Long before anyone spoke of injection molding, ancient Greek bronze casters were already fighting this battle. Building the clay or plaster molds used in indirect lost-wax casting, they divided each mold into as few sections as possible without damaging any undercut modeling: the same hooks and recessed details that make a bronze figure look alive are what make it hard to pull from its mold (Metropolitan Museum of Art, 2003). More than two thousand years later, engineers designing plastic parts are solving the identical problem, with steel tools and thirty-second cycle times instead of clay and kiln fire.
What an undercut actually is
A mold typically opens in two directions, so part geometry has to release cleanly along that single path, says Chris Wlezien, Senior Director of Product Design & Innovation and a McKinsey alum. Most parts get help from a draft angle, a slight taper, usually one to three degrees, that lets the mold pull away without dragging on the cooling plastic. An undercut breaks that rule: a hook, hole, groove, or snap that sits behind the line the mold needs to travel, so pulling straight back would shear the part or wreck the tool (Wlezien, interview).
Moldmakers have standard fixes, each with its own cost. A side-action, or slide, moves sideways before the mold retracts. A lifter travels with the ejector pins to free sharper undercuts, and is generally the most expensive addition to a tool. For threads, an unscrewing core spins as the part releases. For a whole trapped section, a collapsing core folds in on itself: geometrically limited and pricey, but sometimes the only option. On the low end, “pickouts” (loose steel pulled out by hand) work only for low-volume, single-cavity jobs. Some parts are simply forced off the tool: a bottle cap thread is soft and rounded enough to flex and jump free without any mechanical action (Wlezien, interview).
The cost math is blunt. Complex actions like lifters and collapsing cores add real money, degrade the mold faster, and shrink the pool of shops capable of producing the part at all, since not every manufacturer has slide or lifter expertise, notes Jacques Laramie, an entrepreneur, innovator, and product designer. An undercut isn’t just a design decision, it’s a sourcing decision that can quietly narrow who’s eligible to quote your job (Laramie, interview).
Designing it out, or in
Every experienced engineer has a story about a “cheap” undercut that got expensive. Wlezien recalled designing a personal water bottle where dense internal geometry forced an unusually thin unscrewing core; a later decision to enlarge the snap features meant remaking the sliding core steel entirely, compounding a mistake that’s already costly to redo in undercut tooling (Wlezien, interview). The lesson from both interviews: consider undercuts as early as possible, ideally during initial modeling, not after surface models are locked and a batch of unmakeable parts already exists (Laramie, interview).
Modern CAD tools help. A draft analysis flags negative draft, the sign of an undercut, in bright red. From there, a designer can rotate the part, shift the parting line (which doesn’t have to sit at 90 degrees to the part), or reconsider whether the undercut is needed at all. Many exist purely to serve assembly, such as a tab into a slot, and can be replaced by splitting a complex part into two simpler ones that snap or screw together, trading mold and assembly cost against the price of the undercut itself (Laramie, interview; Wlezien, interview).
Discover Material Selection with Prospector Premium, learn more here!
The sustainability wrinkle
That “snap it together” solution has its own fork, and it’s becoming a regulatory issue. Plastic snap-fits are generally classified by how easily they come apart: “live snaps,” built to be opened repeatedly, and “dead snaps,” designed for permanent assembly (FirstMold, 2026). That choice, made at the undercut-design stage, increasingly determines whether a consumer can open a device at all.
The EU’s Right to Repair Directive takes effect July 31, 2026, covering mobile phones, tablets, and household electronics (European Commission, 2026). Since June 2025, smartphones and tablets sold in the EU carry a repairability label graded A to E, and products that score poorly, often because of adhesives or non-removable fasteners rather than accessible screws, face real competitive disadvantage (InsightPulseHub, 2026). A dead snap that once seemed like a clever way to eliminate a screw is now, in some markets, a liability built into the mold.
The extreme case
Medical device miniaturization pushes the same mechanisms (sliders, lifters, collapsing cores) down to features smaller than a grain of resin. Standard injection molding generally breaks down once parts have micro features or tolerances tighter than a human hair (Medical Design and Outsourcing, 2024). Micro molds still use side actions, lifters, and collapsible cores to form undercuts at micron-level tolerances, for applications including insert-molded battery connectors for implantable cardiac devices and thin-wall tips for pacemaker leads (Nissha Medical Technologies, 2026). It’s the same puzzle Greek bronze casters faced, at a scale where error is measured in millionths of an inch.
Where it doesn’t stop
The problem doesn’t disappear outside molding, it flips. In 3D printing, the equivalent challenge isn’t a feature that won’t release from a rigid mold, but an unsupported overhang that sags before it solidifies, a mirror-image version of the same geometry puzzle.
References
Metropolitan Museum of Art. (2003, October 1). The Technique of Bronze Statuary in Ancient Greece. https://www.metmuseum.org/essays/the-technique-of-bronze-statuary-in-ancient-greece
FirstMold. (2026, March 4). Design Guide for Snap-Fits: Product Design Series. https://firstmold.com/tips/snap-fits/
European Commission. (2026, July 31). Right to repair: New consumer rights for easy and attractive repairs. https://commission.europa.eu/news-and-media/news/right-repair-new-consumer-rights-easy-and-attractive-repairs-2026-07-31_en
InsightPulseHub. (2026, February 26). EU Right to Repair July 2026: 3-Year Warranty Extension and Manufacturer Obligations. https://insightpulsehub.com/eu-right-to-repair-july-2026-3-year-warranty-extension-and-manufacturer-obligations/
Medical Design and Outsourcing. (2024, October 11). Innovations in micro molding for medical device miniaturization. https://www.medicaldesignandoutsourcing.com/innovations-in-micro-molding-for-medical-device-miniaturization/
Nissha Medical Technologies. (2026). Isometric Micro Molding Solutions. https://dm.nisshamedical.com/en/isometric-micro-molding/micro-molding-and-miniaturization/
Laramie, J. Personal interview conducted for this article, 08/12/2026.
Wlezien, C. Personal interview conducted for this article,08/14/ 2026.
The views, opinions and technical analyses presented here are those of the author or advertiser, and are not necessarily those of ULProspector.com or UL Solutions. The appearance of this content in the UL Prospector Knowledge Center does not constitute an endorsement by UL Solutions or its affiliates.
All content is subject to copyright and may not be reproduced without prior authorization from UL Solutions or the content author.
The content has been made available for informational and educational purposes only. While the editors of this site may verify the accuracy of its content from time to time, we assume no responsibility for errors made by the author, editorial staff or any other contributor.
UL Solutions does not make any representations or warranties with respect to the accuracy, applicability, fitness or completeness of the content. UL Solutions does not warrant the performance, effectiveness or applicability of sites listed or linked to in any content.
