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The Importance of Density Measurement
Measuring density is crucial in manufacturing many of today’s products. Being able to characterise the structure and quality of solid materials quickly and efficiently can not only help guide development and manufacturing processes, but also influence the quality of the end products.
In ceramic engineering, the most common method for producing ceramic components is to form a green body comprising a mixture of the ceramic material and various organic or inorganic additives, and then to fire it in a kiln to produce a strong, vitrified object.
The measurement of bulk density of, for example, green ceramic tiles during production is a fundamental parameter for the quality of the product. The porosity of freshly pressed ceramic bodies governs green tile behaviour during the process (drying, glazing, and firing) and largely determines the properties of the final product. This makes it necessary to control tile porosity during shaping. Due to the difficulty of measuring ceramic tile porosity, bulk density is the physical magnitude that is actually measured to control the pressing stage. By measuring the density of the product, it becomes easier to regulate porosity and adjust formulations accordingly. For example, decreases in density can signal the presence of voids, whereas increases may indicate that a process, such as a polymer approaching its crystalline state, is optimised. Density measurements must therefore be as accurate as possible, in order to reduce the number of cracks in the material which are proportional to the mechanical resistance and could potentially damage its structural integrity. As such, monitoring density could not only help with quality control, but could also reduce waste for manufacturers. It is therefore important that optimum measurement procedures are followed in determining density.
Measuring Bulk Density – Traditional Methods
Bulk density is defined as the total mass of a body divided by its bulk volume (g/cm3). Although determining mass can be achieved easily with a balance, finding out the volume presents a challenge. Many ASTM methods for determining bulk density of refractory materials and glasses are described in books published by the ASTM. Most of these methods are based on volume displacement by Archimedes’ principle; that is, since both refractory materials and glasses are compatible with water, their bulk density can be easily determined by volume displacement in water. However, this test method is not suitable for green body ceramics, which will disintegrate easily in water.
For green body ceramics, mercury displacement is recognised as the benchmark method for measuring the bulk volume of a body. Mercury is an excellent displacement liquid – its high wetting angle and large surface tension prevents it from penetrating into small pores in green bodies. Other advantages of this method are its ease of use and apparent high precision. Nevertheless, it has the drawbacks of being destructive, discontinuous, and manual. Furthermore, the high toxicity of mercury implies a grave health issue for workers performing industrial compaction controls and as such it is now illegal to use industrially in most parts of the world. This means that companies must look for alternatives to the use of mercury in tile bulk density measurement.
Gas pycnometry is one example of a laboratory technique that uses a displacement medium to determine ceramics’ volumes. An inert gas, such as helium or nitrogen, is introduced into an empty chamber, where the pressure is measured. When the sample is placed in the chamber and resealed, the same quantity of gas is expanded into the area at the same pressure. The difference between the two pressures, combined with the known volume of the empty sample chamber, allows the volume of the sample’s volume to be calculated. Although modern gas pycnometers have improved accuracy, most instruments are limited to testing small samples, making them unsuitable for many ceramic applications. In addition, this method relies on the sample material and analysis gas being completely free of moisture. The sample must also be pre-treated in a vacuum oven to remove volatile substances. Otherwise, contamination could cause pressure instabilities, potentially resulting in inaccurate readings. Errors can also occur due to leaks, temperature instabilities or thermal gradients.
Measuring Density Using a Ceramscan
Given the potential inaccuracies of displacement methods in determining porosity in ceramics, manufacturers have been keen to explore other options. Laser profiling has been pioneered as a non-contact, non-destructive method of measuring density, which is particularly suited to fragile samples, such as green compact ceramics.
Laser scanning equipment removes many of the inaccuracies and limitations of traditional density measurement techniques. Stable Micro Systems’ Ceramscan instrument works by loading a sample onto one of several platform options – one of which is a disposable plate to which the sample can be adhered to support it on its perimeter. The desired settings can be simply entered into the software before running the test. Measurements are automatically produced after each scan, including volume, surface area and density. It is also easier to compare samples of the same batch, as the mean, standard deviation and coefficient of variation are automatically calculated.
During the test, the product is weighed on an external balance – measuring to three decimal places. The laser device, which has been declared safe for eyes, is scanned vertically to measure the contours of the product at selectable intervals while it rotates. Each interval collects 400 data points, to provide a detailed profile. Density scans can be performed quickly using this method; ranging from a few seconds to several minutes depending on the chosen interval (0.05 mm to 50 mm) and the preferred degree of precision. Manufacturers can test samples up to 300 mm in height and 190 mm in diameter, with a weight of up to 10 kg (using the loadcell within the instrument), making it also suitable for tiles and tableware, among others.
Image Credit: Stable Micro Systems
Why Measure Volume, Density and Product Dimensions Using a Ceramscan?
- Fluid displacement techniques are not suitable for porous materials. The Ceramscan is a non-contact laser-based measurement that is not affected by sample porosity.
- Mercury displacement techniques represent a toxic risk to the environment and are therefore illegal in many countries. The Ceramscan has been verified as accurate as mercury displacement methods.
- Gas displacement techniques usually measure very small samples or otherwise attract a high instrument price. The Ceramscan has the largest measurement envelope in its class.
- X-ray techniques are expensive and present a health and safety issue and therefore require fully trained operators. The Ceramscan uses an eye-safe laser and therefore is the safest procedure for density determination.
The Future of Density Measurement and Beyond
Ceramics and other laminate structures need to display minimal porosity in order to maintain their structural integrity and ensure high quality end products. However, the fragile nature of the material means that it is particularly difficult to measure. While traditional methods of measuring density, such as gas pycnometry, are adequate in gaining a reasonable view of the sample, they are also dependent on stable conditions to make optimum calculations.
Having access to a method of measuring density that avoids damaging fragile ceramics is a lower cost and less time-consuming alternative to more traditional techniques. In addition, laser profiling instruments, such as Stable Micro Systems’s Ceramscan, offer promising replacements for traditional toxic methods, allowing manufacturers to make accurate judgements, by the safest means possible, to inform the formulation process and enhance the quality of their end products.
Density measurements of ceramics and advanced materials is amongst many other tests that can be performed on Stable Micro Systems’ physical testing instruments.
Image Credit: Stable Micro Systems
This information has been sourced, reviewed and adapted from materials provided by Stable Micro Systems Ltd.
For more information on this source, please visit Stable Micro Systems Ltd.