Cutting tool manufacturer Paul Horn boasts that its Hartstoffe factory in Tübingen, Germany operates the greatest number of processes to make carbide cutting tools in the world: four. Altogether, they enable the company to pump out about nine million cutting inserts per year.
My visit to the Horn Harstoffe carbide factory started in the mixing room. “It’s like a bakery here,” says export director Harald Haug; machines knead carbide powder in large mixers to make a 120 kg batch for processing in the four forming processes. Like a bakery, the white-painted factory is kept scrupulously clean throughout. It’s quiet too; some 54 employees tend machines that are almost entirely automated.
One of the most advanced processes is injection moulding for making the more complex forms; it took the company several years to master. In the process, an Arburg injection moulding machine forces the carbide, mixed with extra additives that are later burned off, into a die under high pressure in a cycle time of less than a minute. Only this process produces complex moulded chipbreaker geometries.
I pause by two Osterwalder CA-HM 320 servo-electric axial powder presses, one of which was making three-sided inserts. I see the press forming a three-sided insert in a matter of seconds, before a robot arm whisked it away for palletising. The machine performs two to four pressing strokes per insert, squeezing it with a force up to 320 kN, and can make 18 per minute.
On the opposite side of the aisle were two extruders squeezing out long bendy rods of carbide. These bars are made into solid end-mills and wear parts, an area of some growth for the company. They are also used to make the Supermini range of inserts.
The fourth process, hydrostatic isostatic pressing (HIP) forms semi-finished inserts under constant high pressure, that can include a through-coolant hole. Parts made by HIP include drawing tools for drinks can manufacturing or press tools.
Once made, all of these parts go off for sintering. First, a pre-sintering process burns off additives at high temperatures (up to 850°C); this stage can last up to four days. Sintering proper is shorter – about 20 hours – but more intense. It requires temperatures of 1,350-1,550° C and pressure up to 100 bar. During the process, the carbide liquefies somewhat, and reorganises so that there are no pores. The result is not only a harder insert, but also a smaller one; sintered parts shrink, by around 20%, although the precise amount depends on the type of carbide powder used.
Exactly what will happen to the sintered inserts depends on their final use. Many insert types will be finish-ground, and coated, in the main Horn factory down the road, before being despatched around the world.
