Insert mold stainless steel rod using acetal

[sjoseph]

Is it possible to insert mold a stainless steel rod using an acetal-like plastic, and then break the rod lose after molding so the rod freely rotates thereafter? The objective is to eliminate an assembly step and produce a final assembly with a feature not possible otherwise.

The break-free process would need to be done by hand with a force less than a couple pounds or so. The rod diameter is 2.5mm and the insert depth is 8mm.

Thanks, Scott

[MTUHusky]

The bad news is that Acetal material likes to stick to cores (i.e. a steel rod), especially if it is not heated and cooled properly.

So its only 8mm long and 2.5 mm diameter. To minimize deflection you should have a length that is less than 2 x the diameter for a blind hole that has a diameter less than 4.5mm. So you are a little over, if you attempt this I would verify your wall thickness all the way around the part as your rod may shift to one side or another due to plastic pressure deflection. At a 2.5mm diameter, cooling channels are probably not going to be an option but at this small size, it may not be an issue. You could always have several rod inserts made and when one comes out just drop it in a bath of cold water, plastic and all, and it should pop right off. Acetal is Hydrophobic so you dont have to worry about the part absorbing water.

Mold release sprays will be a great help if you can use them on your product.

Hope this helps,

Husky

[sjoseph]

Thanks for the reply. I was thinking that the thermal coefficient of acetal would, upon cooling, enable a separation shrinkage if the rod surface roughness was sufficiently low (i.e. acetal shrink is >10um, Ra of stainless < 1um). The part can cool for a while after molding, so the temperature should be room before trying to break loose the rod. It seems like you are effectively saying the same thing, no?

Also, i don't need the rod to pull out after molding. Each piece has a permanent rod insert. So the process would be (a) insert a rod into the mold tool, (b) mold the part, (c) 24 hours later, free the rod so that it can rotate in the hole. I was thinking that perhaps I need a tool that will hold the piece as the rod is freed so that I don't stress the plastic hole walls.

What do you think?

Scott

The bad news is that Acetal material likes to stick to cores (i.e. a steel rod), especially if it is not heated and cooled properly.

So its only 8mm long and 2.5 mm diameter. To minimize deflection you should have a length that is less than 2 x the diameter for a blind hole that has a diameter less than 4.5mm. So you are a little over, if you attempt this I would verify your wall thickness all the way around the part as your rod may shift to one side or another due to plastic pressure deflection. At a 2.5mm diameter, cooling channels are probably not going to be an option but at this small size, it may not be an issue. You could always have several rod inserts made and when one comes out just drop it in a bath of cold water, plastic and all, and it should pop right off. Acetal is Hydrophobic so you dont have to worry about the part absorbing water.

Mold release sprays will be a great help if you can use them on your product.

Hope this helps,

Husky

[MTUHusky]

Ah, I see what you are trying to accomplish. I have no experience with inserts being able to move after molding but it sounds like it may be possible.
A few things I would try:
-Dropping them in a cold water bath out of the mold to help loosen the rod up right away.
-If you can get away with using mold release, I would as it may be loose right out of the mold.
-Your shrink calculations need to be spot on here, to the point that I would involve an expert level mold flow analysis. As I said before, this material will keep on shrinking for over a day, it shrinks too much and it will lock onto the rod.
-As I mentioned in the previous comment, you are going to have to be aware of the rod deflection during injection as the length of the rod is greater than 2 x the diameter. If the rod deflects, the wall thickness will be greater on one side vs. the other which could lead to binding of the rod when it rotates. Unfortunately, if you run into core deflection, the only tool you will have to fix it is processing it out (keep pressures low, shear rates high all controlled with fill profiling).

Keep me posted on what you learn from this trial, sounds like an interesting project.

Husky