Plastic Joining Inserts: Threaded Inserts and Compression Limiters

Plastic Joining Inserts

Plastic forming and molding processes offer nearly limitless design options for all sorts of consumer products, tools, toys, electronics, medical devices, and much more.  Manufacturers continue to push for lighter, thinner materials in an effort to reduce costs, which in turn pushes designers to find new ways to add strength and resilience to their designs.  Traditional threaded fasteners such as screws and studs are a viable option, but require a few additional components to ensure that they do not damage the lightweight plastic materials that they’re holding together.  These extra components are known as Threaded Inserts and Compression Limiters, and in this article, we’ll explore each of these insertion fasteners and their applications in plastic device manufacturing.   

Plastic Connections Overview 

Joining two components together with a threaded fastener usually consists of either a thru-bolt connection or a tapped hole connection.  Both of these bolted connection types work well in harder materials, but present challenges when used in softer materials such as plastics.  These challenges include:

  • Fracture – holes in plastic materials tend to be weak spots, and are often the starting or ending positions of cracks and fractures.
  • Over-Compression – plastics are often brittle and low density, such that high compression loads (or over-tightening a bolt to begin with) can end up damaging the components. 
  • Strain Deformation – many plastics deform easily under higher temperatures, and become overly brittle under lower temperatures, either of which can lead to permanent physical changes.  Bolted connections are more susceptible to failure in these cases due to the applied compression force of the joint. 
  • Visco-Elastic Flow – many plastics achieve their light weight and high pliability thanks to their unique molecular makeup that performs just like an extremely thick, flowable fluid.  This flow allows plastics to be easily bent or flexed while able to elastically return to their original form, but also sets a low threshold for permanent deformation under just a little more stress than normal, especially at elevated temperatures and pressures.  This means that bolt holes can enlarge, thin, or change shape relatively easily simply due to the nature of this plastic ‘flow’ under load.  

When it comes to making bolted connections, many plastics are inherently too soft or brittle to drill and tap from the start, failing in a combination of ways mentioned above.  Next, let’s discuss hardware options designed to make threaded joining in plastics possible.     

Threaded Inserts

A threaded insert is a piece of hardware made from metallic materials (and sometimes very hard plastic), with the intention to then ‘insert’ this threaded piece into a larger plastic component during manufacturing.  The threaded insert provides all of the performative benefits of its metal material:  high compression strength, non-wearing threads, close tolerance dimensions, and resistance to loads that would otherwise damage the base plastic.  In effect, threaded inserts offer all of the resilience of metallic threaded joints where needed, while leaving the rest of the component to remain a lightweight, lower-cost plastic material. 

Female Threaded Insert w/ Knurls
Fig 1 – Female Threaded Insert w/ Knurls

Threaded inserts offer multiple features that can often be combined to fit any application:

  • Straight – uniform diameter over its entire length
  • Tapered – tapered core with diameter reducing from one end to the other, used for interference retention of the screw
  • Open End – both ends open, where a longer screw could extend beyond the length of the insert
  • Blind End – one end capped, where a longer screw would bottom out in the insert
  • Symmetrical – can be installed with no need to orient the insert before insertion, saving manufacturing time and machine complexity
  • Flanged Head – has a thickened bearing ring on one end, used for added reinforcement and retention of the insert, as well as to bear the full diameter of any mating screw heads or other hardware (so that they don’t touch surrounding plastic, such as in electronics where metallic fasteners should not contact silicon boards)
  • Smooth – smooth outside surface for general and low-load applications
  • Grooved – insert has a groove cut into its outside surface for moderate pull-out resistance (but does not protect against undesired rotation)
  • Knurled – roughened outside surface, used for maximum resistance for both pull-out and rotation 
  • Custom – many custom inserts help designer’s achieve unique configurations, such as dual diameters (for sheath and rod connections), reverse-hand threads, O-ring seals, and more

Another style of threaded insert uses a male threaded stud to extend from the face of the base plastic component, offering a connection point that could slip through a compression limiter (described below) and fasten down directly with a nut and washer.  Many of the same features described above apply to studded inserts as well. 

Male Threaded Stud Inserts
Fig 2 – Male Threaded Stud Inserts

Do you have a plastic joining application but are unsure of which specialty fastener might be best to use?  E&T Fasteners offers expert support and product selection between the above options and more!  Contact us to discuss your application, or to see our standard fastener offerings, visit our product selection here

Compression Limiters Overview

Threaded inserts take care of the threaded end of a fastened joint, but what about the non-threaded end?  For plastic components that only need a hole for a screw to pass through, we turn to Compression Limiters. 

A compression limiter is essentially a female threaded insert without the internal threads.  Also made from resilient metals, compression limiters are provided to protect thru-holes.  Often you’ll find a compression limiter mating up to a threaded insert (either male or female), and in other cases, you’ll find compression limiters alone where a bolt and nut fasten through multiple compression limiters. 

Compression Limiter
Fig 3 – Compression Limiter

Compression limiters can be supplied with many of the same options as their threaded counterparts, as follows:

  • Straight – uniform diameter over its entire length
  • Open End – both ends must be open to pass through a screw or bolt 
  • Symmetrical – can be installed with no need to orient the compression limiter before insertion, saving manufacturing time and machine complexity
  • Flanged Head – has a thickened bearing ring on one end, used for added reinforcement and retention, as well as to bear the full diameter of any mating screw heads or other hardware (so that they don’t touch the plastic, such as in electronics where metallic fasteners cannot touch silicon boards)
  • Smooth – smooth outside surface for general and low-load applications
  • Grooved – has a groove cut into its outside surface for moderate pull-out resistance (but does not protect against undesired rotation)
  • Knurled – roughened outside surface, used for maximum resistance for both pull-out and rotation 
  • Custom – some custom limiter examples include double flanged heads, O-ring ends, and pre-lubricated (for inserts that act as low-load pivot points) 

For both threaded inserts and compression limiters, common material options include brass, stainless steel, steel, and aluminum.  These inserts can be installed in several ways, selected based on the design load, base material, and manufacturing process being utilized.  Generally, mold-in styles are selected for injection molding and thermosetting, and press-in styles selected for mechanical insertion, ultrasonic welding, and heat staking. 

E&T Fasteners is here to help you navigate fastener and accessory options.  Price quotes, availability, lead times, delivery confirmations, unique application suggestions – whatever your need, we’re here to take your call or email.  For support and advice from our fully trained staff, speak with an E&T sales representative today. 

About E&T Fasteners

With sales and support teams spanning the Continental US, E&T Fasteners is here to serve your hardware, fastening, and component needs both domestically and internationally.  We are a stocking distributor of metallic, plastic, and exotic alloy fasteners, backed by our technical staff directly experienced in your unique applications.  We can assist your specifying and selection efforts, providing engineered solutions, record-fast lead times, and stellar customer service.