The gap a single casting process cannot close
Aerospace aluminium housings usually combine thin walls, closed internal oil galleries, shaft bosses and stiffening ribs whose requirements are set independently of one another. Gravity sand casting cannot hold the dimensional and surface tolerance on the internal passages; permanent-mould low-pressure casting gives good feeding but cannot be opened around the undercuts; gypsum and investment casting reproduce surface detail well but are slow and expensive per part. Foundries therefore tend to end up with a compromise route that inherits the weaknesses of each process it borrowed from.
A typical failure pattern appears when a plant that has run low-pressure permanent-mould casting takes on a new family of thin-wall parts that cannot be stripped from a steel mould. The immediate substitution is a 3D-printed resin-bonded sand mould poured under low pressure, and four problems follow: the sand surface is scoured by the metal stream under low pressure, producing inclusion defects; printed cores for the small oil passages come out with a rough surface and limited strength, so they break during handling, core assembly and pouring; thick-to-thin transitions produce shrinkage porosity, cold shuts and misruns; and the sand core trapped inside a closed cavity cannot be removed after solidification.
A workable answer is to keep the printed sand mould as the geometric backbone and to add gypsum chemistry only where it earns its place: as a self-hardening coating on the mould wall, as a transfer coating laid down against the pattern and then backed with sand, and as a water-collapsible core that replaces the fragile printed passages. Because gypsum-bonded material has a thermal conductivity well below that of resin sand, the same coating and core also act as local thermal insulation, which can be positioned deliberately to slow solidification in heavy sections and let the riser feed them.
Alloy and melting practice: AC4B at 720 °C
The demonstrator parts are poured in AC4B, a Japanese grade metallurgically comparable to YL112 and to US 380.0. It combines useful room-temperature mechanical properties with high hot strength and good fluidity in the liquid state, which is why it is used for thin-wall, geometrically complex aerospace castings normally produced by gravity or low-pressure casting.
| Element | Cu | Si | Mg | Zn | Fe | Mn | Ni | Ti | Pb | Sn | Cr | Al |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| wt.% | 2.0–4.0 | 7.0–10.0 | ≤0.5 | ≤1.0 | ≤1.0 | ≤0.5 | ≤0.35 | ≤0.2 | ≤0.2 | ≤0.1 | ≤0.2 | balance |
Melting follows a fixed sequence: the charge is dried at 350 °C for 2 h to remove adsorbed moisture; the crucible is preheated to 500 °C for 2 h, cooled below 300 °C, cleaned and coated; all tools are stripped of residual metal, preheated and coated; the charge is melted under a cover flux, stirred at 740 °C to dissolve the alloying elements, then cooled naturally to a pouring temperature of 720 °C, refined with argon under stirring, held for 10 min and skimmed. Pouring at 720 °C is also the condition that constrains every consumable described below, since coatings, transfer layers and cores all see that temperature in contact with liquid metal.
Investment casting as the reference route, and why gypsum is used only locally
Lost-wax precision investment casting remains the benchmark for aerospace aluminium: a wax or pattern is coated with a refractory shell, dewaxed, fired and poured. Its known weak points are the shell and the interface. Colloidal silica binders give shells of low strength that crack during dewaxing and pouring; soluble organic polymers added to the binder raise shell strength but add cost, and organic fibres dispersed in the ceramic slurry have a similar strengthening effect. Heat transfer across the casting–shell interface has been characterised in three stages: early in solidification the heat flux stays roughly constant while the interfacial heat transfer coefficient rises as the temperature difference between casting and shell falls; in the middle stage the coefficient decreases linearly with solid fraction; late in solidification it decreases only slowly.
Pattern making has moved toward additive manufacturing. Printed PS patterns used with gypsum investment moulds have been documented with a bulk density of 0.45 g/cm³, a skeletal density of 1.04 g/cm³, a glass transition temperature of 100 °C, a Vicat softening point of 90 °C and a heat deflection temperature of 78 °C; laser granulometry gives D10 = 33.38 μm, D50 = 77.87 μm and D90 = 138.82 μm, with a volume mean diameter of 82.52 μm and a surface mean diameter of 58.50 μm. Castings poured in such moulds reproduced the pattern surface without gas porosity.
The limitation of the fully gypsum-bonded route is the same property that makes it attractive. Gypsum conducts heat poorly, so filling is gentle and surface finish is excellent, but the casting solidifies slowly, the secondary dendrite arm spacing coarsens and the as-cast mechanical properties fall short of what a metal or investment mould delivers. The compromise adopted here is to use gypsum-bonded material only as a thin coating, a transfer layer or a core, so that surface finish and collapsibility are gained without imposing a slow-cooling bulk mould on the whole part.
Soluble gypsum cores: formulation window and measured properties
The core base was α-hemihydrate gypsum and water at a mass ratio of 8:3 (40 g gypsum to 15 g water). Three additives were screened in single-factor experiments, each at five levels against an additive-free control, with three specimens per condition and arithmetic means used for comparison. MgSO₄ acts as an accelerator and as a soluble phase; BaSO₄ is refractory, non-wetting toward aluminium and does not react with the melt; chopped basalt fibre, drawn from basalt melted at 1500 °C, retains 85% of its breaking strength at 400 °C and 80% at 600 °C and can be conditioned to resist shrinkage up to 860 °C, which covers a 720 °C pour.
| Series | Additive | Levels added to 40 g α-hemihydrate + 15 g water | Control |
|---|---|---|---|
| A | MgSO₄ | 10, 12.5, 15, 17.5, 20 g | 0 |
| B | Chopped basalt fibre, below 1 mm | 0.1, 0.15, 0.2, 0.25, 0.3 g | 0 |
| C | BaSO₄ | 5, 7.5, 10, 12.5, 15 g | 0 |
Slurries were cast into tube moulds, left 6 h at room temperature to set, dried at 100 °C for 6 h and then tested for water loss, impact resistance and collapsibility. Water loss was computed from the mass before and after baking:
$$Q_i=\frac{m_1-m_2}{m_1}\times 100\%$$
where \(m_1\) is the mass before baking (g), \(m_2\) the mass after baking (g) and \(Q_i\) the water loss (%). Impact resistance was ranked by the pendulum swing angle after fracture on an XJJ-5 simply supported beam machine (2.9 m/s impact velocity, 1 J and 4 J energies, 150° pre-angle, 230 mm span, 1.0 ± 0.1 mm jaw radius), with equal-diameter cylindrical specimens struck at the same position; a larger residual swing means less energy absorbed and therefore a weaker core. Collapsibility was measured as the time a fixed water stream played on the dried specimen produced a visible indentation.
The control lost 8.67% of its mass after 6 h at 100 °C but 30.43% at 200 °C, after which the curve flattens. The two-stage behaviour follows the dehydration path:
$$\mathrm{CaSO_4\cdot 2H_2O}\xrightarrow{125\ ^\circ\mathrm{C}}\mathrm{CaSO_4\cdot 0.5H_2O}\xrightarrow{190\ ^\circ\mathrm{C}}\mathrm{CaSO_4}$$
MgSO₄ behaves differently and dictated the test procedure. Specimens containing it cracked, spalled and effloresced once the temperature reached 300 °C, because the hydrate loses water in successive steps at roughly 55, 100, 150 and 300 °C, going from the heptahydrate to the hexahydrate, tetrahydrate, monohydrate and finally the anhydrous salt before decomposing to MgO with sulfur oxide release above about 900 °C. Baking time was therefore extended to 3 h so that each temperature level reached equilibrium. Below 200 °C, water loss falls as MgSO₄ content rises; above 300 °C it converges with the control, so the additive barely changes the final water loss. The fibre series tracks the control closely and decreases only slightly with fibre content, since basalt fibre bonds to gypsum mechanically and holds no water of crystallisation. BaSO₄ raises slurry consistency and makes mixing harder but leaves water loss essentially unchanged.
Impact results were unambiguous. Adding MgSO₄ reduces impact resistance monotonically, BaSO₄ reduces it slightly over the whole range, and only basalt fibre improves it. A plain gypsum core already outperformed a 3D-printed thin sand core by 43% (residual swing angle 62° versus 110° for the same struck position), which is the reason the gypsum route was pursued at all. Fibre length matters as much as fibre content: above 2 mm the fibres tangle during mechanical stirring and no suitable dispersant was found, while below 0.5 mm the mechanical key with the gypsum matrix is too weak to reinforce. The working window is therefore 0.5–1 mm, obtained by milling 12 mm fibre through a 1 mm screen.
Collapsibility falls steadily with MgSO₄ content. The salt is ionic and does not react with the hemihydrate; during drying and baking it dehydrates and leaves a network of micropores that improve permeability, and after the casting has seen 720 °C that network simply dissolves in water, so the core disintegrates without mechanical force. A 20% addition brings the collapse time to 9.3 min. Higher additions collapse faster still but take strength away, so the level was fixed at the point where both requirements are met.
The resulting formulation is 53% α-hemihydrate, 20% MgSO₄, 0.39% chopped basalt fibre, 6% BaSO₄ and 20% water. A verification batch made to this recipe gave the following properties:
| Property | Value | ||||||
|---|---|---|---|---|---|---|---|
| Water loss (%) at | 100 °C: 12.24 | 200 °C: 23.21 | 300 °C: 27.13 | 400 °C: 27.65 | 500 °C: 27.68 | 600 °C: 27.70 | 700 °C: 27.71 |
| Residual swing angle (°) | 28 | ||||||
| Collapse time in water (min) | 9.3 | ||||||
Mixing order is not a detail. Fibre added to a ready slurry stays agglomerated; the workable sequence is to disperse the weighed fibre in the water at 600 r/min for 5 min, then add the gypsum and the other additives while stirring and continue for 2 min. Sections cut from cores made this way show uniformly distributed fibre with no bundles or knots at 200× under an MM-4C metallurgical microscope.
Drying schedule and behaviour of the core in a real pour
Because the core carries a large water content and sees 720 °C metal, an inadequate bake produces gas porosity and can crack the core outright. The schedule derived from the water-loss curves is 100 °C for 6 h, 150 °C for 3 h, 200 °C for 3 h and 300 °C for 6 h.
The validation pour used a hand-rammed clay-bonded sand mould containing three cores side by side: the soluble gypsum core, a gypsum-plus-fibre core without MgSO₄, and a 3D-printed sand core with coating on one half only. After pouring, the casting was immersed in water at room temperature. The soluble core collapsed within 3 h and could be washed out with an ordinary water stream. The fibre-only gypsum core disintegrated partly but stayed compact inside the cavity, adhered to the cavity wall and was associated with gas porosity. The printed sand core was the hardest to remove and required sectioning the casting; its coated half showed a markedly better surface than the uncoated half, which exhibited sand penetration. Compared on the same casting, the gypsum core left a smoother contact surface than the printed sand core, and the soluble version left a cavity that could be cleaned completely.
A gypsum-based coating for 3D-printed sand moulds
The coating base is a gypsum slurry of α-hemihydrate and water at 1:1, to which a water glass solution equal in mass to the water is added; the water glass has a modulus of 2.2–2.4 and is diluted 1:2 with water before use. All additive percentages below are referred to the mass of that slurry. A four-factor, three-level orthogonal array covered quartz powder (5/10/15%, 200 mesh artificial quartz, 98% purity), mica (1.2/2.5/3.7%), sodium pyrophosphate (1.2/1.8/2.8%) and talc (30/35/40%, 400 mesh).
| Run | Quartz (%) | Mica (%) | Pyrophosphate (%) | Talc (%) | Flow (mm) | Strength grade | Suspension, 2 h (%) |
|---|---|---|---|---|---|---|---|
| 1 | 5 | 1.2 | 1.2 | 30 | 150 | 2 | 96.5 |
| 2 | 5 | 2.5 | 1.8 | 35 | 149 | 1 | 95.8 |
| 3 | 5 | 3.7 | 2.8 | 40 | 143 | 1 | 95.4 |
| 4 | 10 | 1.2 | 2.8 | 35 | 148 | 2 | 96.2 |
| 5 | 10 | 2.5 | 1.2 | 40 | 143 | 1 | 96.1 |
| 6 | 10 | 3.7 | 1.8 | 30 | 139 | 2 | 96.1 |
| 7 | 15 | 1.2 | 1.8 | 40 | 138 | 2 | 92.4 |
| 8 | 15 | 2.5 | 2.8 | 30 | 136 | 3 | 93.8 |
| 9 | 15 | 3.7 | 1.2 | 35 | 131 | 3 | 93.1 |
Flow was measured as the spread diameter after lifting a 50 mm internal diameter, 10 mm high cylinder; suspension as the settled volume fraction in a 100 mL stoppered cylinder after 2 h; surface strength by coin scratch on the dried layer, graded 1 (powder rubs off with a finger), 2 (powder comes off under light scratching) and 3 (powder comes off only under heavy scratching), with grade 3 accepted. The scratch grading is operator-dependent and yields a ranking rather than a value, which is why a pour trial was used as the final arbiter.
| Response | R quartz | R mica | R pyrophosphate | R talc |
|---|---|---|---|---|
| Flow (mm) | 12.33 | 7.66 | 1.00 | 1.34 |
| Strength grade | 1.34 | 0.33 | 0.33 | 1.00 |
| Suspension, 2 h (%) | 3.03 | 0.36 | 0.46 | 0.84 |
Quartz dominates all three responses. Because the gypsum slurry is denser than a conventional water-based wash, adding quartz or mica raises viscosity and suppresses levelling; pyrophosphate disperses the refractory particles and should therefore recover some flow, but at 1.2–2.8% its effect is within measurement noise. Quartz also strengthens the dried layer, whereas talc weakens it, which is consistent with water glass bonding quartz to the sand substrate more strongly than talc. That observation has a direct consequence for the talc-rich transfer coating described in the next section: it should be bonded with silica sol rather than water glass. Suspension stays above 92% in every run and deteriorates only when quartz exceeds about 10%.
Selection was weighted toward strength first, since a layer that fails during handling or pouring cannot improve surface quality at all; flow second, because a wash that puddles in corners displaces the casting dimension; suspension third. Two candidates emerged, and a brushing trial on scrap moulds separated them: the strength-first mix piled up at internal corners, while the flow-first mix stayed level, reached grade 2 strength and was accepted at 27% α-hemihydrate, 3% quartz, 0.64% mica, 0.64% sodium pyrophosphate, 16% talc, 27% water glass solution and 27% water.
The pour trial used a 3D-printed resin sand mould for an aerospace part whose inclined surfaces carried visible stair-stepping. Coating was applied to three quarters of the mould surface and the remaining quarter was left bare as a reference; the mould was dried at 180 °C for 6 h before pouring. The coated region gave a clearly lower roughness than the bare quarter, on which the printed stair-steps and slight sand penetration remained visible. Brush marks were partly smoothed by the scouring metal but not reliably, so 5% industrial anhydrous ethanol was added to the mix. No component of the coating reacts with ethanol; it evaporates in 2–3 h at room temperature and leaves the original solids ratio on the mould, which makes both brushing and dip coating practical.
Transfer coating for the non-occupancy route
The non-occupancy process, also called the transfer coating method, inverts the usual sequence. The coating is applied to the pattern, resin-coated sand is rammed against it and cured, and the pattern is then withdrawn, leaving the coating bonded to the mould face where the metal will flow. The technique was introduced in the 1970s and is known as the K-Y method after Komatsu and Yamanishi; a Soviet variant of the 1980s sprayed the coating into a core box at 240 °C and immediately filled it with resin-coated sand, curing the coating and the core in one step. It has been used for bronze relief art casting with water glass sand and water-based self-hardening coatings, for self-hardening formulations whose hardening time is controlled through the MgO content, and for lost foam, where a transfer coating containing 0.074 mm aluminium powder and 0.019 mm PbO-ZnO-Na₂O glass powder poured in AZ91 at 780 °C under −0.06 MPa produced a 20–30 μm ceramic surface layer and an alloyed layer with about three times the microhardness of the substrate. Published lost foam and full-mould formulations have used high-alumina bauxite with quartz, talc, graphite and Fe₂O₃ powder, bonded with resin and ethyl silicate, suspended with PVB and attapulgite in isopropanol; none of them transfers directly to a gypsum-based system, which is why a separate array was run here.
The transfer coating uses α-hemihydrate and water at 1:1 as its base, with silica sol as binder (SiO₂ 30 ± 1%, Na₂O ≤ 0.2%, pH 6.5–8.0, density 1.19–1.21 g/cm³ at 20 °C, particle size 8–15 nm) and MgSO₄ as the hardening agent. A three-factor, three-level array covered talc (20/30/40%), MgSO₄ (5/10/15%) and silica sol (5/10/15%).
| Run | Talc (%) | MgSO₄ (%) | Silica sol (%) | Initial set (min) | Crack resistance grade | Thickness (mm) |
|---|---|---|---|---|---|---|
| 1 | 20 | 5 | 5 | 21 | 2 | 0.53 |
| 2 | 20 | 10 | 15 | 16 | 2 | 0.92 |
| 3 | 20 | 15 | 10 | 12 | 3 | 1.32 |
| 4 | 30 | 5 | 10 | 20 | 2 | 0.61 |
| 5 | 30 | 10 | 5 | 15 | 1 | 1.31 |
| 6 | 30 | 15 | 15 | 11 | 2 | 1.54 |
| 7 | 40 | 5 | 15 | 20 | 1 | 0.74 |
| 8 | 40 | 10 | 10 | 16 | 1 | 1.53 |
| 9 | 40 | 15 | 5 | 12 | 2 | 1.76 |
| Response | R talc | R MgSO₄ | R silica sol |
|---|---|---|---|
| Initial setting time (min) | 1.00 | 8.66 | 0.33 |
| Crack resistance grade | 1.00 | 1.00 | 0.33 |
| Thickness (mm) | 0.42 | 0.91 | 0.13 |
Setting time is controlled almost entirely by MgSO₄: the anhydrous salt takes water into hydration water and hardens the layer, so its content sets the working window. Between 10 and 25 °C ambient, 10–15 min is the usable range. A layer that sets too fast has not yet keyed mechanically into the sand when ramming begins, and transfer fails at stripping; one that sets too slowly is squeezed during ramming and accumulates on complex pattern surfaces. Crack resistance was graded 1 (smooth, uncracked), 2 (cracks narrower than 0.5 mm without spalling) and 3 (cracks with spalling) after drying at 100 °C and holding at 720 °C for 2 min. All but one run fell in grades 1–2, and run 3, at 15% MgSO₄, spalled: the small range values show that the gypsum matrix, not the additives, governs thermal cracking, but excess MgSO₄ releases its hydration water too quickly under a fast heating ramp and opens the layer. A controlled drying schedule is therefore as important as the formulation. Natural coating thickness after levelling and setting should stay between 0.5 and 1.5 mm: thinner layers are damaged in handling, thicker ones insulate the metal and impede gas escape, and an excess poured on the pattern can form a step at the lower edge that delaminates when sand is rammed over it.
The accepted formulation is 33% α-hemihydrate, 30% talc, 10% MgSO₄, 10% silica sol and 17% water, giving an initial set of 16 min, an average natural coating thickness of 1.53 mm, which is compressed to about 1 mm after ramming, and crack resistance grade 1. As with the mould coating, the transfer layer is dried at 180 °C for 6 h before pouring.
Release agent and moulding sand: the two process details that decide success
The coating adheres to the pattern only by van der Waals attraction, so the release agent has to separate a weak interface without contaminating it. It must not react with the coating, must form a continuous film rather than beading, and must be cheap and environmentally acceptable. Four agents were compared: floor paraffin oil and edible rapeseed oil as oil types, and graphite and an industrial high-efficiency release agent as solution types. Films were assessed after 5 min; the paraffin oil and the industrial agent formed the most uniform films, with the two oils and graphite beading less evenly.
Stripping told a different story. Paraffin oil, rapeseed oil and graphite all damaged the transferred layer to varying degrees. Rapeseed oil failed completely: part of the oil merged into the coating and prevented it from setting, and the rest was squeezed to the coating–sand interface during ramming, lowering adhesion until the layer ruptured. Only the industrial high-efficiency release agent gave an intact coating over the whole pattern face.
The sand is equally decisive. With hand-rammed clay-bonded sand, the ramming force is set by the operator: too high and the transfer layer is squeezed and piles up at corners while thin areas tear; too low and the mould lacks strength, so the layer’s grip on the pattern exceeds the sand’s cohesion and the mould breaks during stripping. Resin-coated sand hardens on its own after filling and needs far less ramming force, so the layer stays uniform and complete, and it also holds dimensional tolerance better. Resin sand is the recommended backing for transfer coating.
Gating design and solidification simulation of a 464 × 512 × 541 mm housing
The demonstrator is a thin-wall housing measuring 464 × 512 × 541 mm and weighing 36.24 kg, with a minimum wall of 10 mm, a top lug with stiffening ribs, and an internal cavity containing a 10 mm wall and three shaft holes. The shaft-hole surfaces are functional and must be structurally sound, so the pouring position places the cavity face with the shaft holes at the bottom and keeps the shaft-hole walls vertical, which lets the metal fill the critical sections first and float slag and gas upward.
Two gating systems were compared. The first is a bottom pour with an open system and a sprue-to-runner-to-ingate area ratio of 1 : 1.8 : 1.8, positioned centrally so that every cavity wall is reached at a similar distance. The second is a vertical slot gating system with the runner at the side and a sprue-to-ingate ratio of 1 : 1, fed from two runners simultaneously because the initial filling rate of a slot gate is low and AC4B is prone to cold shuts at 15 mm average wall thickness. The plant controls pouring time for castings of this size between 15 and 25 s, and 20 s was used in the calculation, which gave a minimum choke cross-section of 1091.72 mm².
| Gating type | Sprue (mm²) | Runner (mm²) | Ingate (mm²) |
|---|---|---|---|
| Bottom pour | 1000 | 1800 | 1800 |
| Vertical slot | 2047 | — | 2033 |
Filling and solidification were simulated in Anycasting with the momentum, continuity and energy equations solved on a finite-difference mesh, using AC4B at a 720 °C pouring temperature, a resin sand mould at 25 °C and a 20 s filling time. In the bottom-pour scheme the metal enters with slight turbulence at 10% fill, the internal thin wall runs ahead of the surrounding cavity at 40% fill, and the flow settles only after 60%. Solidification is worse: the gating system freezes first, the internal thin wall starts at 5%, the top ribs at 10%, and the shaft-hole walls are the last regions to solidify at 70%, by which time the gate can no longer feed them, so shrinkage porosity is expected there. In the vertical slot scheme the two runners fill unevenly at 10%, but the metal passes the narrow slot smoothly without spatter; at 30% the internal wall and the outer cavity are level, and filling remains quiet to 90%. Solidification proceeds sequentially, the shaft holes farthest from the gate are still fed, and the gating system itself solidifies last, acting as a riser.
Defect probability was assessed with the retained melt modulus, which is the ratio of residual melt volume to residual melt surface area:
$$R_M=\frac{V_R}{A_R}$$
where \(R_M\) is the retained melt modulus, \(V_R\) the residual melt volume and \(A_R\) the residual melt surface area. A small modulus means a small, concentrated isolated pool and therefore a high probability of shrinkage. The probability maps confirmed the solidification analysis: the bottom-pour scheme scattered defects over more of the part, while the vertical slot scheme was clean at the gate and concentrated its remaining risk at the side shaft holes, the lug ribs and the internal thin wall. The vertical slot scheme was selected, with a riser placed on the top face, which is also a machining surface, so that gas pressure is held at the riser and porosity in the casting is reduced. Adding further risers at the remaining risk locations would have driven the yield below 37%, which rules the part out of economical batch production.
Local thermal tailoring and RMM-based verification of the composite process
The composite route addresses the residual shrinkage not by adding feeders but by changing the local heat transfer. Gypsum conducts heat far less readily than resin sand, so a transfer-coated gypsum core placed at a heavy section slows its cooling, extends its solidification time and lets the riser keep feeding it after the thin walls have frozen. The interfacial heat transfer coefficients used in the simulation were estimated from the standard duct correlation
$$h=0.023\,\frac{k_f}{D_f}\,Re^{0.8}Pr^{0.3}$$
with \(k_f\) the fluid thermal conductivity, \(D_f\) the equivalent diameter, \(Re\) the Reynolds number and \(Pr\) the Prandtl number, giving the values below.
| Interface | Temperature ( °C) | Heat transfer coefficient (W/m²·K) |
|---|---|---|
| Sand core / casting | ≥577 | 95.72 |
| Sand mould / casting | ≥577 | 95.72 |
| Sand mould / air | ≥25 | 41.9 |
| Transfer coating / casting | ≥577 | 0.748 |
| Transfer coating / sand | ≥577 | 0.748 |
The two risk zones identified by the modulus maps, the shaft-hole base away from the gate and the thin wall on the gate side, were rebuilt as separate cores made by the transfer route and re-simulated with these boundary conditions. The predicted shrinkage probability at both locations dropped below the level at which the part would be rejected, so the design moved to a pour trial.
Production trial: surface quality and microstructure
For the trial, the metal pattern for each risk-zone core was assembled, cleaned and dried, sprayed with the industrial release agent and left 5 min to film, then coated with the transfer mix, backed with resin sand under moderate ramming and stripped. The cores were dried at 180 °C for 6 h and assembled into a 3D-printed resin sand mould. No shrinkage porosity appeared at either predicted location, the thick end face was sound, and the transfer-coated contact surfaces were visibly smoother than adjacent sand-cast surfaces.
For a second, related aerospace part, the full combination was applied: large cores were dip coated to eliminate brush marks, small cores were replaced by soluble gypsum cores so that the internal galleries could be washed out, and the inner ring was formed by transfer coating while the remaining surfaces were brushed. Surface roughness was assessed against a standard comparator block by visual matching. The parts moved from Ra 50–100 μm to Ra 6.3–12.5 μm, meeting the drawing requirement, and the gypsum cores collapsed as intended. Metallographic sections showed a grain structure comparable to that obtained in an unmodified 3D-printed mould, so the hybrid does not degrade the as-cast structure of the alloy.
The economic effect is mostly in downstream operations. Because the gypsum core and the transfer layer provide surface quality and clean cavities, fewer risers and chills are needed, and the yield rises; the printed sand mould remains available for fast geometric iteration, which is where most of the development cost sits.
What remains unresolved
Three limitations are worth stating plainly. First, the soluble gypsum core cannot yet be printed, so a pattern is still required for every core geometry, which limits the design freedom that motivated the printed sand mould in the first place; a printable substitute with the same collapsibility would remove that constraint. Second, the MgSO₄ content sets strength against collapsibility in opposite directions, and no composition has been found that optimises both, so the 20% level is a compromise rather than an optimum. Third, the simulation was run with a conventional mould-filling solver and the gypsum cores were entered as separate bodies with adjusted boundary conditions; a solver that treats the composite mould as a single heterogeneous domain, with temperature-dependent properties for the coating and the core, would predict the shrinkage risk more reliably and reduce the amount of manual intervention still needed between simulation and process design.