What Type of Screw to Use for Plastic: A Complete Guide

For most thermoplastic parts, use a thread-forming screw with a 30° flank angle and larger pitch; brittle, hard, or thermoset plastics take thread-cutting screws or metal inserts instead.

The wrong fastener cracks a boss before it ever holds. Plastic splits along the boss wall when a standard 60° tapping screw drives in, because that profile was built to cut metal, not displace it. Matching the screw to the plastic’s flexural modulus — its stiffness — and to how often the joint comes apart prevents cracking, stripping, and pull-out.

Screw Types And When Each One Fits

Thread-forming screws displace plastic as they drive, creating mating threads without removing material — the right choice for ductile thermoplastics. Thread-cutting screws carve a path instead, which suits brittle or thermoset material that cannot flow around a forming screw.

Three options cover nearly every plastic assembly:

• Thread-forming screws for ductile thermoplastics. These use a narrower, taller thread profile than standard Type B screws, spreading threads wider apart and cutting the outward radial force that splits a boss.
• Thread-cutting screws for thermosets, brittle grades, and unusually hard plastics that cannot absorb the expansion of thread forming.
• Machine screws with metal threaded inserts when the joint comes apart repeatedly, carries heavy load, needs precision threads, or the boss simply won’t hold a formed thread.

One widely specified direct-assembly design swaps the conventional 60° thread angle for a 30° angle, pairs it with a smaller core diameter and larger pitch, and threads cleanly into general thermoplastics.

Matching The Screw To The Plastic

Your plastic’s flexural modulus is the fastest filter for choosing a screw, because it measures how much the material flexes before it cracks. Ductile plastics bend; stiff ones shatter.

This modulus guidance covers conventional plastics up to E = 20,000 N/mm². Reinforced grades above that threshold need testing rather than a table lookup.

The practical fix for a cracked boss or a stripped hole is usually a different thread profile, not a bigger screw. Bossard’s technical library and Pencom’s thread-forming design guide both document the full modulus bands and the pilot-hole geometry that goes with them.

Once you have the modulus band, the pilot-hole size matters just as much — an undersized hole over-stresses the plastic no matter how good the thread profile is. If you’d rather skip the trial and error, this roundup of tested screws for plastic assemblies covers the specific fasteners that hold up in real boss designs.

Pilot Holes, Boss Geometry, And Driving Speed

A working plastic joint depends on the hole and boss as much as the screw. Pilot-hole diameter drives how much the plastic has to stretch, and getting it wrong causes the same cracks a bad thread angle does.

Don’t pick a pilot hole from a generic percentage — plastic type, hole geometry, mechanical load, and installation all shift the final number, so field testing decides it.

Head shape matters too. Wider heads raise friction, lower surface pressure, cut relaxation, and hold residual locking force better on plastic. Countersunk screws are cautioned against here, since a conical head wedges into the plastic and encourages cracks.

Installation comes down to control:

1. Identify plastic type, ductility, wall thickness, boss geometry, temperature, chemical exposure, load, and service life.
2. Pick the screw type — thread-forming for ductile thermoplastic, thread-cutting or inserts for brittle and thermoset material.
3. Form the pilot hole to the manufacturer’s diameter, with adequate engagement, boss diameter, edge distance, and relief.
4. Drive with a torque-controlled or torque-angle-controlled driver. Hand-driving is where joints fail.
6. Set installation torque experimentally near production conditions — suitable torque only comes from testing.
7. Verify insertion torque, stripping torque, pull-out strength, clamp load, vibration resistance, temperature performance, and repeatability.

Friction heat works against you. Spin the screw too fast and the plastic softens right at the boss wall, so the same torque that tightened cleanly on the bench strips the joint on the line. Bossard’s tightening torque resource covers how clamp load and preload behave in these joints. Rigid joint design, corrosion, and torque tolerances interact, and lubricated, plated, or dissimilar-metal assemblies shift the numbers.

A few mistakes cause most plastic fastener failures. Standard 60° metal screws generate excess radial stress, cracking, heat, and boss deformation. Thread-forming screws crack thermosets and brittle grades that cannot flow. Over-tightening strips formed threads and crushes bosses because plastic relaxes under sustained load. And coated steel thread-forming screws carry a hydrogen-embrittlement risk, which ISO 4042 treatment addresses. Ask who cannot use a method before committing: a glass-filled or very hard reinforced plastic above the modulus limit needs its own testing, not a modulus table, and a boss too thin for thread forming needs an insert instead.