If you’ve ever held a silicone phone case with a built-in metal logo, a power tool handle reinforced with a steel insert, or a medical catheter tip molded around a precision brass component, you’ve held a product made via insert molding. For those of us who’ve spent years on the production floor of an insert molding supplier, this process is equal parts precision art and technical science—where every tiny variable, from plastic resin temperature to the fit of a metal insert, can make or break a part. One of the most misunderstood, yet critical, variables in this entire workflow? Lubricants. Too often, new customers walk in thinking lubricants are just “slick stuff to make things slide” — a trivial add-on to be skipped to save a buck or rushed through for speed. Over the last 12 years, I’ve seen what happens when that mindset takes hold: cracked brass inserts that break in the molding press, plastic parts that warp mid-production, and entire batches of rejected parts that cost us and our clients thousands in rework and delays. Today, I want to break down exactly what lubricants do in insert molding for both metal and plastic inserts, why they’re non-negotiable for consistent, high-quality parts, and how we tailor our lubricant choices to every job we run. Insert Molding (Metal & Plastic Inserts)
Let’s start with the two core types of inserts we work with every day: metal inserts (think steel, brass, aluminum, or even stainless steel for medical or automotive parts) and plastic inserts (less common, but used for things like threaded plastic bosses for consumer electronics or reinforced hinge points for outdoor gear). For both, the role of a lubricant isn’t one-size-fits-all—it shifts based on the insert material, the plastic resin we’re molding around it, and the final part’s required tolerances.
First, metal inserts. If you’re molding molten plastic (usually heated to 350–500°F, depending on the resin) directly onto a cold metal insert, two things are happening that can go very wrong very fast. First, the metal insert has small, microscopic imperfections—tiny burrs from stamping, leftover oil from its own manufacturing, or just surface roughness from machining. When molten plastic flows over that rough metal surface under tons of hydraulic pressure (insert molding presses exert anywhere from 50 to 500 tons of force, depending on part size), the plastic can catch on those imperfections, causing a defect called “scoring” — deep, visible scratches in the plastic around the insert that make the part unfit for sale. Second, if the metal insert is even slightly misaligned (a common issue when it’s loaded into the mold, especially for small, high-volume parts like phone fasteners), the edge of the metal can scrape against the mold cavity wall as the press closes. Without a lubricant, that friction generates enough heat to warp the mold, wear down the insert’s shape, or even cause the press to stall mid-cycle—costing hours of downtime.
But here’s the part most people don’t hear: lubricants also prevent adhesive failure. Adhesive failure is exactly what it sounds like—the plastic insert detaches cleanly from the molded plastic, like peeling a sticker off paper. It’s the worst defect for insert molding, because it doesn’t show up during an initial inspection; it can happen weeks or months after the part is shipped, especially when the product is exposed to temperature swings or vibration (think a car door handle that falls off after a year of highway driving). Lubricants create a micro-thin barrier on the metal insert’s surface that’s not too thick, not too thin—it smooths out microscopic roughness so plastic can flow evenly, but doesn’t create a gap large enough to prevent the plastic from bonding. We use a water-based, food-grade lubricant for our medical insert molding jobs because it evaporates completely after the insert is loaded, leaving no residue that could interfere with plastic adhesion. For automotive parts, we use a dry-powdered lubricant made from molybdenum disulfide (a common, high-performance lubricant) that handles the high pressure of molding large structural parts without leaving any mess on the mold.
Now, plastic inserts. This is a trickier case, because plastic inserts react very differently to heat and friction than metal ones. Most plastic inserts used in insert molding are engineering plastics—like nylon, PEEK, or ABS—chosen for their strength and ability to hold threads or structural loads. When we mold another layer of plastic around a plastic insert, the same pressure and heat issues apply, but we also have to worry about the insert melting. If the lubricant is too high in silicone, for example, it can transfer silicone to the insert’s surface, which then prevents the new plastic from bonding. Even worse, excess lubricant can seep into gaps between the plastic insert and mold, causing flash—thin, unwanted plastic strands that form along mold edges, requiring manual trimming and adding hours of labor to every batch.
I learned this lesson the hard way 8 years ago, when a client came to us with a high-volume order of plastic inserts for a line of children’s toy action figures. We tried using a silicone-based lubricant to speed up insert loading, since the inserts were small (less than half an inch wide) and we were running 10,000 parts a day. Within three days, the client sent us a call—every batch had 15–20% of parts with flash around the insert’s edge. We tracked the issue back to the silicone lubricant, which was vaporizing at the mold’s operating temperature (around 250°F) and condensing on the mold cavity walls, creating a barrier that let molten plastic seep into tiny gaps. Switching to a non-silicone, water-soluble lubricant fixed the issue immediately—flash dropped to less than 0.5% per batch, and we hit our production targets without any delays. That’s when I realized: lubricant selection isn’t just about reducing friction—it’s about matching the lubricant to every variable of the job, down to the mold’s temperature and the resin being used.
Another key role of lubricants in insert molding, regardless of insert material, is reducing mold wear. Molds are the heart of insert molding; each one costs anywhere from $5,000 to $50,000, depending on complexity, and replacing them is a huge expense. Without a lubricant, every time an insert is loaded and the mold closes, the metal insert (or even a sharp edge on a plastic insert) rubs against the mold cavity. Over thousands of cycles, that friction wears down the mold’s steel, leaving tiny scratches and imperfections that show up as defects in every part. We track mold cycle counts across every job, and we’ve found that using the right lubricant cuts mold wear by 30–40% on average. For example, on a job we’ve been running for 5 years for a power tool manufacturer, we use a synthetic lubricant formulated specifically for steel molds and brass inserts. We’ve run over 2.5 million parts on that same mold, and it’s still within spec—whereas a similar job without that lubricant would have needed a full mold replacement after 1.2 million cycles.
Wait a second—what about lubricants that are considered “release agents”? I know a lot of folks use those terms interchangeably, but for our team, there’s a clear distinction. A release agent is a heavy, often oily substance designed to get a part out of the mold after molding is complete. A lubricant for insert molding is applied to the insert itself before it’s loaded into the mold, specifically to reduce friction between the insert and the mold during closing and molding, as well as improve plastic adhesion. The right lubricant won’t interfere with the part’s ability to eject from the mold (that’s the mold’s own ejection system’s job), and it won’t leave residue that ruins the final part’s finish. For the food and beverage industry, which we serve heavily, we can’t use release agents at all—because they leave residue that can contaminate food packaging. That’s where the right insert lubricant becomes even more critical: it has to reduce friction, improve adhesion, and leave no residual odor or taste, even after being exposed to years of washing and use.
Of course, there are common mistakes we see with lubricant use, even among experienced insert molding teams. One is over-lubricating. It’s easy to think “more slick = less friction,” but too much lubricant can cause plastic to flow unevenly, leading to voids in the molded part, or seep into the insert’s threaded holes, making it impossible to assemble the final product. Last year, we had a client bring us parts from another supplier where the threaded brass inserts had excess lubricant in their holes, so when the client tried to screw a bolt in, the bolt stripped the plastic around the insert because the lubricant prevented the plastic from gripping the metal threads. That was a $20,000 rework bill for the client, all because the supplier used twice as much lubricant as needed. The other mistake is using a one-size-fits-all lubricant. I’ve seen teams use the same general-purpose lubricant for metal inserts, plastic inserts, medical parts, and automotive parts—and every time, it leads to avoidable defects. Medical parts can’t have silicone residue, automotive parts need lubricants that handle extreme temperature swings, and small consumer electronics parts need lubricants that won’t cause flash in tiny mold gaps.
At our facility, we’ve built a system for every job that starts with a lubricant consultation before we even quote a project. We ask clients three key questions: what’s the insert material, what plastic resin are you using, and what’s the final part application? For a medical catheter molded around a titanium insert, we use a non-silicone, USP Class VI lubricant that evaporates at low temperatures, leaving no residue. For a automotive door handle molded around a steel insert, we use a dry lubricant that withstands -40°F to 250°F, so the part won’t fail in extreme weather. For a kids’ toy molded around an ABS plastic insert, we use a water-based lubricant that’s non-toxic and leaves no messy residue that could harm kids.
I also want to address the elephant in the room: why not just skip the lubricant to cut costs? That’s the question every new client asks, and it’s the easiest one to answer with real numbers, not just theory. Let’s say a customer is quoting a 10,000-part job, and skipping lubricant saves them $0.01 per part—total savings of $100. But over the last year, we’ve had 12 clients come to us after working with suppliers who skipped lubricant, each with an average of $8,000 in rework and delayed shipment costs from defects caused by friction and poor adhesion. For that same 10,000-part job, skipping lubricant can lead to 10–15% defective parts, which means 1,000–1,500 parts that can’t be used—costing $1,500 in wasted material, plus overtime to rework or replace the parts, plus lost revenue from delayed shipments. That’s a net loss of thousands of dollars, all to save $100 upfront. Lubricants aren’t a line item to cut—they’re an investment in consistent quality and on-time delivery.
Over the years, I’ve worked with clients who once swore they could do insert molding without lubricants, only to come back to us after a batch of failed parts. I’ve also had clients who test different lubricants with us, and see their defect rates drop by 90% almost overnight. At the end of the day, insert molding is all about attention to detail—every tiny part of the process, even a seemingly trivial lubricant, adds up to a part that works as intended for years. If you’re working on an insert molding project, and you’re not sure what lubricant is right for your parts, or you’ve been dealing with insert defects that keep slowing you down, we’re here to help. We’ve worked on thousands of insert molding jobs for industries from medical to automotive to consumer electronics, and we can help you troubleshoot lubricant-related issues, select the right materials, and get consistent, high-quality parts every time.
Over Molding References:
- Rosato, D. V., & Rosato, M. G. (2004). Insert Molding Handbook. Hanser Gardner Publications.
- Harper, C. A. (2002). Modern Plastics Handbook. McGraw-Hill Education.
- International Organization for Standardization. (2019). ISO 10993-5: Biological Evaluation of Medical Devices – Part 5: Tests for In Vitro Cytotoxicity. ISO.
- American Society for Testing and Materials. (2018). ASTM D4066 – Standard Specification for Plastic Additives. ASTM International.
Yangzhou Dingyue Plastic & Electronics Co.,Ltd
Address: No.28 Yiju Road, Yunxi Town, Hanjiang District, Yangzhou, China.
E-mail: monica.pan@yzdingyue.com
WebSite: https://www.plastic-injection-molding.com/
