SMC, aluminium and cast iron — housing materials

In short

Electrically insulating glass-fibre SMC avoids metallic corrosion and can allow built-in radio, but radio performance depends on formulation and construction. Aluminium offers higher thermal conductivity and established remelting routes. Cast iron’s high density needs to be weighed against form and structural requirements. Select materials for the actual product, environment and operation.

Overview table

The following table summarises the five principles at material-principle level. Exact values for a specific product should always be verified against the manufacturer's datasheet.

Overview table
Property	SMC composite	Aluminium	Cast iron
Electrical conductivity	Electrically insulating	Electrically conductive	Electrically conductive
Metallic corrosion	None in matrix	Corrodes; galvanic risk	Rusts
RF transparency	Possible; verify in the assembly	No (shields)	No (shields)
Thermal conductivity	Low; formulation dependent	High; alloy dependent	Depends on cast-iron grade
Weight (relative)	Low–medium	Medium	High
Recyclability	Thermoset; needs available recovery routes	Good	Good
Typical use	Luminaire housing	Housing, heatsink	Columns, heritage

1. Electrical insulation — Class I vs Class II

IEC 60598 divides luminaires into protection classes based on the complete luminaire construction. Class I uses protective earthing as a protective measure. Class II instead relies on double or reinforced insulation and does not require a protective earth conductor to the luminaire. The housing material alone does not determine the protection class. The luminaire's marking and documentation must substantiate it. (IEC 60598-1)

Aluminium and cast iron are electrically conductive, while SMC is electrically insulating. A metal housing does not automatically make a luminaire Class I. A luminaire with metal parts may also be Class II if the complete construction meets the requirements. The SMC housing contributes to VALDUR's insulation construction. The material alone does not determine the protection class. It does not replace the requirements for double or reinforced insulation in the complete luminaire.

VALDUR is a Class II luminaire. According to the installation instructions, it is connected without a protective earth conductor, and there is no earth conductor in its connection cable. The luminaire's Class II design must be verified against its marking and documentation.

Note

Class II applies to the luminaire, not the entire electrical installation. The rest of the electrical installation must still be designed and installed in accordance with applicable rules. Installation and any adaptations must be carried out by a qualified electrical contractor. Always follow VALDUR's marking and installation instructions.

2. Corrosion — matrix, surface, and galvanic risk

SMC composite (Sheet Moulding Compound) is a thermoset material of cured polyester resin reinforced with glass fibres. The matrix is not metallic and therefore cannot undergo metallic corrosion. This is the difference from aluminium. SMC is not “corrosion-proof” in all respects. It can age through UV exposure, heat, moisture, and chemical attack. These processes differ fundamentally from metal corrosion. (European Alliance for SMC BMC)

Aluminium is not particularly corrosion-sensitive in neutral environments, thanks to its oxide layer. But in coastal environments with chlorides and high humidity, corrosion accelerates. In addition, galvanic corrosion occurs when aluminium is in electrical contact with a nobler metal (e.g. stainless steel fixings and screws) in the presence of electrolyte (rainwater with salt). Aluminium is the less noble metal in the couple and corrodes preferentially. (TWI)

Cast iron can rust when exposed to moisture. Suitable coatings and maintenance can protect it. Applications include columns and decorative elements; its high density is a significant constraint where low weight is a priority.

See also moisture, condensation, and failure modes in street luminaires for a review of the most common corrosion mechanisms in the field.

3. RF transparency — what it means for wireless controls

Electrically insulating, glass-fibre SMC can allow an antenna inside the housing. Signal attenuation depends on formulation, fillers, wall thickness, frequency and antenna placement. Metal can shield radio signals, so performance must be verified in the complete luminaire.

A metal housing can use an external antenna or a radio window made from a suitable material. An SMC housing can reduce the need for external parts, but the complete construction must still meet its stated IP requirements.

In VALDUR, the SMC housing enables the built-in AirGlow module, where RF transparency is a prerequisite for the function.

4. Thermal conductivity — SMC's limitation and the solution

Glass-fibre SMC generally conducts heat much less readily than aluminium. Conductivity depends on the specific formulation or alloy, so a universal SMC value is misleading. Heat from the LED module needs a designed thermal path validated in the complete luminaire.

An internal aluminium heatsink can provide a separate thermal path for the LED module. An SMC housing does not remove the need for heat dissipation. The product design and thermal verification must establish the driver's location and its own heat path. The housing material does not show that the driver is mounted against the LED heatsink.

The combined construction is technically more complex than a pure aluminium housing, but enables functions that an all-metal housing cannot provide. This is a design choice, not a material deficiency.

5. Recyclability — an honest trade-off

Aluminium is one of the most recyclable materials in technical use. Recycled aluminium requires only ~5 % of the energy needed for primary production. This is a real environmental advantage at end of product life. (IAI)

Thermoset SMC cannot be remelted. Routes for glass-fibre composites include mechanical grinding and co-processing in cement production. Collection, material composition and local treatment capacity determine which route is actually available. End-of-life handling therefore needs to be agreed with the waste processor. (EuCIA)

The trade-off that is rarely mentioned

A fair environmental comparison needs product-specific lifecycle data for the same lighting function, assessment period and system boundaries. Operating energy, maintenance, actual replacements and end-of-life treatment must be included. The material name alone does not establish which option has the lowest total environmental impact.

Recycled aluminium, however, is straightforward environmentally. The difference between primary and secondary aluminium origin on a product declaration is therefore relevant for a correct lifecycle assessment.

SMC recycling is more complex than aluminium recycling. Material type alone does not show which option has the lower total environmental impact. The comparison needs product-specific lifecycle data using the same functional unit, service conditions and system boundaries. No service-life scenario should be treated as a product comparison without documented input data.

Cast iron — columns and heritage

Cast iron is an option for columns and decorative lighting where form and appearance are priorities. Compare the actual product mass, corrosion protection, maintenance and structural requirements.

VALDUR and material choice

VALDUR uses SMC thermoset composite as the luminaire housing. The electrically insulating housing contributes to the luminaire's Class II construction and enables a built-in AirGlow module without an external node, while the housing is not subject to metallic corrosion. The complete luminaire construction determines the protection class. Marking and documentation must substantiate it; the SMC material alone does not. The internal heatsink is aluminium — SMC replaces the housing, not the heatsink. Full material documentation is available in the technical guide: materials.

Next level of understanding

Corrosion freedom in the housing is one thing. Moisture inside the luminaire is another.

The SMC housing does not undergo metallic corrosion. Condensation can still form inside a luminaire when moist air cools below its dew point, regardless of housing material.

Lifespan & reliability

Moisture, condensation and failure modes in street luminaires

How moisture gets in, what it does there, and what the IP rating actually guarantees.

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Frequently asked questions

Does aluminium corrode?

Yes. Aluminium is normally protected by an oxide layer, but chlorides and high humidity can accelerate corrosion. If aluminium is in electrical contact with a more noble metal, such as stainless steel, and an electrolyte such as salt water is present, galvanic corrosion can occur.

Does a Class II luminaire need to be earthed?

No. VALDUR is a Class II luminaire whose protection relies on the complete luminaire's double or reinforced insulation construction. According to the installation instructions, VALDUR is connected without a protective earth conductor and its connection cable has no earth conductor. Do not make improvised earth connections to the luminaire. Always check the luminaire's Class II marking and follow the installation instructions. The rest of the electrical installation must be installed in accordance with applicable rules by a qualified electrical contractor.

Does SMC age faster than aluminium outdoors?

SMC and aluminium have different degradation mechanisms. UV, heat, moisture and chemicals can affect an SMC formulation; alloy, surface protection and exposure affect aluminium. Outdoor durability needs evidence for the actual material grade and conditions. The material name or addition of UV stabilisers alone does not establish product life.

Which material should be chosen for coastal environments with high salt content?

At high chloride exposure, the absence of metallic corrosion in the housing is the most important property. SMC composite has no metallic matrix and is not subject to the pitting and chloride-induced corrosion that can rapidly damage aluminium housings near saltwater. Metal fixings, screws and cable entries still need to be selected with galvanic compatibility in mind. For chemically aggressive environments such as wastewater plants and chemical industry, specific chemical resistance requirements should be verified per chemical agent.

Summary

The luminaire housing material affects installation, operation, and service life in five concrete ways: electrical material properties, corrosion behaviour, RF transparency, thermal conductivity, and recyclability. No material wins on all points.

VALDUR combines an electrically insulating SMC housing with an internal aluminium heatsink. The housing avoids metallic corrosion; radio performance and thermal management still need validation in the complete construction. SMC contributes to the Class II design, but the protection class belongs to the complete luminaire and is supported by its marking and documentation.

Cast iron is used for applications including columns and decorative lighting. Material selection should account for product mass, installation environment, wireless controls, protection class, intended service period and maintenance. No material is universally best for every design.

SMC, aluminium and cast iron — luminaire housing materials