Sink marks

Pls give your expert opinion on the attached query for sink mark.
Even though it looks like a very basic question but we need to provide answers to the younger generation that yes sink comes in the rib area because ... waiting to hear from you all

Kripal

If this is in a casting, it's because of metal shrinkage. Expect to find porosity in those areas as well, since there's no feed for the metal to draw from as it cools, so it will draw from small pockets of molten metal. The fix is to add some sprues or risers there, that have a larger cross-section than exists in those intersections.

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Originally Posted by awerby

If this is in a casting, it's because of metal shrinkage. Expect to find porosity in those areas as well, since there's no feed for the metal to draw from as it cools, so it will draw from small pockets of molten metal. The fix is to add some sprues or risers there, that have a larger cross-section than exists in those intersections.

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Thanks for the reply Awerby.
The section in question is for a injection molded plastic part.

It's still due to shrinkage, although plastic doesn't behave exactly like metal - it won't develop porosity like metal does.

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Originally Posted by kripal

Pls give your expert opinion on the attached query for sink mark.
Even though it looks like a very basic question but we need to provide answers to the younger generation that yes sink comes in the rib area because ... waiting to hear from you all

Kripal

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On the outside of the part you get a uniform cooling, for the rib that will always get a sink if it is designed like you have it, to reduce the sink you need to have the rib a max thickness of 60% of the main body wall thickness

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Originally Posted by mactec54

On the outside of the part you get a uniform cooling, for the rib that will always get a sink if it is designed like you have it, to reduce the sink you need to have the rib a max thickness of 60% of the main body wall thickness

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Plastic will always tend to flow along the path of least resistance. Therefore, the area of the nominal wall thickness will fill up first as the plastic is injected. Only after this section is full and the cavity pressure rises will the plastic then start to fill the rib section. This can create a surface blemish since some of the plastic has had a chance to cool while it was filling the wall section. This cooler plastic may push against the cavity side and create some cosmetic blemishes, especially on textured surfaces.

Is this analysis in right direction ?