Overmolding and insert molding
Two parts become one, without screws and without glue
Connect metal, magnets, or a second plastic directly in the mold. This saves assembly steps, makes the product stronger and seals better than any secondary operation.
What is overmolding
A product made of two materials
Overmolding is an injection molding technique where plastic is molded around an already existing part. That part can be a metal insert, a magnet, a threaded insert, or a previously molded plastic part. A product comes out of the mold in which both materials are firmly bonded together.
The benefit is rarely in the part itself, but in what is eliminated: no assembly line, no screws, no glue, and no sealing that might end up leaking in the field. That is exactly why we factor this choice into the design, not only after the mold is made.
The terms side by side
Insert, outsert or 2K
Insert part is overmolded
An insert, usually metal, is placed in the mold and is fully overmolded with plastic. Typical for threaded inserts, contacts, shafts, and magnets.
Plastic on a large carrier
The same technique, reversed ratio: the insert is larger than the plastic part. On a metal plate, you create features like guides, clips, and bearings.
Two plastics, one cycle
Two plastics in one machine, usually hard with soft. This gives a TPE grip on a durable housing, without anyone assembling anything.
The practical difference: 2K requires a machine with two injection units, while overmolding and insert molding can also be done in two steps on a standard machine. This makes the entry much cheaper and is almost always the better route for smaller series.
When it pays off
What you gain
- Less assembly
What is bonded in the mold doesn’t need to be assembled by anyone after. This saves actions, loose parts, and chances for errors in the chain.
- Stronger and better sealed
The plastic flows around and encloses the insert. That bond is stronger than glue and seals off without a separate gasket.
- Lighter than metal
Replacing a metal part with an overmolded core lowers the weight and keeps the strength exactly where you need it.
- Added insulation
Plastic around metal insulates electrically and thermally. For contacts and housings that is often the real reason for the choice.
What matters most
Four points that make it succeed or fail
Do the materials bond to each other?
Not every combination bonds chemically. PP with TPE-V and PA with TPE-E go well together, many other pairs don't. If it doesn't fit chemically, we anchor mechanically with undercuts and through holes.
Do they shrink differently?
Two materials shrink differently when cooling. If you don’t account for that, the product warps or the bond cracks. Plastic over steel is the trickiest case here.
Will the insert stay in place?
The injection pressure is substantial. The insert must be fixed in the mold, otherwise it will shift and the dimensions will be wrong. That also helps determine how the mold is built.
Who inserts, and how often?
Inserting is manual or robotic and is part of the cycle time, thus reflected in your unit price. For large series, automation pays off; for small series, manual work is cheaper. We calculate both.
We answer these four questions before making the mold, not after. More design rules are on the Design for Manufacturingpage.
Frequently asked questions
Overmolding in brief
What is overmolding?+
Overmolding is an injection molding technique where plastic is molded around an already existing part, such as a metal insert, a magnet, or a previously molded plastic part. A product comes out of the mold in which both materials are firmly bonded together.
What is the difference between insert and outsert molding?+
With insert molding, an insert is fully overmolded with plastic, such as a threaded insert in a housing. With outsert molding, the insert is larger than the plastic part, such as plastic features on a metal plate. Technically it's the same approach, with the ratio reversed.
What’s the difference with 2K injection molding?+
2K injection molding processes two plastics in one machine with two injection units, in one cycle. Overmolding can also be done in two steps on a standard machine, because the first part is inserted again. This makes the entry cheaper and is usually the better route for smaller series.
Which materials bond to each other?+
Not every combination bonds chemically. PP with TPE-V and PA with TPE-E go well together. If the combination doesn’t fit, we mechanically anchor: undercuts, through holes, and grooves where the plastic flows and locks. We check which route makes sense before the mold is built.
Can I have metal overmolded?+
Yes, metal is the most common insert: threaded inserts, contacts, shafts, and plates. Two points are key: the part must remain fixed in the mold because injection pressure is high, and the shrinkage difference between plastic and steel must be included in the design.
How much does a mold for overmolding cost?+
A mold at QDP starts at 3,000 euros, with most between 3,000 and 5,000 euros. Overmolding is usually somewhat higher than a single-cavity mold, because fixation and mold construction require more. You can get a direct estimate for your product using the online calculator.
From what quantities is it attractive?+
The sweet spot at QDP is 500 to 20,000 units per run, and within that range overmolding is often cost-effective sooner than you'd expect, because saved assembly is directly offset against the mold costs. We calculate both options, with and without overmolding.
And if the materials don’t bond?+
Then we solve it by means of shape instead of chemistry. With undercuts, through holes, and surrounding grooves, the plastic mechanically locks the insert in. That bond holds under load, even if both materials don’t bond chemically.