Which screw should I use in plastic?
Which screw should I use in plastic?
A thread-forming screw designed for plastic, or a heat-set threaded insert where the joint will be undone more than once.
Plastics fail at fixings in two ways: brittle materials such as polystyrene and acrylic crack from the hoop stress a thread creates, and ductile ones such as polypropylene creep, so a joint tightened today is loose in six months. Thread-forming screws for plastics are designed around both, with a wide thread angle to reduce hoop stress and a large flank area to spread load. The other rule is torque: plastic bosses strip at a fraction of the torque a metal thread takes, so a power driver on its highest clutch setting will destroy the boss before you feel anything. Anything intended to be opened more than a handful of times should have a metal insert rather than a thread cut in plastic.
The options, ranked
| Option | Good at | Bad at |
|---|---|---|
| Thread-forming plastic screw | A wide, shallow, single-lead thread displaces material rather than cutting it, so it does not crack the boss. | Specific to plastics and not stocked everywhere. |
| Heat-set threaded insert | Melts into a moulded boss and gives a metal thread that can be assembled repeatedly — the standard in 3D-printed parts. | Needs a soldering iron and the right hole size. |
| Machine screw and captive nut | Strongest and fully demountable. | Needs access to both sides or a designed-in nut trap. |
| Standard woodscrew | Works once in soft plastics. | Its sharp, deep thread wedges brittle plastics apart, and every reassembly strips the hole further. |
Technique that changes the outcome
- Use the boss and pilot dimensions the screw maker publishes; they are specific and they matter
- Set the driver clutch low and finish by hand
- For repeated assembly, install a heat-set insert with a temperature-controlled iron
- Support brittle plastic around the fixing so the hoop stress has somewhere to go
What goes wrong
- Overtightening, which strips the boss in an instant
- Woodscrews in brittle plastics
- Reusing a stripped hole; move to an insert or a machine screw
Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.
Other screw questions
| For | Use |
|---|---|
| Wood | A pilot hole matching the screw's core diameter in the lower piece and a clearance hole in the upper one — that is what makes a joint tight. |
| Decking | Stainless steel — 304 inland, 316 near the coast — or a screw specifically rated for modern treated timber, which eats ordinary zinc plating. |
| Masonry | A concrete screw into a precisely sized hole, or a plug and screw — and hollow block needs a different anchor entirely. |
| Plasterboard | Coarse-thread bugle-head drywall screws into timber studs, fine-thread into metal studs — and never use them for anything structural. |
| MDF | A straight-shank MDF screw with a parallel thread, always into a pilot hole — the edge splits without one. |
| Sheet metal | A self-drilling (Tek) screw with a drill point matched to the material thickness, or a self-tapping screw into a correctly sized pilot hole. |
| Chipboard | A twin-thread chipboard screw with an unthreaded shank under the head, so it pulls the joint together instead of bursting the board. |
| Roofing sheets | A hex-head screw with a bonded EPDM washer, driven until the washer just spreads — no further. |
| Brass hardware | Cut the thread with a steel screw of the same size first, wax the brass one, and use a driver that fits the slot exactly. |
Common questions
- Which screw should I use in plastic?
- A thread-forming screw designed for plastic, or a heat-set threaded insert where the joint will be undone more than once.
- What should I avoid?
- Overtightening, which strips the boss in an instant Woodscrews in brittle plastics Reusing a stripped hole; move to an insert or a machine screw
Related pages
- Which screw and pilot hole should I use for wood? A pilot hole matching the screw's core diameter in the lower piece and a clearance hole in the upper one — that is what makes a joint tight.
- Which screw should I use for decking? Stainless steel — 304 inland, 316 near the coast — or a screw specifically rated for modern treated timber, which eats ordinary zinc plating.
- Which screw should I use in masonry? A concrete screw into a precisely sized hole, or a plug and screw — and hollow block needs a different anchor entirely.
- Which screw should I use for plasterboard? Coarse-thread bugle-head drywall screws into timber studs, fine-thread into metal studs — and never use them for anything structural.
- Which screw should I use in MDF? A straight-shank MDF screw with a parallel thread, always into a pilot hole — the edge splits without one.
- Which screw should I use for sheet metal? A self-drilling (Tek) screw with a drill point matched to the material thickness, or a self-tapping screw into a correctly sized pilot hole.
- Which screw should I use in chipboard? A twin-thread chipboard screw with an unthreaded shank under the head, so it pulls the joint together instead of bursting the board.
- Which screw should I use for roofing sheets? A hex-head screw with a bonded EPDM washer, driven until the washer just spreads — no further.
- How do I drive brass screws without snapping them? Cut the thread with a steel screw of the same size first, wax the brass one, and use a driver that fits the slot exactly.
- Which glue works on plastic? Glue and adhesive
- Which drill bit should I use for plastic? Drill bit
- Which sandpaper grit for plastic? Sandpaper grit
- Which paint sticks to plastic? Paint
Sources
- Typical range compiled from manufacturer specifications — At One Place