Customers almost never see the most important part of sand casting, because that is the model. The model forms a cavity in the sand, which is subsequently filled with molten metal. If the model is designed correctly, the sand casting process can produce high-quality parts that require minimal machining. If the design is wrong, you will always be troubled by casting defects, low production and customer complaints.
The sand casting model is the bridge between the design drawings and the final castings. The rationality of its design directly affects the shape accuracy, surface quality, defect rate and production cost of the castings. With the development of digital technology (such as 3D printing sand molds), the limitations of traditional models have gradually been broken, but its core role has always been the key to the success of sand casting.
1. Punching holes in the sand
Unlike die casting, which requires precision molds to be machined from large pieces of steel, sand casting only requires sand, two boxes and a wooden model. This makes it an affordable process for small and medium-volume orders.
The model nominally has the same size and shape as the finished part, but it needs to be adjusted to adapt to the sand casting process and the characteristics of the poured metal. In addition, gates, runners, sprues, and risers must also be included. These are channels for metal to flow into the cavity formed by the pattern in the sand.
The sand for sand casting is contained in two boxes. After the pattern is removed, the two boxes are stacked one on top of the other. The lower box, the lower mold, always has a void formed by the pattern. It will also have channels for metal to flow into.
The upper box, the upper mold, may not have any shape, in which case it is called a "flat back". However, most complex castings require partial shape to be formed in the upper mold, which can create cracks and parting lines in the casting.
2. Model considerations
There are several key points to consider when designing and making a model.
The model is released without damaging the sand - this is achieved by giving the sides of the model a draft angle instead of vertical.
The cavity fills smoothly - the gate is usually located where the cavity is filled from the bottom and is an opening that limits the speed of metal inflow. The best filling speed is when there is no turbulence or bubbles, and the metal does not freeze before reaching all the internal corners and surfaces.
Shrinkage and deformation allowance - Metal shrinks as it cools, so to produce a part of the required size, the model must create a slightly larger cavity. In addition, metal does not cool at the same rate everywhere.
Riser size - The riser is an open area on the top of the mold and is the last part to be filled. This provides a visual indication that the cavity is full. It also forms a reservoir for molten metal, which flows back into the cavity as the metal cools and shrinks in the cavity.
3. Key parameters of model design
Material selection: wooden mold (low cost, easy to process), metal mold (high durability), resin mold (high precision).
Parting surface design: It should be as simple as possible to reduce the number of sand mold blocks.
Draft angle: adjusted according to model height and material (1°~5°).
Shrinkage allowance: calculated according to metal type and casting size (usually 1%~3%).
Surface treatment: polishing or coating to improve the surface quality of the sand mold.
4. Plan how to cast the part
The first thing to consider is to ensure that the model has no pockets where air can be trapped when the cavity is filled. One aid to achieve this is to place the riser at the highest point. However, a feature of sand casting is that, unlike die casting, trapped air can penetrate the sand to some extent.
When the casting comes out of the sand, it comes with the gate and riser. These must be cut off before the part can be machined.
A split casting will have a parting line around the periphery. The fit tolerances of the upper and lower molds and the pattern placement tolerances mean that there may be a slight offset between the top and bottom halves of the casting.
The casting team will work with the part designer before the model is made to minimize the impact of these features on the machining and the final part.
In sand casting, the pattern (i.e. the mold used to make the sand mold) is a core element that determines the quality, accuracy and production efficiency of the casting.
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