1. The Fundamental Process Dichotomy
High-pressure green sand moulding has been the traditional workhorse of the global foundry industry. By bonding silica sand with bentonite clay, coal dust (sea coal), and moisture (3%–4%), green sand lines achieve rapid cycle rates for compact automotive castings.
However, when casting size increases (>50 kg up to several tonnes), green sand encounters severe operational penalties: high pattern wear from abrasive sand squeeze pressures, mould wall movement (dilation) leading to dimensional drift, high gas blowhole defects from steam generation, and heavy environmental sludge handling costs.
The Vacuum Sealed Moulding Process (V-Process) completely removes water, bentonite clay, and chemical resins. Mould rigidity is established purely by differential atmospheric pressure pressing unbonded dry silica sand against a heated 0.05–0.1 mm EVA plastic film covering the pattern.
2. Head-to-Head Technical & Operational Comparison
| Parameter | V-Process (Vacuum Moulding) | High-Pressure Green Sand |
|---|---|---|
| Moulding Sand Formulation | 100% Dry unbonded silica sand (AFS 50–70) | Sand + 8–10% Bentonite + 4–6% Coal dust + Water |
| Dimensional Tolerance (ISO 8062) | CT5 – CT7 (High accuracy, near net shape) | CT8 – CT11 (Subject to mould wall expansion) |
| Surface Roughness (Ra) | Ra 3.2 – 6.3 μm (Plastic film smoothness) | Ra 12.5 – 25 μm (Sand grain indentations) |
| Draft Angle Requirement | 0° to 0.5° (Zero draft possible on many features) | 1.5° to 3.0° necessary to strip green sand |
| Pattern Tooling Wear | Virtually Zero: Film protects pattern from sand abrasion | High: High-pressure abrasive sand wear requires frequent re-machining |
| Sand Reclamation Rate | 98% direct re-use (Dry screening and cooling only) | Heavy dead-clay waste dumping; fresh additions required |
| Gas & Inclusions Defects | Zero steam blowholes; vacuum continuous evacuation | Water vapor & volatile coal gases create pinhole risks |
3. Economics of Unbonded Dry Sand Reclamation
In a 10,000 TPA foundry, bentonite and additive purchases represent a continuous operational expenditure ranging from ₹1.2 to ₹1.8 Crore annually, plus substantial municipal tipping fees for spent black sand disposal.
In V-Process, when the vacuum is shut off after casting solidification, the sand returns instantly to a free-flowing dry fluid state. The sand slides smoothly out of the flask with zero mechanical vibration hammering, passing through magnetic separation, vibrating screens, and fluidized bed cooling before returning directly to overhead batch hoppers.
Because V-Process produces castings with 0°–0.5° draft angles and tight CT6 tolerances, machine shop stock allowances can be trimmed from 5–8 mm down to 2–3 mm. For high-alloy wear parts or ductile iron housings, this saves 15%–25% in raw molten metal pouring weight per piece.
4. When Should an Indian Foundry Choose V-Process?
V-Process is exceptionally advantageous for medium to large castings (50 kg to 5,000 kg) featuring flat expansive surfaces, thin ribs, or costly machining requirements—such as counterweights, piano plates, sanitary bathtubs, valve housings, mining pump volutes, and machine tool beds.