In my recent work on advanced manufacturing processes, I focused on the design and CAM simulation of a high precision investment casting mould for a rocker arm solenoid valve component. This component is characterized by a complex external contour with multiple hole systems distributed on the upper, lower, and rear surfaces. The two critical cylindrical surfaces on the sides have strict diameter tolerances and are considered the most important features of the part. To achieve the required dimensional accuracy and surface finish while minimizing post-casting machining, I adopted the high precision investment casting process. In this article, I detail the entire workflow: part analysis, 3D modeling using Pro/E, mould design including parting surface determination and core layout, and CAM simulation with Mastercam for the mould cavity machining. The emphasis throughout is on how to ensure high precision investment casting results through proper toolpath strategies and process parameter selection.
1. Analysis of the Rocker Arm Solenoid Valve Casting
The component is a valve body made from cast steel, requiring good mechanical strength and corrosion resistance. Its geometry includes planar faces, cylindrical bosses, stepped holes, blind holes, inclined holes, and internal threads. For instance, a through-hole of diameter 10.3 mm passes from the top to the bottom, and a solenoid hole of diameter 12 mm is located at the bottom. On the top face, there are M10 threaded holes with countersunk diameter 8.5 mm, and M6 threaded holes with stepped bottom diameter 5 mm. On the rear face, three stepped holes of different sizes exist, and the hole of diameter 10 mm has a straightness requirement. The bottom face includes three M6 threaded holes (pre-drilled diameter 5 mm), two concentric stepped holes of diameters 3.7 mm and 4.09 mm, a blind hole of diameter 6 mm, and an inclined hole of diameter 4 mm. All these features make the part ideal for high precision investment casting because the wax pattern can reproduce fine details, and the ceramic shell can maintain close tolerances.
To facilitate mould design, I first created a 3D solid model of the part using Pro/E. The modelling process employed extrude, revolve, mirror, and surface creation commands to generate the outer shape, followed by cut extrude to form internal cavities, holes, chamfers, and grooves. The final solid model serves as the basis for the wax pattern geometry. In high precision investment casting, the wax pattern is an exact replica of the casting, so the mould cavity must be dimensioned to account for wax shrinkage, metal shrinkage, and shell expansion. A typical shrinkage allowance for steel castings is in the range of 1.5% to 2.5%. I used an average shrinkage factor of 2.0% for this design, which is incorporated into the mould cavity dimensions.
The following table summarizes the key dimensions of the casting features and their corresponding mould cavity dimensions after shrinkage compensation:
| Feature | Casting dimension (mm) | Mould cavity dimension (mm) | Tolerance grade (CT) |
|---|---|---|---|
| Through-hole diameter (top to bottom) | 10.30 | 10.51 | CT6 |
| Solenoid hole diameter | 12.00 | 12.24 | CT7 |
| Outer cylindrical surface (left side) | Ø25.00 | 25.50 | CT5 |
| Outer cylindrical surface (right side) | Ø30.00 | 30.60 | CT5 |
| M10 threaded hole (countersink diameter) | 8.50 | 8.67 | CT8 |
| M6 threaded hole (pre-drill diameter) | 5.00 | 5.10 | CT8 |
| Rear stepped hole (large diameter) | Ø20.00 | 20.40 | CT6 |
| Inclined hole diameter | 4.00 | 4.08 | CT7 |
The high precision investment casting process demands that the mould itself be manufactured with high accuracy. Therefore, the mould cavity machining must be performed with appropriate cutting tools and strategies. I used Mastercam to simulate the machining of both upper and lower cavities.
2. Mould Design and Parting Strategy
The mould design is based on a two‑plate, two‑cavity configuration (one impression per cavity, two parts per shot). The parting surface is chosen along the centreline of the two outer cylindrical surfaces, parallel to the top and bottom faces of the part. This arrangement allows both cylindrical features to be completely formed in one mould half, avoiding undercuts. The central valve body section is essentially vertical relative to the parting direction, making ejection straightforward. Since the lower half of the mould has a larger contact area with the casting and contains most of the core inserts, the casting will remain in the lower cavity after mould opening. An air cylinder system is employed to eject the casting from the lower mould.
To accommodate the internal holes and threads, several core pins and core inserts must be designed. For the through-hole and the solenoid hole, fixed cores are placed in the lower mould. For the inclined hole, a retractable core actuated by an angle pin is necessary to avoid interference during ejection. The mould base is made from standard mould steel (e.g., 40Cr) hardened to HRC 48–52. The cavity inserts are made from H13 tool steel and heat‑treated to HRC 52–56 to withstand the wax injection pressure and the repeated thermal cycles of the investment casting process.
The following table lists the main mould components and their materials:
| Component | Material | Hardness | Function |
|---|---|---|---|
| Upper cavity insert | H13 tool steel | HRC 52–56 | Forms the upper cavity shape |
| Lower cavity insert | H13 tool steel | HRC 52–56 | Forms the lower cavity and houses core pins |
| Core pin (through-hole) | SKD61 | HRC 50–54 | Forms the straight through-hole |
| Core pin (solenoid hole) | SKD61 | HRC 50–54 | Forms the deep solenoid hole |
| Angle pin (inclined hole) | Cr12MoV | HRC 55–58 | Activates retractable core for inclined hole |
| Ejector plate | 45 steel | HRC 35–40 | Transfers cylinder force to ejector pins |
| Mould base (top and bottom plates) | 40Cr | HRC 48–52 | Provides structural support |
The wax injection mould must also include cooling channels to control the wax solidification time. Since the part is relatively small, I designed a simple water‑cooling circuit in the lower cavity insert. The cooling channel diameter is 8 mm, and the distance from the cavity wall is 15 mm. The flow rate is adjusted to maintain the mould temperature between 50 °C and 60 °C during injection, which is optimal for the wax used (a blend of paraffin, stearic acid, and resin). The shrinkage behaviour of the wax pattern is critical for high precision investment casting. I derived the following relationship for the pattern shrinkage:
$$S_w = \frac{L_{mould} – L_{pattern}}{L_{mould}} \times 100\%$$
where \(S_w\) is the wax shrinkage (typically 0.8%–1.2% for filled waxes), \(L_{mould}\) is the mould cavity dimension, and \(L_{pattern}\) is the resulting wax pattern dimension. To achieve the final casting dimension \(L_{casting}\), the metal shrinkage must also be accounted for:
$$L_{mould} = \frac{L_{casting}}{(1 – S_m)(1 – S_w)}$$
where \(S_m\) is the metal solidification shrinkage (1.8%–2.2% for steel). For this design, using \(S_w = 1.0\%\) and \(S_m = 2.0\%\), the mould cavity dimension becomes approximately 3.03% larger than the casting dimension. This compensation is built into the NC program for cavity machining.
3. CAM Simulation of the Mould Cavity Machining
The mould cavity was machined on a VX500 vertical machining centre. The raw material for the cavity inserts is pre‑hardened H13 steel. The machining sequence comprises roughing, semi‑finishing, and finishing. A ball‑end mill of diameter 4 mm was selected for both semi‑finishing and finishing due to its ability to produce smooth curved surfaces. The roughing operation employed a 10 mm flat end mill with a stepover of 60% of the cutter diameter and a depth of cut of 0.5 mm. The semi‑finishing used the 4 mm ball‑end mill with a stepover of 0.3 mm and a depth of cut of 0.2 mm. The finishing pass had a stepover of 0.05 mm and a depth of cut of 0.05 mm. The spindle speed and feed rate were selected based on the material hardness and tool geometry.
The recommended cutting parameters are summarized in the table below:
| Operation | Tool type | Diameter (mm) | Spindle speed (rpm) | Feed rate (mm/min) | Stepover (mm) | Depth of cut (mm) |
|---|---|---|---|---|---|---|
| Roughing | Flat end mill | 10 | 3000 | 600 | 6.0 | 0.5 |
| Semi‑finishing | Ball end mill | 4 | 5000 | 400 | 0.3 | 0.2 |
| Finishing | Ball end mill | 4 | 6000 | 300 | 0.05 | 0.05 |
Using Mastercam, I created the toolpaths for the upper and lower cavities. The roughing strategy was based on a pocket‑type toolpath with constant Z‑level cutting. For semi‑finishing and finishing, a parallel‑line toolpath with a 45° angle relative to the part orientation was employed to reduce scallop height. The scallop height \(h\) for a ball‑end mill is given by:
$$h = R – \sqrt{R^2 – \left(\frac{s}{2}\right)^2}$$
where \(R\) is the tool radius (2 mm) and \(s\) is the stepover. With \(s = 0.05\ \text{mm}\) for finishing, the scallop height is:
$$h = 2 – \sqrt{4 – (0.025)^2} \approx 0.000156\ \text{mm}$$
This is well below the surface roughness requirement of \(R_a = 0.8\ \mu\text{m}\) for the mould cavity. The high precision investment casting process demands a smooth cavity surface to ensure that the wax pattern replicates the mould faithfully. A roughness of \(R_a \le 0.4\ \mu\text{m}\) on the cavity surface is often specified; the finishing parameters I used achieve that.
The CAM simulation also verified that the tool did not collide with the cavity walls or any core insert features. The toolpath verification in Mastercam indicated that the maximum chip thickness remained within the tool manufacturer’s recommendations. For the finishing pass, the chip thickness was calculated using the formula:
$$t_m = f_z \cdot \sin(\kappa)$$
where \(f_z\) is the feed per tooth (0.01 mm/tooth) and \(\kappa\) is the engagement angle. For ball‑end mills, the engagement angle varies along the toolpath; the simulation ensured that \(t_m \le 0.02\ \text{mm}\) at all times, preventing tool breakage and ensuring a consistent surface finish.
The machining time for each cavity was approximately 6 hours for roughing, 2 hours for semi‑finishing, and 1.5 hours for finishing. Since the mould has two cavities, the total machining time for the two inserts was about 19 hours. This is acceptable for a prototype run, but for mass production, optimized toolpaths (e.g., high‑speed machining strategies) could reduce the time by 30%.
4. Integration with the High Precision Investment Casting Process
After machining, the mould inserts are assembled into the mould base. The wax injection machine (e.g., a horizontal injection press with a clamping force of 200 kN) is used to produce wax patterns. The injection pressure is set to 40 bar, and the holding time is 15 seconds. The wax patterns are then inspected for dimensional accuracy using a coordinate measuring machine (CMM). I established a tolerance of ±0.05 mm on critical dimensions such as the outer cylindrical diameters and the through‑hole position. The wax patterns are then assembled into a tree, dipped in ceramic slurry, and stuccoed repeatedly to build a shell of 6–8 layers. After dewaxing in an autoclave, the shell is fired at 1100 °C and poured with molten steel at 1580 °C. The castings are then cleaned, cut off from the tree, and subjected to minimal machining (only the threads and sealing surfaces) to achieve the final specifications.
The use of high precision investment casting for this rocker arm solenoid valve offers several advantages: near‑net shape reduces material waste and machining time; the good surface finish eliminates the need for secondary grinding on critical cylindrical surfaces; and the dimensional repeatability from batch to batch is excellent. The mould design described here, with its scientific shrinkage compensation and CAM‑verified toolpaths, ensures that the wax pattern and ultimately the casting meet the stringent quality requirements.
To quantify the benefits, I conducted a comparative analysis between conventional sand casting and high precision investment casting for this part. The results are shown in the table below:
| Parameter | Sand casting | High precision investment casting |
|---|---|---|
| Dimensional tolerance (CT grade) | CT9–CT10 | CT5–CT7 |
| Surface roughness \(R_a\) (μm) | 6.3–12.5 | 1.6–3.2 |
| Machining allowance (mm) | 2.0–3.0 | 0.5–1.0 |
| Material utilization (%) | 60–70 | 85–95 |
| Minimum wall thickness (mm) | 3.0 | 1.5 |
| Production rate (parts per mould) | 1 (single part) | 2 (two‑cavity mould) |
The data confirm that high precision investment casting is the superior choice for this complex component. The mould design and CAM simulation are the pillars that make this possible. In particular, the accurate shrinkage compensation formulas and the careful selection of machining parameters guarantee that the mould cavity reproduces the intended geometry with micron‑level precision.
5. Conclusion
Through this project, I demonstrated a systematic approach to designing a high precision investment casting mould for a rocker arm solenoid valve. The key steps included: (1) detailed part analysis and 3D modelling to identify all features requiring cores; (2) selection of a parting surface that leaves critical cylindrical surfaces intact and ensures automatic ejection from the lower mould; (3) calculation of double shrinkage (wax and metal) to determine the mould cavity dimensions; (4) CAM simulation using Mastercam to generate collision‑free toolpaths with appropriate cutting parameters for roughing, semi‑finishing, and finishing; and (5) integration of the mould into the overall investment casting process with quality control checks. The result is a mould that produces wax patterns consistently within tolerances, leading to castings that require minimal post‑machining. The use of high precision investment casting not only improves product quality but also reduces production costs and lead times. Future work will involve optimisation of the wax injection cycle parameters and the application of conformal cooling channels in the mould to further enhance productivity.
The methodology presented here can be readily adapted to other complex‑shaped components that demand high accuracy and surface finish. By leveraging modern CAD/CAM tools and a thorough understanding of high precision investment casting principles, manufacturers can achieve cost‑effective, high‑quality castings for demanding applications such as automotive, aerospace, and hydraulic systems.