Neck Finish Guide
One of the most common questions we receive from customers is how to determine the neck finish of round containers and the proper closures. Below is a quick guide to understanding neck finishes.
Screw thread or continuous thread closure sizes are expressed with two numbers separated by a hyphen or slash. The first number refers to the millimeter diameter measured across the inside of the cap’s opening or the outside of the bottle’s threads. The second number refers to the thread style, “GPI” or “SPI” finish. The Glass Packaging Institute (GPI) and Society of the Plastics Industry (SPI) are responsible for establishing uniform standards for glass and plastic container neck finishes. The closure industry does not necessarily adhere to the same standards, so it is often best practice to purchase containers and caps from the same distributor when possible.
The inside diameter of the bottle neck. Specifications require a minimum “I” to allow sufficient clearance for filling tubes. Linerless closures, with a plug or land seal, and dispensing plugs and fitments require a controlled “I” dimension for a proper fit.
The outside diameter of the thread. The tolerance range of the “T” dimension will determine the mate between bottle and closure.
The outside diameter of the neck. The difference between the “E” and “T” dimensions divided by two determines the thread depth.
The height of the neck finish. Measured from the top of the neck to the point where the diameter “T” extended down, intersects the shoulder.
Measured from the top of the finish to the top edge of the first thread. The “S” dimension is the key factor determining the orientation of the closure to the bottle and the amount of thread engagement between the bottle and cap.