How Colorant Affects Shrinkage

I received a detailed question via email the other day, I will do my best to convey the nature of the question in this blog…

I am running as part in acetal and am having difficulty in maintaining the same dimensions as natural when running colorant:

When molded with natural: overall length = 49.33

Natural + yellow 130C: overall length = 49.32

Natural + green 326C: overall length = 49.20

To increase the overall length of the green part…
Mold temp was increased from 60 to 70ÂșC
Hold pressure was increased 65 to 110 bar
Hold time was increased to 6.0 to 7.5 seconds

As a result, the part was the correct length, but the weight increased from 23.96 to 24.29 grams.

My question is, is there any technical information regarding colors, pigments, etc. and their affects on materials?
My Response
There is much information available online, but there are a few things that you can also investigate in-house and with your supplier.
1. Any additive will change the melting characteristics of the polymer. You should always perform a tact temperature study as well as check and document the temperature of your melt with each polymer/additive combination. Some additives may cause the polymer to stick to the screw rather than the barrel, requiring an unnecessarily high screw speed to recover the shot. Adversely, one colorant may cause the polymer to melt very smoothly, resulting in a lower melt temperature. To better explain this, one of my customers (who processes only one base resin) showed me a screw with different colored stains along the transition zone. Each stain indicates the melting characteristic of that material/colorant combination. Some colors caused softening near the feed zone, while others softened closer to the metering zone with a rainbow of colors in between.
2. In additional to documenting the process outputs such as fill weight, peak pressure at fill, back pressure, etc. you should also consider taking a picture of the short so that you can visually match the appearance of the colored first stage short shots to the natural first stage short shot. Although most additives act as plasticizers, some additives will actually increase the viscosity of the polymer melt… especially if it reduces the melt temperature. This consequence can be improved by optimizing the screw recovery for each formulation you use.
3. Contact your material supplier and request any Technical Service Bulletins regarding acetals and/or the colorants you use. Also request drying specifications and annealing suggestions. In many cases, materials like acetals will provide more consistent dimensions when annealed.
Additional Thoughts
In semi-crystalline polymers, such as nylon, colorants and additives can also affect semi-crystalline site nucleation and growth. 
For, more information about tact temperature studies, please read: Optimizing Screw Recovery…
For free multimedia presentations on process documentation, scientific molding, and in-mold rheology, please visit:

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