SRS - Technical Centre: Helpful Hints

Dewax/Firing - Firing Cycles

1. One of the major causes of porosity related defects is due to incomplete burnout or a reducing atmosphere inside the burnout oven. Keep in mind that the burnout oven is an incinerator not a kiln. In order to completely eliminate all the wax and carbon residues requires a temperature of 730 degrees Celsius, a good airflow and exhaust for combustion gasses. If any of these are lacking you will have problems with your castings.

2. Carbon residues in the mould cavity create little explosions on contact with the molten metal causing irregular shaped holes. Poor airflow in the oven creates a reducing atmosphere. At elevated temperatures this atmosphere causes the Calcium Sulphate within the investment to decompose at the cavity surface releasing sulphur. When the molten metal is cast into the mould sulphur dioxide is formed and is absorbed by the metal whilst still liquid creating sulphur gas porosity.

3. Check burnout oven to ensure there is adequate air supply and exhaust. Electric ovens rarely have inlet vents and the exhausted vents are often too small. Drilling a few holes in the bottom of the door and making sure the exhaust vents are open and large enough will improve airflow.

BURNOUT OVEN DOOR

4. If you notice a yellow ring on the investment around the button after casting, a rotten egg smell when quenching or a dark colour on the investment next to the casting, you more than likely are getting a sulphur gas reaction in the oven. Improve airflow and extend the burnout time.

SULPHUR GAS

5. The temperature controller on the burnout oven should be checked and calibrated on a regular basis.

6. Make sure that the bottom plate in the oven has grooves in it to allow the wax to flow out, off, and away from the mould allowing air to flow in. If there are not any grooves in the bottom plate then it is advisable to raise the flasks off the bottom plate by standing them on ceramic supports. Empty wax from the furnace on a regular basis.

TRAY OF WAX	EMPTY TRAY

7. Avoid anything that can create a reducing atmosphere during burnout such as flames being too high, overloading the oven, back drafts or lack of exhaust.

8. When double stacking flasks in the burnout oven make sure to stagger the flasks to keep sprue holes over open areas. This will allow wax to flow out and air to get into the flasks.

OVERLOADED	GOOD STACK

9. Once the wax starts to melt out of the flask the process must be continued. If the oven cools and the wax solidifies, mould erosion can occur when the wax melts again.

10. Flasks should be placed sprue down even after steam dewaxing. Any residual wax left in the flask will boil and erode the investment if the wax can't flow out.

11. Follow the recommended firing cycles keeping in mind the size of can, amount of cans in oven and the type of oven being used.

12. Once the oven is down to the required casting temperature leave for 1 hour to allow the temperature of the can to stabilize.

View other helpful hints online or download (Acrobat Reader PDF) versions by selecting relevant links below:

HELPFUL HINTS

	Casting		Download PDF (92k)
	Dewax/Firing - Firing Cycles		Download PDF (148k)
	Investing		Download PDF (112k)
	Modelmaking		Download PDF (56k)
	Proportioning Charts		Download PDF (204k)
	Sprueing		Download PDF (204k)
	Wax Patterns		Download PDF (124k)