In this article we will discuss about the manufacturing process types. The types are:- 1. Micro Manufacturing 2. Generative Manufacturing Processes 3. Self-Assembly.
Manufacturing Process Type # 1. Micro Manufacturing:
Silicon technology and extension of that form an important component of micro manufacturing, there are other varieties of processes used to produce micro components. It should also be remembered that ultra-precision devices to measure and control dimensions at the micro level need to be also developed to make micro manufacturing possible. It is further important to note that in micro manufacturing machine tools / processing equipment and dimensional and surface quality measuring instruments must be integrated into a closed loop control system.
Micro manufacturing and conventional precision manufacturing use many of the same techniques and equipment, but the objectives are different. In precision manufacturing, the objective is to produce parts with a size to tolerance ratio greater than 10,000. On the other hand, in micro manufacturing, the goal is to produce parts typically 1-100μm in size with sufficient accuracy to achieve functional goals with required repeated satisfaction. Quite often, the accuracy and tolerances in micro manufacturing may be beyond the range of those prescribed in precision engineering.
Figure 7.4 indicates the dimensional magnitudes covered by micro manufacturing along with the wavelengths of different types of electromagnetic radiation. This is relevant as such radiations are used as tools for micro manufacturing of a few types as indicated. X-ray lithography can produce micro components with ease because of its very small wavelength.
It is also to be noted that in MEMS applications one of the key features is integration of components covering a range of a few orders of magnitude. Therefore, for MEMS, the required metrology has to also cover several orders of magnitude. So, the progress of MEMS and micro manufacturing has been closely associated with a parallel development in micro metrology.
Developments in Metrology:
Features smaller than the wavelength of visible light cannot be seen directly. Scanning Electron Microscopes (SEM) can be considered to be the first development which made viewing of features smaller than 0.5μm possible. In SEM, the wavelength of a high energy electron (accelerated by kilovolts) is very small. We know –
we get –
The short wavelength of an accelerated electron gives rise to a very high lateral resolution, typically 1 nm.
Optical interference techniques also yield good lateral resolution (in the micron range). Contact profile meters using pointed stylus with a few microns tip radius are also used for measurements with similar precision. High degree of vertical resolution can be obtained by interference microscopy method. Two beams are produced from one source; one of the two beams is considered as a reference wave while the other is the object wave which is modulated in phase by the object surface.
When the two waves recombine, a fringe pattern results that depends on the modulation of the object wave. With this arrangement, it is possible to measure heights upto 50μm with an accuracy of the order of ±5μm However, for this technique to be successful, fairly reflective surfaces are required.
For mechanical type stylus instruments, the stylus movement is measured relative to a reference that conforms to the nominal shape of the surface under inspection. As mentioned earlier, a sharp conical or pyramidical diamond tip is used with about 2-10μm radius. In Scanning Tunnelling Microscopes (STM), noncontact type stylus is used. These instruments are capable of lateral resolution sufficient to resolve protruding atoms. The vertical resolution is as small as 0.02μm.
Figure 7.5 shows the basic principle of STM in which a very sharp stylus tip is brought very close to the surface of the sample and the gap is less than 1 nm. At such close proximity, the free electrons from work surface tunnel to the conductive sharp stylus tip, resulting in a very small current given by –
I α e-2kd, (7.1)
Where I is the tunnelling current, k is a constant, and d is the distance between the tip and the surface. The typical value of k is 1.0 Å-1.
Another major development in micro metrology is the invention of Atomic Force Microscopes (AFM). This is also a noncontact type instrument which uses a very fine stylus attached to the end of a thin cantilever. As the fine stylus tip is brought close to a surface (within 30-150μm), an attractive force between the atoms is generated. This causes the cantilever to bend and a capacitive sensor measures its small deflection. Since the atomic force itself depends on the gap between the stylus tip and the work surface, profiling of the surface can be done.
Quite a few other types of micro metrology techniques like vibroscanning method, elastic transmission method, etc., have recently emerged. Of course, it should be remembered that MEMS do not necessarily require a very high resolution measurement. MEMS generally have relatively large vertical dimensions and measuring large vertical dimensions of features possessing smaller lateral dimensions is a typical challenge in MEMS micro metrology.
The typical ranges of micro metrology tools are shown in Fig. 7.6.
Types of Micro Manufacturing Processes:
It is needless to mention that a whole range of processes are used for the purpose of micro manufacturing. A few of these are extreme precise form of conventional machining operations. Some are based on the nontraditional machining processes like ultrasonic machining, electric discharge machining, laser beam machining, etc., but with much higher degree of tolerance and more precise control. At the same time, two-dimensional (or even quasi two-dimensional) microsystems use marking and etching technology.
A new concept in manufacturing has emerged recently which suggests development of shaped objects by adding material in small quanta. It is felt that at the highest level of sophistication, shaped objects will grow by itself utilizing the self-assembling capacity of smart materials. Before this becomes applicable to macro sized parts, for producing micro sized parts such techniques may prove to be very useful.
Manufacturing Process Type # 2. Generative Manufacturing Processes:
The Generative Manufacturing Processes (GMPs) do not fit in with the basic concepts of the traditional manufacturing processes and represent a major breakthrough. Unlike the manufacturing processes in the old era, the shape of a work piece is not achieved by removal of excess material in the form of chips or by forming / casting. Instead, it is done by addition of material in small quanta without any particular form. One important aspect which makes these processes so eminently suitable for the future is its basic nature being so amenable to computer control.
In GMPs, material is added or created (by solidification / bonding) where it is needed. The first commercial GMP was based on solidification of a liquid by a laser beam (called stereo lithography) and was developed in 1987 by an American firm “3D-Systems” at Auto fact, Detroit. Since then, many other techniques for GMP have been developed and commercial machines are now available in the market. Currently, the materials used for these processes are mostly nonmetallic and often do not possess the requisite amount of density and strength necessary for functional purposes.
Thus, the major usage of these processes has remained confined to rapid development of prototypes and models. However, the ongoing research indicates that in the near future it may be possible to produce actual parts made of materials suitable for functional components. Hence, the day is not far off when these processes will make desktop manufacturing possible.
Basic Principle of Generative Manufacturing:
In all types of GMPs, first a computer model (CAD model) of an object component is developed. This CAD model is next split into thin layers as indicated in Fig. 7.43a. The direction of slicing and slice thickness can be varied for convenience of generation. Next, in order to generate a solid object of the same shape as that of the CAD model, material is added (or grown) layer wise, the layers being of the same shape and thickness as obtained from slicing the CAD model. The thickness of a layer grown (t) must be the same as the distance between the corresponding consecutive slicing planes.
Though most of the commercially developed generative manufacturing processes use the layer-by-layer approach to build a three-dimensional solid object, a direct three-dimensional building up technique is also under serious consideration. In the direct method, it will not be required to decompose the three-dimensional bodies into two-dimensional layers and an object will be built directly point by point.
It will undoubtedly enhance the freedom and flexibility in shape generation. However, a number of technological challenges need to be overcome before a direct method becomes a technological and commercial success.
Thus, the generalized representation of all generative manufacturing processes can be as indicated in Fig. 7.43b. The hierarchy of the steps for shape generation is shown in Fig. 7.43c. As per the hierarchy described in Fig. 7.43c, a three- dimensional model is decomposed along a direction to yield a series of discrete entities like surfaces, lines, and points, which are then generated by a particular process.
After a three-dimensional model is decomposed to layers (surfaces), some processes will require further decomposition of the surface into lines which are actually deposited / generated. In some cases, even the lines need to be decomposed into points for physical generation of the solid object.
General Features and Classification of Generative Manufacturing:
The basic principle followed by the generative manufacturing process is radically different from the basic concepts of manufacturing processes which have remained prevalent since the beginning of human civilization. The world had to wait for the necessary achievements in computer technology for the GMPs to become commercially viable. These processes are also free from the traditional problems of manufacturing as the material is created wherever needed during a GMP.
The major advantages of the generative manufacturing processes are as now given:
1. Advantages during Design:
Manufacturing process is quite independent of the part features and there is no need for feature based design.
2. Advantages in Planning:
No blanks are required and no planning for the blanks is required. The GMPs are based on single operation only and no complicated scheduling and routing problems are faced.
3. Advantages during Shaping:
The GMPs being tool less processes the complex tasks of tool selection, tool management, requirements for jigs, fixtures, moulds, dies are all eliminated.
4. Advantages in Automation:
The process being completely computer- oriented, integration and automation of the manufacturing process is easy and relatively inexpensive.
A large number of techniques and procedures have already been developed and a systematic classification of the whole spectrum of the generative manufacturing processes is desirable. However, the classification can be based on the nature of the state of the raw material (i.e., solid, liquid, or powder), the way material is created / solidified, and the geometric character of shape generation procedure. Table 7.8 shows the classification based on the nature of the raw material.
The classification of processes is also possible based on the techniques of shape generation (see Table 7.9). The development of three-dimensional shapes is possible either by direct three-dimensional technique or by depositing layer upon layer. Layers are developed either as agglomeration of points and lines created gradually or the whole layer is created simultaneously.
In cases of shape generation through solidification of a liquid polymer, the two-dimensional layer-by-layer technique is appropriate. In this approach, the lower layers need to be created first. The direct three-dimensional techniques do not require creation of lower layers first and the flexibility in shape generation is more. Of course, programming becomes more demanding and the computational load increases.
Issues Related to CAD and GMP Software:
The first inevitable requirement for generative manufacturing processes is representation of the desired three-dimensional object in the form of a computer generated model. Certain issues related to CAD and subsequent processing of CAD model is discussed in brief.
In most cases, the GMP systems receive their data from CAD systems in either three-dimensional surface models or three-dimensional solid models. 3D Systems Inc., who first marketed a GMP machine based on stereo lithography (STL), developed an STL file format. Since such machines outnumber all other types of machines, the STL format has become the de facto standard for almost all types of GMP machines. The technique is based on creating a mesh of interconnected triangles oriented three-dimensionally.
Figure 7.44 shows the representation of three-dimensional surfaces by triangular mesh. The vertices of the triangles are ordered to indicate which side of the triangle contains material, and thus need to be generated during the process. It is not difficult to realize that an increase in the number of triangles results in improvement of accuracy when curved surfaces are involved.
The computational slicing of the CAD model is done by using a ray tracing algorithm which scans through a particular z-level of the model. The resulting cross-section can be one or more closed paths and a complete representation of the area to be filled with material. Such areas are to be generated by suitable cross-hatching algorithms to generate the trajectory of the creating element (the laser beam or material ejecting nozzle, etc.). Cross-hatching is important and should be optimal so as to generate the object in the shortest possible time, maintaining the required density and strength.
Distortion also depends to some extent on the hatching pattern and should be taken note of. The orientation of the object needs to be chosen judiciously to optimize time and accuracy. The software needed for slicing and generation of data to control the GMP system movements is not a general one and depends on the specific system. The effect of orientation on build time and accuracy is indicated by the example shown in Fig. 7.45.
It is seen that the orientation of the part as shown in Fig. 7.45a results in stair-step appearance and to generate a smooth curved surface the thickness of the layers has to be very small increasing the time required to generate the object. If the orientation of the object is changed to that shown in Fig. 7.45b, a smoother curved surface can be generated without making the layer thickness too small.
The slice axis is defined as the normal to the plane created by slicing and this also represents the build direction. The thickness of the slice ultimately becomes the thickness of the corresponding layer created and therefore dictates the texture, accuracy, and builds time. Normally, layer thickness is in the range 0.0625-0.75 mm. However, it should be noted that the use of thicker layers does not necessarily reduce the build time though the number of layers to be created reduces.
This is so as the scanning speed, while creating the material, depends on the thickness. Figure 7.46 shows the dependence of build time on layer thickness. As can be seen, the build time reduces with the beam power for a given layer thickness. This is so because the scanning can be done at higher speeds with increased beam power. However, the dependence of build time on layer thickness suggests the existence of an optimum thickness with which the build time is minimum. The range 0.125 mm to 0.25 mm is recommended for such operations.
Internal hatching is used to solidify (or create) the area of a layer inside the outside boundaries of the material object. A properly chosen hatching pattern can generate a part in minimum time with the right kind of properties like strength, density, etc., and reduces distortion to a minimum. Initially, the boundary lines are created and then the interior is criss-crossed with lines, giving the part adequate stiffness.
Figure 7.47 shows a commonly-used hatching pattern that is called Tri-Hatch. It consists of parallel lines making 0°, 60°, and 120° with the x-axis, resulting in an internal structure that consists of equilateral triangles. The spacing between the consecutive lines is about 0.625 mm. When liquid photopolymers are used in the process, the material trapped inside the triangles remains liquid. It is solidified when the part is subjected to a curing operation.
Recently, another hatch pattern, called WEAVE™, has become popular. In this, the scanning lines are parallel to the x- and y-axis, the spacing being about 0.28 mm when the layer thickness is about 0.25 mm. With WEAVE™ hatching, the trapped liquid volume is less. The distortion of the part is also substantially reduced. However, a much better control of curing depth while scanning is necessary.
It is obvious that the outer surface of the generated part cannot end up being porous. Therefore, skins are created by skin fills which consist of closely spaced scan lines. The spacing between the scan lines is in the range 0.0762 mm to 0.127 mm. The skin fills are created after the internal hatching is complete. With the introduction of WEAVE™, the importance of skin fills is reduced as the volume of trapped liquid is quite small.
In many cases while slicing the CAD model of a part into layers, isolated regions may be generated as indicated in Fig. 7.48. The overhanging projection is connected to the main body of the part from the top.
Since the building up is from the bottom, the connection is generated at a later time. Hence, it becomes essential to design a support for the overhanging part to prevent it from falling down under the action of gravity. Of course, such situations arise when a liquid based process, e.g., stereo lithography, is used. Sometimes, supports are necessary even though no isolated region is created. For example- parts having cantilever beams or simple beams require a support structure.
Otherwise, when the overhanging beam is just started, the initial few layers will not be able to hold itself under the action of gravity. After the generation of the complete part is over, such supports are removed. The programming of the job has to involve the creation of such supports. By using proper orientation of the part, the necessity of support can be either reduced or eliminated. For example, if the part shown in Fig. 7.48 is placed upside down or sideways, no isolated island is generated and supports can be eliminated.
Self-assembly (or self-organization) is a process in which components automatically come together to form aggregates. All living objects and biological systems as well as a large number of nonliving physical systems exhibit self-organizing behaviour. Examples of non-biological systems formed by self-assembly are many, the most common being crystals, micelle, colloids, and self-assembled monolayers.
The key point in this mode of ordered structure formation is that all the information required to specify the desired shape and size is available within the process itself. Self-assembly can involve components from the molecular to the planetary (weather systems) scales and can be based on many different kinds of interactions. Traditionally, self-assembly has been associated with atoms and molecules only and its study has been a common subject among the chemists.
However, the subject has gained importance recently with the advent of nanotechnology. There are mainly three scales in which the process operates—molecular, nano-scale (colloids, nanowires, etc.), and meso to macroscopic (objects with dimensions from microns to centimeters).
Of course, the subject is at its infancy and neither our understanding nor our control of self-assembly is anywhere near the stage when it will be possible to map out the road to the final destination—self-assembly of micro devices and -machines. It is hoped that the ultimate self-assembled structures will be multifunctional, self- correcting and, perhaps, self-replicating.
The ultimate in manufacturing is the process through which three-dimensional devices and objects will be manufactured by self-assembly of material(s) without any continued intervention of human action. Intelligent micron-sized (even nano-sized) machines will be a major development in future engineering activities. However, to successfully exploit the potential of such devices, it is essential that the manufacturing cost be low.
It has also been mentioned that the top-down manufacturing processes are limited in their capacity to manufacture micron-sized (or nano-sized) three-dimensional features. The currently practised photolithographic techniques are capable of producing submicron level features (as required in VLSI chips) but the capability of lithographic processes is limited to two-dimensional systems primarily. For producing truly three-dimensional objects, the scope of lithography is very doubtful.
There have been some recent developments like focused ion beam (FIB) technologies, using which even nano-sized three-dimensional features can be produced. But, unfortunately, this technique is not very suitable for batch production. Thus, the cost of the objects produced using FIB is prohibitive. The other top-down approaches for micro manufacturing have limited capabilities like micro drilling, micro slot cutting, etc. Besides, the cost of manufacturing is generally high.
It is clear that the bottom-up approach can be very suitable for developing very small-size three-dimensional objects. However, the usual philosophy of adding (or generating) micro-sized elements in succession to develop the whole object is not only time-consuming but also not amenable to batch production.
If, on the other hand, the process of material addition takes place spontaneously without human intervention, not only large scale batch production will be possible but the time taken for manufacturing will also be within reasonable limits. The process will be somewhat like producing a huge number of bacteria in a vat. The addition of material will be spontaneous as in the case of the growth of a living object.
Like any spontaneously occurring process (like rolling of a ball down an inclined plane resulting in lower potential energy), minimization of energy is the key motivating factor in self-assembly and self-organization. The process is governed by different types of interaction forces acting between assembling components and the resulting structure formation is associated with minimization of energy.
The self-assembly processes can be classified into two main groups as now explained:
1. Un-Coded Self-Assembly:
In these processes, minimization of energy is the only motivation while forming aggregates. No very specific geometric features are involved.
2. Coded (Directed) Self-Assembly:
With un-coded self-assembly, formation of three-dimensional features is generally not possible. In coded self-assembly, information on the geometric features of the desired object is encoded into the basic components. A suitable combination of the motivation for energy minimization and the geometric features produces complex three-dimensional objects. The best example is the recombination of two DNA strands. There is only a unique way two strands can join.
Successful self-assembly depends on Six important requirements, viz.:
i. Components / Elements:
Groups of molecules, segments of macro- molecules, nanoparticles / micro particles that interact with one another are the components of a self-assembly process. The primary objective is to create a more ordered structure from the randomly-oriented large number of these basic building blocks.
ii. Inter Component Interactions:
The creation of an ordered agglomeration of the components requires a suitable interaction among the components which brings them together and keeps them assembled. These interactions should lead to weak bonding so that readjustment of the orientations becomes possible. These interactions motivate the components to come together, overcoming the energy barrier due to thermal fluctuations.
iii. Ability to Readjust Positions:
If the interactive forces are too strong, the components get attached to one another in an irreversible manner, i.e., once attached, no further spontaneous change of orientation of their relative positions is possible. Such a situation leads to the formation of amorphous and disordered structures. Thus, to provide the ability to readjust positions, the strength of the bonds must be comparable to those which try to readjust the positions. Figure 7.63a shows a situation where adjustability is not possible.
The assembly process leads to disordered amorphous states. On the other hand, Fig. 7.63b shows a self-assembly process with reversibility which leads to an ordered regular shaped object.
All self-assembly processes require mobility of the components and therefore are facilitated in solutions instead of dry environment.
v. Mobility and Transportation:
The components in a self-assembly process need to be mobile for obvious reason.
Reaching a state of equilibrium is necessary in self-assembly processes. Otherwise, the assembled object may not possess structural stability.
As in the case of any spontaneously occurring process, e.g., rolling down of a ball on an inclined plane, resulting in lower potential energy, minimization of total energy is the key motivation for any self-organizing or self-assembly process. The process is governed by different types of interactions among the components with a desire to minimize the overall energy. In appropriate cases, this is achieved through the realization of a particular pattern or shape.
The concept of self-organization based on energy minimization can be explained with the classic example of monolayer and micelle formation with surfactant molecules in aqueous solutions. Surfactant molecules have spatially different domains within the structure, and, in the simplest form, consist of a hydrophilic head group and a hydrophobic tail as shown in Fig. 7.64a. The hydrophilic head prefers to remain in the proximity of water whereas the hydrophobic end tries to avoid contact with water.
Such hydrophilic and hydrophobic characteristics can be attributed to their surface tensions. In general, the surface energy (or surface tension) of a hydrophilic material is higher than that of water and is preferentially wetted by water. In contrast, a hydrophobic material has surface energy less than that of water and is not wetted by water. When such a surfactant molecule is put in water, the hydrophilic end is wetted and prefers to remain surrounded by water.
Conversely, the hydrophobic end prefers to avoid contact with water. Under the influence of the resulting antagonistic force field originating from the interfacial tensions of different domains of the molecule, it is most suited if the molecule migrates to the water-air interface and orients itself so that the hydrophilic head remains in water and the tail is in the air as shown in Fig. 7.64b. This is the most basic example of self-organization as the orientation of the molecule is predetermined by the interfacial tensions.
As more such molecules are added, all of them migrate to the water surface and organize themselves in a head-down position, forming an ordered monolayer (Fig. 7.64c). When the concentration of these molecules is increased, the water-air interface gets fully covered by the monolayer (at this stage, the distance between the neighbouring molecules is governed by the steric repulsion between the head groups—i.e., the heads nearly touch each other as indicated in Fig. 7.64c).
The newly added molecules are forced to stay within water, but instead of getting distributed in a random fashion (which is thermodynamically not favourable); the surfactant molecules form spherical aggregates called micelles. The structure of a micelle is shown in Fig. 7.64d which hides all the hydrophobic ends from water. Depending on the situation, a bilayer can also form as shown in Fig. 7.64e. There exists a critical concentration [known as critical micellar concentration (cmc)] above which all the surfactant molecules added to the solution will form micelle.
If an organic solvent is used instead of water, the scenario becomes just the opposite as the hydrocarbon tail is energetically favoured and the hydrophilic head is repelled. Thus, depending on the concentration, either a monolayer or reverse micelle is formed automatically. However, many other kinds of other structural aggregates like rods, lamella, etc., can also form depending on the situation.
Many different types of patterns and structures can be self-assembled by designing the basic components. Figure 7.65 shows a particular case where a porous structure is developed. When hexagonal components with alternate faces as hydrophobic and hydrophilic are used, again a porous nanostructured pattern is generated as shown in Fig. 7.66a. On the other hand, if all the faces are made hydrophobic, a nonporous structure is formed as shown in Fig. 7.66b.
Thus, it is seen that more complex shape generation can be achieved when the basic components are designed suitably. Such type of self-organization is termed as templated self-assembly. In such cases, the interactions between the components with a pre-existing regular pattern determine the formation of the final structures. In essence, such type of templated self-assembly combines both the top-down and bottom-up concepts to some extent. We now give examples of templated self-assembly.
White sides and his group have attempted such templated self-assembly. They prepared millimeter-size components of different regular shapes and coated the chosen faces with a low melting metallic alloy, and suspended the resulting particles in aqueous KBr solution kept at the melting temperature of the alloy. On agitation, the components collide and interact through the capillary forces between the drops of liquid alloy.
The assembly of the components takes place in a manner that minimizes the area of contact of KBr solution with the alloy, resulting in the lowest free energy of the aggregate. The shape and character of the resulting aggregate depend on the shape of the components and the locations and geometry of the alloy coated faces when regular polyhedra and cubes are chosen. Figures 7.67a and 7.67b show the correct and incorrect matching of faces for minimizing the exposed surface area.
The configurations resulting from the assembly indicated in Fig. 7.67a are energetically more favourable than those in Fig. 7.67b because those in Fig. 7.67a minimize the alloy-KBr interface area. White sides’ group also experimented with a shape-selective lock-and-key geometry. The geometry of the component is indicated in Fig. 7.68a.
Out of the three favourable choices for self-assembly (Fig. 7.68b), the head-to-tail configuration leads to best choice as – (i) it minimizes the area of exposed hydrophobic faces, and (ii) it leads to better kinematic stability. Thus, when the solution is agitated to the correct level, the bonding for tail-to-tail and head-to-head configurations are not strong enough to hold. Thus, only the head-to-tail configuration of self-assembly survives, leading to a long strip-like aggregate to form as shown in Fig. 7.68c.
Generally, self-assembly is a manifestation of information coded as shape, surface properties, charge, etc., in individual components and these characteristics determine the interactions among them. Molecular self-assembly involves weak covalent interactions like van der Waals, electrostatic, acid-base interactions, interactions based on hydrophobicity and hydrophilicity, hydrogen bonds, etc. In contrast, in the case of components of larger sizes (tens of nanometers to hundreds of microns), interactions such as electric and magnetic fields, capillary and entropic interactions are of interest.
For self-assembly to generate structures more complex than crystals, it is important that the components must come together only in some predetermined unique way.