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Posted By: Joann

They are mold gates. They are not related to the sonic welding. If you had two halves of a slab, in raw state never having been welded together, they would have these marks.

The halves are injection molded. So there is a channel that the melted plastic runs through until it gets to the actual cavity that is shaped like the slab, and it fills that cavity. When the two halves of the mold are separated, you have the slab half but also the the channel part still attached.

It's like those little cheap plastic toys or whatever, where you have a plastic "spine" off of which are several plastic "ribs" with a piece of the toy at the end of each rib. You break the piece off of the end of the rib and assemble the pieces.

Each piece of the toy you break off would have a little piece of the rib still attached. That's what this is. It's the piece of the channel (runner, actually) that the material flowed through. Operators or robots trim these pieces off, and the clipping can be more or less flush. I've seen sprues on my slabs that are as big as .015" or so. Even if they are flush, they will still have the white stress mark.

There's your manufacturing essay for the day. Not evidence of tampering. Not an artifcact of the weld process. Just where the sprue was clipped.

J