In the research and production of large, thin‑walled, and complex castings, our foundry has achieved significant progress in casting processes, which simultaneously imposes stricter requirements on mould design. This article presents my design approach for a high precision investment casting mould used to produce wax patterns for a large cabin body component. The mould structure, parting method, positioning system, locking mechanism, and hollow structure are optimized to facilitate easy operation, reduce weight, shorten manufacturing time, and ensure reliable production. Throughout the design, the principles of high precision investment casting guide every decision.
I begin by analyzing the casting characteristics. The cabin body casting has overall dimensions of 700 mm × 650 mm × 500 mm, with a typical wall thickness of only 2.5 mm. Two flanges exist at both ends, and nine lugs with holes are distributed inside the larger end. To improve mold filling and compensate for insufficient strength of the thin wall, twelve stiffening ribs (8 longitudinal and 4 transverse, each 8 mm × 5 mm) are arranged on the outer surface. The wax pattern for high precision investment casting must be dimensionally accurate and is manually operated. Wax is fragile and has poor elasticity, so the mould must avoid any undercuts that hinder demolding. The loose blocks must not be too large or heavy, their surfaces must be smooth and easy to machine and inspect, and inter‑block positioning must be reliable.
The casting has two open ends and a closed middle section that bulges outward, forming a waist‑drum shape. The height of the cavity is 640 mm, and the wall thickness remains only 2.5 mm. To ensure uniform wall thickness, the mould must be a single piece in the vertical direction within the 640 mm height range, avoiding any mismatch. Only radial (horizontal) parting is acceptable, which not only reduces block weight but also makes demolding convenient, simplifies assembly and inspection, and guarantees the 2.5 mm wall thickness.
For the nine lugs and holes at the larger end, the blocks must be withdrawn either inward or outward. The outward nine blocks are positioned on the half‑moulds; the inward nine blocks shape both the lugs and the internal flange. Their withdrawal direction differs from the direction of the blocks forming the 2.5 mm wall. Therefore, a separate group of blocks must be designed above the lower surface of the internal flange to extract the complex cavity independently without damaging the thin wall. Reliable positioning is essential.
Because the casting is large, the mould must have sufficient strength to withstand injection pressure while allowing manual demolding quickly and conveniently. Hence, weight reduction measures and lifting facilities are necessary. In summary, the key considerations for this high precision investment casting mould are: block parting, inter‑block positioning, easy demolding, weight reduction, and manufacturability/maintenance.
Mould Structure Overview
Using UG software to build the 3D model, I adopted a hollow structure with top‑bottom locking, left‑right opening, and two sets of internal blocks plus positioning blocks. The main components are shown conceptually in the following table, which summarizes the functions and design features of each part.
| Component | Function | Design features |
|---|---|---|
| Upper locking mechanism (1) | Clamp upper and lower halves together | Provides vertical clamping force |
| Positioning pins (2) – 18 pieces | Align blocks radially | Inserted after assembly, removed before demolding |
| Upper mould fixing plate (3) | Carries upper cavity surface | Locates and fixes the upper positioning block |
| Upper positioning block (4) | Centers the first inner block group | First to be removed during demolding |
| Inner block group I (5) – 11 pieces | Forms internal features near the top flange | 5° draft angle on mating faces; includes ejector mechanism |
| Left/right half‑moulds (14, 17) | Form outer cavity surfaces | Located on base, locked by side locking mechanism |
| Side locking mechanism (18) | Secure left/right half‑moulds | Prevents opening during wax injection |
| Support rods (8) and pins (7) | Transmit downward force to lower positioning block; lifting | Used to lift lower positioning block during demolding |
| Inner block group II (10) – 9 pieces | Forms lower internal flange and lugs | Long‑strip shape; 5° draft; hollow interior for weight reduction |
| Lower positioning block (11) | Centers the second inner block group | Located in bottom base |
| Lower mould base (12) | Supports all components | Provides datum for locating half‑moulds and internal blocks |
| Ejector mechanism (19) | Assist demolding of initial blocks | Screws or rods push the first block to break wax adhesion |
Demolding Sequence
After wax is injected under pressure and held for a certain time, the demolding process follows a carefully designed order. First, all 18 positioning pins (2) are pulled out. Then the upper locking mechanism (1) is opened, and the upper mould fixing plate (3) is removed, exposing the upper positioning block (4). That block is lifted out, revealing the first inner block group (5). The 11 blocks of group I are withdrawn radially toward the center axis A in a specific sequence (a, b, c, …), as illustrated in the schematic. Each block has a 5° draft angle on its mating surfaces to avoid dragging neighboring blocks and damaging the wax pattern. After group I is removed, the nine internal lugs and the upper flange cavity are completely freed.
Next, the side locking mechanism (18) is released. The left and right half‑moulds (14, 17) are moved outward by 1–5 mm each. By attaching lifting ropes to the cylindrical pins (7) on support rods (8), the lower positioning block (11) is hoisted upward. This exposes the second inner block group (10), which consists of nine long‑strip blocks that form the lower internal flange and the nine lug cavities. These blocks are withdrawn toward the center A in the order shown in the plan view (a, b, c, d, …, f). Because the wax shrinkage creates a tight grip on these blocks, the first two blocks to be removed are equipped with ejector mechanisms (19). A slight push from the ejector breaks the adhesion, after which the block can be lifted using threaded rods (13) inserted into dedicated lifting holes. The remaining blocks are removed sequentially. Finally, all internal blocks are out, and the wax pattern sits freely on the lower mould base (12). The outer half‑moulds can then be moved away completely.
Positioning and Locking System
Reliable positioning is critical for high precision investment casting to maintain the 2.5 mm wall thickness and avoid mismatch. The two inner block groups (5 and 10) are positioned relative to each other using two precision steel dowel pins (2) per pair of mating faces. These pins are inserted after the blocks are assembled and are removed before demolding. The top face of group I is located by the upper mould fixing plate (3) and the upper positioning block (4); the bottom face of group II is located by the lower positioning block (11) and the lower mould base (12). The left and right half‑moulds (14, 17) are positioned radially against the steps on the lower mould base. The side locking mechanism (18) clamps the half‑moulds together horizontally, while the upper locking mechanism (1) provides vertical force. Together, these elements ensure that all cavity surfaces remain aligned under wax injection pressure, which can be up to several megapascals.
The following table summarizes the positioning methods for each group of blocks.
| Block group | Primary positioning | Secondary positioning | Draft angle on mating faces |
|---|---|---|---|
| Inner block group I (5) | Upper positioning block (4) + upper mould fixing plate (3) | Dowel pins (2) between blocks | 5° |
| Inner block group II (10) | Lower positioning block (11) + lower mould base (12) | Dowel pins (2) between blocks | 5° |
| Left/right half‑moulds (14, 17) | Recess in lower mould base (12) | Side locking mechanism (18) | N/A |
| Upper/lower positioning blocks (4, 11) | Fixing plate or base with screws | Support rods (8) for lower block | N/A |
The locking force must overcome the wax injection pressure. Assume the injection pressure is $$P_{\text{inj}} = 5\ \text{MPa}$$. The total projected area perpendicular to the locking direction is approximately $$A_{\text{proj}} = 0.7\ \text{m} \times 0.5\ \text{m} = 0.35\ \text{m}^2$$. The required clamping force is then:
$$F_{\text{clamp}} = P_{\text{inj}} \times A_{\text{proj}} = 5 \times 10^6\ \text{Pa} \times 0.35\ \text{m}^2 = 1.75 \times 10^6\ \text{N}$$
This force is provided by the combination of the upper and side locking mechanisms. In practice, mechanical toggle clamps or hydraulic cylinders can be used, but for manual operation, heavy‑duty screw‑type locks are preferred.
Hollow Structure for Weight Reduction
Given the large dimensions (640 mm height in the cavity), the internal blocks could become extremely heavy if made solid. To reduce weight and facilitate handling, a hollow structure was adopted for the core region between the upper positioning block (4) and the lower positioning block (11). Over the 600 mm height, the blocks are designed with internal cavities, leaving only the necessary wall thickness for strength. The support rods (8) also serve as lifting points. This design reduces the total mould weight by approximately 30% compared to a solid design, shortens machining time, and saves material. The hollow spaces also allow better heat dissipation during wax cooling, which improves dimensional stability – a key requirement in high precision investment casting.
The weight reduction can be quantified. Assume a solid block occupying a cylindrical volume of diameter 200 mm and height 600 mm. The volume of a solid cylinder is:
$$V_{\text{solid}} = \pi r^2 h = \pi \times (0.1\ \text{m})^2 \times 0.6\ \text{m} \approx 0.01885\ \text{m}^3$$
Using the density of tool steel $$\rho = 7.85 \times 10^3\ \text{kg/m}^3$$, the mass is:
$$m_{\text{solid}} = \rho V_{\text{solid}} = 7850 \times 0.01885 \approx 148\ \text{kg}$$
The hollow design retains only a 20 mm thick outer shell. The internal cavity has radius 80 mm. The volume of the shell is:
$$V_{\text{shell}} = \pi (R^2 – r^2) h = \pi \times (0.1^2 – 0.08^2) \times 0.6 \approx 0.00679\ \text{m}^3$$
$$m_{\text{shell}} = 7850 \times 0.00679 \approx 53.3\ \text{kg}$$
The reduction factor is:
$$\frac{m_{\text{solid}} – m_{\text{shell}}}{m_{\text{solid}}} = \frac{148 – 53.3}{148} \approx 64\%$$
In reality, additional ribs and bosses are required, so the actual weight saving is about 50%. This dramatic reduction makes manual demolding feasible.
Key Design Formulas and Parameters
To ensure the success of high precision investment casting, several geometric and process parameters must be carefully selected. The following table collects the essential design parameters used in this mould.
| Parameter | Symbol | Value | Unit |
|---|---|---|---|
| Casting overall dimensions | L×W×H | 700×650×500 | mm |
| Nominal wall thickness | t | 2.5 | mm |
| Internal cavity height | Hcav | 640 | mm |
| Number of stiffening ribs | Nribs | 12 (8 longitudinal + 4 transverse) | — |
| Rib cross‑section | b×h | 8×5 | mm |
| Draft angle on block mating faces | α | 5 | degree |
| Number of inner blocks (group I) | NI | 11 | pieces |
| Number of inner blocks (group II) | NII | 9 | pieces |
| Number of positioning dowel pins | Npins | 18 | pieces |
| Injection pressure (typical) | Pinj | 5 | MPa |
| Side locking force per screw | Fside | ~50 | kN |
| Required clamping force | Fclamp | ~1.75×106 | N |
| Weight reduction ratio (core region) | η | ~50 | % |
The relationship between wall thickness and the mould cavity dimensions can be expressed as a simple check. For a uniform wall thickness t, the internal cavity volume Vcavity is related to the outer volume Vouter and the casting volume Vcast:
$$V_{\text{cavity}} = V_{\text{outer}} – V_{\text{cast}} \approx V_{\text{outer}} – (V_{\text{outer}} – t \cdot A_{\text{surface}}) = t \cdot A_{\text{surface}}$$
For a complex shape, the exact formula is not trivial, but the principle ensures that the cavity envelope must be exactly offset from the casting surface by the wax shrinkage allowance (typically 0.5%–1%). In this design, the mould’s cavity dimensions were offset by 0.8% of the casting dimensions to account for wax shrinkage, and then machined to final tolerance.
Advantages and Conclusion
The mould described above has been successfully manufactured and put into production without any rework. The wax patterns produced meet all dimensional requirements for high precision investment casting. The key innovations include:
- Radial parting of the tall cavity (640 mm) eliminates vertical mismatch and ensures uniform wall thickness.
- Two separate inner block groups arranged in two layers allow independent demolding of the internal flange and the thin wall.
- A 5° draft angle on all mating faces, supplemented by ejector mechanisms, guarantees reliable demolding without damaging fragile wax.
- The hollow structure reduces mould weight by approximately 50%, making manual operation practical and accelerating machining.
- Robust positioning by dowel pins and multiple locking mechanisms maintains cavity alignment under high injection pressure.
This design provides a valuable reference for future large, complex, thin‑walled castings produced by high precision investment casting. The systematic approach to parting, positioning, and demolding sequence can be adapted to similar components. The use of tables and formulas throughout the design process clarifies the engineering decisions and facilitates communication among team members.
In the demanding field of high precision investment casting, every detail matters. By focusing on the specific challenges of the cabin body – its size, thin walls, and internal features – I was able to create a mould that is both simple to operate and reliable in production. This experience reaffirms the importance of thorough analysis and creative structural solutions in achieving high precision investment casting success.