Measuring Core Thickness: Finding the Right Tool for the Job
By Harley Dakin, Empire Plastics
This article is intended as a follow up to the May/June 2004 article by Barry Cowles on the importance of material specifications. I think everyone can agree on the need for material specifications on core stock. This need is brought about by higher customer demands for quality and the ever tightening of price margins in every link in the supply chain. In this article, I will discuss and explain the importance of measuring the output of the process and how to choose a measuring tool that will give you useful information. I will focus on sheet thickness or what is generally known in the card industry as gauge.
Understanding the need for a specification on thickness of the (core) sheet.
The sheet thickness is the single most important contributing variable in the finished card thickness followed by overlay and then variation caused in the lamination process.
The finished card specification for thickness is covered under ISO IEC 7810.
The thickness tolerance becomes even more critical, considering the affect of embedding chips into a card. The cavity that is machined into the card body and used to secure the chip can cause problems when the thickness of the card is incorrect. This can cause over-height or under-height conditions of the chip location. Other core stock thickness variation can lead to embossing height problems and in extreme cases could even lead to overall poor print quality caused by uneven pressure from the blanket in the press.
How to measure the core or sheet thickness. The card industry generally uses micrometers to measure thickness of core and cards. This measuring tool has been used for years in the printing industry and like most industries, changing from the familiar to the unknown becomes a leap of faith. Micrometers come in many designs and price ranges. With my experience in the industry, you can often see people using micrometers with .001-inch resolution and interpreting to a measurement to the nearest .0001-inch. A micrometer may be a tool of choice for a “quick and dirty” method of roughly determining a sheet thickness but I do not believe a micrometer can be used to consistently establish a valid data point for decision-making. The older micrometers are typically vernier scale and are susceptible to parallax affect; simply put, different people see different values when looking at the same lines on the scale. This is addressed, to a degree, by use of digital or electronic micrometers, which eliminates the parallax affect.
To use a micrometer, you must hold the unit or the sample in you hand. This can transfer body heat to the sheet or micrometer resulting in less than true readings, especially when you are measuring thickness to the nearest .00001- inch. I believe it is best to use a measurement method that allows you to condition the sheet with a degree of confidence you are not affecting the final reading by temperature changes. A hands-off approach is best and using a single or multiple series of digital indicators allows you to take several readings at one time across the web or width, of the sheet without encountering the parallax or conditioning problem.
How do you choose the right measuring tool to do the job?
In this case, we need to determine which of the two methods yield the best results when measuring sheet gauge. To look at measurement with respect to capability, you must not only include the equipment being considered but you must also treat the entire process as a system. The purpose in taking a measurement is to determine variation within the lot of sheets or cards. The observed variation can come from three major sources. These include:
Using statistical methods, we can break out the measurement system variation and determine which of the two measuring methods best fit our application. For those familiar with six sigma, this is more well-known as MSA or Measurement System Analysis. This analytical method uses two or three operators measuring ten random samples, three times each.
A more detailed and comprehensive definition can be found in many statistical quality control handbooks and other sources on the Internet. Table A shows the results of an analysis on a Micrometer and Digital dial indicators (mounted to surface plate).
As you can see from the chart, the micrometer measuring tends to contribute more variation to the measuring system as compared to the Digital indicator. Another consideration to make when selecting a gauge is to apply the 10-division rule when determining the proper resolution for measuring equipment. Generally you should do this before beginning any MSA. This is a simple calculation and is done by subtracting the lower specification from the upper specification and dividing by 10 to calculate the value.
Table A
| Variation Source |
Micrometer |
Digital dial indicator |
| Equipment (Repeatability) |
38% |
7% |
| Operator/Appraiser (Reproducibility) |
5% |
6% |
| Sheet |
56% |
86% |
| Total |
43% |
13% |
The resolution of the measuring device or equipment must be equal to or less than the calculated value for 10 divisions of the specification. In our case, equipment with resolution or incremental measurements of .00005-inch is sufficient. Remember the cost of a measuring device is greatly affected by its resolution, so make sure the attribute being measured is important enough to warrant the monetary expenditure. As a side note, this measuring method is a supplement to many electronic Beta or constant scanning devices used to control gauge thickness of sheet during the manufacturing process.
Our exercise was a simplistic look at selecting measuring equipment used in the audit of core sheet thickness. It is important to remember that calibration of measuring equipment and training of operators is needed to maintain a proper measurement system.