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Lesson 2.2ShapeChapter 2 · Molding plastics and rubber

Overmolding and two-shot molding

A second plastic molded onto the first

Overmolding molds a second plastic onto a first one, so a single part comes out made of two materials. The soft rubbery grip on a toothbrush or a cordless drill is made this way. It replaces glue and assembly, and the two layers do not peel apart in normal use.

Overmolding and two-shot molding: a real photo
Photo: Swoolverton, CC BY-SA 3.0, via Wikimedia Commons
A second plastic is molded onto the first, making one two-material partTwo-shot molding: one machine, two injection units and a mold half that turnsFirst shothard plastic fills cavity Aunit 1 · hardunit 2 · softfixed halfturning halfcavity Acavity BOpen and turnthe part moves from A to Bhalf turns 180°Second shotsoft plastic fills the gap in Bnext partgrip addedEject: one part, two materialssoft grip (2nd shot)fused where they touchhard core (1st shot)Same idea, other waysOvermolding:the first part is made inone mold, then moved to a second one.Insert molding:a metal part is placedin the mold and plastic flows around it.1234hard plastic (1st shot)soft plastic (2nd shot)molten plasticmold steel
Two-shot molding · one mold cycle in 3 panels, plus the finished partCircled numbers match the steps below. Panels 1–3 show the same mold during one cycle; panel 4 enlarges the finished part.

How it works

  1. First shot. One injection unit fills cavity A with a hard plastic. This makes the rigid base part, called the substrate.

  2. Open and turn. The mold opens and the half holding the substrate turns 180°. The substrate now faces cavity B, which has extra room where the grip will go.

  3. Second shot. The mold closes and a second injection unit fills that extra room with a soft plastic, often TPE (thermoplastic elastomer, a rubbery plastic that can be melted). The hot melt fuses with the surface of the substrate. At the same moment, cavity A molds the next substrate.

  4. Eject. The mold opens and the finished part is pushed out. From then on, every cycle delivers one complete two-material part.

The bond comes from the two plastics melting together, which only works for compatible pairs. Otherwise designers add holes or grooves so the soft material locks on mechanically.

Two variants use the same idea. Overmolding in two molds works the same way, but a robot or an operator moves the substrate from the first mold to the second. Insert molding places a metal part (a threaded insert, a blade, electrical contacts) in the mold instead of a first shot.

Key facts

Materials
Hard plastics (polypropylene, ABS, polycarbonate, nylon) paired with soft TPE or silicone; or two colors of the same plastic; inserts are often brass or steel
Typical parts
Toothbrush handles, power tool grips, two-color buttons and keys, housings with built-in seals
Part size
Mostly small to medium, from a few grams to a few kilograms
Precision
Like injection molding: typically about ±0.05 to ±0.1 mm on the hard part; the soft layer is looser
Surface finish
As molded, glossy or textured; soft areas feel grippy
Batch size it suits
Two-shot: tens of thousands to millions; two molds or hand loading: also smaller runs
Tooling and machine cost
Higher than for one material: a more complex mold, often tens of thousands of dollars or more, and a machine with two injection units and a turning platen
Speed
One finished part per cycle, typically about 20–60 s

Strengths and limits

Strengths

  • One finished part, no glue or assembly
  • Strong, sealed joints between hard and soft areas
  • Soft-touch grips, seals and crisp two-color looks
  • Fast and consistent at high volume

Limits

  • Molds and machines cost more than for one-material parts
  • The two plastics must be compatible to bond, which limits the pairs
  • More to tune: two materials, two temperatures, different shrinkage
  • Two-shot only pays off at high volume; two-mold overmolding is slower per part

Where it's used

  • Personal careToothbrushes, razor handles
  • ToolsPower tool housings, screwdriver and plier handles
  • AutomotiveTwo-color buttons and knobs, tail-light lenses, seals molded onto housings
  • ElectronicsPhone cases with soft bumpers, waterproof housings with molded-in seals, cable strain reliefs
  • MedicalDevice housings and instrument handles with soft grips

How to recognize it

  • A soft, rubbery zone that cannot be peeled off, with a crisp edge against the hard plastic.
  • Two colors meeting at a perfectly sharp boundary, with no glue line or gap.
  • Small holes or slots in the hard part filled with the soft material (mechanical locks).
  • Gate marks in both materials, one for each shot.

Compare with

Injection molding2.1the single-material base process; overmolding simply runs it twice, on top of the first part.Compression molding2.6another common way to bond rubber onto metal or plastic parts, by pressing it around them in a heated mold.

Watch it

Star Rapid · Insert molding, overmolding and two-shot molding compared for beginners

SIMTEC Silicone Parts, Inc. · Two-step overmolding versus one-machine two-shot molding

Injection Molding Skills & More · A two-shot cycle, step by step

Injection mold maker · Real footage of a hard polycarbonate part getting a soft TPE layer

See real photos

Remember

  • A second plastic is molded onto the first: one part, two materials, no glue.
  • The layers hold by fusing or by mechanical locks, so material pairs matter.
  • Two-shot does it in one machine; overmolding uses two molds; insert molding starts from metal.

More in Molding plastics and rubber

All 100 processes
2.1ShortInjection moldingMelt forced into a closed steel moldOpen the lesson2.3Blow molding (extrusion and injection blow molding)Hot tube or preform inflated in a moldOpen the lesson2.4ThermoformingHeated sheet sucked onto a moldOpen the lesson2.5Rotational moldingPowder coats a hot, turning moldOpen the lesson2.6Compression molding (SMC, BMC, rubber)A charge pressed and cured in a hot moldOpen the lesson2.7CalenderingCompound squeezed into sheet between rollsOpen the lesson

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