Methods for inserting threaded metal inserts

Ultrasonic insertion
The insert is pressed into a hole in the plastic by a horn that vibrates at ultrasonic frequency. The ultrasonic energy melts the plastic around the insert. Once the insert is pressed in, the plastic freezes off evenly around the insert.

Inserts as shown below specially developed for ultrasonic insertion, are commercially available in various types and sizes.

Ultrasonic insertion gives a shorter molding cycle than parts with molded-in inserts. However, it also represents an additional manufacturing process. Care should always be taken to ensure the insert is solidly embedded in the substrate.

Inserts for ultrasonic insertion

Heated inserts

A special press that pre-heats the insert is used for hot pressing-in. The inserts are pressed into the hole when they have reached the desired temperature. The heat-transfer melts the polymer and the molten polymer flows into the undercuts and secures the insert after cooling down. The advantages of this method are:
- strong connection, high loads can be absorbed
- low internal stresses in the plastic if well-designed and executed
- low equipment costs compared to ultrasonic insertion.

A longer insertion time is needed for cooling-down the plastic.

Cold pressed in inserts

Inserts can also be pressed directly into a hole. This can be done cold as well as hot.Pressing-in cold is done with a small press. The inserts are provided with knurls under an angle at the outside. It is the fastest and easiest way of insertion, but high stresses will be present in the material round the insert making the connection weak.

Cold pressing in is carried out with knurled inserts

Molded in inserts

The insert is put into the mold (cavity) during the injection molding cycle. It is important to heat the inserts to a temperature close to the mold temperature before molding to remove differences in thermal expansion that can lead to a stress build up at the metal/plastic interface. It is also essential that the inserts are clean and free of any process lubricants. Molded-in inserts have the following advantages:
- strong connection, high loads can be absorbed
- low internal stresses in the plastic if well-designed and executed.

There are some disadvantages in this process:
- a longer injection cycle is required to position the inserts in the mold
- heating of the inserts is necessary
- thermal stresses in the plastic if not well executed
- severe damage to the mold is possible if inserts move.

Coil inserts

Coil inserts offer a better wear resistance and strength than the surrounding polymer, but high stresses may be introduced into the boss. Coil inserts offer only limited connection strength.

Thread cutting inserts

Thread-cutting inserts are comparable to self-tapping screws. The insert is screwed into a drilled or injection molded hole without internal thread. A cutting edge at the outside of the insert cuts a thread in the plastic.

Expansion inserts

Expansion inserts can simply be pushed into a hole in the product. They are provided with a saw cut at the bottom, thus forming separate segments. The two segments are spread after putting the insert in place, either before or during insertion of the screw.

Expansion inserts offer limited connection strength.


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