Excerpt from the March 2016 issue of Metalworking World Magazine
Sometimes, we believe foundries are similar. However, there are dozens of technical distinctions that differentiate green sand casting plants. Due to these large or small differences, we should define the technological advantages that transform a process from inefficient to concise. From this observation, foundry customers should know the productive processes in more depth to choose the most suitable one for producing the most competitive casting products.
There are several methods to obtain excellent green sand castings, no matter whether made of cast iron, steel, or aluminum. Some of the most popular methods include sand/resin, green sand, die- and lost-wax casting, as well as fully different processes like pressure die-casting. Each process features application pros and cons. In this article, we address the sand/resin casting process, also called “no bake”, a highly widespread solution because it allows:
- Low equipment costs (compared to other foundry processes)
- Possibility of obtaining very complex geometries
- Small batches
- Excellent finish
On the other hand, this process sometimes involves a higher cost per casting compared to other technologies, more easily automated or with fewer auxiliary materials to be managed. Therefore, it is especially suitable for limited samples and pre-series.
Sand casting represents a standard foundry machining. It is called this because the melted metal is cast into a mold composed of special molding sand, which at the process end breaks down to permit extraction of the piece. The first phase is the creation of a pattern made of various materials, around which the mold is implemented. They generally provide a symmetrical plan for the correct opening of the mold without damaging the pattern. Once the mold is complete with all necessary cores and cavities for the demanded geometry, they pore the liquid metal through gravitational force. After the solidification, it is necessary to clean the piece from the deposits and the residues of the casting. The molds after the casting are destroyed and must be treated. They are generally constituted by refractory materials or compacted sands. The used sands lose their qualities due to the effects of high-temperatures; therefore, it is possible to use them partially and only after treatment. Molding sands, when exhausted, can be used as base materials for further applications (for instance, production of bitumen), thus avoiding their disposal in dump.
SMC Group's Rimini foundry resin devision demonstrates the no bake process where they implement medium-to-small-size green sand iron castings. The resin division of the company is subdivided into three zones:
- the molding through no-bake process and painting
- the core assembling and fastening of flasks
- the shakeout
Before the flask filling, they position:
- Kalmins--commonly called risers
- the gate stick
- wood pieces as sand reinforcement
- chillers (used to avoid the occurrence of defects in critical areas of the casting)
Kalmins keep the cast iron hot and assure a correct casting feeding in the most critical areas: aluminum silicate or a mixture of resin and sand (exothermic Kalmins), characterized by a higher seal.
The hardening time that elapses between the end of the molding and the shakeout varies according to the type of alloy element and the catalyst (generally, from 40 to 60 minutes). After controlling the perfect mold hardening to avoid successive deformations, the mold is separated from the pattern and afterward painted with a refractory paint. An excessive hardening time of the mold or its excessive exposure to infrared radiation, caused by an excessive cooling time of the metal, can result in a detachment of the molding sand with the production of a piece differing from the desired product. Painting is necessary because the mold surface is excessively porous and subjected to serious problems of metal penetration, with unacceptable consequences for the component surface quality. The mold drying takes place in furnace.
The core assembling can be executed in chain or on the ground, according to the complexity of the core assembling itself. During the core assembly they arrange:
- filters (ceramic or sponge-type)
- Germaloy (inoculants in Fe-Si alloy)
- ogives (to grant a better centering of the two flasks)
- beads (to avoid the cast iron leakage)
- and make holes to allow gas to escape
Once the core assembly is complete, the upper semi-flask is rotated and superimposed on the lower one. Finally, the two semi-flasks are fastened, both with bolts and with apposite fasteners.
After fastened, flasks are positioned on the tracks in the casting zone: the fastening is fundamental to avoid metal leakages while casting, especially in the gate.
The cast molding box, after the metal has cooled at temperatures under 300 °C, is moved to the shakeout, where the piece is separated from the mold.
Siliceous sand is used for molding. An example of the chemical composition of this kind of sand, in the specific case called PMG sand, is reported in table 2. The sand is mixed with resin and acid. The nominal flow of sand is 530 kg/min, the resin one of 3.5 kg/min (therefore the resin is about the 0.66% in weight compared to the sand), while the acid one is included between 40 and 70% of resins.
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