Sand casting and investment casting are two common casting processes, each with its own advantages and disadvantages. The topic of investment casting vs sand casting is often discussed by manufacturers. Choosing which one to use for producing their castings is a dilemma that troubles many manufacturers when placing orders. To solve this problem, understanding the differences between the two casting processes becomes crucial. This article will provide a detailed introduction to the similarities and differences between investment casting and sand casting, along with their respective pros and cons, hoping to offer you some assistance.
Sand casting
Sand casting is a metal casting process that primarily uses foundry sand as the mould material. Foundry sand comes in various types, including commonly used silica sand and chromite sand etc. To enhance cohesion between sand particles, binders are typically added to the sand material. Depending on the type of binder used, sand casting processes can be categorised into primary methods such as clay sand, resin sand, and water glass sand casting. As a highly adaptable and relatively simple casting method, sand casting is one of the most widely used techniques. Statistics indicate that over 60% of metal castings are produced using sand casting processes.
Sand mould casting begins with the fabrication of a master pattern, typically made of wood or metal. The master pattern is then placed on a base plate, enclosed within upper and lower sand boxes, and filled with casting sand. After compaction, the pouring system channels are established. The lower box is rotated 180 degrees, the upper sand box is fitted, and sand is added again. The upper sandbox is subsequently removed, and the master pattern is extracted, completing the cavity formation. For parts with internal cavities, sand cores are inserted into the mould cavity. Finally, the moulds are assembled and secured using locating pins. Molten metal is then poured into the cavity through the pouring system. After complete cooling, a series of post-processing steps are performed, including sand removal, cleaning, gate and sprue removal, and grinding, ultimately yielding the target casting.
Investment Casting
Investment casting is a casting process based on the lost-wax technique.
The investment casting process begins with the creation of high-precision metal moulds. Wax is then injected into the mould and allowed to cool, forming a wax pattern. If the casting contains internal cavities, water-soluble or ceramic cores may be inserted. After trimming, the wax patterns are assembled into a wax tree with a pouring system to enhance production efficiency. The wax tree is then repeatedly dipped in ceramic slurry and coated with sand, layering until a sufficiently thick, rigid ceramic shell is formed. The completed shell assembly is subsequently placed in an autoclave for dewaxing. The melted wax is recycled, leaving a hollow ceramic shell. Molten metal is then poured into the preheated ceramic shell. After the metal cools and solidifies, the shell is removed to yield a casting tree with its pouring system intact. Following processes such as shot blasting, cutting, and grinding, the target casting is obtained.
The primary methods of lost-wax casting currently are the water glass method and the silica sol method. The latter is more costly but provides superior surface quality for the castings.
Investment Casting vs. Sand Casting Comparison
After gaining a basic understanding of investment casting and sand casting, we know both processes involve pouring molten metal into refractory moulds and allowing it to cool to produce metal castings. Next, we will conduct a more detailed discussion of their similarities and differences.
Casting Time
Compared to investment casting, sand casting (especially clay sand) offers a significant advantage in production cycle time, with relatively simpler mould fabrication. Investment casting, however, involves multiple processes, including pattern making, pattern assembly, shell construction, dewaxing, and metal melting, resulting in a production cycle considerably longer than sand casting.
Surface Quality
Overall, the surface quality of castings produced by investment casting far surpasses that of sand casting.
Sand castings typically exhibit surface roughness ranging from Ra 12.5 to 25 μm, featuring a visible granular texture. The finished parts usually have parting lines formed by the mould’s parting surface. Additionally, sand casting may result in surface defects such as sand inclusions, sand wash, and porosity. In contrast, investment casting achieves surface roughness as low as Ra 1.6–3.2 μm or even lower. This exceptional surface quality is fundamental to investment casting’s reputation.
Dimensional Tolerance
Investment casting achieves tighter dimensional tolerances than sand casting. Typically, sand casting operates within CT10–13 tolerance grades, meaning a 100mm dimension has a tolerance of approximately ±1mm. In contrast, investment castings can achieve tolerances of CT4-6, meaning a 100mm dimension has a tolerance of approximately ±0.4mm. Investment casting’s dimensional tolerances are one order of magnitude tighter than sand casting.
Due to the high precision and surface finish requirements for thin-walled components, sand casting is unsuitable for producing such parts. The minimum wall thickness achievable is typically 3-5 mm, whereas investment casting enables extremely thin walls of 0.5-1.0 mm.
Production Materials
Refractory materials used in investment casting—such as specialised waxes, zircon sand, and silica sol—carry higher costs than the silica sand and binders employed in sand casting.
Design Flexibility
The use of liquid slurry to form moulds in investment casting grants engineers exceptional design freedom. Whether producing hollow parts with internal passages or components featuring undercut designs, investment casting can achieve your requirements with high precision. While sand casting can produce some complex structures, the inherent limitations of parting lines and mould removal in sand casting inherently restrict design flexibility.
Castable Materials
Sand casting is suitable for nearly all casting alloys, ranging from steel and iron to brass and most non-ferrous alloys. Similarly, numerous materials can be used in investment casting. Beyond the aforementioned ferrous and non-ferrous metals, investment casting is widely employed for high-performance alloys with high melting points and difficult machinability, such as high-temperature alloys and titanium alloys.
Consistency in Mass Production
Since all investment casting wax patterns originate from the same mould, mass-produced parts exhibit exceptional consistency. In contrast, sand casting’s manual production process makes it challenging to guarantee the consistency of components in large batches. However, modern automated moulding lines can now produce hundreds or thousands of highly consistent castings.
Ease of Mould Modification
Sand casting requires lower costs and shorter lead times for mould modifications than investment casting. Sand moulds, typically made of wood or resin, are easier and more economical to alter. Although they are prone to damage during casting, new moulds can be produced swiftly. In contrast, investment casting’s metal moulds involve high fabrication costs and longer lead times than sand moulds. This is the primary reason for investment casting’s higher prototype development expenses.
Castable Weight & Dimensions
Sand casting imposes no upper limit on castable weight. Components ranging from a few kilograms to hundreds of tons can be produced. Consequently, sand casting remains a critical technology for heavy equipment manufacturing. Investment casting, constrained by processes like shell formation, is typically used for small to medium-sized castings, with weight ranges generally spanning from a few grams to approximately 100 kilograms. Casting extremely large parts is challenging.
Material Utilization
Both sand casting and investment casting allow for the recycling of their gating systems. In investment casting, the pouring system and initial wax patterns can be reused. The metal recovery rate can exceed 90%. Its near-net-shape characteristics reduce subsequent machining losses, further enhancing the material utilisation rate of investment casting. Although sand casting also involves a large gating system, the subsequent machining allowance is substantial, resulting in a metal recovery rate generally between 50% and 70%.
Post-Processing
Post-processing for investment casting is relatively straightforward. After shell removal, the primary steps involve cutting, a series of grinding operations, and heat treatment. Due to the inherent dimensional accuracy and surface finish of the castings, minimal machining is required. In contrast, sand casting post-processing is relatively complex, involving desanding, cutting away the large riser system, and grinding. Given the poor initial surface finish and dimensional tolerances, extensive machining is typically necessary to achieve the target dimensions and shape.
Common Defects
Defects arising during the casting process are an unavoidable aspect of any casting process. For sand casting, the quality control of moulding sand directly determines whether defects like sand holes (insufficient strength of sand casting material), gas pores, and shrinkage porosity (improper gate/sprue placement or cooling) appear on the casting surface. For investment casting, improper control during dewaxing or calcination may cause shell cracking. An ill-designed gate/sprue system can lead to shrinkage porosity on the casting surface.
Secondary Assembly
Investment casting’s single-piece advantage eliminates extensive assembly processes while reducing risks from assembly errors. Sand casting, however, often requires complex components to be divided into multiple simple parts for separate casting, followed by subsequent welding or mechanical joining for assembly. This increases secondary assembly workload and time costs.
Environmental Sustainability
Regarding environmental impact, while sand moulds can be recycled, sand casting generates substantial solid waste and dust during production. Although investment casting wax can be recovered, the large volume of discarded shells produced during shell formation constitutes industrial waste. Certain manufacturing stages may also introduce chemical pollution. Consequently, both processes require significant efforts in waste management.
Investment Casting vs Sand Casting Cost
Investment casting involves complex processes and extensive preparatory work, typically resulting in significantly higher costs than sand casting. The cost differences manifest specifically in the following aspects:
Mold Costs
Sand casting moulds are usually made from wood or plastic, which are extremely low-cost materials.
Investment casting moulds are high-precision steel or aluminium dies, which are very expensive.
Material Costs
Sand casting primarily uses inexpensive moulding materials like silica sand and clay, with most used sand being recyclable.
Investment casting requires specialised mould materials, such as proprietary waxes. Its shell-forming materials consist of refractory materials like zircon sand and silica sol, costing significantly more than foundry sand.
Processing Costs
The sand casting process is relatively simple with a short production cycle. Although automated moulding lines require significant initial investment, high-volume production reduces per-unit costs.
The investment casting process is lengthy and complex, with extended production cycles. The wax injection stage requires a constant-temperature environment, while dewaxing and calcination consume substantial energy.
Although investment casting has higher costs than sand casting in the above aspects, its near-net-shape advantage results in significantly lower post-processing costs compared to sand casting.
Sand Casting vs Investment Casting, Which One Is Right For You?
Although this article is titled “Investment Casting vs. Sand Casting,” the two processes are not in competition but rather complementary. The choice of which casting process to use for your casting project depends entirely on a comprehensive assessment of your part’s design complexity, precision requirements, order volume, and budget.
Generally speaking, investment casting is more suitable for parts with complex structures or those demanding exceptionally high surface quality. Its primary applications are concentrated in high-precision industries, including aerospace, medical devices, and petrochemicals. Sand casting, on the other hand, is ideal for producing simpler structures where strict surface quality requirements are not paramount. Sand-cast components are predominantly used in heavy machinery and infrastructure sectors.
Pingheng Machinery Offers Investment Casting Solution for You
Sand casting and investment casting are both casting processes with a long history. With the progress of the times, both have made significant advancements in casting procedures and material utilisation. Meanwhile, as a company with 26 years of deep expertise in the investment casting field, Pingheng Machinery has evolved from a foundry initially providing only rough castings to a one-stop investment casting solution provider. Leveraging our extensive casting experience and proven track record in order fulfilment, we can deliver satisfying solutions for your investment casting projects.
Embark on your next casting project with Pingheng Machinery.
