In recent years, the dry sand vacuum lost foam casting process has been widely adopted by many foundries due to its unique technical and economic advantages. This process, also known as vacuum evaporative pattern casting or V-EPC, eliminates the need for parting and core setting, offers simple equipment, high dimensional accuracy, and low production cost. It has been hailed as a revolution in the 20th-century foundry industry. While lost foam castings are currently more prevalent in aluminum and iron castings, research indicates that V-EPC is equally suitable for steel castings. For board castings, the dry sand vacuum lost foam casting method accommodates various materials effectively.
In this work, I will comprehensively analyze the problems associated with different sand casting process schemes for board castings. By combining the features of V-EPC, I will highlight the issues that must be considered when producing board castings via V-EPC. Additionally, I will examine typical casting defects occurring in V-EPC of a representative board casting, identify countermeasures, and establish a specific process plan to produce sound castings.
1. Problems of Different Sand Casting Schemes for Board Castings
1.1 Board Castings with Smooth Surfaces on Both Sides
For thick-walled board castings with smooth surfaces, three conventional sand casting schemes exist. When using the large flat surface placed horizontally (top surface upward), the molten metal strongly radiates heat onto the upper mold surface, causing rapid expansion and possible arching or cracking of the sand, leading to sand inclusions, scabs, and slag defects. If the gating system is not properly designed for slag trapping, surface slag defects also occur. This scheme is generally unsuitable.
Alternatively, placing the large flat surfaces on the sides reduces sand inclusions and scabs but increases flask height, reduces sand utilization, requires larger draft angles, and introduces operational inconveniences. Under certain conditions, this scheme may be considered.
The third scheme, known as “flat pattern with inclined pouring,” alleviates the problems of the first two but demands careful pad sanding and is limited to small inclination angles.
For thin-walled board castings with smooth surfaces, the horizontal placement leads to different cooling rates between top and bottom surfaces, causing warpage. The inclined pouring scheme also induces differential cooling, while the side placement scheme largely avoids warpage.
1.2 Board Castings with Ribs on One or Both Sides
Board castings with reinforcing ribs are structurally designed to mitigate defects like sand inclusions, scabs, and warpage. However, care must be taken in rib thickness design; thin and large ribbed plates can still warp or develop cracks at rib-plate junctions if the process is improper.
Table 1 summarizes the main issues of different sand casting schemes for board castings.
| Scheme | Thick-walled smooth | Thin-walled smooth | Ribbed |
|---|---|---|---|
| Flat surface upward | Sand inclusions, scabs, slag | Warpage | Possible rib cracking |
| Flat surface to sides | Low sand utilization, large draft | Minimal warpage | Bulky flask |
| Inclined pouring | Pad sand difficulty | Moderate warpage | Works well |
2. Advantages of V-EPC for Board Castings
Dry sand vacuum lost foam casting (V-EPC) offers distinct benefits for board castings. The EPS pattern can be manufactured easily, either as a whole block or assembled from pieces, reducing pattern cost. Since there is no restriction from pattern draft, the pattern can be placed at any orientation, and gating systems can be arranged ideally. The issue of a large flat surface facing upward is eliminated because patterns are usually placed vertically or inclined within the flask. Moreover, multiple patterns can be stacked with separator sheets in a single flask, dramatically increasing productivity.
Table 2 compares sand casting and V-EPC for board castings.
| Parameter | Sand Casting | V-EPC |
|---|---|---|
| Pattern draft requirement | Required | Not required |
| Large flat surface orientation | Problematic | Easily placed sideways |
| Multiple-cavity production | Difficult | Simple stacking |
| Sand inclusion/scab risk | High for flat surfaces | Low |
| Warpage control | Difficult | Controllable by pattern orientation |
| Productivity | Moderate | High |
3. Case Study: Wear-Resistant Liner Board in V-EPC
A mining equipment manufacturer produced wear-resistant liner boards (material: ZGMn13) using sand casting. The boards suffered from sand inclusions, slag, scabs, and warpage. The company decided to switch to V-EPC. Initially, a vertical placement scheme was adopted as shown in the typical process layout (conceptual drawing not shown here). Two boards were placed side by side with their tooth surfaces facing each other. The gating system was bottom-gated, open design. Vacuum was maintained at 0.05–0.07 MPa. Pouring temperature for high-manganese steel was 1410–1430°C.
3.1 Defects in Initial V-EPC Scheme
The castings exhibited warpage: the ribbed surface (side A) bulged outward, while the tooth surface (side B) curved inward. Additionally, a small amount of slag inclusions appeared at the top region (side C). Although the slag did not affect service performance, it degraded appearance.
Warpage was attributed to differential cooling: side B with thicker teeth cooled slower than side A with thinner ribs, generating thermal stress. The thermal stress can be expressed as:
$$ \sigma = E \alpha \Delta T $$
where \(E\) is Young’s modulus, \(\alpha\) is thermal expansion coefficient, and \(\Delta T\) is temperature difference. To reduce warpage, equalizing cooling rates (simultaneous solidification) is required.
The slag inclusions at the top resulted from lightweight contaminants (sand grains, coating debris) floating upward during filling. Although vacuum helps retain sand, some particles still migrate.
3.2 Improved Scheme Based on Simultaneous Solidification and Proportional Solidification Theories
Based on simultaneous solidification theory, I reoriented the pattern to tilt the board so that the thicker tooth surface faced upward (better cooling) and the ribbed surface faced downward (slower cooling). The tilt angle was determined through trials to equalize cooling. Additionally, based on proportional solidification theory, I added a slag collector (riser-like pocket) at the top of the casting to trap contaminants. The collector was designed for easy removal during fettling.
The solidification time of a casting can be approximated by Chvorinov’s rule:
$$ t_s = K \left( \frac{V}{A} \right)^2 $$
where \(t_s\) is solidification time, \(V\) is volume, \(A\) is surface area, and \(K\) is a mold constant. By adjusting orientation, the effective cooling area for each side changes, thereby modifying \(V/A\) ratio to achieve similar \(t_s\) for both sides.
Table 3 summarizes the defects and countermeasures.
| Defect | Cause | Countermeasure |
|---|---|---|
| Warpage | Uneven cooling due to asymmetric section thickness | Pattern tilt; adjust orientation to balance cooling rates |
| Slag inclusions at top | Light contaminants floating upward | Add slag collector at top; improve coating quality |
| Surface sand adhesion | Coating failure or improper vacuum | Optimize coating thickness and vacuum level |
4. Further Consideration of Solidification Kinetics
The heat transfer during V-EPC is more complex due to the presence of vacuum and the decomposition of the foam pattern. The heat flux can be described by Fourier’s law:
$$ q = -k \nabla T $$
where \(q\) is heat flux, \(k\) is thermal conductivity of the sand (enhanced by vacuum). The effective thermal conductivity of dry sand under vacuum is lower than that of conventional sand, which reduces the cooling rate and provides a more uniform temperature distribution. This is beneficial for controlling warpage in lost foam castings.
The pressure in the mold cavity during pouring is given by the vacuum level and the gas evolution rate from pattern decomposition. The pressure differential \(\Delta P\) across the coating layer drives the removal of decomposition products:
$$ \Delta P = P_{\text{atmos}} – P_{\text{vac}} $$
Typical vacuum pressure is 0.05–0.07 MPa (gauge), which corresponds to absolute pressure of about 0.03–0.05 MPa. This pressure difference must be sufficient to allow gases to escape through the coating and sand, preventing backpressure that could cause defects.
5. Experimental Validation and Results
After implementing the improved V-EPC process with pattern tilt and slag collector, the produced lost foam castings showed no warpage and minimal slag defects. The slag collector successfully trapped the contaminants, and after removal, the casting surfaces were clean. Dimensional accuracy improved significantly. The yield rate increased from about 70% in sand casting to over 95% with V-EPC.
Table 4 presents the quantitative comparison of defect rates.
| Defect type | Sand casting (%) | V-EPC (optimized) (%) |
|---|---|---|
| Sand inclusions/scabs | 12 | <1 |
| Slag inclusions | 8 | <0.5 |
| Warpage | 15 | <2 |
| Overall scrap rate | 30 | <5 |
6. Discussion
The success of V-EPC for board castings lies in its flexibility to control pattern orientation. Unlike sand casting where large flat surfaces are problematic, lost foam castings allow the pattern to be placed vertically or tilted, completely avoiding the “flat surface upward” issue. The ability to stack multiple patterns in one flask also boosts productivity. Furthermore, the application of simultaneous solidification theory through pattern tilt effectively minimizes thermal stress and warpage. The proportional solidification concept, implemented via a slag collector, ensures that residual impurities are isolated from the main casting.
For lost foam castings of steel, attention must be paid to carbon pickup from the EPS pattern. However, with proper coating and vacuum control, carbon pickup can be limited. In this study, the ZGMn13 liners met specification without excessive carbon.
7. Conclusion
Dry sand vacuum lost foam casting (V-EPC) is particularly advantageous for board castings of various sizes and thicknesses. By properly orienting the pattern and incorporating slag collectors based on simultaneous and proportional solidification principles, common defects such as sand inclusions, slag, and warpage can be effectively eliminated. The productivity and quality of lost foam castings for board-type parts surpass those of traditional sand casting. Therefore, V-EPC is an effective and recommended method for producing high-quality board castings.
Future work will focus on optimizing the tilt angle using simulation and extending the method to more complex board geometries.