On this blog, we love plastics — all sorts of plastic, especially the plastic-injected molded kind. But there comes a time when plastic doesn’t have the metal, eh, mettle, to get the job done right. Plastic parts that have to be assembled and re-assembled multiple times, for example. Plastic threads just won’t cut it, in those applications.
That’s where metal comes in. You heard right — real metal, like brass, aluminum and stainless steel. The critical factor is force, of which there are two kinds:Pull-out Force: this is the resistance of the insert pulling out of the part. You want this as high as possible because you don’t want the insert to be jerked out of the part by brute force; you do want it to be unscrewed, using the threads as guides.
Torque-out Force: Ideally, you want this kind of force to be as low as possible while still allowing the insert to “sock-up” — get tightened to the point where the pull-out force is as high as possible (see above).
So what kinds of materials are best for the plastic part and the metal insert? Depends on the application. Only thermoplastics will work with the two most popular insertion methods—heat staking and ultrasonic — because these two processes involve re-melting already molded plastics. For thermosets, there are two options: 1) making the insert part of the original molding process; and 2) cold pressing the insert in later. If you choose the latter option, the elasticity of the plastic resin will be crucial.
Brass is the most common material for the threaded inserts themselves. However, the movement toward “green” alternatives has led to increased use of alternatives such as stainless steel or aluminum which don’t have any lead. Stainless steel, not surprisingly, is stronger and more corrosion-resistant than brass, and aluminum inserts are approximately 70% lighter. So there are advantages and disadvantages to both metals if you decide not to go with brass.
Heat Staking Threaded Inserts
Best for:
- thermoplastics
- high overall performance and low installation cost
Both ultrasonic and heat-staking insertion starts with an already formed hole (the size determined by the insert manufacturer), that can either be drilled after molding or formed in the molding process itself. The insert’s outside walls can be straight or slanted. Both straight and slant-walled inserts offer excellent alignment of the fastener (as long as the hole is made properly.), with the self-aligning tapered inserts being simpler and faster to press in.
Ultrasonic Assisted Threaded Inserts
Best for:
- thermoplastics
- high overall performance
Molded-In Threaded Inserts
Best for:
- thermosets & thermoplastics
- best pull-out and torque performance
Cold Pressed-In Threaded Inserts
Great for:
- thermosets
- easy installation
- lowest cost
- only a simple press requried