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Colour problems in injection-moulded silicone parts
Colour problems in injection-moulded silicone parts
Colour differences around the injection point are a known phenomenon in silicone injection moulding (LSR and LIM). On this page you will read the cause, why it mainly occurs with mixed colours and what measures you can take to achieve an even colour.
The problem
The phenomenon of colour differences around the injection point is usually caused by density differences of the pigments used and can be aggravated by a limited degree of dispersion, that is, the pigment particle size. Silicone in motion can carry pigments with different densities and sizes or shapes along in the flow in different ways.
Causes
Especially in and around the injection point, the flow of the silicone changes with each injection, sometimes the material even moves back and forth. Much depends on the injection system, but in general the material flow is high at the start of an injection and slows down as the end approaches. When the gate is closed, the material can even flow back a little due to the back pressure of the material in the mould. This makes the flow around the injection point relatively turbulent and multidirectional.
If pigments move along with the flow in different ways and in different directions, a kind of colour separation can occur, visible as concentration differences of those pigments in the parts. Concentration differences of pigments are unfortunately very visible, and this is what is observed in this case.
This happens mainly with mixed colour combinations and rarely with a single colorant that contains only one pigment. This does not mean that there are no pigment concentration differences with a single colorant, but because only one pigment is used it is hardly visible.
A green pigment combined with white often remains visually acceptable because the density of both pigments is close together. They move along with the flow in the same way, so no visible defect occurs. This is the case, for example, with cobalt green combined with titanium dioxide (white).
Solutions
The question is how to maintain or achieve colour homogeneity across the final part. There is no single solution, and often some trial and error is needed to determine which measure works best. Not all solutions are considered feasible or desirable by every user. Below are the measures found so far.
Stir the colorant thoroughly
Most buckets of colorant have a label with the note stir well before use. Doing this a little longer than usual can reduce colour separation effects.
Circulate the colorant continuously
Continuously circulating the colorant in the reservoir of the dosing unit on the injection moulding machine can also help.
Use a longer static mixer
A longer static mixer on top of the injection moulding machine improves mixing by increasing the mixing intensity and thus improving the initial homogeneity. Note: this can result in a slight capacity reduction due to the higher pressure drop across the longer mixer.
Extend the cycle time
A longer cycle time provides a longer mixing time. This lowers the output slightly but usually compensates for high waste percentages.
Adjust the mould design
For larger LSR or LIM parts, mould designs can contain double or even triple injection points per cavity instead of one. This reduces the material flow and the distance to be covered within the mould. Given the cause, this approach is mainly suitable for larger parts.
Improve the degree of dispersion of colorants
The degree of dispersion is crucial. The smaller the pigment particles in the colorant dispersion, the more uniform the final colour. Maximising the degree of dispersion costs extra time, labour and energy and can lower output, making it a trade-off between efficiency and quality. If none of the measures works sufficiently, discuss with your colorant manufacturer the options for increasing the degree of dispersion of the LSR colorants.
Mix pigments under vacuum
Mix pigments under vacuum. Larger air bubbles leave the material fairly quickly, but microscopic air bubbles can remain and grow exponentially when injected hot into the mould. This can lead to large air bubbles and thus to casting defects.
This information is based on years of experience and may not cover all causes and solutions. We cannot give any guarantee of a result and accept no liability. It remains up to the customer to test what works best.