How to measure the colour of Metal

How to measure the colour of Metal

Measuring the colour of metal

There are a variety of different forms of metal and many different uses of these. The forms touched on in this article will be bare metals and powdered metals, although the measurement methods can be modified to suit most different forms of metal.

Possible quality issues

The importance of colour measurement within this industry is extremely important; with the types of products made, such as metal cabinets or even car panels, discrepancies in product colour and appearance would be considerably noticeable. With some products being manufactured, they may not always be completed and assembled at the same time, meaning some panels could be constructed a while after the rest of the product. The issue caused by this could be that the hue of the paint used could be slightly different over time from batch to batch. This could cause a noticeable discrepancy when all the panels are assembled to form one unit and some panels could stand out as being different in colour.

This issue would be found more in processes that use materials such as powder coatings as there could be a variety of colours available and, as stated, the differing hues could cause an issue in terms of consistency from batch to batch. The quality of these colours may not be reliable and noticeable differences could be seen between batches of the intended same colour from a supplier.

How the metal is prepared and treated could also cause a quality issue; if different amounts of chemicals or coatings are applied, they could have an effect on the final appearance of the product. The presence of gloss or shine on the finished products can also cause a variation in appearance depending on the light and also the angle it is viewed from.

Product-wise, if we think of metal sheets or cabinets, simply analysing their appearance by eye would not be very accurate. The overall appearance of the surface would change depending on the angle they were viewed from, the lighting they were viewed under and even the observer analysing the product. If colour measurement analysis is currently only being quantified by eye, a more accurate methodology needs to be put in place.

Considering a method

When thinking of a method to suit the current manufacturing and quality control processes, factors to take into account would be the physical characteristics of the products requiring measurements. Are the surfaces smooth and flat? Are they textured? How large is the available surface for measurement? Are the products glossy or have a shine to them? Other factors to take into account should be the time available for measurements, practicality, the number of operators and what the results will be used for.

All of the above factors need consideration when thinking of using an instrument for appearance analysis. Good, consistent sample preparation is key so factors such as where on the product the measurement should be taken from should be discussed and a convenient sample area for all product measurements should be agreed upon.

A good methodology to follow would be the following:

Instrument used

The instrument used could either be a diffuse spectrophotometer or a spectrophotometer with directional geometry. For portability and ease of use, a MiniScan EZ of either 4000S/L or 4500L models could be used as this will give results using CIE L*a*b* and provide Pass and Fail notifications for samples when related to a standard.

Sample Size

Depending on the size of the available space for measurements on the products, an instrument should be chosen that can accommodate a port size for the corresponding sample size. Whatever the port size, this should remain consistent for all future measurements of the samples to keep data consistent.

Quantity of measurements

Generally speaking, the more measurements the better. If using either diffuse or directional geometry, an average measurement should be taken. By taking three or more measurements of the sample, particularly on larger samples, and using this data to calculate an average, it gives a broader view of the sample as a whole. Particularly as the samples being analysed can be slightly glossy in appearance, taking average measurements would provide a more accurate representation of the product.

This method is an idea of the procedure that could be used for appearance analysis of the product. This method can, of course, be modified to suit an individual as long as the method chosen remains the same for all measurements being taken, particularly those of a standard and the samples being compared to that standard.
The data collected using such measurements can then be used as required. This data could be sent to other sites (if applicable) for deliberation or for altering the manufacturing process in some way.
This methodology should utilise a spectrophotometer that can give quantifiable data that can be used to ensure the correct colour of any coatings but also aid in the rest of the production process to help increase the quality of the processes and the units they produce.

Uses of colour measurement and appearance analysis

The most obvious use of measuring colour used by most applications is to ensure the consistency of the products made. By using numerical data from each product, it is very clear to see how alike they are to products from previous batches. Another use of colour analysis could be to ensure a consistent production process. By taking colour measurements at suitable stages, the most effective manufacturing method could be produced; a manufacturing process that causes minimal errors and more panels of consistent quality ready for coating.

It is possible to work backwards from a finished product to see where, if any, errors were made; once measured for colour to ensure it is of the level of quality. If in-depth details are kept of the entire production processes, it is possible to make certain alterations according to the finished products. For example, if the final product appears darker than normal, each individual stage could be evaluated to see how they compare to the production stages of a good final product. Utilising a spectrophotometer for this function can help reduce the time spent recoating the unacceptable products and also reduce the cost of having to recoat panels.

The instrument chosen should be capable of being taken to the production line for measuring and storing sample data and even comparing that sample data to pre-determined standard data already stored on the instrument, with the ability then to export the data onto a PC for a more visual analysis. For a portable instrument that can give reliable results, it is worth having a look at the MiniScan EZ instruments. These instruments are available in both geometries with small or large area views, depending on sample size and are handheld so can easily be used at the production line.

The final question to address would be if you wanted to measure the colour of the product or the appearance. Diffuse instruments tend to give accurate colour measurements, negating the specular reflections whereas an instrument that uses directional geometry will give data that best represents what a human sees when looking at the product surface. In this industry, both geometries have been used with both being capable of providing the necessary data reliably. The choice of which one to use is dependent on the individual manufacturer.

Recommended Product
How to measure the colour of Metal

HunterLab MiniScan EZ Spectrophotometer

The MiniScan EZ 4000S is a light weight hand-held spectrophotometer used for measuring the reflected colour of samples. This instrument is designed for one handed operation with thumb-tip navigation of functions. This portable spectrophotometer can be used right at the production line at any appropriate moment as required and gives easy to read colour data.

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