Threaded Screw Inserts.

What are Threaded Screw Inserts?

Threaded screw inserts, also known as screw thread inserts, are internally threaded metal bushes from Accu's threaded inserts range that give plastic, wood and non-ferrous metal parts a machine thread able to survive repeated assembly. Most are short cylinders with a metric internal thread and a textured or threaded outer body, many also carry a flange that seats on the surface of the part. That outer form locks the insert into the host material against torque and pull-out, so the screw wears against metal rather than against the part itself.

A screw thread insert earns its place wherever a fastener has to come out and go back in without damaging the part, from electronics enclosures and 3D printed prototypes to moulded automotive trim, medical device housings and furniture.

How Threaded Screw Inserts Work.

Every type anchors its outer body in the host so it resists rotation as the screw is tightened and also pull-out once the joint is loaded. Heat-Fix and Tech-Fix inserts are pressed hot into a slightly undersized hole, so the thermoplastic softens, flows around the body and grips it as it cools; Sonic-Fix and some Multisert versions achieve the same with ultrasonic energy. Broach-Fix and Fin-Fix press in cold on interference and Expansion-Fix expands against the wall of a through hole, while Flow-Fix and Himould are moulded in. Self tapping threaded inserts, including Thread-Fix, Trisert, Trisert-3 and the stainless steel Self Tapping range, cut their own thread into a plain pilot hole, so the host never needs tapping.

Take, as a working example, the moulded ABS rear housing of a handheld patient monitor, where M3 screws hold the circuit board and a battery cover that comes off at every service. Driven straight into a plastic boss, each screw recuts the thread a little more until it strips and the housing is scrap. A brass Heat-Fix or Sonic-Fix insert fitted after moulding gives those screws a metal thread instead and the same logic holds for a PLA prototype bracket, an automotive centre console panel or a stainless Self Tapping insert in a plywood cabinet.

Materials Threaded Screw Inserts Are Made From

Accu's threaded screw inserts are made from engineering metals chosen to suit the host material and service environment:

• Brass: The main material of the range, with high thermal conductivity that carries heat quickly into the plastic during heat-set and ultrasonic installation.
• Stainless Steel - (A2): Pairs good general corrosion resistance with the strength to cut a thread in wood, plastic and non-ferrous metal.
• Stainless Steel - (303): A free-machining austenitic grade used for holding accurate threads for general industrial use.
• Stainless Steel - Marine (A4): Molybdenum-bearing stainless steel for resistance to chlorides, salt spray and washdown chemicals.
• Titanium - (Grade 5): Ti-6Al-4V, a high strength, corrosion resistant alloy at little more than half the density of steel.

Four finishes are offered:

• Natural: The uncoated standard for every brass insert and for Stainless Steel - (A2).
• Clear Passivated: A chemical treatment on the stainless steel and titanium Trisert-3 inserts that supports their protective oxide layer.
• AccuBlack: An option on Stainless Steel - (A2) Self Tapping inserts from M3 to M20, giving visible fixings a low-reflectivity look.
• Zinc Nickel plated and Trivalent Passivated: Used on steel Trisert-3, where the zinc nickel protects sacrificially and the trivalent passivate seals it without hexavalent chromium.

Sizes & Types of Threaded Screw Inserts Available

Accu's threaded screw inserts are available in a broad spread of metric sizes and installation types:

• Insert Type: Broach-Fix, Expansion-Fix, Fin-Fix, Flow-Fix, Heat-Fix, HiMould, Multisert, Self Tapping, Sonic-Fix, Tech-Fix, Thread-Fix, Trisert and Trisert-3.
• Installation Method: Heat Press, Heat Press / Ultrasonic Press, Cold Press / Heat Press, Cold Press / Heat Press / Ultrasonic Press, Press-In, Press-In Expansion, During Moulding and Self Tapping.
• For Use With: Thermoplastics, or Wood, Plastic & Non-Ferrous Metal (Requires Pilot Hole).
• Internal Thread Size: M1.6 (1.6 mm) to M20 (20 mm).
• Length (L): 1.35 mm to 27 mm.
• Outer Diameter (OD): 2.65 mm to 26 mm.
• Recommended Hole Size (HS): 2.3 mm to 25.4 mm.
• Flange Diameter (FD): 3.2 mm to 12.7 mm.
• Minimum Wall Thickness: 1.3 mm to 6 mm.

Not every combination exists. M10 to M20 is Stainless Steel - (A2) Self Tapping only, with M8 adding a single brass Thread-Fix, while M1.6 is one flanged Multisert and M2 and M2.5 are brass only. Stainless Steel - (303), Stainless Steel - Marine (A4) and Titanium - (Grade 5) are limited to flanged Trisert-3 from M3 to M6. Flow-Fix and HiMould suit new tooling rather than finished parts, and Minimum Wall Thickness is only specified on the heat-set and press-in Fix types.

Whichever insert fits, machine screws complete the joint, while Brass Threaded Inserts narrows the choice to heat-set brass. Where a wall is too thin for any insert, rivet nuts are the alternative for sheet and tube, Polyfix screws thread straight into plastic instead and Threaded Inserts For Wood cover timber and board furniture.

FAQ

Q: What size hole do threaded screw inserts need?

A: Each insert lists its Recommended Hole Size (HS) in the tech spec and it varies by type as much as by thread size. For M3, Broach-Fix needs 3.8 mm, Heat-Fix, Sonic-Fix and Tech-Fix need 4 mm, Trisert needs 4.1 mm to 4.4 mm and Self Tapping needs 4.6 mm to 4.8 mm. Most Fix types hold a -0/+0.1 mm tolerance, so drill or mould to the figure. Accu's hole size charts cover 3D printed parts.

Q: How do you install threaded inserts in metal?

A: The threaded inserts for metal in this range are the Stainless Steel - (A2) Self Tapping types, specified for non-ferrous metal as well as wood and plastic. Drill a plain pilot hole to the Recommended Hole Size (HS), 4.6 mm to 4.8 mm at M3 rising to 24.9 mm to 25.4 mm at M20, then drive the insert in square so it cuts its own mating thread. Start it straight: an insert driven at an angle cuts a crooked thread. Accu's installation guide walks through the process.

Q: Can self tapping threaded inserts be used in steel?

A: Not as specified. Accu's stainless steel threaded inserts for metal list wood, plastic and non-ferrous metal as their host materials, so steel and cast iron fall outside that specification. Solid steel normally takes a tapped thread directly, and for thin steel sheet or tube, rivet nuts are the usual route.

Q: What is the best threaded insert for plastic?

A: It depends on when the insert goes in. Flow-Fix and Himould are moulded in, so the plastic forms around them. Heat-Fix, Tech-Fix, Sonic-Fix and Multisert go into a moulded or 3D printed hole with heat or ultrasonic energy, and Multisert can also be cold pressed. Without heating equipment, Broach-Fix, Fin-Fix and Expansion-Fix press in cold, and Thread-Fix, Trisert and Trisert-3 cut their own thread.

Q: What are the disadvantages of using threaded inserts?

A: Mainly an extra installation step and tighter control of the hole, often to -0/+0.1 mm, plus the equipment that heat-set and ultrasonic types need. The boss must also be thick enough: where specified, Minimum Wall Thickness runs from 1.3 mm to 6 mm depending on type and size, and a thinner wall risks splitting. Moulded-in types also commit the design at the tooling stage.

Q: Do threaded inserts need to be glued?

A: Not with this range, because none of its installation methods relies on adhesive. Heat-set and ultrasonic inserts are held by reflowed plastic, press-in types by interference, moulded-in types by the plastic that sets around them and self tapping types by the thread they cut, so an accurate hole does the job glue would otherwise do.

Bespoke Threaded Screw Inserts Manufacture.

High precision, bespoke manufacture of Threaded Screw Inserts to customer specification. State of the art facilities specialising in both small batch prototyping and large scale manufacture.