If you’ve ever handled a power tool, a kitchen utensil grip, or a handheld medical device, you’ve interacted with an over molded part. As an over molding supplier, I’ve spent 12 years troubleshooting why some of these parts hold up under daily use, and the question I get most often from clients is: “How well will this over molded part resist fading, staining, or color shift over time?” That’s where color fastness comes in—and it’s not just about matching a brand’s Pantone code for a launch event; it’s about designing a part that works as hard as the product it’s attached to, even when exposed to harsh environments. Let’s break down what color fastness actually means for over molded parts, the factors that can break it, and what we do at our shop to make sure your parts stay looking and performing like new. Over Molding
First, let’s get one thing straight: color fastness isn’t a single test. It’s a set of performance metrics that measure how well a material retains its original color when subjected to specific stressors. For over molded parts, which are made by injecting a second material (usually a softer elastomer like TPE, TPU, or silicone) onto a rigid base part (often ABS, PC, or nylon), color fastness depends on two intersecting systems: the base resin, the over mold material, and the bond between them. A lot of people assume color comes from just adding dye to plastic, but for over molding, the pigments and how they interact with both materials make all the difference.
Let’s start with the core of color fastness for over molded parts: the pigments themselves. Not all pigments are created equal. For example, organic pigments (which give bright, vivid reds, blues, and yellows) are more prone to fading from UV exposure than inorganic pigments like titanium dioxide, iron oxide, or carbon black. In our shop, we’ve seen clients come to us after a failed batch where they used a cheap organic red pigment that faded to pink after 30 days of outdoor testing—especially for over molded parts on patio furniture or construction equipment. We always recommend pigment selection that aligns with the part’s end use: if it’s a handheld power tool that will spend most of its life in a garage or workshop, a balanced mix of organic and inorganic pigments works for color and durability. If it’s an over molded part for marine equipment or outdoor signage, we opt for 100% light-stabilized inorganic pigments, even if the client has to accept a slightly softer, less neon color. The trade-off is worth it when the part doesn’t fade after two years of sun exposure.
But pigments only tell half the story. The other big factor is the compatibility between the over mold material and the base resin, and how that compatibility affects color migration. Color migration is one of the most common (and frustrating) color fastness issues we deal with. That’s when pigment from the over mold layer seeps into the base part, or vice versa, leaving streaks, discoloration, or a cloudy haze. A few years ago, we had a medical device client who designed an over molded silicone grip on a polycarbonate base. They used a bright blue silicone pigment that looked perfect on our test bench, but after autoclaving (high-temperature steam sterilization) for medical use, the blue pigment migrated into the polycarbonate, turning the base part a dull, unmarketable gray. The problem? The silicone and PC weren’t properly prepped, and the pigment wasn’t encapsulated. We fixed it by switching to a solvent-resistant pigment for the silicone, adding a thin adhesion promoter layer between the two materials to create a barrier, and running a 10-cycle autoclave test before the part left our facility. By the time we sent the first production run, the client didn’t have a single migration failure.
Thermal stability is another critical color fastness factor for over molded parts. Over molding requires heating materials to high temperatures—usually 180–300°C, depending on the resin—so pigments that break down at those temperatures will degrade during molding, leading to faded, discolored parts right out of the tool. We once had a client who used a yellow pigment optimized for extrusion molding, where temperatures stay around 200°C. When we over molded it at 260°C for a nylon base, the yellow pigment broke down during injection, resulting in parts that were pale greenish-yellow instead of the bright sunflower yellow they’d specified. The issue wasn’t the pigment’s inherent light fastness—it was thermal degradation during the molding process. For that client, we switched to a heat-stabilized pigment rated for 300°C, which could withstand our molding temperatures without breaking down, and matched their exact Pantone color on the first try. We always run a small-scale color validation test before full production, where we mold 10–15 test parts with the exact materials and process parameters the client will use, to catch these thermal degradation issues early.
Now, let’s talk about the real-world stressors that test color fastness, because lab tests only matter if they mirror how a part will be used in the wild. The four most common stressors for over molded parts are UV exposure, chemical contact, abrasion, and temperature cycling. Let’s break each down, and how we test for them at our facility.
UV exposure is the big one for parts used outdoors, automotive interiors/exteriors, or recreational equipment. We use ASTM G154 testing, which simulates 1000 hours of full sun exposure using a fluorescent UV lamp, to measure color delta (ΔE) — that’s the numerical value that tells you how much the color has shifted. A ΔE of 1 or less is undetectable to the human eye, which is what we target for all our over molded outdoor parts. For example, a lawn care equipment client recently needed over molded grips for their trimmer that would stay black even after a full season of summer sun. We ran the ASTM G154 test on their part, and our initial test had a ΔE of 2.8, which was slightly visible as a dull gray. We adjusted the pigment package to add a UV stabilizer to the over mold TPE, brought the ΔE down to 0.7, and the part has held its color through two full summers of field testing with no fading.
Chemical contact is another major issue, especially for parts that come into contact with cleaning products, oils, food, or household chemicals. Kitchen utensil over molds, for example, might touch dish soap, baking soda, or food acids, while automotive interior over molds might encounter gasoline, windshield washer fluid, or sunblock. We test chemical resistance using ISO 105-X12, which involves exposing parts to common chemicals for 24 hours at room temperature, then checking for color change or staining. A few months ago, we had a food service client with over molded spatulas that were staining after being used with tomato sauce and bleach-based dish soap. The problem was their TPE over mold didn’t have chemical-resistant pigment. We switched to pigments formulated to resist food acids and bleach, and ran a 72-hour chemical soak test to validate—no staining, and the color stayed consistent across 5000 uses in their test kitchen.
Abrasion is often overlooked, but it’s a huge factor for parts that get handled a lot: tool grips, remote controls, phone cases, or medical device handles. When the outer layer of an over molded part wears away, the pigment can flake off, or the underlying base part’s color can show through, creating an uneven, faded look. We test abrasion resistance using Taber Abrasion, where we rub a weighted wheel across the part for 1000 cycles, then measure color change or pigment loss. A construction equipment client recently had over molded gear shifts that were fading after just 6 months of use because the abrasive dirt on job sites wore away the outer pigment layer. We added a scratch-resistant topcoat to the over mold TPE, and ran the Taber test—our parts showed a ΔE of 0.3 after 1000 cycles, with zero pigment flaking, while the client’s original parts had a ΔE of 4.2 and visible wear.
Temperature cycling is the final big stressor, especially for parts used in extreme climates or medical applications that require sterilization. When a part is exposed to repeated hot and cold temperatures, the two materials in the over molded assembly expand and contract at different rates, which can cause pigment cracking, peeling, or migration. We test temperature cycling using IEC 60068-2-1, cycling parts between -40°C and 85°C for 100 cycles, then checking for color change or bond failure. For a winter sports equipment client with over molded ski boot grips, this test was critical—their parts went from freezing mountain tops to warm indoor ski lodges multiple times a day. We worked with their TPE supplier to select a flexible pigment that wouldn’t crack as the material expanded and contracted, and after testing, the parts showed no color shift or pigment damage.
Here’s the thing I tell every new client: color fastness isn’t something you can add on as an afterthought. It’s a design choice that needs to be part of the over molding process from day one. Too often, clients come to us with a set color (often from a competitor’s part) and get frustrated when our over molded version fades, because they didn’t account for how the over mold material and pigment interact. That’s why at our shop, we start every project with a color fastness consultation: we ask about the part’s end use, the environments it will be exposed to, and any industry standards it needs to meet (like FDA for medical parts, or UL for electrical products). We then recommend the right pigments, materials, and process parameters to hit those targets, and run validation tests before production begins.
I also want to address a common misconception: more expensive materials don’t always equal better color fastness. We’ve used low-cost TPEs with the right pigment packages that outperformed high-end TPUs in UV testing, because the pigment and stabilizer were matched to the specific resin. It’s not about cutting corners—it’s about matching the right components to your application.
If you’re currently sourcing over molded parts and have had issues with fading, staining, or color shifting, or you’re designing a new product and want to make sure your parts stay looking like new for years, I’m here to help. We work with clients across industries, from medical devices and power tools to outdoor recreation and consumer goods, and we tailor every project to meet their color fastness needs. No generic solutions, no one-size-fits-all tests—just over molded parts that hold up to whatever they go through, and keep their color as intended. Don’t let color fastness be an afterthought in your product design. Reach out to discuss your next over molding project, and we’ll make sure your parts look as good as they perform.
Over Molding References
ASTM G154, Standard Practice for Operating Fluorescent Ultraviolet (UV) Lamp Apparatus for Exposure of Nonmetallic Materials
ISO 105-X12, Textiles – Tests for colour fastness – Part X12: Colour fastness to rubbing
IEC 60068-2-1, Environmental testing – Part 2-1: Tests – Test A: Cold
Yangzhou Dingyue Plastic & Electronics Co.,Ltd
We’re well-known as one of the leading over molding manufacturers and suppliers in China, specialized in providing high quality products and service for global clients. We warmly welcome you to buy customized over molding at competitive price from our factory.
Address: No.28 Yiju Road, Yunxi Town, Hanjiang District, Yangzhou, China.
E-mail: monica.pan@yzdingyue.com
WebSite: https://www.plastic-injection-molding.com/
