Color Streaks
Color differences caused by poor pigment dispersion, masterbatch incompatibility or thermal damage. Materials with metallic pigments are particularly prone.
Color streaks are differences in shade running in the flow direction on the part surface. Their source is mostly the colorant not being dispersed homogeneously in the melt; one or more of the dosing, mixing and plasticizing stages has fallen short. Streaks seen in the first shots after a color change, on the other hand, are caused by residue and disappear over time.
How to Recognize It
- Streaks on the surface where the color shade changes in the flow direction; they may concentrate around weld lines.
- With metallic/effect pigment materials, orientation of the pigment with the flow is typical.
- If the streak is caused by thermal damage, the characteristics of burn marks are also seen.
Physical Causes
- Pigment agglomerates are not sufficiently dispersed in the melt (insufficient shear/mixing).
- The masterbatch carrier is incompatible with the base plastic or its MFI value is very different.
- The colorant particles have not dissolved fully in the melt.
- Pigment damage from excessive temperature/residence; excessive deformation of the part also creates color differences.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- If the processing range of the material allows, increase the back pressure and screw speed (mechanical homogeneity improves).
- Try increasing the injection speed.
- If there are signs of thermal damage, apply the burn mark procedure (lower the temperature/residence).
Mold, Design and Material Solutions
- Use a smaller pigment or a pigment paste/masterbatch; the MFI value of the carrier should equal that of the base plastic.
- Try a smaller granule.
- A permanent solution requires a screw with a high L/D ratio, shear-mixing sections or a non-return valve with an intensive mixer.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Gloss Differences | A gloss difference concerns the way the surface reflects light; the color is the same. With a color streak the shade itself changes. |
| Tiger Stripes | Tiger stripes are bands repeating from the gate and arise from flow instability. A color streak is a dispersion problem. |
Frequently asked questions
Is increasing the back pressure enough?
Back pressure improves mixing and in most cases is the first step to try. But too much raises the shear heat and leads to burn marks. Screw speed and mixing elements should also be assessed together.
How much effect does the dosing system have?
It can be decisive. With volumetric dosing, when the granule size or bulk density changes the ratio drifts and a difference in shade appears. A gravimetric system weighs every dose; the comparison is in the gravimetric and volumetric dosing article.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice