Masterbatch are used to colour plastic products to the desired appearance.
Plastic goods manufacturers are well aware that Masterbatch Pellets may change colour during extrusion or processing due to excessive temperatures or other processing conditions. As these changes can be a challenge to control, it is important that plastic processors also measure the final object that the masterbatch is used for, say a PET water bottle.
It is however very useful to also manage the Masterbatch Deliveries on arrival, for conformity.
Regardless of the industry or application, if the input colour is close from batch to batch, then, all other conditions being equal, the output colour should be the same.
This is also a cheaper solution than say an in-line process control, as the chances are that you already have a bench-top instrument to measure colour.
As measuring masterbatch pellets can be a challenge, we have listed the following 10 tips to improve repeatability.
- Use a quality instrument with top-port measuring capabilities and precise accuracy.
We suggest here either the Konica Minolta CM-5 Spectrophotometer, or the less expensive Konica Minolta CR-5 Colorimeter. Both of these instruments allow the masterbatch to be measured directly on the top-port.
Konica Minolta have released the CM-36d/G Spectrophotometer range, which by nature has horizontal sample alignment, but can be used in a vertical position for top-port measurements. For this blog, we will specifically discuss the CM-5 and CR-5 and how they are used for masterbatch.
- Both of these instruments can operate without software, but for long term accurate corporate record keeping, as well as Template Managed conditions, we also suggest a quality software like Konica Minolta’s SpectraMagic NX Pro.
- You will also require the appropriate accessories to ensure reproducible results. These will work with the Standard Mask (30mm) supplied with both Instruments, along with the recommended basic accessory, the CM-A124 Zero Calibration Box.
The CR-502 Tube Cell (60mm x 40mm depth) accessory Glass Cell is required to contain a suitable quantity of pellets for a good accurate average reading.
Note: that these are not just any glass, but perfectly clear colourless glass, so as to not influence the colour measured. Standard glass Petri Dishes will add Yellow / Green to the colour, and as these cheaper glass accessories vary in colour, reproducibility will be poor.
- Plastics have the capacity to transfer light, so to reduce this “edge Loss Effect” we suggest the use of a CM-A519 Retaining Ring which eliminates this effect.
6. If you have been using the Instrument in another mode or setting, like in Transmittance Mode, you now need to set the Instrument to the correct settings: Select MENU | Measurement Conditions
> Select Reflectance Mode
> Select Target Mask size as ∅ 30mm
> Select SCI (Specular Component Included = Maximum Data)
> Some customers will select SCE (Specular Component Excluded) as the object is measured through a glass cell and they want to eliminate the effects of the glass on the measurement.
7. You may not have to change these, but check Color Settings:
Select MENU | Color
Most common Colour Space is L*a*b
Observer 10 degrees
8. You are now ready to take measurements. User Calibrate the Instrument according to the new settings using first the Black or Zero Calibration Box, and then the White Tile Calibration. Fill the cell with Masterbatch to be measured. Ensure that you always fill to the same level, and that there is enough in the cell for CONSTANT readings, i.e if the layer is too thin, light will escape through the pellets.
9. As the pellets are quite large, large shadow and light differences occur. To overcome this, it makes sense to measure a sample a number of times and take an average reading, say 4-6 time, each time turning the cell on its axis, to get a different viewpoint each time. Averaging can be pre-set at:
Select MENU | Meas option
Use Auto or Manual according to your preference
10. If you compare ABSOLUTE values, it is very difficult to visualise what a certain value difference means in visual terms. Also, as colour
measurements may vary slightly, to avoid confusion, we suggest to use a TARGET | SAMPLE procedure.
Take a KNOWN acceptable batch, and carefully measure in the TARGET VALUES.
Select a visually visible TOLERANCE between TARGET and SAMPLE typically Δ *a*b 1.5 say.
Measure samples AGAINST the target, and even with slight changes, the DIFFERENCE will always be accurate.
SAMPLE ABSOLUTE VALUES
TARGET | SAMPLE COMPARISON
These procedures are all clearly shown in our Konica Minolta CM-5 Spectrophotometer / CR-5 Colorimeter Manual available on OPEN RESOURCES on our website under MANUALS or contact us for further information.