Light Line

Working With Light Line in Practical Manufacturing

I keep running into people who don't know what light line actually means until they get burned on a batch. The short version is that light line refers to a visible seam or trace left behind when the molten material doesn't fully fuse along the parting line or weld line during injection molding. It shows up as a thin, lighter-colored line on the surface of the finished part, and it's usually a sign that something in the process needs adjusting. Light line isn't a defect category you file a complaint about in most QC departments. It's more of a cosmetic inconsistency that becomes a structural problem only when it's severe enough to indicate incomplete fusion of the polymer chains. In practice, you'll see it on parts that have long flow paths, thin walls, or multiple gate locations. The material arriving at the far end of the cavity is cooler, the viscosity is higher, and the two fronts don't merge as cleanly as they should. The line itself is a region where the molecular entanglement across the interface is weaker than the surrounding material. That means the part can crack along that line under stress, even if it looks fine to the naked eye. You catch it early by running a peel test or a bend test on sample parts, not by looking at the shelf.

How to Identify and Fix It

The first thing I do is check the melt temperature and the mold temperature separately. Light line often appears when the mold is too cold relative to the melt, which causes the leading edge of the flow front to skin over before it meets the opposing stream. In my experience, bumping the mold temperature by about 10 to 15 degrees Celsius resolves the issue in roughly 60 percent of cases. It's a small change and it doesn't always work, but it's worth trying before you start tweaking screw speed or back pressure. If that doesn't fix it, look at the gate design. A gate that's too thin forces the material through at high shear, which can generate localized heating but also cause premature cooling at the gate entry point. I had a project once where the light line appeared only on the second cavity of a dual-cavity tool, and it turned out the runner balance was off by about 0.3 millimeters. The second cavity was filling slightly slower, the melt front was cooler, and the weld line formed further down the part than in the first cavity. Reworking the runner geometry to within 0.05 millimeters of balance eliminated the variation completely. Another common fix is adjusting the injection speed profile. Instead of running a single fast fill, splitting the injection into two stages with a switchover point near the gate eliminates the problem in many thin-wall applications. The first stage pushes the melt through the gate and initial cavity volume at a moderate speed. The second stage accelerates to fill the remaining volume before the material cools too much. This approach usually cuts the incidence of light line by half or more, depending on the resin and wall thickness.

When Light Line Isn't Fixable

Sometimes the material itself is the problem. Polycarbonate and ABS are more forgiving because they have broader processing windows. Polypropylene and nylon tend to show light line more easily, especially if the resin has been exposed to moisture or if the regrind content is too high. I've seen parts with 30 percent regrind develop light line within the first hundred cycles, while virgin material from the same batch ran clean. If you're running a high-regrind formula and the light line persists after temperature and speed adjustments, the regrind is likely the bottleneck. Reducing it to 10 to 15 percent usually clears things up. There are also cases where the part geometry makes light line unavoidable. Long, thin ribs or deep pockets create natural weld lines no matter how you tune the process. In those situations, you're not going to eliminate the line entirely, but you can make it less visible by using a darker colorant, texturing the surface, or orienting the gate so the weld line falls in a non-aesthetic area. This is standard practice in consumer electronics housings, where the appearance matters more than structural perfection in that specific zone.

A Note on Testing

Don't rely solely on visual inspection. I've seen teams approve parts that looked acceptable until a drop test or a thermal cycle revealed cracking along the light line. Run a simple tensile test across the weld line area and compare the results to a reference sample from a known-good part. If the strength drops by more than 15 to 20 percent, you have a real problem regardless of how the part looks. That threshold varies by application, so adjust it based on your end-use requirements, but having a number to track is better than eyeballing every batch.