1.Location of the parting line/plane. By properly locating the parting plane.
2.The number of cores can be reduced.
3.The gating can be made less elaborate.
4.The material wasted can be reduced.
5.The dimensional accuracy can be increased.
6.Use of uniform thicknesses in a casting, where possible. Uniform thicknesses lead to uniform cooling and solidification. This leads to stress free and distortion free castings. Heavier sections cool more slowly, and may have shrinkage cavities, porosities and large grain structures. Voids, porosities and cracks can be sites of subsequent failures and should gestation be prevented by minimizing variations in cross sections.
7.When uniform cross-sections cannot be maintained, then changes in cross-sections must be gradual. A recommended way to achieve this is to use a transition radius of 1/3 of the thicker section and blend in the radius with a 15-degree slope line.
8.When two or more uniform sections intersect, they create a region of heavy cross-section, resulting in the problems mentioned earlier. One way to minimize this is to core the intersection by a hole, similar to a hub hole in a wheel with spokes.
9.When sections intersect to form continuous ribs, contraction occurs in opposite directions as the material cools down. This leads to a high stress area at the intersections, causing cracking immediately, or in service. The way to avoid this is to stagger the ribs and thereby maintain uniform cross-sections.
10.Large unsupported areas tend to warp, so they should be avoided.
11.In addition, a die casting,minimum wall thickness must be maintained to avoid voids and non-fill areas. See casting allowance table for minimum wall thickness for some common metals.
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2007年11月28日星期三
Sand casting in aluminium
A decorative plaque, a copy of an iron casting, approx imately 500mm by 350mm has been sand cast in aluminium by Norse Precision Castings of Bedford, demonstrating an ability to accurately reproduce very intricate detail. Specialising in complex shapes, in both aluminium and solid polyurethane, the key lies in the pattern making and the ability to produce near net shape finished components.
Sand casting lends itself to economic low volume production, even one-offs.
Working from originals, drawings or CAD files, castings up to one metre diameter are well within the production range of Norse Precision Castings and some specials have been produced two metres diameter.
Engineers at Norse Precision Castings are always available for technical and applications advise on all aspects of prototype and short production runs of aluminium castings. die casting ,pressure die casting,aluminum die casting,sand casting ,investment casting,lost wax casting,metal machinings,Aluminum Casting
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2007年11月21日星期三
plaster mold casting
Similar to plaster mold casting, the pattern used in ceramic mold casting is made of plaster, plastic, wood, metal or rubber. A slurry of ceramic is poured over the pattern. It hardens rapidly to the consistency of rubber. This can be peeled of the pattern, reassembled as a mold. The volatiles are removed using a flame torch or in a low temperature oven. It is then baked in a furnace at about 1000 °C (1832 °F) yielding a ceramic mold, capable of high temperature pours. Additionally, the pour can take place while the mold is until hot.
Tolerances can be held to 0.4 %, surface finishes can be better than 2 - 4 µm (.075 - .15 µin). Add 0.3 mm (.012 in) for parting line tolerances. Wall thickness can be as small as 1.25 mm (.050 in), and the weights can range from 60 g (2oz) to a ton. Draft allowance of 1° is recommended.
This process is expensive, but can eliminate secondary machining operations. Typical parts made from this process include impellers made from stainless steel, bronze, complex cutting tools, plastic mold tooling.
Tolerances can be held to 0.4 %, surface finishes can be better than 2 - 4 µm (.075 - .15 µin). Add 0.3 mm (.012 in) for parting line tolerances. Wall thickness can be as small as 1.25 mm (.050 in), and the weights can range from 60 g (2oz) to a ton. Draft allowance of 1° is recommended.
This process is expensive, but can eliminate secondary machining operations. Typical parts made from this process include impellers made from stainless steel, bronze, complex cutting tools, plastic mold tooling.
2007年11月16日星期五
Alternative casting methods
Sand casting for mass production has largely been superseded by other methods.
* Modern mass production methods can produce thin but accurate molds—of a material superficially resembling paper mache, such as is used in egg cartons, but that is refractory in nature—that are then supported by some means, such as dry sand surrounded by a box, during the casting process. Due to the higher accuracy it is possible to make thinner and hence lighter castings, because extra metal need not be present to allow for variations in the molds. These thin-mold casting methods have been used since the 1960s in the manufacture of cast-iron engine blocks and cylinder heads for automotive applications.
* Various automotive mechanical components are now frequently made of aluminum, which for appropriately shaped components may be made either by sand casting or by die casting, the latter an accurate process that greatly reduces both materials use and machining and finishing costs. While the material and the processing setup is more expensive than the use of iron this is one of the most straightforward ways to reduce weight in a vehicle, important as a contributor to both fuel economy and acceleration performance. For front engine vehicles with rear wheel drive the improvement in weight distribution can improve both handling and traction. For all configurations weight saved in the engine is multiplied in that this enables use of lighter suspension components which in turn improves suspension response by reducing unsprung weight
* Starting in the early 1980s, some castings such as automotive engine blocks have been made using a sand casting technique conceptually similar to the lost wax process, known as the lost foam process. In this process, the pattern is made of polystyrene foam, which the sand is packed around, leaving the foam in place. When the metal is poured into the mold, the heat of the metal vaporizes the foam a short distance away from the surface of the metal, leaving the molding cavity into which the metal flows. The lost-foam process supports the sand much better than conventional sand casting, allowing greater flexibility in the design of the cast parts, with less need for machining to finish the casting. This technique was developed for the clay mold casting of abstract art pieces and was first adopted for large quantity commercial production by the Saturn Corporation.
* Modern mass production methods can produce thin but accurate molds—of a material superficially resembling paper mache, such as is used in egg cartons, but that is refractory in nature—that are then supported by some means, such as dry sand surrounded by a box, during the casting process. Due to the higher accuracy it is possible to make thinner and hence lighter castings, because extra metal need not be present to allow for variations in the molds. These thin-mold casting methods have been used since the 1960s in the manufacture of cast-iron engine blocks and cylinder heads for automotive applications.
* Various automotive mechanical components are now frequently made of aluminum, which for appropriately shaped components may be made either by sand casting or by die casting, the latter an accurate process that greatly reduces both materials use and machining and finishing costs. While the material and the processing setup is more expensive than the use of iron this is one of the most straightforward ways to reduce weight in a vehicle, important as a contributor to both fuel economy and acceleration performance. For front engine vehicles with rear wheel drive the improvement in weight distribution can improve both handling and traction. For all configurations weight saved in the engine is multiplied in that this enables use of lighter suspension components which in turn improves suspension response by reducing unsprung weight
* Starting in the early 1980s, some castings such as automotive engine blocks have been made using a sand casting technique conceptually similar to the lost wax process, known as the lost foam process. In this process, the pattern is made of polystyrene foam, which the sand is packed around, leaving the foam in place. When the metal is poured into the mold, the heat of the metal vaporizes the foam a short distance away from the surface of the metal, leaving the molding cavity into which the metal flows. The lost-foam process supports the sand much better than conventional sand casting, allowing greater flexibility in the design of the cast parts, with less need for machining to finish the casting. This technique was developed for the clay mold casting of abstract art pieces and was first adopted for large quantity commercial production by the Saturn Corporation.
2007年11月13日星期二
Sand casting
Sand casting requires a lead time of days for production at high output rates (1-20 pieces/hr-mold), and is unsurpassed for large-part production. Green (moist) sand has almost no part weight limit, whereas dry sand has a practical part mass limit of 2300-2700 kg. Minimum part weight ranges from 0.075-0.1 kg. The sand is bonded together using clays (as in green sand) or chemical binders, or polymerized oils (such as motor oil.) Sand in most operations can be recycled many times and requires little additional input.
Preparation of the sand mold is fast and requires a pattern which can "stamp" out the casting template. Typically, sand casting is used for processing low-temperature metals, such as iron, copper, aluminum, magnesium, and nickel alloys. Sand casting can also be used for high temperature metals where other means would be unpractical. It is said to be the oldest and best understood of all techniques. Consequently, automation may easily be adapted to the production process, somewhat less easily to the design and preparation of forms. These forms must satisfy exacting standards as they are the heart of the sand casting process - creating the most obvious necessity for human control.
Plaster casting is similar to sand molding except that plaster is substituted for sand. Plaster compound is actually composed of 70-80% gypsum and 20-30% strengthener and water. Generally, the form takes less than a week to prepare, after which a production rate of 1-10 units/hr-mold is achieved with items as massive as 45 kg and as small as 30 g with very high surface resolution and fine tolerances.
Once used and cracked away, normal plaster cannot easily be recast. Plaster casting is normally used for nonferrous metals such as aluminium-, zinc-, or copper-based alloys. It cannot be used to cast ferrous material because sulfur in gypsum slowly reacts with iron. Prior to mold preparation the pattern is sprayed with a thin film of parting compound to prevent the mold from sticking to the pattern. The unit is shaken so plaster fills the small cavities around the pattern. The form is removed after the plaster sets.
Plaster casting represents a step up in sophistication and requires skill. The automatic functions easily are handed over to robots, yet the higher-precision pattern designs required demand even higher levels of direct human assistance.
Preparation of the sand mold is fast and requires a pattern which can "stamp" out the casting template. Typically, sand casting is used for processing low-temperature metals, such as iron, copper, aluminum, magnesium, and nickel alloys. Sand casting can also be used for high temperature metals where other means would be unpractical. It is said to be the oldest and best understood of all techniques. Consequently, automation may easily be adapted to the production process, somewhat less easily to the design and preparation of forms. These forms must satisfy exacting standards as they are the heart of the sand casting process - creating the most obvious necessity for human control.
Plaster casting is similar to sand molding except that plaster is substituted for sand. Plaster compound is actually composed of 70-80% gypsum and 20-30% strengthener and water. Generally, the form takes less than a week to prepare, after which a production rate of 1-10 units/hr-mold is achieved with items as massive as 45 kg and as small as 30 g with very high surface resolution and fine tolerances.
Once used and cracked away, normal plaster cannot easily be recast. Plaster casting is normally used for nonferrous metals such as aluminium-, zinc-, or copper-based alloys. It cannot be used to cast ferrous material because sulfur in gypsum slowly reacts with iron. Prior to mold preparation the pattern is sprayed with a thin film of parting compound to prevent the mold from sticking to the pattern. The unit is shaken so plaster fills the small cavities around the pattern. The form is removed after the plaster sets.
Plaster casting represents a step up in sophistication and requires skill. The automatic functions easily are handed over to robots, yet the higher-precision pattern designs required demand even higher levels of direct human assistance.
2007年11月8日星期四
Common Alloys in Die Casting
Aluminum, Zinc and Copper alloys are the materials predominantly used in die-casting. On the other hand, pure Aluminum is rarely cast due to high shrinkage, and susceptibility to hot cracking. It is alloyed with Silicon, which increases melt fluidity, reduces machinability. Copper is another alloying element, which increases hardness, reduces ductility, and reduces corrosion resistance.
Zinc can be made to close tolerances and with thinner walls than Aluminum, due to its high melt fluidity. Zinc is alloyed with Aluminum (4%), which adds strength and hardness. The casting is done at a fairly low temperature of 425 ºC (800 ºF) so the part does not have to cool much before it can be ejected from the die. This, in combination with the fact that Zinc can be run using a hot chamber process allows for a fast fill, fast cooling (and ejection) and a short cycle time. Zinc alloys are used in making precision parts such as sprockets, gears, and connector housings.
Zinc can be made to close tolerances and with thinner walls than Aluminum, due to its high melt fluidity. Zinc is alloyed with Aluminum (4%), which adds strength and hardness. The casting is done at a fairly low temperature of 425 ºC (800 ºF) so the part does not have to cool much before it can be ejected from the die. This, in combination with the fact that Zinc can be run using a hot chamber process allows for a fast fill, fast cooling (and ejection) and a short cycle time. Zinc alloys are used in making precision parts such as sprockets, gears, and connector housings.
2007年11月3日星期六
Sand Casting Technical introduction
Sand casting is used to make large parts (typically Iron, but also Bronze, Brass, Aluminum). Molten metal is poured into a mold cavity formed out of sand (natural or synthetic). The processes of sand casting are discussed in this section, include patterns, sprues and runners, design considerations, and casting allowance.
patterns
The cavity in the sand is formed by using a pattern (an approximate duplicate of the real part), which are typically made out of wood, sometimes metal. The cavity is contained in an aggregate housed in a box called the flask. Core is a sand shape inserted into the mold to produce the internal features of the part such as holes or internal passages. Cores are placed in the cavity to form holes of the desired shapes. Core print is the region added to the pattern, core, or mold that is used to locate and support the core within the mold. A riser is an extra void created in the mold to contain excessive molten material. The purpose of this is feed the molten metal to the mold cavity as the molten metal solidifies and shrinks, and thereby prevents voids in the main casting. sand casting, cast iron casting, aluminum sand casting, grey iron casting, ductile iron casting
In a two-part mold, which is typical of sand castings, the upper half, including the top half of the pattern, flask, and core is called cope and the lower half is called drag. The parting line or the parting surface is line or surface that separates the cope and drag. The drag is first filled partially with sand, and the core print, the cores, and the gating system are placed near the parting line. The cope is then assembled to the drag, andthe sand is poured on the cope half, covering the pattern, core and the gating system. The sand is compacted by vibration and mechanical means. Next, the cope is removed from the drag, and the pattern is carefully removed. The object is to remove the pattern without breaking the mold cavity. This is facilitated by designing a draft, a slight angular offset from the vertical to the vertical surfaces of the pattern. This is usually a minimum of 1?or 1.5 mm (0.060 in), whichever is greater. The rougher the surface of the pattern, the more the draft to be provided. sand casting, cast iron casting
Sprues and Runners
The molten material is poured in the pouring cup, which is part of the gating system that supplies the molten material to the mold cavity. The vertical part of the gating system connected to the pouring cup is the sprue, and the horizontal portion is called the runners and finally to the multiple points where it is introduced to the mold cavity called the gates. Additionally there are extensions to the gating system called vents that provide the path for the built up gases and the displaced air to vent to the atmosphere. The cavity is usually made oversize to allow for the metal contraction as it cools down to room temperature. This is achieved by making the pattern oversize. To account for shrinking, the pattern must be made oversize by these factors, on the average. These are linear factors and apply in each direction. These shrinkage allowance are only approximate, because the exact allowance is determined the shape and size of the casting. In addition, different parts of the casting might require a different shrinkage allowance. See the casting allowance table for the approximate shrinkage allowance expressed as the Pattern Oversize Factor. sand casting, cast iron casting
Sand castings generally have a rough surface sometimes with surface impurities, and surface variations. A machining (finish) allowance is made for this type of defect. See casting allowance table for the finish allowance.
patterns
The cavity in the sand is formed by using a pattern (an approximate duplicate of the real part), which are typically made out of wood, sometimes metal. The cavity is contained in an aggregate housed in a box called the flask. Core is a sand shape inserted into the mold to produce the internal features of the part such as holes or internal passages. Cores are placed in the cavity to form holes of the desired shapes. Core print is the region added to the pattern, core, or mold that is used to locate and support the core within the mold. A riser is an extra void created in the mold to contain excessive molten material. The purpose of this is feed the molten metal to the mold cavity as the molten metal solidifies and shrinks, and thereby prevents voids in the main casting. sand casting, cast iron casting, aluminum sand casting, grey iron casting, ductile iron casting
In a two-part mold, which is typical of sand castings, the upper half, including the top half of the pattern, flask, and core is called cope and the lower half is called drag. The parting line or the parting surface is line or surface that separates the cope and drag. The drag is first filled partially with sand, and the core print, the cores, and the gating system are placed near the parting line. The cope is then assembled to the drag, andthe sand is poured on the cope half, covering the pattern, core and the gating system. The sand is compacted by vibration and mechanical means. Next, the cope is removed from the drag, and the pattern is carefully removed. The object is to remove the pattern without breaking the mold cavity. This is facilitated by designing a draft, a slight angular offset from the vertical to the vertical surfaces of the pattern. This is usually a minimum of 1?or 1.5 mm (0.060 in), whichever is greater. The rougher the surface of the pattern, the more the draft to be provided. sand casting, cast iron casting
Sprues and Runners
The molten material is poured in the pouring cup, which is part of the gating system that supplies the molten material to the mold cavity. The vertical part of the gating system connected to the pouring cup is the sprue, and the horizontal portion is called the runners and finally to the multiple points where it is introduced to the mold cavity called the gates. Additionally there are extensions to the gating system called vents that provide the path for the built up gases and the displaced air to vent to the atmosphere. The cavity is usually made oversize to allow for the metal contraction as it cools down to room temperature. This is achieved by making the pattern oversize. To account for shrinking, the pattern must be made oversize by these factors, on the average. These are linear factors and apply in each direction. These shrinkage allowance are only approximate, because the exact allowance is determined the shape and size of the casting. In addition, different parts of the casting might require a different shrinkage allowance. See the casting allowance table for the approximate shrinkage allowance expressed as the Pattern Oversize Factor. sand casting, cast iron casting
Sand castings generally have a rough surface sometimes with surface impurities, and surface variations. A machining (finish) allowance is made for this type of defect. See casting allowance table for the finish allowance.
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