{"id":3490,"date":"2026-10-09T06:37:01","date_gmt":"2026-10-08T22:37:01","guid":{"rendered":"http:\/\/www.weissmanwonders.com\/blog\/?p=3490"},"modified":"2026-10-09T06:37:01","modified_gmt":"2026-10-08T22:37:01","slug":"how-to-measure-the-glossiness-of-in-mold-labels-4534-db8930","status":"publish","type":"post","link":"http:\/\/www.weissmanwonders.com\/blog\/2026\/10\/09\/how-to-measure-the-glossiness-of-in-mold-labels-4534-db8930\/","title":{"rendered":"How to measure the glossiness of In – Mold Labels?"},"content":{"rendered":"

If you\u2019ve ever held a plastic detergent bottle, a food container, or a personal care tube with a crisp, scratch-resistant design that\u2019s bonded directly to the surface\u2014not stuck on with separate adhesive\u2014you\u2019ve likely interacted with an in-mold label (IML). As an IML supplier who\u2019s worked with brands from small craft beverage makers to multinational packaged goods companies over the past seven years, I\u2019ve seen firsthand how glossiness of these labels isn\u2019t just a cosmetic detail. It\u2019s a make-or-break factor that can make or break a product\u2019s shelf appeal, print durability, and even how well the label adheres to the plastic part once it\u2019s molded. In-Mold Labeling<\/a><\/p>\n

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Wait, I get it\u2014for a lot of folks, \u201cglossiness\u201d feels like a vague, subjective term. When a marketing team says they want a \u201chigh-gloss\u201d finish for their new line of stainless steel-cleaning spray bottles, that\u2019s not just a preference. It\u2019s a requirement that directly impacts production timelines, material costs, and the final product\u2019s quality. Over the years, I\u2019ve had more than one brand manager call me panicking after their first sample batch arrived, saying the label looked \u201cflat\u201d instead of \u201cluxury\u201d like they pictured. More often than not, the issue came down to not measuring glossiness correctly before full production.<\/p>\n

Today, I want to pull back the curtain on how we measure IML glossiness\u2014stuff I\u2019ve learned from testing hundreds of batches, troubleshooting with our in-house quality team, and collaborating with contract labs that specialize in packaging materials. I\u2019ll skip the overly technical jargon, but I\u2019ll cover the science that matters, the tools we actually use (not just theory), and the mistakes I see brands (and even some IML suppliers) make all the time. Let\u2019s start with why IML gloss is different from, say, the gloss of a printed paper label.<\/p>\n

In-mold labels aren\u2019t like self-adhesive paper or film labels that you slap on a plastic part after molding. When we produce IMLs, they\u2019re loaded into an injection molding or thermoforming machine, and molten plastic is either injected around them (for rigid parts like tubs or bottles) or formed over them (for flexible parts). That means the label\u2019s finish isn\u2019t just sitting on top\u2014it\u2019s fused to the plastic\u2019s surface. Variables like the mold\u2019s surface texture, the temperature of the molten plastic, and the type of film we use to make the label can all change how glossy the final part looks, even if we printed the label at a consistent gloss on our end. That\u2019s why measuring glossiness post-molding isn\u2019t just a \u201ccheck\u201d step\u2014it\u2019s a critical part of quality control (QC) for the entire IML-to-part assembly.<\/p>\n

Now, let\u2019s talk about how we actually measure it. The core tool here is a gloss meter, and if you\u2019re in packaging, you\u2019ve probably heard of one, but not everyone knows how to use it correctly for IMLs. First, gloss meters measure reflected light at a specific angle, because different surfaces reflect light differently depending on the angle you\u2019re looking at (or measuring) from. For packaging films and labels, the standard angles are 20\u00b0, 60\u00b0, and 85\u00b0. Here\u2019s why that matters: a 60\u00b0 angle is the workhorse for most general packaging, including IMLs. It\u2019s the angle that gives a reliable reading for mid to high gloss levels, which is what most brands want for their IML products.<\/p>\n

A 20\u00b0 angle is for ultra-high-gloss finishes\u2014think car trim or luxury cosmetics packaging\u2014where even small variations in gloss show up as a big difference in brightness. If a brand is going for a super sleek, mirror-like IML, we use a 20\u00b0 reading because the 60\u00b0 angle might not pick up the tiny differences in reflected light that make a finish look truly glossy. On the flip side, 85\u00b0 is for low-gloss or matte IMLs, where you want to avoid glare. For example, a kid\u2019s sippy cup that\u2019s meant to be held outdoors in bright sunlight would need a low-gloss IML, and the 85\u00b0 angle is better at measuring the subtle non-reflective properties there.<\/p>\n

When I\u2019m training our quality techs, I always emphasize that you can\u2019t just wave a gloss meter over a label and take one reading. IML surfaces aren\u2019t perfectly uniform\u2014there might be tiny variations from the mold\u2019s texture, or from how the molten plastic flowed during injection. The industry standard is to take at least 10 separate readings across the surface of the IML (or the molded part, once it\u2019s pulled from the machine) and then calculate the average. You also have to mark down where you took the readings\u2014for example, 5 readings on the label\u2019s front, 2 near the edge, 2 near the hole if it\u2019s for a hang tab, and 1 on the molded plastic part itself if the label overlaps. I once worked with a snack brand that complained their IML gloss was inconsistent, and turns out their QC team only took one reading in the exact center of the label, where the gloss was 12 GU (gloss units) higher than the edges. That\u2019s a huge difference on a shelf.<\/p>\n

Another big mistake: not calibrating the gloss meter regularly. I\u2019ve seen techs skip this step because it feels like a hassle, but if your meter is off by even a few GU, you could end up with batches that don\u2019t meet the brand\u2019s specs. We calibrate our gloss meters every morning before production starts using a standard gloss plate\u2014usually a 94 GU plate for 60\u00b0 readings, which is the common mid-gloss reference. If the meter is off, we adjust it before we test any samples. We also calibrate it after every 50 sample readings, or if we drop it (even a small bump can throw off calibration).<\/p>\n

Now, let\u2019s get into what the numbers actually mean. Gloss units (GU) aren\u2019t arbitrary\u2014they\u2019re a standardized scale where 0 GU is a perfectly matte, non-reflective surface, and 100 GU is a mirror-like surface that reflects all incoming light at the measurement angle. For IMLs, here\u2019s a rough breakdown of what we see in the industry, based on the product type:<\/p>\n