The inner surface of the induction furnace coil and the ceramic upper construc- tion (upper concrete ring) must form a smooth surface, without any deposits, which tapers gradually to form a cone at the top. If a crucible removal device is used, the conicity should be 0.8%. When working with an outer lining with approx. 40 mm overlap at the bottom, the coil plastering should not be applied conically. In this case, the conicity of the outer lining is sufficient. The coil plas- tering and the outer lining are applied with a water content of 5 – 8%. This mois- ture leads to a long drying time, and thus also to extended starting times. In order to remove this moisture from the coil plastering and the outer lining quick- ly, these can be dried out with a gas burner and the tamping form. The coil plas- tering can be dried out at a maximum of 150 °C and the outer lining at a maxi- mum of 350 °C in the lower area.
The drying time in both cases should be at least 24 hours, if the outer lining has been cast immediately after application of the coil plastering, the drying time should be at least 36 hours. The temperature increase should be between 30 to 50 K/h.
If no burner is available, the drying can also be carried out inductively for medi-
um-frequency furnaces.
If neither gas nor inductive heating are to be used, the coil plastering and the
outer lining should be allowed to dry out normally in air for approx. 12 hours,
and then dried out for a further 24 hours with a 3 kW or 6 kW fan heater placed
on the bottom of the furnace, depending on the furnace size. Some customers use
resistance-heated “baskets”, which are supplied with a line through the furnace
bottom. In the case of electrical heating, the heating should be carried out with
the furnace cover closed.
Before applying the tamping compound, the inner surface of the coil is “papered” with 0.5 mm of Cogemikanit or similar material up to the edge of the furnace. Micanites with added flow materials are expensive, and provide hard- ly any heating benefits.
The furnace bottom with the bottom electrodes is now prepared for the applica- tion of the bottom compound. The tamping compound is filled in to such a level that the bottom electrodes can be tamped over to a depth of approx. 10 mm. This is usually carried out after manual ventilation using a bottom agitator, which is powered electrically or pneumatically. Agitation continues for 5 – 10 minutes depending on the furnace size. For bottom thicknesses of over 350 mm the work should be carried out in 2 layers, paying special attention to the bottom elec- trodes. After the bottom agitator has been removed, the bottom height from the furnace edge must be checked and recorded. Now an outer ring is well keyed around the furnace bottom up to the cone of the tamping form (at least 15 mm), in order to provide a good connection to the crucible wall. The tamping form is installed, centred and fixed in place at the crucible edge using wooden wedges. The crucible wall is filled up to the edge of the furnace in layers of 300 to 400 mm. After every layer, “Neptune zinc” ventilation is carried out to ensure better final compaction. Compaction must now be carried out very carefully in the area of the bottom cone. For the crucible wall, the self-rotating wall agitator is used. This must be installed in the lowest possible position using a crane, and agitation applied in this area for approx. 5 minutes. Agitation then continues for 2, 3 or 4 minutes for furnaces with up to 800 mm diameter and 100 mm height difference, up to 1,200 mm diameter and 125 mm height difference and over 1,200 mm diam- eter and 150 mm height difference respectively. Since the crucible wall will be subjected to high mechanical stresses in the upper area, the greatest care must be exercised here. The upper 200 mm are compacted by hand after removal of the wooden wedges. Die tamping compound should finish approx. 50 – 70 mm below the furnace edge. Tamping compound should be added continually during agitation to avoid the formation of layers between the compacted compound and the added compound. A thin patch layer is then applied to the ring of the crucible wall to prevent the tamping compound trickling out at the 1st tapping. The com- pound is not scraped out of the casting spout. In the case of disposable and per- manent templates, the casting spout should also be filled with compound in order to achieve a higher filling level during sintering. In the case of a disposable template, the compound is removed from the casting spout following completion of the sintering process, and the area dried with a welding burner. After approx. 10 minutes, this part of the tamping form becomes so soft that a “breakthrough” can be made to the casting spout using a crowbar. The iron now flows into the casting spout and the furnace can be tipped for the 1st time.
When using a permanent template, this must be provided with a supporting film
of micanite or with a separating agent prior to installation. There are conical
complete templates available, as well as folding templates.
In the case of the conical complete template, this is heated up to approx. 450 °C
after agitation at a rate of approx. 150 K/h, and then cooled down after 1 hour
to 200 °C with compressed air or cooling fans. To remove it, the template must
be pulled directly upward in a single movement. The folding permanent tem-
plate is not heated, and is removed by folding in one of the 120° segments.
Following removal of the templates, an “impact protection” layer of recycled
material is applied to the furnace bottom. The height of this layer should be 200
– 300 mm, depending on the furnace size. The crucible must now be filled up to
the casting spout as quickly as possible. No power should be switched to the
furnace during this time. Once the maximum filling level has been reached,
power is switched to the furnace, providing a temperature increase of approx.
100 K/h, i.e. about 40 kW/t, or 200 kW for a 5-t furnace. At this output, the melt
is brought up to sintering temperature and maintained at this level for approx.
2 hours. The 1st tapping is then carried out, and a total of 3 charges should be
processed.