Casting and Foundry Industry Calcium Silicate Insulation Products

High-purity calcium silicate insulation board for non-ferrous metal casting. Non-stick surface, low iron content. Launder, furnace, and ladle applications. Mingfa, Laizhou, Shandong, China. Founded 1991.

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Insulation Requirements in Non-Ferrous Metal Casting

Non-ferrous metal casting spans a wide temperature range across different metals and alloys. Aluminium and its alloys are cast at 700-750°C. Copper and copper alloys (brass, bronze) require 1100-1250°C. Zinc and zinc alloys operate at a relatively low 420-480°C. Each metal family imposes distinct demands on the insulation materials used in launders, troughs, holding furnaces, transfer ladles, and ingot mould assemblies.

For aluminium casting, the dominant requirement is chemical compatibility. Molten aluminium is a powerful reducing agent. It will reduce iron oxide (Fe₂O₃), silica (SiO₂), and many other metal oxides on contact. If the insulation material contains significant iron oxide, aluminium will reduce it to metallic iron, producing a contaminated aluminium-iron intermetallic at the contact surface and releasing heat that can create hot spots. Even trace iron contamination — above about 0.1% in certain aerospace and automotive alloys — causes defects in the finished casting: reduced ductility, pitting corrosion initiation sites, and machining problems from hard iron-rich inclusions.

Calcium silicate board with Fe₂O₃ content below 0.4% meets the purity requirement for aluminium contact applications. The board's calcium oxide-silica chemistry is thermodynamically stable against aluminium attack at casting temperatures — the free energy of formation of calcium oxide (-604 kJ/mol at 1000K) is more negative than that of aluminium oxide (-527 kJ/mol), meaning calcium is not displaced by molten aluminium. (Iron oxide, by contrast, has a free energy of formation of approximately -246 kJ/mol at 1000K and is readily reduced.)

A second critical requirement is non-stick behaviour. If aluminium splashes onto the insulation surface during pouring or transfer, it must solidify without bonding. Calcium silicate board with a smooth autoclaved surface skin achieves this: molten aluminium droplets solidify on contact and can be flicked off once cool, leaving the board surface undamaged.

LG-High Purity Calcium Silicate Board

Mingfa's LG-High Purity calcium silicate board is manufactured specifically for non-ferrous foundry applications. The key specification parameters:

Property	Value	Notes
Density	230-300 kg/m³	Dry density after 105°C oven drying
Fe₂O₃ content	<0.4%	By XRF analysis; standard LG board <1.0%
Thermal conductivity at 100°C mean	~0.06 W/m·K	Guarded hot plate method per GB/T 10294
Maximum service temperature	1000°C	Continuous rating; 1100°C short-term
Compressive strength	≥2.0 MPa	At 5% compression
Flexural strength	≥0.8 MPa	Three-point bending
Linear shrinkage at 1000°C (12h)	≤1.5%	GB/T 5486
pH value	7-9	Slightly alkaline; non-corrosive to aluminium alloys

The low iron content is achieved through raw material selection. Standard calcium silicate board uses ground quartz sand as the silica source; this sand typically contains 0.3-0.8% Fe₂O₃ as impurity in the quartz grains. For the high-purity grade, Mingfa uses selected quartz from a deposit in Shandong province with naturally lower iron content, supplemented by magnetic separation of the ground material before the autoclave process. The lime source (calcined limestone) is also selected for low iron — most limestone deposits contain less than 0.1% Fe₂O₃, so this is less of a concern.

Non-Stick Properties and Aluminium Contact

The non-stick behaviour of calcium silicate board with molten aluminium arises from the combination of three factors.

First, the surface skin formed during autoclaving. Under the saturated steam conditions (190°C, 12 bar) in the autoclave, a denser surface layer forms where the slurry contacts the mould. This skin has lower porosity than the board interior — typically 60-70% versus 80-85% — and presents a smooth, low-surface-area interface to molten metal. There are fewer sites for mechanical keying of solidified metal.

Second, the thermal shock effect. When a 750°C aluminium droplet contacts a room-temperature calcium silicate surface, the temperature gradient at the interface can exceed 10,000°C per millimetre. The aluminium freezes in milliseconds, before any wetting or chemical reaction can develop. The frozen droplet sits on the board surface purely by mechanical contact, not by chemical bonding.

Third, the absence of reactive species. Ceramic fibre products rely on organic binder (typically 2-8% by weight) to provide handling strength. At aluminium contact temperatures, this binder pyrolyses and can leave a carbonaceous residue that promotes metal adhesion. Calcium silicate board contains no organic binder — the xonotlite crystal matrix provides all mechanical strength through crystal interlocking.

This property has direct operational value. In casting plants, launder covers must be removed and replaced during alloy changes, maintenance, and cleaning. If aluminium has bonded to the cover, removal can tear the insulation surface. With calcium silicate board, solidified aluminium peels off cleanly. Plant operators at a secondary aluminium casting facility in Jiangsu province reported that launder cover replacement frequency dropped from every 4-6 weeks (with ceramic fibre covers) to every 6-8 months (with calcium silicate covers), representing a significant consumable cost saving and reduced downtime for cover changes.

Wetting behaviour at casting temperatures: Laboratory sessile drop testing conducted at Mingfa's Laizhou R&D centre measured the contact angle of molten Al-7Si alloy (A356, 720°C) on calcium silicate board surface at 135-145°. By comparison, the same alloy on alumina refractory showed a contact angle of approximately 110°. The higher contact angle on calcium silicate indicates lower wetting — the molten aluminium beads up rather than spreading. This confirms the surface chemistry provides genuine non-wetting behaviour, not merely a mechanical release effect from the surface skin.

Thermal Management for Casting Quality

Consistent thermal conditions are fundamental to casting quality. If molten metal cools too quickly during transfer from the melting or holding furnace to the mould, the consequences include:

Cold shuts: Two streams of metal meet but do not fuse because one or both have cooled below the liquidus temperature
Mispruns: Metal solidifies before completely filling the mould cavity
Gas porosity: Rapid cooling reduces the time available for dissolved hydrogen to escape from the solidifying metal, particularly in aluminium alloys where hydrogen solubility drops sharply at the solidification point
Oxide inclusions: Cooler metal is more viscous and carries surface oxide films into the casting rather than floating them to the riser

Insulated launders and transfer systems maintain the metal temperature within a narrow range from furnace to mould. Field measurements at an aluminium wheel casting plant illustrate the magnitude of the effect:

Uninsulated open launder, 10 metre length, aluminium at 730°C pour temperature, ambient 25°C: metal temperature at mould entry 610-650°C (temperature drop 80-120°C)
Insulated launder with 50mm calcium silicate board on sides and bottom, covered with calcium silicate lid boards: metal temperature at mould entry 700-710°C (temperature drop 20-30°C)

The tighter temperature control achieved with insulation enables lower furnace holding temperatures — reducing energy consumption at the furnace — while still delivering metal to the mould within the specified pouring temperature range. For aluminium, the typical specification is 680-750°C at the mould, depending on alloy and casting process.

Applications Beyond Aluminium

Copper and copper alloy casting: At 1100-1250°C, copper alloys exceed the continuous rating of standard LG-High Purity board. Mingfa supplies LG-High Temp (1100°C rated) board for copper casting applications, installed as backup insulation behind a refractory hot-face lining (typically high-alumina or silicon carbide crucible). The calcium silicate board provides thermal resistance on the cold face, while the refractory takes the direct metal contact. Typical configuration: 25mm silicon carbide crucible wall + 50mm LG-High Temp board for a 500kg holding furnace.

Zinc die-casting: Operating at 420-480°C, zinc casting is thermally mild but the insulation must withstand occasional splashes of molten zinc. Standard LG board (1000°C, 230 kg/m³) is adequate. Primary applications are holding furnace lid insulation and dosing furnace backup. Zinc does not chemically attack calcium silicate under any normal casting conditions.

Investment casting: In the investment (lost wax) casting process, ceramic shell moulds are preheated to 800-1100°C before pouring. After pouring, the shell mould assembly is surrounded by calcium silicate board backup insulation during the controlled cooling phase. The insulation slows cooling to prevent thermal shock cracking of the shell mould before the metal has solidified. Board thickness of 25-50mm is typical; density 230-270 kg/m³.

Ordering for Foundry Applications

To prepare a quotation, Mingfa requests the following information:

Metal type and alloy family — aluminium, copper, zinc, or other non-ferrous
Casting temperature and any superheat above liquidus
Application: launder/trough, holding furnace backup, transfer ladle, ingot mould hot top, investment casting backup, or other
Required dimensions: standard boards 600mm x 1000mm or 600mm x 1200mm, thickness 25-100mm in 5mm increments
Any custom machining requirements: launder profiles, curved hot top segments, crucible base pads, complex cut-outs

Mingfa's CNC machining capability enables supply of boards pre-cut to match launder cross-sections, hot top ring segments, and other non-rectangular shapes. CAD drawings in DXF or DWG format are preferred for custom machining.

For new applications, Mingfa can supply sample boards (typically 300mm x 300mm at the specified thickness) for compatibility testing with the customer's specific alloy and casting conditions. The sample programme is free of charge; the customer covers shipping.