Questions and Issues Regarding the Use of Investment Casting Plaster

Things to Keep in Mind in Casting (The Secrets of the Trade)

1. Plaster Molding

• Ensure the correct ratio of water to plaster; the standard mix is 40:100, and for vacuum mixing, it’s 38:100.
• To achieve a perfect and uniform plaster slurry, carefully weigh the water and plaster before mixing and ensure the correct powder-to-water ratio.
• Never add water to plaster, as it will form lumps during mixing (always add plaster to water).
• Maintain the slurry temperature at 22°C for optimal casting results.
• Waxes that are too close to the sides or top of the cylinder create a very thin casting wall; ensure a minimum distance of 5 mm or more from the sides and top of the cylinder.
• Use clean ionized or distilled water for plaster molding.
• Instead of using any other measurement method, weigh the powder and water.
• Regularly calibrate the scale to achieve the optimal weight for plaster and water.

۲. Air Venting

• Ensure the air venting is free from air bubble leaks.
• There is a possibility of leakage at the base of the cylinder during casting. Ensure a good seal between the wax sprue and the base; it is recommended to seal it with molten wax.
• Ensure that when waxes or wax trees are made, they are stored in a dry, clean, and dust-free environment. Do not store waxes or trees in a dusty environment or casting room.
• Do not use excessive talc or release agents, as they can cause the production of rough waxes and, consequently, rough casting.
• For bubble-free castings, regularly service and recharge the oil in your vacuum pump.
• Check for leaks in the vacuum tubes and chambers.

۳. Drying

• Do not move the cylinder on the bench set during the setting process after plaster molding; remove the base and tape at least 90 minutes later.
• Allow the cylinder to rest for approximately 2 to 12 hours. The later you place the cylinder in the furnace or wax remover, the better the result will be.
• If the cylinders are to remain for more than 12 hours before wax removal, place a damp towel over them to prevent excessive drying. Failure to do so may result in casting defects.

۴. Wax Removal

• Start the wax removal process 90 minutes after plastering.
• Ensure that the temperature during the first stage of wax removal is maintained between 220°C and 250°C, and that it lasts for at least 4.5 hours. If the issue persists, increase the holding time.
• It is better to use dry wax removal instead of steam wax removal to prevent damage to the plaster.
• If the wax removal process using steam lasts for more than 90 minutes, it will start to erode the surface of the mold. Therefore, avoid steam de-waxing in a sealed container.

۵. Firing Furnace

• Ensure a minimum of 5 cm distance between the cylinders and the furnace walls.
• The cylinder should be placed in the furnace in a way that allows the vapors to escape easily (ensure the sprue is facing upward after wax removal).
• The furnace temperature should start from ambient temperature or match the wax removal temperature to prevent thermal shock to the plaster.
• Ensure that the furnace ramp temperature does not exceed 250°C per hour.
• Be careful that after firing in the furnace and before casting, the cylinder should not be struck during handling, as there is a risk of cracking or displacement of the mold.
• After reaching the final temperature, keep the cylinder in the furnace for at least 1 hour, as it takes 60 minutes for the core of the cylinder to reach the furnace indicator temperature.
• During casting, at no stage of the plaster firing process should the temperature exceed 630°C.
• As a standard, calibrate your furnace every 6 months.
• Ensure that you have a high-quality digital temperature controller for your furnace.

۶. Casting

• Before casting, ensure that the equipment is fully prepared.
• Make sure the molten material has reached its complete melting temperature before removing the cylinder from the furnace.
• After removing the cylinder from the furnace, cast quickly to prevent the plaster from cooling down.
• Ensure that the type of casting machine is suitable for the type of casting models (centrifugal, vacuum, induction).

۷. Maintenance and Care

• For best results, reseal the bag after use. Otherwise, moisture from the air will damage the powder, leading to rough casting and cracking.
• After resealing the bag, store the powder at room temperature between 10°C and 35°C. Temperatures outside this range may cause product degradation and casting errors.
• Ensure minimal contact of the bag with moisture and store it in a low-humidity environment with good air circulation.
• Do not store the product on the floor to prevent moisture infiltration through surface contact. It is preferable to store the product on a pallet.

۸. Temperature and Ratio

• Metal Type * Metal Pouring Temperature (°C) * Cylinder Temperature in Furnace (°C) * Metal to Wax Ratio