Bell Jar for Vacuum Casting: What It Is and How It Works
By Pepetools - Precision Engineered Professional Equipment | Published: 2026-10-30
Category: How-to Guides
Learn what a bell jar is, how it works in vacuum casting, and key safety tips for using a 9x8 acrylic bell jar in jewelry casting.
Quick answer
A bell jar is a clear acrylic chamber that sits on a vacuum casting table to create a sealed environment. When the vacuum pump removes air from inside, the bell jar helps pull trapped air bubbles out of the investment material, resulting in smoother, more detailed castings.
Why use a bell jar instead of a vacuum chamber?
A bell jar is essentially a vacuum chamber designed for casting work. Its open-bottom design lets you place it directly over the flask or mold on the vacuum table. The transparent acrylic walls let you watch the process—something a metal chamber cannot offer. This visibility helps you confirm when bubbles stop rising, so you know the cycle is complete.
Are there trade-offs with an acrylic bell jar?
Acrylic is lighter and clearer than glass or metal, but it can scratch or crack if handled roughly. The Pepetools 9x8 bell jar has thick, sturdy walls and a heavy ground bottom rim for a reliable seal. Still, you must never use a chipped or damaged jar—it can shatter under vacuum. Inspect the rim before every use.
Frequently asked questions about bell jars for vacuum casting
- How does a bell jar remove air bubbles from investment?
- You place the filled flask under the bell jar on the vacuum table. The vacuum pump pulls air out of the jar, lowering the pressure. The reduced pressure causes dissolved air in the investment slurry to expand and rise to the surface as bubbles, which then pop. The process typically takes 2–5 minutes depending on the material and vacuum strength.
- How do I set up a bell jar for vacuum casting?
- Place the bell jar on a clean, flat vacuum table with a rubber gasket (if included). Connect the vacuum pump hose to the table port. Position the filled flask inside the jar. Turn on the pump and watch through the clear walls until bubbles stop rising. Slowly release the vacuum before lifting the jar. Always release vacuum slowly to avoid disturbing the investment.
- What safety precautions should I take?
- Keep the bell jar rim and vacuum table clean of investment debris—any grit can break the seal or scratch the acrylic. Never use a chipped or damaged bell jar; it can fail under vacuum. Never press or hit the bell jar during operation. Always wear eye protection in case of sudden failure. Inspect the bell jar rim before every use. Replace immediately if cracked or chipped.
Product facts
- Bell Jar 9" x 8" For Vacuum Casting Machines: Pepetools high quality bell jars are manufactured from acrylic for lasting use. Perfectly clear to see through when running a cycle. Thick, sturdy walls. Heavy bottom rim cut ground and smooth to create a perfect seal. The clear acrylic lets you visually confirm when air bubbles stop escaping, so you know exactly when the vacuum cycle is done. The heavy ground rim ensures a tight seal against the table, preventing vacuum leaks.
Bell Jar 9" x 8" For Vacuum Casting Machines
