The process is somewhat similar to stereolithography in principle as can be seen from )ig. E. /n this case however a laser beam is traced over the surface of a tightly compacted powder made of thermoplastic material &A(. The powder is spread by a roller &:( over the surface of a build cylinder &C(. A piston &D( moves down one object layer thickness to accommodate the layer of powder.
The powder supply system &( is similar in function to the build cylinder. /t also
comprises a cylinder and piston. /n this case the piston moves upward incrementally to supply powder for the process.
3eat from the laser melts the powder where it strikes under guidance of the scanner system &)(. The C"E laser used provides a concentrated infrared heating beam. The entire fabrication chamber is sealed and maintained at a temperature just below the melting point of the plastic powder. Thus heat from the laser need only elevate the temperature slightly to cause sintering greatly speeding the process. A nitrogen atmosphere is also maintained in the fabrication chamber which prevents the possibility of e$plosion in the handling of large quantities of powder.
After the object is fully formed the piston is raised to elevate the object. $cess powder is simply brushed away and final manual finishing may be carried out. That8s
not the complete story though. /t may take a considerable time before the part cools down enough to be removed from the machine. 'arge parts with thin sections may require as much as two days of cooling time.
>o supports are required with this method since overhangs and undercuts are supported by the solid powder bed. This saves some finishing time compared to
stereolithography. 3owever surface finishes are not as good and this may increase the time. >o final curing is required as in stereolithography but since the objects are
sintered they are porous. Depending on the application it may be necessary to infiltrate the object with another material to improve mechanical characteristics.
*uch progress has been made over the years in improving surface finish and porosity. The method has also been e$tended to provide direct fabrication of metal and ceramic objects and tools.
olid ;round +uring
olid ;round +uring was developed and sold by +ubital 'td. of /srael. 2hile the method offered good accuracy and a very high fabrication rate it suffered from high acquisition and operating costs due to system comple$ity. This led to poor market acceptance. 2hile the company no longer e$ists and its intellectual property has been acquired by "bjet 7eometries 'td. it%s still an interesting e$ample of the many
technologies other than stereolithography that utili1e photopolymer materials. Also notable is that a similar electrostaticaly+based layer e$posure scheme is used by Speed -art A: &Sweden( in its powder+based technology being commerciali1ed in E@@,. The early versions of the Cubital system weighed several tons and required a sealed room. Si1e was made more manageable and the system sealed to prevent e$posure to photopolymers but it was still very large. &-lease see the discussion on
stereolithography for a description of photopolymers.(
/nstead of using a laser to e$pose and harden photopolymer element by element within a layer as is done in stereolithography S7C uses a mask to e$pose the entire object layer at once with a burst of intense 9< light. The method of generating the masks is based on electrophotography &$erography(.
This is a two cycle process having a mask generation cycle and a layer fabrication cycle. /t takes about E minutes to complete all operations to make a layer!
. )irst the object under construction &A( is given a coating of
photopolymer resin as it passes the resin applicator station &:( on its way to the e$posure cell &C(.
E. A mask is generated by electrostatically transferring toner in the required object cross sectional image pattern to a glass plate &D( An electron gun writes a charge pattern on the plate which is developed with toner. The glass plate then moves to the e$posure cell where it is positioned above the object under construction.
,. A shutter is opened allowing the e$posure light to pass through the mask and quickly cure the photopolymer layer in the required pattern. :ecause the light is so intense the layer is fully cured and no secondary curing operation is necessary as is the case with stereolithography.
G. The mask and object under fabrication then part company. The glass mask is cleaned of toner and discharged. A new mask is
electrophotographically generated on the plate to repeat the cycle.
?. The object moves to the aerodynamic wiper &( where any resin that wasn%t hardened is vacuumed off and discarded.
H. /t then passes under a wa$ applicator &)( where the voids created by the removal of the unhardened resin are filled with wa$. The wa$ is hardened by moving the object to the cooling station &7( where a cold plate is pressed against it.
B. The final step involves running the object under the milling head &3(. :oth the wa$ and photopolymer are milled to a uniform thickness and the cycle is repeated until the object is completely formed within a wa$ matri$.
Secondary operations are required to remove the wa$. /t can either be melted away or dissolved using a dish+washing+like machine. The object is then sanded or otherwise finished as is done in stereolithography. The wa$ matri$ makes it unnecessary to generate e$tra support structures for overhangs or undercuts. This and the large volume capacity of the system also makes it easy to nest many different objects within the build volume for high throughput.