CNC Machining Design Guide

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Materials

Design

Processes

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Formulas

Math
Inserts
•  Inserts are used in plastic parts, to allow the use of fasteners such as machine screws. The advantage of this is that since these inserts are made out of metal, they are robust. Further, machine threads also allow great many cycles of assembly and disassembly
•  Inserts are installed using one of the following methods:
-  Ultrasonic insertion. The insert is vibrated using an ultrasonic transducer, called the "horn" mounted in an ultrasonic machine.

The horn has to be specially designed for each application for optimum performance. The ultrasonic energy is converted to thermal energy due to the vibrating action, which allows the insert to be melted inside the hole.

This type of insertion can be done rapidly, with short cycle times, low residual stresses. Good melt flow characteristics for the plastic are necessary for the process to be successful.

The ultrasonic equipment is relatively expensive, and also needs a custom horn for optimal production rates.

-  Thermal Insertion. The inserts are heated by placing them over the hole and pressing them in with a heated tool. The tool first heats the insert, then the insert is pressed in.

The advantage of this method is that the special tooling necessary is relatively simple, usually a cylindrical tool, which can be easily, fabricated in the machine shop. The cycle times are usually short.

However, care has to be taken, not to overheat the insert or the plastic, or it will lead to local plastic degradation.
 

-  Press Fitting.The inserts are designed with barbs (straight knurls that are interrupted) and can be press fitted inside the hole in the boss.

This process is fast and requires no special tooling. However, high hoop stresses are generated, so the boss design has to be robust. Also, the retention is strictly based on press fitting and the small amount of material that flows inside the barbs. Thus retention is not very high.
 

-  Molded-in Inserts. The inserts are placed in the mold prior to the injection of plastic. The injection of the plastic completely encases the insert on the outer diameter and provides very good retention. In fact retention of such inserts is the best compared to other process (barb/knurl design being the same).

This process slows down the operation of the mold, since the inserts have to be manually placed inside the mold. Inserts can be automatically placed in a mold, but this greatly increases the complexity and cost of the mold. This can only be justified if the volume of production is very high to offset the cost savings in shorter cycle times.

Glossary