Overmolding
Also: two-shot molding, two-shot injection molding, 2K molding, double-shot molding, multi-shot molding, multi-material molding, insert molding, soft-touch overmolding, TPE overmolding, LSR overmolding
Overmolding molds a second material onto a first: soft TPE on rigid plastic by two-shot or pick-and-place, and plastic around metal by insert molding.
87 shops tagged Overmolding among 768 injection molding companies in the Noramark directory
Overmolding molds one material onto another so they come out of the press as one part. The common forms are a soft thermoplastic elastomer (TPE, TPU) or liquid silicone rubber over a rigid plastic substrate, for grips, seals, bumpers and buttons, and plastic molded around a metal or other preformed part, called insert molding. Two-shot (2K) molding does both shots in one cycle on a press with two injection units: the substrate is molded, then the mold rotates or a core pulls back and the second material is shot against it while it is still warm. Pick-and-place overmolding molds the substrate in one mold and loads it, by hand or robot, into a second mold on an ordinary press.
Insert molding loads metal parts into the mold before each shot: threaded brass inserts, bushings, pins, electrical contacts and lead frames, blades, magnets and cable ends. Vertical presses with rotary or shuttle tables let one set of inserts be loaded while another is molded. The plastic shrinks onto the insert and locks through knurls, grooves and holes. It replaces post-molding assembly (heat staking, press fits, ultrasonic insertion, adhesives) with a stronger, sealed joint, at the cost of slower cycles and loading labor or robots.
The joint is the design problem. A chemical (melt) bond forms only between compatible pairs, where the second shot wets and partly melts the first: TPU on polycarbonate or ABS, TPE grades formulated for PP, ABS, PC or nylon, PP on PP. Incompatible pairs, such as most TPEs on acetal (Delrin) or on polyethylene, need a mechanical interlock (through holes, undercuts, dovetails or a wrap-around) that holds the soft material even though it does not stick. Metal inserts do not bond chemically without primers and rely on mechanical locking. The substrate also has to survive the melt temperature of the second shot without softening, which is why the rigid, higher-melting material is molded first.
At a glance
| Typical tolerances | Rigid substrates hold injection-molding tolerances, about ±0.1 mm (±0.004") on typical features. A soft TPE layer holds about ±0.15-0.25 mm (±0.006-0.010"), and its edges are only as clean as the steel shutoff against the substrate. Insert position ±0.05-0.1 mm (±0.002-0.004") from the mold locating pins; the insert itself must be held to about ±0.025-0.05 mm (±0.001-0.002") where the mold closes on it, or the part flashes or the insert is crushed. |
|---|---|
| Size limits | Two-shot presses are fewer than standard presses; job shops typically run 50-500 tons, for parts from a few grams to about 1 kg. Insert molding runs on vertical presses from about 30 to 300 tons. Overmold layers about 0.5-3 mm (0.020-0.120") thick, with 1-2 mm (0.040-0.080") ideal for TPE: thin layers do not fill and thick ones sink. Inserts range from small pins and contacts to machined parts of a few hundred grams. |
| Surface finish | Each material takes the surface of its own mold steel: gloss or texture on the rigid substrate, usually a matte or textured soft-touch finish on the TPE. The boundary between materials is a shutoff line, crisp when the substrate has a step or groove at the edge, flashed and prone to peeling when it does not. Metal inserts keep their own finish; plate them (nickel, tin, gold) before molding, because the plastic prevents plating afterwards. |
| Lead time | Insert and pick-and-place molds 4-10 weeks, similar to single-shot molds; two-shot molds 10-16 weeks, plus time to schedule a two-shot press. Aluminum prototype overmold tools in 3-5 weeks. Allow extra sampling rounds to tune the bond and the shutoff. Production after approval 2-4 weeks, longer when inserts are sourced separately. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Substrates: ABS, polycarbonate, PC/ABS, polypropylene, nylon 6 and 6/6 (filled or not), PBT and acetal. Overmold materials: styrenic (SEBS) TPEs, typically Shore 30A-80A for grips, TPU for tougher wear surfaces, TPV for seals, and liquid silicone rubber, which needs its own cold-runner equipment and self-bonding grades. Hard-on-hard two-shot parts pair two rigid resins, such as a clear polycarbonate lens in an opaque housing. Insert metals: C360 brass threaded inserts, steel and stainless pins and bushings, copper and beryllium copper contacts, aluminum heat spreaders; PVC and TPE are molded over copper wire for cable ends and strain reliefs. Pick the overmold grade from the TPE or LSR supplier adhesion data for the exact substrate grade, and test the bond on first articles.
What drives the cost
- Tooling: a two-shot mold with a rotating platen or core-pull costs roughly 1.5-2 times a single-shot mold for the same part
- Press availability and rate: two-shot and vertical insert presses are fewer and bill more per hour
- Labor or automation to load inserts and substrates, and the cycle time it adds
- Materials: bondable TPE grades and LSR cost more than general-purpose resins, and inserts carry their own cost
- Quantity: two-shot pays at high volume; pick-and-place and insert molding at lower volumes
- Scrap from weak bonds, flash at the shutoff and misloaded inserts
- Testing: peel, pull-out and torque tests, and leak tests on sealed parts
When to use it
- Soft-touch grips, seals, gaskets, bumpers and buttons molded onto a rigid housing
- Threaded metal inserts, pins and bushings that must carry high pull-out and torque loads or be sealed
- Electrical connectors, sensors and cable strain reliefs where the plastic insulates and seals the metal
- Replacing assembly steps (gluing, press fits, separate gaskets) at volumes of tens of thousands a year and up
- Two-color or clear-and-opaque parts in one molded piece
When not to
- Low volumes: heat-set or ultrasonic inserts, glued-on grips and separate gaskets cost less than a second tool or a two-shot tool
- Material pairs that do not bond and leave no room for a mechanical interlock
- Parts that must come apart for repair or recycling
- Inserts that cannot take the molding temperature and pressure, such as many electronic assemblies; low-pressure molding or potting suits them
- Substrates that soften at the melt temperature of the overmold material
Design tips
- Choose the overmold grade from the TPE or LSR supplier adhesion data for the exact substrate grade, and ask for peel test results.
- Add mechanical interlocks (through holes, undercuts, grooves) even with bonding pairs; with non-bonding pairs they are the only hold.
- Keep the soft layer 1-2 mm (0.040-0.080") thick and uniform, and end it in a groove or against a step about 0.5-1 mm (0.020-0.040") deep so the steel shuts off cleanly; avoid feather edges, which peel.
- Support the substrate with mold steel behind thin sections, or the second shot will push it out of shape.
- Give metal inserts knurls, grooves or flats for pull-out and torque, a chamfer for loading, and a sealing land where the mold closes, held to about ±0.025-0.05 mm (±0.001-0.002").
- Keep plastic around an insert at least about one wall thick so it does not crack as it shrinks onto the metal, and preheat large inserts toward mold temperature.
- Mold the rigid, higher-melting material first and the softer, lower-melting material second.
- Put bond strength, pull-out force or torque requirements on the drawing so they are tested on first articles.
Frequently asked questions
- What is the difference between two-shot molding and overmolding?
- Two-shot molding is one way to overmold. Overmolding is any process that molds a second material onto a first; two-shot does both shots in one cycle on a special press, while pick-and-place overmolding molds the substrate first and loads it into a second mold. Two-shot costs more in tooling and gives a lower part price and a better bond at high volume.
- What is insert molding?
- Molding plastic around a preformed part, usually metal, that is placed in the mold before each shot: threaded inserts, pins, bushings, contacts, blades or cable ends. The plastic shrinks onto the insert and locks through knurls and grooves, giving a sealed joint that is stronger than pressing or heat-setting the insert afterwards.
- What plastics can be overmolded with TPE?
- Standard TPEs bond to polypropylene; specially formulated grades bond to ABS, polycarbonate, PC/ABS and nylon, and TPU bonds well to polycarbonate and ABS. Acetal (Delrin) and polyethylene bond poorly to most TPEs and need a mechanical interlock. Always pick the grade from the supplier adhesion table for the exact substrate and test peel strength.
- How much does two-shot injection molding cost?
- A two-shot mold typically costs 1.5-2 times a single-shot mold for the same part, often $30,000-150,000 for production tooling, and it runs on presses that bill more per hour. In exchange the part comes out finished in one cycle, so at tens of thousands of parts a year it usually costs less than molding two parts and assembling them.
- Should I use insert molding or heat-set inserts?
- Heat-set or ultrasonic inserts go in after molding, need no special tooling and suit low to moderate volumes. Insert molding gives higher pull-out and torque strength and a sealed joint, and suits high volumes or loads, at the cost of insert loading in every cycle.
