Excellent sand casting design is the key to the success of the final product. Otherwise, it will be very costly. In addition to considering the budget, we must meet the actual functional requirements. That is, we can apply it to real product casting. Therefore, based on decades of casting experience, we found that there are eight crucial points in the sand casting design process. Below we will analyze them in detail one by one according to their importance.
Key elements of sand casting design:
- Draft angle
- Undercuts and cores
- Parting line
- Corners and angles
- Wall thickness
- Cross sections
- Junction design
- Casting allowances
Sand Casting Draft Angle
The sand casting draft angle refers to the angle perpendicular to the model that can easily remove the contents from the fragile and fragile sand mold without damaging the outer wall. The basis for judging the sand casting draft angle is the molding process of the product, how we design the casting, and the depth of the pattern inside the mold. Nevertheless, in sand casting design, many designers tend to overlook the importance of sand casting draft angle. Choosing suitable sand casting draft angle will increase the usability of the tapered surface in sand casting design.
In addition, it can reduce processing costs due to high metal flow. Therefore, the standard sand casting draft angle should comply with the ISO standard and will not affect the function of the sand casting design. In this way, your company can produce more and cheaper sand molds with consistent standards. Usually, the standard sand casting draft angle is 2 degrees. The minimum draft with external and internal features is about 1 degree.
Determining the position of the parting line is a crucial step in sand casting design. It is the dividing line of demoulding. We rely on it to distinguish whether we should change the direction. The engineering designer must understand the parting line placement and mark it on the casting drawing because it determines the final production cost and the quality of castings. It is not enough to rely solely on the practical experience of the metal foundry workers on placing the parting line.
We know that when placing the parting line, it should be as low as possible, and it should be wide, short, and flat. If the parting line does not match, the extension margin between it and the seam burrs should not exceed 0.020. In addition, the maximum seam flash extension material of the parting line we selected is about 0.015. Pay attention if the position of the parting line changes. The core usage, pouring position, casting weight, and dimensional accuracy will also be affected.
Undercuts and Cores
The undercut plays a role in the sand casting design to prevent removing the model during the mold manufacturing stage. The use of core will increase the time and cost of casting production. So we are in sand casting design should reduce the use frequency of core sand parts or completely avoid it. In fact, the definition of the parting line given by the experts in the early days is helpful for us to understand the characteristics of the product and avoid the undercut situation. Now that it is more refined, it is not clear about the balance and interaction between the various parts. Because of this, we need to understand the rules and standards of sand casting design.
Generally, a uniform cross-section or uniform wall thickness is an advantage. But it should not appear in sand casting design because sudden section changes are not feasible in the creation of many casting products. In principle, only the thicker part of the casting should not be formed in isolation when it is cooled, and the cross-section can be considered effective. That happens because these thicker parts take more time to cool down. When the surrounding metal has solidified, they remain intact. The thicker part will solidify next, but it cannot be screed from the surroundings. It will cause defects such as porosity or tearing in the casting. Therefore, we recommend that you discuss the thickness limit of your product with your local metal foundry before conducting a sand casting design.
In sand casting design, it is vital to obtain uniform solidification of the model and avoid the formation of cavities. So how to do this? This is related to the ratio of the volume to the surface area of the sand mold. Usually, we require that the solidification rate of castings is proportional to the square of their ratio. That’s because the part with a smaller volume-specific surface area solidifies faster during the casting of the product—Vice versa. Many metal foundries will increase the overall thickness or add some materials in the load-bearing position to solve this problem. However, the correct approach should be to use stiffeners and gussets. Because although the former increases the strength, the local thick wall is reduced. And in this way, the molten material in the sand mold will flow, and the heavy section will also be stressed or cracked.
Corners and Angles
As we all know, the cooling characteristics of the material used to make sand molds will also seriously affect the quality of sand castings. Therefore, this problem should be considered when sand casting design. When the cooling effect of the casting itself or the sand mold is not good, the intersection of the sharp corner and the corner of the part will locally generate heat. As a result, the casting feels a concentrated source of stress, which will deform, shrink and crack in the subsequent process, which will affect the quality of the final product. Sand casting mold design should avoid this.
Due to the particularity of sand casting mold design, the shape of the part can be very complicated. There are usually many connection points. We call them junction. There are five types of a junction: L, X, V, Y, and X-T. As mentioned earlier, these junctions also produce local mass concentrations. And there will be cracks, shrinkage, stretching, etc. The ideal sand casting design method is to reduce the excessive local mass concentration caused by the junction. In the end, it must fit into the product very smoothly.
First of all, we should understand that most metals undergo a shrinkage reaction during solidification, such as common steel, copper, aluminum, magnesium, and zinc. When considering the management of sand casting design, it should be noted that there should be a machining allowance at the interface between two sand castings. In other words, we should avoid curved edges at the interface of sand castings. That’s because their shrinkage is related to the material’s freezing point and the volume ratio to the surface area of the casting. For details, you can refer to the machining allowance recommendations issued by ISO 8062.