Identifying, Understanding and Preventing Optical Defects in Injection Moulding

Weld lines occur wherever two melt fronts meet within the mould and merge again. They typically appear in geometries with obstacles, such as housings with holes or moulded parts with inserts.

Flow lines, on the other hand, follow the direction of melt flow. They can become visible as wavy, curved streaks or surface irregularities caused by uneven flow conditions, such as cooling effects or viscosity variations. Technically, these effects cannot be completely avoided. They are therefore often accepted – under one condition: their visibility should be reduced as much as possible. Processors can mainly influence these effects via the process.

Well-designed tooling with appropriately sized gate cross-sections, optimised gate positions, controlled flow front management, as well as adjusted melt temperature, mould temperature and injection speed all help to improve melt fusion and visual appearance.

At Lifocolor, we support you by jointly analysing materials, masking colours, effects, particle sizes and rheological behaviour. Based on this, we develop a masterbatch that minimises the visibility of weld lines and flow lines as far as possible.

Cloudy parts or visible streaks are classic examples of defects where both masterbatch and processing conditions may play a role. In most cases, the visual effect is caused by uneven pigment distribution or viscosity differences within the polymer matrix.

If streaks occur in specific, repeatable positions, the root cause often lies in the process – for example in mixing performance, shear effects, dosing or flow behaviour.

To prevent uneven distribution originating from the masterbatch, we focus on one of our core competencies at Lifocolor: highly homogeneous, agglomerate-free masterbatch formulations. At the same time, we precisely match carrier, colourants and additives to your polymer and processing conditions.

This ensures optimal pigment dispersion during processing. Where required, we also develop combination masterbatches with suitable additives, such as flow improvers.

Black specks or brown streaks are sometimes misinterpreted as contamination or masterbatch-related issues. In reality, they are usually caused by thermally degraded polymer melt.

Excessive temperatures, long residence times, high shear or trapped air can lead to local material degradation, visible as dark inclusions or streaks.The key influencing factors lie in the process and tooling: temperature control, screw design, venting and material flow must be carefully aligned.

We tailor our masterbatches to your processing parameters and ensure high thermal stability of our formulations, so they do not contribute to such defects under normal processing conditions.