One of the most common uses for induction heating is the heat staking of threaded metal inserts into plastic. Most thermo-plastics are too soft to sufficiently hold a thread, so brass or steel threaded inserts are added. Post-molded installation is more cost-effective than molding in place and induction is a proven way to pre-heat the inserts prior to installation.
For heat staking, the insert is preheated with induction and then pressed into a hole in the plastic part. This is accomplished by positioning the induction coil over the hole and then holding the insert in the coil for a short period of time. When the correct temperature is achieved, the insert is pressed into the plastic. A narrow zone of plastic then melts and flows into the knurls of the insert. The plastic re-solidifies, resulting in a complete assembly with much better mechanical properties than inserts implanted with other techniques.
The insert material is usually brass or steel; each has advantages and disadvantages. Brass is non-magnetic and will not corrode as easily as steel. However, brass is a softer material and will anneal at temperatures as low as 450°F., whereas steel starts to anneal at 1200°F.
Some glass-filled plastics require inserts to be heated to 700°F for correct installation, so the brass inserts must be heated and inserted quickly to prevent thread annealing. Brass has low electrical resistivity and therefore requires more power to heat with induction than steel.
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