Have you ever pressed a brass threaded insert into a plastic housing...

And the moment you tightened the bolt down, the brass insert ripped entirely out of the plastic?

You probably used a hammer or a cold arbor press. Here is why "cold pressing" is destroying your assemblies. 👇

When new engineers need strong threads in a plastic part, they often design a tight hole and physically force a knurled brass insert into it.

But plastic is not meant to be stretched like a rubber band at room temperature.

When you cold-press an insert, the sharp brass knurls act like miniature wedges. They do not interlock with the plastic—they just shear, scrape, and fracture it.

The outward pressure creates massive internal "Hoop Stress."

Over time, or the moment it gets exposed to a chemical cleaner, that invisible stress causes the entire plastic mounting boss to shatter.

To get maximum pull-out strength, you have to flow the plastic, not force it. You need Thermal Staking (Heat Staking).

By heating the brass insert, you melt a microscopic layer of the surrounding plastic. The liquid polymer flows perfectly into all the tiny undercuts and knurls of the brass.

When it cools, the plastic acts like glue, locking the insert permanently in place with ZERO residual stress.

If you are designing for heat-staked inserts, follow these 3 rules:

𝟭. 𝗧𝗵𝗲 𝟴-𝗗𝗲𝗴𝗿𝗲𝗲 𝗧𝗮𝗽𝗲𝗿

Never use perfectly straight inserts in a perfectly straight hole. The insert will tilt and melt in crooked. Always specify "Tapered Inserts" and design an 8-degree draft angle into your plastic hole. This allows the insert to self-center perfectly before the melting even begins.

𝟮. 𝗧𝗵𝗲 𝗠𝗲𝗹𝘁 𝗭𝗼𝗻𝗲 𝗩𝗼𝗹𝘂𝗺𝗲

You cannot displace material that has nowhere to go. The hole in your CAD must be precisely sized so the volume of the displaced plastic exactly matches the volume of the knurled grooves on the insert. If the hole is too tight, molten plastic will spew out the top and ruin your mating surface.

𝟯. 𝗧𝗵𝗲 𝗖𝗼𝗹𝗱 𝗣𝗹𝗮𝘁𝗲 𝗙𝗶𝗻𝗶𝘀𝗵

Never push the insert 100% flush with your hot iron. Stop about 1 mm above the surface, remove the heat, and immediately press a flat, cold block of metal against the insert to push it the final 1 mm. This instantly freezes the top layer of plastic perfectly flat against the brass, guaranteeing a flush fit for your bolts.

Have you ever had a 3D printed or molded part crack because an insert was forced in too hard? Let me know below! 👇