Standards for plastic bottle neck and cap threads have been established to ensure safety, quality control and performance. While plastic bottles can vary greatly in size and by industry-specific use there are terms and ratios that are consistent in their engineering and manufacturing.
Who Sets the Standard?
Industry standards for plastic bottle threads, among other related manufacturing issues, are promoted by industry trade groups such as the SP and the Closure Manufacturers Association.
Standards Definitional Terms
Key definitional terms in discussion of plastic bottle threads are: The height or H dimension, which is the the neck portion of the bottle from the top of the neck to where the neck meets the horizontal shoulder spread of the bottle; the screw or S dimension, which is measured from the end of the bottom thread to the top edge of the first thread, which is what makes the tight connection between the bottle and the cap; the inner diameter of the bottle neck or I dimension for filling and pouring; the thread or T dimension, which is the outer diameter of the thread at their utmost protuberance from the bottle neck; and the E dimension at the narrowest diameter taken at the base of each thread.
Depending on the size of the bottle neck, thread standards range from between 5 threads per inch and 12 threads per inch.
A tightly fitting bottle and cap that are well engineered will protect the bottle contents by keeping them fresh and free from being tampered with and from leakage or spilling, which would not only lose product but damage bottle and products shipped or stored adjacent to them.
Proprietary Thread Design
While there are many thread standards at use in the plastic bottle manufacturing industry, some commercial enterprises do have their bottle and thread designs custom-engineered solely for their own proprietary use either to achieve a unique functional or aesthetic effect.