Casting is a metal working process that converts molten metal into the desired shape by using a mold with negative impression.
In this guide, we will compare two vital metal casting processes – investment casting vs sand casting. So, before choosing any technique for your next project, read this guide.
Definition: Investment Casting and Sand Casting
The two processes involve pouring molten metal in mold. Usually, the differences comes in mold designs as you will see in the processes shortly.
Key Metal Casting Processes: Lost Wax Casting and Sand Casting
Steps in investment casting:
1. Pattern Making
This is typically done via three-dimensional printing using computer software, machining or injection molding.
2. Assembly
Involves making a cluster by attaching several wax patterns to a wax sprue allowing for multiple castings in a single pour. A cluster can have up to hundreds of wax patterns.
3. Making the Ceramic Shell
The cluster (tree) is thereafter inundated in a ceramic slurry that adheres to the wax pattern. After hardening, it is immersed in another slurry composition consisting smaller ceramic particles that fills the voids and smoothens the surface. You repeat his process until you achieve the desired thickness.
4. Dewaxing
Involves heating the cured ceramic molds in a furnace, usually upside down, to drain out the wax pattern by melting. The heating process is fast to initiate quick melting of the surface preventing mold stress application by the expanding wax.
5. Preheating
Executed at high temperatures exceeding 800 oC, preheating serves to eliminate any moisture content, wax traces and mold sintering.
It is important to stave off cooling.
6. Pouring and Solidification
At this stage, you fill up the mold cavity with the molten material before allowing cooling into solid state. The pouring process shouldn’t’ be too fast or too slow to prevent casting flaws such as inconsistent cooling.
7. Divesting
Here, the ceramic mold shell is removed to obtain the casting by vibrating, hammering, blasting or even dissolving. Next, cut off the sprue.
It is the sprue that provides the pathway for pouring molten metal.
8. Finishing
The casting is cleared of any ceramic traces by applying finishing techniques such as etching, sandblasting or grinding. An inspection process of the component follows, where it is examined for its fulfillment of desired standards.
The key sand casting stages include:
1. Make a Pattern
You can make your pattern using plastic or metal.
In making the pattern, it is made larger compared to the desired part to provided allowance for shrinkage and machining.
It is important to provide pathways for the transfer of the liquid metal in to the mold when designing the pattern. A system of gates and risers also controls the flow of molten material while providing vents for gas release.
2. Making the Core
Cores are useful in the molding of components with hollow features as they prevent the molten metal from filling such parts. In making the core, you employ a core box which consists strong material with good hardness, temperature tolerance and permeability.
3. Mold Creation
Usually, a mixture of binding agent and sand make a good mold.
These molds have two sections:
- Cope – which forms the top section
- Drag – it is the bottom section
In creating the mold, the pattern is set between the cope and drag before being removed leaving a desired impression. The core and drag are then reassembled having a cavity instead of the pattern with the addition of a core where necessary.
4. Filling the Mold
Filling the mold can involve direct injection of liquid metal into the mold system. A smooth flow reduces turbulence thus preventing flaws such as the formation of oxides. When handling very reactive metals, shielding gasses such as argon ae employed.
5. Casting Removal
Once the item cools and solidifies, you will disintegrate the mold.
The sand can always be reused in another sand casting process.
6. Finishing
The casting features the gates, runners and risers which are not part of the product and hence are cut off. Other finishing operations such as grinding, sandblasting and even machining can be undertaken to achieve the desired dimensional tolerances.
Choosing Metals to Cast
| Investment Casting | Sand Casting |
| Aluminum Alloys such as A-356, A-357, C-355, and F-357 | Aluminium Alloys e.g. Tenzaloy, Almag 35, A357.0, A356.0, A355.0, A319.0 |
| Carbon Steel, Tool Steel and Stainless Steel grades like Austenitic 300 and Martensitic 400 | Copper Alloys such as Bronze and Brass Alloys e.g. Red brass, Yellow Brass, C83300, C83600, C89833, C85400, C85700, C93700, C92600 |
| Titanium, Gold and Silver alloys | Steel (BS3100)
High tensile strength and wear resistance |
| Super Alloys like Nickel Alloys (High temperature Resilience) and Cobalt Alloys (Superior wear, oxidation and corrosion resistance) | Spheroidal Graphite Iron (BS EN 1563)
Heavy loads capability and impact resistance |
| Cast Iron e.g. Gray Iron and Ductile Iron (High strength and heat resistance) | Nickel and it alloys. |
| Copper Alloys e.g C-85800, C-87000, and C-95000. | Iron and Iron Alloys these may include gray iron, ductile iron or malleable iron |
| Magnesium Grades and Alloys: AM50A, AE42, AS41B, AZ91D and AM60B. | Zinc and its Alloys |
Types of Investment Casting Techniques vs Sand Casting Techniques
Investment Casting Techniques
The variations in investment casting are courtesy of the patterning material used and the way the pattern is made as follows:
Lost Foam Casting
Make use of polystyrene foam with the capacity for automation and thus large volume production. Polystyrene foam is formed when polystyrene beads are melted in the mold via steam heating.
Thereafter, the formed pattern is assembled to a sprue with others and sprayed with ceramic coating to form a refractory shell. The casting process follows but instead of melting the polystyrene foam first, you directly add the hot molten material which induces evaporation.
Direct Casting
When using die formation, you will use wax to make a pattern. The pattern forming process can be through hand carving and using computer aided 3-D engineering design via stereolithography.
The wax pattern is thereafter duplicated before assembly to a sprue and immersed in a ceramic slurry to make the mold. A dewaxing process follows followed by material casting.
Water Glass Casting
During this casting, water glass replaces ethyl silicate. Remember, the ethyl silicate is a biding agent.
Replacing binding agent reduces production cost and material.
Using water glass improves the surface finish requiring no secondary processes thanks to flaw reduction.
Technique in Sand Casting
Green Sand Casting
Here, you fill wet sand with molten metal. Usually, the mold is in uncured state. After the casting process, you can reuse the sand. However, since it is uncured during casting, it can disintegrate jeopardizing the casting process.
Sodium Silicate Casting
Utilizes sodium silicate in the mold making process through a dehydration process using carbon (IV) oxide. For hollow casting, use sodium silicate.
When using sodium silicate, it is advisable to combine it with other materials when making solid cores. This allows you to break off the core when extracting the casting.
Resin Sand Casting
Resin sand as the mold material is heated in the appropriate mix resulting in a sound mold featuring a uniform surface. While using resin sand results in near perfect castings, it is slow and expensive given it requires more material for succeeding processes.
Quality and Accuracy: Investment Casting compared to Sand Casting Process
The quality of castings made from investment casting is superior to that of castings from sand casting owing to reduced flaws. Investment casting results in high tolerance surfaces with regard to dimension making secondary machining or finishing unnecessary.
Total Cost for Casting
Sand Casting is affordable than lost wax casting. That is considering the availability of mold material (sand).
Investment casting features a rather elaborate mold making process which makes it marginally costly. However, the high quality castings resulting from investment casting are a reprieve to the high costs.
Investment Casting Mold Design vs Sand Casting Mold Design
Use of an identical impression pattern in the mold making process of investment casting allows for even complex casting.
Making cores in sand molds can take time not mentioning the difficulty in assembling them within the mold. Additionally, when making sand molds, you have to consider allowance for shrinkage and machining.
Surface Finishes: Investments Casting compared to Sand Casting
The surface finish obtained in investment casting is superior given the high dimensional tolerance achieved. Minor finishing touches serve to remove the gate and runners with the casting simply a duplicate of the pattern.
Castings in sand blasting feature surface inadequacies stemming from the mold systems such a parting lines and tapering. The non-uniformity of the sand particles will also manifest on the casting surface as roughness.
Advantages of Investment and Sand Casting
| Sand Casting | Investment Casting |
| The availability of materials makes it a low cost operation. | Capable of making ferrous castings and non-ferrous castings. |
| Can make castings of both ferrous and non-ferrous materials. | Achieves high accuracy levels with tighter tolerances and fewer defects. |
| Works well for making very large castings over hundreds of pounds in weight. | Highly flexible in terms of design allowing complex parts with unique features. |
| Has short lead time allowing for faster turnarounds. | Feature an impressive surface finish requiring less secondary processes. |
| Can easily alter the mold design without incurring great loss. | Reduced wastage given the high dimensional accuracy. |
| Capable of supporting large scale productions due to faster turnarounds. | |
| No size limitations as castings can weigh between a few ounces and hundreds of pounds. |
Production Volume: Sand Casting Vs Investment Casting
Investment casting is suitable for large volume production.
Contrarily, sand casting poses a design challenge in making castings especially with cavities making it unsuitable for high volume productions.
Key Applications:
| Applications of Investments Casting | Applications of Sand Casting |
| Making complex and precise parts and components for aerospace and defense: such as turbine blades. | Used in the automotive industry to produce parts such as engine blocks and cylinder heads. |
| Medical devices like implants and surgical instruments with high accuracy and surface finish. | Industrial machinery components like valves, pumps and compressors. |
| Automotive components with complex design such as brake parts, engine parts and suspension components. | Large agricultural machinery parts like plows and cultivators. |
| Detailed jewelry items such as medals. | Making art pieces such as sculptures. |
| Fine artistry depicting intricate shapes and features. | High strength aerospace components such as engine components and landing gears. |
Conclusion
Whether to choose investment casting or sand casting will depend on your unique application requirements.
More importantly, the choice of any process should be based on facts highlighted in this guide.
For any questions or inquiry on investment or sand casting, contact us now.
