Permanent mold casting and sand casting can be thought of as two sides of the same coin: similar processes, with each best-suited for certain requirements and scenarios. Today, we’ll examine permanent mold casting more closely: What is it? When should it be used? What are its benefits?
What Is Permanent Mold Casting?
Permanent mold casting refers to the fact that the same mold is used for each casting cycle, rather than being broken away after each use as with sand casting. Although the mold is not truly “permanent” — as with any process and tooling, it will degrade over time and eventually need to be refurbished or replaced — the name applies to the continually used nature of the mold.
What Is the Permanent Mold Casting Process?
In a broad sense, the basis of the permanent mold casting process is similar to any casting process: A molten raw material is fed into a mold cavity, allowed to cool, and then removed from the mold. In the case of permanent molds, the mold is created in two halves and is separated when the material has cooled.
For a more detailed look at the process, as well as some variations on the basic, gravity-based feeding process, read on:
- Mold creation: A permanent mold will be created around a pattern, which is a single piece in the form to be cast, typically created through machining. Permanent molds are most often made from iron or steel.
- Mold preheating: Once the mold has been created and the process commences, the mold will be preheated to allow the molten material to flow freely through the mold, without premature cooling and solidification. Preheating has the benefit of attenuating some of the thermal shocks that naturally occur with molten material. A “cold” mold would react much more harshly to the vast difference in temperature of the raw material.
- Optional core placement: Cores can be placed within a permanent mold to achieve part features such as cavities and undercuts.
- Material pour: In this step, the liquefied raw material (such as aluminum) is poured into the mold cavity via one or more sprues. While this step can be carried out through the force of gravity alone, other methods such as low-pressure casting and vacuum casting are also available.
- Cooling: Based on the size and shape of each part, a minimum cooling time is required for full solidification. Once the cooling time has been reached, the next and final step occurs.
- Part removal/ejection: As mentioned above, permanent molds are created in two halves, which are separated once the part has completely solidified and is ready to be removed. The interior surfaces of the cavity are sometimes coated with a material to prevent the piece from sticking to the mold. Ejector pins may also be present in the mold. After ejection, any excess material, such as flash or material from runners, will be trimmed. From here parts will be moved through processes such as heat treat, age, and other necessary finishing requirements.
When Is Permanent Mold Casting Used?
The process is frequently drawn upon for parts requiring greater strength and durability. Some typical applications of permanent mold cast parts include:
- HVAC and ventilation components including fan blades and blowers
- Valve bodies
- Pump components
- Insert castings
- Electronic housings
- Brake castings
What Are the Benefits of Permanent Mold Casting?
If you’re choosing between sand and permanent mold casting, the following advantages of permanent mold casting may help make your decision a bit easier:
- Increased strength: The porosity of a sand mold plays a role in decreasing the strength of the finished part. Iron or steel permanent molds do not have this same issue.
- Smooth finish: Parts manufactured through permanent mold casting yields a smoother finish when compared to the sand casting process. Additional processes, such as polishing, can be used to increase the smoothness of either casting process.
- Accurate dimensions: Permanent molds are able to produce parts with tighter tolerances than sand.
- Tool life: Depending on the material used in casting, the pour temperature, mold maintenance, and more, permanent molds may remain effective for more than 50,000 cycles.
With this information, you’re now able to make a well-informed decision about the right mold casting process for you. If you have any additional questions, contact LeClaire Manufacturing today.