Advantages of Permanent Mold Casting
The permanent mold casting process is the production of castings by pouring molten metal into permanent metal molds under gravity pressure. Castings produced in permanent molds have finer grain structure and better strength properties than those cast from similar alloys in sandcasting and diecasting (see advantages below).
Major savings through reduced machining costs are often obtainable because the castings have better dimensional uniformity, machining allowances are smaller, and holes may be accurately located and dimensioned with metal cores.
In addition, a smooth surface finish is achieved right out of the mold, therefore, eliminating some secondary polishing operations. The permanent mold process is frequently chosen because it can reduce secondary operations. Permanent metal molds have a production life up to 120,000 or more castings. Casting sizes range from a few ounces up to 50 pounds.
Permanent Mold Casting Process
The permanent mold casting process is the production of castings by pouring molten metal into permanent metal molds using gravity or tilt pouring. The process involves the following steps:
- A refractory wash or mold coating is brushed or sprayed onto those surfaces of the preheated mold (usually 250-500°F or 121-260°C) that will be in direct contact with the molten metal alloy. The proper operating temperature for each casting is set.
- Cores, if applicable, are inserted, and the mold is closed either manually or mechanically.
- The alloy, heated above its melting temperature, is poured into the mold through the gating system.
- After the casting has been allowed to solidify, cores and other loose mold members are withdrawn, the mold is opened, and the casting is removed.
- The usual foundry practice is followed for trimming gates and risers from the castings.
The basic difference between sand casting and permanent mold casting is the metal (or permanent) mold used in the p.m. process. Permanent mold castings usually have better mechanical properties than sand castings because solidification is more rapid and fill is more laminar. The basic difference between permanent mold and die-casting is that permanent mold is a gravity feed process versus injection yielding a denser casting.
Metal molds (or dies) usually are made of high-alloy iron or steel and have a production life of up to 120,000 or more castings.
It is desirable and generally more economical to use permanent steel cores to form cavities in a permanent mold casting. When the casting has re-entrant surfaces or cavities from which one-piece permanent metal cores cannot be withdrawn, destructive cores made of sand, shell, plaster and other materials are used. This variation is called the semipermanent mold method. Sectional steel cores are used in some instances.
Due to the chilling nature of a solid steel permanent mold tool, the castings are sounder than sand castings. For pressure-tight and nonporous castings, the permanent mold process is excellent. Permanent mold castings can be stronger than die and sand castings and less porous than die-castings.
Permanent mold aluminum castings generally range from a few ounces to more than 100 lbs. Aluminum castings weighing several hundred pounds have been produced commercially by the permanent mold process.
Permanent mold castings usually need less finish machining and polishing than sand castings. If the parting lines on the outside of a casting can be held to a minimum, high quality as-cast finishes approaching 100 rms. can be achieved and many aluminum castings with these good as-cast finishes are used without subsequent finishing for cooking utensils, hardware items, automotive parts, highway bridge railings and ornamental work.
Because of the various factors that must be considered when choosing a process, the decision to use permanent molding should be based on thorough engineering and production cost studies for each casting.
Castings produced in permanent molds have finer dendrite arm spacing (DAS) and grain structure and better strength properties than those cast from a similar alloy in sand do. Therefore, the casting often can be designed with thinner walls and lower weight. In addition, the cast surfaces are generally smoother than sand castings and closer dimensional tolerances can be maintained.
When properly produced, permanent mold castings can be subject to less shrinkage and gas porosity than sand castings. They do not ordinarily contain entrapped gas (such as in die-castings); thus, they are superior to die-castings in soundness and pressure tightness. For these reasons, permanent molding is sometimes chosen for better quality.
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