In our sand casting foundry, we have extensive experience working with a wide variety of copper alloys, each possessing distinct characteristics. Some alloys exhibit excellent fluidity, while others have high shrinkage rates, and still others are prone to oxidation, forming thin films that lead to inclusions like dross. For example, aluminum bronze tends to oxidize rapidly, creating oxide films that result in slag inclusions. Therefore, when designing sand casting foundry processes, we must adopt different technical methods to address these challenges. Because the pouring performance of these alloys varies significantly, the gating system must be selected carefully according to the casting properties of the alloy, and risers and chillers must be correctly configured to ensure casting quality.
Copper alloys can be broadly divided into two main categories. The first category is tin bronze, which has relatively low shrinkage. Examples include alloys like CuSn10, CuSn12, CuSn5Zn5Pb5, and CuSn3Zn8Pb5. The second category includes lead-tin bronze and brass, which exhibit higher shrinkage or are prone to oxidation during pouring. Examples include CuPb10Sn10, CuZn33Pb2, CuZn40Pb2, CuZn38Al1, and others.
Below, we take the sand casting foundry production of a worm wheel made from CuAl10Fe3 aluminum bronze as an example to briefly explain the steps and methods of our process design.
1. Machining Allowance
Based on the maximum dimensions of the casting, we refer to a standard table of machining allowances (Table 1) to select a relatively large allowance. Then, according to the technical requirements, complexity, quality requirements, casting conditions, and machining equipment, we appropriately reduce it. For example, when using chillers, the machining allowance can be reduced by 3 mm or even eliminated. Reducing the machining allowance not only saves metal but also guarantees casting quality. Therefore, we always strive to minimize it. Generally, for a worm wheel made of aluminum bronze with a diameter of 200–300 mm, we use a machining allowance of 4–6 mm.
| Maximum Casting Dimension (Length or Height) | Maximum Machining Allowance (Top & Bottom) |
|---|---|
| Up to 500 | 3–5 |
| 500–1000 | 4–6 |
| 1000–2000 | 5–8 |
| 2000–3000 | 6–10 |
2. Risers
Commonly used risers in our sand casting foundry include blind risers and open risers. Open risers are simpler in form. Based on the alloy type, open risers are divided into two categories: those for tin bronze and those for lead-tin bronze and brass, with standardized dimensions. See Tables 2 to 5.
| Standard A | Standard B |
|---|---|
| When < condition > … | When < condition > … |
For a more complex shape like that of a worm wheel, we locate the hot spot first. After adding the machining allowance, we find the thermal center. Using the geometric method of drawing circles, we pass through the center of the inscribed circle of the hot spot and draw tangential circles to determine the riser diameter and height. For example, the riser width D and height H are determined such that H = (1.2 \text{ to } 1.5) D. When the riser feeds from the side, we use H = (1.5 \text{ to } 2.0) D.
For blind risers, many sources recommend a hemispherical top. However, through our trials, we found that using a blind riser with a height ratio given by H/D = 1.5 works well for aluminum bronze worm wheels.
| Casting Height (mm) | Riser Width (mm) | Riser Height (mm) |
|---|---|---|
| … | … | … |
| Contact Area Fraction | Riser Diameter (mm) | Riser Height (mm) |
|---|---|---|
| 0.4–0.6 | … | … |
3. Chills
In our sand casting foundry, we use external chills to accelerate solidification and eliminate shrinkage porosity. For the worm wheel, we place chills at the root of the teeth. The chill thickness is determined by the casting section thickness: if the casting thickness is t, the chill thickness is t or 1.5t (0.8t to 1.2t for one chill, 1.2t to 1.5t for another). Tests have shown that using these thicknesses provides excellent results, completely eliminating shrinkage at the tooth root. To prevent shrinkage at the “X” location, we use a blind riser for feeding. The blind riser dimensions are determined by the casting geometry, with H = 1.5D and the riser must be slightly above the rim (by about 10 mm).
When using chills, we must avoid leaving them in the sand mold too long to prevent moisture absorption and gas porosity. If the chill hinders free contraction of the casting, we leave gaps between chills and fill them with asbestos rope to allow free contraction after pouring.
4. Gating System
Historically, our sand casting foundry used the same gating system specification for all non-ferrous alloys, with a sprue:runner:gate cross-section ratio of 1:2:1.5. After years of experience, this ratio proved suitable for tin bronze, though we often increased it by one grade. For example, a casting weighing 10 kg would use a system designed for 20 kg. However, for lead-tin bronze and brass, we found this ratio inadequate after a year of trials. Therefore, we developed a new specification with a ratio of:
$$ \text{Sprue} : \text{Transition Runner} : \text{Runner} : \text{Ingate} = 1 : 0.5 : 2 : 1.5 $$
The cross-sectional area of each element is selected from Tables 5 and 6 based on the gross weight of the casting including the riser.
| Gross Casting Weight (kg) | Sprue | Transition Runner | Runner | Ingate |
|---|---|---|---|---|
| 5–10 | 1.5 | 0.8 | 3.0 | 2.3 |
| 10–25 | 2.5 | 1.2 | 5.0 | 3.7 |
| 25–50 | 3.5 | 1.8 | 7.0 | 5.2 |
| 50–100 | 5.0 | 2.5 | 10.0 | 7.5 |
| 100–200 | 7.0 | 3.5 | 14.0 | 10.5 |
| 200–400 | 10.0 | 5.0 | 20.0 | 15.0 |
| Sprue Diameter | Transition Runner Slot Dimensions | Runner Dimensions | Ingate Dimensions |
|---|---|---|---|
| … | … | … | … |
Example: Worm Wheel of AlCu10Fe3
Consider a worm wheel made of aluminum bronze with gross weight 40 kg. From Table 5, the required cross-sections are:
- Sprue: 3.5 cm² ⇒ diameter = 21 mm (single round sprue)
- Transition runner: 1.8 cm² ⇒ use a filter with 6 holes of Ø7 mm (total area 2.3 cm² approx.)
- Runner: 7.0 cm² ⇒ choose a single runner with dimensions shown in the figure (height 20 mm, width 35 mm)
- Ingate: 5.2 cm² ⇒ if using 6 gates, each gate area = 0.87 cm², giving dimensions (say 10 mm × 9 mm)
The sprue height should be slightly above the top of the riser, forming a 30° angle at the pouring cup. This angle is critical for large castings. Our experience shows a 30° angle is most appropriate.
The number of ingates is usually even for easy calculation, and we consider the fluidity of the alloy. For a worm wheel with an inner diameter of 250 mm, the circumference is about 785 mm. Using 6 ingates, the spacing is about 130 mm, ensuring the metal can flow without cold shuts.
5. Coating for Permanent Molds Used in Sand Casting Foundry
In our sand casting foundry, we also extensively employ permanent molds (iron molds) for copper alloy castings. To improve surface finish, reduce machining allowance, and extend mold life, we apply a coating to the mold surface. Whether using centrifugal or stationary iron molds, the following copper alloys require coating: CuAl10Fe3, CuSn10, CuSn12, CuPb10Sn10, CuZn33Pb2, etc.
The coating composition is: about 60 g rosin, 500 g graphite powder, and 500 g alcohol. To prepare, weigh the rosin and graphite, add alcohol, and stir thoroughly until completely dissolved. The viscosity can be tested with a finger; it should feel slightly tacky. Before heating the mold, clean it thoroughly, removing all previous residues and rust, and polish the inner surface with sandpaper until bare metal shines. Then apply a thin, uniform layer of coating with a cotton wad or cloth. Heat the mold to 150–250°C before pouring.
Important notes: After mixing, let the coating stand for 30 minutes to fully dissolve. Apply the coating evenly. Only prepare the amount needed to avoid evaporation of alcohol.
Mathematical Summary of Design Procedure
We can encapsulate the key design formulas used in our sand casting foundry for copper alloy worm wheels as follows:
Machining allowance (mm): M = 4-6 for worm wheel Ø200-300 mm. When chill is used, M_{chill} = M – 3.
Riser dimensions (for blind riser):
$$ D = \text{determined by geometric construction from hot spot} $$
$$ H = 1.5 D $$
Riser must extend at least 10 mm above the rim.
Chill thickness (mm):
$$ t_{chill} = 0.8 t \text{ to } 1.2 t \quad \text{or} \quad t_{chill} = 1.2 t \text{ to } 1.5 t $$
where t is the casting section thickness at the point of application.
Gating system cross-sectional area ratio:
$$ A_{sprue} : A_{transition} : A_{runner} : A_{gate} = 1 : 0.5 : 2 : 1.5 $$
Sprue diameter (round sprue):
$$ d_{sprue} = \sqrt{\frac{4 A_{sprue}}{\pi}} $$
Number of ingates n is chosen so that the distance between adjacent gates is no more than 150–200 mm, ensuring complete filling:
$$ \text{Spacing} = \frac{\text{inner circumference}}{n} \le 150 \text{ mm} $$
All these guidelines have been refined through years of practice in our sand casting foundry, and they have proven reliable for producing high-quality copper alloy castings with minimal defects such as shrinkage, porosity, and cold shuts.
The table below summarizes the transition runner (filter) standard we use in our foundry:
| Filter Type | Diameter of Central Hole | Number of Filter Holes | Filter Hole Diameter | Total Filter Area (cm²) |
|---|---|---|---|---|
| Type I | 10 | 6 | 6 | 1.70 |
| Type II | 12 | 8 | 4 | 1.01 |
| … | … | … | … | … |
Finally, we emphasize that the success of casting in our sand casting foundry depends on a thorough understanding of alloy properties, careful design of risers and chills, and precise gating system calculation. By following these principles, we consistently achieve sound castings with high dimensional accuracy and excellent mechanical properties.