Sand Casting vs Die Casting
What is Sand Casting?
Sand casting is the most widely used casting process. It uses consumable sand to form complex metal parts that can be made from almost any alloy. Since sand molds must be destroyed to remove parts, called castings, sand mold casting usually has a low productivity.
The sand casting process includes the use of furnaces, metals, molds and sand molds. The metal is melted in the furnace and then poured into the cavity of the sand mold with a ladle. The sand mold is separated along the parting line and the solidified casting can be removed. The next section describes the steps in this process in more detail.
Sand casting is used to produce various complex metal parts. The size and weight of these parts vary greatly, from a few ounces to several tons. Some smaller sand mold parts include gears, pulleys, crankshafts, connecting rods and propellers.
Larger applications include housings for large equipment and heavy machine bases. Sand casting is also very common in the production of automotive parts, such as engine blocks, engine manifolds, cylinder heads and gearbox shells.
What is Die Casting?
Die casting is a manufacturing process that allows the production of geometrically complex metal parts by using reusable molds (called molds).
The die casting process includes the use of furnaces, metals, die casting machines and molds. The metal is usually a non-ferrous alloy such as aluminum or zinc, melted in a furnace, and then injected into the mold of the die casting machine.
There are two main types of die casting machines: hot chamber die casting machines (for low melting point alloys such as zinc) and cold chamber die casting machines (for high melting point alloys such as aluminum). The differences between these machines are explained in detail in the equipment and tools section.
However, in both machines, when the molten metal is injected into the mold, it quickly cools and solidifies into the final part, the casting. The next section describes the steps in this process in more detail.
The size and weight of the castings produced during this process vary greatly, from a few ounces to 100 pounds. A common application of die castings is the outer shell-thin-walled outer shell, which usually requires the installation of many ribs and bosses inside. The metal shells of various appliances and equipment are usually die-cast. Some automotive parts are also manufactured using die castings, including pistons, cylinder heads and engine blocks. Other common die castings include propellers, gears, bushings, pumps and valves.
The Process Cycle for Sand Casting
Mold-making – The first step in sand casting is to make molds for castings. In a consumable mold process, this step must be performed for each casting.
The sand mold is formed by loading sand into each half of the sand mold. Sand is filled around the pattern, which is a replica of the external shape of the casting. When the pattern is removed, the cavity that will form the casting remains.
Before the mold is formed, any internal features of the casting are formed by a separate sand core. More detailed information about mold making will be introduced in the next section. Mold making time includes positioning the mold, filling the sand, and removing the mold. Molding time is affected by part size, number of cores and sand type. If the mold type requires heating or baking time, the mold manufacturing time will increase significantly.
In addition, in order to facilitate the disassembly of castings, lubrication is usually performed on the surface of the mold cavity. The use of lubricants can also improve the fluidity of metals and the surface finish of castings. The lubricant used is selected according to the temperature of the sand and molten metal.
Clamping – Once the mold is formed, it must be ready for casting molten metal. First lubricate the surface of the mold cavity to facilitate the removal of the casting. Then, the core is positioned, the mold halves are closed and firmly clamped together. The mold halves must be kept firmly closed to prevent any loss of material.
Pouring – The molten metal is maintained at the set temperature in the furnace. After the mold is clamped, the molten metal can be scooped from the vessel in the furnace and poured into the mold. The pouring can be performed manually or by an automatic machine. Enough molten metal must be poured to fill the entire cavity and all channels in the mold. The filling time is very short to prevent any part of the metal from solidifying prematurely.
Cooling – Once the molten metal injected into the mold enters the cavity, it begins to cool and solidify. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. The mold can only be opened after the cooling time has elapsed. The required cooling time can be estimated based on the wall thickness of the casting and the temperature of the metal. Most of the possible defects are the result of the solidification process. If part of the molten metal cools too quickly, parts may shrink, crack, or be incomplete. When designing parts and molds, precautions can be taken and will be discussed in later chapters.
Removal – After the predetermined solidification time has passed, the sand mold can be simply broken and the casting can be removed. This step, sometimes called shaking out, is usually done by a vibrating machine, which shakes out the sand and pours it out of the flask. Once removed, some sand and oxide layers may adhere to the surface of the casting. Shot peening is sometimes used to remove any residual sand, especially from the inner surface, and to reduce surface roughness.
Trimming – During the cooling process, the material in the mold channel solidifies and adheres to the part. The excess material must be cut from the casting manually by cutting or sawing or using a trimming press. The time required to trim excess material can be estimated based on the size of the casting shell. Larger castings require longer trimming time. The scrap generated by this dressing is either discarded or reused in the sand casting process. However, the scrap may need to be readjusted to the appropriate chemical composition before it can be used in conjunction with non-recyclable metals.
The Process Cycle for Die Casting
Clamping – The first step is to prepare and clamp the two halves of the mold. Each mold half is first cleaned from the last injection and then lubricated to facilitate the discharge of the next part. Lubrication time increases with the increase in part size, cavity and the number of side cores. In addition, depending on the material, lubrication may not be required after each cycle, but may be required after 2 or 3 cycles. After lubrication, the two mold halves connected in the die casting machine are closed and clamped firmly together.
When injecting metal, sufficient force must be applied to the mold to make it close securely. The time required to close and clamp the mold depends on the machine-larger machines (machines with greater clamping force) will require more time. This time can be estimated based on the dry cycle time of the machine.
Injection – The molten metal is kept at a set temperature in the furnace and then transferred to a cavity where the mold can be injected. The method of transferring molten metal depends on the type of die casting machine, regardless of whether a hot chamber die casting machine or a cold chamber die casting machine is used. The difference of this kind of equipment will be explained in detail in the next section.
Once transferred, the molten metal is injected into the mold under high pressure. Typical injection pressure range is 1000 to 20000 psi. During solidification, this pressure keeps the molten metal in the mold. The amount of metal injected into the mold is called a projectile. The injection time is the time required for the molten metal to fill all channels and cavities in the mold.
This time is very short, usually less than 0.1 second, to prevent premature solidification of any part of the metal. The appropriate injection time can be determined by the thermodynamic properties of the material and the wall thickness of the casting. Larger wall thickness requires longer spray time. In the case of using a cold chamber die casting machine, the spraying time must also include the time of manually wrapping the molten metal into the shot blasting chamber.
Cooling – Once the molten metal injected into the mold enters the mold cavity, it begins to cool and solidify. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. After the cooling time is over, the mold can only be opened after the casting is solidified. The cooling time can be estimated based on the thermodynamic properties of the metal, the maximum wall thickness of the casting, and the complexity of the mold. Larger wall thickness requires longer cooling time. The geometric complexity of the mold also requires longer cooling times because of the additional resistance to heat flow.
Ejection – After a predetermined cooling time, the mold half can be opened, and the injection mechanism can push the casting out of the mold cavity. The time to open the mold can be estimated based on the dry cycle time of the machine, the injection time is determined by the size of the casting shell, and should include the time when the casting falls off the mold. The ejection mechanism must apply a certain force to eject the part, because the part shrinks and adheres to the mold during the cooling process. Once the casting is ejected, the mold can be clamped for the next injection.
Trimming – During the cooling process, the material in the mold channel will solidify and adhere to the casting. The excess material and any burrs must be cut from the casting manually by cutting or sawing or using a trimming press. The time required to trim excess material can be estimated based on the size of the casting shell. The scrap generated by such trimming is either discarded or can be reused during the die casting process. Recycled materials may need to be readjusted to the appropriate chemical composition before they can be combined with unrecycled metals and reused during the die casting process.
Advantages of Sand Casting
Design flexibility – the size and weight of parts range from a few millimeters and a few grams to a few meters and a few tons. The size and weight of castings are only limited by the handling and supply of molten metal.
High complexity shapes – no other process can provide the same range of possibilities as casting to shape complex features, and casting can produce components that are close to net shape.
Wider material choice – almost all types of engineering alloys can be cast as long as it can be melted.
Low cost tooling – Compared with some other metal manufacturing processes, the cost of tools and equipment is lower. Therefore, it becomes one of the cheapest ways to achieve near net shape components.
Short lead time – Compared with other products, the lead time is short, so it is very suitable for short time production.
Disadvantages of Sand Casting
Low strength – compared to machined parts, due to the high porosity, the material strength is low.
Low dimensional accuracy – due to shrinkage and surface finish, dimensional accuracy is very poor.
Poor surface finish – due to internal sand mold wall surface texture.
Defects unavoidable – like any other metallurgical process, there are inevitable defects or quality changes, such as shrinkage, porosity, casting metal defects, surface defects.
Post processing – if tighter tolerances are required to connect with other mating parts, a secondary machining operation is usually required.
Advantages of Die Casting
High production efficiency – the production process is easy to mechanize and automate. Generally speaking, cold chamber die casting machines die-cast 50 to 90 times per hour. Hot chamber die casting machines die-cast 400 to 900 times per hour.
Excellent dimensional accuracy and smooth cast surfaces – the general tolerance level of die casting is IT13 ~ IT15 in GB / T 1800-2009, and the accuracy is as high as IT10 ~ W 11. The surface finish (Ra) is 3.2 ～ 1.6μm, and the local can reach 0.8μm. Due to the high dimensional accuracy and high surface finish of the die-casting parts, the die-casting parts with lower requirements can be directly used, which avoids machining or less machining, and improves the alloy Utilization rate, saving a lot of machining costs.
High mechanical properties of die castings – the metal melt cools rapidly during the die casting process and crystallizes under pressure, so a layer of grains close to the surface of the die castings is small and dense. Both strength and hardness are high.
Die-casting complex thin-walled parts – compared with sand castings and metal castings, die castings can have complex part shapes, and the wall thickness of the parts can be smaller. The minimum wall thickness of aluminum alloy die castings is 0.5mm. The minimum wall thickness of zinc alloy die castings can reach 0.3mm.
Parts of other materials can be embedded in die castings – complex fasteners or inserts can be included in the final component. Therefore, precious material cost and processing cost are saved. Parts with complex shapes can be obtained to improve the performance of the parts. In addition, assembly work is reduced.
Economical and easy for mass production – produce-durable and dimensionally stable die castings with a specific shape, so no or less machining is required.
Variety of surface textures – die castings can have many finishing techniques and surfaces. This process can obtain smooth or textured parts surface.
Disadvantages of Die Casting
Porosity is easily generated in die castings – because the metal melt fills the mold cavity at a very high speed during the die casting process, and the mold material is not breathable. The die-casting parts produced by the general die-casting method are prone to air holes. Due to the existence of pores, die castings cannot be strengthened by heat treatment, nor can they be used at high temperatures. At the same time, the machining allowance of the parts should not be too large, otherwise the hardened layer on the surface of the die-casting part will be removed and the surface of the die-casting part will be exposed.
Not suitable for small batch production – the type of die casting is complex and costly, so it is usually only suitable for large batch production. Small-scale production is not economical.
Low die life when die casting high melting point alloy – some metals (such as copper alloys) have high melting points, high requirements for resistance to thermal deformation and thermal fatigue strength of die casting materials, and relatively low die life. At present, the materials of die castings are mainly aluminum alloy, zinc alloy and magnesium alloy. Ferrous metals are rarely processed by die casting.
Large capital investment – compared with most other casting processes, the cost of foundry machines, molds and related equipment is very high. Therefore, to make die casting an economical process, mass production must be carried out.
Cost Drivers of Sand Casting
The material cost of sand casting includes metal cost, molten metal cost, casting sand cost and core sand cost. The cost of metal is determined by the weight of the part, calculated by the part volume and material density, and the unit price of the material. For larger part weights, the melting cost will also be higher, and is affected by the material, because some materials have higher melting costs. However, the melting cost is usually negligible compared to the metal cost. The amount of molding sand used, and the resulting cost, are also proportional to the weight of the part. Finally, the cost of core sand depends on the number and size of cores used to cast the part.
Production costs include various operations for casting parts, including core making, mold making, pouring, and cleaning. The cost of making the core depends on the volume of the core and the number of parts used for casting. When using automated equipment, the cost of mold manufacturing is not affected by part geometry. However, the inclusion of cores will slow down this process slightly, thereby increasing costs. Finally, the cost of pouring metal and cleaning the final casting are determined by the weight of the part. It takes longer to pour and clean larger and heavier castings.
Mold cost has two main components-the model and the core box. The cost of the pattern is mainly controlled by the size of the part (envelope and projected area) and the complexity of the part. The cost of the core box depends first on its size, which is the result of the number and size of cores used to cast the part. Very similar to the model, the complexity of the core will affect the time to manufacture this part of the tool (in addition to the core size), and thus affect the cost.
The number of casting parts also affects the mold cost. Larger production volumes will require the use of mold materials, both the model and the core box, will not wear out in the required number of cycles. The use of stronger or more durable tooling materials will greatly increase the cost.
Cost Drivers of Die Casting
The material cost is determined by the weight of the required material and the unit price of the material. The weight of the material is obviously the result of the part volume and material density; however, the maximum wall thickness of the part can also play a role. The weight of the required material includes the material that fills the mold channels. Thin-walled parts require a larger channel system to ensure that the entire part is filled quickly and evenly, thereby increasing the amount of material required. However, this extra material usually saves less material than the reduced part volume due to thin walls. Therefore, despite the larger channels, the use of thinner walls generally reduces material costs.
The production cost is mainly calculated based on the hourly rate and cycle time. The hourly rate is proportional to the size of the die casting machine used, so it is important to understand how the part design affects machine selection. Die-casting machines are usually expressed in terms of the tonnage of clamping force they provide. The required clamping force is determined by the projected area of the part and the pressure of the molten metal injection. Therefore, larger parts require greater clamping force and therefore more expensive machines. In addition, certain materials that require high spray pressure may require larger tonnage machines. The dimensions of the parts must also meet other machine specifications, such as clamping stroke, platen size, and shot peening capability. In addition to the size of the machine, the type of machine (hot and cold room) also affects the cost. The use of high melting temperature materials, such as aluminum, will require cold room machinery, which is usually more expensive.
The cycle time can be divided into injection time, cooling time and reset time. By reducing these times, production costs will be reduced. The injection time can be shortened by reducing the maximum wall thickness of the part. Moreover, some materials are injected faster than others, but the injection time is very short, and the cost savings are negligible. Using hot chamber machines can save a lot of time because in cold chamber machines, the molten metal must be wrapped into the machine. The bandaging time depends on the weight of the shot. For lower wall thicknesses, the cooling time will also be shortened because they require less time to cool all the way. Some thermodynamic properties of the material also affect the cooling time. Finally, the reset time depends on the machine size and part size. Larger parts require larger movements of the machine to open, close, and eject parts, and larger machines require more time to perform these operations. In addition, using any edge core will slow down this process.
Tooling cost is mainly composed of two parts: mold group and cavity processing. The cost of the module is mainly controlled by the size of the part shell. Larger parts require larger and more expensive mold sets. The cost of machining the cavity is affected by almost all aspects of the part geometry. The main cost driver is the size of the cavity that must be processed, measured by the projected area of the cavity (equal to the projected area of the part and projection hole) and its depth. Any other component that requires additional processing time will increase the cost, including feature count, parting surface, side core, tolerance and surface roughness.
The number of parts and materials used will affect the life of the mold and thus the cost. Materials with a high casting temperature, such as copper, will cause a short die life. Zinc, which can be cast at lower temperatures, allows longer tool life. As production increases, this effect becomes more costly.
The last thing to consider is the number of lateral movement directions, which will indirectly affect the cost. The additional cost of the side core depends on the quantity used. However, the number of directions can limit the number of cavities that can be contained in the mold. For example, a mold that requires 3 parts in the direction of the side core can only contain 2 cavities. Although there is no direct cost increase, using more cavities can further save costs.