Measuring Small Plastic Parts Using the LabScan XE

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 sample port 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 of 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 LabScan XE 0/45 spectrophotometer can be used to measure the reflectance of individual small samples carefully positioned over a special, small port plate, or batches of samples can be read with the instrument in the port-up orientation and the samples in a glass or plastic sample cup or dish.

The LabScan XE

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. The proper size port insert must be used to limit the area measured by the instrument, and a flat area must be located for measurement, or else a group of samples must be measured through the clear window of a glass or plastic cup or dish 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 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.
Plastic pieces may be slightly fluorescent, which means that they will be sensitive to the UV content of the light source. If fluorescence is an issue, use of the UV control option is recommended.

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 opening in the port plate chosen for measurement. The variable sample illumination (VSI) option must be available with the instrument in order to use this measurement method.

1. Configure your software to read using the desired color scale, illuminant, and observer.
2. Install the 0.20-inch (5-mm) port plate and standardize the instrument for the 0.125-inch (3.175-mm) area of view. If the area of the sample to be measured will not completely cover the hole in this port plate without presenting excessive curvature to the instrument, you may install the CMR 2562 port plate (HunterLab Part Number CMR-2562) with a 3-mm opening instead. Other special port plates are also available that may suit your needs, such as capsule holders (HunterLab Part Numbers 04-7135-00 and CMR-2713; different sizes) and a tablet holder (HunterLab Part Number CMR-2714). First standardize on the black glass, then the white tile.
3. Locate a flat area on the sample, place it face-down over the sample port, and hold it in place, if required, using the sample clamp (HunterLab Part Number D02-1009-200), if available, with the white disk installed. Make sure that the area of the sample to be measured faces the port and completely covers the port opening. Use the retroviewer, if available, to check that the desired sample area is correctly positioned over the port opening.
4. Take a single color reading of the sample. Rotate the sample 90° or move the sample to another flat area containing the same color and read it at least once more. Alternatively, you may measure another part of the same color. 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.
5. Record the average color values.

Procedure #2: Measuring Small Plastic Parts in Batches. This method should be used to obtain an overall average of the color of a batch, particularly when samples are too small or too curved to be measured using Procedure #1.

1. Configure your software to read using the desired color scale, illuminant, and observer.
2. Orient the instrument with the port facing up. Replace the regular port insert with the special port insert for the sample cup (HunterLab Part Number 02-3081-14) if your parts are smaller than about 1/4-inch size. Otherwise, install the regular 1.75-inch port insert.
3. Standardize the instrument for the 1.75-inch area of view using the black and calibrated white standards that come with the instrument.
4. Scoop up or pour pieces from the sample batch to fill the glass sample cup (HunterLab Part Number 04-7209-00) to the top. The 2-inch (50-mm) sample thickness makes the translucent, irregular pieces effectively opaque for reflectance measurement. If your parts are larger than about 1/4-inch size, better opacity will be achieved if you use a 5-inch glass sample cup (HunterLab Part Number 04-7767-00) or a large Petri dish instead.
5. Place the sample cup in the port insert so that the pieces will be read through the glass bottom of the cup or place the dish over the port insert.
6. Cover the sample cup with the opaque cover (HunterLab Part Number 04-4000-00). The cover minimizes the possibility of ambient light reaching the detector through the piece sample when the instrument is in the port-up orientation. (This cover will not be used if you are measuring your sample in a larger dish.)
7. Take a single color reading of the pieces through the bottom of the sample cup. Dump, refill, and read the pieces at least five times from the same batch or, if you are using a large sample dish, move the dish so that a different area of sample is measured. 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.
8. Record the average color values for the sample batch.