Most foundries can produce a casting that looks like a bogie. Fewer can produce a bogie casting that holds the tolerances that the RDSO drawing requires — dimension after dimension, heat after heat, without drift.
The difference between a foundry that delivers consistent dimensional quality and one that does not comes down to the moulding process before the steel ever goes in.
No-Bake sand casting — also called cold-box or chemically-bonded sand casting — is the process that LCPL uses for railway bogie components. This article explains what it is, how it differs from green sand casting (the most common alternative), and why the choice between them matters for bogie quality.
What Is No-Bake Sand Casting?
In No-Bake casting, sand is mixed with a chemical binder — typically a furan or phenolic resin system — that cures at room temperature when a catalyst is added. The mould is formed by packing the catalysed sand mixture around the pattern, waiting for the binder to cure (typically 15–45 minutes depending on formulation and temperature), and then drawing the pattern.
The result is a rigid, dimensionally stable mould that retains its shape during metal pouring. The cured sand can be stored for extended periods without dimensional change — unlike green sand moulds which begin losing moisture and strength immediately after moulding.
Green Sand Casting — The Common Alternative
Green sand casting uses a mixture of silica sand, clay (bentonite) and water as the binding system. The clay-water system gives the sand its “green strength” — the ability to hold shape before any metal enters. Green sand is fast, recyclable, and cost-effective.
For many casting applications — automotive, agricultural equipment, general engineering — green sand delivers acceptable results at the lowest cost per kilogram of casting.
For railway bogies, green sand has limitations that compound to create quality problems:
1. Dimensional drift from moisture loss A green sand mould begins drying from the moment it is formed. As moisture evaporates, the clay-water bond weakens and the mould experiences micro-contraction — typically a few tenths of a millimetre over an hour. For a simple casting, this is negligible. For a CASNUB side frame where the journal pocket dimensions must be within ±0.5mm and the pedestal brace width controls the axle assembly fit, this drift matters.
2. Mould erosion during pouring High-velocity liquid steel erodes green sand mould surfaces during the pour. Eroded sand particles enter the steel melt, creating oxide inclusions in the casting surface. These surface inclusions — called “sand inclusions” — are visible as hard, rough spots after machining. In non-critical areas, they are cosmetic. In the journal pocket where the adapter sits, they create stress raisers that contribute to fatigue crack initiation.
3. Gas evolution Green sand contains moisture — and moisture becomes steam when liquid steel at 1600°C contacts it. The steam must escape through the mould; if the mould permeability is inadequate or the pour is too fast, steam is trapped in the casting as blow holes — internal porosity that reduces mechanical properties.
4. Surface finish limitations The surface finish of a green sand casting is limited by the grain size of the sand and the moisture content of the mould surface. Typical green sand Ra values (roughness average) for steel castings are 12.5–25 μm. No-Bake castings routinely achieve 6.3–12.5 μm — a surface finish that requires less machining to reach the specified finish on wear-critical surfaces.
How No-Bake Overcomes These Limitations
Dimensional stability: The cured No-Bake mould is rigid and does not drift with time. Moulds can be checked dimensionally after curing and before pouring — any out-of-tolerance mould is identified before it becomes a casting defect.
Erosion resistance: The cured resin binder is much more resistant to the thermal and mechanical erosion of liquid steel than clay-bonded green sand. Sand inclusions are significantly reduced.
Lower gas evolution: No-Bake systems use resin binders rather than water. Gas evolution during pouring is from resin decomposition — controllable through binder system selection and mould outgassing design. There is no steam flash.
Better surface finish: The rigidity of the No-Bake mould maintains sharp detail from the pattern surface. Internal mould surfaces show less friability and erosion, producing a cleaner casting surface.
Complex geometry: The rigid No-Bake mould can accurately reproduce complex geometries — undercuts, thin walls, close-tolerance cores — that green sand struggles with. The CASNUB bolster’s fish-belly profile (thicker at midspan, tapering to the ends) is exactly the type of geometry that benefits from No-Bake precision.
The RDSO Dimension Set for a CASNUB Side Frame
To understand why dimensional accuracy matters, consider the critical dimensions specified in the RDSO drawing for a CASNUB 22HS side frame:
- Wheelbase tolerance: The distance between the two pedestal saddle centres must be 2000mm ±2mm
- Journal pocket width: Controls the fit of the adapter, which transfers the wheel load into the side frame
- Column width and parallelism: Controls the wedge fit and damping geometry
- Column wear surface flatness: Deviation from flat increases edge loading on the wedge friction surface
A green sand casting with typical dimensional scatter might produce wheelbase variation of ±3–4mm lot-to-lot. A No-Bake casting from a well-controlled process achieves ±1–1.5mm.
That difference sounds small. In service, a side frame with wheelbase 3mm oversize means the wheel set assembly is forced into position with a pre-stress on the adapter arrangement — which reduces fatigue life from the first revolution.
No-Bake at LCPL’s Andal Foundry
LCPL’s No-Bake casting facility at Andal is part of our fully integrated manufacturing sequence. The mould-making, pouring, and shakeout operations are on the same site as the EAF and the machining and testing facilities.
This matters for quality traceability — every casting produced at LCPL can be traced from the mould (identified by cavity number and pour date) to the heat of steel (from the EAF heat record) to the dimensional inspection (from the inspection report). This chain does not break because we do not send patterns to external mould shops or castings to external machining facilities.
Our 3D modelling and process simulation capability — using casting simulation software — allows us to verify gating and riser design before committing a new component to production. The simulation predicts where shrinkage porosity is likely to form, allowing gating design adjustment to prevent it.
FAQ — No-Bake Casting and Railway Bogies
Q: Is No-Bake sand casting more expensive than green sand? Yes, per kilogram of casting, No-Bake is more expensive due to resin binder cost and longer mould preparation time. However, the reduction in rework, machining stock, and defect rejection rates often offsets the binder premium for complex, tight-tolerance components like bogie castings.
Q: Can green sand castings meet RDSO dimensional requirements for CASNUB bogies? Yes, with careful process control — some foundries produce acceptable dimensional results with green sand. However, the process window is narrower, heat-to-heat consistency is harder to maintain, and surface finish is inferior.
Q: What is the No-Bake binder system used by LCPL? LCPL uses a furan-based No-Bake system, which provides good strength, reasonable pot life for large moulds, and acceptable shakeout characteristics after pouring. The specific formulation is production-tuned to our component geometry and pour temperatures.
Q: Does No-Bake affect the environment in the foundry? Furan binders release furfuryl alcohol and fume during curing and shakeout. Adequate ventilation is required — LCPL’s moulding area is equipped with local exhaust ventilation. Sand reclamation from No-Bake systems is less efficient than green sand, which is a disposal consideration managed through our foundry waste programme.
Q: Does LCPL use No-Bake for all components or just bogies? All CASNUB and LWLH bogie castings use No-Bake. Smaller components (wedges, pivot castings) may use shell moulding for certain production runs — a variant that also uses a chemically bonded sand system and provides similar dimensional advantages for smaller, simpler geometries.