Sand Mold Processes
Fundamentally, a mold is produced by shaping a refractory material to form a cavity of a desired shape such that molten metal can be poured into the cavity. The mold cavity needs to retain its shape until the metal has solidified and the casting is removed. This sounds easy to accomplish, but depending on the choice of metal, certain characteristics are demanded of the mold.
Green Sand Molding
The most common method used to make metal castings is green sand molding. In this process, granular refractory sand is coated with a mixture of bentonite clay, water and, in some cases, other additives. The additives help to harden and hold the mold shape to withstand the pressures of the molten metal.
The green sand mixture is compacted through mechanical force or by hand around a pattern to create a mold. The mechanical force can be induced by slinging, jolting, squeezing or by impact/impulse.
The following points should be taken into account when considering the green sand molding process:
- for many metal applications, green sand processes are the most cost-effective of all metal forming operations;
- these processes readily lend themselves to automated systems for high-volume work as well as short runs and prototype work;
- in the case of slinging, manual jolt or squeeze molding to form the mold, wood or plastic pattern materials can be used. High-pressure, high-density molding methods almost always require metal pattern equipment;
- high-pressure, high-density molding normally produces a well-compacted mold, which yields better surface finishes, casting dimensions and tolerances;
- the properties of green sand are adjustable within a wide range, making it possible to use this process with all types of green sand molding equipment and for a majority of alloys poured.
Chemically Bonded Molding Systems
This category of sand casting process is used widely throughout the metalcasting industry because of the economics and improved productivity each offers. Each process uses a unique chemical binder and catalyst to cure and harden the mold and/or core. Some processes require heat to facilitate the curing mechanism, though others do not.
Gas Catalyzed or Coldbox Systems—Coldbox systems utilize a family of binders where the catalyst is not added to the sand mixture. Catalysts in the form of a gas or vapor are added to the sand and resin component so the mixture will not cure until it is brought into contact with a catalyst agent. The sand-resin mixture is blown into a corebox to compact the sand, and a catalytic gas or vapor is permeated through the sand mixture, where the catalyst reacts with the resin component to harden the sand mixture almost instantly. Any sand mixture that has not come into contact with the catalyst is still capable of being cured, so many small cores can be produced from a large batch of mixed sand.
Several coldbox processes exist, including phenolic urethane/amine vapor, furan/SO2, acrylic/SO2 and sodium silicate/CO2. In general, coldbox processes offer:
- good dimensional accuracy of the cores because they are cured without the use of heat;
- excellent surface finish of the casting;
- short production cycles that are optimal for high production runs;
- excellent shelf life of the cores and molds.
Shell Process—In this process, sand is pre-coated with a phenolic novalac resin containing a hexamethylenetetramine catalyst. The resin-coated sand is dumped, blown or shot into a metal corebox or over a metal pattern that has been heated to 450-650F (232-343C). Shell molds are made in halves that are glued or clamped together before pouring. Cores, on the other hand, can be made whole, or, in the case of complicated applications, can be made of multiple pieces glued together.
Benefits of the shell process include:
- an excellent core or mold surface resulting in good casting finish;
- good dimensional accuracy in the casting because of mold rigidity;
- storage for indefinite periods of time, which improves just-in-time delivery;
- high-volume production;
- selection of refractory material other than silica for specialty applications;
- a savings in materials usage through the use of hollow cores and thin shell molds.
Nobake or Airset Systems—In order to improve productivity and eliminate the need for heat or gassing to cure mold and core binders, a series of resin systems referred to as nobake or airset binders was developed.
In these systems, sand is mixed with one or two liquid resin components and a liquid catalyst component. As soon as the resin(s) and catalyst combine, a chemical reaction begins to take place that hardens (cures) the binder. The curing time can be lengthened or shortened based on the amount of catalyst used and the temperature of the refractory sand.
The mixed sand is placed against the pattern or into the corebox. Although the sand mixtures have good flowability, some form of compaction (usually vibration) is used to provide densification of the sand in the mold/core. After a period of time, the core/mold has cured sufficiently to allow stripping from the corebox or pattern without distortion. The cores/molds are then allowed to sit and thoroughly cure. After curing, they can accept a refractory wash or coating that provides a better surface finish on the casting and protects the sand in the mold from the heat and erosive action of the molten metal as it enters the mold cavity.
The nobake process provides the following advantages:
- wood, and in some cases, plastic patterns and coreboxes can be used;
- due to the rigidity of the mold, good casting dimensional tolerances are readily achievable;
- casting finishes are very good;
- most of the systems allow easy shakeout (the separation of the casting from the mold after solidification is complete);
- cores and molds can be stored indefinitely.