Conductividad térmica frente a expansión térmica en el diseño de piezas a medida

In the manufacturing process, mastering heat management determines whether the component will fail on the first day or last for years. Understanding heat in terms of thermal conductivity and thermal expansion helps engineers understand the concept of heat transfer during the manufacturing of components. Temperature gradient is also an important factor for showing the difference in temperature between two points in a material.

Understanding how these two properties interact helps avoid real production problems. For instance, if you choose a mold steel due to its hardness but ignore its thermal conductivity, you will experience extended cycle time. Moreover, using a plastic-metal assembled design without checking CTE mismatch will wrap or bind in the field.

This guide shall lead us in understanding the importance of thermal conductivity and expansion and the impact on metals and engineering plastics.

Defining Thermal Conductivity and Thermal Expansion: Why it matters?

Thermal conductivity of a material indicates its ability to transfer heat under a temperature change. It is denoted by (k) and is measured in Watts per meter Kelvin (W/mK) ^[1]. In mold making, this property impacts the cooling cycle times, while in CNC machining, the heat dissipation during machining.

Thermal expansion describes how changes in size and volume occur as temperature changes. It is mathematically defined as the coefficient of linear thermal expansion CTE (∝=∆L/Lθ∆T) and expressed in units of   ×10⁻⁶/°C,10⁻⁶/°K or ppm/K.

∝: Coefficient of linear thermal expansion (CTE).

∆L: Change in material length (L-Lθ).

Lθ: Original material length.

∆T: Temperature change (T final-T initial)

This means a change of 1 K is equivalent to 1°C. To contextualize it, 1 ×10⁻⁶/°C= 1 ppm/K. To illustrate further, a CTE of 23 ppm/K means a 1-meter-long beam expands by 23 µm for every 1 K rise in temperature.

Diag. Thermal Expansion illustration.

CTE is more effective in combination rather than in isolation, such that two mating components with different CTEs expand differently with changes in temperature. This phenomenon causes several thermal fit failures in mixed-material assemblies.

Thermal Conductivity Variance across different metal like Mold Steels, Aluminum, and Copper Alloys

Metals exhibit different thermal conductivities. Determining their variation in thermal conductivity is crucial, especially in manufacturing processes where thermal management or temperature sensitivity is involved.

This table shows their thermal conductivity and the role they play in various industrial applications.

Thermal conductivity variance in metals is due to factors such as:

• Atomic structure: materials like metals conduct heat more easily because of highly ordered atomic lattices, which cause free movement of electrons.
• Material density: higher density means higher thermal conductivity. Closely packed atomic molecules transfer heat quickly. Silver density of 10.49 g/cm³ has high thermal conductivity compared to Aluminum of Density 2.70 g/cm³.
• Temperatura: A change in temperature affects thermal conductivity. When subjected to high temperatures, some materials tend to reduce conductivity, such as copper and iron, while others increase conductivity, like Silicon.

Why Thermal Expansion Mismatch is an Engineering Threat More Than Absolute CTE Values?

The engineering significance of CTE mismatch occurs when you assemble two materials with different CTEs together. As they experience temperature change, they expand and contract at different rates, resulting in the accumulation of mechanical stress at their interface. Repeated cycles lead to material failure, cracking, or thermal fatigue. In severe cases where delamination occurs, contaminants or moisture may penetrate and compromise the component.

Taking two metals like Steel and aluminum, Steel’s CTE spans through 12.6×10⁻⁶/°C while aluminum expands at around 23.6 ×10⁻⁶/°C, nearly double ^[2]. A 1°C rise in a 1-meter piece of each would have a mismatch of 11μm if clamped together.

Hence, pairing aluminum mold plates with steel guide pins requires clearance calculations occurring at operating temperatures, not just room temperature.

In electronic assembly, the PCB and conformal coating expand and contract during cycling. Expansion of FR-4 PCB is at 14 to 17 ppm/°C in the plane of the board, while on Z-axis falls between70 to 90 ppm/°C.  Different coatings can be used, such as Acrylic coating (70 to 120 ppm/°C), urethane (60 to 100 ppm/°C), and silicone (150 to 300 ppm/°C). This shows that many coatings expand and contact much faster than the PCB beneath them. This difference causes specific failures such as

1. Solder Joint Fatigue: Since coating surrounds solder joints and surface-mount components, as it expands and shrinks with heat, it pulls up on small chip pins (like BGA or QFP pins), breaking joints.
2. Glass Transition (Tg) Spikes: exceeding temperature of the coating’s glass transition temperature (Tg) can increase the CTE, which accelerates mechanical strain.

In insert molding, most metal insert components are enclosed in plastic housings. Brass threaded inserts have ABS enclosures, engineered with standoff geometry to absorb CTE gap. This is attributed to plastics expanding five to ten times faster than metals.

A single-part CTE rarely poses a problem since thermal change in a homogeneous part is predictable and easy to compensate.

How Should Engineers Design around CTE Mismatch in Multi-Material Assemblies?

Product designers can attribute to fact that they have encountered problems caused by differences in thermal expansion at least once in their operations. A component is supposed to fit perfectly under room temperature, wrap, crack, or distort when temperature changes. These failures are caused by CTE (α) mismatch.

Our engineers consider CTE from the earliest stages of designing components in material selection, tolerance analysis, and thermal evaluation.

Fits and joints calculation at operating temperature, not just room temperature

A fit can perform perfectly during assembly at 20°C but fail to maintain the same interference at 80–150°C if the materials differ in CTEs.

Matching material CTEs where possible

This is one of the most effective ways to minimize thermal stress on multi-material assemblies. They are likely to maintain dimensional stability if they expand and contract at the same rates. Beryllium copper (BeCu) and steel have nearly similar CTE values; thus, mixed steel/BeCu mold bases maintain stability in repeated thermal cycles.

Compliant Features at Plastic-to-Metal Interfaces

Incorporating compliant geometries like standoffs, ribs, or bosses manages thermal expansion differences better than rigid metal inserts.

Account for nonlinear CTE behavior near a plastic’s glass transition temperature (Tg)

Plastics like PEEK and other semi-crystalline polymers have CTEs that change significantly as they approach or exceed Tg. Relying on a single constant CTE value produces inaccurate design calculations.

Simulation and Prototyping to validate thermal performance

Design calculation provides a strong baseline, but real-world thermal behavior is dictated by dynamic operational variables. FEA and thermal simulations can predict how the material will behave when the temperature changes.

Engineering Plastics Comparison in Thermal Conductivity and Expansion

Beyond metals, plastics play an important role in manufacturing; therefore, determining their thermal conductivity is equally important. When it comes to thermal conductivity and expansion, plastics occupy the opposite end of both scales. The Transient Plane Source (TPS) gives accurate and fast measurements of a polymer’s thermal conductivity. Transient Line Source (TLS) measures the thermal conductivity of molten polymers.

ABS denotes acrylonitrile, butadiene, and styrene. The acrylonitrile component helps ABS improve its thermal stability.  Thermal conductivity of ABS ranges between 0.14 and 0.21 W/m·K. CYCOLAC™ resins, a brand of ABS, deliver excellent dimensional stability and a low coefficient of linear thermal expansion (CTE) among unfilled grades. Hence, it addresses the need for consumers’ aesthetics and comfort in vehicle interiors and other decorative components like grilles, door handles, and appliques.

POM (Delrin/acetal) has a thermal conductivity of 0.31 W/m·K with a Coefficient of Thermal Expansion (CTE) of 1.1 × 10⁻⁴ /°C. This expansion rate is among the highest compared to other common plastics. This property makes it a good thermal insulator and helps prevent heat build-up.

PEEK has an outstanding performance in dimensional and thermal stability. Its low thermal conductivity, ranging 0.24-0.26 W/m.K makes it a good insulator. With a Coefficient of Linear Thermal Expansion (CTE) of 45–55 × 10⁻⁶ /°C, PEEK’s Tg of 143°C can withstand high temperatures up to 260°C, above most polymers. This property makes it ideal for various engineering applications in the automotive and aerospace industries.

Nylon has very low thermal conductivity, falling between 0.22 and 0.25 W/(m·K); that’s why it makes a better thermal insulator than a heat conductor. For this reason, it is useful in the industrial production of components like engine covers and air intake manifolds.

Conclusión

This article has determined that thermal conductivity and expansion are more than material properties because they play a critical role in influencing product performance. Understanding how material transfer heat empowers engineers and innovators to create effective thermal solutions across industries.

At First Mold, with the help of manufacturers and engineers, we address these challenges at the earliest stages of our product development. Our components perform reliably under real-world operating conditions since we incorporate material expertise, DFM principles, and careful thermal analysis.

We’re passionate about helping you navigate these challenges with the help of our manufacturing team. We do not manufacture designs that simply work, but one that performs consistently throughout their life cycle.

Referencia

[1] Pelleg, J. (2012). Mechanical Properties of Materials. Springer. https://doi.org/10.1007/978-94-007-4342-7

[2] ASM International. (1990). ASM Handbook, Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys. ASM International. https://doi.org/10.31399/asm.hb.v01.9781627081610