GlossaryKind of term: Manufacturing termInsert installation (heat, ultrasonic)
What is insert installation?
Insert installation is fitting metal bushes with an internal thread into a plastic part, by heat, ultrasound or pressing.
The bushes are usually brass, sometimes stainless steel, and let a screw be fastened and released repeatedly without damaging the plastic. The outside of the bush carries knurls or ribs that lock it against pull-out and rotation. Common methods are heat-setting, where the plastic melts round the heated insert and solidifies again, ultrasonic insertion, where vibration melts the plastic locally, and cold press-fitting. Inserts placed in the mould during injection moulding fall outside this step.
What does insert installation depend on?
Heat-setting and ultrasonic insertion only work in thermoplastics that melt and flow. In thermosets, expansion, self-tapping or bonded inserts are usual; in PTFE and other plastics that flow poorly, press-in inserts too. Heat-setting leaves little residual stress; press-fitting is usually faster but stresses the wall and can crack a thin one. The hole follows the insert supplier's specification for material and method: diameter, depth and sometimes a slight taper. That size differs from the tapping drill size. Pull-out and torque strength depend on material, wall thickness around the insert and the quality of installation.
How does insert installation work in practice?
The hole is usually part of the machining program; installing each insert and checking that it sits square and firm is a handling step of its own. That time grows with the number of inserts per part and depends on the method and on any installation press with a fixture; it bears no fixed relation to the milling time. A mark-up on milling time therefore fits poorly as soon as the number of inserts or the milling time changes. The inserts themselves are bought parts. On thin walls, glass-filled plastics and inserts close to an edge, the method can decide whether the wall holds and how much pull-out strength remains.
What is insert installation often confused with?
Insert installation is the operation; the bush itself is one type of thread insert. Another well-known type, a stainless steel coil, is used mainly in metal to strengthen or repair a thread. Nor is insert installation tapping: a thread tapped into plastic wears faster when a screw is fastened and released often.
Frequently asked questions
When do you choose heat-setting over pressing inserts?
On thin walls, brittle or filled plastics and where the insert must hold a high pull-out load. The plastic flows round the insert with little residual stress. Pressing is usually faster, but stresses the wall and needs enough wall thickness.
How do you price insert installation?
As a separate item: time per insert times the number per part, plus the inserts as purchases and any fixture or installation press. A mark-up on milling time does not follow the number of inserts.
Which hole size belongs with an insert?
The size the insert supplier specifies for your plastic and method, with the matching depth. It differs from the tapping drill size; the wrong size gives a loose insert or a cracked wall.