Date | 2026-06-26 07:42:17
Engineers designing multi-material assemblies run into the same problem repeatedly: after a temperature change, parts no longer fit — the joint has gone loose, or it has seized completely.
Whether it is a metal insert inside a circuit breaker housing, the fit between a BMC divider and an aluminium frame in an energy storage module, or the assembly of an automotive headlamp reflector to its housing, temperature-driven dimensional change can affect both function and service life. Dimensional stability is one of the core advantages separating BMC/SMC from thermoplastic engineering plastics, and a major reason these materials are used so widely in precision electrical insulation parts.
This article starts from the basic concept of thermal expansion and works through the physical basis of dimensional stability in BMC/SMC and its engineering value.
Thermal expansion is the physical phenomenon in which a body increases in size as its temperature rises. Nearly all materials expand when heated and contract when cooled.
The coefficient of linear thermal expansion (CLTE) is the core parameter describing how much a material expands. It is defined as the relative change in length per unit length for each 1 °C change in temperature, expressed in K⁻¹ or 10⁻⁶/°C.
In elasticity theory, the thermal strain produced by a change in temperature is described by:
ε_thermal = α · ΔT
where α is the coefficient of linear expansion and ΔT the temperature change. When dissimilar materials are assembled together — a BMC housing and a metal insert, for example — a difference in expansion coefficient means the two deform by different amounts, generating thermal stress at the interface. If that stress exceeds the strength of the material, the result ranges from a loosened fit to cracking and failure.
The coefficients vary widely between materials:
Steel — approximately 12×10⁻⁶/°C
Aluminium — approximately 24×10⁻⁶/°C
Copper — approximately 16–18×10⁻⁶/°C
Thermoplastics (PA, PP and similar) — 50–200×10⁻⁶/°C
BMC/SMC — 13–35×10⁻⁶/°C
The reason behind the gap: thermoplastic molecules are linear chains. As temperature rises, the van der Waals forces between chains weaken and segmental motion intensifies, which appears macroscopically as large expansion. In BMC/SMC the thermoset resin cures into a three-dimensional crosslinked network, and chain movement is strongly constrained by chemical bonds; at the same time, the high loading of glass fibre and inorganic filler has a very low expansion coefficient of its own, which suppresses bulk expansion. Together these two factors put the expansion coefficient of BMC/SMC far below that of thermoplastics.
BMC has a coefficient of linear expansion of (1.3–3.5)×10⁻⁵ K⁻¹, that is 13–35×10⁻⁶/°C. The upper end of that range is above steel and aluminium, but in practice BMC is usually used in combination with metal and the temperature excursion is often limited (for example −40 °C to +85 °C or +125 °C), so the design clearance at the mating face can compensate for the difference.
SMC typically shows 12–16×10⁻⁶/°C, very close to steel. A part made from it therefore changes size almost in step with a steel structure, leaving very little thermal stress — one of the main reasons SMC is widely adopted where parts must mate with metal frames, such as automotive body panels and switchgear partitions.
The implication: over the same 50 °C temperature swing, a 100 mm BMC part expands by roughly 0.065–0.175 mm, while a PA66 part of the same size expands by roughly 0.4–0.5 mm. For a precision-fit electrical insulation part, that difference is enough to decide whether the assembly passes.
Beyond thermal expansion, moulding shrinkage is the other key parameter governing the dimensional accuracy of BMC/SMC parts.
Moulding shrinkage is the volumetric contraction that occurs as the material cools from cure temperature to room temperature during compression moulding. A thermoset resin undergoes chemical shrinkage as it crosslinks, and the addition of filler and glass fibre suppresses that shrinkage effectively.
With low-profile additives, BMC/SMC shrinkage can be held at a very low level. High-quality SMC can achieve zero shrinkage or no more than 0.5 mm/m, meaning a 1 m part shrinks by no more than 0.5 mm. The company's 17 series BMC holds shrinkage at ≤0.2%, which is a precision moulding grade.
Shrinkage occurs during the cooling stage of moulding, is one-time, and sets the initial dimensional accuracy of the part.
Thermal expansion occurs during service, is reversible, and sets how stable the part remains as temperature changes.
Together the two define the dimensional behaviour of a BMC/SMC part across its entire life cycle.
BMC/SMC parts are frequently used with metal inserts, copper busbars and aluminium frames. An expansion coefficient close to that of metal means no excessive thermal stress when temperature changes, which avoids cracking around inserts or loosening of the fit. This is particularly critical for threaded inserts in circuit breaker housings and for busbar supports in energy storage modules.
In high-voltage apparatus, clearance and creepage distance are directly safety-related dimensions. If an insulating part moves too much with temperature, clearance can shrink and raise the risk of breakdown. The low expansion of BMC/SMC keeps these critical dimensions stable across the full temperature range.
Thermoplastics, with their high shrinkage and high expansion, often require stock to be left for a second machining operation. The low shrinkage and low expansion of BMC/SMC allow a part to be moulded to final dimension in one step, with no downstream machining, which lowers production cost substantially.
One caution: temperature has little effect on the dimensional stability of BMC/SMC, but humidity does — the material swells as it absorbs moisture. Parts used in humid environments therefore need either a suitable assembly clearance allowed for at the design stage, or a low-moisture-absorption formulation.
Principles for grade choice: for parts with very tight dimensional requirements, prefer a low-shrinkage range (17 series); for parts that must mate with metal over the long term, prefer a grade whose CLTE matches the metal; for hot and humid applications, allow for moisture expansion in the design.
From the physics of thermal expansion to the actual figures for BMC/SMC and on to practical design considerations, dimensional stability is not a vague claim — it is a material property that can be measured, calculated and optimised.
BMC has a coefficient of linear expansion of 13–35×10⁻⁶/°C and SMC 12–16×10⁻⁶/°C, far below thermoplastics and close to metal. High-quality SMC can reach zero shrinkage or no more than 0.5 mm/m. The company's 17 series holds shrinkage at ≤0.2%. These figures underpin the wide use of BMC/SMC in precision electrical insulation parts, automotive components and energy storage structural parts.
Understanding the relationship between thermal expansion and dimensional stability lets engineers predict how a part will behave across the full temperature range at the material and design stage — rather than discovering at assembly that it is a little off.
BMC is 13–35×10⁻⁶/°C and SMC 12–16×10⁻⁶/°C. Steel is about 12×10⁻⁶/°C and aluminium about 24×10⁻⁶/°C, so SMC sits very close to steel and BMC is close to aluminium. Thermoplastics are far higher: unreinforced PA66 is 80–100×10⁻⁶/°C and PP 100–200×10⁻⁶/°C.
Thermoplastics have a linear molecular chain structure: as temperature rises, the van der Waals forces between chains weaken and segmental motion intensifies, which shows up macroscopically as large expansion. In BMC/SMC the thermoset resin cures into a three-dimensional crosslinked network, so chain movement is strongly constrained by chemical bonds, and the high loading of glass fibre and inorganic filler has a very low expansion coefficient of its own. The two factors together suppress bulk expansion.
No. Moulding shrinkage occurs during the cooling stage of moulding and is one-time; it sets the initial dimensional accuracy of the part. Thermal expansion occurs during service and is reversible; it sets how stable the part remains as temperature changes. Together they define the dimensional behaviour of a BMC/SMC part across its entire life cycle.
Temperature has little effect on the dimensional stability of BMC/SMC, but humidity does — the material swells as it absorbs moisture. Parts used in humid environments therefore need either a suitable assembly clearance allowed for at the design stage, or a low-moisture-absorption formulation.
For precision parts that mate with metal, prefer SMC or BMC 17 series (shrinkage ≤0.2%). For high-temperature environments such as an automotive engine bay, choose 18 series, with a temperature index ≥170 °C and stable dimensions at elevated temperature. For high-humidity environments, allow clearance for moisture expansion or specify a low-absorption formulation. Where clearance is critical, the low coefficient of expansion helps keep creepage distance compliant across the full temperature range.
For technical data sheets, grade recommendations, or a discussion about your specific requirements:
📧 wendy.qiu@smcbmc.com 📞 +86-13868305300
