The two terms are used loosely and often interchangeably, which causes real confusion in quoting. Insert moulding places a component, usually metal, into the tool and moulds around it. Overmoulding moulds one polymer onto another. Two shot moulding does the overmoulding in one machine with one tool. Knowing which you are asking for changes the tooling quote substantially.
Insert moulding
A pre made component, most often metal, is loaded into the mould before the shot and the plastic forms around it. The insert can be a threaded bush, a stamping, a pin, a bearing or a magnet. It needs one tool, features to locate the insert, and a loading operation each cycle, which is manual at low volume and robotic at high volume.
Overmoulding
A soft polymer is moulded onto a rigid one, usually for grip, sealing or damping. The rigid substrate is itself a moulded part, which means either two tools with a transfer between them, or a two shot tool. Bond strength comes from the material pair and, where no compatible pair exists, from mechanical interlock designed into the substrate.
Two shot moulding
Both materials are moulded in one machine with a rotating or shuttling tool, so the substrate never leaves the mould between shots. It is fast, repeatable and the most expensive to tool, because you are effectively paying for two tools that must register perfectly. It becomes the right answer once the volume amortises that, which is usually well into the hundreds of thousands.
Choosing between them
If the second material is going onto metal, that is insert moulding whatever anyone calls it. If it is going onto plastic, the question is whether the volume justifies a two shot tool or whether two separate tools with a transfer operation is cheaper. Below high volume, two tools nearly always wins, and it also lets each part be sourced separately.
Questions people ask about overmolding vs insert molding
What is the difference between insert moulding and overmoulding?
Insert moulding places a pre made component, usually metal, into the tool and moulds around it. Overmoulding moulds one polymer onto another.
Is two shot moulding the same as overmoulding?
Two shot is one way of doing overmoulding, in a single machine with a rotating or shuttling tool. The alternative is two separate tools with a transfer between them.
Which is cheaper to tool?
Insert moulding needs one tool. Overmoulding with two separate tools is next. Two shot tooling is the most expensive and pays back only at high volume.
How do I decide?
Metal insert means insert moulding. Polymer on polymer means overmoulding, and volume decides whether it is two tools or a two shot tool.