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Customized
NHD
A graphite crucible with an eccentric hole is a high-density graphite container featuring a non-centrally symmetrical pouring hole located at the bottom or top of the crucible. This design is not simply a matter of drilling a hole, but rather a precision structure based on fluid dynamics calculations—the eccentric hole allows molten metal to enter the mold cavity tangentially, creating a spiral filling path that thoroughly expels air from the cavity.
In traditional center-hole crucibles used in vacuum suction casting, molten metal strikes the bottom of the mold vertically and rebounds in all directions, easily causing porosity and cold shuts in intricate structures such as complex jewelry designs and dental crowns and bridges. The eccentric-hole design alters the angle of entry of the molten metal to achieve laminar filling, increasing the density of castings by more than 30% and significantly reducing the scrap rate.
Ningheda Graphite uses fine-grained isostatic graphite billets that are precision-machined via CNC to ensure an eccentric hole positional accuracy of ±0.1 mm. This ensures a perfect, seamless fit with the equipment’s discharge pipe, eliminating the risk of liquid leakage.
Single Outlet Graphite Melting Crucible
Graphite Crucible & Stirring Rod Assortment
| Items | Specifications |
Material | High-Purity Isostatic Graphite |
Bulk Density | 1.78-1.90 g/cm³ |
Ash Content | ≤0.06% |
Compressive Strength | ≥70 MPa |
Flexural Strength | ≥35 MPa |
Resistivity | ≤10 μΩ·m |
Maximum Operating Temperature | 3,000°C (in an inert atmosphere) |
Positioning Accuracy of Eccentric Holes | ±0.1mm |
Standard Specifications | ∅30 mm–∅120 mm (customizable) |
Advantage 1: Eccentric-hole flow channel design ensures zero porosity in castings
The eccentric-hole structure, optimized through CFD fluid simulation, allows molten metal to fill the mold smoothly in a spiral pattern, effectively preventing turbulence-induced entrapment of air. According to customer tests, the porosity defect rate for complex jewelry castings has been reduced from 12% to less than 2%.
Advantage 2: High-Purity Graphite Material, No Contamination of Precious Metals
High-purity isostatic graphite with an ash content of ≤0.06% is used, ensuring no carbon impurities are introduced during the melting process. The purity of finished gold and platinum products can be consistently maintained at 99.9% or higher, meeting the purity requirements for investment gold bars and high-end jewelry.
Advantage 3: Excellent Thermal Shock Resistance—No Cracking Under Rapid Cooling or Heating
With a thermal expansion coefficient as low as 2.5×10⁻⁶/°C, the graphite can be heated directly from room temperature to 1,600°C without preheating or a temperature ramp-up phase, and can withstand repeated thermal cycling. Laboratory testing: 50 cycles from 1,000°C to room temperature water quenching, with a 98% integrity rate.
Advantage 4: Strength Increases Rather Than Decreases at High Temperatures
Unlike ceramic crucibles, which soften and deform at high temperatures, graphite maintains a compressive strength of ≥70 MPa even at temperatures above 2,000°C. When filled with high-density platinum solution (density 21.4 g/cm³), the crucible walls show no bulging or cracking.
Advantage 5: Rapid Heat Conduction, Short Smelting Time, Energy Savings, and Reduced Consumption
Graphite has a thermal conductivity of 80–150 W/(m·K), far exceeding that of refractory clay materials. Smelting 1 kg of gold takes only 8–12 minutes, reducing smelting time by 40% compared to clay crucibles and saving over 25% in electricity per batch.
Advantage 6: Exceptionally Long Service Life and Low Overall Cost
Under vacuum or inert gas protection, eccentric-hole graphite crucibles used for gold and silver smelting have an average service life of 80–120 smelting cycles—2–3 times that of clay-graphite composite crucibles. The average daily cost per unit is reduced by 60%.
Horizontal Graphite Crucible With Flat Stir Rod
Multi-Spec Standard Graphite Crucible Blanks
Vertical Graphite Crucible Flat Stirring Rod Kit
In the vacuum pressure casting (VPC) process, molten metal rapidly fills the plaster mold under the combined effects of vacuum suction and positive pressure from an inert gas. During this process, the flow pattern of the molten metal directly determines the quality of the casting. The vertical pouring method used with traditional center-hole crucibles has three critical flaws:
First, air entrapment. As the molten metal falls vertically from the center, it creates a counter-rotating vortex at the bottom of the mold. The air at the top of the mold cavity is “trapped” by the molten metal and cannot escape, ultimately forming porosity at the top of the casting and in thick, large cross-sections.
Second, temperature inconsistency. The vertical impact causes molten metal to splash; the molten metal that first contacts the mold wall cools rapidly, while the molten metal falling later has a higher temperature, resulting in an uneven casting microstructure and significant variations in hardness.
Third, oxidation inclusions. Under turbulent conditions, the surface area of the molten metal increases dramatically, increasing its exposure to residual oxygen and forming oxide scale inclusions, which appear as black pitting after polishing.
Ningheda Graphite's eccentric orifice design fundamentally resolves the above issues. The eccentric orifice is located on one side of the crucible’s base; molten metal enters the mold at a tangential angle of 15°–30°, forming a stable spiral flow along the mold wall. This flow pattern offers three major mechanical advantages:
Thorough degassing: The spiral flow drives air from the center of the mold cavity toward the vent, allowing the vacuum pump to reduce residual pressure to below 10 Pa within 0.3 seconds.
Optimal temperature gradient: As the molten metal flows close to the mold wall, the outer layer cools first to form a solidification shell, while the inner molten metal continuously fills the shrinkage, eliminating shrinkage porosity.
Laminar flow protection: The low Reynolds number flow minimizes contact between the molten metal and air, reducing the oxidation and burn-off rate by 50%.
Open Inner Cavity Graphite Crucible Threaded Rod
Vertical Graphite Crucible With Pointed Thread Rod
Although clay-bonded graphite crucibles commonly found on the market are inexpensive, their graphite content is only 30%–50%, with the remainder consisting of refractory clay and asphalt binder. This composite material has significant drawbacks:
Low maximum temperature resistance: The clay components begin to soften at temperatures above 1200°C, making them unsuitable for melting platinum (melting point 1768°C) and cobalt-chromium alloys (melting point 1350°C).
Poor thermal shock resistance: The thermal expansion coefficients of clay and graphite differ significantly. Rapid cooling and heating generate shear stress at the interface, causing cracks to form after just 3–5 uses.
High risk of contamination: SiO₂ and Al₂O₃ impurities in the clay react with precious metals at high temperatures, resulting in gray inclusions on the surface of the castings.
Ningheda graphite crucibles are made from isostatically pressed high-purity graphite with a purity of ≥99.9%, which undergoes isotropic pressing at 200 MPa and high-temperature graphitization at 2,800°C. Their microstructure is dense and uniform, with consistent properties in all directions, eliminating the interlaminar weak planes found in traditional molded graphite. Under scanning electron microscopy, the material exhibits a porosity of ≤12% with a concentrated pore size distribution, ensuring both erosion resistance and prevention of molten metal penetration.
Precision Machining Process: Eccentricity Control of ±0.1 mm
The positional accuracy of the eccentric hole is critical to successful casting. If the eccentricity deviation exceeds 0.3 mm, the angle of incidence of the molten metal changes, disrupting the spiral flow pattern and actually exacerbating turbulence.
Ningheda Graphite is equipped with a five-axis CNC machining center and uses diamond-coated cutting tools for dry cutting. The machining process is as follows:
Blank Inspection: Each graphite blank undergoes ultrasonic testing to eliminate internal cracks and inclusions.
Rough Machining: The outer shape is turned, leaving a 0.3 mm finishing allowance.
Finishing: The inner and outer circles are CNC-turned to ensure concentricity ≤ 0.05 mm.
Eccentric Hole Machining: Using the bottom surface as the reference, the center distance is offset according to the customer’s drawings; after drilling, the hole is precision-bored to the target dimensions.
Chamfering and Polishing: The hole edges are rounded to an R0.5 radius, burrs are removed, and the process prevents scratching of equipment seals.
Coordinate Measuring Machine (CMM) Inspection: 100% inspection of eccentricity, bore diameter, and perpendicularity, with a dimensional report issued.
Jewelry Gold Silver Casting Application
Lab Semiconductor Metal Test Use
Industrial Precious Metal Smelting Factory
Precious Metal Casting in the Jewelry Industry (Primary Application)
Suitable for the smelting, ingot casting, and vacuum casting of gold, silver, karat gold, platinum, and palladium jewelry. The eccentric hole design minimizes precious metal residue, reduces waste of valuable raw materials, and is compatible with automated mass-production equipment in jewelry factories.
Precious Metal Refining Laboratories
Small-scale smelting for precious metal purification and alloy formulation experiments; residual metal can be easily and completely drained, facilitating frequent changes in alloy formulations and reducing cross-contamination.
High-Temperature Smelting of Non-Ferrous Metals
Small-scale smelting of copper alloys and rare alloy samples; a stable thermal field ensures uniform alloy composition.
▍Usage and Maintenance Guide
Storage: Store in a dry, well-ventilated warehouse. Graphite absorbs moisture very easily; if the crucible becomes damp, it may shatter during the initial heating process.
Preheating: For first-time use, heat the crucible gradually. Hold at 200°C for 30 minutes to thoroughly remove moisture.
Use: Do not allow the molten material level to exceed 2/3 of the inner cavity to prevent high-temperature molten material from spilling over. Do not strike the crucible body or the edges of the eccentric hole with hard objects.
Cleaning: Use a soft-bristled brush to remove residual metal. Do not scrape the inner walls of the crucible with hard tools, as this will damage the dense surface layer.
Recommendations for Extending Service Life: Use the crucible under vacuum, nitrogen, or argon protective atmospheres to significantly slow down graphite oxidation and extend its service life.
Tapered Single Spout Graphite Crucible
Slant Opening High Capacity Crucible
Multi-Cavity Custom Graphite Crucible
▍Packaging Method
Inner Packaging: Each crucible is individually wrapped in shock-absorbing bubble wrap, with a foam protective core inserted into the eccentric hole to prevent damage to the precision channels during transit.
Outer Packaging: Five-layer reinforced corrugated cardboard box with internal foam dividers. Bulk orders are reinforced with fumigation-free plywood crates.
Labeling: The outer carton is labeled with “Fragile,” “Keep Dry,” and “This Side Up,” and includes a product specification label and a copy of the material test report.
Samples/Small Orders: DHL/FedEx/UPS international express delivery, with delivery to major cities worldwide within 7–15 business days.
Large-Volume Orders: Full container load (FCL) or less than container load (LCL) by sea freight; FOB, CIF, and DDP terms supported. Can be shipped to designated ports or overseas warehouses.
Special Requirements: We can handle export customs clearance, Certificates of Origin (CO), and commodity inspection procedures according to customer requirements.
Q1: What is the difference between a graphite crucible with an eccentric hole and a standard crucible with a central hole?
A1: Conventional crucibles have a centrally located hole, which makes it easy for precious metals to remain at the bottom of the crucible after the molten metal is poured out, resulting in raw material loss. The offset hole design guides the molten metal to gather on one side, allowing for more thorough discharge and reducing precious metal residue. At the same time, the structural dimensions can be matched to the discharge interfaces of automated casting equipment, making them suitable for assembly line production. In contrast, center-hole crucibles are mostly only suitable for simple manual smelting.
Q2: Can the position and size of the eccentric hole be modified to fit my equipment?
A2: We support full customization. We can adjust the eccentric distance and the inner diameter of the flow channel based on the location of your casting equipment’s discharge port, while also adjusting the overall height and outer diameter of the crucible. We will provide CAD drawings for your confirmation before production.
Q3: Will melting highly reactive precious metals such as platinum and palladium cause contamination?
A3: We use ultra-high-purity graphite products with a total impurity content of ≤0.01%. These materials are chemically stable and will not react with precious metals at high temperatures, preventing issues such as carbon infiltration or discoloration, and meeting the requirements for high-end jewelry casting.
Q4: Will the crucibles break during shipping?
A4: Although graphite is hard and brittle, our packaging has passed drop tests. Each piece features independent cushioning supports at the eccentric hole locations, and the breakage rate during normal logistics transport is less than 0.5%. If you receive a damaged item, please take photos and contact us within 24 hours for a replacement.
Q5: What is the minimum order quantity? What is the lead time?
A5: Standard sizes (∅40/∅50/∅60 mm) require a minimum order of 10 pieces; custom sizes require a minimum order of 50 pieces. Standard products ship within 10–15 days, and custom products ship within 15–30 days. Expedited shipping for urgent orders is available upon request.
Q6: Besides precious metals, can common metals like copper and aluminum be smelted in these crucibles?
A6: Yes, but it is not recommended. Copper and aluminum have low melting points, so using standard clay-graphite crucibles is more cost-effective. High-purity isostatic graphite crucibles are more expensive, and their purity advantages are best utilized when processing precious metals.
Q7: Can this crucible be used at high temperatures in the open air for extended periods?
A7: We do not recommend prolonged use at high temperatures in an open environment. Graphite oxidizes slowly in a high-temperature, oxygen-rich environment. We strongly recommend using it under a vacuum or an inert protective atmosphere (such as nitrogen or argon), which not only extends the crucible’s service life but also prevents the oxidation of precious metals.
If you are looking for eccentric-flow graphite crucibles compatible with vacuum casting machines and induction furnaces to reduce losses during precious metal smelting, please feel free to send your equipment model, product dimension drawings, and smelting material requirements to our technical engineers. We provide free solution selection and parameter quotes, along with graphite material test reports. Our company can supply a full range of high-temperature graphite products, including standard straight-hole graphite crucibles, graphite pouring spouts, and graphite molds, offering a one-stop shopping experience.
+86-795-4605783 
sales@nhdcarbon.com 
No. 966 of Tiangong South Avenue, High-tech Industrial Park, Fengxin County, Yichun City, Jiangxi Province, China.