Overmolding vs Insert Molding vs Two-Shot Molding

Concept render

Insert molding places a preformed part, usually metal such as a threaded insert or contact, in the mold and shoots plastic around it. Overmolding adds a second material, often a soft TPE, over a molded plastic part. Two-shot molding makes both shots in one tool and machine. Pick-and-place uses two molds and suits lower volumes.

A wearable often needs two materials in one part: a soft band or grip on a rigid shell, a soft button cap, or a brass thread in a plastic boss. Three molding routes can do this, and they differ in tooling, bond strength and volume fit. This guide compares them for small devices, using design rules that molders and resin suppliers publish.

This is an engineering summary of published guides. Resin grades and molder limits differ, so confirm every number with your molder and your resin supplier.

Overmolding vs insert molding: what is the difference?

Protolabs defines overmolding as adding a layer of resin to an existing molded part, most often a TPE over a hard substrate. It defines insert molding as injecting resin over another material, usually metal, with a rigid plastic as the injected material (as of November 2026). Xometry treats overmolding as a type of insert molding in which plastic is molded over another molded part (as of November 2026).

The words overlap in practice. Avient's overmolding guide uses insert molding for any route where a pre-molded insert is placed into a mold and TPE is shot over it. So when you talk to a molder, describe the stack instead of the label: what the first part is made of, what the second material is, and whether it runs in one tool or two.

Sources: Protolabs design guide, Xometry, Fictiv, Avient, Teknor Apex.

How does two-shot molding compare with pick-and-place overmolding?

Protolabs describes two-shot molding as a highly automated process with three primary methods: transfer, rotational and core-back, which works only with very specific linear geometries. In pick-and-place, a batch of substrates is molded and cooled, then placed by hand into a second mold (as of November 2026).

Volume and tooling

Protolabs says two-shot needs specialized equipment and expensive molds, and pays off at high volume. Pick-and-place uses simpler molds and less complex equipment, with faster setup, but more labor per part. Fictiv puts the line at about 10,000 parts and says pick-and-place is used mainly for prototypes and low-volume runs (both as of November 2026).

The resin suppliers frame it by annual volume. Avient favors multi-shot above 250,000 units a year or where labor costs more, and insert molding below that. Teknor Apex gives the same rule of thumb and notes that insert molding runs on a conventional single-shot machine. One pair counts a run and the other a year, but for a pilot of a few hundred units all four point to pick-and-place.

Bond strength changes with the process

Protolabs names reduced chemical bonding as the biggest challenge in pick-and-place, where the substrate cools and is stored and handled before the second shot. It says all three two-shot methods put the overmold on a warm substrate, which aids chemical bonding. In a Teknor Apex and Covestro study of medical-grade TPEs on PC, PC+ABS and PC+polyester, direct two-shot gave slightly higher peel strengths than insert molding, and all specimens failed cohesively in the peel tests (all as of November 2026). If your samples use pick-and-place and production uses two-shot, plan to repeat the peel test. For soft-over-rigid samples before any tool is cut, see urethane casting for low volumes.

Which overmolding materials bond to PC, ABS and PC/ABS?

For the shell resin itself, see PC vs ABS vs PC/ABS. For the soft layer, the grade matters more than the family name.

• Teknor Apex says standard styrenic TPEs and TPVs generally adhere to PP and PE but show poor adhesion to engineering plastics like PC and ABS. Grades for those substrates are chemically modified for them.
• Avient says not all of its TPEs bond to all substrates. Its example is a Dynaflex TPE that bonds to PP but will not adhere to PC. Its overmolding grades are formulated for PC, ABS, PC/ABS, nylons and others.
• Fictiv says the substrate's surface energy must exceed the overmold's. It lists PC at 46 dynes/cm, ABS at 35 and PP at 30, and says hard pairs may need flame, corona or plasma treatment.
• Protolabs publishes a chart that marks each substrate and overmold pair as a chemical or a mechanical bond. Look up your exact grades there and in the resin supplier's data sheet.

Additives can undo a good pair. Protolabs says fillers, additives and some surface treatments can reduce bonding. Avient warns that a color concentrate carrier that does not suit the TPE chemistry can seriously hurt adhesion, and that PP-based carriers may not be the best choice for TPEs made to bond to engineering plastics. Avient also says hygroscopic materials must be dried before molding (all as of November 2026).

Feel is a separate decision. Avient says it depends on hardness, modulus, friction, texture and wall thickness, and a thinner overmold feels harder. Hardness alone does not predict flexibility: a 65 Shore A SEBS is much more flexible than a 65 Shore A TPU, per Avient (as of November 2026).

How do you design an overmold that does not peel?

Protolabs says peeling usually starts at an edge and spreads along the interface (as of November 2026). Most of the rules below protect the edges or add mechanical grip.

Walls and thickness

Avient's starting points, as of November 2026: keep walls as uniform as possible, and walls of 0.060 to 0.120 in (1.5 to 3 mm) give good bonding in most applications. Keep the TPE no thicker than the substrate to prevent warpage, use corner radii of at least 0.020 in (0.5 mm), and give long draws 3 to 5 degrees of draft per side. For draft on the rigid shell, see injection molding draft angles. Avient puts the absolute maximum flow length at 150 times the wall thickness for most overmolding, and advises shorter flow or interlocks where bond strength matters more.

Edges and shutoffs

Protolabs says to avoid exposed overmold edges, and a raised substrate edge can protect them. Avient is more specific. Keep the TPE surface flush with the bare substrate, and set the TPE edge deeper than that surface. Never run the TPE edge even with or over the edge of the part. To limit flash, cut a groove 0.015 to 0.030 in (0.38 to 0.76 mm) deep in the substrate along the TPE edge, with the steel shutting off positively in that groove (both as of November 2026).

Mechanical interlocks

Protolabs and Fictiv both describe overmold flowing into holes in the substrate, best when the hole widens at the bottom like a dovetail. Wrapping the overmold around the substrate, or adding grooves, posts or bosses, increases the bonded area. Teknor Apex adds that a textured substrate finish helps too. Avient says properly designed deep undercuts are possible in its TPEs under 60 Shore A if there are no sharp corners, an advancing core is used and the elastomer can flex as it is ejected (all as of November 2026). For undercuts in the rigid shell, see undercut injection molding.

Clean, dry substrates

Avient says a dirty insert is likely to lose adhesion. Move inserts straight to the second mold where possible, protect stored ones from dust, wear gloves because skin oil affects adhesion, and use no mold release. Preheating can help, but Avient strongly discourages it for nylon inserts (as of November 2026).

How do you design insert molding for threaded inserts and metal contacts?

Insert molding puts metal inside the plastic during the shot. Protolabs calls insert molding an alternative to installing metal parts by heat staking or ultrasonic welding, and says molded-in inserts remove that secondary step. It also says there is no chemical bonding between metal inserts and plastic, so the insert has to hold by its shape (as of November 2026). Many inserts are knurled or shaped for that reason. Brass is common, and steel and stainless steel are also available.

Protolabs gives these points for its own hand-loaded aluminum tools, as of November 2026. Treat them as one molder's rules.

• Inserts smaller than 0.125 in across can be a problem, and so can inserts too large to hold in one hand.
• Hold locating surfaces to plus or minus 0.002 in (0.05 mm) to avoid flash.
• Keep about a 2:1 ratio of hand clearance to insert depth, so no insert sits too deep in the mold.
• Add slightly thicker walls or ribs around the insert to support it. Glass-filled resins help reduce the risk of cracking at knit lines.
• Send the insert supplier's CAD file with the part design and assembly drawings when you request a quote.

Plastic inserts move too. Avient says a plastic substrate in an insert-molding tool needs support, or injection pressure can deform or displace it and cause flash. Two-shot avoids this because the first shot stays supported in the tool (as of November 2026).

Charging contacts need one more check. Metal does not bond chemically to the plastic around it, so the joint around each contact is a possible leak path in a sealed device. Design the seal there on purpose and test it on molded parts. IP67 vs IP68 test conditions covers what a water test asks for.

Which process fits each feature of a wearable?

The rows below are engineering reasoning from the guides above. None describes a specific project.

Each choice also changes the cycle. Xometry says overmolding takes two steps, which adds cycle time and cost, and parts with metal inserts usually cost more than plastic-only parts. Avient says the TPE cools from one side only, roughly 15 to 40 seconds for every 0.100 in (2.5 mm) of overmold wall. Protolabs quotes about 15 days for its quick-turn overmolded and insert-molded parts (all as of November 2026). For how the mechanical work fits with the electronics and firmware, see engineering.

What should you ask your hardware team?

Bring this list to the enclosure review before the first tool is cut, and record which supplier confirmed each answer.

• Which features need a second material, and could any of them be a separate part held by snaps or screws instead?
• For each soft zone, which substrate grade and which elastomer grade, and does the elastomer supplier list that pair as bonding?
• Is the bond chemical, mechanical or both, and where are the interlocks?
• Where does each soft edge end? Is it below the substrate surface in a groove with a positive shutoff?
• Which process makes the samples and which makes production? If they differ, when is the peel test repeated?
• Which peel or pull test, on how many parts, and which failure mode counts as a pass?
• For each metal insert: the supplier part number, its CAD file, the retention shape, the locating tolerance and how the mold holds it.
• Are the substrate tool, the overmold tool and the insert drawings all in the file handover?

If you want a two-material enclosure checked for bonding, inserts and tooling before you commit to molds, tell us about your device.

Sources. All pages accessed November 6, 2026. Guides and resin grades change, so confirm current values before you decide.

• Protolabs, Overmolding and Insert Molding Design Guide (link)
• Protolabs, Building Strength into Parts with Insert Molding (link)
• Protolabs, Insert Molding service page (link)
• Xometry, The Differences Between Insert Molding vs. Overmolding (updated October 30, 2026) (link)
• Fictiv, Overmolding Part Design (December 21, 2022) (link)
• Avient, GLS TPE Overmolding Guide (PDF, file dated October 2020) (link)
• Teknor Apex, The Essential Guide to Overmolding Process and Materials (link)
• Teknor Apex with Covestro, Adhesion of TPEs on Polycarbonates for Medical Wearables (link)