Dimensional Stability of Polyester Film

Polyester-based photographic films are highly sensitive to environmental conditions, and dimensional accuracy can be significantly affected during handling, storage, or imaging. Four main factors influence dimensional changes in polyester film masks: temperature, relative humidity, mechanical stress, and ageing. Among these, humidity is the most critical and least predictable factor, whereas temperature changes are easier to control in a production environment.

Unlike glass components, such as soda lime or quartz, polyester film exhibits dimensional instability under environmental variations. Glass responds only to temperature loads, and has no response to changes in ambient humidity. Film, however, undergoes dimensional shifts due to both its polyester base and the gelatine emulsion layer, both of which are hygroscopic.

Response to Temperature Changes

All materials expand with increasing temperature, but the degree of expansion depends on the Coefficient of Thermal Expansion (CTE). Polyester and gelatine layers both exhibit a CTE of approximately 18 mm/m/°C, meaning that a 1-meter target will expand by 18 microns per degree Celsius increase. In contrast, soda lime glass has a CTE of 9 mm/m/°C, and quartz is extremely stable at just 0.05 mm/m/°C.

The thermal response of polyester film is relatively fast; when exposed to ambient air on both sides, it reaches thermal equilibrium (2/3 of total expansion) in just a few minutes. Glass, due to its higher thermal mass, responds more slowly. This quick thermal response of polyester must be considered during alignment and imaging processes.

Response to Humidity Changes

Humidity is a more complex and time-dependent influence. The dimensional change due to humidity is characterized by the Coefficient of Relative Humidity Expansion (CRH), typically around 1 µm/m/%RH for polyester film. The gelatine absorbs and desorbs moisture almost instantly—while the polyester base reacts slowly, over several hours.

This results in a two-phase expansion: roughly 50% of the dimensional change occurs within a few seconds (gelatine), while the remainder evolves gradually (polyester). The humidity sensitivity makes it essential to acclimatise film for a minimum of 8 hours at controlled conditions (21°C, 50% RH) before critical use.

Mechanical Tension Effects

Polyester film also expands under mechanical stress. If held under vacuum or tension during imaging, its dimensions may appear temporarily stable. However, once the vacuum is released, latent expansion or contraction can occur rapidly. Mechanical forces must be applied uniformly to avoid warping or localized strain.

Ageing and Irreversible Changes

Over time, polyester film undergoes minor but measurable dimensional shifts due to internal stress relaxation and material recovery. Additionally, extreme humidity conditions (<30% or >70% RH) can cause irreversible dimensional changes, a phenomenon known as hysteresis. At high RH, gelatine chains swell and lose mechanical coherence; at low RH, they form hydrogen bonds and lock the film into a reduced size state.

Recommended Handling Practice

• Store and use film in controlled conditions (21°C, 50% RH)
• Allow 8 hours for full acclimatisation prior to use
• Minimize mechanical stress and uneven vacuum exposure
• Avoid humidity extremes to prevent permanent dimensional drift
• Use quartz or soda lime glass masks where dimensional precision is critical