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\r\n Bad surface finish on really thick part\r\n
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\r\n 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.
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\r\nAny 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).
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\r\n I would guess the material was too cold, or not enough hold pressure / time
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\r\n Gate looks awful small for .3" thick part. How fast does it fill?\r\n
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\r\n takes about 2-3 seconds to fill. It\'s actually slightly ticker at 0.316" (8mm).\r\n
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\r\n Are the white streaks splay (usually from moisture or burned material), blush (material dragging), or unmixed color?
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\nI\'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.\r\n
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\r\n 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.\r\n
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\r\n 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.
\nHere\'s a picture of a similar set-up:
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![Quote](/heidenhain/forum/images/misc/quote_icon.png)
Originally Posted by
cordvision\r\n
![View Post](/heidenhain/forum/images/buttons/viewpost-right.png)
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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.
\nHere\'s a picture of a similar set-up:
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If you have a shut off pin then it is called a valve gate
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\nHow hot do you have the mold, it will need to have the correct heat, for the material you are using
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\nYour material is cooling off to quick
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\nThe pin gate is too small,so you can\'t fill the mold quick enough, you are getting what is called cold flow\r\n
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\r\n 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.\r\n
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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
Attachment 330416