Bad surface finish on really thick part

I did some preliminary testing for injection molding a 1/3 inch thick part (disk shaped) from POM. I use a hot sprue bushing for direct injection. The surface finish is terrible. I suspect it is because the part shrinks a lot as it cools down so I have the hold pressure on for a long time and the POM that gets injected as the part shrinks moves the already cooled down skin along the mold surface. I hope that makes sense. Not sure if that is really what is going on, but it looks like it in my opinion.

Any suggestion how to get a good surface finish on that part? (BTW, I can't re-design the part, it needs to be that thick all the way through).

Attachment 330414

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I would guess the material was too cold, or not enough hold pressure / time

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Gate looks awful small for .3" thick part. How fast does it fill?

takes about 2-3 seconds to fill. It's actually slightly ticker at 0.316" (8mm).

Are the white streaks splay (usually from moisture or burned material), blush (material dragging), or unmixed color?

I'm leaning towards too small of a gate. What's the diameter? At 8mm thick, it should ideally be almost 8mm in diameter, especially if you are cutting it out anyway.

Yes, the gate is much smaller. I'm using a hot sprue bushing with a ring gate that is 1.5mm in diameter. How does the gate diameter influence the creation of those ripples? I don't think the hot sprue bushing would work with a 8mm gate, I would have to buy a regular sprue bushing.

I'm actually not sure if I used the right terminology (ring gate). It's a hole with a diameter of 1.5mm and a little needle in the middle.
Here's a picture of a similar set-up:

Attachment 330424

Quote:

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Originally Posted by cordvision

I'm actually not sure if I used the right terminology (ring gate). It's a hole with a diameter of 1.5mm and a little needle in the middle.
Here's a picture of a similar set-up:

Attachment 330424

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If you have a shut off pin then it is called a valve gate

How hot do you have the mold, it will need to have the correct heat, for the material you are using

Your material is cooling off to quick

The pin gate is too small,so you can't fill the mold quick enough, you are getting what is called cold flow

A small gate can cause all kinds of surface finish issues. The Troubleshooter on PlasticsToday is always harping about gate size. I think half of his articles talk about the original gate being too small.