Resume
Essays
Resources
Home
Up
Manipulating material digitally
Javier Díaz Reinoso
Abstract.
T
he techniques of “Layered Manufacturing Technology” (LMT) promise to produce versatile machines, but at the moment any of this techniques can "close the loop", that is build itself. A preprocess to produce “cubes” of agglomerates of different materials is proposed, in essence "digitalize” material, with that is possible to simplify process of manipulation of materials, similar to the use in electronics where the use of digital signals simplify electronics designs.
January, 1998
Introduction.
With the possibility of having models of physical objects in a computer, it is possible now to use these models to create a physical object using technologies related to “Layered Manufacturing Technology” (LMT) and “Rapid Prototyping Technology” (RPM) , such as Stereolithography, Selective sintering, Droplet deposition and Laminated object manufacturing [Stucki et al 95] . These technologies are quickly evolving, but all of them have problems such as:
-
Only one material at the time.
-
Need special materials (e.g. in stereolithography, liquid photo polymers, in selective sintering, thermoplastic power) .
-
Problems creating empty spaces (in stereolithography the use of “support pillars”) .
To resolve these problems, a new technology is proposed, which has the following properties:
-
The use of a preprocess which can create “cubes” of aggregates of different materials. With the use of these “cubes” it is possible to completely fill the target volume.
-
One or more of these materials are “fillers” which are removed in a postprocess. In this manner it is possible to create empty spaces in the object.
-
Each “cube” is controlled individually and “injected” in a specific point of a plane. After the plane is completed, the next layer of material is deposited until the full 3D part is built.
-
When all the volume is completed, a postprocess, such as sintering, is applied to obtain a consolidated object.
Preprocess of the material.
In certain of these technologies a preprocess is needed. In selective sintering as an example, the material is a thin powder of thermoplastics. This powder is really a lot of small irregular particles with a resolution always smaller than the resolution of the final object.
For example, in a laser printer with a resolution of 300 DPI, which is equivalent to 84 µm, normally the particles of the toner are approximately 1 µm in size.
But in this manufacturing technology, a precise minimum is needed, for example (using a minimum resolution of 50 µm equivalent to 508 DPI) if a conducting layer is separated from other conducting layer with a nonconductor layer, all the layers of 50 µm, then this layers cannot be created with irregular particles or spheres because is not possible to fill all the space, instead if a "cube" of an aggregate of smaller particles is used the problem is resolved.
As an example, start with a metallic power with a resolution of 10 µm, add an organic thermal glue, extrude this mixture in a funnel with a square hole of 50 µm per side, use heat to activate the glue, finally cut the column with a knife (Figure 1) . An array of funnels can obtain the material in bulk.
Figure 1.
Other techniques which can be used are: partial sintering, using high pressure to unite the material, obtain thin films of the material and then cut it in cubes, mold beam of square profiles and then cut in cubes. It all depends on the material used.
In any case the objective of this preprocess is to create a physical analog of the "voxels" in the computer models [Chandru et al 95] , in this manner it is possible to fill all the spaces and simplify the design.
New problems can occurs with this technique, such as thermal deformation in a sintering postprocess, but decreasing in a small amount the size of the cubes can resolve those new problems. By the way, an exact precision in the size of the "cubes" is not necessary, it is possible to have small gaps around the "cubes" (5-10% of the size) without destroying the overall order.
The problem of empty spaces, is resolved using one or more materials of filler which can be removed in a postprocess. Typical filler materials should be inert in chemical or thermal reaction with the other "active" materials. The postprocess can be vacuum cleaning, liquid dissolvent, and materials which evaporate in the sintering process, similar to the "loss wax" process used in jewelry.
Injecting the material.
In the technologies of stereolithography and selective sintering the material is put over a substrate and then a laser is used to process the material. In this technique the analogy is similar to an "ink-jet" printer, in which each drop of ink is controlled individually. A few techniques can be used, the simpler probably is to use piezoelectric "fingers" which can stop o resume the flow of "cubes".
This piezoelectric "fingers" can be built using bimorph films of piezoelectric material [Pennwalt 87] . These films are two or more (multimorph) layers of piezoelectric material surrounded by conducting electrodes, analogous to the bimetallic strip used in thermostats. An applied voltage will cause a bender motion.
Using these "fingers" it is possible to open and close a conduit which contain the "cubes" of material (Figure 2) . The cubes can be added to the conduit from a reservoir above, a background vibration is needed probably. Notice that this process depends on gravity.
Figure 2.
Then it is possible to create a matrix of actuators ordered by rows of materials (Figure 3) , because the time needed in order to complete a layer depends on the ratio between the total area of the layer over the sum of the areas of the injectors, it is useful to minimize this ratio; a practical ratio can be within 10:1 and 100:1.
To drop a "cube", the actuator is switched off, allowing the cubes to flow down, then the activator is switched on in the time needed for a cube to travel its length, for example in the case of a cube 50 µm the time is about 3 milliseconds.
Because the injector is near the surface the drop is “soft”. The cubes do not jump over. If the cubes stick to the walls of the conduit then a background vibration can help loosen it.
Taking a ratio of 100:1 of area of injectors and 3 msec to drop each cube, that is about 300 Hz, then it is possible to inject a maximum of about 3 layers per second; because the use of materials is asymmetric and the movement of the assembly needs time, then 1 layer per second is a better maximum possible, 1 layer/sec is 1 mm each 20 seconds or 3:20 minutes per centimeter. This speed is acceptable for building an object of 50 µm of resolution.
Figure 3.
The matrix of injectors can scan the XY plane allowing each material to touch each position in a cycle. When a layer is completed the elevator can move in the Z direction. The information in the computer is used to control this movements, taking a work area of 10 cms per side with a 100:1 ratio, then a 1 cm
2
of area of injectors is needed, that is 40,000 injectors of 50 µm, if the frequency is 100 Hz then 4
bits/sec are required to control the injectors.
The feeders of material can be tubes with compressed air which move materials from outside to the top of the injectors. Another possibility is using loaded cassettes sitting over the injectors. These cassettes can then be changed on the fly when empty. One advantage of this method is the possibility of controlling the orientation of the cubes.
Consolidation.
After filling the target volume a process is required to consolidate the individual cubes into solid parts. Here two process are analyzed, the first is sintering the block in a furnace at high temperature and the second is using pressure with epoxy materials.
Sintering is a common process used in powder metallurgy but in this case the problems arise using many materials at the time. A few of these problems can be:
-
Problems mixing materials because of different sintering temperatures.
-
Problems in the interface: undesirable chemical reactions can occurs in the interface of two materials.
-
Problems because of thermal expansion.
-
The high temperature need for one material can destroy another, for example mixing metals and plastics.
The other consolidating process is using epoxies. These materials can be obtained with various fillers, such as steel, titanium, ceramics and also silver for a conductive material. Using techniques of micro encapsulation of the epoxy and hardener can produce powder which can then be agglomerated in "cubes". Using pressure or ultrasound to rupture the encapsulation can initiate the hardening of the material. The advantages are the low temperature and the speed. Epoxies can harden in a few minutes. The principal disadvantage is probably the high cost.
Finally a technique for the future can be adapting the lasers used in selective sintering to "weld" a layer at the time. If the substrate and the raw material is cooled to a low temperature and the laser is focused into a small spot, then neighborhood material can be protected, metal and plastic can be processed side by side at different temperatures. A very low speed could however become a problem .
Filler removal.
After forming the solid parts a filler removal process is required to create the empty spaces. If the filler material is not consolidated itself and can be accessed from the outside, then a vacuum cleaner can be used to recover the filler. Otherwise a liquid solvent may be required.
Another possibility here is using two fillers, each one with a different solvent. After the first filler is removed a mold is created between the object and the second filler. This mold can then be filled with a liquid material. For example, suppose a prosthetic hand with metal and ceramic has been created but needs a soft silicone coating. In summary an extra step (or two) of molding is easily added.
Limitations.
One economic limitation is the higher cost because of the need of using micro powders and the extra processing in order to produce the "cubes. Part of this problem is demand because the majority of the manufacturing technologies are
subtractive
processes [Stucki et al 95] , the raw materials in the market come in as blocks, rods and sheets of materials which are cut and fold. This new
additive
process needs the materials in a new form. And with an increased demand the cost may be reduced. Today however the extra processing of the "cubes" is justified by the additional precision obtained.
A few of the technical problems are already described, but there exist other problems which arise because of the equivalence of "voxels" and "cubes", that is the digitalization of real material. The "jaggies" found in computer graphics are now real mechanical defects, building a sphere or cylinder can only be approximate.
The "brute" force way to solve the problem is to increase the resolution, but using sintering the problem can be less than imagined since heat can smother the surfaces, in effect "antialiasing" the jaggies.
Take notice also that not all the materials need be cubes. Balls can be mixed with cubes without destroying the order provided the balls are a minority. As a matter of fact, if other geometries are coated with a filler and formed as a "cube" then these other geometries can be inserted in an object. The limitation here is the total number of materials possible.
Other problematic structures are sub micrometer coatings, large fiber composite materials and materials which require special postprocess.
Possible applications.
While the actual prototyping technologies are more useful building models, this technique can create "real" and useful objects. This technique can be used to build complex objects, difficult by any of the actual manufacturing techniques. As an example, the machine described here is a lot more difficult to build without itself, therefore in the Appendix a description of a machine built using itself is described.
Another example already described is a prosthetic hand. If it is possible to build a machine full of motors, sensors, structural elements and sheathing in one fast process then that is an interesting possibility.
An obstacle to these projects is the necessity to include electronics elements in the process, using motors and sensors always need control and amplification.
One alternative is cutting cubes from a processed silicon wafer, that is cubical chips with transistors, diodes and sub circuits. The problems here are the orientation and the connections. A diode is easy but for a three element circuit a double cube is necessary. This can be done if first all the single cubes are deposited and then the double cubes are inserted. This has a penalty of a decrease to half the speed of the process.
A far away possibility is to obtain cubes of semiconductor material and combine then directly to obtain basic components, such as diodes and bipolar transistors (MOSFET is really an sub micrometer technology) . A process have been described [Wallenberger 95] in which three dimensional structures of semiconductor material are obtained directly from a gaseous phase. This technique can probably be used to obtain fibers of a square profile to produce cubes of about 5 µm of side.
These size is about one order of magnitude larger than the current resolution of microelectronics, but it is a 3D technique, can be useful for example, in flat panel TV, permanent solid state memories, artificial neuron networks and others.
Integrating electronics in the process also permit the construction of micro robots, as described in [Flynn et al 89] , which open a lot of other applications.
Conclusion.
A technique related to LMT has been presented. This technique uses "cubes" of aggregate materials analogous to the "voxels" in computer graphics. These "cubes" can be of different materials. Using one or more of these materials as a filler for creating empty spaces and then consolidating the block using different postprocesses. The process looks feasible because it depends only on obtaining "cubes" of aggregates and the operation of actuators of piezoelectrical material.
References.
[Chandru et al 95]
Voxel-Based Modeling for Layered Manufacturing
,Vijay Chandru, Swami Manohar, C. Edmond Prakash, IEEE Computer Graphics and Applications, November, 42-47
[Flynn et al 89]
Twilight Zones and Cornerstones, A Gnat Robot Double Feature
, Anita M. Flynn, Rodney A. Brooks, Lee S. Tavrow, MIT A.I. Memo 1126
[Pennwalt 87]
Pennwalt Corporation, Kynar Piezo Film, Technical Manual, 24-25
[Stucki et al 95]
Computer Graphics in Rapid Prototyping Technology
, Peter Stucki, Jack Bresenham, Rae Earnshaw, IEEE Computer Graphics and Applications, November, 17-19
[Wallenberger 95]
Rapid Prototyping Directly from the Vapor Phase
, Frederick T. Wallenberger , Science, Vol 267, 3 March, 1274-1275
Extra reference
Volume Graphics,
Arie Kaufman, Daniel Cohen, Roni Yagel, IEEE Computer, July 1993.
Subfield of computer graphics wich represent 3D models using voxels directly.
© 2000-2003 Javier Diaz R. Up