SI® brand inserts employ press-in, molded-in, or heat/ultrasonic installation methods to provide strong, reusable, permanent threads in plastic.

New ways to find your product.


SI® Externally Threaded Studs

Key Insert Features

  • Knurling provides torque out resistance
  • A hex shaped body also provides rotational torque resistance
  • Opposing knurls and diamond knurling provide pull-out resistance. The longer the insert, the better the pull-out value
  • Undercuts and barbs also improve pull-out resistance

Slots found in cold press inserts collapse upon installation and expand during mating hardware assembly to provide torque out and pull-out resistance

Ultrasonic / Heat Staking Inserts

  • Ultrasonic - Frictional heat caused by the vibration melts the plastic surrounding the insert allowing easy insertion. When the vibration ceases, the plastic solidifies, locking the insert permanently in place.
  • Heat Staking - Installed by pressing the insert into the mounting hole with a thermal press to melt the plastic surrounding the insert.


  • Installed during the molding process, the inserts are located in the mold cavity by core pins. When the mold opens, the core pins are withdrawn leaving the inserts permanently encapsulated in the plastic section with only the threads exposed.
  • Installing the inserts during the molding process eliminates the need for secondary steps or installation equipment.


  • Installed by simply pressing the inserts into pre-molded or drilled holes. Installation is accomplished using any standard press at any time during the production process.
  • Eliminates the need for molding-in inserts.
  • Eliminates the need for heat or ultrasonic equipment.

Try our Compression Limiter Configuration Tool


Thermal Heat Staking Insertion

During this process, the insert is placed into a molded or predrilled hole

  • A heated probe contacts the insert heating it to at or above the melt temperature of the plastic
  • Localized melting occurs as the insert is driven into place
  • The probe is retracted and the plastic re-solidifies around the insert

Ultrasonic Insertion vs Thermal/Heat Insertion

Ultrasonic Insertion Advantages

  • Fast and reliable
  • Works well for most thermoplastics
  • Uses standard 20kHz welders - easy to source
  • Repeatable results

Thermal/Heat Insertion Advantages

  • Non-Abrasive compared to ultrasonic insertion
  • Provides better plastic flow during installation
  • Metal dust is not created
  • Multiple inserts can be installed simultaneously
  • Better method used for larger inserts
  • Inserts can be installed on different levels simultaneously
  • Tooling, equipment and maintenance costs are lower

Ultrasonic Insertion Disadvantages

  • Insertion is noisy
  • More abrasive, risk damage to insert
  • Can create metal dust during installation
  • Limited to the number of inserts that can be installed at once

Thermal/Heat Insertion Disadvantages

  • Slower when installing 1-insert at a time
  • Longer cool-down time necessary for the insert
SI Inserts


Inserts are installed during the injection mold operation.

The insert is placed on a core pin in the injection mold and the plastic is molded around the insert. Proper pin size is important to retain insert in place.

Advantages and Disadvantages of a Mold-in Insert


  • Excellent holding strength
  • Excellent plastic flow around insert
  • No secondary assembly
  • Multiple inserts can be installed during the molding cycle


  • Longer cycle times - more costly
  • Plastic can get into the threads
  • Mold Damage possible if misloaded
  • Inserts need to be removed before plastic can be reground if plastic part is defective
  • Molded-in Stress & Possible Sink Marks
SI Inserts

Cold Press Insertion

Press-in Inserts for Plastics

  • Install by simply pressing the insert into pre-molded or drilled holes
  • Eliminates the need for heat or ultrasonic equipment
  • No melting of the plastic. Holding values may be lower

Flanged Threaded Inserts for Ultrasonic and Thermal/Heat Insertion

Flanged, Straight Wall, Thru-Threaded, IUTFB™ Inserts

  • Provides larger surface area and high pullout in reverse entry applications
  • Brass flange offers a contact surface for electrical connections
  • Flange helps distribute the load applied when fastening prior to installation
  • Aluminum and stainless steel available upon request

Compression Limiters

We do not currently sell equipment for installing SI® brand threaded inserts for plastics. For your convenience, we have listed below suppliers of ultrasonic and thermal insertion presses.

Sonics & Materials
Branson Ultrasonics
Sonobond Ultrasonics
Ultra Sonic Seal

Rinco Ultrasonics

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