The Purpose of Vent Drops

This question arrived in my e-mail the other day…

In a previous blog you mentioned the term ‘Vent Drop’. Could you explain how these are typically incorporated into the mold design?
Note: Jason is referring to the post Venting Issues which states:
One commonly overlooked aspect to venting is the vent drop. Behind any well designed vent is a substantive vent drop.  These drops are deep grooves, which channel the gas from behind the vent to the outside the mold. Think of the vent as a gate designed to transfer the gas from the mold to the vent drop.
My Response
One of the most common vent drop is a deep channel machined around the perimeter of the mold cavity. This method is often preferred because it is easy to add more venting since it will automatically transfers the gas to the vent drop.
Another common vent drop method is to machine a channel 5-10 times deeper and at least 2 times wider than the actual vent.
If runners or actions prevent the vent drop from reaching the to the perimeter of the mold through the parting line, holes are often be drilled through the core to vent the air to the ejection housing.
Additional Thoughts
Many mold designers make the mistake of assuming that the air will vent away from a core block which is ‘proud’ (sticks out from the base a little). When clamped, most of this metal is designed to focus the clamp tonnage, and does not provide enough clearance to properly vent the gas from the mold during injection.

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