Casting is a widely used manufacturing process for producing metal parts and components. The two most common casting processes are sand casting and investment casting. Both processes have their own advantages and disadvantages. This article will analyze the differences between sand casting and investment casting in several aspects.
Sand casting, also known as sand molded casting, is a metal casting process that involves pouring molten metal into a mold cavity formed within a sand mold. The sand mold is made by packing sand around a pattern and then removing the pattern to leave the mold cavity. Sand casting is suitable for making large, low precision metal parts at low costs.
Investment casting, also called lost-wax casting, is a metal casting process that involves producing a wax pattern, creating a ceramic mold around the wax pattern, melting out the wax, and finally pouring metal into the hollow mold. Investment casting can produce small, complex, and high-precision metal parts. It is an expensive process compared to sand casting.
Both sand casting and investment casting have their unique advantages in different applications. By comparing them in terms of part size, surface finish, dimensional accuracy, design flexibility and cost, we can have a clearer understanding of their differences and make an informed choice between these two casting processes.
sand casting can produce much larger size metal parts than investment casting
The size limit of sand cast parts is much larger than investment cast parts. Sand casting is capable of producing metal parts weighing several tons, with lengths up to 2-3 meters. This is because the sand molding process can use large patterns and mold flasks without much restriction. On the other hand, the investment casting process is limited to smaller sized wax patterns and ceramic molds. The size limit is around 0.5-1 meter and 50 kg in weight. Beyond these limits, issues like pattern collapsing, mold cracking, and metal liquidity will arise. Therefore, when large metal parts are needed, sand casting should be the first choice over investment casting.
investment casting offers better surface finish than sand casting
Surface finish of the castings is an important consideration for many applications. In general, investment casting can produce cast metal parts with better surface finish compared to sand casting. This is attributed to the following reasons:
Firstly, the ceramic mold from investment casting can capture intricate details with good mold wall surface finish. While sand molds have relatively coarse surface due to sand grain structure.
Secondly, sand mold uses mold release agents that can burn-on imperfections onto the casting surface. Mold release is not needed for investment casting.
Thirdly, sand casting is more prone to gas and shrinkage porosity defects that roughen the surface. The fine ceramic mold in investment casting allows better control of porosity issues.
However, sand casting may utilize green sand that offers better surface finish than chemically bonded sand. And surface finishing operations like grinding and polishing can improve the finish of sand castings to required standard.
investment casting offers higher dimensional accuracy and tolerances than sand casting
Dimensional accuracy and tolerances are critical for metal parts that require precise fits or mating with other components. Investment casting generally offers higher dimensional accuracy and tighter tolerances compared to sand casting.
The primary factor is the different mold wall finishes – the coarse sand mold wall in sand casting restricts the casting accuracy to CT10-CT12, while the fine ceramic mold in investment casting can achieve CT6-CT8 accuracy.
Also, the collapsibility of sand molds can more easily introduce distortions. The rigid ceramic mold in investment casting maintains its shape and accuracy better. Things like pattern material and process parameters also play a role here.
However, sand casting has the advantage that sand is more permeable which avoids metal casting defects caused by mold gas entrapment. This enhances the consistency in dimensions for sand castings. If extremely high precision parts are needed, secondary machining operations may be required for both sand and investment castings.
investment casting offers better design flexibility than sand casting
Design flexibility refers to the capability of the casting process to produce complex part geometries and features. Investment casting offers far better design flexibility compared to sand casting.
The investment casting uses wax patterns that can be made into almost any geometry with little restriction. Complex internal passages and external shapes can be replicated precisely in the ceramic mold. Sand casting uses solid patterns which limit the complexity of cavity geometries. Limited overhangs, deep pockets and thin sections can be difficult to mold and cast.
Moreover, investment casting can produce castings with excellent surface finish, dimensional accuracy and little finishing allowance needed. This enables greater freedom in part design to fully exploit the process capability. While sand castings require more machining stock and design allowances restricting the design options.
However, sand casting also has some flexibility advantages with the option of multi-piece patterns and cores to produce complex sand mold cavities.
sand casting has much lower costs than investment casting for large volume production
When it comes to part cost, sand casting is generally more economical than investment casting, especially for large production volumes.
The sand casting method utilizes simple reusable patterns and sand molds that have low material and processing costs. While investment casting uses disposable wax patterns and ceramic molds that require costly materials, multiple steps and special equipment.
For small batch prototyping, investment casting may be cheaper due to no need for pattern making. But the breakeven point is typically around 50-100 units, beyond which sand casting becomes more cost effective. Also, the more complex the part geometry is, the higher the breakeven point would be.
Secondary processing required after casting also affects part cost. Sand castings often require extensive machining to achieve final dimensions. Investment castings require little finishing work and thus have lower associated costs.
In conclusion, sand casting and investment casting have their distinct advantages suitable for different applications. Sand casting is better for large size metal parts, while investment casting suits small, complex and high precision parts. By considering factors like part specifications, production volume and cost, the proper casting process can be selected.