History of 3D printing
3D printing is currently an extremely important branch of R&D. Due to the possibility of rapid prototyping of designed models, it has become possible to quickly eliminate design errors and improve the product at the design stage. This approach greatly accelerates the implementation of new solutions without incurring significant costs of production and testing of untested models. With 3D printing, making a prototype model with a complicated geometry has become possible in a short time and with unprecedented precision.
In the early stages of the development of 3D printing using photopolymerization, flat stencils with a shape corresponding to the printed part were used, through which an area of photo-cured resin was illuminated. The spatial depth of the element was obtained in a similar way as in today’s printers – by moving up and down the printer table. This solution, however, did not find wide interest because of limited possibilities of shape setting and the length of the process. It became necessary to find a solution allowing to obtain objects with more complex geometry.
The initial development of 3D printing is considered to be the invention of the stereolithography method by Chuck Hull. The method involved curing a liquid photo-curable resin with an ultraviolet laser beam. This approach made it possible to change the geometry of the model as its height increased, and consequently to produce complex 3D parts in a single process. The process of creating a print was based on scanning the photosensitive material with a UV laser beam – section by section, layer by layer until a complete print was obtained. This approach required the creation of a method of data transfer between a computer and a printing device (3D printer). A special STL (stereolithography) file format was used for this purpose.
Another milestone in the development of 3D technology was the development of FDM printing – Fused material Deposition by S. Scott Crump. The method was based on the principle of CNC milling machines using a head that applies a molten polymer. In the initial stages of development, almost exclusively ABS was used which at that time was indicated as a universal engineering material. In a very short time, other materials joined it including soluble support materials. An important date for the development of this technology was the year 2005 when the patent covering the principle of FDM printers was released, which enabled the creation of open-source projects and contributed to the popularization of the technology among hobbyists. On the wave of releasing patent claims, many companies offering ready to use 3D printers for various purposes emerged, from large industrial machines to miniature desktop printers.
An important stage in the development of 3D printing techniques was the emergence of DLP (digital light processing) printers. In this process, the mapping of the shape of the cross-section of the object follows by displaying it using a projector – by displaying successive cross-sections, successive layers of print with changing geometry are obtained. Initially, illumination of the photo-cured resin was performed from the top of the resin bath, while the resulting model was immersed in the liquid composition. This approach, however, required significant amounts of photo-curable material and limited the height of the printout to a large extent, so a method was developed in which the bottom of the bath with the photo-curable resin is a transparent release film FEP (Fluorinated ethylene propylene). Thanks to such an arrangement of the 3D printer, the amount of photosensitive material was minimized, and the height of the printout is limited only by the height of the printer arm because the printout itself is “pulled out” from the bath with liquid resin. The direct use of projectors also carried some limits. In the case of the DLP process, the resolution of printouts depends on the design of the projector, and the optics used to display the image. The resolution of the displayed image also decreases with the size of the printer’s work area. The lamps used in projectors emitted light with a broad spectrum, but only a small fraction of the energy radiated by the lamp is in the range below 400 nm, where most photoinitiators exhibit absorbance.
The main type of projectors used in the early development of this technique was equipped with a matrix of multiple microscopic mirrors (DMD digital micromirror device), whose positioning was controlled by the projector software. The limiting factor in the resolution of the system was, of course, the number and size of the mirrors, and the size of the projected image. The amount of light reaching the resin bath could not be too high, because too much light falling on the mirrors caused burning of the reflectors and resulted in artefacts in the displayed image.
Another approach uses an LCD screen instead of a micro-mirror array by positioning it in the optical path between the lamp and the photosensitive material bath. Such a system typically has a higher resolution than DMD systems and is called an LCD-DLP system. For LCD matrices, the amount of energy delivered to the photosensitive resin is limited by the high absorbance of the matrix itself. The absorbance of the matrix material varies with wavelength and almost completely blocks radiation below 400nm. For this reason, modern printer designs use LED lamps with a wavelength of 405 nm or higher. The amount of light delivered to the resin is still small relative to the intensity of the illumination used. This problem affects the limitation of photopolymerization of materials requiring high-intensity illumination.
The next stage in the development of 3D printing processes is continuous printing processes, where photosensitive material is illuminated by successive sections of the model with a smooth transition between them, while the movement of the working table is smooth and resembles a slow drawing of the finished model from the resin bath. In this approach, the high reactivity of the photo-cured resin, as well as the intensity of illumination, are of great importance. An important aspect is a material from which the bottom of the bath is made, through which the resin is exposed. This material must be permeable to light and must prevent polymerization of the material on the resin contact surface. For this purpose, permeable membranes, for example, are used; their function is to prevent polymerization on their surface by oxygen inhibition of radical photopolymerization. This inhibition takes place effectively only at a small distance from the membrane, while oxygen diffusion into deeper layers of liquid resin is impeded so that free radical photopolymerization can take place there.
However, this method has its limitations – it is not suitable for printing parts containing large flat surfaces, because the suction force when pulling the model out of the liquid resin causes deformation of the oxygen permeable membrane and leads to deformation of the model.
The history of 3D printing in its very turbulent beginnings has shown many ideas and solutions that have been verified over time by the market. Among the ideas that lived to see the development and commercialization are technologies based on the polymerization of liquid resin, thermal processing of thermoplastic materials, melting/sintering of metals and methods creating prints from composite materials or rubber.
Each 3D printing method requires the preparation of an appropriate set of instructions for the 3D printer. These instructions vary depending on the type of printer and printing technique used, but they have a common element which is a 3D model. 3D models are created using CAD (computer-aided design) software, but to process them into a set of instructions understood by the 3D printer, STL (from stereolithography) file format is adopted as a standard.
STL format files store the geometry of the 3D object as a mesh of polygons forming all of its surfaces. However, to communicate with a 3D printer it is necessary to prepare a set of commands to be executed by the printer based on the geometry read from the STL file. Programs called slicers are used for this – their task is to divide the existing 3D model into a series of sections according to the parameters set by the operator. Printing technologies using laser or print heads require additional division of cross-sections of the model into paths along which the print head will follow. The output file after this operation is a set of commands saved in the *.gcode text format.
With a set of commands, the printer can reproduce a geometric object using the appropriate material but depending on the design of the device and the quality of the machine code, the results may have a different spatial resolution.
