Troublesome undercut?

[yomama]

Hi all,
I'm a novice on these boards. I have a part that my toolmaker insists can't be made as it is designed. It has an undercut as shown in the attachment, a "ring" groove. The dia of the hole where the undercut groove lives is about 12.5mm.

The toolmaker says I can't have an undercut here, because it's too difficult to have the part extracted from the tool. I figure it's because the hole is too small to make the slide in the mold inject material, then release out the hole.

Is this true? It really affects the design to have such a limitation.

Thanks for any help you can provide.
Steve

[DwightDixon]

What kind of undercut you can pull depends on the modulus of the material you are dealing with and whether the material has sufficient properties when it is ejected. It also depends on whether the part is still trapped in the cavity when it is ejected. You can not cause the material to truly reduce its volume when it is ejected. The part can be deformed and allowed to change its shape, if it is not trapped in the cavity. You would need to open the mold and then try to eject the part. There are such things as collapsing cores which might make the mold design easier. You should have a free consultation with your material supplier's technical people, to assure that they believe the part can be made.

[Will Call]

Ditto what Dwight said but DME carries a collapsing core that will work.

[yomama]

Thanks guys. Got to ask the vendor about that. They said it's "impossible" so it might be the kind of mold where the part is not ejected from the cavity....does that make the undercut harder to mold?

[Will Call]

There are other options if the undercut isn't on the ejector side. Look at "inventing" a spring loaded, or side activated.

NOTHING IS IMPOSSIBLE.

[Louis]

The impossible sometimes takes a bit longer and costs a bit more

[Will Call]

Steve,

Where are you located? I may know a shop in the area that can help.

[brian_shih]

Will Call is probably right. DME collapsible core may do, but all about the cost.

Dwight's more conservative, but may save you more on the tooling cost if the under cut is not too much, say 0.6 or even more for some materials like TPR, EVA. With the right plastic material, there is a chance that it is very possible it can be extracted from the tool.

By the way, the collapsible core Will mentioned belongs to the pre-enginerred components category in DME's catalogue.

[Yoda]

That doesn't look like much of an undercut. There are several methods of colllapsing the core that might work.

[Rocky Huber]

I know that this is an old post, but I felt the need to reply for anyone else who may read it in the future. A .6mm undercut, on a 12.5mm diameter core that is molding polypropylene or polyethylene, is not a problem to eject from the mold if it's done properly. As mentioned before the part needs to be free to expand, and can't be stripped if it is contained by the cavity.

The problem with this particular part, from what I can see in the picture, is that the external ribs that attach to the round boss that contains the undercut will prevent the ring from being able to expand enough to release the part from the core without distortion.

rhuber at ivanhoetool.com
rhuber at dzynsource.com

[Chanson]

yes, the hole is too small for the slider come out.

[Bader]

what is the material ???

it would be waste of time to discuss this issue without knowing the material first !

Bader

[Yoda]

If the material allows it, Bader is correct, the open the mold, pull the core and eject the part. I have done thie many times. Do not try and use the part ejection to pop the part off the core.

[Mason_Dixon_Design]

If this is a low volume part the ideal thing would be to machine the undercut in as a secondary operation. Depending on the material these area's can be stripped. This would likely distort the undercut. The mold would cost some more to allow this stripping. This feature can possibly be modified to be formed open close?
John.
[email protected]

[Ray Chan]

what is the material ???

it would be waste of time to discuss this issue without knowing the material first !

Bader

I agree with Bader, if the material is PP or PE, it is possible.