Caps & Closures

Caps & Closures

Frequently Asked Questions

Choosing the right septa and closures for your analysis is critical to ensure chemical compatibility with your sample and solvents and protect sample integrity and prevent contamination. Screw and snap caps are usually made of plastic, mostly polypropylene (PP) or polyethylene (PE), whereas seals for crimp-top vials are made from aluminum. For applications involving automated transport of vials, magnetic caps are recommended. The cap liner or septa is the part of the cap which establishes a tight seal with the vial. Liners and septa can be laminated with PTFE on one or both sides, which results in high chemical resistance and a nearly inert barrier between sample and septa material.

Our closures and liners are designed for a variety of applications. Our chemical compatibility chart will help you to find containers and cap materials that fulfill the requirements of your application/experiment. However, please keep in mind that product performance can vary depending on conditions. It is recommended that proper tests be performed to determine the best product combination for the application. If you still have questions, we’re here to help.

The American Glass Packaging Institute (GPI), American Society German Institute for
Standardization of the Plastics Industry (SPI), and German Institute for Standardization (DIN - GL thread) are responsible for establishing the international standards for glass and plastic container neck finishes that are commonly used in laboratories. Despite the standardized dimensions, we need to caution you that there may still be compatibility issues between containers and caps coming from different manufacturers; therefore, it is advantageous to buy both container and screw closure from the same source whenever possible. Similar to the packaging industry, when a closure finish is designated as 33-400, it means that the nominal diameter measured across the inside of the closure at the opening is approximately 33 mm. The 400 designates a specific style of thread. Keep in mind, the thread finish of the closure and container must be the same. A container with a 33-400 thread finish should be used with a closure that has a 33-400 thread finish. A similar nomenclature guideline applies to the GL thread system in which the number (25, 32 or 45) corresponds to the outer diameter of the thread in mm, for example, a GL 45 thread has an outer diameter of 45 mm. For containers with crimp finish, seals and stoppers are selected based on the mouth OD of the vial. For example, a 20 mm crimp top vial (mouth OD = 20 mm) has to be combined with a 20 mm stopper and a 20 mm seal. A corresponding 20 mm crimping and decapping tool is needed to apply and remove the seal from the vial.

Your choice should be based on the application’s chemical and temperature exposure
requirements. For example, the blue polypropylene caps are liner-less, made of chemical
resistant material, and well suited to many general laboratory applications. If a higher chemical or temperature resistance (to 180 °C) is required we recommend the red PBT (polybutylene terephthalate) cap. Our PBT caps have PTFE-faced liners, meaning that they are an excellent option for long-term sample storage or shipping (ASTM D4991-07 (2015)). GL 25 and GL 45 bottle caps made of PFA are available as liner-less or with cap liners are suitable for the most demanding applications in the pharmaceutical industry including dry heat sterilization (to 200°C).

Crimp closures are generally well known for their excellent sealing characteristics — an absolute must to maintain parenteral content integrity and long-term sample storage. It has to be noted, however, that the quality of a crimp seal is strongly dependent on the right calibration of the crimping tool used to apply the closure to the vial. To maintain good results, make sure to re-calibrate your crimping tool regularly and check the quality of the seal every time you crimp. The best way to know if your vial has been crimped properly is to visually inspect the overall appearance of the crimp.

An optimal crimp result is characterized by the following features:

  • The cap surface is flat.
  • The sides of the cap fit tightly around the vial neck and do not show any deformations.
  • The septa is not being sucked into the vial nor does it emerge out of the center hole.

Many of our customers rely on the so-called “turning test” or “twist test” to verify the crimp result. To test the quality of the crimp, the vial is held in one hand while the other hand is used to test whether the crimped cap can be turned or not. A cap which does not turn indicates a good crimp.

Warning: this test is not very meaningful for two reasons:

  1. Many septa come with a PTFE lamination. Due to the smooth surface of the PTFE membrane, the cap can be turned even if the cap was crimped correctly.
  2. An overcrimped closure which passes the turning test will have impaired sealing characteristics due to the deformation of the cap and septa.

Take into account that resealing characteristics vary amongst septa and that repeat injections may cause coring and/or tearing of some septa. In general, septa made of silicone sandwich coated with PTFE, demonstrate excellent resealing characteristics. They are the product of choice for repeated injections and sample storage. Septa made of PTFE / rubber are a more economical solution for single injection applications that do not require long-term sample storage.

In general, crimp caps, which are attached or removed using crimping or decapping tools,
provide best quality seals for long-term sample storage. If you are looking for a convenient closure system that does not require additional tools, screw caps can offer a good compromise of secure fit and ease of use.

In contrast to crimp top and screw cap vials, snap cap vials are chosen strictly for convenience and for specific fill and finish equipment requiring nested snap cap configurations. Snap caps may be adequate for applications that do not require heating of the sample above room temperature and where moderate sealing performance can be tolerated.

The main function of the PTFE part of the liner is to create a nearly inert barrier between sample and septa material prior to injection. Therefore, this membrane always faces in the direction of the sample.

The hardness of a material is called durometer and measured in ° Shore. Different scales are used to specify what type of substance is described. For example, the “A” in “Shore A 45°” refers to the scale for flexible mold rubbers/plastic. Numbers go from 0 to 100. A high shore grade describes a hard liner whereas a low shore grade refers to a softer liner.