Guide To Cap and Neck Finishes

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From customers to engineers, we all know that the cap on your bottle must fit correctly in order to prevent leakage evaporation and possibly dangerous spillages. In reality, what they don’t know is how to make sure that the cap is the right size. Nowadays, there are so many types of containers available on the market that selecting the correct closure can be very confusing. It is essential to ensure that the size you need is available when you decide on a style.

Cap sizes are not interchangeable. Bottles with a capsize in the neck finish have a unique measurement system. The first thing you need to know is how the sizes of the caps are determined. The first number refers to the diameter of the cap or opening in millimeters, which is the nominal diameter of the inside of the cap or opening or of the outer diameter of the bottle. The second number represents the GPI / SPI thread finish and refers to the height and style of the threads on the cap. In the glass and plastic container industry, GPI stands for the Glass Packaging Institute and SPI stands for Society Of Plastics Industry.

A 28 by 400 cap, for example, has a diameter of 28 millimeters and will fit a container with a 400 GPI thread finish. It is crucial that both numbers are correct. Caps measuring 28 by 400 will not fit container measurements of 28 by 410. Although the diameter is the same, there may be some leakage due to the thread size being different.

GPI / SPI Neck Finishes

Experts in glass packaging (GPI, Glass Packaging Institute) and plastic packaging (SPI, The Society of Plastics Industry) developed the guidelines for round plastic containers and closures, along with glass containers, in order to ensure consistency between the various types of packaging.

400	A 400 neck finish consists of a single thread turn.
410	A 415 neck finish consists of two thread turns, but the threads are thinner and the neck is typically taller.
415	A 415 neck finish consists of two thread turns, but the threads are thinner and the neck is typically taller.
425	A 425 neck finish consists of two thread turns but is most commonly found on smaller capacity containers such as vials.
430	A 430 neck finish consists of one or two thread turns. The threads are also deeper than the standard 400 and 415 neck finish. The unique shape of the neck is commonly known as a buttress and is designed to help the user pour product with better accuracy.

You may come across a variety of neck finishes other than the ones listed above. Many different types of neck finishes and variations are available that have all been designed to accommodate specific caps. For more information on what cap works best for your product, please contact a packaging specialist.

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