Steps of process
We have combined some physical/thermodynamic elements and practical operating informations with the desire to improve the quality of the castings obtained with the lost-wax casting method which ancient origins and today permits to obtain complex shapes of a very good quality. The optimal result is obtained only by respecting all stages necessary during this high technology process and using equipment which, if properly used, satisfy all various physical, mechanical and thermodynamic laws wich interact during casting. Operator experience, gained by working with such appropriate equipment, is also an indispensable ingredient. We will descrive shortly the working steps of the lost-wax casting process.
The casting master must always be produced to a very high standard of finish, since this will determine the quality of the resulting casting and the lengthy and expensive finishing operation will be reduced to a minimum. Also, excessive loss of precious metal during this operation will also be eliminated.
The master pattern maker has to bear in mind:
- The dimensional shrinkage which occurs during vulcanisation of the rubber mould, during solidification of the wax pattern and also solidification of the cast metal.
- Feed sprue or sprues with respect to the configuration of the model has also The location of the to be carefully considered.
Most expensive jewellery casting are mainly hand finished, but if the production is for a more common market, it tends to be finished by a variety of automatic process
Rubber Mould Preparation
In recent years, the use of the elastomer (silicon rubber) has improved the quality of the wax patterns offering the following advantages:
- Elimination of release agents, talc or silicon spray
- High flexibility
- Good memory
- Long life maintaining the reproduction fidelity
We have not to neglect the time saved not to use release agents which tend to inhibit filling of thinner sections and then prevent to the wax to complete the model. This material, supplied by us, replaces the usual organic rubber and is designed to satisfy the exacting requirements of manufacturers of high quality jewellery in their mouldmaking for lost-wax casting The sequence of photos shows the simple operation of the elastomer and also the practicality of using locator pins to simplify the register of the moulds avoiding wax defects caused by the not perfect closing of the mould.
After the advent, during these last years, of the wax injector under vacuum it would seem impossible to use the old method of the injection only, with all the problems that it involves, like air bubbles, incomplete filling and difficulties in reproducing thin sections. Today’s available equipments are of extreme reliability and give very good accurancy. Pressure and vacuum are supplied by an air compressor and by a small vacuum pump and regulated in seconds. The injection time, vacuum time and nozzle temperature are automatically controlled by a compact electronic circuit board. The rubber mould is pushed to the injector nozzle. The mould is then air evacuated for one or more seconds and then the wax immediately injected. It is easy to conclude that under these conditions it is obligatory to obtain waxes without defects increasing remarkably the production and the quality.
The system to use of the gypsum-bonded investments poured around the wax patterns to create castings is relatively recent compared to the long history of the lost-wax process. We want to remember that the famous Florentine goldsmith Benvenuto Cellini in the year 600 used an investment prepared by him that contained also chalk. The modern powders are easy to use and permit the rapid preparation of good quality moulds. The quality of the investment must be taken in a very good consideration in order to avoid dangerous consequences on casting pieces.
The investment material absorbs the humidity of the air, but to the contrary, it must always be very dry before using, otherwise more time for working will be necessary and also the shape can be weakened remarkably. Moreover, the surface of the fusion may become rough or result “flashing”. • Additives and modifiers contained in modern powders may disapear after a certain time, altering the characteristics of the material. Shelf life should thus be observed. One result of this can be the appearance of greater fluidity of the investment “slurry”. Attempts to correct this by adding more powder to the mix will alter the water-powder ratio from recommended by the manufacturer and create a problems. This is normally 1 Kg powder to 360-400 grammes or millilitres of water. • Powder should always be added to water, not vice-versa, otherwise a “lumpy” mix will result with pockets of unmixed investment producing weakness in the final mould. Mixing in a modern mixer under vacuum takes care of this type of problem, guaranteeing a smooth and homogeneous mix.
For these purpose we can offer you our mixers. The new design of the mixer place the motor on the top. This permits to eliminate the problems caused by the conventional vacuum mixers present on the market. The advantages and the characteristics are following:
• Different investment quantities are always mixed in the same mixing time
- The motor remaining protected by airborne investment powder
- Rapid and easy cleanig of the unit
- Investment is poured into slightly vibrated flasks under vacuum ensuring a complete physical homogeneity and absence of air.
- Automatic mixing time control.
When the investment which contains the wax tree is very hard, the flask is placed in an appropriate furnace, which is called “steam dewaxing furnace” and in wich the wax trees is melted and evacuated through the hole in the bottom of the flask. When the wax has been evaquated the flask which is still hot will be placed in a burnout furnace. In this second furnace the investment acquires the hardness necessary to resist to the melting.
With this method of casting the melted metal receive some pressures according to the acceleration which is variable and regulated according to the metal and to the pieces which shall be produced. The acceleration is progressive in order to avoid turbulence and cracks of the investment. Our centrifugal casting machines are equipped with the most up-graded medium/low frequency power generators in order to guarantee an optimal mixing of the metal; moreover these generators substitute the old oscillatory tubes whose operation was get exausteded in the time. These casting machines are equipped with a double-swing centrifugal arm which gives a higher inlet pressure of the metal in the flask avoiding, thanks to these variable and regulated forces, negative turbulences. Thanks to the most technological studies, and to to the application of all these new technologies the results of the casting with GALLONI centrifugal CASTING MACHINES will be extraordinary.
One new solution to obtain high quality castings in gold, silver and their alloys is the bottom pour melting and casting with vacuum assistance in the flask chamber.
With this system is applied a depression to the flask that taking advantage of the porosity of the investment allow the metal to join perfectly to the internal cavities.
All impurities which are generated by reaction of the investment are absorbed by the vacuum and does not contaminate the surface of the metal which will fill up the cavities of the object avoiding the possible defects.
This casting system, applied to our machines since ten years, is a GALLONI result obtained thanks to various studies and researches of the most high technology and the obtained improvements are now well-known all over the world. However, it is necessary to understand and explain the two types of pressure applied to the metal. In a type of machine the crucibles chamber is subjeted to pressure in order to push the metal outside from the crucible increasing its injection in the flaks which is completely under vacuum.
In the other type, the pressure is applied on the melting metal already inside the flask in order to compress its molecules and push it in the cavities to the inside of the flask. In this way the thinner parts of the cavities will be filled up at lower temperatures. All the process of both systems is set up and controlled by an industrial computer with “touch-screen” automating in this way all the operations. When all casting parameters are programmed the operator simply has to charge the crucible, place the casting flask in position and press one button!
This system is offered to the operators who need to make the production of ingots, bars, shots and filings recovery, rejections. Metal is melted always thanks to the induction heating in fast time and is poured manually by the operator or through an automatical pouring equipment mounted on the furnace. With this system it is possible to melt silver, gold, steel and platinum. It is often recommended for the preparation of the prealloy.
After casting it is necessary to extract the casting tree from the flask. The solution offered by GALLONI is a water jet cabin which removes the investment from the casting with a high pressure Water Jet Cabin.