Thermal Expansion and Photo Etch Railings for Model Ships

Over the years I’ve read numerous tips to avoid attaching long, continuous lengths of photo etch (PE) railings. The reasoning usually follows that it’s easier to work with railing no longer than 3 inches. I also recall that I would occasionally read a warning that a long length (> 3 inch) of railing may break away from the model due to changes in temperature. As I’m about to start work on a model ship, I thought I would first examine this effect and share what I learned.

The property of materials to expand and contract with changes in temperature is a well-known physical phenomenon. We can estimate how much a given material’s length will change using a simple correlation:

Δl=α·l₀·ΔT

Where Δl is the change in length, α is a coefficient of thermal expansion for the material of study, l₀ is the initial length of the material, and ΔT is the change in temperature (T_final-T_initial).

Given the equation above, we can now study the dimensional changes in both the styrene plastic as well as the brass PE railing. Experimentally derived values of α for styrene and brass are 70x10^-6m/m∙°C and 18.7x10^-6m/m∙°C, respectively; values for other materials are also available.

Let’s consider a model ship stored in two different locations: one where we control temperature and one where we do not. In this analysis I’ll say that a room with and without temperature control has a change in temperature of 11°C (27°C-16°C or 80°F-20°F) or 35°C (35°C-0°C or 95°F-32°F), respectively. We can then calculate the change in length for various materials and for different lengths of each material. See Figure 1.

Figure 1a. Linear thermal expansion for different lengths of
styrene and brass heated from 16 to 27°C and 0 to 35°C, respectively.

Figure 1b. Same as Figure 1a in English units.
The figure illustrates the following:

Styrene expands nearly 4 times more than brass. So, it's not that you need to worry about movement of the railing, but rather the model to which it's attached. You can calculate this as a ratio of the coefficients of thermal expansion (70/18.7 = 3.7).

Expansion/contraction is 3 times less in a room with temperature control compared to one without. You can calculate this as a ratio of the changes in temperature (35/11 = 3.2).

Despite the calculated differences, the magnitude of the change in length is always small (<< 1 mm for most conditions). This is true for all storage conditions, materials, and lengths examined.

It’s worth stating that this basic analysis lacks insight on several notable issues, including the adhesive strength, construction technique, and effect of repeated temperature cycling. There may very well be compromises where spots of a PVA adhesive offers mechanical flexibility or perhaps a continuous bead of CA glue proves quite rigid and strong. One may even speculate that years of seasonal temperature changes could cause small cracks to form and allow the railings to break away from the ship - it’s unclear.

For short lengths (< 3 inch) it would appear that you have little to worry about regarding changes in temperature. However, for longer lengths (< 6 inch) it may be worthwhile to slightly overlap the ends of the railing to allow for seasonal movement and avoid the potential formation of gaps between each run.

Good luck!