Choosing Between Vertical and Horizontal Green Sand Casting Processes
Sand casting is often thought of as a process for small to medium quantities, but with one modification it’s an economical process for large quantity production. That modification is to part the mold vertically rather than horizontally. Understanding why this makes a difference, and how you might benefit from it, requires a closer look at the two green sand casting methods: horizontally-parted vs. vertically-parted casting processes.
One Mold, One Part
Sand casting entails creating a cavity in sand and then filling it with liquid metal. The metal solidifies to form the part, after which the sand is broken away and reused. The cavity is formed by a pattern that, with the addition of casting allowances and draft angles, replicates the part to be made.
As the pattern has to come out of the sand to make way for the metal, the mold cavity is always made in two halves. Sand is packed around one half and the pattern removed. Then more sand is packed around the other side and the two halves are locked together.
In most casting operations the sand is called, “green sand”. “Green” in this context refers to it having a degree of moisture that binds the particles together. The alternative is a resin sand. This is coated with a binding material that holds it all together. Resin sand casting is a little more expensive than green sand, but yields a smoother surface finish and higher geometric accuracy. That in turn enables casting of thinner walls and finer features.
Horizontal Vs. Vertical Parting
Historically, sand casting was done with a horizontal split between the two halves of the mold. The bottom half is a box called the drag and the top half is the cope. The pattern is used to make a cavity in the top half of the drag and the bottom of the cope. Together they are referred to as the flask.
In the 1960s a variation of this process was developed where the parting line between the two halves is vertical. This eliminates the flask, reduces the amount of sand needed, and can be automated for higher production rates.
The pattern is made in two halves, which face each other vertically. Sand is deposited between them, and then they close up to create cavities in both sides of the sand. One pattern, referred to as the swing plate, lifts up and the other pattern – the pressure plate – pushes the formed sand cavity out.
The patterns then go back into their start position, more sand is dropped between them, and the process repeats. The sand molds form a line that pass under the metal pouring station. Once filled they continue moving as the metal solidifies. At the end of the line the sand is shaken out and the cast parts retrieved.
Pros and Cons of Horizontal vs. Vertical Sand Casting
The big advantage of the vertical technique is that it’s capable of higher production rates than the traditional horizontal method. In addition, it produces consistently high quality castings. The downsides are that the equipment requires substantial capital investment and pattern tooling is more expensive than for the horizontal method.
In contrast, horizontal sand casting is very cost-effective for smaller quantity production. The quality may be slightly more variable but the process can produce larger and heavier castings than would usually be made on an automated vertical casting line.
Selecting the Right Green Sand Casting Process for Your Parts
When either large or small quantities of castings are needed it’s usually obvious which process to use. However, there are many parts where the projected volumes mean the decision requires a combination of analysis and experience. The sand casting specialists at Impro have deep experience with both methods and are well-qualified to advise on the best approach. If you’re exploring options for producing parts by sand castings, ask what we recommend.