Guide issued by Attilio Galloni
The best casting result is attained only by following all the required phases by the process technology which demands suitable equipment and proper methods to fulfill the several physical, mechanical and thermodynamical rules affecting the operation. Experience is the essential factor which is achieved in the right way by having available the most appropriate instruments and products. We will describe in short the working steps to reach the casting phase. Casting signifies to combine, blend or melt, liquefy. The means to heat and melt metals are fire, electricity turned into heat, electricity as physical mean or turned into magnetic energy. To reach a perfect casting is indispensable to follow the basic operations we will indicate. The wax forming the pattern shall be sufficiently plastic, at the same time shall have the capacity to melt and burn completely without solid residues. Even the least residue spoils the casting result. The pattern particles left inside become ash that will be included in the casting, producing micro-porosity.
2. How to position the sprues in the wax quality.
The dimensions of these sprues are essential for the best result. It has to be evaluated their size and position because they shall deliver liquid metal for the mass shrinkage during the cooling phase to avoid final product reduction.
We recommend using products of ascertained quality. The use of investments poured around wax patterns to create castings is relatively recent compared to the long history of the lost-wax process. The modern powders are easy to use and enable the rapid preparation of good quality moulds. Powders consist mainly of the following: Silicon dioxide (SiO2) Calcium sulphate – hydrate (CaSO4. ½ H2O) In recent years it has been common to use phosphatic investments that are without sulphur. The investment material absorbs the humidity of the air, while it must always be very dry before use. Without the utmost precision, we are not sure to reach best results. As general indication, 1 kg powder to 360-400 grams of water. Castings with perfect surfaces require the liquid investment to be smooth and homogeneous; the mechanical mixing under vacuum guarantees to reach the purpose; after mixing, the investment shall be poured into the flasks where it receives a further vacuum action. The equipment of picture 1 is effective and at reasonable cost. Working time is approx. 9 minutes at 23°C (73.4°F) temperature. Water shall always be at ambient temperature.
4. Wax removal from the flask.
The first step is removing the wax by placing the flasks into an electrical furnace at approx. 200°C (392°F). The furnace of picture 2 features also a drawer to collect the melted wax. The flask opening shall be positioned downward and the retention time depends on the dimensions: approx. 4 hours for medium size flasks and 6 hours for the biggest.
5. Thermal treatement of the flask.
Position the flasks in the furnace with the opening upward to make gas to be released easier. The heating process shall strictly follow the manufacturer instructions. We can apply a quite accurate test about the adequate thermal treatment of the investment by checking carefully the investment mass near the melted pieces. If it is grey colour, it means the thermal treatment is un-complete and the reaction gases have not fully dissipated. This can cause anomalies and micro-porosity in the casting. In this case check whether the temperature is the one indicated by the maker, otherwise increase the retention time at the pre-set maximum temperature. Picture 4. They use oversized electrical furnace to avoid the flasks too close to furnace walls and to themselves. Take care the furnace internal volume is well aerated to eliminate the gases produced in the process, which contain compounds of sulphur dioxide and sulphur oxides. Now it is useful a clarification about the temperature indicated by the furnace pyrometer and the actual one of the flasks. The flasks consist of refractory mass that delates the decrement of their temperature. Therefore experience has to be accumulated on the suitable time that depends in a proportional way on the flask dimensions. It is almost compulsory to check the flask internal temperature before melting by using a portable thermometer, picture 5. The flask heating is very important and we need a furnace which can be adjusted with good accuracy. Picture 6. The coating porosity makes possible to receive these compressed gases from the metal, that are not always released and sometimes remain included in the metal. As logical consequence, at the pouring of the melted metal this physical impediment must be removed. The picture 7 is clear enough to prove that when a liquid fills a certain volume, the air or the gas occupying this volume must somewhere evacuated and, if do not come out completely, will eventually be incorporated in the liquid metal that rapidly consolidate. The only possibility is removing gases from the flask. When the flask heating is completed, the process continues with the metal casting by a centrifugal method or much better with an electronic vacuum/pressure casting machine such our PRESSOVAC that is considered the technologically most advanced equipment in the world. Picture 8 After filling the metal in the crucible and positioned the flask in the chamber, a fully automatic electronic control system ensures the complete removal of gas and air from the flask followed by the vacuum melting. By a single operation the molten metal is poured into the flask within a few seconds with vacuum, then pressurizing just after filling it. After 60 seconds the pressure is released, casting chamber is opened and the machine is ready for another working cycle. We resume all the procedures for flawless casting:
- Good quality and characteristics of the wax.
- Investment preparation.
- Wax removal from the flask.
- Flask thermal treatment.