Pressure indicating films use microencapsulated inks or transfer coatings to visually indicate, measure, and/or map the contact stress or pressure changes across a surface. They consist of one or two sheets and have separate color-forming and color-developing layers.
Types of Pressure Indicating Films
Pressure indicating films are sold by the roll or the sheet and differ in terms of size, materials, and performance. In terms of product type, suppliers may designate pressure indicating films as high pressure, super high pressure, medium pressure, low pressure, super low pressure, ultra-low pressure, or extreme low pressure.
Products for low-pressure applications consist of two sheets (two-ply), both with a base of a plastic such as polyethylene terephthalate (PET). One film is coated with a color-forming material and the other is coated with a color-developing material. The coated sides face each other so that when pressure is applied, the microcapsules on the color-forming film transfer the color to the color-developing material. Medium and high pressure films are usually single-sheets.
Pressure indicating films carry product specifications for various properties. The most important of these are pressure range, temperature range, reusability, and permanence.
- Pressure range is the range of pressures that the indicating film can accurately respond to, typically measured in megapascals (MPa) or pounds per square inch (psi).
- Temperature range is the range of temperatures at which the film will function properly.
- Reusability is whether the indicating film must be discarded after being used, or can be used multiple times.
- Permanence is whether the indication's on a film are permanent or temporary.
Thickness, precision, and recommended humidity range also need to be considered to fit the environment the film is being used in and application it is used for.
Applications for pressure indicating films range from industrial to experimental applications including brake pads, bolted joints, fuel cells, heat sealing, liquid crystal displays (LCDs), tire tread measurements, ultrasonic welding, and wafer polishing.