What's the point?

[iamsuperdan]

What is the point of these little plastic tab things on plastic parts? 

Placement on kits seems pretty random to me.

 

And why would this window piece require four of them?

[Tom Geiger]

Those are the ejector pins. Much better than having them visible on the part itself

[Danno]

The more a part has, the more assured complete flow of the molten plastic is to all areas of the part. 

[Jantrix]

4 hours ago, Tom Geiger said:

Those are the ejector pins. Much better than having them visible on the part itself

Or more exactly, they are extra material added for the ejector pins within the mold housing to push against, to push the part out of the mold. Like Tom said, much prefered than having pin marks in the plastic part itself.

[peteski]

I don't know the official name for those, but like others have said, these are areas where the ejector pins will push the part out of the mold. If you look at every nub, you will see ejector mark on it. Those are used where it is not convenient to have ejector pins under the part itself. This either done on very small or thin cross-section parts that are too small to have ejector pin under them, or on clear parts where they would be marred by ejector marks.  They have nothing to do with flow of the molten plastic.

[smellyfatdude]

I worked as a molder in a foundry, for many years. In metalcasting, those are known as risers. As already noted, they are there to ensure that as the plastic cools, the part in the mold cavity doesn’t shrink. That little bit of plastic pushes against the part until it solidifies.

[Deuces ll]

We grind those risers off before straighting aluminum castings at work....

Edited October 10, 2020 by Deuces ll