5 Common Defects in 2K Injection Moulded Light Guides and How to Avoid Them

First confirm what the two shots are intended to achieve

Not every automotive light guide is a two-shot component. Some designs use a single transparent material; others combine an optical region with another colour or material for a specific function. This article addresses the latter projects. It describes five useful defect categories, not a ranking based on production statistics or a universal diagnosis for every light guide.

Before troubleshooting, label the first shot, the second shot and their intended interface. ARBURG’s multi-component guidance highlights material selection and chemical and thermal compatibility. Two materials fitting into one mould does not establish that they will bond as required. Agree whether the design needs adhesion, mechanical retention or another interface function, and establish suitable acceptance checks for that specific construction.

1. Splay, streaks and bubbles in the transparent region

Transparent components can make small material or processing inconsistencies easy to see. Covestro explains that moisture can cause streaks and bubbles during moulding, and may also damage moisture-sensitive polymers. However, a bubble is not a diagnosis by itself. Röhm’s processing guidance also identifies filling, insufficient shrinkage compensation and entrapped air as possible causes of internal bubbles.

Examine the first shot separately and compare it with the completed component. This establishes whether the symptom already exists or appears during the second operation. Keep photographs in consistent lighting and record the location, material batch and trial sequence. That evidence is more useful than describing every transparent defect as poor polishing.

• Check material preparation against the exact grade instructions and actual storage history.
• Review the filling and packing evidence before deciding whether the mark is moisture-related or a void.
• Change one documented variable at a time and retain labelled comparison samples.

2. Weld lines or visible changes in the filling pattern

A line where flow fronts meet differs from the designed boundary between two materials. Covestro’s weld-line guidance explains how fronts meeting after an opening or from separate gates can affect both appearance and mechanical behaviour. Diagnose the line within the relevant shot before deciding that it is an adhesion problem between shots.

Compare the mark with the gate layout and predicted filling pattern. Review whether the location coincides with an optical path, a visible surface or a loaded feature. Gate changes, venting and processing conditions should be evaluated together within the material’s recommended range. A setting that improves the photograph may still leave an unacceptable functional result.

• Distinguish a moulded flow-front line from a scratch, interface boundary or handling mark.
• Record the observation with both unlit inspection and the agreed optical test arrangement.
• Validate any proposed improvement against the original acceptance criteria, not appearance alone.

3. Separation or inadequate retention at the interface

If the component requires the two regions to remain joined, visible separation or weak retention needs an interface review. Start with the specified material pair and the intended joining mechanism. Do not assume that all clear and opaque grades are compatible, or that mechanical engagement and adhesion provide interchangeable performance.

Ask the material supplier to review the proposed combination and its use conditions. Document the interface geometry, shot sequence and preparation of the first shot. Compare the actual part with that agreed construction. A successful demonstration made from a different polymer pair cannot validate this component, and a fresh sample may not represent performance after the required environmental exposure.

• Define what interface performance must be demonstrated and how it will be tested.
• Check for handling contamination or an unapproved material or colour substitution.
• Review any interlock change with the designer because it can also affect the optical geometry.

4. Colour bleed or washout near the second-shot flow

A coloured streak in a clear region may originate from the first shot rather than from mixed pellets. Covestro’s polycarbonate overmoulding guidance describes local remelting and displacement of a thin first-shot feature by the incoming material. This washout mechanism is a reason to examine the position of the mark relative to the second-shot gate and flow.

Compare an intact first shot with a completed sample and identify the feature that appears changed. Review the selected material guidance, gate arrangement and local geometry with the process team. Do not respond by applying a universal lower temperature: an unsuitable adjustment can create a different filling or interface problem. The corrective proposal must remain compatible with both materials.

• Locate the start of the streak and check whether it follows the second-shot flow direction.
• Separate local feature damage from contamination already present in the material path.
• Confirm that a geometry change preserves the required light path and mounting interfaces.

5. Warpage or a shift in the finished component geometry

Two shots have different thermal histories. Covestro notes that overmoulding shrinkage can impose stress and distortion on an existing substrate. Its temperature-control guidance also connects mould thermal conditions with shrinkage and dimensional behaviour. These are reasons to review the complete sequence rather than judging the first shot alone.

Measure the first shot and finished component using an agreed datum and fixture method. Ask whether the observed movement comes from the moulded geometry, the locating arrangement or restraint during measurement. Record the condition of the sample at inspection so comparisons are meaningful. Assess any support, cooling or geometry change against both the dimensional and optical requirements.

• Review how the first shot is located and supported before the second shot is added.
• Compare free-state and assembled-state requirements where both are specified.
• Check the result after the agreed conditioning or exposure, not only immediately after removal.

Use a controlled investigation instead of a universal recipe

Build a trial log that connects each sample to the drawing revision, material identity, shot sequence and process change. Include an unchanged reference when practical. Agree which symptom is being addressed and what would count as improvement before the next run. This keeps a change intended to solve one defect from quietly redefining the part’s acceptance requirements.

For a project discussion with Yuya, describe the component’s function and provide non-confidential images of the issue where appropriate. Include the two material grades, the affected shot and the checks already completed. Contact the team to establish the tooling scope and a suitable method for exchanging the detailed engineering brief.