The type of final casting (jewellery product obtained) is greatly influenced by the type of the investment mould made and the burnout cycle followed.Below we provide the general rules and recommendations that you can follow for getting high quality investment mould to better cast the jewellery of your desire.
INVESTMENT SLURRY CASTING
It is highly recommended to follow the steps mentioned by the investment powder maker while making the investment slurry.The water-powder ratio and setting time is very important, because it is the basis for performing all the operations involved in creating the invested flask (mould) and it must be followed very strictly.
The basis of setting of the investment slurry is due to hydration of calcium sulphate hemihydrate (CaSO4.1/2H20) to gypsum crystal (calcium sulphate dihydrate – CaSO4.2H2O).
For obtaining highly quality slurry, general steps that should be followed are mentioned as below,
- Use water at the recommended temperature, typically about 20°C/68°F, to ensure a consistent behaviour of the investment. Investment made with water that is too hot will set faster. Water that is too cold will slow down the setting time and lead to weak moulds and defects such as watermarks.
- As for water quality, it is preferable to use deionised water, because the setting time can be appreciably changed (lengthened) by the substances dissolved in tap water. By using deionised water, the consistency of the investment slurry can be maintained very easily.
- It is important that the recommendations of the investment manufacturer on powder/water ratio, mixing times, temperatures, etc. are followed.
Just as an indication, the data for an investment powder with about 8 minutes are:
- Powder to water ratio: 100:38.
- Mixing time: about 3 minutes.
- Vacuuming: about 1.5 minutes.
- Pouring the slurry in the flask: about 1.5 minutes.
- Vacuuming and vibrating the flask: 2 minutes.
Total working time: 8 minutes.
- According to most recent theory, after vacuuming we should let the flasks sit undisturbed from a minimum of 1 hour to a maximum of 2 hours, before dewaxing. The flasks should never become thoroughly dry: if this happens, they should be abundantly sprayed with water before dewaxing. It is not advised to prepare batches of filled flasks and use them in subsequent days. If the flasks become completely dry, there is a high risk of crack formation, rupture or even major blowouts during casting.
- The preferred practice is to prepare the flasks, to let them set and to send them directly to dewaxing and burnout. The programmed burnout cycle will be carried out overnight and, on the following day, when equilibrated at the casting temperature, the flasks will be cast.
Recent research carried out by the producers of investment powder suggests that after complete setting of the investment, i.e. 1 to 2 hours after flask filling, the wax of the patterns should be removed, to empty the mould cavities where the liquid metal will be poured. Dewaxing can be carried out in two ways:
- Dry dewaxing – it is done as part of the burnout cycle in the burnout oven/furnace.
- Steam Dewaxing – it was recommended for jewellers working in close proximity to avoid air pollution prior to actual burnout cycle.Steam also fill in the pores of the investment and makes sure no wax is entrapped in the pores and guarantees it's complete removal.
Though the effect of steam on the mould is still not totally clear, it is also said that steam can reduces the permeability of the mould which is desired during the casting operation.
Also during casting of gemstone, boric acid is used to protect the gemstone .Using Steam dewaxing boric acid is removed and makes the gemstones to get damaged.
Hence in major of the casting industry dry dewaxing directly in the burnout oven is practised.
BURNOUT CYCLE :-
Burnout, as the name implies, is carried out to burn out the last traces of wax and to give the investment mould the refractoriness and characteristics required for casting. The final characteristics of the mould will depend strongly on the burnout cycle selected and particularly on the heating rate and temperature homogenisation in the holding periods. Therefore, it is important to accurately follow the burnout cycle recommended by the producer of the investment.
There are two critical points in the heating cycle.
- The first one is at about 100-120°C (212-248°F), when absorbed water and part of the gypsum crystallisation water evaporate and this process causes the gypsum to contract.
- The second critical point is around 250°C (482°F), when the gypsum changes its form and this causes the gypsum to expand.
- Gypsum is highly non-conductive in nature and it takes a long time for the heat to reach from the outside of the flask to the centre and reach a homogenous temperature throughout the flask.
In the Industry, mainly two variants of the burnout furnace is found
- In a Rotating type burnout furnace, the platform on which the flask is placed rotates at certain RPM (Rotation Per Minutes) as to assure a uniform heating from all side so as to ensure that the mould don't crack during the above mentioned critical points, ensure complete removal of the wax and perfect curing of the gypsum so as to provide you with porosity free and moisture free mould.
- Whereas in the static burnout , homogenization of the temperature throughout the gypsum mould takes time and there is some little chance of porosity or the mould getting a minute crack due to temperature differences which may cause problems during the casting.
- As a general rule in gypsum-bonded investment, we should not exceed the maximum temperature of 750°C (1382°F). Above 750°C (1382°F), because of the presence of silica, calcium sulphate decomposition can start, with consequent degradation of investment strength whereas to guarantee complete combustion of carbonaceous residues left by the wax, we should exceed 690°C (1274°F). Therefore, as a general consideration 730°C is the maximum recommended temperature for burnout cycle.
- Also to guarantee complete burning of carbonaceous residues, the oven atmosphere must be maintained as highly oxidising. For the same reason, we need to provide sufficient gap between the flasks kept in the oven, to provide free air circulation.
The following is a typical burnout cycle:-
- After dewaxing, ramp slowly to 250°C (482°F) in 1 hour.
- Hold at 250°C (482°F) for 2 hours,
- Ramp to 450°C (842°F) in 1 hour,
- Hold at 450°C (842°F) for 2 hours,
- Ramp to 730°C (1346°F) in 1.5 hours,
- Hold at 730°C (1346°F) for 3 hours,
- Then slow cooling to the selected flask casting temperature and equilibrate at the casting temperature for at least 1.5 hours.
The casting temperature of the mould is chosen as a function of the pattern being cast and the alloy used. The timing given for the cycle will vary, depending on the size of the flask. Larger flasks require longer cycle times and one need to follow the cycle recommended by the casting experts.