Design for Manufacturing / Screws and Bosses
DfM Deep Dive
Screws and Bosses: Three Routes to a Connection
There are three routes to attach something to an injection molded product: molded-in threads, thread formed afterward with a self-tapper, or a metal insert. The right choice depends on assembly frequency, load, and quantities.
Molded-In Thread
Thread can be molded in directly, with requirements: a coarse, rounded profile instead of fine metric thread (sharp thread peaks fill poorly and create notches), a thread-free lead-in of 0.5 to 1 mm at the top and bottom, and external thread on the parting line (releases automatically). Internal thread requires an unscrewing unit or collapsible core; at low volumes, better to avoid that. Molded threads are strong for rarely disassembled, coarse connections such as lids and caps.
Self-Tappers in Plastic Bosses
| Characteristic | Indicative Rule |
|---|---|
| Hole Diameter (Core) | 0.8 to 0.85× the nominal screw diameter |
| Outer Diameter Boss | 2 to 2.5× the screw diameter |
| Screw-In Depth | at least 2× the screw diameter |
| Lead-In Countersink | about 0.5 mm to prevent upsetting of the first thread |
| Wall Connection | boss to wall via ribs, not solid (sink) |
| Reuse | indicative 5 to 10 times mounted; more often = choose insert |
Metal Inserts
| Type | When |
|---|---|
| Heat embedded (heat staking) | Standard choice after molding: inexpensive, good pull-out values |
| Ultrasonically embedded | Same principle, faster in series |
| Insert molded (insert molding) | Highest strength and best positioning, costs cycle time |
| Expansion/press-in insert | Cold placed afterward, lowest pull-out values |
Don’t forget that the best screw is sometimes no screw: snap-fits, bayonets, ultrasonic welding or an integrated clamping detail save on inserts, screws, and assembly time. “Can this screw go away?” is a standard question in every DfM review.
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