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I'm getting some plastic parts (made with HDPE) CNC machined. A few of these parts require the addition of threaded inserts. I've printed the parts myself using a 3d printer and when I add the threaded insert, it's impossible to insert it perfectly in the plastic part (perfectly straight in the hole) since I'm using a soldering iron (not very professional) to heat up the insert.

I know threaded inserts are used in professionally manufactured plastic products, so I'm wondering how they make sure the threaded insert is perfectly inserted into the plastic part. Which tool do they use to add the insert? What type of inserts are used?

Thanks everyone for the help.

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  • $\begingroup$ Can we have a photo of the insert and link to the datasheet? $\endgroup$
    – Transistor
    Mar 24 at 9:11
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    $\begingroup$ look up various PEM nut products... overmolding, press-in, ultrasonic, and "anchor" type inserts (that expand when you put a screw into them), are some methods. // I can tell you that simple press fit in plastic is not the best. $\endgroup$
    – Pete W
    Mar 24 at 13:04
  • $\begingroup$ What am I missing here? Plastics are glued together, press fits will be problematic due to plasticity of the inserted piece. But if you had to, a press and jigs would be used for perpendicular fitting; probably you would need an oven to heat up HDPE, I can't figure out how you would use a soldering iron to do it. $\endgroup$
    – Tiger Guy
    Mar 24 at 23:49
  • $\begingroup$ @Tiger Guy, AFAIK the inserts are typically metal, other than some clever expanding types, vaguely similar to drywall or masonry anchors. Plain interference fit is weak vs being pulled out, but if it has a flange and the screw comes from opposite side that is ok. Ultrasonic press inserts are the "fancy" solution. Ultrasonic vibration generates heat very locally and melts or softens the material most just where needed. The inserts are ribbed or textured to help grip. Homebrew pre heating could possibly add too much heat to reach the desired surface's temperature, might mess up the hole... $\endgroup$
    – Pete W
    Mar 25 at 0:01
  • $\begingroup$ @PeteW, that's what I was missing - thought he was 3d printing the inserts. $\endgroup$
    – Tiger Guy
    Mar 25 at 16:38
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The methods available to a hobbyist are akin to that of the production world, in that one needs only a controlled heated implement traveling in a restricted manner.

A drill press with a long bolt threaded to match the insert provides such restrictions. Heat can be applied to the insert and the quill lowered into the part, which is secured appropriately to the bed. Even a small butane or propane torch would provide suitable heating.

It's no stretch to imagine a high end CNC grade machine in which the heating element is incorporated into the portion holding the insert and the part into which the insert is to be installed is on a belt or other transport system.

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  • $\begingroup$ heating the insert makes the inserted part bigger, not smaller $\endgroup$
    – Tiger Guy
    Mar 25 at 16:38
  • $\begingroup$ @TigerGuy it looks like you already realized this, but for others benefit we're talking about metal threaded inserts being inserted into plastic. The plastic flows around the hot insert as you insert it. $\endgroup$
    – Drew
    Apr 2 at 4:19
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Some threaded inserts have flanged faces for a few reasons, one being that if you press the flange flush to the surface around the hole you're tapped hole will be relatively perpendicular. These can just be hammered in.

Others, typically "self-clinching", are installed with arbor presses and some degree of attention by the operator to maintain square.

Not related to your actual question but an aside: An alternative to heating up the plastic (or whatever the housing material is) is to cool the insert. Assuming it's metal you should be able to get it pretty cold without cracking. Dry ice is cheap and readily available, let the part sit for a while, should give you some more clearance to easily align the insert.

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