Measuring Small Plastic Parts Using the MiniScan EZ

In the plastics industry, the color of small extruded plastic pieces must often be assessed, primarily to ensure lot-to-lot color consistency. Plastic pieces are typically translucent and non-uniform in size and shape. Therefore, special accessories and presentation techniques are required to provide repeatable results. In some cases, a flat area large enough to cover the nose cone may be available so that the pieces can be measured individually as described in Procedure #1, below. Other times, a sampling of a number of plastic pieces should be measured together in order to obtain an overall average of the color for the batch. Several readings of the part or group of parts should be averaged for the final result, preferably with replacement of the sample between measurements.

A HunterLab MiniScan EZ spectrophotometer can be used to measure the reflectance of small samples carefully positioned over the sample port either individually or, if that is not possible, in a batch. This is the method advocated by HunterLab for the measurement of small plastic parts if a LabScan XE with UV control is not available. The diffuse/8° SAV model MiniScan EZ has a 0.3-inch (8-mm) viewed area and the 45/0 SAV model has a 0.2-inch (5-mm) viewed area. These models can measure individual parts slightly larger than their viewed areas. If the individual part is not large enough to completely cover the hole in the nose cone of the MiniScan EZ you have available, then you should measure your samples in a batch. This method also applies to other small, translucent or opaque items, such as tablets, capsules, nuts, beans, and small candies.

The MiniScan EZ

The Application

Plastic parts have several non-uniform characteristics that require compensating preparation and presentation techniques in order to ensure a repeatable sample measurement.

The available measurement area may be very small and/or curved. A flat area on the sample that will completely cover the hole in the nose cone must be located for measurement, or else a group of samples must be pressed into a flat layer at the sample port in order to be effectively made into a solid.
These pieces are irregular in size and shape, requiring the averaging of several readings with replacement.
Plastic pieces may be slightly fluorescent, which means that they will be sensitive to the UV content of the light source. If this is the case, consideration should be given to using an instrument with a UV control option, such as the LabScan XE.
Plastic pieces may be translucent � not opaque � and will be sensitive to ambient light and to small differences in the optical configuration of the instrument. Using a sufficient sample thickness and/or constant background will minimize these effects.

Note: If inter-instrument agreement is a concern when measuring translucent plastic pieces, all the instruments used for these measurements MUST be the same model to minimize measurement differences.

Recommended Color Scale	CIE Lab* or Hunter L, a, b as a full color descriptor
Recommended Single-Number Indices	YI E313 for indication of yellowness, WI E313 for indication of whiteness, dE CMC for visible total color difference.
Recommended Illuminant/Observer	D65/10°. C/2° may also be used.

Measurement Methods

Procedure #1: Measuring Small Plastic Parts Individually. This method is preferred so long as a relatively flat sample area is available that can completely cover the hole in the nose cone of the available instrument.

1. Configure your software or the instrument firmware to read using the desired color scale, illuminant, and observer.
2. Standardize the instrument, first using the black glass or light trap to set the bottom of the scale. Make sure the black glass is in solid contact with the port or the light trap completely covers it.
3. Complete the standardization using the calibrated white standard.
4. Locate a flat area on the sample and place it at the instrument nose cone with the side to be measured toward the instrument. Make sure that the area of the sample to be measured completely covers the hole in the nose cone without presenting excessive curvature to the instrument.
5. Take a single color reading of the sample. Rotate the sample 90° or move the nose cone to another flat area on the sample (or another sample of the same color) and read it at least once more. Average the multiple color readings for a single color measurement. Averaging multiple readings with rotation or movement between readings allows you to better estimate the color of the piece as a whole.
6. Record the average color values.

Procedure #2a - Measuring Plastic Parts as a Batch Directly in a Cup. This method has the advantage of measuring the piece color directly, but is dependent on the operator making sure that the pieces are flush against the measurement port. A white cup, such as a Styrofoam or white plastic drinking cup, the 45/0 LAV model of MiniScan EZ, and the MiniScan EZ nose cone with cover glass (HunterLab Part Number A02-1002-129) are required.

1. Configure your software or the instrument firmware to read using the desired color scale, illuminant, and observer.
2. Standardize the instrument, first using the black glass to set the bottom of scale. Make sure the black glass is in solid contact with the port.
3. Complete the standardization using the calibrated white standard.
4. Fill the white cup with plastic pieces.
5. Place the nose cone with cover glass of the MiniScan EZ into the pieces and read. The cover glass keeps the pieces from entering the measurement port and the thickness of the layer of pieces makes it effectively opaque. Dump, refill, and read the pieces at least five times from the same batch. Average the five color readings for a single color measurement representing the color of the batch. Averaging multiple readings minimizes measurement variation associated with non-uniform samples.
6. Record the average color values for the sample batch.

Procedure #2b - Measuring Plastic Parts as a Batch Directly in a Barrel or Container. This method allows you to measure pieces directly in the shipping container but is dependent on the operator making sure that the pieces are flush against the measurement port. The 45/0 LAV model of MiniScan EZ and the MiniScan EZ nose cone with cover glass (HunterLab Part Number A02-1002-129) are required.

1. Configure your software or the instrument firmware to read using the desired color scale, illuminant, and observer.
2. Standardize the instrument, first using the black glass to set the bottom of scale. Make sure the black glass is in solid contact with the port.
3. Complete the standardization using the calibrated white standard.
4. Place the MiniScan EZ into a crate or large box filled with plastic pieces and read. The cover glass keeps the pieces from entering the measurement port and the thickness of the layer of pieces makes it effectively opaque.
5. After one measurement, lift the MiniScan EZ, replace the instrument in a different spot, and read. Remove, replace, and remeasure the sample to a total of at least five times from the same batch. Average the five color readings for a single color measurement representing the color of the batch. Averaging multiple readings minimizes measurement variation associated with non-uniform samples.
6. Record the average color values for the sample batch.