Definition
A caliper is a laboratory testing instrument used to evaluate the flow resistance of fluid materials such as coatings, inks, adhesives, etc. on vertical or inclined surfaces. The instrument quantifies the sagging properties of materials by simulating their behavior under gravity, providing key data for product formulation development and quality control.
Principle
The core working principle of the caliper is based on the shear flow behavior of the fluid under the action of gravity. The instrument typically comes with a set of standardized scrapers or multi-slot dies with varying gap thicknesses. For testing, the sample is coated on the test substrate and a squeegee is used to form a series of parallel coatings with decreasing thickness. The coated substrate is immediately placed vertically and left to rest for a specified period of time under the specified environmental conditions. The material has a downward flow tendency under the action of gravity, and the thicker the coating, the greater the shear stress. After resting, the material's resistance to sagging is evaluated by observing or measuring the critical thickness at which the coating stops flowing, i.e., the maximum non-sagging thickness. This process involves the investigation of the equilibrium relationship between fluid yield stress and viscosity.
Measurement method
The measurement of sagging performance mainly follows a series of international and national standards, such as ASTM D4400, ISO 16862, etc. The general procedure is as follows: First, the sample is evenly filled into the scraper slot of the hanging device. Subsequently, the squeegee is smoothly pulled across the substrate at a constant speed, forming a series of wet films of equal width but with continuous step-by-step thickness variations on the substrate. Quickly place the substrate vertically and start timing. Let stand for a specified period of time (usually 10 to 30 minutes) under standard temperature and humidity conditions. Finally, the maximum wet film thickness without significant falling flow is determined by visual observation or using a thickness gauge, which is the measurement result. Some precision instruments will be combined with optical sensors for automatic interpretation. The results are usually expressed in micrometers (μm) or mils (mil).
Influencing factors
The results of the sagging measurement are influenced by a variety of factors. The rheological properties of the material itself are decisive factors, including the yield value, thixotropy, and viscosity recovery rate of the fluid. The type and ratio of thickeners, fillers, and solvents in the formulation directly affect these properties. In terms of test conditions, ambient temperature and humidity affect the volatilization rate of solvents and the viscosity of the material. The scraping speed and angle during the coating process also affect the uniformity of the initial coating. In addition, the surface roughness and absorbency of the substrate can have an impact on the fluid interface state. Therefore, the standard method has strict regulations on the test environment, tool specifications, and operating procedures to ensure comparable and repeatable results.
Applications
Hanging gauges are widely used in industries that need to control the coating effect of vertical surfaces. In the field of architectural coatings, it is used to evaluate the sagging resistance of interior and exterior wall coatings, putty and waterproof coatings in construction. In industrial coatings, it is used to detect the leveling and sagging balance of automotive coatings, marine coatings and steel structure anti-corrosion coatings after spraying. In the adhesives and sealants industry, it is used to ensure that products do not slide excessively during vertical joints or façade construction. The printing ink industry uses it to control the printing quality of inks on packaging materials or signage facades. Together, these applications serve product performance optimization, construction process guidance, and final quality acceptance.
Instrument selection considerations
When choosing a dagger meter, it is necessary to consider multiple technical parameters and functional requirements. First, the squeegee specification of the instrument needs to match the viscosity range of the material to be measured and the expected film thickness, and common multi-slot molds should cover the applicable thickness from thin to thick. Secondly, the material of the instrument should be corrosion-resistant and easy to clean, usually stainless steel. For scenarios with high accuracy requirements, models equipped with automatic readouts or image analysis systems can be considered to reduce human error. The instrument should be designed to meet the specified dimensions of the relevant international or industry standard. In addition, it is necessary to consider whether it is necessary to be equipped with a constant temperature and humidity environment chamber to meet strict standard test conditions. Users should conduct a comprehensive evaluation based on the type of material they routinely test, standard compliance requirements, and laboratory budget.