Development of a hot press for grinding tools requires a good match between the system, the sintering moulds and the powder compounds-and above all the high degree of flexibility and specialist knowledge of the system engineer.
Grinding tool manufacturers can find this knowledge at Dr. Fritsch, which has been involved in engineering and developing innovative powder mixtures for over 50 years.This article by G. Weber and S. Bongartz.

Nowadays there are several competing processes for making grinding tools. In general, either batch furnaces without a special atmosphere or hot presses are used,a further distinction
being made in hot presses between sintering presses with a direct flow of current and indirectly heated sintering presses.Currently the most widespread are batch furnaces: they are
relatively cheap to purchase, building the mould for the tools is somewhat less complex and the principle has been in use a relatively long time-so manufacturers have very broad experience of this method. The mould is placed in a preheated
furnace, the temperature being set approximately 50 to 100 °C above the sintering temperature being aimed for. The mould is provided with a thermocouple and often also with a form of
hood and a supply of protective gas. As soon as the desired sintering temperature is reached in the mould, it is removed from the furnace and immediately compressed to final thickness
under a hydraulic press. As heating and compression follow one another, it is important to find precisely the right time for pressing-this is a fairly critical point with this method.
In sintering furnaces with indirect heating, a relatively large number of tools can be sintered simultaneously one above the other, but the drawbacks of the design (it is slow to heat up and
press forces vary from top to bottom) mean that only a few systems are in use.


Hot pressing with a directly heated mould has likewise been used for a long time. A great many key technologies have been introduced into the latest generation of sintering presses to increase the economy and quality of tools.


Considerably shortened cycle times-direct heating makes cycle times of approximately 20 minutes possible (compared to 50 minutes in the furnace or 3-5 hours in the case of indirect heating).


Longer mould lives-as compressing and heating take place at the same time, typical pressing forces of 200-350 kg/cm2 are used instead of approximately 1000 kg/cm2. This is gentle on
the expensive moulds (which are made of special alloys) and increases their life cycle.


Automatic production cycle-the sintering programmes can be calculated, with heating and cooling performed automatically.
Once the sintering programme is complete, the product is fully sintered and compressed to final thickness. Operating staff are not required during the sintering process.


Customised process technology-the optimum sintering programme (including individual pressure and temperature values) is compiled for each metal bond. Vacuum and protective gases
can be controlled.


Optimum process control-the temperature, time, force and travel of the sintering cycles are recorded.


Quality and performance of grinding tools-many hot-pressed grinding tools have much longer life time than their furnacesintered counterparts.
Dr. Fritsch developed the new GSP 230 to produce grinding tools using hot press technology with a direct current flow; it complements the line of grinding tool production machines in
an optimal manner. The machine is designed for tools up to 300 mm in diameter. The large opening makes it possible to process moulds up to 300 mm high or also several smaller moulds one above the other.


The advantage of this technology is the even heating of the workpieces, with the maximum output of 180 kVA having been selected deliberately generously. Several thermocouples
control and monitor sintering temperature. The press force is 46 to 600 kN (approximately 60 tonnes). Should higher press forces be required, customised solutions may be offered.
The GSP 230 works with protective gas, forming gas or vacuum to prevent the oxidisation of grinding wheels.


The hot press can be networked using the PC software IPA-NT;
in other words, the PC is used to visualise the process, to save process data for quality assurance purposes and to programme offline. By IPA-NT Messenger, the operator may keep in touch with the machine (this requires a mobile telephone or a BlackBerry).