Color Part V

On the continuing saga of color and the opportunities we have.

Shift in color during a production run tends to be an issue. There are a few things that can cause a shift in color, these being temperature, base materials and contamination / stabilization.
The temperature that one processes the material at is key. How we tell this is a problem is simple. One way is that upon start up the color is fine and after running for a period of time we note a shift. Or upon purging the machine the color shifts.  In both of these cases we have time, and temperature calculations.
1- We have an interruption in processing and let the material soak in the barrel. Upon start up we note that the color shifted but is going back to shade / hue we desire. This is what we expect, in some cases. The time factor here is key, i.e. how long did we leave the machine down?
2- We move the mold to a free machine noting that the barrel capacity is greater than where we normally run the mold or process. After processing for a while we note that the shade/hue is off from the initial product produced, or shifted from what is usually produced on the smaller machine.
3- We move the mold to a smaller machine size using more of the barrel capacity. But we increase heats due to through put and also the RPM for recovery time, and note a shift in the color / or don’t.
4- We compound up a color concentrate and use a base material that is clean in color. We than purchase brand X that is not as clean and note that the shade / hue is off. Or we note that the base resin comes in various shades of yellow each lot.
5- The base resin tends to be a temperature sensitive material, is the pigment shifting or the base resin.  Test by producing product without pigment.
6- We note that on certain colors for multicolor product we get black streaks. The streaks occur at a temperature increase point, i.e. the weld line area. This than goes back to a stabilization of the material / color and or contamination type problem.

In the above are examples of what is common.  When ordering color as stated earlier in this series, the temperature that the material is run at is extremely important. This need be transferred to the people manufacturing the color.  It is also important to note what is going on the floor. When we use a smaller press or larger press, or even going from the 10 ounce barrel to a 15 ounce barrel on the same tonnage machine what is happening to the melt temperature of our material.  How are we treating the base resin, an example is nylon which has oxidized in the dryer and we can’t seem to get a good white out of it. The use of higher heat stabilization for our color concentrates is not cheap and these cost monies. So if you find that you have a new grade/supplier for the same old color and tend to run into problems of color shift be sure that it is stabilized to the same degree as the previous.

Thanks for the time.

Steven L Silvey
Sr. Technical Service
General Polymers / Ashland Distribution Company