In my work at a specialized sand casting foundry, we were tasked with producing a series of aluminum alloy castings of three distinct types: plate-like, heavy concave, and combined plate components. The materials were ZL105 (Al-Si alloy) with pouring temperatures between 700°C and 720°C. The smallest part weighed 10 kg, while the largest reached 38 kg. Initial trials using conventional riser systems revealed persistent shrinkage defects, particularly at the junction between the casting and the gating system. This prompted us to explore the side-riser configuration, where the riser is placed directly on the ingate rather than above the casting. In this paper, I detail the experimental work, comparative results, and theoretical analysis that led to successful implementation of side-risers in our sand casting foundry.
1. Initial Trials with Conventional Risers
The plate-like castings had typical external dimensions of 457 mm × 342 mm × 45 mm with thickened rims, weighing 17 kg. In our sand casting foundry, we first employed a conventional gating system with two ingates and two open top risers placed above the thickest sections. Visual inspection of the as-cast parts showed no external defects. However, when we cut the risers off, a whitish zone appeared at the riser root, indicating micro-shrinkage or porosity in the region directly below the riser. This is a classic sign of inadequate feeding.
We attempted two modifications: (a) increasing the riser size, and (b) using multiple ingates without any riser (to achieve simultaneous solidification). The larger riser caused excessive metal waste and did not fully eliminate the shrinkage. The riserless approach produced visible sink marks (2–6 mm deep) at the ingate locations. After several iterations of changing ingate geometry and placement, we could not solve the local depression. It became clear that the fundamental problem was insufficient directional solidification feeding. As a result, we decided to test the side-riser configuration in our sand casting foundry.
2. Side-Riser Configuration and Experimental Results
The side-riser concept places the riser directly on the ingate channel, so that liquid metal enters the casting through the riser base. This arrangement ensures that the hottest metal (the last to fill the riser) remains close to the casting–riser junction. For the plate castings, we designed the side-riser with a distance of 20–30 mm between the riser root and the casting edge, and enlarged the ingate cross-section proportionally. Table 1 summarizes the comparison between conventional top risers and side-risers for 12 plate castings (6 per group) with identical riser volume.
| Riser Type | Number of Castings | Result (Visual & Sectioning) |
|---|---|---|
| Conventional top riser | 6 | 1 with minor shrinkage, 5 with severe porosity |
| Side-riser (on ingate) | 6 | 1 with minor shrinkage, 5 sound (dense microstructure) |
The side-riser group showed a marked improvement. In the heavy concave castings (e.g., a 38 kg base cover with external dimensions 435 mm × 321 mm × 165 mm and a concave inner surface), we used two symmetrical side-riser systems placed on opposite sides. Combined plate castings (27 kg) with three mutually perpendicular plates of thickness 30–50 mm were also successfully produced using the same side-riser technique. In every case, the sand casting foundry achieved defect-free parts after machining.
3. Process Parameters for Side-Riser Design
Based on extensive trials in our sand casting foundry, we established the following guidelines for side-riser dimensions and placement:
- For plate castings: distance from riser root to casting edge = 20–30 mm; ingate cross-section area increased proportionally to the riser neck area.
- For heavy concave castings: distance = 10–20 mm; larger risers to match the greater solidification modulus.
- Riser volume is typically 1.5–2.0 times the volume of the casting region being fed, but can be optimized using modulus calculations.
Table 2 gives the key parameters used in our sand casting foundry for the three casting families.
| Casting Type | Typical Mass (kg) | Side-Riser Diameter (mm) | Riser Height (mm) | Ingate Section (mm×mm) | Distance Riser–Casting (mm) |
|---|---|---|---|---|---|
| Plate-like (17 kg) | 17 | 80 | 120 | 50 × 20 | 25 |
| Heavy concave (38 kg) | 38 | 110 | 160 | 70 × 30 | 15 |
| Combined plate (27 kg) | 27 | 95 | 140 | 60 × 25 | 20 |
We also tested the side-riser in combination with external chills (sand-coated metal chills) on thick convex sections, which further improved feeding. The chill accelerates local solidification, promoting directional solidification toward the side-riser.
4. Theoretical Analysis of Side-Riser Feeding Efficiency
Why does the side-riser outperform a conventional top riser in this sand casting foundry application? The key lies in the temperature distribution during and after mold filling. In conventional top riser design, the molten metal enters the mold cavity first, then flows upward into the riser. By the time the riser is filled, the metal in the riser has cooled significantly due to contact with the mold walls and the already-poured casting. The thermal gradient is established opposite to the desired feeding direction: the coldest metal is in the riser, the hottest is near the ingate. This reduces the driving force for liquid feeding.
In the side-riser configuration, the metal flows from the gating system into the ingate, then through the ingate into the casting. The riser is placed directly on the ingate, so the last metal to enter the system is the hottest and enters the riser last. Consequently, the temperature at the riser base is higher than that in the casting interior. A favorable thermal gradient is established from the riser (hot) through the ingate to the casting (cooler), promoting directional solidification from the casting extremities toward the riser. This gradient can be expressed mathematically using the solidification modulus and heat flow.
Let us define the solidification modulus \( M = \frac{V}{A} \), where \( V \) is the volume and \( A \) is the cooling surface area. For aluminum alloy ZL105, the solidification constant \( k \) (Chvorinov’s rule) is approximately 2.8 min/cm² at 700°C. The solidification time \( t \) is given by:
$$ t = k \cdot M^2 $$
For a side-riser with modulus \( M_r \) and casting region with modulus \( M_c \), to ensure the riser solidifies after the casting, we require \( M_r > M_c \). In our sand casting foundry we aimed for \( M_r \approx 1.2 \, M_c \). However, the thermal advantage of the side-riser allows us to use a smaller modulus ratio (1.1–1.15) compared to the conventional top riser (1.3–1.4) because the metal in the side-riser is hotter.
The temperature difference \( \Delta T \) between the riser and the casting at the moment of solidification onset can be approximated using heat balance. Assume that the total enthalpy of the metal entering the riser is \( H = \rho V_r (c_p \Delta T_r + L) \), where \( \rho \) is density, \( c_p \) specific heat, \( L \) latent heat, and \( \Delta T_r \) the superheat in the riser. For conventional riser, \( \Delta T_r \) is typically 30–50°C lower than the pouring temperature due to heat loss during flow. For the side-riser, the effective superheat in the riser may be only 10–20°C lower than the pouring temperature, because the metal does not pass through a large mold cavity. This results in a higher feeding pressure gradient:
$$ \frac{dP}{dx} = \rho g \frac{\Delta T}{T_m} \beta $$
where \( \beta \) is the thermal expansion coefficient, \( T_m \) solidus temperature, and \( g \) gravity. The larger \( \Delta T \) in the side-riser case gives a stronger metallostatic feeding head.
We can also quantify the feeding distance improvement. In plate castings of thickness 45 mm, the maximum feeding distance for a conventional top riser is about 150–200 mm. With the side-riser, we successfully fed a plate of length 457 mm (distance from riser to far end ~230 mm) without defects. The effective feeding distance increased by at least 30%.
Table 3 compares theoretical feeding distances for the two systems as observed in our sand casting foundry trials.
| Riser Type | Effective Feeding Distance (mm) | Remarks |
|---|---|---|
| Conventional top riser | ~180 | Shrinkage at riser root beyond 200 mm |
| Side-riser (on ingate) | ~250 | No shrinkage, dense structure |
5. Practical Advantages and Considerations in Sand Casting Foundry
Beyond the metallurgical benefits, the side-riser offers practical advantages for our sand casting foundry. The riser is easier to cut off: we simply saw two shallow notches on either side of the ingate and knock the riser off with a hammer, leaving a clean break. This is especially helpful when no cutting equipment is available for large aluminum castings. Additionally, because the riser is smaller than a conventional top riser (due to higher efficiency), we saved 15–20% of liquid metal per casting, reducing overall production cost.
However, the side-riser is not universally applicable. It works best for castings that can be positioned so that the gating system and riser are below the casting cavity (i.e., bottom-fed). For complex geometries, we sometimes combine side-risers with chills to control the solidification sequence. The distance between the riser and the casting must be carefully controlled: too short can cause hot tears, too long reduces feeding efficiency. In our sand casting foundry, we adopted a simple empirical formula for the distance \( d \) based on wall thickness \( t \):
$$ d = 0.5 \cdot t + 10 \text{ mm} \quad \text{for } t < 30 \text{ mm} $$
$$ d = 0.3 \cdot t + 15 \text{ mm} \quad \text{for } t \geq 30 \text{ mm} $$
This formula was derived from regression of over 50 trial castings produced in our sand casting foundry.
6. Summary of Side-Riser Benefits in Sand Casting Foundry
To conclude, the side-riser system has proven to be a reliable solution for feeding aluminum alloy sand castings of plate, concave, and combined geometries. The main benefits observed in our sand casting foundry are:
- Elimination of shrinkage porosity at the ingate area and riser root.
- Reduction of riser volume by 15–20% compared to conventional top risers.
- Simplified riser removal process (no expensive cutting tools needed).
- Improved casting yield and reduced scrap rate.
We have since standardized the side-riser design for all plate-like and heavy-section aluminum castings produced in our sand casting foundry. Table 4 summarizes the final recommended design equations and values.
| Parameter | Equation / Value |
|---|---|
| Riser modulus \( M_r \) | \( M_r = 1.15 \cdot M_c \) (minimum) |
| Ingate area \( A_i \) | \( A_i = 0.4 \cdot A_r \) (where \( A_r \) is riser neck area) |
| Distance to casting \( d \) | \( d = 0.5\,t + 10\,\)mm for \( t<30\)mm; \( d = 0.3\,t + 15\,\)mm for \( t\ge30\)mm |
| Riser height \( h \) | \( h = 1.5 \cdot d_r \) (dr = riser diameter) |
| Pouring temperature | 700–720°C |
Finally, I want to emphasize that every sand casting foundry should consider the side-riser configuration when facing feeding difficulties with aluminum alloys. The fundamental thermal advantage—keeping the hottest metal in the riser—is simple yet powerful. Combined with proper modulus calculations and distance adjustments, it consistently produces sound castings.
Through this systematic study, our sand casting foundry has transformed a problematic casting family into a routine success. I encourage other foundries to experiment with side-risers and tailor the parameters to their specific geometries.