If you need to make plastic parts with strong joints and reusable threads, metal threaded inserts are the way to go. The best method depends on the part requirements.
Using Heat Set Inserts
50 pieces 1/4-20 threaded inserts for wood working applications, 10mm length, 6mm Allen key included. Threaded insert head diameter: 0.461 / 11.7mm,
Threaded Inserts for Wood 1/4-20 x 10mm 50 Pieces
Threaded Inserts for Plastics
SI® - Threaded Inserts for Plastic
Lots to Say About Recycling
Round Tubing Threaded Insert fits 1 outside diameter tubing with 16/18 gauge wall thickness Threaded Round Plastic Tubing Insert with 1/4-20 metal
Round Plastic Threaded Tube Inserts - 4 Pack Threaded 1(OD) Round Plastic Tubing Insert 1/4-20 Threads - Threaded Tubing Inserts Threaded Tube
Insert # 256A4-6BR Molded-In Closed End Threaded Inserts
Four Ways to Tackle Threaded Inserts for Plastics
Equipment Solutions Incorporate Recycled Plastics into Wood and Fiberglass Composite Materials
Robert Gallagher - Product Development Manager - PennEngineering®