Basic Knowledge of Metal Casting
Metal casting has been widely used in our residential and industrial fields. Casting is the foundation of modern machinery manufacturing industry.
Casting process is to pour the melted metal into the mould, after the solidification by cooling, to get the curtain shape, size and performance of metal castings.
In 513 BC, we found the world’s oldest iron casting – Cast Jin Ding, weighing about 270 kg. Europe began the production of cast iron around the eighth century AD. The emergence of cast iron expanded the application range of metal castings. For example, in the 15th to 17th Century, Germany, France and other countries have laid cast iron pipes to the resident’s drinking water. After the industrial revolution in 18th century, steam engines, textile machines, railways and other industries have developed very fast, so iron casting developed into a new period of industrial services for large industries. The casting technology began to have great development.
In 20th century, the development of casting is very fast, one important factor is the product of new technology, requires better casting with a variety of mechanical and physical properties, while still have good machining properties; Another reason is the mechanical industry itself and other industries such as chemicals, instruments and other development, all these factors created a favorable material conditions for the foundry industry. Such as the detection devices, to ensure the casting quality improvement and stability, and to provide the condition to the casting theory development. The invention of electronic microscope helped people to deep into the metal of the microscopic world, exploration of metal crystal mystery.
During this period, there was large development for various metal castings with superior performance, such as ductile iron, malleable welded cast iron, low carbon stainless steel, aluminium, copper, aluminium, aluminium-magnesium alloy, titanium-based, nickel-based alloys , and the invention of the gray cast iron inoculation of new technology, so that metal casting has a wider adaptability.
After middle of 20th century, there was high pressure moulding with wet sand, chemical hardening sand moulding and core making, moulding and other special vacuum casting, shot blasting and other new technology to make castings with high shape, dimensional accuracy and good surface finish, foundry working conditions and environmental health is also greatly improved.
Since the 20th century, for foundry industry, gray iron inoculation treatment and chemical hardening sand moulding process have two new special significance. These two inventions has broken the traditional methods for thousands of years, and has opened up new areas to improve the competitiveness of the casting.
Casting generally is divided into ordinary and special castings. Ordinary sand casting, including wet sand, dry sand, chemical hardening sand categories. Special casting material according to the different shapes can be divided into two categories: natural sand and gravel as the main form of mineral materials, such as investment casting, shell casting, vacuum casting, clay-type casting, solid casting, ceramics casting, etc.; a class of metal as the main mold material, such as metal casting, centrifugal casting, continuous casting, pressure casting, low pressure casting.
Casting process can be divided into three basic parts, namely, preparation of metal casting, mold preparation and casting processing. Metal casting is the production of metal castings for the cast, which is a metallic element as the main ingredient, and adding other metals or non-metallic elements and the composition of the alloy, traditionally known as the casting alloy, there are cast iron, Steel and cast non-ferrous alloys.
After the cooling, the castings have to be removed for the gate, riser and metal burrs, so the clean-up process is necessary. Equipments for such work are shot blasting machines, gate riser cutting machines. Some castings for special requirements have to be treated by post-casting processing, such as heat treatment, plastic surgery, anti-rust treatment, rough machining and so on.
Casting is an economical method of rough shape, complex shape parts for more demonstrated its economy. Such as automotive engine block and cylinder head, ship propellers and exquisite works of art. Somewhat difficult to cut the parts, such as nickel-based alloy gas turbine parts can not be formed without casting method.
In addition, the casting of the part size and weight to adapt to a wide range of metal species is almost unlimited; parts in the mechanical properties in general also has the wear and corrosion resistance, shock-absorbing and comprehensive performance. Sometimes, you can not get those characters by other metal forming methods such as forging, rolling, welding and punching, etc. So casting methods used in the machine manufacturing industry produced rough parts, in the number and tonnage thus far is the greatest.
Casting product development trend is to ask the cast a better overall performance, higher accuracy, less margin and more smooth surface. In addition, energy requirements and restore the natural environment of the community are increasingly vocal. To meet these requirements, the new cast alloy will be developed, refining new processes and new equipment will accordingly appear.
With the continuous improvement of the electronics and test methods, foundry workers will have deeper research for the metal crystals and in-depth exploration, to improve casting performance and internal quality. Robotics and computer will also become very popular in the field of casting production and management applications. Metal castings will have a better development and wider applications in the future.
How to prevent sand holes in iron castings
As for the resin sand molding, water glass sand molding, although the intensity of the two molding are very high, sand holes will still appear in iron castings.
Reason: Sand cleaning is not thorough. There are residual sand in the mould. Secondly, the liquid iron wash the wall of mould to cause the sand runner.
Some iron foundries use painting to prevent the brushing, which takes the good results, but still there will be sand holes. First, because the brushing paint in the gates is more difficult. The second is the high pouring temperature, which cause paint erosion, and high temperature immersion will cause the sand holes too. Casting plant did not attach great importance to the gates design. Casting process ignored the erosion of the runner wall. All these factors lead to the casting sand holes and sand trachoma.
Improving Method: Using the ceramic pipes for the cast gates. This product easy to be used, with smooth flow, resistance to molten iron, molten steel erosion, does not absorb liquid steel, sawing of good features. You can simplify the molding process, eliminating the paint on the inside of the gate, and the gating system to improve the layout, to avoid sand washing, sand holes and sand defects in iron castings.
Pattern making is the heart of foundry practice.
Patterns for bonded sand molding:
Traditionally made of wood, patterns can be made of anything strong enough to withstand the pressures of being rammed in the sand.
There are a few considerations in making good patterns.
• Patterns need to have draft. That is to say the sides need to be slightly tapered so the pattern will release from the sand.
• Patterns cannot have undercuts that will prevent them being pulled from the sand. If your pattern must have an undercut, a two piece pattern can be used.
• Patterns should be painted to give a smooth surface and to seal them from moisture.
• Avoid sharp corners in your patterns. They make weak areas in the casting. Use a wax or putty to round off right angles (filets).
Allow your pattern to be slightly oversize to make up for the natural shrinkage in the metal you are casting. Consult a shrink table to determine how much oversize you need.
Sand castings mean the metal castings produced by sand molds. There are many types of sand castings, such as gray iron, ductile iron sand castings, carbon steel sand castings and aluminum sand castings etc.
There are many types of sands that can be used for sand casting process, including green sand, resin sand, pre-coated resin sand, water glass dry sand etc. Accordingly, the sand casting processes include floor molding, automatic molding and shell molding process.
In the total metal casting output in the whole world, 60% to 70% of metal castings are sand castings. Their unit weight could from dozens of gram to dozens of tons, and their dimensions could be from dozens of millimeters to several meters. Moreover, according to the castings produced by other casting processes, sand castings are the cheapest and have very high production rate.
However, sand castings have many defects, such as sand holes, air holes, shrinkage, cracks, uneven materials, deformation etc. These defects are the main disadvantages for sand castings. However, as for the sand castings made by shell molding process, their defects are very few. So, more and more clients have required their suppliers to use shell molding process.
For your reference, if your products need high dimensional tolerance for rough castings, then you should use shell molding process, although this process is more costly than others.
If your products are large, such as from 1 meter to 2 meter long, from 100 kg to 2 tons, then you should use the resin sand casting process.
If your products are larger than 2 tons, such as the dryer cylinder for paper machinery, then you still need to use the green sand casting process.
If you need very high production rate, such as the automotive parts, then you should use the automatic molding line.
It is easy to find the metal foundries for producing sand castings however, the technical levels have large difference between them., I think it is really not an easy word to produce qualified sand castings.
All of the following products were sand castings.
Ian M. Kay, Cast Metals Institute (CMI), Des Plaines, Illinois
Industry people will tell you that a casting only can be as good (and cost-effective) as its design. While there are numerous variables that impact casting quality, none can make up for a poor pattern design.
Thankfully, patternmakers have a few “tricks” in their toolbox to make designs more easily castable and, therefore, cost-effective. Draft, fillets and radii (each depicted in Fig. 1) are three such modifications that can be applied to foundry patterns to assist mold production and help eliminate casting defects. An explanation of each of these three “tricks” is given below:
• Draft—is the angle applied to the vertical surfaces of a pattern to aid in the removal of the pattern from the molding media;
• Fillets—are the rounding-out of internal corners of the pattern;
• Radii—are added to round-out the external edges of the pattern.
Since the casting is a replica of the pattern, draft, fillets and radii appear on the casting as well.
Design engineers should understand the reasons for these modifications and, when possible, consider them in their cast component designs. The result will be more easily manufactural, higher quality and more cost-effective castings.
Fig. 1. This drawing shows three of the tools—draft, fillet and radius—that patternmakers arm themselves with to improve the casting manufacturing process.
Fig. 2. The positive draft (l) will enable the pattern to be extracted from the sand mold after compaction. If a pattern was constructed with negative draft (r), it would tear the mold apart during the pattern removal process and cause a defect in the casting.
The term “draft” comes from the Old English word dragan, which means to pull or draw, as when a draft horse was used to pull a wagon.
Draft angles are normally factored into the pattern design to help the foundry extract the pattern from the mold prior to closing and pouring. Draft angles are used in most processes in which a pattern is withdrawn the flexibility of the rubber pattern allows it to be easily removed from the mold.
Some cast shapes have “natural” draft. This simply means that the inherent shape provides the necessary draft angle (Fig. 3), without requiring any further draft modification. Cylindrical shapes have natural draft if the diameter of the cylinder lies on the parting line of the mold (the parting line shows the separation of the casting into the two sides of the mold). Polygonal shapes also contain natural draft if the part is correctly oriented to the parting line of the mold.
The application of draft is not limited to patterns. Permanent mold processes (in which the mold consists of a reusable metal die) also require draft to facilitate the removal of the solidified casting from the die. Also, coreboxes may require draft to allow the sand core to be withdrawn from the box after the core has cured.
Fig. 3. These six component shapes, if used in the illustrated parting line orientation, do not require any additional considerations for draft.
Sharp, internal corners on a casting design should be avoided whenever possible. The small volume of mold media located in a casting’s internal corner quickly becomes superheated and limits the casting’s ability to transfer heat at this location. Most mold media are poor conductors of heat and, once heated to metal temperature, have no way of transferring the heat energy to other parts of the mold. This creates localized areas that solidify more slowly than the rest of the part, resulting in “hot spots” in the casting. Hot spots can cause many casting defects, such as hot tears, cracking, shrinkage, metal penetration and gas-related defects. Also, such sharp edges on the mold are prone to breaking off and can produce sand inclusions in the casting.
To avoid this problem, the patternmaker usually adds a concave junction, or fillet, where the two casting surfaces meet. This fillet blends the two intersecting surfaces and modifies what would otherwise be a harsh internal corner into a rounded corner. This effectively removes the small volume of sand in the corner, reducing the hot spot in the casting and eliminating the resulting problems. Fillets also aid in the removal of the pattern from the mold and strengthen the pattern.
The size of a fillet is determined by its radius—it is not measured by its width. Fillet sizes vary, but it is possible for a fillet to be too large. A large fillet adds extra metal to the junction, which also can create shrinkage concerns due to the volume of metal. The best design is to maintain uniform section thickness whenever possible.
A radius is the rounding of external edges of a pattern to aid in removing the pattern from the mold and eliminating the sharp edges from the casting. It also can make the casting more aesthetically pleasing than a component with sharper corners. Sharp external corners also can produce casting defects. External corners rapidly lose heat to the surrounding mold material and, as a result, the corner may not fill completely, resulting in a misrun defect.
In gray and ductile iron castings, the microstructure of the casting also may be affected due to the rapid loss of heat. Rapid cooling of the external corners often can produce an inadvertent chilled structure of white iron or iron carbides. These chilled edges are extremely hard and brittle and can result in reduced tool life or breakage during machining.