3-Plate Tool - Pin Gates: Witness / gate marks

[rtaylor]

Hi,

We were just wondering on a 3 plate tool with pin gates to mould round cylindrical parts; would there always be a witness / gate marks left behind on the plastic part or can it be avoided completely so that it is smooth finish with no mark left behind?

Its a spoke fed (multipoint) centre fed with pin gates. The material is random copolymer PP - plastic injection moulded.

I don't suppose anyone has a image of what the gate mark can look like when moulding via pin gates?

Thanks


Robert

[Suhas]

It will always leave a mark. However if it is designed correctly it will have no protrusion and will be relatively smooth.
Suhas

[rtaylor]

It will always leave a mark. However if it is designed correctly it will have no protrusion and will be relatively smooth.
Suhas

Hi, so just to check if as you say it will always leave a witness mark then that means there is some protrusion?

[JayDub]

Typically you can gate into a dimple or depression on the part surface so any protrusion is still below flush with the nominal wall.

[rickbatey]

Hot air gun will eliminate witness mark and can help with gate vestige. Gating into a small depression is about the best idea with this type part. By the way: What kind of press will this run in? You really need a toggle with an adjustable clamp stop.
Rick

[rtaylor]

Hot air gun will eliminate witness mark and can help with gate vestige. Gating into a small depression is about the best idea with this type part. By the way: What kind of press will this run in? You really need a toggle with an adjustable clamp stop.
Rick

Hi, thanks. The press I'm assuming you mean moulding machine, it would be a 80Tonne injection moulding press.

Gating into a depression, just for clarification purposes what does that mean or entail?

[iautry1973]

Basically you have a small divot in the part that you are gating into. So when the part is pulled away from the gate if there is a remnant left it doesn't protrude above the adjacent surface. It would be something you usually design into the mold in the beginning and negotiate with the customer. So long as it isn't a cosmetic issue and the part is thick enough at that point it will work rather well.

[rtaylor]

Basically you have a small divot in the part that you are gating into. So when the part is pulled away from the gate if there is a remnant left it doesn't protrude above the adjacent surface. It would be something you usually design into the mold in the beginning and negotiate with the customer. So long as it isn't a cosmetic issue and the part is thick enough at that point it will work rather well.

Thanks for the clarification. So basically its something that is incorporated into the mould tool that works along with a pin gate method? Do you know the technical name of this process that you are describing? I will try and explain to our moulders about the small depression / divot, but do you know what this process is called?

Our cylindrical round part is 2mm thick walls and works with a plunger a bit like a syringe barrel / plunger.

[chrisprocess]

Hi rtaylor,

I've attached photos below of pin gate witness line, gating into a depression, a side view of gating into a depression, and the under side of gating into a depression with reference parts that happen to be on my desk:o
as you can see the underside of the part has a dome-like feature to maintain the wall-thickness.
302
303
304
305

edit: photo came out bad, colored it in with sharpie to see the depression feature
306

[rtaylor]

Thanks for sharing. Much clearer now to see. It looks much like a recess similiar to that left behind with a hot tip gate. I was thinking it was smooth all over and no change to wall thickness as mentioned by someone above in a previous post.

[chrisprocess]

Thanks for sharing. Much clearer now to see. It looks much like a recess similiar to that left behind with a hot tip gate. I was thinking it was smooth all over and no change to wall thickness as mentioned by someone above in a previous post.

The recess pictured above is rather aggressive, others may only be 0.020" deep or so. Deep enough if there's gate vestige it does note protrude beyond the flat surface. The part above is used on the inside of a finished product so cosmetics were not critical, but the top surface had to be flat for movement so the recess is deep and wall thickness is kept consistent.
Depending on your part design and function wall thickness may or may not be kept consistent.. A 0.020" recess in a 0.200" wall section is less significant than 0.030" recess in a 0.060" wall thickness. If that makes sense.

[rtaylor]

The recess pictured above is rather aggressive, others may only be 0.020" deep or so. Deep enough if there's gate vestige it does note protrude beyond the flat surface. The part above is used on the inside of a finished product so cosmetics were not critical, but the top surface had to be flat for movement so the recess is deep and wall thickness is kept consistent.
Depending on your part design and function wall thickness may or may not be kept consistent.. A 0.020" recess in a 0.200" wall section is less significant than 0.030" recess in a 0.060" wall thickness. If that makes sense.


Thanks for the claification. Just a thought, a hot runner with a valve gate, does this leave any witness marks on the part and compared to pin gate / 3 plate tool - does it give better overall surface finish on the part?

[JayDub]

A valve gate will leave a witness mark very similar to an ejector pin.