a precision of less than 5 μ m. The system of floating cavity requires some pins for a pre-centre.

 

7.2.2.3 Microinjection mold manufacture

For the manufacture of molds for microinjection high production, it nowadays exist specific mechanization equipment, very sophisticated, that provides the high precision required. Among these technologies are ultraprecisión machining diamond-tipped, machining with special cutting tools (with diameters on the order of 0.3 mm compared to 4 mm diameter of the smallest conventional tools), systems of micro eroding EDM (electrical discharge machining), laser machining equipment, microrectifying, etc.

However, with a centre of a conventional high-speed machining, available in most mold-maker’s workshops, it is also possible to obtain high precision on the range of micro technologies, which will be later supplemented by microerosion EDM technology in the cases in which it’s necessary to mechanize edges or deep grooves, such as the capillary runners, or in certain hard false pieces.

Another of the technologies with a great potential for this level of requirements is the laser manufacturing, which provides great advantages in the manufacture of delicate or polished up pieces, and generally, in too small geometries for conventional machining.


Mold mechanized

The mold mechanization requires techniques adapted to micro-machining. These micro technologies are usually classified intwo groups:

The first group is formed by lithographic processes: most of these processes are applied in clean room or workloads sealed inside the machine process. These are processes that use various masks and light to print geometries like photographic material. They can process a limited group of materials and the "aspect ratio" that can reach is limited to values around 1:100 (higher in some cases and for specific techniques as the LIGA). Its high cost is holding back its expansion and use.

The second group is the ultra-precision machining, comprising the evolution of the techniques usually used in companies which manufacture precision components. These are cutting machines (milling, turning), laser, electro-erosion and penetration, specially adapted to carry out small movements. These machines can equip minimum size tools, they have assistance systems for positioning and making references. They are also designed to minimize the environmental impact on the component machining.

Apart from these two groups, there are some technologies, such as the laser machining, which are applied in both processes, in one case to manufacture masks and in the other directly on material to be mechanized. In addition, there are common issues such as micro-precision positioning (necessary both to manufacture the masks and to align them) or micro-metrology.

The ultra precision machining has the advantage of being able to generate complex 3D forms, perhaps not with all technologies but in some of them. The lithographic processes are limited to perform mechanized parts with similar dimensions to those of a wafer and on large areas of flatness, while ultraprecision processes have an important market in generating high-resolution details in localized areas of parts not necessarily micrometric, as may be areas of molds or texturing surfaces.

In fact, there are different technologies to implement micro-machining. Some of these are:

­Mechanical Systems:

­Combined systems:

Electrical systems:

Chemical devices:

7.2.3 Plastic material for microinjection

When choosing a material for the injection of plastic micro parts, one must take into account, among other aspects, that this material will suffer more than in a conventional injection because the injection speed and shear are higher, and cooling speeds are faster due to the high aspect ratio. Therefore, it is possible that a material chosen by its performance has to be discarded because it does not support the necessary processing conditions to obtain that piece with the required quality.

LCP, COC, PC, PS, PPE, PMMA, PEEK, PSU, PAI, PEI, PBT, PA y POM are the most common materials used in micro-injection.

For example, if a high reproduction is required, one will choose PCL or COC. If the injection process requires materials with high fluidity, one will use PC, PA, POM, PBT, PEI, EPP and PSU. And if it is required a high temperature resistance, the polymers used would be PEEK and PEI.


7.2.3.1 Pellets selection

As already mentioned, the conventional pellets can not be used in microinjection as a matter of size. It is necessary to use pellets of 0.05 to 0.5 mm of diameter to prevent slipping in the screw feeding area.


7.2.3.2 Nanocomposites

In recent years, compounds that use nanometric fillers have undergone considerable interest to both research and industry. One of the main reasons has been the possibility to increase significantly a large number of properties compared with the virgin polymer or polymer loaded with conventional fillers (micro and macro). For instance, some of the new advantages are an increase of the tensile module, the elastic break modulus [Biswas-01], the heat resistance and the flammability [Gilman-00]; also it is obtained a decrease of the permeability to gases.

Although nanoparticles used as fillers can be made of various materials, the most commonly used in combination with polymeric matrixes are ceramic origin and are characterized by being particles a few nanometers thick and with very high "aspect ratio (length / thickness): between 10 and 1000. The scattering of such particles in a polymer matrix enable to achieve a combination of new properties, not achievable through conventional fillers because improvements are attributable to high dispersion, low particle size, high aspect ratio and low content of nanofiller (less than 10%). One of the more used filler so far has been the montmorillonite, aluminium structure and laminar aspect ratio of 1:1000.

Depending on the type of interfacial interactions that occur between the matrix and the silicate sheet, you can get two types of structures in nanocomposites [Komori-01]. In interspersed structures the inserting of the polymer matrix in the structure of silicate occurs following a regular crystallographic order and keeping the multilaminar structure of silicate. In the exfoliated nanocomposites, the polymer completely disperses the clay original sheeting.

In this case, the sheets are separated within the polymer matrix in a distance that depends on the quantity of nanfiller added. The ideal state of exfoliation is that in which the flake of silicates is evenly dispersed. However, it is known that the difficulty lies on exfoliating the sheets, as they are strongly linked by Van der Waals interactions, being necessary an adequate nanofiller-matrix interaction.

The methods to achieve the incorporation and exfoliation of ceramic nanofillers have been based on the filler dispersion by dissolution or by in-situ polymerization of monomer. Recently the possibility of obtaining matrix polymer nanocomposites through direct mixing in molten polymer has helped to stimulate an increasing research on these compounds, achieving the extension to a larger number of polymeric matrixes [Vai-97].

The technology of direct mixing has the advantage that it does not need solvents and uses conventional equipment, such as extruders and mixers. Therefore this is very attractive both from an environmental and an economic point of view. However, the intercalation by direct mixing is highly specific and depends on the nature and characteristics of the polymer, so that its implementation as well as the identification and optimization of processing parameters is not clearly described neither is known for a large number of polymeric materials.


7.2.4 Manipulation and verification of the micropieces produced

One of the main problems in the field of microinjection is the manipulation and verification of the injected pieces, not only dimensionally but also in a structural level.


7.2.4.1 Manipulation

When working with mili and micro sizes it appears a number of difficulties that does not exist in greater sizes: cohesion forces, such as electrostatic forces, Van der Waals and capillary. Often the objection is not to pick up the micro-piece but to leave it without any difficulty as the above-mentioned forces are more important in these scales.

The working environment has a great importance: it must be a controlled environment of temperature, humidity, vibration and environment dust, ie a clean room.

Basically there are two systems of manipulation: with and without contact, although the latter solves the problem of the forces of cohesion, it is limited because of the magnitude of the forces that can be applied to the pieces.

One way to reduce cohesion forces is to try to reduce the forces of contact, as seen in the following figures.

 

You should not forget that in the case of manipulation with contact the force applied to the piece has to be controlled. A solution is to incorporate a piezo-resistive sensor:
 

Some specific machines for micro-injection as the Battenfeld Microsystem 50 provide handling and inspection systems micro-injected pieces, but you must bear in mind that once the parts are manufactured they are part of a package, that is, they must be assembled. What’s more, it would be perhaps interesting to do afterwards modifications.

With a global movement of 1cm3 and a positioning resolution of 1 μm, it is capable of incorporating different tools at its edge in terms of work to be done: tweezers, clips, cutting tools, trimming and chips removing tools.


7.2.4.2 Inspection of micropieces


7.2.4.3 Surface finishing analysis

El AFM (Atomic Force Microscopy) allows to obtain photos where it can see the lines of 5 µm deep in the surface of injected part in POM. This lines have been made by mechanized tools.

The principle of operation consists of a silicon micro-cantilever with a micropoint at its end. Its deflection is monitored by a laser source and a detector.

This system not only gives "topographic" information; depending on the operation mode, ie using the overhanging part as a system for transmitting vibrations generated by the device with a certain amplitude and phase, you can measure the response (amplitude and phase) and thus have structural characteristics and material type.

The white light interferometer is another technique for obtaining a map of profiles of the piece, but without contact, through optic reflection. It's basically a divergent laser source, which, to find a beam splitter, is separated into two identical wave fronts, spreading in perpendicular directions. These beams are reflected in two flat mirrors, recombining after the beam splitter. If the mirrors were placed at the same distance from the beam splitter, then, disregarding the differences due to the thickness of the mirror, the beams would be recombined in phase, and would not be forthcoming from any interference pattern. If they move mirrors away, then the differences will produce optical path of interference fringes, which both depend on the distance between the mirrors as the wavelength of radiation used. For this reason, the interferometer is used both for determining distances and to determine wavelengths.

 

You must not forget that all the techniques of optical measurement have two basic limitations, as shown in the drawing, multi-reflection due to surface roughness and the inability to detect the return beam by geometric motifs of the piece to measure.

Morphological analysis

There are techniques that allow dissolution of selectively remove the amorphous phase of a semi-crystalline material and then analyze its structure. For this test we must choose a semi-crystalline material that resists the chemical attack of the solvent.

 

The outcome of the test (Figura 7.16¡Error! No se encuentra el origen de la referencia.) shows how inside of the piece, at 125 μm from the surface, there are crystalline spherulite formations, but as we move towards the surface, the crystal growth diminishes, until reach the surface (last image on the right) where the crystal growth is nil. This is due to a too rapid cooling of the piece, and therefore one can conclude that mechanically they will not be homogeneous.


Mechanical analysis

The micro hardness tester allows analysis of surface hardness in micro parts as well as visco-elasticity (conducting cyclic loading), elastic modulus and creep.

 

MicroDAC (micro deformation analysis by means of correlation) is a technique that allows viewing surface tensions in micro-pieces, doing previously a digitalization of the piece, scanning all the areas through mathematical algorithms and comparing them with the same areas at different levels of tension.

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