Precision Castings

Also called investment casting or lost wax casting, it’s very suitable for producing precision castings with complex shapes and high dimensional accuracy & quality. In addition, it offers a large freedom in both design and material choice of your desired castings. Precision castings are known as the highest quality castings available on the market.

SIPX is your reliable partner in the field of precision metal casting. We are a China precision casting manufacturer located in Chongqing. As a proffesional precision casting foundry, we would like to share our thoughts and ideas about product design, material choice, measurements or machining.

What is Investment(Precision) Casting

Investment casting is one of the oldest manufacturing processes, in which molten metal is poured into an expendable ceramic mold. The mold is formed by using a wax pattern – a disposable piece in the shape of the desired part. The pattern is surrounded, or “invested”, into ceramic slurry that hardens into the mold. Investment casting is often referred to as “lost-wax casting” because the wax pattern is melted out of the mold after it has been formed. Lox-wax processes are one-to-one (one pattern creates one part), which increases production time and costs relative to other casting processes. However, since the mold is destroyed during the process, parts with complex geometries and intricate details can be created.

Investment casting can make use of most metals, most commonly using cast iron, stainless steel, and tool steel. This process is beneficial for casting metals with high melting temperatures that can not be molded in plaster or metal. Parts that are typically made by investment casting include those with complex geometry such as turbine blades or firearm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries. The only process that matches this breadth of materials is machining, but it cannot produce the complex geometries that precision casting can deliver.

Precision Casting Processes

1. Creating A Mold

Unless investment casting is being used to produce a very small volume (as is common for artistic work or original jewelry), a mold or die from which to manufacture the wax patterns is needed.

investment casting process-Produce Wax Pattern-s

2. Produce Wax Pattern

The wax is injected into the metal mold or die and allowed to solidify. The number of wax patterns is the same as the castings to be produced, each individual casting requires a new wax pattern.

investment casting process-assemble wax pattern tree

3. Assemble Wax Patterns Tree

Assemble the wax patterns to a gate or system. Multiple patterns are assembled to allow for large quantities to be produced at one time.

investment casting process-creating shell

4. Creating The Shell

Dip the pattern tree into ceramic slurry, the thickness of the ceramic shell depends of the size and weight of the part being cast. The average wall thickness is approximately 0.375 in. (9.525 mm)

investment casting process-wax removal

5. Wax Removal

The hardened ceramic mold is turned upside down, placed in an oven, and heated until the wax melts and drains away. The result is a hollow ceramic shell.

6. Melt And Cast

The ceramic mold is preheated to around 1000 – 2000°F (550 – 1100°C) and filled with molten metal, liquid metal flows into the pouring cup, through the central gating system, and into each mold cavity on the tree.

investment casting process-casting removal

7. Shakeout And Casting Removal

Once the casting solidifies, break the ceramic shell, and cut the individual investment castings from the sprue. Finally remove the excess metal from casting body by grinding.

8. Finishing

If necessary, final post-processing sandblasting, grinding, and machining is performed to finish the casting dimensionally. To improve the mechanical properties of the parts, heat treating may be applied.

Factors That Affests the Accurary of Precision Castings

Under normal circumstances, the dimensional accuracy of precision castings is influenced by many factors, such as the structure of the castings, material, molding, shell making, and metal cast. The setting and unreasonable operation of any of them will change the shrinkage rate of the casting. Then the dimensional accuracy of the casting deviates from the requirement. The following are the factors that may cause precision defects in precision castings:

1. Structure of the Castings

  • Thick wall casting parts usually hava large shrinkage rate while a casting part with a thin wall has a small shrinkage rate.
  • If the free shrinkage rate is large, it definitely will hinder the shrinkage rate.

2. Materials

  • The higher the carbon content of the material, the smaller the line shrinkage. The lower the carbon content, the greater the line shrinkage.
  • There’s a universal casting shrinkage formula for common materials: Casting shrinkage K = (LM-LJ) / LJ × 100%

LM represents the cavity size, and LJ is the casting size. K is affected by the following factors: wax mold K1, casting structure K2, alloy type K3, casting temperature K4.

3. Mold making

  • The effects of wax temperature, wax pressure and dwell time on melt size are the most obvious. Holding time has little effect on the final size after the injection molding is finished.
  • The linear shrinkage of the wax (molding) material is about 0.9-1.1%.
  • When the molds are stored, further shrinkage occurs, with a shrinkage value of about 10% of the total shrinkage. However, after 12 hours of storage, the dimensions of the molds were essentially stable.
  • The radial shrinkage rate of the wax mold is only 30-40% of the longitudinal shrinkage rate, and the effect of wax temperature on the free shrinkage rate is much greater than the effect on the resistance shrinkage rate (the optimal wax temperature is 57-59℃, the higher the temperature, the greater the shrinkage rate).

4. Shell material

Zirconia sand and zirconia powder are used because their coefficient of expansion is small, only 4.6×10-6/°C, so they are negligible.

5. Shell roasting

The expansion coefficient of the shell is negligible because it is only 0.053% when the shell temperature is 1150°C.

6. Casting temperature

The higher the casting temperature, the higher the shrinkage rate. The lower the casting temperature, the smaller the shrinkage. Therefore, the pouring temperature should be appropriate.

Linear Tolerance of Investment Casting

Each casting process’s linear tolerances are not the same. Below is a list of liner tolerance standards for casting:

ISO8062(GB/T6414) CT4-CT8—Nominal dimension (mm) Linear dimension tolerance (ISO8062)

CT4 CT5 CT6  CT7  CT8 
0100.26 ±0.130.36±0.180.52±0.260.74±0.371±0.5
10160.28±0.140.38 ±0.190.54±0.270.78±0.391.1±0.55
16250.3±0.150.42±0.210.58 ±0.290.82 ±0.411.2±0.6
25400.32±0.160.46±0.230.64±0.320.9±0.451.3±0.65
40630.36±0.180.5±0.250.7±0.351±0.51.4 ±0.7
631000.4±0.20.56±0.280.78±0.391.1±0.551.6±0.8
1001600.44±0.220.62±0.310.88±0.441.2±0.61.8±0.9
1602500.5±0.250.7±0.351±0.51.4±0.72±1
2504000.56±0.280.78±0.391.1±0.551.6±0.82.2±1.1
4006300.64±0.320.9±0.451.2±0.61.8±0.92.6±1.3

Linear tolerancing is normally applied to the investment castings features of Length, Concentricity, Fillet radii, Holes, Straightness, Corner Radii, Flatness, and Curved Holes.

General rule: Up to 1” +/- .010”, for each additional inch up to ten inches +/- .003” per inch. For dimensions greater than ten inches allow +/- .005” per inch. Secondary operations such as straightening and sizing will produce closer dimensional tolerance.

Dimensions Normal Tolerance of Investment Casting
Up to 1” +/- .010”
Up to 2” +/- .013”
Up to 3” +/- .016”
Up to 4” +/- .019”
Up to 5” +/- .022”
Up to 6” +/- .025”
Up to 7” +/- .028”
Up to 8” +/- .031”
Up to 9” +/- .034”
Up to 10” +/- .037”
> 10” allow +/- .005” per inch

An exception to the linear tolerance exists on wall thickness where the tolerance must be a minimum of +/- .020”.

Materials for Precision Casting

Material Grade Common Application
Stainless Steel 304 Housings, bodies. SS304 is widely used in medical, plumbing, watering, mining and petrochemical industries as the standard for cleanliness as well as corrosion resistance.
316 Housings, gears, plates. Superior corrosion resistance of SS316 makes it ideal for many applications including automotive, food & dairy and petrochemical environments.
304L/316L Similar to other 300 series steel, low carbon makes it softer but more corrosion resistant. Ideal industries are such as food & dairy, medical and petrochemical.
410 & 416 Housings, bushings, handles, brackets. 400 series steels are high strength and very machinable. They will tend to break rather than bend under intense pressure. Ideal industries are automotive, food & dairy and various machine tools.
17-4 Very strong. Can be used in military, various machine tools, etc.
Duplex Stainless Steel 2205 Duplex stainless steel 2205 consists of a two-phase microstructure — both ferritic stainless steel and austenitic stainless steel. Nitrogen is enhanced in Duplex 2205 compared to other duplexe stainless steel.
Super Duplex Stainless Steel 2507 Desalination, marine, pulp & paper mill, chemical process pressure vessels, heat exchangers and piping, oil and gas industry, offshore oil industry.
Zeron 100 Desalination, flue-gas desulfurization, oil and gas industry, pollution control, chemical, pharmaceutical, mining and mineral industries, pulp and paper, power generation, marine industries
Titanium Grade 2 Chemical process, marine etc.
Grade 5 Aerospace, medical, marine and chemical processing industries and oil field services.
Carbon Steels 1000 series Softest of common carbon steels. More likely to bend rather than break under intense pressure. Commonly used to make cranks, handles, and braces.
4100 series High strength material is used when the deformation of a part is not allowed. Commonly used to cast firearm components and ratchets.
8600 series 8600 series is stronger than 1000 series but will bend prior to breakage. Commonly used to cast firearm components, housings, and nozzles.
Tool Steels A2, S7, D2 Used when parts will be under higher heat due to less distortion. Not corrosion resistant like stainless steel.
Aluminum Alloys A356 Housing, covers, plates. Aluminum is lightweight and extremely corrosion resistant like stainless steel.

Advantages of Precision Casting

Due to its complexity and labor requirements, investment casting is a relatively expensive process – however the benefits often outweigh the cost.

Large Size Range

Parts manufactured by investment casting are normally small, but the process can be used for parts weighing up to 250kg.

Versatile And Intricate Shapes

Great versatility, suitable for casting most metals. Very intricate castings can be produced easily. For example, where machine tools cannot reach.

Smooth Surfaces And Superior Dimensional Accurate

Net-shape parts are easily achievable, and finished parts are often produced with no seam line so machining and finishing are reduced or eliminated. A 125 micro finish is standard, and even finer finishes are not uncommon.

High Strength

The metal load of the investment casting process is 90-95%, a value similar to that of Injection Moulding.

High Cost Efficiency In Large Volume

For high-volume production, the time and labor saved by eliminating or decreasing secondary machining easily makes up for the cost of new tooling. Small casting runs are less likely to make up for the investment. Generally, investment casting is a logical choice for a run of 25 parts or more.

Typical Applications

Why Choose SIPX

With over 10 years of experience, Sipx is well-versed in this field. Yes, you are dealing with masters. You can trust us with your needs and expectations.

You will save 20-30% of the cost because of our excellent production capacity. Furthermore, we only insist on making high-quality casting products.

  • Rapid Prototyping
  • One-Stop Services Supplier
  • On-demand manufacturing
  • ISO16949 Certificated
  • Strict Quality Control System

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