The combination of a specially prepared powder (originally used for isostatic pressing) and highly developed axial pressing technology enables the production of dimensionally accurate blanks without reworking - in just one step. The S-DT, S-DTML
from KOMAGE is a press system specially developed for the production of zirconia blanks in dental ceramic.
In addition to the desired advantages - such as a uniform density in the product - isostatic pressing processes also have serious disadvantages in the production of high-quality ceramics. For example, the process is complex, requires extensive reworking and produces relatively high material losses in the process. Depending on the intended shape of the green compact, the starting material is filled into a bag or a tube. For example, to obtain round, disc-shaped products, cylindrical tubing would be used. Then the tube is placed in the pressure vessel of the isostatic press, which is filled with liquid. The subsequent slow increase of the liquid pressure in the vessel up to, for example, 2000 bar, ensures largely isostatic compression conditions on the green compact and must then slowly decrease again to ambient pressure. The liquid is drained off. The compact is removed from the hose and trimmed so that it can be machined, e.g. slices can be cut from it. Undefined compacted areas or so-called "elephant feet" - unevenness at the bottom and at the top - have to be cut off. Numerous work steps are therefore necessary, and trimming results in valuable material that cannot always be fully recycled. Machining must be carried out with a diamond saw blade, as the zirconium dioxide used in dental ceramics has almost the hardness of a precious stone. For this reason, after isostatic pressing, the desired parts are also often first cut to rough dimensions and then turned over on the lathe so that they become radially plane.
The axial pressing process for dental ceramics by KOMAGE
The powder material used in the isostatic process requires no flow improvers or lubricants, only a binder. Also, the grain structure does not play such a big role, because the material - no matter what consistency - is simply pressed into the tubes. Axial production is different. In order to use this efficient, residue-free one-step process, KOMAGE adapted the traverse program for the hydraulic test press. After only two optimization steps, both the press technology and the tool technology could be adapted in such a way that the desired parts could also be produced with poorer, i.e. coarser-grained, flow-tough material. However, not yet optimally.
The new press procedure in application
After the publication of the first, promising results on the KOMAGE website, a dental laboratory became interested in the new process. Previously, the lab had milled parts out of pre-produced ceramic plates. Both the plates and the process were extremely costly. The blanks produced, i.e. semi-finished products, were between 400 and 500 euros for a diameter of 100 and a height of 20 millimeters, because the relatively high material and machining losses from the process were included in the price of the pre-produced plates. The aim was therefore to provide a process with which dental ceramics could be produced as quickly as possible and with as little loss as possible. To solve the flowability problem, a starting material was used that is used to manufacture hip prostheses. In this segment, homogeneity and dimensional accuracy are paramount. It is therefore impossible to achieve these with a simple powder containing only binder material.
Measurable and cost-saving
The starting material from hip prosthetics was used on a trial basis. The first pressed parts were discs of 100 millimeters in diameter. In further steps, the process was optimized - such as sintering to avoid breakage, or in the area of pressing parameters, which had to be finely balanced to achieve the desired densities. The material is compacted with a KOMAGE S120/200, i.e. a hydraulic press with 120 or 2000 kN (approx. 200 tons) pressing force. Although - as experience shows - it would not be absolutely necessary, products in the field of dental ceramics are partly still isostatically post-compacted after the axial pressing - simply because the conviction prevails in the market that only isostatically pressed materials achieve the highest quality. With the KOMAGE process at the beginning of the production chain, high-quality, dimensionally stable dental ceramics can be produced in a single step at a quarter of the manufacturing costs for the isostatic process. The cost of a 10mm blank was less than 100 euros in 2017.